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Laboratory Research in
Environmental Engineering
Laboratory Manual
Monroe L. Weber-Shirk
Leonard W. Lion
James J. Bisogni, Jr.
Cornell University
School of Civil and Environmental Engineering
Ithaca, NY 14853
ii
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
iii
Laboratory Research in
Environmental Engineering
Laboratory Manual
Monroe L. Weber-Shirk
Instructor
mw24@cornell.edu
Leonard W. Lion
Professor
lwl3@cornell.edu
James J. Bisogni, Jr.
Associate Professor
jjb2@cornell.edu
School of Civil and Environmental Engineering
Cornell University
Ithaca, NY 14853
Fifth Edition
iv
© Cornell University 2001
Educational institutions may use this text freely if the title/author page is included.
We request that instructors who use this text notify one of the authors so that the
dissemination of the manual can be documented and to ensure receipt of future
editions of this manual.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
5
Table of Contents
Table of Contents ...............................................................................................................5
Preface .................................................................................................................................9
Laboratory Safety ............................................................................................................10
Introduction ....................................................................................................................10
Personal Protection ........................................................................................................10
Laboratory Protocol .......................................................................................................12
Use of Chemicals ...........................................................................................................13
References ......................................................................................................................19
Questions........................................................................................................................19
Laboratory Measurements and Procedures ..................................................................20
Introduction ....................................................................................................................20
Theory ............................................................................................................................20
Experimental Objectives ................................................................................................22
Experimental Methods ...................................................................................................22
Prelab Questions ............................................................................................................24
Questions........................................................................................................................25
Data Sheet ......................................................................................................................27
Lab Prep Notes ...............................................................................................................29
Reactor Characteristics ...................................................................................................30
Introduction ....................................................................................................................30
Reactor Classifications...................................................................................................30
Reactor Modeling...........................................................................................................30
Mass Conservation .........................................................................................................34
Conductivity Measurements ..........................................................................................35
Procedures ......................................................................................................................36
Prelab Questions ............................................................................................................39
Data Analysis .................................................................................................................39
Lab Prep Notes ...............................................................................................................41
Acid Precipitation and Remediation of Acid Lakes......................................................43
Introduction ....................................................................................................................43
Experimental Objectives ................................................................................................47
Experimental Apparatus.................................................................................................48
Experimental Procedures ...............................................................................................48
Prelab Questions ............................................................................................................53
Data Analysis .................................................................................................................53
Questions........................................................................................................................54
References ......................................................................................................................54
Lab Prep Notes ...............................................................................................................55
Table of Contents
6
Measurement of Acid Neutralizing Capacity ................................................................57
Introduction ....................................................................................................................57
Theory ............................................................................................................................57
Procedure .......................................................................................................................60
Prelab Questions ............................................................................................................61
Questions........................................................................................................................61
References ......................................................................................................................62
Lab Prep Notes ...............................................................................................................63
Phosphorus Determination using the Colorimetric Ascorbic Acid Technique ..........64
Introduction ....................................................................................................................64
Experimental Objectives ................................................................................................66
Experimental Procedures ...............................................................................................66
Prelab Questions ............................................................................................................67
Data Analysis .................................................................................................................67
Questions........................................................................................................................68
References ......................................................................................................................68
Lab Prep Notes ...............................................................................................................69
Soil Washing to Remove Mixed Wastes .........................................................................70
Objective ........................................................................................................................70
Introduction ....................................................................................................................70
Theory ............................................................................................................................71
Apparatus .......................................................................................................................77
Experimental Procedures ...............................................................................................78
Prelab Questions ............................................................................................................82
Data Analysis .................................................................................................................82
References ......................................................................................................................82
Lab Prep Notes ...............................................................................................................85
Oxygen Demand Concepts and Dissolved Oxygen Sag in Streams .............................87
Introduction ....................................................................................................................87
Theory ............................................................................................................................87
Streeter Phelps Equation Development .........................................................................88
Zero Order Kinetics .......................................................................................................92
Experimental Objectives ................................................................................................93
Experimental Methods ...................................................................................................94
Prelab Questions ............................................................................................................95
Data Analysis .................................................................................................................96
References ......................................................................................................................97
Lab Prep Notes ...............................................................................................................98
Methane Production from Municipal Solid Waste .....................................................100
Introduction ..................................................................................................................100
Theory ..........................................................................................................................100
Experiment description ................................................................................................110
Experimental methods .................................................................................................112
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
7
Prelab questions ...........................................................................................................114
Data analysis ................................................................................................................114
References ....................................................................................................................115
Lab Prep Notes .............................................................................................................117
Volatile Organic Carbon Contaminated Site Assessment ..........................................119
Introduction ..................................................................................................................119
Experiment Description ...............................................................................................119
Experimental Procedures .............................................................................................120
Procedure (short version) .............................................................................................122
Prelab Questions ..........................................................................................................123
Data Analysis ...............................................................................................................123
References ....................................................................................................................123
Lab Prep Notes .............................................................................................................124
Volatile Organic Carbon Sorption to Soil ...................................................................126
Introduction ..................................................................................................................126
Theory ..........................................................................................................................126
Analysis of the Unsaturated Distribution Coefficient ( K GS ) ........................................131
Analysis of the Saturated Distribution Coefficient ( K LS ) ............................................133
Experimental procedures .............................................................................................135
Procedure (short version) .............................................................................................136
Prelab Questions ..........................................................................................................137
Data Analysis ...............................................................................................................137
References ....................................................................................................................138
Additional References Relevant to Data Reduction ....................................................139
Symbol List ..................................................................................................................140
Lab Prep Notes .............................................................................................................141
Enhanced Filtration .......................................................................................................142
Introduction ..................................................................................................................142
Theory ..........................................................................................................................142
Previous Research Results ...........................................................................................144
Filter Performance Evaluation .....................................................................................145
Experimental Objectives ..............................................................................................145
Experimental Methods .................................................................................................145
Prelab Questions ..........................................................................................................147
Data Analysis ...............................................................................................................147
Questions for Discussion .............................................................................................148
References ....................................................................................................................148
Lab Prep Notes .............................................................................................................149
Gas Transfer ...................................................................................................................150
Introduction ..................................................................................................................150
Theory ..........................................................................................................................150
Experimental Objectives ..............................................................................................152
Table of Contents
8
Experimental Methods .................................................................................................153
Prelab Questions ..........................................................................................................154
Data Analysis ...............................................................................................................154
References ....................................................................................................................155
Lab Prep Notes .............................................................................................................156
Instrument Instructions.................................................................................................157
Compumet software .....................................................................................................157
pH Probe Calibration ...................................................................................................157
pH Probe Storage .........................................................................................................158
Procedure for Cleaning pH Gel-Filled Polymer Electrode ..........................................158
Dissolved Oxygen Probe..............................................................................................158
Gas Chromatograph .....................................................................................................160
UV-Vis Spectrophotometer .........................................................................................160
Index ................................................................................................................................161
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
9
Preface
Continued leadership in environmental protection requires efficient transfer of
innovative environmental technologies to the next generation of engineers.
Responding to this challenge, the Cornell Environmental Engineering faculty
redesigned the undergraduate environmental engineering curriculum and created a
new senior-level laboratory course. This laboratory manual is one of the products of
the course development. Our goal is to disseminate this information to help expose
undergraduates at Cornell and at other institutions to current environmental
engineering problems and innovative solutions.
A major goal of the undergraduate laboratory course is to develop an atmosphere
where student understanding will emerge for the physical, chemical, and biological
processes that control material fate and transport in environmental and engineered
systems. Student interest is piqued by laboratory exercises that present modern
environmental problems to investigate and solve.
The experiments were designed to encourage the process of “learning around the
edges.” The manifest purpose of an experiment may be a current environmental
problem, but it is expected that students will become familiar with analytical methods
in the course of the laboratory experiment (without transforming the laboratory into
an exercise in analytical techniques). It is our goal that students employ the
theoretical principles that underpin the environmental field in analysis of their
observations without transforming the laboratories into exercises in process theory.
As a result, students can experience the excitement of addressing a current problem
while coincidentally becoming cognizant of relevant physical, chemical, and
biological principles.
At Cornell, student teams of two or three carry out the exercises, maximizing the
opportunity for a hands-on experience. Each team is equipped with modern
instrumentation as well as experimental reactor apparatus designed to facilitate the
study of each topic.
Computerized data acquisition and instrument control are used extensively to make
it easier for students to learn how to use new instruments and to eliminate the
drudgery of manual data acquisition. Software was developed at Cornell to use
computers as virtual instruments that interface with a pH meter/ion (Accumet 50), gas
chromatograph (HP 5890A), UV-Vis Spectrophotometer (HP 8452) This code is
available at the course web site.
The development of this manual and the accompanying course would not have
been possible without funds from the National Science Foundation, the DeFrees
Family Foundation, the Procter and Gamble Fund, the School of Civil and
Environmental Engineering and the College of Engineering at Cornell University.
Monroe L. Weber-Shirk
Leonard W. Lion
James J. Bisogni, Jr.
Ithaca, NY
March 7, 2016
Preface
10
Laboratory Safety
Introduction
Safety is a collective responsibility that requires the full cooperation of everyone in
the laboratory. However, the ultimate responsibility for safety rests with the person
actually carrying out a given procedure. In the case of an academic laboratory, that
person is usually the student. Accidents often result from an indifferent attitude,
failure to use common sense, or failure to follow instructions. Each student should be
aware of what the other students are doing because all can be victims of one
individual's mistake. Do not hesitate to point out to other students that they are
engaging in unsafe practices or operations. If necessary, report it to the instructor. In
the final assessment, students have the greatest responsibility to ensure their own
personal safety.
This guide provides a list of do's and don'ts to minimize safety and health problems
associated with experimental laboratory work. It also provides, where possible, the
ideas and concepts that underlie the practical suggestions. However, the reader is
expected to become involved and to contribute to the overall solutions. The following
are general guidelines for all laboratory workers:
1) Follow all safety instructions carefully.
2) Become thoroughly acquainted with the location and use of safety facilities such
as safety showers, exits and eyewash fountains.
3) Become familiar with the hazards of the chemicals being used, and know the
safety precautions and emergency procedures before undertaking any work.
4) Become familiar with the chemical operations and the hazards involved before
beginning an operation.
Personal Protection
Eye Protection
All people in the laboratory including visitors must wear eye protection at all times,
even when not performing a chemical operation. Wearing of contact lenses in the
laboratory is normally forbidden because contact lenses can hold foreign materials
against the cornea. Furthermore, they may be difficult to remove in the case of a
splash. Soft contact lenses present a particular hazard because they can absorb and
retain chemical vapors. If the use of contact lenses is required for therapeutic reasons
fitted goggles must also be worn. In addition, approved standing shields and face
shields that protect the neck and ears as well as the face should be used when
appropriate for work at reduced pressure or where there is a potential for explosions,
implosions or splashing. Normal prescription eyeglasses, though meeting the Food
and Drug Administration's standards for shatter resistance, do not provide appropriate
laboratory eye protection.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
11
Clothing
Clothing worn in the laboratory should offer protection from splashes and spills,
should be easily removable in case of accident, and should be at least fire resistant.
Nonflammable, nonporous aprons offer the most satisfactory and the least expensive
protection. Lab jackets or coats should have snap fasteners rather than buttons so that
they can be readily removed.
High-heeled or open-toed shoes, sandals, or shoes made of woven material should
not be worn in the laboratory. Shorts, cutoffs and miniskirts are also inappropriate.
Long hair and loose clothing should be constrained. Jewelry such as rings, bracelets,
and watches should not be worn in order to prevent chemical seepage under the
jewelry, contact with electrical sources, catching on equipment, and damage to the
jewelry.
Gloves
Gloves can serve as an important part of personal protection when they are used
correctly. Check to ensure the absence of cracks or small holes in the gloves before
each use. In order to prevent the unintentional spread of chemicals, gloves should be
removed before leaving the work area and before handling such things as telephones,
doorknobs, writing instruments, computers, and laboratory notebooks. Gloves may be
reused, cleaned, or discarded, consistent with their use and contamination.
A wide variety of gloves is available to protect against chemical exposure. Because
the permeability of gloves of the same or similar material varies from manufacturer to
manufacturer, no specific recommendations are given here. Be aware that if a
chemical diffuses through a glove, that chemical is held against the worker's hand and
the individual may then be more exposed to the chemical than if the glove had not
been worn.
Personal Hygiene
Everyone working in a chemistry laboratory should be aware of the dangers of
ingesting chemicals. These common sense precautions will minimize the possibility
of such exposure:
1) Do not prepare, store (even temporarily), or consume food or beverages in any
chemical laboratory.
2) Do not smoke in any chemical laboratory. Additionally, be aware that tobacco
products in opened packages can absorb chemical vapors.
3) Do not apply cosmetics in a laboratory.
4) Wash hands and arms thoroughly before leaving the laboratory, even if gloves
have been worn.
5) Wash separately from personal laundry, lab coats or jackets on which chemicals
have been spilled.
6) Never wear or bring lab coats or jackets into areas where food is consumed.
7) Never pipette by mouth. Always use a pipette aid or suction bulb.
Laboratory Safety
12
Laboratory Protocol
The chemistry laboratory is a place for serious learning and working. Horseplay
cannot be tolerated. Variations in procedures including changes in quantities or
reagents may be dangerous. Such alterations may only be made with the knowledge
and approval of the instructor.
Housekeeping
In the laboratory and elsewhere, keeping things clean and neat generally leads to a
safer environment. Avoid unnecessary hazards by keeping drawers and cabinets
closed while working. Never store materials, especially chemicals, on the floor, even
temporarily. Work spaces and storage areas should be kept clear of broken glassware,
leftover chemicals and scraps of paper. Keep aisles free of obstructions such as
chairs, boxes and waste receptacles. Avoid slipping hazards by keeping the floor clear
of ice, stoppers, glass beads or rods, other small items, and spilled liquids. Use the
required procedure for the proper disposal of chemical wastes and solvents.
Cleaning Glassware
Clean glassware at the laboratory sink or in laboratory dishwashers. Use hot water,
if available, and soap or other detergent. If necessary, use a mild scouring powder.
Wear appropriate gloves that have been checked to ensure that no holes are present.
Use brushes of suitable stiffness and size. Avoid accumulating too many articles in
the cleanup area. Usually work space around a sink is limited and piling up dirty or
cleaned glassware leads to breakage. Remember that the turbid water in a sink may
hide a jagged edge on a piece of broken glassware that was intact when put into the
water. A pair of heavy gloves may be useful for removing broken glass, but care must
be exercised to prevent glove contamination. To minimize breakage of glassware,
sink bottoms should have rubber or plastic mats that do not block the drains.
Avoid the use of strong cleaning agents such as nitric acid, chromic acid, sulfuric
acid, strong oxidizers, or any chemical with a "per" in its name (such as perchloric
acid, ammonium persulfate, etc.) unless specifically instructed to use them, and then
only when wearing proper protective equipment. A number of explosions involving
strong oxidizing cleaning solutions, such as chromic sulfuric acid mixtures, have been
reported. The use of flammable solvents should be minimized and, when they are
used, appropriate precautions must be observed.
Unattended Operation of Equipment
Reactions that are left to run unattended overnight or at other times are prime
sources for fires, floods and explosions. Do not let equipment such as power stirrers,
hot plates, heating mantles, and water condensers run overnight without fail-safe
provisions and the instructor's consent. Check unattended reactions periodically.
Always leave a note plainly posted with a phone number where you and the instructor
can be reached in case of emergency. Remember that in the middle of the night,
emergency personnel are entirely dependent on accurate instructions and information.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
13
Fume Hoods and Ventilation
A large number of common substances present acute respiratory hazards and
should not be used in a confined area in large amounts. They should be dispensed and
handled only where there is adequate ventilation, such as in a hood. Adequate
ventilation is defined as ventilation that is sufficient to keep the concentration of a
chemical below the threshold limit value or permissible exposure limit.
If you smell a chemical, it is obvious that you are inhaling it. However, odor does
not necessarily indicate that a dangerous concentration has been reached. By contrast,
many chemicals can be present at hazardous concentrations without any noticeable
odor.
Refrigerators
Chemicals stored in refrigerators should be sealed, double packaged if possible,
and labeled with the name of the material, the date placed in the refrigerator, and the
name of the person who stored the material A current inventory should be maintained.
Old chemicals should be disposed of after a specified storage period. Household
refrigerators should not be used for chemical storage.
If used for storage of radioactive materials, a refrigerator should be plainly marked
with the standard radioactivity symbol and lettering, and routine surveys should be
made to ensure that the radioactive material has not contaminated the refrigerator.
Food should never be stored in a refrigerator used for chemical storage. These
refrigerators should be clearly labeled "No Food". Conversely food refrigerators,
which must be always outside of, and away from, the chemical work area, should be
labeled "Food Only—No Chemicals".
Radioactive Materials
Radioactive materials are used in the Environmental Engineering laboratories.
Doors of rooms containing radioactive materials are clearly labeled. Areas where
radioactive materials are used are clearly delineated with labeling tape and signs. All
equipment within areas labeled radioactive are potentially contaminated and should
not be touched or removed. Do not place anything into or take anything from an area
labeled radioactive.
Working Alone
Avoid working alone in a building or in a laboratory.
Use of Chemicals
Before using any chemical you need to know how to safely handle it. The safety
precautions taken are dependent on the exposure routes and the potential harmful
effects.
Routes of Exposure
1) ingestion
2) inhalation
3) absorbed through skin
Laboratory Safety
14
4) eye contact
Each potential exposure route requires different precautions. Chemical exposure
may have acute (immediate, short term) or chronic (long term potentially cumulative)
affects. Information on health hazards can be found on chemical labels and in
Material Safety Data Sheets.
Material Safety Data Sheets
MSDS sheets for most chemicals used in the laboratory are located on the
bookshelf in the entrance hallway of the Environmental Laboratory. Electronic
versions (potentially more current) can be found using the world wide web at:
http://www.cee.cornell.edu/safety/
MSDS provide extensive information on safe handling, first aid, toxicity, etc.
Following is a list of terms used in MSDS:
TLV—Threshold Limit Value—are values for airborne toxic materials that are to be
used as guides in control of health hazards. They represent concentrations to
which nearly all workers (workers without special sensitivities) can be exposed to
for long periods of time without harmful effect. TLV's are usually expressed as
parts per million (ppm). TLV's are also expressed as mg of dust or vapor/m3 of air.
TDLo—Toxic Dose Low—the lowest dose of a substance introduced by any route,
other than inhalation, over any given period of time and reported to produce any
toxic effect in humans or to produce carcinogenic, neoplastigenic, or teratogenic
effects in animals or humans.
TCLo—Toxic Concentration Low—the lowest concentration of a substance in air to
which humans or animals have been exposed for any given period of time and
reported to produce any toxic effect in humans or to produce carcinogenic,
neoplastigenic, or teratogenic effects in animals or humans.
TDLo—Lethal Dose Low—the lowest dose (other than LD50) of a substance
introduced by any route, other than inhalation, over any given period of time in
one or more divided portions and reported to have caused death in humans or
animals.
LD50—Lethal Dose Fifty—a calculated dose of a substance that is expected to cause
the death of 50% of an entire defined experimental animal population. It is
determined from the exposure to the substance by any route other than inhalation
of a significant number from that population.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
15
LCLo—Lethal
Concentration
Table 1. NFPA hazard code ratings.
Low—the
Code
Health
Fire
Reactivity
lowest
Very short exposure
Will rapidly or
Capable of detonation
concentration
can cause death or
completely
or explosive reaction at
4
of a substance
major residual
vaporize at normal
normal temperatures
in air, other
injury
pressure and
and pressures
temperature
than LC50, that
Short exposure can
Can be ignited
Capable of detonation
has
been
cause serious
under almost all
or explosive reaction
3
reported
to
temporary or
ambient
buy requires a strong
have
caused
residual injury
temperatures
initiating source or
death
in
must be heated under
confinement before
humans
or
initiation
animals. The
Intense or continued
Must be
Undergoes violent
reported
exposure can cause moderately heated
chemical change at
2
concentrations
temporary
or exposed to high elevated temperatures
may be entered
incapacitation or
temperature before and pressures or reacts
possible residual
ignition
violently with water.
for periods of
injury
exposure that
Can cause irritation Must be preheated
Normally stable but
are less than 24
but only minor
before ignition
can become unstable at
1
hours (acute) or
residual injury
elevated temperatures
greater than 24
and pressures.
During a fire offers
Will not burn
Stable even under fire
hours (subacute
no hazard beyond
conditions.
0
and chronic).
combustion
LC50—Lethal
Concentration Fifty—a calculated concentration of a substance in air, exposure to
which for a specified length of time is expected to cause the death of 50% of an
entire defined experimental animal population. It is determined from the exposure
to the substance of a significant number from that population.
Chemical Labels
All chemicals must be labeled. Unlabeled containers of mystery chemicals or
chemical solutions are a nightmare for disposal as well as a potential safety hazard.
The OSHA Hazard Communication Standard and the OSHA Lab Standard have
specific requirements for the labeling of chemicals. In a laboratory covered under the
Lab Standard, if a chemical is designated as a hazardous material, that is having the
characteristics of corrosivity, ignitability, toxicity (generally meaning a highly toxic
material with an LD50 of 50 mg/kg or less), reactivity, etc., and if it is made into a
solution or repackaged as a solid or liquid in a concentration greater than 1% (0.1%
for a carcinogen) it needs to have a so called Right-To-Know (RTK) label that
duplicates the hazard warnings, precautions and first aid steps found on the original
label. All other chemicals must have at minimum a label with chemical name,
concentration, and date prepared. "Right to Know Labels" will be made available for
your use when necessary.
Laboratory Safety
16
National Fire Protection Association (NFPA) ratings are included to indicate the
types and severity of the hazards. The NFPA ratings are on a scale of 0-4 with 0 being
nonhazardous and 4 being most hazardous. The ratings are described in Table 1.
Chemical Storage1
There has been much concern, and some confusion, about the proper storage of
laboratory chemicals. Here “proper” means the storage of chemicals in such a manner
as to prevent incompatible materials from being accidentally mixed together in the
event of the breakage of one or more containers in the storage area or to prevent the
formation of reactive vapors that may require vented chemical storage areas. Below is
a concise guide to the storage of common laboratory chemicals.
1) Perchloric acid is separated from all other materials.
2) Hydrofluoric acid is separated from all other materials.
3) Concentrated nitric acid is separated from all other materials.
4) Highly toxic materials (LD50 of 50 mg/kg or less) are stored separately.
5) Carcinogenic chemicals are stored separately.
6) Inorganic acids (except for 1, 2, 3 above) are stored separately.
7) Bases are stored separately.
8) Strong oxidizing agents are stored separately.
9) Strong reducing agents are stored separately.
10) Water reactive, pyrophoric and explosive materials are stored separately.
11) Flammable organic materials (solvents, organic acids, organic reagents) are stored
separately.
Guidelines for separating incompatible chemicals:
1) Place the chemicals to be stored separately in a heavy gauge Nalgene (or similar
plastic) tub. Plastic secondary containers must be compatible with the material
being stored.
2) Strong acids, especially perchloric, nitric and hydrofluoric are best stored in
plastic containers designed to store strong mineral acids. These are available from
lab equipment supply houses.
3) Bottle-in-a-can type of containers are also acceptable as secondary containment.
Small containers of compatible chemicals may be stored in a dessicator or other
secure container. Secondary containment is especially useful for highly toxic
materials and carcinogens.
4) Dry chemicals stored in approved cabinets with doors may be grouped together by
compatibility type on separate shelves or areas of shelves separated by taping off
sections of shelving to designate where chemicals of one type are stored.
Physically separated cabinets may be used to provide a barrier between groups of
1
Prepared by Tom Shelley (Chemical Hygiene Officer) 7/96. For additional
information or questions you may have, please contact Tom Shelley at 5-4288.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
17
stored incompatible chemicals. Strong mineral acids may be stored in one cabinet
and strong bases stored in a second cabinet, for example. Flammable solvents
should be stored in a rated flammable storage cabinet if available.
If you are uncertain of the hazardous characteristics of a particular chemical refer
to the MSDS for that material. A good MSDS will not only describe the hazardous
characteristics of the chemical, it will also list incompatible materials.
Transporting Chemicals
Transport all chemicals using the container-within-a-container concept to shield
chemicals from shock during any sudden change of movement. Large containers of
corrosives should be transported from central storage in a chemically resistant bucket
or other container designed for this purpose. Stairs must be negotiated carefully.
Elevators, unless specifically indicated and so designated, should not be used for
carrying chemicals. Smoking is never allowed around chemicals and apparatus in
transit or in the work area itself.
When moving in the laboratory, anticipate sudden backing up or changes in
direction from others. If you stumble or fall while carrying glassware or chemicals,
try to project them away from yourself and others.
When a flammable liquid is withdrawn from a drum, or when a drum is filled, both
the drum and the other equipment must be electrically wired to each other and to the
ground in order to avoid the possible buildup of a static charge. Only small quantities
should be transferred to glass containers. If transferring from a metal container to
glass, the metal container should be grounded.
Chemical Disposal
The Environmental Protection Agency (EPA) classifies wastes by their reaction
characteristics. A summary of the major classifications and some general treatment
guidelines are listed below. Specific information may be found in the book, Prudent
Practices for Disposal of Chemicals from Laboratories, as well as other reference
materials.
Ignitability: These substances generally include flammable solvents and certain
solids. Flammable solvents must never be poured down the drain. They should be
collected for disposal in approved flammable solvent containers. In some cases it may
be feasible to recover and reuse solvents by distillation. Such solvent recovery must
include appropriate safety precautions and attention to potentially dangerous
contamination such as that due to peroxide formation.
Corrosivity: This classification includes common acids and bases. They must be
collected in waste containers that will not ultimately corrode and leak, such as plastic
containers. It often may be appropriate to neutralize waste acids with waste bases and
where allowed by local regulations, dispose of the neutral materials via the sanitary
sewer system. Again, the nature of the neutralized material must be considered to
ensure that it does not involve an environmental hazard such as chromium salts from
chromic acid neutralization.
Reactivity: These substances include reactive metals such as sodium and various
water reactive reagents. Compounds such as cyanides or sulfides are included in this
Laboratory Safety
18
class if they can readily evolve toxic gases such as hydrogen cyanide. Their collection
for disposal must be carried out with particular care. When present in small
quantities, it is advisable to deactivate reactive metals by careful reaction with
appropriate alcohols and to deactivate certain oxygen or sulfur containing compounds
through oxidation. Specific procedures should be consulted.
Toxicity: Although the EPA has specific procedures for determining toxicity, all
chemicals may be toxic in certain concentrations. Appropriate procedures should be
established in each laboratory for collection and disposal of these materials.
The handling of reaction byproducts, surplus and waste chemicals, and
contaminated materials is an important part of laboratory safety procedures. Each
laboratory worker is responsible for ensuing that wastes are handled in a manner that
minimizes personal hazard and recognizes the potential for environmental
contamination.
Most instructional laboratories will have clear procedures for students to follow in
order to minimize the generation of waste materials. Typically reaction byproducts
and surplus chemicals will be neutralized or deactivated as part of the experimental
procedure. Waste materials must be handled in specific ways as designated by federal
and local regulations. University guidelines for waste disposal can be found in
chapter
7
of
the
Chemical
Hygiene
Plan
(available
at
http://www.cee.cornell.edu/safety/ )
Some general guidelines are:
1) Dispose of waste materials promptly. When disposing of chemicals one basic
principle applies: Keep each different class of chemical in a separate clearly
labeled disposal container.
2) Never put chemicals into a sink or down the drain unless they are deactivated or
neutralized and they are allowed by local regulation in the sanitary sewer system.
[See Chemical Hygiene Plan for list of chemicals that can be safely disposed of in
the sanitary sewer.]
3) Put ordinary waste paper in a wastepaper basket separate from the chemical
wastes. If a piece of paper is contaminated, such as paper toweling used to clean
up a spill, put the contaminated paper in the special container that is marked for
this use. It must be treated as a chemical waste.
4) Broken glass belongs in its own marked waste container Broken thermometers
may contain mercury in the fragments and these belong in their own special
sealed "broken thermometer" container.
5) Peroxides, because of their reactivity, and the unpredictable nature of their
formation in laboratory chemicals, have attracted considerable attention. The
disposal of large quantities (25 g or more) of peroxides requires expert assistance.
Consider each case individually for handling and disposal.
A complete list of compounds considered safe for drain disposal can be found in
Chapter 7 of the Chemical Hygiene Plan (http://www.cee.cornell.edu/safety/).
Disposal techniques for chemicals not found in this list must be disposed of using
techniques approved of by Cornell Environmental Health and Safety. When possible,
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
19
hazardous chemicals can be neutralized and then disposed. When chemicals are
produced that cannot be disposed of using the sanitary sewer, techniques to decrease
the volume of the waste should be considered.
References
Safety in Academic Chemistry Laboratories. A publication of the American Chemical
Society Committee on Chemical Safety. Fifth edition. 1990
Cornell University Chemical Hygiene Plan: Guide to Chemical Safety for Laboratory
Workers. A publication of the Office of Environmental Health, 2000.
(http://www.ehs.cornell.edu/lrs/CHP/chp.htm)
OSHA Laboratory Standard
One of the best books to get started with regulatory compliance is a publication
from the American Chemical Society entitled, "Laboratory Waste Management. A
Guidebook."
Questions
1) Why are contact lenses hazardous in the laboratory?
2) What is the minimum information needed on the label for each chemical? When
are right to know labels required?
3) Why is it important to label a bottle even if it only contains distilled water?
4) Find an MSDS for sodium nitrate.
a) Who created the MSDS?
b) What is the solubility of sodium nitrate in water?
c) Is sodium nitrate carcinogenic?
d) What is the LD50 oral rat?
e) How much sodium nitrate would you have to ingest to give a 50% chance of
death (estimate from available LD50 data).
f) How much of a 1 M solution would you have to ingest to give a 50% chance
of death?
g) Are there any chronic effects of exposure to sodium nitrate?
5) You are in the laboratory preparing chemical solutions for an experiment and it is
lunchtime. You decide to go to the student lounge to eat. What must you do
before leaving the laboratory?
6) Where are the eyewash station, the shower, and the fire extinguishers located in
the laboratory?
Laboratory Safety
20
Laboratory Measurements and Procedures
Introduction
Measurements of masses, volumes, and preparation of chemical solutions of known
composition are essential laboratory skills. The goal of this exercise is to gain
familiarity with these laboratory procedures. You will use these skills repeatedly
throughout the semester.
Theory
Many laboratory procedures require preparation of chemical solutions. Most
chemical solutions are prepared on the basis of mass of solute per volume of solution
(grams per liter or Moles per liter). Preparation of these chemical solutions requires
the ability to accurately measure both mass and volume.
Preparation of dilutions is also frequently required. Many analytical techniques
require the preparation of known standards. Standards are generally prepared with
concentrations similar to that of the samples being analyzed. In environmental work
many of the analyses are for hazardous substances at very low concentrations (mg/L
or µg/L levels). It is difficult to weigh accurately a few milligrams of a chemical with
an analytical balance. Often dry chemicals are in crystalline or granular form with
each crystal weighing several milligrams making it difficult to get close to the desired
weight. Thus it is often easier to prepare a low concentration standard by diluting a
higher concentration stock solution. For example, 100 mL of a 10 mg/L solution of
NaCl could be obtained by first preparing a 1 g/L NaCl solution (100 mg in 100 mL).
One mL of the 1 g/L stock solution would then be diluted to 100 mL to obtain a 10
mg/L solution.
Absorption spectroscopy is one analytical technique that can be used to measure
the concentration of a compound. Solutions that are colored absorb light in the visible
range. The resulting color of the solution is from the light that is transmitted.
According to Beer's law the attenuation of light in a chemical solution is related to the
concentration and the length of the path that the light passes through.
P
log o
P
bc
2.1
where c is the concentration of the chemical species, b is the distance the light travels
through the solution, is a constant Po is the intensity of the incident light, and P is
the intensity of the transmitted light. Absorption, A, is defined as:
P
A log o
P
2.2
In practice Po is the intensity of light through a reference sample (such as deionized
water) and thus accounts for any losses in the walls of the sample chamber. From
equation 2.1 and 2.2 it may be seen that absorption is directly proportional to the
concentration of the chemical species.
A bc
CEE 453: Laboratory Research in Environmental Engineering
2.3
Spring 2001
21
The instrument you will use to measure
absorbance is a Hewlett Packard (HP) model
8452A diode array spectrophotometer. The Table 1. Wavelengths of light
color
wavelength (nm)
diode array spectrophotometer uses a broadultra
violet
190-380
spectrum source of incident light from a
violet
380-450
deuterium lamp. The light passes through the
blue
450-490
sample and is split by a grating into a spectrum
green
490-560
of light that is measured by an array of diodes.
yellow
560-590
orange
590-630
Each diode measures a bandwidth of 2 nm with
red
630-760
316 diodes covering the range from 190 nm to
820 nm. The wavelengths of light and their colors are given in Table 1. The light path
for the diode array spectrophotometer is
shown in Figure 1.
The HP 8452A spectrophotometer has a
Diode Array
photometric range of 0.002 - 3.3
absorbance units. In practice absorbance
Spectrograph Lens
measurements greater than 2.5 are not very
meaningful as they indicate that 99.7% of Grating
Slit
Shutter
the incident light at that wavelength was
Sample Cell
absorbed. Conversely, an absorbance of
Source Lens
0.002 means that 0.5% of the incident light
at that wavelength was absorbed.
Deuterium Lamp
When measuring samples of known
concentration
the
Spectrophotometer
software (page 160) calculates the Figure 1. Diagram of light path in
relationship between absorbance and diode array spectrophotometer.
concentration at a selected wavelength. The
slope (m), intercept (b) and correlation
coefficient (r) are calculated using equation
2.4 through 2.6.
The slope of the best fit line is
m
xy
x
x n
x y
n
2.4
2
2
The intercept of the line is
b y mx
2.5
The correlation coefficient is defined as
r
x2
xy
x
x y
n
2
n
y
2
y
n
2
Laboratory Measurements and Procedures
2.6
22
where x is the concentration of the solute (methylene blue in this exercise), y is the
absorbance, and n is the number of samples.
Experimental Objectives
To gain proficiency in:
1) Calibrating and using electronic balances
2) Digital pipetting
3) Preparing a solution of known concentration
4) Preparing dilutions
5) Measuring concentrations using a UV-Vis spectrophotometer
Experimental Methods
Mass Measurements
Mass can be accurately measured with an electronic analytical balance. Perhaps
because balances are so easy to use it is easy to forget that they should be calibrated
on a regular basis. It is recommended that balances be calibrated once a week, after
the balance has been moved, or if excessive temperature variations have occurred. In
order for balances to operate correctly they also need to be level. Most balances come
with a bubble level and adjustable feet. Before calibrating a balance verify that the
balance is level.
The environmental laboratory is equipped with balances manufactured by Denver
Instruments. To calibrate the Denver Instrument balances:
1) Zero the balance by pressing the tare button.
2) Press the MENU key until "MENU #1" is displayed.
3) Press the 1 key to select Calibrate.
4) Note the preset calibration masses that can be used for calibration on the bottom
of the display.
5) Place a calibration mass on the pan (handle the calibration mass using a cotton
glove or tissue paper).
6) The balance will automatically calibrate. A short beep will occur and the display
will read CALIBRATED for three seconds, and then return to the measurement
screen.
Dry chemicals can be weighed in disposable plastic "weighing boats" or other
suitable containers. It is often desirable to subtract the weight of the container in
which the chemical is being weighed. The weight of the chemical can be obtained
either by weighing the container first and then subtracting, or by "zeroing" the
balance with the container on the balance.
Temperature Measurement
Use the Accumet™ pH/ion meter to measure the temperature of distilled water.
The temperature probe is the 4-mm diameter metallic probe. Place the probe in a
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
23
100-mL plastic beaker full of distilled water. Wait at least 15 seconds to allow the
probe to equilibrate with the solution.
Pipette Technique
1) Use Figure 2 to estimate the mass of 990 µL of distilled water (at the measured
temperature).
2) Use a 100-1000 µL digital pipette to transfer 990 µL of distilled water to a tared
weighing boat on the 100 g scale. Record the mass of the water and compare with
the expected value (Figure 2). Repeat this step if necessary until your pipetting
error is less than 2%, then measure the mass of 5 replicate 990 µL pipette
samples. Calculate the mean ( x defined in equation 2.7), standard deviation (s
defined in equation 2.8), and coefficient of variation, s/ x , for your measurements.
The coefficient of variation (c.v.) is a good measure of the precision of your
technique. For this test a c.v. < 1% should be achievable.
x
s
x
2.7
n
x2
x
n
n
2.8
Measure Density
1000
Kg/cubic meter
1) Weigh a 100 mL volumetric flask
999
with its cap (use the 400 g or 800 g
balance).
998
2) Prepare 100 mL of a 1 M solution
997
of sodium chloride in the weighed
flask. Make sure to mix the
996
solution and then verify that you
995
have exactly 100 mL of solution.
15
20
25
30
Note that the volume decreases as
Temperature (°C)
the salt dissolves.
3) Weigh the flask (with its cap) plus Figure 2. Density of water vs. temperature.
the sodium chloride solution and
calculate the density of the 1 M
NaCl solution.
Prepare methylene blue standards of several concentrations
1) A methylene blue stock solution of 1 g/L has been prepared. Use it to prepare 100
mL of each of the following concentrations: 1 mg/L, 2 mg/L, 3 mg/L, 4 mg/L, and
5 mg/L.
Laboratory Measurements and Procedures
24
2) Note any errors in transfer of mass as you prepare these dilutions (the color will
make it easy to see).
Prepare a standard curve and measure an unknown
1) See page 160 for instructions on using the UV-Vis Spectrophotometer software.
2) Measure the absorbance of the methylene blue solutions using a UV-Vis
spectrophotometer. Analyze the 5 methylene blue samples plus a distilled water
sample (0 mg/L methylene blue) as standards. Select Measure Standards from
the computer control palette. Fill in the information for the six samples (starting
with distilled water and ending with the highest concentration of methylene blue)
and follow instructions as you are prompted.
3) Save the data as \\enviro\enviro\Courses\453\fundamentals\netid_blue.
4) Record the absorbance at 660 nm for each of the solutions. You can drag the blue
cursor on the “standard graph” to the wavelength of choice and read the exact
absorbance (and wavelength) in the digital display to the right of the graph. Note
that you can do this after you have analyzed all of the standards.
5) Record the correlation coefficient (equation 2.6), slope (equation 2.4), and
intercept (equation 2.5) for the absorbance at 660 nm vs. methylene blue
concentration. These values are shown next to the “calibration graph” and
correspond to the wavelength selected using the blue cursor on the “standard
graph.”
6) Measure the absorbance of a methylene blue solution of unknown concentration.
Select Measure Samples from the control palette. Save the data as
\\enviro\enviro\Courses\453\fundamentals\netid_unknown. Record its absorbance
at 660 nm and the calculated concentration. These values are given in the digital
displays in the bottom left of the window.
7) Print the results by selecting Print from the control palette.
8) Export your standards spectra to the \\enviro\enviro\Courses\453\fundamentals
folder.
9) Turn on the pump and place the sipper tube in distilled water to clean out the
sample cell by selecting Run Pump from the control palette.
Prelab Questions
1) You need 100 mL of a 1 µM solution of zinc that you will use as a standard to
calibrate an atomic adsorption spectrophotometer. Find a source of zinc ions
combined either with chloride or nitrate (you can use the world wide web or any
other source of information). What is the molecular formula of the compound that
you found? Zinc disposal down the sanitary sewer is restricted at Cornell. How
does the disposal restriction for zinc influence how you prepare the zinc standard?
How would you prepare this standard using techniques readily available in the
environmental laboratory? Note that we have pipettes that can dispense volumes
between 10 L and 1 mL and that we have 100 mL and 1 L volumetric flasks.
Include enough information so that you could prepare the standard without doing
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
25
any additional calculations. Consider your ability to accurately weigh small
masses. Explain your procedure for any dilutions.
2) The density of sodium chloride solutions as a function of concentration is
approximately 0.6985C + 998.29 (kg/m3) (C is kg of salt/m3). What is the density
of a 1 M solution of sodium chloride?
Questions
1) Create a graph of absorbance at 660 nm vs concentration of methylene blue in
Excel using the exported data file. Does absorbance at 660 nm increase linearly
with concentration of methylene blue?
2) Plot as a function of wavelength for each of the standards on a single graph.
Make sure you include units and axis labels on your graph. If Beer’s law is
obeyed what should the graph look like?
3) Did you use interpolation or extrapolation to get the concentration of the
unknown?
4) What colors of light are most strongly absorbed by methylene blue?
5) What measurement controls the accuracy of the density measurement? What
should the accuracy be? What was your percent error in measuring the density of
the 1 M NaCl solution?
Laboratory Measurements and Procedures
27
Data Sheet
Balance Calibration
Balance ID .................................................................___________
Mass of calibration mass ............................................___________
2nd mass used to verify calibration ...........................___________
Measured mass of 2nd mass ......................................___________
Temperature Measurement
Distilled water temperature ........................................___________
Pipette Technique (use DI-100 balance)
Density of water at that temperature ..........................___________
Actual mass of 990 µL of pure water ........................___________
Mass of 990 µL of water (rep 1) ................................___________
Mass of 990 µL of water (rep 2) ................................___________
Mass of 990 µL of water (rep 3) ................................___________
Mass of 990 µL of water (rep 4) ................................___________
Mass of 990 µL of water (rep 5) ................................___________
Average of the 5 measurements .................................___________
Standard deviation of the 5 measurements ................___________
Precision
Percent coefficient of variation
of the 5 measurements ...............................................___________
Accuracy
average percent error for pipetting
100
actual mass - measured mass
............................___________
actual mass
Where actual mass is calculated from the density of water and the setting of the
pipette and measured mass is measured with the balance.
Laboratory Measurements and Procedures
28
Measure Density (use DI-800 balance)
Molecular weight of NaCl .........................................___________
Mass of NaCl in 100 mL of a 1-M solution ...............
Measured mass of NaCl used .....................................___________
Measured mass of empty 100 mL flask .....................___________
Measured mass of flask + 1M solution ......................___________
Mass of 100 mL of 1 M NaCl solution ......................___________
Density of 1 M NaCl solution ....................................___________
Prepare methylene blue standards of several concentrations
Volume of 1 g/L MB diluted to 100 mL to obtain:
1 mg/L MB .................................................................___________
2 mg/L MB .................................................................___________
3 mg/L MB .................................................................___________
4 mg/L MB .................................................................___________
5 mg/L MB .................................................................___________
Measure absorbance at 660 nm using a spectrophotometer.
Spectrophotometer (computer name)? .......................___________
Absorbance of distilled water ....................................___________
Absorbance of 1 mg/L methylene blue ......................___________
Absorbance of 2 mg/L methylene blue ......................___________
Absorbance of 3 mg/L methylene blue ......................___________
Absorbance of 4 mg/L methylene blue ......................___________
Absorbance of 5 mg/L methylene blue ......................___________
Slope at 660 nm (m)...................................................___________
Intercept at 660 nm (b) ...............................................___________
Correlation coefficient at 660 nm (r) .........................___________
Absorbance of unknown at 660 nm ...........................___________
Calculated concentration of unknown .......................___________
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
29
Lab Prep Notes
Table 2. Reagent list.
Description
NaCl
Methylene blue
Supplier
Fisher Scientific
Fisher Scientific
Catalog
number
BP358-1
M291-25
Table 3. Equipment list
Description
Calibra 100-1095
µL
Calibra 10-109.5
µL
DI 100 analytical
toploader
DI-800 Toploader
100 mL volumetric
Supplier
Fisher Scientific
Catalog
number
13-707-5
Fisher Scientific
13-707-3
Fisher Scientific
01-913-1A
Fisher Scientific
Fisher Scientific
UV-Vis
spectrophotometer
Hewlett-Packard
Company
01-913-1C
10-198-50
B
8452A
Table 4. Methylene Blue Stock Solution
Description MW (g/M)
319.87
C16H18N3SCl
conc. (g/L)
1
100 mL
100.0 mg
Setup
1) Prepare stock methylene blue solution and distribute to student workstations in 15
mL vials.
2) Prepare 100 mL of unknown in concentration range of standards. Divide into two
bottles (one for each spectrophotometer).
3) Verify that spectrophotometers are working (prepare a calibration curve as a test).
4) Verify that balances calibrate easily.
5) Disassemble, clean and lubricate all pipettes.
Laboratory Measurements and Procedures
30
Reactor Characteristics
Introduction
Chemical, biological and physical processes in nature and in engineered systems
usually take place in what we call "reactors." Reactors are defined by a real or
imaginary boundary that physically confines the processes. Lakes, segments of a
river, and settling tanks in treatment plants are examples of reactors. Most, but not all,
reactors experience continuous flow (in and out). Some reactors, experience flow
(input and output) only once. These are called "batch" reactors. It is important to
know the mixing level and residence time in reactors, since they both affect the
degree of process reaction that occurs while the fluid (usually water) and its
components (often pollutants) pass through the reactor.
Tracer studies can be used to determine the hydraulic characteristics of a reactor
such as the disinfection contact tanks at water treatment plants. The results from
tracer studies are used to obtain accurate estimates of the effective contact time.
Reactor Classifications
Mixing levels give rise to three categories of reactors; completely mixed flow
(CMF), plug flow (PF) and flow with dispersion (FD). The plug flow reactor is an
idealized extreme not attainable in practice. All real reactors fall under the category
of FD or CMF.
Reactor Modeling
Both the CMF and the PF reactors are limiting cases of the FD reactor. Therefore the
FD reactor model will be developed first. Equation 3.1 is the governing differential
equation for a conservative (i.e., non-reactive) substance in a reactor that has
advective transport (i.e., flow) and some mixing in the direction of flow (x dimension).
C
C
2C
-U
Dd 2
t
x
x
C = concentration of a conservative substance
U = average fluid velocity in the x direction
Dd = longitudinal dispersion coefficient
t = time
The dispersion coefficient is a measure of the mixing in a system.
3.1
Flow with Dispersion
One of the easiest methods to determine the mixing (dispersion) characteristics of a
reactor is to add a spike input of a conservative material and then monitor the
concentration of the material in the reactor effluent.
Assuming complete mixing in y-z plane then transport occurs only in the x
direction and the concentration of tracer for any x and t (after t=0) the solution to
equation 3.1 gives:
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
31
C(x,t)
x 2
exp
A Dd t
Dd t
M
3.2
where M = mass of conservative material in the spike, Dd = axial dispersion
coefficient [L2/T], x' = x - Ut, U = longitudinal advective velocity in the reactor, and
A is the cross-sectional area of the reactor. A measure of dispersion can be obtained
directly from equation 3.2. From this equation we expect a maximum value of C at t =
M
x/U. At this time C(x,t)
. If the mass of the tracer input (M) and reactor
A Dd t
cross-sectional area (A) are known, then Dd can be estimated.
The form of equation 3.3 is exactly like the normal distribution curve:
x2
CA
1
exp 2
M
x
x
3.3
x2 2 Dd t
3.4
where
The variance in concentration over space ( x2 ) is the variance in concentrations
taken from many different positions in the reactor at some single moment in time, t.
The variance in x ( x2 ) has dimensions of length squared.
The variance of tracer concentration versus time ( t2 , with dimensions of time
squared) can be measured by sampling at a single point in the reactor at many
different times and can be computed using the following equations.
Ct t t dt t
2
2
t
0
Ct dt
2
C t dt
0
t 2
3.5
C t dt
0
0
where
t C (t )dt
t
0
3.6
C (t )dt
0
For discrete data points:
n
2
t
t
2
i
i 0
Ci t
n
C t
i 0
t 2
i
and
Reactor Characteristics
3.7
32
n
t
t
i 0
n
i
Ci t
C t
i 0
3.8
i
Inlet and outlet boundary conditions affect the response obtained from a reactor.
Closed reactors have little dispersion across their inlet and outlet boundaries whereas
"open" reactors can have significant dispersion across their inlet and outlet
boundaries. Typically open systems have no physical boundaries in the direction of
flow. An example of an open system would be a river segment. Closed systems have
small inlets and outlets that minimize dispersion across the inlet and outlet regions.
An example of a closed system is a tank (or a lake) with a small inlet and outlet. The
reactors used in the lab are closed. The t in equation 3.8 is the measured average
residence time for the tracer in the reactor. For ideal closed reactors the measured
residence time, t , is equal to the theoretical hydraulic residence time ( = reactor
volume/flow rate).
The above equations suggest that from the reactor response to a spike input we can
compute the dispersion coefficient for the reactor. We have two options for
measuring reactor response:
1) synoptic measurements: at a fixed time sampling many points along the axis of
the reactor will yield a Gaussian curve of concentration vs. distance. In practice
synoptic measurements are difficult because it requires sampling devices that are
time-coordinated. By combining equations 3.4, 3.7, and 3.8 it is possible to
estimate the dispersion coefficient from synoptic measurements.
2) single point sampling: measure the concentration at a fixed position along the x
axis of the reactor for many times. If the reactor length is fixed at L and
measurements are made at the effluent of the reactor (observe the concentration of
a tracer at x = L as a function of time) then x is no longer a variable and C(x,t)
becomes C(t) only. The response curve obtained through single point sampling is
skewed. The curve “spread” changes during the sampling period and the response
curve is skewed.
Peclet Number
The dimensionless parameter Pe (Peclet number) is used to characterize the level of
dispersion in a reactor. The Peclet number is the ratio of advective to dispersive
transport.
Pe
U
Dd L
3.9
In the limiting cases when Pe = 0 (very high dispersion) we have a complete mix
regime (CMFR) and when PE = ∞ (Dd = 0, no dispersion) we have a plug flow
reactor (PF).
For single point sampling of the effluent response curve, skew increases as the
dispersion level in the reactor increases. The degree of skew depends on the
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
33
dispersion coefficient, the velocity in the x-direction, and the length of the reactor.
Peclet values in the range 100<Pe<∞ result in a symmetric response curve.
Response curve skew makes the assumption of a symmetrical normal distribution
curve inappropriate and a new relationship between the variance and the dispersion
coefficient (or Pe) has to be determined. Boundary conditions affect the determination
of the dispersion coefficient. The relationship between the Peclet number and
variance for open systems is given by:
8
2
2 2
Pe Pe
t2
3.10
For closed systems the relationship is:
2
2
t2 2 1 e
Pe Pe
Pe
2
3.11
2
in equations 3.10 and 3.11 is dominant for Peclet numbers much
Pe
greater than 10 as is shown in Figure 1. The additional terms in equations 3.10 and
3.11 are corrections for skewedness in the response curve. These skewedness
corrections are not very significant for Peclet numbers greater than 10. Thus for
Peclet numbers greater than 10 the Peclet number can be determined using equation
3.12 for both open and closed systems.
The term
Pe
2 2
3.12
t2
Flow through Porous Media
Flow through porous media (such as groundwater through soil) is a type of flow
with dispersion. The above equations can
be applied by recognizing that the relevant
water velocity is the pore water velocity.
10000
Q
2/Pe
The pore water velocity is U =
where
1000
A
open
A is the cross sectional area of the porous
closed
100
media and (volume of voids/total 2
10
volume) is the porosity of the porous
2
1
media.
0.1
Completely Mixed Flow Reactor
0.01
In the case of CMF reactors, there is not
0.001
an analytical solution to the advective
0.001 0.01 0.1
1
10
100 1000 10000
dispersion equation so we revert to a
Pe
simple mass balance. For a completely
mixed reactor a mass balance on a Figure 1. Relationship between equations
conservative tracer yields the following 3.10 through 3.12.
differential equation:
Reactor Characteristics
34
dC
Cin C Q
3.13
dt
where Q is the volumetric flow rate and V is the volume of the reactor.
Equation 3.13 can be used to predict a variety of effluent responses to tracer inputs
such as the pulse input used in this experiment. If a mass of tracer is discharged
directly into a reactor so that the initial concentration of tracer in the reactor is C0 =
M
and the input concentration is zero (Cin = 0) the solution to the differential
V
equation is:
V
-t
C C0 exp
t
3.14
If a reactor has a complete mix flow regime its response (C/C0 vs. time) to a pulse
input should plot as a straight line on a semi-logarithmic plot. The slope of this plot is
the negative inverse of the average hydraulic residence time, t , of the reactor.
Complete mix flow regimes can be approximated quite closely in practice.
Plug Flow Reactor
Plug flow regimes are impossible to attain because mass transport must be by
advection alone. There can be no differential displacement of tracer relative to the
average advective velocity. In practice some mixing will occur due to molecular
diffusion, turbulent dispersion, and/or fluid shear. For the case of the plug flow
reactor the advective dispersion equation 3.1 reduces to:
C
C
U
t
x
The velocity U serves to
transform
the
directional
concentration gradient into a
temporal concentration gradient.
C
In other words, a conservative
substance moves with the Co
advective flow of the fluid. The
solutions to this differential
equation for a pulse input and for
a step input are shown graphically
in Figure 2.
Mass Conservation
3.15
C
C
o
U
U
X
p ul se i np ut
X
st ep i np ut
Figure 2. Pulse and step input in a plug flow
reactor.
When a pulse of conservative
tracer is added to a continuous
flow reactor, all of the tracer is
expected to leave the reactor eventually. The mass of a substance that has left the
reactor is given in equation 3.16.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
35
n
M QCi ti
3.16
i 0
where Q is the flow rate and M is the mass of any substance whose concentration is
given by C. If Q and ∆t are constant, then equation 3.16 can be rewritten as
n
M Qt Ci
3.17
i 0
Equation 3.17 can be used to determine if all of the tracer was measured in the reactor
effluent.
Conductivity Measurements
We will use a tracer containing salt (NaCl) and red dye # 40 (for visualization).
The concentration of NaCl will be monitored using a conductivity probe.
Conductivity is the measure of a material's ability to conduct electric current.
Conductivity is measured by passing an electrical current between two electrodes and
then measuring the voltage. The electrodes can be made of platinum, titanium,
gold-plated nickel, or graphite. Conductivity is defined as:
I
3.18
E
where G is conductivity, I is the current, and E is the measured voltage.
If the current is held constant, as the conductivity of the solution increases the
voltage between the electrodes will decrease. For a given current, the measured
voltage will increase as the size of the electrodes decreases and as the distance
between the electrodes increases. We are interested, however, in measuring properties
of the solution, not properties of the conductivity probe! Specific conductivity, C, is a
property of the solution.
G
L
3.19
A
where L is the distance between the electrodes and A is the area of the electrodes. The
L
term is a property of the conductivity cell and is called the cell constant. In
A
practice, the cell constant is determined during calibration by measuring the
conductivity (G) of a solution with known specific conductivity (C). The units of
specific conductivity are Siemens/cm where Siemens are the inverse of Ohms.
For a solution to be conductive, it must have ions that can transport the charge
between the electrodes. In pure water, the only ions available are H+ and OH-. Adding
species that disassociate into charged ions increases both the concentration of ions
and the specific conductivity. At low concentrations, specific conductivity increases
linearly with the concentration of ions. At very high concentrations ion-ion
interactions become significant and the relationship is no longer linear. The specific
conductivity of several common solutions is given in Table 1.
C G
Reactor Characteristics
36
Conductivity measurements are
Table 1. Conductivity of some common
temperature
dependent.
The
solutions.
conductivity of a solution will
Solution
Specific Conductivity
increase as the temperature increases.
pure
water
0.055 µS/cm
The Accumet™ meter that you will
distilled water
0.5 µS/cm
use in this laboratory compensates
deionized water
0.1-10 µS/cm
for this effect by also measuring the
typical drinking water
0.5-1.0 mS/cm
temperature and reporting the
wastewater
0.9-9.0 mS/cm
maximum
drinking
water
1.5 mS/cm
solution specific conductivity at
ocean
water
53 mS/cm
25°C.
10% NaOH
355 mS/cm
In this lab sodium chloride will
495 mg/L NaCl
1 mS/cm
increase the specific conductivity of
the water in the reactors. The concentration of sodium chloride will be low enough so
that specific conductivity will be linearly related to the concentration of sodium
chloride.
Procedures
A conservative tracer will be used to characterize each of the reactors. A
conservative tracer with 20 g NaCl/L and 4 g red dye # 40/L will be used. The salt
will increase the conductivity of the water and conductivity will be measured to
monitor the salt concentration. The red dye was added to the tracer to make it possible
to see the tracer.
A common problem when using tracers is that the tracer may have a different
density than the fluid that is in the reactors. In this case the salt and dye add
significantly to the density of the tracer. The tracer would tend to sink to the bottom
of the reactors. To compensate for this problem the density of the water being
pumped into some reactors will be increased by using a glucose solution (37 g
glucose/L). Glucose is nonionic and thus will not increase the conductivity of the
solution.
Calibrate Conductivity probe
Calibrate the conductivity probe by placing it in a 495 mg NaCl/L standard. Press
the conductivity button on the Accumet™ meter if it is not already in the
conductivity mode. Press standardize and enter 1000 µS/cm. Press enter and the
meter will calibrate and return to the normal display mode.
Measure Conductivity of tracer
Prepare a calibration curve for conductivity vs. concentration of the tracer
(expressed as mg/L of NaCl). The tracer has 20 g/L of NaCl. Measure the
conductivity of tracer diluted with distilled water so that the final concentrations of
NaCl are 500, 200, and 100 mg/L. As a zero point measure the conductivity of
distilled water.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
37
Measure Reactor Response to Pulse Input
For each reactor add a pulse input of sodium, measure conductivity vs. time in the
reactor effluent and use the Compumet™ software to monitor the conductivity vs.
time (see discussion below). Save the collected data for later analysis using a
spreadsheet program. The experimental setup is shown in Figure 3. Specific
instructions for each type of reactor
are detailed below.
Injection port
Porous Media
The porous media column is 2.5
Feed
Flow with
cm in diameter, 60 cm long and solution
dispersion
(glucose
contains 60 cm of glass beads. The solution)
Peristaltic
or
overall
porosity
including
pump
headspace and underdrains is
Completely
mixed
approximately 0.4. Use this
reactor
information to estimate the volume
stirrer
of water in the reactor. The
conductivity probe should be
or
plumbed into the effluent line.
Plug flow
reactor
1) Verify that the flow rate is set to
10 mL/min.
2) Inject 10 mg NaCl (0.5 mL of
Figure 3. Reactor schematic. Only one reactor
tracer) into the influent line.
at a time will be connected to the peristaltic
3) Select Set Method from the pump.
Compumet™ control palette.
Use automatic data transmission
with a timed interval of 1
second. Set channel A to Conductivity and channel B to Off.
4) Select Monitor Sample from the control palette.
5) Start the pump and press the enter key on the keyboard to begin data acquisition.
6) Measure the actual flow rate by collecting a timed sample from the effluent. To
get an accurate flow rate you should collect a sample for several minutes.
7) Estimate the width of the tracer pulse when the pulse nears the top of the reactor
and record the corresponding time. This information will be used to obtain an
estimate of the dispersion coefficient.
8) Turn off the pump when the conductivity returns to the baseline conductivity.
9) Stop data acquisition by clicking on the Stop Sampling button.
10) Save the data to \\Enviro\enviro\Courses\453\reactors\netid_porousmedia by
selecting Save data from the control palette. The data will be saved in a file (tab
delimited format) that can be opened by any spreadsheet program.
Completely Mix Flow Reactor (CMFR)
1) Verify that the flow rate is set to 300 mL/min.
Reactor Characteristics
38
2) Fill the CMFR with distilled water to within about 2 mm of the overflow drain .
3) Measure the conductivity of the distilled water.
4) Set the stirrer to the highest setting that doesn't cause splashing (setting 8) and
place the conductivity probe near the stir bar.
5) Add 800 mg NaCl (40 mL tracer) directly to the CMFR.
6) Select Set Method from the Compumet™ control palette. Use automatic data
transmission with a timed interval of 10 second. Set channel A to Conductivity
and channel B to Off.
7) Select Monitor Sample from the control palette.
8) Start the pump and press the enter key on the keyboard to begin data acquisition.
9) Record the time when water begins flowing out the overflow (this is your actual
time zero!)
10) Measure the flow rate by collecting a timed sample from the effluent. To get an
accurate flow rate you should collect a sample for several minutes.
11) Turn off the pump after 2 residence times.
12) Stop data acquisition by clicking on the Stop Sampling button.
11) Save the data to \\Enviro\enviro\Courses\453\reactors\netid_CMFR by selecting
Save data from the control palette. The data will be saved in a file (tab delimited
format) that can be opened by any spreadsheet program.
13) Determine the volume of water in the CMFR.
Baffled Tank Reactor
The baffled tank reactor is a simple attempt to reduce mixing and short-circuiting.
The channels are approximately 4.5 cm wide, 4.8 cm deep and have a total length of
80 cm.
1) Verify that the flow rate is set to 300 mL/min.
2) Determine the volume of water in the baffled tank.
3) Fill the baffled tank with glucose water.
4) Measure the conductivity of the glucose water.
5) Select Set Method from the Compumet™ control palette. Use automatic data
transmission with a timed interval of 10 second. Set channel A to Conductivity
and channel B to Off.
6) Select Monitor Sample from the control palette.
7) Inject 200 mg NaCl (10 mL of tracer) into the influent line with a syringe.
8) Start the pump and press the enter key on the keyboard to begin data acquisition.
9) During data acquisition, it is important to gently move the conductivity probe up
and down to continually bring the probe into contact with the changing solution.
While moving the probe up and down, do not lift the probe so high that the
platinum contacts leave the solution
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
39
10) Measure the actual flow rate by collecting a timed sample from the effluent. To
get an accurate flow rate you should collect a sample for several minutes.
11) Turn off the pump when the conductivity returns to the baseline conductivity.
12) Stop data acquisition by clicking on the Stop Sampling button.
13) Measure the average conductivity of the remaining solution in the baffled tank.
12) Save the data to \\Enviro\enviro\Courses\453\reactors\netid_baffled by selecting
Save data from the control palette. The data will be saved in a file (tab delimited
format) that can be opened by any spreadsheet program.
Pipe Flow
The pipe flow setup consists of 15.24 m of 6 mm ID tubing.
1) Verify that the flow rate is set to 50 mL/min.
2) Fill the pipe with glucose water.
3) Select Set Method from the Compumet™ control palette. Use automatic data
transmission with a timed interval of 1 second. Set channel A to Conductivity and
channel B to Off.
4) Select Monitor Sample from the control palette.
5) Inject 10 mg NaCl (0.5 mL of tracer) into the influent line with a syringe.
6) Start the pump and press the enter key on the computer keyboard to begin data
acquisition.
7) Measure the actual flow rate by collecting a timed sample from the effluent. To
get an accurate flow rate you should collect a sample for several minutes.
8) Turn off the pump when the conductivity returns to the baseline conductivity.
9) Stop data acquisition by clicking on the Stop Sampling button.
13) Save the data to \\Enviro\enviro\Courses\453\reactors\netid_pipe by selecting
Save data from the control palette. The data will be saved in a file (tab delimited
format) that can be opened by any spreadsheet program.
Prelab Questions
1) Why is a 37 g/L glucose solution used for the plug flow reactor? Why is the
glucose solution not needed for the completely mixed flow reactor?
2) Why is the conductivity of pure water not zero?
Data Analysis
Use a consistent set of units throughout your data analysis and include the units in
your spreadsheet and report!
1) Derive an equation relating concentration of NaCl in the tracer to conductivity
based on the 4 point calibration curve. Use the slope from the equation and the
baseline conductivity of each of the reactors to convert the conductivity data to
concentration of NaCl for each reactor.
Reactor Characteristics
40
2) Perform a mass balance on the salt. When applicable include the measurements of
residual salt left in the reactors at the end of your experiments. Use equation 3.17
to calculate the mass of NaCl measured in the effluent from each reactor and
compare with the mass of NaCl added.
3) Calculate the volume of the pipe flow and porous media reactors based on their
dimensions and porosity.
4) From the data determine t for each reactor (use equation 3.8 for the baffled tank,
pipe flow, and porous media column and use equation 3.14 for the CMFR) and
compare with the hydraulic residence time ( = V/Q). Discuss any discrepancies.
5) The width of the plume as measured by eye for the porous media column is
approximately 4 standard deviations (2 on each side of the peak). Use your
measurement of the width of the plume and equation 3.4 to estimate the
dispersion coefficient for the porous media reactor.
6) Estimate the Peclet numbers (equation 3.12) and the dispersion coefficients
(equation 3.9) for the baffled tank, pipe flow, and porous media column. Compare
the dispersion coefficient for the porous media reactor with the estimate obtained
in the previous step.
7) Plot actual and theoretical effluent tracer concentration versus time for the three
reactors. Use the calculated dispersion coefficient (equation 3.9) substituted into
equation 3.2 to model the baffled tank, pipe flow, and porous media column. On
the graphs also show , and t (these can be added to your graph in Excel as
additional plots). Use the CMFR volume to obtain the theoretical value of the
initial concentration.
8) Compare your results with theory. Give possible reasons for deviations from
theoretical expectations.
9) Discuss what you learned.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
41
Lab Prep Notes
Table 2. Equipment list
Description
Supplier
Accumet™ 50
meter
floating stir bar
magnetic stirrer
6 L container
Conductivity
Cell 1/cm
column
glass shot
297-420 µm
6 L container
with baffles
3-port leur
manifold
variable flow
digital drive
Easy-Load
pump head
PharMed tubing
size 18
20 liter HDPE
Jerrican
Fisher Scientific
Catalog
number
13-635-50
Fisher Scientific
Fisher Scientific
Fisher Scientific
Fisher Scientific
14-511-99A
11-500-7S
03-484-22
13-620-160
Fisher Scientific
Sunbelt
Industries
CEE shop
K420830-6010
Mil 5
Cole Parmer
H-06464-82
Cole Parmer
H-07523-30
Cole Parmer
H-07518-00
Cole Parmer
H-06485-18
Fisher Scientific
02-961-50C
Table 3. Reagent list
Description
Supplier/Source
NaCl
red dye #40
glucose
1000 µS
solution
Tracer
Fisher Scientific
MG Newell
Aldrich
495 mg NaCl/L
glucose feed
solution
Catalog
number
BP358-1
07704-1
15,896-8
20 g NaCl/L
4 g red dye #40/L
37 g glucose/L
1)
2)
3)
4)
5)
Prepare glucose solution for the baffled tank and pipe flow reactors in Jerricans.
Prepare 1 L of tracer.
Prepare 1 L of 1000 µS/cm solution (conductivity standard).
Distribute tracer and conductivity standard for each setup.
Use # 18 tubing for CMFR and baffled tank. Use #16 tubing for pipe flow and
porous media reactors.
6) Place all conductivity probes in distilled water so they are conditioned. Otherwise
the readings will drift.
Reactor Characteristics
42
1040
density (g/L)
7) 0.5 mL tracer/100 mL = 100 mg/L.
The concentration of glucose
required to achieve the same density as
a sodium chloride solution is 1.848
times as great.
density = 0.378C + 998.215
1030
1020
1010
1000
990
0
20
40
60
80
100
C (g/L)
Figure 4. Density of glucose solution as a
function of glucose concentration.
1025
density = 0.6985C + 998.29
density (g/L)
1020
1015
1010
1005
1000
995
0
10
20
30
C (g/L)
Figure 5. Density of sodium chloride
solution as a function of concentration.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
43
Acid Precipitation and Remediation of Acid Lakes
Introduction
Acid precipitation has been a serious environmental problem in many areas of the
world for the last few decades. Acid precipitation results from the combustion of
fossil fuels, that produces oxides of sulfur and nitrogen that react in the earth's
atmosphere to form sulfuric and nitric acid. One of the most significant impacts of
acid rain is the acidification of lakes and streams. In some watersheds the soil doesn’t
provides ample acid neutralizing capacity to mitigate the effect of incident acid
precipitation. These susceptible regions are usually high elevation lakes, with small
watersheds and shallow non-calcareous soils. The underlying bedrock of
acid-sensitive lakes tends to be granite or quartz. These minerals are slow to weather
and therefore have little capacity to neutralize acids. The relatively short contact time
between the acid precipitation and the watershed soil system exacerbates the problem.
Lakes most susceptible to acidification: 1) are located downwind, sometimes
hundreds of miles downwind, from major pollution sources–electricity generation,
metal refining operations, heavy industry, large population centers; 2) are surrounded
by hard, insoluble bedrock with thin, sandy, infertile soil; 3) have a high runoff to
infiltration ratio; 4) have a low watershed to lake surface area. Isopleths of
precipitation pH are depicted in Figure 1.
Figure 1. The pH of precipitation in 1999.
Acid Precipitation and Remediation of Acid Lakes
44
In acid-sensitive lakes the major parameter of concern is pH (pH = -log{H+},
where {H+} is the hydrogen ion activity, and activity is approximately equal to
concentration in moles/L). In a healthy lake, ecosystem pH should be in the range of
6.5 to 8.5. In most natural freshwater systems, the dominant pH buffering
(controlling) system is the carbonate system. The carbonate buffering system is
composed of four components: dissolved carbon dioxide ( CO 2 aq ), carbonic acid
( H 2 CO3 ), bicarbonate ( HCO3- ), and carbonate ( CO 3-2 ). Carbonic acid exists only at
very low levels in aqueous systems and for purposes of acid neutralization is
indistinguishable from dissolved carbon dioxide. Thus to simplify things we define
H 2 CO*3 CO 2 aq H 2 CO3
4.1
The CO 2 aq >> H2 CO3 and thus H 2 CO*3 CO 2 aq (all terms enclosed in []
are in units of moles/L).
The sum of all the molar concentration of the components of the carbonate system
is designated as CT as shown in equation 4.2.
CT H 2 CO*3 HCO3- CO3-2
4.2
The carbonate system can be considered to be a "volatile" system or a
"non-volatile" system depending on whether or not aqueous carbon dioxide is
allowed to exchange and equilibrate with atmospheric carbon dioxide. Mixing
conditions and hydraulic residence time determine whether an aquatic system is
volatile or non-volatile relative to atmospheric carbon dioxide equilibrium. First,
consider the "non-volatile" system.
Non-volatile System
For a fixed CT, the molar concentration of each species of the carbonate system is
determined by pH. Equations 4.3-4.8 show these functional relationships.
H 2 CO*3
CT
K
KK
1 1 1 22
[H ] [H ]
0 CT
4.3
where
0
1
K
KK
1 1 1 22
[H ] [H ]
CT
HCO3-
1CT
K2
[H ]
1
K1
[H ]
4.4
4.5
where
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
45
1
4.6
CT
CO3-2
2 CT
2
[H ] [H ]
1
K1 K 2
K2
4.7
where
2
4.8
K1 and K2 are the first and second dissociation constants for carbonic acid and 0, 1,
and 2 are the fraction of CT in the form H 2 CO*3 , HCO3- , and CO 3-2 respectively.
Because K1 and K2 are constants (K1 = 10-6.3 and K2 = 10-10.3), 0, 1, and 2 are only
functions of pH.
A measure of the susceptibility of lakes to acidification is the acid neutralizing
capacity (ANC) of the lake water. In the case of the carbonate system, the ANC is
exhausted when enough acid has been added to convert the carbonate species HCO3- ,
and CO 3-2 to H 2 CO*3 . A formal definition of total acid neutralizing capacity is given
by equation 4.9.
ANC HCO3- 2 CO3-2 OH - - H
4.9
ANC has units of equivalents per liter. The hydroxide ion concentration can be
obtained from the hydrogen ion concentration and the dissociation constant for water
Kw.
Kw
OH -
H
4.10
Substituting equations 4.5, 4.7, and 4.10 into equation 4.9, we obtain
ANC CT
Kw
H
H
4.11
For the carbonate system, ANC is usually referred to as alkalinity.2
2
Alkalinity can be expressed as equivalents/L or as mg/L (ppm) of CaCO3. 50,000
mg/L CaCO3 = 1 equivalent/L.
Acid Precipitation and Remediation of Acid Lakes
46
Volatile Systems:
Now consider the case where aqueous CO 2 aq is volatile and in equilibrium with
atmospheric carbon dioxide. Henry's Law can be used to describe the equilibrium
relationship between atmospheric and dissolved carbon dioxide.
CO 2 aq PCO2 K H
4.12
where KH is Henry's constant for CO2 in moles/L-atm and PCO2 is partial pressure of
CO2 in the atmosphere (KH = 10-1.5 and PCO2 = 10-3.5). Because CO 2 aq is
approximately equal to H 2 CO*3 and from equations 4.1 and 4.3
PCO2 K H 0 CT
CT
4.13
PCO2 K H
4.14
a
Equation 4.14 gives the equilibrium concentration of carbonate species as a function
of pH and the partial pressure of carbon dioxide.
The acid neutralizing capacity expression for a volatile system can be obtained by
combining equations 4.14 and 4.11.
ANC
PCO2 K H
a
Kw
H
H
4.15
In both non-volatile and volatile systems, equilibrium pH is controlled by system
ANC. Addition or depletion of any ANC component in equation 4.11 or 4.15 will
result in a pH change. Natural bodies of water are most likely to approach equilibrium
with the atmosphere (volatile system) if the hydraulic residence time is long and the
body of water is shallow.
Lake ANC is a direct reflection of the mineral composition of the watershed. Lake
watersheds with hard, insoluble minerals yield lakes with low ANC. Typically
watersheds with soluble, calcareous minerals yield lakes with high ANC. ANC of
freshwater lakes is generally composed of bicarbonate, carbonate, and sometimes
organic matter ( A -org ). Organic matter derives from decaying plant matter in the
watershed. When organic matter is significant, the ANC becomes (from equations
4.11 and 4.15):
ANC CT
ANC
PCO2 K H
a
Kw
H
Kw
H
H A -org
H A -org
CEE 453: Laboratory Research in Environmental Engineering
4.16
4.17
Spring 2001
47
where equation 4.16 is for a non-volatile system and equation 4.17 is for a volatile
system.
During chemical neutralization of acid, the components of ANC associate with
added acid to form protonated molecules. For example:
H HCO3- H 2 CO*3
4.18
H A -org HA org
4.19
or
In essence, the ANC of a system is a result of the reaction of acid inputs to form
associated acids from basic anions that were dissolved in the lake water. The ANC
(equation 4.9) is consumed as the basic anions are converted to associated acids. This
conversion is near completion at low pH (approximately pH 4.5 for the bicarbonate
and carbonate components of ANC). Neutralizing capacity to another (probably
higher) pH may be more useful for natural aquatic systems. Determination of ANC to
a particular pH is fundamentally easy — simply add and measure the amount of acid
required to lower the sample pH from its initial value to the pH of interest.
Techniques to measure ANC are described under the procedures section of this lab.
Neutralization of acid precipitation can occur in the watershed or directly in the
lake. How much neutralization occurs in the watershed versus the lake is a function of
the watershed to lake surface area. Generally, watershed neutralization is dominant.
Recently engineered remediation of acid lakes has been accomplished by adding
bases such as limestone, lime, or sodium bicarbonate to the watershed or directly to
the lakes.
Experimental Objectives
Phase I: Acid Neutralization by Soil Column
In this experiment, we will study the effect of watershed soil on neutralization of
acid precipitation. Simulated acid precipitation (dilute sulfuric acid) will be applied to
a column of watershed soil that will discharge to a volume of water that represents a
lake. The column will contain an artificial soil amended with CaCO3 representing a
highly calcareous soil. Influent and effluent lake pH and ANC will be measured over
time to document the depletion of the ANC of the soil column and the subsequent
acidification of the lake. Organic matter as an ANC component will not be
incorporated into these experiments.
Phase II. Acid Lake Remediation
Remediation of acid lakes involves addition of ANC so that the pH is raised to an
acceptable level and maintained at or above this level for some design period. In this
experiment sodium bicarbonate (NaHCO3) will be used as the ANC supplement.
Since ANC addition usually occurs as a batch addition, the design pH is initially
exceeded. ANC dosage is selected so that at the end of the design period pH is at the
acceptable level. Care must be taken to avoid excessive initial pH — high pH can be
as deleterious as low pH.
Acid Precipitation and Remediation of Acid Lakes
48
The most common remediation procedure is to apply the neutralizing agent directly
to the lake surface, instead of on the watershed. We will follow that practice in this
lab experiment. Sodium bicarbonate will be added directly to the surface of the lake
that has an initial ANC of 50 µeq/L and is receiving acid rain with a pH of 3. After
the sodium bicarbonate is applied, the lake pH and ANC will be monitored for
approximately one hour.
Experimental Apparatus
The experimental apparatus consists of an acid rain storage reservoir, peristaltic
pump, soil column, and lake (Figure 2). The pH of the lake influent and of the lake
will be monitored using pH probes connected to a pH meter. The soil column will be
removed for phase II of the
experiment.
The soil column is filled with
glass beads. The ANC of the soil
column is the result of adding
CaCO3 slurry to the top of the soil
column. Undissolved CaCO3
quickly settles to the glass beads
and thus CaCO3 is only removed
from the column as it dissolves. Figure 2. Schematic drawing of the experimental
For this experiment, 250 mg of setup.
CaCO3 has been added to the soil
column. Detail of the soil column
is shown in Figure 3.
Experimental
Procedures
Stainless steel fastener attaching
Plexiglas cover to the filter base
1/8 in 'O' ring
Plexiglas cover
Calibration of pH Meter
6 mm diameter
Plexiglas baffle
Calibrate both pH probes
Plexiglas dowel
attached to the pH meter. To
Glass
calibrate the probe attached to
beads
channel A toggle the display
10 cm ID
by pressing Channel until
Wire cloth
Wire screen
Plexiglas tube
only the pH from channel A
PVC cap
is in the display. (The display
PVC grid
toggles between channel A,
channel
B,
and
both
Underdrain
Adaptor
channels.) Then follow the
calibration procedure outlined Figure 3. Soil column used for neutralizing acid rain.
in the Appendix of this
manual (page 157). Repeat
the procedure for the channel
B probe (if you are doing the soil column experiment).
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
49
Acid Neutralization by Soil Column Experiment
In this experiment, the "acid rain" will pass through the soil column before running
into the lake. Flow should be controlled to give a 15 minutes hydraulic retention time
in the lake. With a lake volume of 5 liters, the required flow rate is 334 mL/min.
Since surface waters are seldom at equilibrium or steady state, this experiment will be
run under dynamic conditions. The lake will start out with no ANC. Data will be
taken from the onset of flow.
1) Measure the volume of the lake (The lake weighs too much for the 5 kg balance
but it can be weighed in two parts. Weigh the empty lake container, fill the lake,
pour half of the lake into a weighed container, weigh both the lake container and
the second container, add the weights of the containers with water and subtract the
container weights.)
2) Fill lake with distilled water.
3) Add 1 mL of bromocresol green indicator to the lake.
4) Preset pump to give desired flow rate of 334 mL/min (5 L/15 minutes).
5) Measure the pH of the acid rain.
6) Prepare to monitor the pH of two probes using the Compumet™ software. Set the
method for automatic sampling of pH probes on both channel A and channel B
every 10 seconds. See section on "Compumet™ software" (page 157) for
information on using the Computer™ software to monitor pH.
7) Place the probe attached to channel A to monitor influent to the lake. Place the
probe attached to channel B to monitor the pH of the lake.
8) Label sample bottles (see step 13).
9) Set stirrer speed to setting 8.
10) Observe the CaCO3 on top of the soil column.
11) At time equal zero start the pump and begin monitoring pH at the lake influent
and in the lake.
12) Measure the flow rate using 100-mL volumetric flask and a stopwatch.
13) Take 50-mL grab samples from the lake influent and effluent at 5, 10, 15, and 20
minutes. The sample volumes do not need to be measured. Collect the samples in
125-mL bottles.
14) Observe the depletion of the CaCO3 on top of the soil column.
15) After the 20-minute sample turn off the pump and stop sampling pH.
14) Save the pH data to \\Enviro\enviro\Courses\453\acid\netid_column by selecting
Save data from the control palette. The data will be saved in a file (tab delimited
format) that can be opened by any spreadsheet program.
Acid Lake Remediation Experiment
In this experiment sodium bicarbonate will be added to a lake to mitigate the
deleterious effect of acid rain. Usually sodium bicarbonate is added in batch doses (as
opposed to metering in). The quantity of sodium bicarbonate added depends on how
Acid Precipitation and Remediation of Acid Lakes
50
long a treatment is desired, the acceptable pH range and the quantity and pH of the
incident rainfall. For purposes of this experiment, a 15-minute design period will be
used. That is, we would like to add enough sodium bicarbonate to keep the lake at or
above its original pH and alkalinity for a period of 15 minutes (i.e. for one hydraulic
residence time).
By dealing with ANC instead of pH as a design parameter, we avoid the issue of
whether the system is at equilibrium with atmospheric carbon dioxide. Keep in mind
that eventually the lake will come to equilibrium with the atmosphere. In practice,
neutralizing agent dosages may have to be adjusted to take into account
non-equilibrium conditions.
We must add enough sodium bicarbonate to equal the negative ANC from the acid
precipitation input plus the amount of ANC lost by outflow from the lake during the
15-minute design period. Initially (following the dosing of sodium bicarbonate) the
pH and ANC will rise, but over the course of 15 minutes, both parameters will drop.
Calculation of required sodium bicarbonate dosage requires performing a mass
balance on ANC around the lake. This mass balance will assume a completely mixed
lake and conservation of ANC. Chemical equilibrium can also be assumed so that the
sodium bicarbonate is assumed to react immediately with the incoming acid
precipitation.
d(ANC)
dt
Q ANCin - ANCout V
4.20
where:
ANCout = ANC in lake outflow at any time t (for a completely mixed lake the
effluent ANC is the same as the ANC in the lake)
ANCin = ANC of acid rain input
V = volume of reactor
Q = acid rain input flow rate.
If the initial ANC in the lake is designated as ANC0, then the solution to the mass
balance differential equation is:
ANCout ANCin 1 - e
-t/
ANC e
-t/
0
4.21
where:
= V/Q
We want to find ANC0 such that ANCout = 50 µeq/L when t is equal to . Solving
for ANC0 we get
ANC0 ANCout - ANCin 1 - e
-t/
e
t/
4.22
The ANC of the acid rain (ANCin) can be estimated from its pH. Below pH 6.3
most of the carbonates will be in the form H 2 CO*3 and thus for pH below about 4.3
equation 4.9 simplifies to
ANC H
CEE 453: Laboratory Research in Environmental Engineering
4.23
Spring 2001
51
An influent pH of 3.0 implies the ANCin = - H = -0.001
Substituting into equation 4.22:
-1
1
ANC0 0.000050 0.001 1 - e e = 1.854 meq/L
The quantity of sodium bicarbonate required can be calculated from:
[NaHCO3]0 =ANC0
4.24
where [NaHCO3]0 = moles of sodium bicarbonate required per liter of lake water
1.854 mmole NaHCO3 84 mg NaHCO3
×
× 5 Liters = 779 mg NaHCO3
liter
mmole NaHCO3
4.25
1) Verify that the system is plumbed so that the “acid rain” is pumped directly into
the lake.
2) Take a 50-mL sample from the acid rain container. Collect the sample in a
125-mL bottle.
3) Preset pump to give desired flow rate of 334 mL/min (5 L/15 minutes).
4) Fill lake with distilled water.
5) Set stirrer speed to 8.
6) Add 1 mL of bromocresol green indicator solution to the lake.
7) Weigh out 779 mg (not grams!) NaHCO3.
8) Add NaHCO3 to the lake.
9) After the lake is well stirred take a 100 mL sample from the lake.
10) Prepare to monitor the pH of one probe using the Compumet™ software. Set the
method for automatic sampling of the pH probe on channel A every 10 seconds.
See section on "Compumet™ software" (page 157) for information on using the
computer to monitor pH.
11) Place the probe attached to channel A to monitor the pH of the lake.
12) Label sample bottles (see step 13).
13) At time equal zero start the pump and begin monitoring the lake pH.
14) Take 100-mL grab samples from the lake effluent at 5, 10, 15, and 20 minutes.
The sample volumes do not need to be measured. Collect the samples in 125-mL
bottles.
15) Measure the flow rate.
16) After the 20-minute sample turn off the pump and stop sampling pH.
17) Save the pH data to \\Enviro\enviro\Courses\453\acid\netid_remediate by
selecting Save data from the control palette. The data will be saved in a file (tab
delimited format) that can be opened by any spreadsheet program.
18) Measure the lake volume.
Acid Precipitation and Remediation of Acid Lakes
52
Analytical Procedures
pH. pH (-log{H+}) is usually measured electrometrically with a pH meter. The pH
meter is a null-point potentiometer that measures the potential difference between an
indicator electrode and a reference electrode. The two electrodes commonly used for
pH measurement are the glass electrode and a reference electrode. The glass electrode
is an indicator electrode that develops a potential across a glass membrane as a
function of the activity ,~ molarity, of H+. Combination pH electrodes, in which the
H+-sensitive and reference electrodes are combined within a single electrode body
will be used in this lab. The reference electrode portion of a combination pH
electrode is a [Ag/AgCl/4M KCl] reference. The response (output voltage) of the
electrode follows a "Nernstian" behavior with respect to H+ ion activity.
RT H
EE
ln
nF H 0
0
4.26
where R is the universal gas constant, T is temperature in Kelvin, n is the charge of
the hydrogen ion, and F is the Faraday constant. E0 is the calibration potential (Volts),
and E is the potential (Volts) measured by the pH meter between glass and reference
electrode. The slope of the response curve is dependent on the temperature of the
sample and this effect is normally accounted for with simultaneous temperature
measurements.
The electrical potential that is developed between the glass electrode and the
reference electrode needs to be correlated with the actual pH of the sample. The pH
meter is calibrated with a series of buffer solutions whose pH values encompass the
range of intended use. The pH meter is used to adjust the response of the electrode
system to ensure a Nernstian response is achieved over the range of the calibration
standards. Refer to the appendix (page 157) for specific procedures.
To measure pH the electrode(s) are submersed in at least 50 mL of a sample.
Samples are generally stirred during pH reading to establish homogeneity, to prevent
local accumulation of reference electrode filling solution at the interface near the
electrode, and to ensure the diffusive boundary layer thickness at the electrode
surface is uniform and small.
ANC. The most common method to determine ANC for aqueous samples is
titration with a strong acid to an endpoint pH. A pH meter is usually used to
determine the endpoint or "equivalence point" of an ANC titration. Determination of
the endpoint pH is difficult because it is dependent on the magnitude the sample
ANC. Theoretically this endpoint pH should be the pH where all of the ANC of the
system is consumed, but since the ANC is not known a-priori, a true endpoint cannot
be predetermined. However, if most of the ANC is composed of carbonate and
bicarbonate this endpoint is approximately pH = 4.5 for a wide range of ANC values.
A 100-mL sample is usually titrated under slowly stirred conditions. Stirring is
accomplished by a magnetic stirrer. pH electrodes are ordinarily used to record pH as
a function of the volume of strong acid titrant added. The volume of strong acid
required to reach the ANC endpoint (pH 4.5) is called the "equivalent volume" and is
used in the following equation to compute ANC.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
53
ANC =
(equivalent vol.)(normality of titrant)
(vol. of sample)
4.27
A more accurate technique to measure ANC is the Gran plot analysis. This is the
subject of a subsequent experiment. All ANC samples should be labeled and stored
for subsequent Gran analysis.
Prelab Questions
1) How many grams of NaHCO3 would be required to keep the ANC levels in a lake
above 50 µeq/L for 3 hydraulic residence times given an influent pH of 3.0 a lake
volume of 5 L, if the current lake ANC is 50 µeq/L?
Data Analysis
-6.3
K1 = 10 , K2 = 10
-10.3
, KH = 10
-1.5
mol/atm L, PCO2 = 10-3.5 atm, and Kw = 10-14.
Soil Column Experiment
1) Plot pH of both lake and soil column effluent versus time.
2) How long did it take for the ANC of the soil column to be depleted? (At a pH of
4.5 the ANC is approximately zero.)
3) How long did it take for the ANC of the lake to be depleted?
Acid Lake Remediation Experiment
1) Plot measured pH of the lake versus time.
2) Given that ANC is a conservative parameter and that the lake is essentially in a
completely mixed flow regime equation 4.21 applies. Graph the predicted ANC
based on the completely mixed flow reactor equation with the plot labeled (in the
legend) as “conservative ANC”.
3) Derive an equation for CT (the concentration of carbonate species) as a function of
time based on the input of NaHCO3 and its dilution in the completely mixed lake
assuming a non-volatile system (the equation will be the same form as equation
4.21).
4) Combine your equation for CT with equation 4.11 and plot the predicted ANC of
the lake vs. time for a non-volatile system based on the measured pH (plot on the
same graph as #2) with plot labeled as “non-volatile model”).
5) Plot the predicted ANC of the lake vs. time for a volatile system using equation
4.15 based on the measured pH (plot on the same graph as #2) with plot labeled as
“volatile model”).
6) Compare the plots and determine whether the lake is best modeled as a volatile or
non-volatile system. What changes could be made to the lake to bring the lake
into equilibrium with atmospheric CO2?
Acid Precipitation and Remediation of Acid Lakes
54
Questions
1) What do you think would happen if enough NaHCO3 were added to the lake to
maintain an ANC greater than 50 µeq/L for 3 residence times with the stirrer
turned off?
2) What are some of the complicating factors you might find in attempting to
remediate a lake using CaCO3? Below is a list of issues to consider.
extent of mixing
solubility of CaCO3 (find the solubility and compare with NaHCO3)
density of CaCO3 slurry (find the density of CaCO3)
References
Driscoll, C.T., Jr. and Bisogni, J.J., Jr., "Weak Acid/Base Systems in Dilute Acidified
Lakes and Streams of the Adirondack Region of New York State," in Modeling of
Total Acid Precipitation Impacts J.L. Schnoor (ed.), Butterworth, Stoneham, MA.,
53-72 (1983).
Driscoll, C.T., Baker, J.P., Bisogni, J.J., And Schofield, C.L., "Aluminum Speciation
and Equilibria in Dilute Surface Waters of the Adirondack Region of New York
State," in Geological Aspects of Acid Deposition O.P. Bricker (ed.), Butterworth,
Stoneham, MA., 55-75 (1984).
Barnard. T.E., And Bisogni, J.J., Jr., "Errors in Gran Function Analysis of Titration
Data for Dilute Acidified Water," Water Research, 19, No. 3 393-399 (1985).
Bisogni, J.J., Jr. and Barnard, T.E., "Numerical Technique to Correct for Weak Acid
Errors in Gran Function Analysis of Titration Data," Water Research, 21, No. 10,
1207-1216 (1987).
Bisogni, J.J., Jr., "Fate of Added Alkalinity During Neutralization of an Acid Lake,"
Journal Environmental Engineering, ASCE, 114, No. 5, 1219-1224 (1988).
Bisogni, J.J., Jr., and Kishbaugh, S.A., "Alkalinity Destruction by Sediment Organic
Matter Dissolution During Neutralization of Acidified Lakes," Water, Air and Soil
Pollution, 39, 85-95 (1988).
Bisogni, J.J., Jr. and Arroyo, S.L., "The Effect of Carbon Dioxide Equilibrium on pH
in Dilute Lakes," Water Research, 25, No. 2, 185-190 (1991).
Olem, H. Liming Acidic Surface Waters. Lewis Publishers, Chelsea, MI. (1991).
Stumm, W. and Morgan, J.J., Aquatic Chemistry, John Wiley & Sons, Inc. NY, NY
1981.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
55
Lab Prep Notes
Bromocresol Green Indicating
Solution
Prepare solution of 400 mg
Bromocresol green/100 mL ethanol.
Add 0.2 mL of indicator solution per
liter of acid rain or lake.
Acid rain
Acid rain is at pH 3.0. Prepare
from distilled water. Add 1 meq
H2SO4/L ([H+] at pH 3.0) to obtain a
pH of 3.0. To acidify 20 liters of
distilled water using 5 N H2SO4:
20 L
L
5 N H2SO4 1000 meq
= 4 mL of 5 N H2SO4
1
1N L
1 meq H2SO4
Add 4 mL of bromocresol green
indicating solution to 20 L of acid
rain solution.
Flow Rate
The residence time of the lake
should be 15 minutes. The lake
volume is 5 L. thus the flow rate is
334 mL/min. Use # 18 PharMed
tubing.
Table 1. Reagents
Description
Supplier
HCL 5.0 N
H2SO4 5N
CaCO3
Na2CO3
Buffer-Pac
NaHCO3
Bromocresol
Green
ethanol
Fisher Scientific
Fisher Scientific
Fisher Scientific
Fisher Scientific
Fisher Scientific
Fisher Scientific
Fisher Scientific
Catalog
number
LC15360-2
LC25840-2
C63-3
S263-500
SB105
S233-500
B383-5
Fisher Scientific
A962P-4
Table 2. Equipment list
Description
Supplier
magnetic stirrer
floating stir bar
Accumet™ 50
pH meter
100-1095 µL
pipette
10-109.5 µL
pipette
pH electrode
6 L container
(lake)
Easy load pump
head
digital pump
drive
PharMed tubing
size 18
soil column
Fisher Scientific
Fisher Scientific
Fisher Scientific
Catalog
number
11-500-7S
14-511-99A
13-635-50
Fisher Scientific
13-707-5
Fisher Scientific
13-707-3
Fisher Scientific
Fisher Scientific
13-620-108
03-484-22
Cole Parmer
H-07518-00
Cole Parmer
H-07523-30_
Neutralizing Column
Cole Parmer
H-06485-17
Neutralization of the acid rain will
see design
be accomplished in a porous media
schematic,
column amended with CaCO3. The
Figure 3
porous media will be glass beads or
20 liter HDPE Fisher Scientific
02-961-50C
sand
(approximately
200-µm
Jerrican
diameter) with no ANC. If new glass
beads are used, they should be washed with distilled water to remove ANC. To
neutralize the acid rain 1 meq/L of CaCO3 will be added to the column. This will be
enough ANC to raise the pH to near neutral. To neutralize 5 liters (15 minutes):
5 L
1 meq H2SO4 1 meq CaCO3 50 mg CaCO3
250 mg CaCO3
L
1meq H2SO4 meq CaCO3
Acid Precipitation and Remediation of Acid Lakes
56
The porous media column amended with CaCO3 will be prepared as follows.
1)
2)
3)
4)
Add 6 cm of glass beads (approximate) to 10 cm diameter column.
Fill from bottom of column with distilled water to 1 cm above glass bead surface.
Attach cover and fill column to top with distilled water.
Prepare CaCO3 slurry (approximately 25 mL distilled water + 250 mg CaCO3).
5) Pour slurry into 50 mL syringe.
6) Connect syringe to manifold at influent of column.
7) Inject slurry into column 15 minutes before use. If prepared too long in advance
the CaCO3 washes out of the column.
8) Allow CaCO3 to settle for 10 minutes before beginning to use.
Setup
1)
2)
3)
4)
5)
6)
7)
8)
9)
Prepare 20-L acid rain for each group.
Prepare neutralizing columns.
Prepare bromocresol green solution if necessary.
Attach one Easy-Load pump head to the pump drives and plumb with #18 tubing.
Plumb Jerrican to pump to soil column to lake using quick connectors (see Figure
2).
Verify that pH probes are operational, stable, and can be calibrated. If you are
doing the soil column experiment you need 2 probes per group.
Verify that buffers (pH = 4, 7, 10) are distributed to each student group.
Provide a mount for the pH probe in the lake.
Set up some lakes with aeration!
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
57
Measurement of Acid Neutralizing Capacity
Introduction
Acid neutralizing capacity (ANC) is a measure of the ability of water to neutralize
acid inputs. Lakes with high ANC (such as Cayuga Lake) can maintain a neutral pH
even with some acid rain input whereas lakes with an ANC less than the acid input
will not maintain a neutral pH. In the Adirondack region of New York State, lakes
typically receive large inputs of acids during the spring thaw when the accumulated
winter snow melts and runs off into the lakes. The ANC of Adirondack lakes is not
always sufficient to neutralize these inputs.
Theory
The ANC for a typical carbonate-containing sample is defined as:
ANC [HCO3- ] 2[CO3-2 ] [OH - ] - [H ]
5.1
This equation can be derived from a charge balance if ANC is considered to be the
cation contributed by a strong base titrant and if other ions present do not contribute
significantly.
Determination of ANC or Alkalinity involves determination of an equivalence
point. The equivalence point is defined as the point in the titration where titrant
volume that has been added equals the "equivalent" volume (Ve). The equivalent
volume is defined as:
Ve =
Vs ·N s
Nt
5.2
where:
Ns = normality (in this case Alkalinity or ANC) of sample, equivalents/L
Vs = volume of sample, liters
Nt = normality of titrant, equivalents/L.
The titration procedure involves incrementally adding known volumes of
standardized normality strong acid (or base) to a known volume of unknown
normality base (or acid). When enough acid (or base) has been added to equal the
amount of base (or acid) in the unknown solution we are at the "equivalence" point.
(Note: the point at which we add exactly an equivalent or stoichiometric amount of
titrant is the equivalence point. Experimentally, the point at which we estimate to be
the equivalence point is called the titration endpoint).
There are several methods for determining Ve (or the equivalence point pH) from
titration data (titrant volume versus pH). The shape of the titration curve (Vt versus
pH) can reveal Ve. It can be shown that one inflection point occurs at Vt Ve . In the
case of monoprotic acids, there is only one inflection in the pH range of interest.
Therefore, an effective method to find the equivalence volume is to plot the titration
curve and find the inflection point. Alternately, plot the first derivative of the titration
plot and look for a maximum.
Measurement of Acid Neutralizing Capacity
58
Gran Plot
Another method to find the ANC of an unknown solution is the Gran plot
technique. When an ANC determination is being made, titration with a strong acid is
used to "cancel" the initial ANC so that at the equivalence point the sample ANC is
zero. The Gran plot technique is based on the fact that further titration will result in
an increase in the number of moles of H+ equal to the number of moles of H+ added.
Thus after the equivalence point has been reach the number of moles of H+ added
equals the number of moles of H+ in solution.
Nt Vt Ve Vs Vt H
5.3
Solving for the hydrogen ion concentration:
H
Nt Vt Ve
5.4
Vs Vt
Equation 5.4 can be solved directly for the equivalent volume.
H Vs Vt
Ve Vt
Nt
5.5
Equation 5.5 is valid if enough titrant has been added to neutralize the ANC. A better
measure of the equivalent volume can be obtained by rearranging equation 5.4 so that
linear regression on multiple titrant volume - pH data pairs can be used.
Vs Vt
Vs
NtVt N tVe
H V V
s
s
5.6
We define F1 (First Gran function)
as:
Vs Vt
[H ]
Vs
5.7
If F1 is plotted as a function of Vt the
result is a straight line with slope =
Nt
and abscissa intercept of Ve
Vs
(Figure 1).
The ANC is readily obtained
given the equivalent volume. At the
equivalence pt:
First Gran Function
F1
0.0009
0.0008
0.0007
0.0006
0.0005
0.0004
0.0003
0.0002
0.0001
0
Gran Function
Gran Function Linear Region
Linear (Gran Function Linear
y = Region)
9.57E-04x - 4.62E-03
R 2 = 9.99E-01
0
1
2
3
4
5
6
Volume of Titrant (mL)
Figure 1. Gran plot from titration of a weak
Vs ANC Ve Nt
5.8 base with 0.05 N acid. Ct = 0.001 moles of
carbonate and sample volume is 48 mL. The
Equation 5.8 can be rearranged to
equivalent volume is 4.8 mL. From equation
obtain ANC as a function of the
5.9 the ANC is 5 meq/L.
equivalent volume.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
59
ANC
Ve Nt
Vs
5.9
pH Measurements
The pH can be measured either as activity ({H+} as measured approximately by pH
meter) or molar concentration ([H+]). The choice only affects the slope of F1 since
[H+] = {H+}/.
Vs Vt
V Vt
[H ] s
Vs
Vs
F1
{H }
t
Vt Ve
Vs
5.10
where is the activity correction factor and the slope is Nt/V0. If H+ concentration is
used then
F1
where the slope is
Vs Vt
V Ve
{H } N t t
Vs
Vs
5.11
t
.
Vs
(This analysis assumes that the activity correction factor doesn't change
appreciably during the titration).
There are many other Gran functions that can be derived. For example, one can be
derived for Acidity or the concentration of a single weak or strong acid or base.
To facilitate data generation and subsequent Gran plot construction and analysis
pH versus titrant volume can be read directly into a computer, that can be
programmed to analyze the data using the Gran, plot theory. The program generates
the Gran function for all data and then systematically eliminates data until the Gran
function (plot) is as linear as possible. The line is then extrapolated to the abscissa to
find the equivalent volume.
ANC Determination for Samples with pH < 4
After the equivalence point has been reached (adding more acid than ANC = 0) the
only significant terms in equation 5.1 are H and ANC.
H HCO3- 2 CO3-2 OH -
5.12
When the pH is 2 pH units or more below the pKs of the bases in the system the only
species contributing significantly to ANC is the hydrogen ion (equation 5.12) and
thus the ANC is simply
ANC - [H ]
5.13
For a sample containing only carbonates, if the pH is below 4 the ANC is
approximately equal to -[H+] and no titration is necessary.
Measurement of Acid Neutralizing Capacity
60
Titration Techniques
Operationally, the first few titrant volumes can be relatively large increments since
the important data lies at pH values less than that of the equivalence point
(approximately pH = 4.5 for an Alkalinity titration). As the pH is lowered by addition
of acid the ionic strength of the solution increases and the activity of the hydrogen ion
deviates from the hydrogen ion concentration This effect is significant below pH 3
and thus the effective linear range is generally between pH 4.5 and pH 3.0. The
maximum incremental titrant volume (∆Va) that will yield n points in this linear
region is obtained as follows.
If Vs » Vt then equation 5.3 reduces to
Nt
(Vt Ve )
[H ]
Vs
5.14
Let [H+]e be the concentration of hydrogen ions at the equivalence point and [H+]f be
the final concentration of hydrogen ions at the end of the titration.
Nt
(Ve Ve ) (V f Ve )
Vs
[H ]e - [H ]f
5.15
Thus the volume of acid added to go from [H+]e to [H+]f is
Vf - Ve
Vs [ H ] f [ H ]e
5.16
Nt
To obtain n data points between [H+]e - [H+]f requires the incremental titrant volume
(∆Vt) be 1/n times the volume of acid added between the equivalence point and the
final titrant volume. Thus by substituting n∆Vt, and typical hydrogen ion
concentrations of [H+]e = 10-4.5 and [H+]f = 10-3.0 into equation 5.16 the maximum
incremental titrant volume is obtained.
Vt
(0.001 0.00003)Vs
0.001Vs
n Nt
n Nt
5.17
Procedure
Calibrate the pH Meter
Calibrate the pH meter using a pH probe connected to channel A. Use 3 standards
(pH = 4, 7, and 10).
Determine ANC of a Known Standard
1) Weigh a 100 mL plastic beaker.
2) Add approximately 50 mL of a 2.5 mM solution of Na2CO3 to the beaker.
3) Weigh the beaker again to determine the exact volume of Na2CO3 solution.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
61
4) Place the beaker on the magnetic stirrer, add a stir bar and stir slowly.
5) Place both the pH electrode and the temperature probe in the Na2CO3 solution
using the probe holding arm attached to the Accumet™ meter.
6) Analyze the sample using Gran plot analysis as detailed at
http://www.cee.cornell.edu/mws/Software/Compumet.htm) Add 0.05 N HCl (the
titrant) using a digital pipette in increments of 0.25 mL.
15) Save the pH data to \\Enviro\enviro\Courses\453\acid\netid_gran by selecting
Save data from the control palette. The data will be saved in a file (tab delimited
format) that can be opened by any spreadsheet program. You will use this data to
plot a titration curve and to verify that the Gran technique accurately measures the
ANC of a sample.
7) Record the ANC and the equivalent volume.
Determine ANC of Acid Rain Samples
Determine ANC for all samples collected from the previous week's lab. Use the
same technique as outlined above (Determine ANC of known standard) except
substitute the samples collected last week and use titrant increment of 0.1 mL in the
linear region. For samples that have a high ANC you can reduce the analysis time by
adding titrant in larger volumes initially until the pH approaches 5. If the initial pH is
less than 4.5 no titration is necessary and equation 5.13 can be used to calculate the
ANC.
Record the initial pH (prior to adding any titrant) and initial sample volume. After
the Gran plot analysis record the alkalinity (ANC) and equivalent volume for each
sample. There is no need to save the data to disk.
Prelab Questions
1) Compare the ability of Cayuga lake and Wolf pond (an Adirondack lake) to
withstand an acid rain runoff event (from snow melt) that results in 20% of the
original lake water being replaced by acid rain. The acid rain has a pH of 3.5 and
is in equilibrium with the atmosphere. The ANC of Cayuga lake is 1.6 meq/L and
the ANC of Wolf Pond is 70 µeq/L. Assume that carbonate species are the
primary component of ANC in both lakes, and that they are in equilibrium with
the atmosphere. What is the pH of both bodies of water after the acid rain input?
Remember that ANC is the conservative parameter (not pH!).
2) What is the ANC of a water sample containing only carbonates and a strong acid
that is at pH 3.2?
3) Why is [H+] not a conserved species?
Questions
1) Plot the titration curve of 2.5 mM Na2CO3 with 0.05 N HCl (plot pH as a function
of titrant volume). Label the equivalent volume of titrant. Label the 2 regions of
the graph where pH changes slowly with the dominant reaction that is occurring.
Note that in a third region of slow pH change no significant reactions are
occurring (added hydrogen ions contribute directly to change in pH).
Measurement of Acid Neutralizing Capacity
62
2) Prepare a Gran plot using the data from the titration curve of the 2.5 mM Na2CO3.
Use linear regression on the linear region or simply draw a straight line through
the linear region of the curve to identify the equivalent volume. Compare your
calculation of Ve with that calculated by the Compumet™ computer program.
3) Compare the measured ANC with the theoretical value for the 2.5 mM Na2CO3
solution. Note that ANC can be defined as the excess of positive charges over the
anions of strong acids.
4) Plot the ANC of the influent and the lake from phase I of the previous lab.
5) Plot the ANC of the lake from phase II of the previous lab on the same graph as
was used to plot the conservative ANC model (see questions 2) to 5) on page 53).
Did the measured ANC values agree with the conservative ANC model?
References
Sawyer, C.N., P.L. McCarty and G.F. Parkin Chemistry for Environmental
Engineering, 4th ed., McGraw-Hill (1994).
Pankow, J.F. Aquatic Chemistry Concepts, Lewis Publishers (1991).
Morel, F.M.M. and J.G. Hering Principles and Applications of Aquatic Chemistry
Wiley-Interscience (1993).
Stumm, W. and J.J. Morgan Aquatic Chemistry 2nd ed. Wiley Interscience (1981).
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
63
Lab Prep Notes
Table 1. Reagent list.
Description
Supplier
HCl 5.0 N
Buffer-Pac
Na2CO3
Fisher Scientific
Fisher Scientific
Fisher Scientific
Catalog
number
LC15360-2
SB105
BP357-1
Table 2. Equipment list
Description
Supplier
Accumet™ 50
pH meter
pH electrode
7x7 stirrer
stirbar 1/2" long
100 mL Fisher
beaker
Fisher Scientific
Catalog
number
13-635-50
Fisher Scientific
Fisher Scientific
Fisher Scientific
Fisher Scientific
13-620-108
11-500-7S
14-511-62
02-593-50B
Setup
1) Prepare 1 L of the known standard (2.5 mM solution of Na2CO3). The MW is
105.99 g/mole.
2.5mM 105.99mg
·
= 265 mg Na2CO3/L
L
mM
2) Prepare 1 L of the titrant (0.05 N HCl from 5.0 N HCl). Dilute 10 mL of 5.0 N
HCl to 1 L. Distribute 100 mL titrant to each student group.
3) Verify that the pH probes are operational, stable, and can be calibrated.
4) Verify that buffers (pH = 4, 7, 10) are distributed to each student group
Measurement of Acid Neutralizing Capacity
64
Phosphorus Determination using the Colorimetric
Ascorbic Acid Technique
Introduction
Phosphorus has been identified as a prime nutrient needed for algae growth in
inland environments. In 1992, the EPA reported that accelerated eutrophication was
one of the leading problems facing the Nation's lakes and reservoirs. Eutrophication
caused by the overabundance of nutrients in water can result in a variety of waterquality problems, including fish kills, noxious tastes and odors, clogged pipelines,
and restricted recreation. In freshwater, phosphorus is often the nutrient responsible
for accelerated eutrophication. Many algae blooms in rivers and lakes are attributed to
elevated phosphorus concentrations resulting from human activities. Phosphorus
enters surface waters from agricultural and urban runoff as well as from industrial and
municipal wastewater treatment plant effluent.
No national criteria have been established for concentrations of phosphorus
compounds in water; however, to control eutrophication, the EPA makes the
following recommendations:
Total phosphates should not exceed 50 g/L (as phosphorus) in a stream at a
point where it enters a lake or reservoir.
Total phosphorus should not exceed 100 g/L in streams that do not discharge
directly into lakes or reservoirs.
Municipal wastewater treatment plants in many areas are required to remove
phosphorous in their treatment process. While the biological treatment process
removes some phosphorus, in most cases precipitation as an insoluble metal
phosphate is required to meet discharge regulations. This precipitation step is
normally accomplished with a metallic salt such as ferric sulfate, ferric chloride or
aluminum sulfate. This precipitation step may be accomplished in the primary or
secondary clarifiers.
Phosphorus Quantification Techniques
Quantification of phosphorous requires the conversion of the phosphorus to
dissolved orthophosphate followed by colorimetric determination of dissolved
orthophosphate. The analysis of different phosphorous forms (e.g. particulate or
organic-P) is obtained by various pretreatment steps. Pretreatment may consist of
filtering to remove suspended matter or various digestion techniques designed to
oxidize organic matter.
Phosphorus can be present in surface waters as organic phosphorus,
orthophosphate (an inorganic form of PO4), or as condensed (solid) phosphates. The
phosphorus may be in solution or as a component of suspended particulates. The wet
chemical colorimetric analysis of phosphorus only works for orthophosphates and
thus other forms of phosphorus must be converted to this form if they are to be
analyzed. Organic phosphorus can be oxidized (digested) using perchloric acid, nitric
acid-sulfuric acid, or persulfate with the persulfate technique being the safest and
least time consuming. The digestion methods are detailed in APHA method 4500-P
B.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
65
Three techniques for colorimetric analysis of phosphorus are available. The
technique most commonly used is the ascorbic acid method, which can determine
concentrations of orthophosphate in most waters and wastewater in the range from 2200 g P/L. Ammonium molybdate and antimony potassium tartrate react in an acid
medium with dilute solutions of orthophosphate-phosphorus to form an intensely
colored antimony-phospho-molybdate complex. This complex is reduced to an
intensely blue-colored complex by ascorbic acid. The color is proportional to the
phosphorus concentration. The complex is not stable and thus analysis must be
performed within 30 minutes of adding the ammonium molybdate and antimony
potassium tartrate.
Barium, lead, and silver interfere by forming a precipitate. The interference from
silica, which forms a pale-blue complex is small and can usually be considered
negligible. Arsenate is determined similarly to phosphorus and should be considered
when present in concentrations higher than phosphorus.
Method Detection Limit
"Method detection limit" is the smallest concentration that can be detected above
the noise in a procedure and within a stated confidence level. Several types of
detection limits are used including instrument detection limit (IDL), method detection
limit (MDL), and practical quantitation limit (PQL). The IDL is strictly instrument
noise and does not include variability due to sample preparation steps. The MDL
includes both instrument noise and sample preparation variability. The MDL is
obtained by making a standard that is near the MDL and dividing it into at least 7
portions. Each of the portions is then processed through all sample preparation steps
and then analyzed. The MDL is calculated using the following equation.
MDL stn 1,
6.1
where n is the sample size and =0.01 is generally the required confidence. The
student t distribution function is available in Excel as a two sided test statistic (so use
TINV(2,n-1)) and the standard deviation, s, can be computed in Excel as STDEV().
The PQL is about five times the MDL and represents a practical and routinely
achievable detection limit with reasonable assurance that any reported value greater
than the PQL is reliable. According to Standard Methods the method detection limit
for phosphorus when using a 1 cm light path is approximately 150 g P/L.
Spectrophotometer Limitations
The diode array spectrophotometer has 316 diodes that cover the wavelength range
of 190 nm to 820 nm. Each diode generates a voltage output that is proportional to the
number of incident photons. The voltage is then digitized, but the manufacturer of the
instrument in the Cornell Environmental Laboratory, Hewlett-Packard, doesn't report
the resolution of the analog to digital converter. At very low concentrations the
difference between the intensity of light transmitted through the reference and the
intensity of light transmitted through the sample approaches zero. At some low
concentration the difference in light intensity approaches the resolution of the analog
to digital converter. Another source of instrument error is drift in lamp intensity over
Phosphorus Determination using the Colorimetric Ascorbic Acid Technique
66
time. The lamp intensity is measured when a reference sample is made. The light
intensity recorded by the diodes will vary proportionally to any lamp intensity drift.
The IDL should decrease as the number of diodes used in the analysis increases (as
in Spectral analysis) for the same reason that replicate analysis of samples decreases
the standard deviation. The "Spectral analysis" feature, which can be used to measure
either single or multiple components, uses as much of the spectrum as the user desires
and thus potentially decreases the IDL. Spectral analysis uses general least squares
regression to add multiples of extinction coefficient arrays for each component to
obtain the best curve fit for the sample. The extinction coefficient arrays are obtained
from the slope of the linear regression line for absorbance as a function of
concentration at each wavelength.
Experimental Objectives
1) Measure the concentration of phosphorus in several samples to test the precision
of the ascorbic acid technique.
2) Compare the results obtained using conventional analysis at a single wavelength
with spectral analysis.
3) Analyze the data using spectrophotometer software outside the lab.
4) Analyze multiple samples so that confidence intervals can be calculated.
5) Estimate the method detection limit (MDL).
6) Discuss methods to improve the method detection limit.
7) Discuss method automation.
Experimental Procedures
Standards Preparation Method
1) Use 100 g P/L stock.
2) Use a digital pipet and prepare 1 mL of each standard.
3) Use E-pure water to dilute the 100 g P/L stock.
Reagent Addition for Samples and Standards
1) Pipette 1 mL sample into a disposable microcuvet using a 1 mL digital pipette.
2) Add 160 L combined reagent and mix thoroughly by swirling.
3) After at least 10 minutes but no more than 30 minutes, measure absorbance of
each sample using a reagent blank as the reference solution.
Samples and Standards to Prepare
1) Reagent blank to be used as reference samples.
2) Prepare 6 standards containing 0 (reagent blank), 1, 3, 10, 30, 100 g P/L.
3) Prepare 6 additional 10 g P/L standards.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
67
4) Prepare 5 samples such as Cayuga Lake water, tap water, Chem lawn runoff, local
creeks…
Spectrophotometer Method
1) Use Sample Cuvettes. (Make sure to orient all cuvettes with the arrow on the left
because the cuvettes are not symmetrical and have different absorbance when
turned 180.)
2) Use the reagent blank as the reference sample for all samples.
3) Use units of g P/L.
4) Label all samples with descriptions that your classmates will understand!
5) Fill in the general description with your initials and a description of the type of
samples
Samples Analysis
1) Measure the reference using a reagent blank. (The reagent blank is also the 0
mg/L standard.)
2) Analyze the reagent blank as a sample and verify that the absorbance deviates less
than 0.004 AU (absorbance units) from zero. If the absorbance deviates more than
0.004 AU reanalyze the reference sample.
3) Analyze 6 standards as standards using the spectrophotometer and save the data as
\\Enviro\enviro\Courses\453\phosphorus\netid_Pstd.
4) Analyze 6 standards as samples using the spectrophotometer and save the data as
\\Enviro\enviro\Courses\453\phosphorus\netid_Pstdsam.
5) Analyze 7 10-g P/L standards as samples and save the data as
\\Enviro\enviro\Courses\453\phosphorus\netid_10Pstdsam.
6) Analyze 5 samples as samples using the spectrophotometer and save the data as
\\Enviro\enviro\Courses\453\phosphorus\netid_Psam.
Prelab Questions
1) You will be creating 1 mL standards by diluting a stock of 100 g P/L. Create a
table showing how you will prepare 1 mL of each of the standards.
2) All of the samples are diluted with a small amount of combined reagent. How is
this dilution accounted for when calculating the concentration of samples?
Data Analysis
1) Plot the absorbance spectra of the standards. What is happening in the UV region?
Are there any absorbance peaks?
2) Choose an appropriate wavelength (perhaps an absorbance peak) and use Excel to
create a calibration curve. For the calibration curve, absorbance should be a
function of phosphorus concentration.
Phosphorus Determination using the Colorimetric Ascorbic Acid Technique
68
3) Use the 10-g/L standards that were analyzed as samples to evaluate the Method
Detection Limit using single wavelength analysis. Use your Spreadsheet to
calculate the concentration of each of the replicates.
4) Use the 10-g/L standards that were analyzed as samples to evaluate the Method
Detection Limit using spectral analysis. Report the wavelength range used for
your analysis. You will need to analyze each sample and copy the results into a
spreadsheet. (Note that within a data file you change which sample the software is
analyzing with the sample number control.)
5) Use the method with the lowest MDL to analyze your samples. Create a bar graph
showing the concentration of each of the samples. Report the MDL in the figure
caption.
Spreadsheet requirements
Your spreadsheet must contain all of the analysis requested above as well as the
following capabilities:
1) A well-marked cell containing the analytical wavelength for single wavelength
analysis. Changes to this cell must be reflected in all calculations and graphs.
2) The graph showing the absorbance spectra of the standards must have a vertical
line indicating the analytical wavelength.
3) All of the graphs must be on the same page as the analytical wavelength control
so the effect of changing the wavelength can be easily observed.
4) A separate sheet where you answer the 4 questions.
Hints
If you haven't already learned how to use Vlookup() now is the time!
The row() function returns the number of the row. I found it useful for this analysis!
The slope() and intercept() functions eliminate the need to type equations off of
graphs!
Questions
1) Which analysis technique gave the best results? Explain why. If the analytical
technique didn't significantly affect the MDL, explain why not.
2) What types of errors dominated your ability to measure phosphorus?
3) What method modifications do you propose to improve phosphorus
measurements?
4) Total phosphorus concentration in Cayuga Lake varies between 10 and 50 ppb.
Would the techniques used in this lab be able to measure these phosphorus levels?
References
http://wwwrvares.er.usgs.gov/nawqa/circ-1136/h6.html#PHOS
http://www.epa.gov/glnpo/lmmb/methods/index.html#Volume 3
Standard Methods for the Examination of Water and Wastewater.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
69
Lab Prep Notes
Reagents
A Sulfuric acid solution, 4.9 N: Add
136 mL concentrated H2SO4 to
800 mL E-pure water. Cool and
dilute to 1 L with E-pure water.
B Ammonium molybdate solution:
Dissolve 40 g of (NH4 )6
Mo7O24•4H2O in 900 mL E-pure
water and dilute to 1 L. Store at
4°C.
C Ascorbic acid: Dissolve 9 g of
ascorbic acid (C6H8O6) in 400 mL
E-pure water and dilute to 500
mL. Store at 4°C. Keep well
stoppered. Prepare fresh monthly
or as needed.
D Antimony potassium tartrate:
Dissolve
3.0
g
of
K(SbO)C4H4O6•½H2O in 800 mL
E-pure water and dilute to 1 L.
Store at 4°C.
Combined color reagent: Combine
the following solutions in order,
Table 1. Reagents
Description
Supplier
concentrated
H2SO4
(NH4 )6
Mo7O24•4H2O
C6H8O6
K(SbO)C4H4O6•
½H2O
sodium lauryl
sulfate
KH2PO4
Fisher Scientific
Catalog
number
Fisher Scientific
Fisher Scientific
Fisher Scientific
Fisher Scientific
Table 2. Equipment list
Description
Supplier
100-1095 µL
Fisher Scientific
pipette
10-109.5 µL pipette Fisher Scientific
Disposable cuvets Fisher Scientific
Cuvet holder
Fisher Scientific
UV-Vis
Hewlett-Packard
spectrophotometer
Company
Catalog #
13-707-5
13-707-3
14-385-942
14-385-939
8452A
mixing (but do not entrain air as oxygen oxides the ascorbic acid) after
each addition: (Prepare fresh weekly. Store at 4°C)
Stock A, (4.9 N H2SO4) 50 mL
Stock B, (Ammonium molybdate solution) 15 mL
Stock C, (Ascorbic acid solution) 30 mL
Stock D, (Antimony-tartrate solution) 5 mL
Water diluent solution: Add 4.0 g sodium lauryl sulfate and 5 g NaCl per L of E-pure
water.
Stock phosphorus standard: Dissolve 0.4394 g of Potassium phosphate monobasic
(KH2PO4) (dried at 105°C for one hour) in 900 mL E-pure water. Add 2 mL of
concentrated H2SO4 and dilute to 1 L. 1.0 mL = 0.100 mg P (100 mg P/L).
Standard phosphorus solution: Dilute 1 mL of stock solution to 1 L. (1.0 mL = 0.1 g
P) (100 g P/L).
Phosphorus Determination using the Colorimetric Ascorbic Acid Technique
70
Soil Washing to Remove Mixed Wastes
Objective
The goal of this laboratory exercise is to acquaint students with some of the
chemical reactions that result in the binding of inorganic and organic pollutants in
subsurface materials. Extractants used by engineers to release contaminants at
hazardous waste sites (where mixtures of both types of contaminants are present) may
or may not prove effective, depending upon their mechanism of action. In this
laboratory exercise, students will test the efficacy of a variety of proposed extractants
in the removal of a mixture of an inorganic metal cation, and an organic compound
from a contaminated porous medium.
Introduction
Many Superfund site soils are contaminated with a mixture of contaminants
including toxic metals and organic compounds. A pressing environmental problem is
to devise clean-up strategies that can effectively remove mixed wastes. Many kinds of
contaminants bind to soils and aquifer media (collectively referred to here as porous
media). Binding reactions limit the effectiveness of “pump and treat remediation” in
which a contaminated porous medium is flushed with water to remove contaminants.
In such cases, it can prove useful to engineer the properties of the aqueous phase to
improve the mobility of the pollutants of interest.
In the case of toxic metals, release of medium-bound or “adsorbed” metals can be
enhanced by introduction to the pore solution of a dissolved compound that will bind
to the metal in the aqueous phase and form a dissolved “complex”. Such compounds
are referred to as “ligands”, and ligands that bind metals very strongly are called
“chelating agents”. Metal solubility and adsorption can also be strongly influenced by
the oxidation state of the metal, and use of oxidants or reductants to alter the redox
conditions in a porous medium can modify metal mobility both directly and
indirectly. Direct effects would be observed if oxidized and reduced metal species
have different adsorption characteristics (ex. Cr2O7-2 vs. Cr+3). Indirect effects would
be observed if a metal were bound to a solid phase that would be dissolved under
different redox conditions (ex. Fe(OH)3 may dissolve under reducing conditions).
Addition of acids or bases could also alter metal mobility. Adsorption of metals is
very sensitive to pH shifts, with a decrease in pH favoring the release of cationic
metal species (ex. Cd+2, Pb+2) and an increase in pH favoring release of anionic
species (ex. Cr2O7-2, SeO3-2).
Organic cations and anions will have a pH dependent adsorption behavior similar
to that described above for metal ions. However, many organic pollutants of interest
are nonionic and their binding to the matrix of the porous medium is not greatly
influenced by pH. “Hydrophobic interactions” of nonionic organic compounds with
organic matter in porous media appear to be a major driving force for their binding.
The addition of surfactants to the pore solution can help to release sorbed nonionic
organic pollutants. Under suitable conditions many organic pollutants can be
degraded by addition of oxidants or by indigenous or added bacteria [i.e.; given that
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
71
the bacteria have the necessary genetic capabilities, nutrients (N, P, etc.) and a
suitable electron acceptor]. Metals, however, are elements and cannot be degraded.
Porous media is not inert. The mineral and organic constituents of the porous
matrix can react with added ligands, acids, bases, oxidants, reductants, and
surfactants. A consequence, in some cases, is that a desired addition may be rendered
impractical.
Given the variability and possible dissimilarity of conditions that influence the
mobility of metal vs. organic pollutants, it is a challenging task to identify a
remediation strategy that will successfully treat a given medium that is contaminated
with mixed wastes. In this laboratory exercise, students will evaluate the utility of
several alternative extractants for remediation of a soil that is contaminated with both
a metal cation and an organic compound.
Theory
Binding Reactions
The binding reactions of pollutants to the porous matrix may be classified, at least
in part, by where and how the binding reaction takes place. The term “adsorption” is
used for reactions that take place at the interface between the solid and the solution.
All other factors being equal, solids with a greater specific surface area (ex. units:
m2/gram) will adsorb greater amounts of a dissolved solute. In adsorption reactions,
the surface is referred to as an “adsorbent” and the solute as an “adsorbate”. Some
adsorption reactions are driven by electrostatic attraction between the surface and the
solute. “Ion exchange” is the term used for this type of reaction. All other factors
being equal, surfaces with a greater number of charged sites per unit surface area will
be able to bind greater quantities of dissolved ions. The concentration of surface
exchange sites is commonly quantified as an “ion exchange capacity”. Surfaces with
a high density of negatively charged sites (cation exchangers) will selectively bind
positively charged ions while those with a high density of positively charged sites
will be selective for anions.
“Absorption” is a process in which a solute penetrates within the solid matrix.
“Partitioning” is a term that is synonymous with absorption. As an example, we
would carry out a partitioning process if we were to add a pollutant to a separatory
funnel containing water and an organic liquid such as octanol and then observe the
resulting distribution of the contaminant between the aqueous and octanol liquid
phases. The distributed contaminant would exist as a dissolved solute in each phase.
As is noted below, the phase distribution behavior of nonionic organic pollutants in
soils and aquifer media displays many characteristics of absorption reactions. The
absorption of nonionic organics appears to be primarily into the organic matter
content of the porous medium. This reaction is driven by the water loving nature of
the solute, or lack thereof (i.e., pollutant “hydrophobicity”). All other factors being
equal, porous media with higher organic carbon contents would have greater uptake
of nonionic organic pollutants.
The term “sorption” is somewhat loosely used when the exact mechanistic nature
of the pollutant’s distribution between the solution and the porous medium is not
Soil Washing to Remove Mixed Wastes
72
understood, or when both adsorption and absorption reactions may contribute to the
contaminant’s phase distribution.
Contaminant sorption reactions result from an reaction between a material that is
dissolved in an aqueous solution with a solid phase. The physical/chemical properties
of the contaminant, the solution and the sorbent all influence the resulting
contaminant phase distribution. These influences are discussed below.
Sorbent Surface Charge
As noted above, if the sorbate is an ion, then electrostatic attraction to the surface
can play an important role in contaminant adsorption. Virtually all soil surfaces are
charged.
Oxide Minerals
Surface charge can result from the ionization of surface functional groups in
response to the hydrogen ion concentration of the aqueous phase. Oxide minerals are
often modeled as diprotic acids (Westall and Hohl, 1980). Accordingly the surface
may donate two hydrogen ions as indicated by the following reactions:
K1
SOH 2
SOH H
K2
SOH
SO H
7.1
7.2
where SO represents the oxide surface that may exchange two hydrogen ions, and K1
and K2 are equilibrium constants for the first and second acid dissociation reactions.
Note, each dissociation constant can be thought of as a expression of the
relationship between the concentration of protonated and deprotonated surface sites
and the solution hydrogen ion concentration. Accordingly:
SOH H
K1
SOH 2
7.3
SO H
SOH
7.4
K2
K1 therefore represents the solution hydrogen ion concentration at which the
concentration of positively charged, diprotic, surface sites SOH 2 is equal to that of
surface sites containing a single proton SOH . Similarly, when H equals K2 then
SO = SOH .
Although other models of the acid base behavior of oxide surface are conceivable,
the above model is helpful in that it predicts that the surfaces can have both positively
and negatively charged sites. With this model, H+ release from the surface will occur
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
73
in response to a decrease in the solution H+ concentration (i.e., an increase in pH,
where pH is defined as -log H ). Accordingly, we would expect increasingly
higher solution pH conditions to favor formation of negatively charged surfaces, and
this is observed. Different surfaces would have different acidity constants (K1 and K2)
and would be expected to have different surface charges at the same solution pH.
Each surface, at one unique pH, would have an equal concentration of SOH 2 and
SO sites and would have no net charge. This is also observed and is referred to as
the pH point of zero charge (PZC). SiO2, a common oxide in porous media (the main
component of sand), has a low PZC (≈ pH 2 to 3) while iron and aluminum oxides
(that commonly occur as surface coatings) have considerably higher PZCs (≈ pH 7 to
8) (Parks and DeBruyn, 1962).
Soil Organic Matter
Another pH-dependent origin of surface charge is the ionization of the acidic
functional groups in soil organic matter. The carboxyl groups of humic-type organic
matter typically have acidity constants ≤ 10-5 (pK ≤ 5) and are therefore highly
ionized at circumneutral pH.
Isomorphic Substitution
A final source of charge in soil is isomorphic substitution in the crystalline lattice
of some clay minerals. Substitution of Al+3 for Si+4 and Mg+2 for Al+3 will result in a
net negative charge for the clay mineral phase.
The combined effects of isomorphic substitution, ionization of organic functional
groups and the low PZC of silicon oxide minerals make it likely that many porous
media will have a net negative charge. Consequently, stronger binding of cationic
contaminants is generally anticipated.
Sorbent Ion Exchange Reactions
Ion exchange reactions involve the exchange of ions of the same charge at an
oppositely charge site on the solid surface. Exchange reactions are often characterized
by “selectivity coefficients” that may be thought of as equilibrium constants for the
exchange reaction. For example, in the exchange of two monovalent cations, the
exchange reaction may be depicted as:
Kx
SO x y
SO y x
y
7.5
where:
K xy
SO y x
SO x y
7.6
The magnitude of the selectivity coefficient, K xy , reflects the extent to which ion x+
vs. y+ will accumulated at the surface. Ions with high selectivity coefficients can
displace more weakly held ions from an exchange site.
Soil Washing to Remove Mixed Wastes
74
In a negatively charged soil, anionic compounds (ex. ionized organic acids, NO3-,
Cr2O7-2, etc.) will be repelled from the surface and therefore may be highly mobile.
Cationic species (ex. quaternary ammonium organic compounds, divalent transition
metals, etc.) will be attracted to the surface and have restricted mobility. In principle,
exchangeable pollutant cations may be mobilized by introduction of high
concentrations of an innocuous cation. The practicality of such an approach would be
dictated by the extent to which other exchangeable cations (that are not of
environmental concern) are also exchanged. Since cations such as Na+, K+, Ca+2 Mg+2
are abundant in porous media, the amount of a cation added for exchange of a trace
pollutant would have to be in great excess of the pollutant cation. As a result, release
of contaminant cations by an ion exchange mechanism does not appear to be
economically feasible.
Sorbent Hydrophobic Interactions
The mechanisms responsible for the adsorption of charged species differ
considerably from those for nonionic compounds. Adsorption of charged ions may, in
some cases, involve more than the simple electrostatic attraction of ions to a surface
of opposite charge. Transition metal cations, for example, will often adsorb to oxide
surfaces even under solution conditions that confer a positive charge on the surface
(see additional discussion below under the topic of solution characteristics).
The sorption of nonionic organic pollutants behaves as if it is a partitioning process
into the organic matter that is present as part of the soil matrix. Some of the general
characteristics that lead to this conclusion are the observance of linear sorption
isotherms at high solution concentrations (that can approach the solubility limit of
solute compounds). [Note, an “isotherm” is simply the relationship between the
quantity of pollutant that is bound (per unit mass or unit surface area of the sorbent)
and the concentration of contaminant in solution.] In contrast, adsorption reactions
are limited by the availability of surface sites and adsorption isotherms are typically
non-linear at high solute concentrations. Partition reactions are also relatively free
from competition (i.e., the presence of a second solute does not effect the sorptive
uptake of the first) while competition for surface sites is an expected characteristic in
an adsorption process. The extent of sorption of a given nonionic organic onto a
variety of sorbents is highly correlated with their organic content as expressed by the
weight fraction of organic carbon, foc (Karickhoff, 1984). For the same sorbent, the
sorption of different nonionic solutes is highly correlated with their octanol-water
partition coefficients (Kow) (Karickhoff, 1984). Collectively, these observations lead
to the conclusion that the sorption of nonionic organic pollutants is primarily driven
by hydrophobic interactions between the solute and the organic matter in the sorbent.
Solution pH
Solution conditions can have dramatic effects on the adsorption of cationic
contaminants. For example the adsorption of cationic transition metals to oxide
surfaces typically increases markedly over a narrow range of 1 to 2 pH units referred
to as the “adsorption edge”. The pH dependence of metal ion adsorption can be
explicitly accounted for by writing the adsorption reaction as:
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
75
Kd
z
SOH x Meaq
SOMe z x xH
7.7
where K d is the pH-dependent metal distribution coefficient, and according to
Honeyman and Santschi (1988)
Kd
SOMe z x H
z
SOH x Meaq
x
7.8
SOMe z x
versus pH, is referred to as a “Kurbatov plot”
A plot of log
z
SOH x Meaq
(after Kurbatov et. al., 1951), and may be used to reveal the magnitude of the
SOMe z x
is the
exponent, x for H in the distribution coefficient. The ratio
SOH x
quantity of adsorbed metal per unit surface. Since the above reaction and its
equilibrium constant, Kd, are an over simplification of the actual adsorption
mechanism, measured values of x are rarely integers. Nevertheless, x values ranging
from 1 to 2 are common for adsorption of metal cations on oxide surfaces and
demonstrate the strong dependence of the adsorption processes on pH. For example,
if x = 2, an increase of 1 pH unit would result in a 100 fold increase in the amount of
bound metal per unit surface (at the same solution concentration of metal ion). In
general, adsorbed metal cations will be released as a consequence of a decrease in
solution pH. Since the surfaces in the porous medium also have acid/base properties,
and because many porous media contain acid-reactive components (such as carbonate
minerals) a very large acid dose may be required to effectively alter the pH of the
pore water. For this reason, acid extraction of adsorbed metals may not always be
feasible.
Metal-Ligand Complexes
Another influence of solution conditions on metal adsorption is through the
reactions of metals with ligands to form complexes. In some cases, metal-ligand
complexes adsorb weakly or not at all (ex. Cl- complexes of Cd and Hg), in other
cases metal-ligand complexes may adsorb with a binding strength greater than that of
the free metal (ex. organic complexes of Cu) (Benjamin and Leckie, 1982). Judicious
selection of a ligand for introduction into a porous medium may, therefore, be used to
accomplish the release of adsorbed cations. Added ligands may, in some cases,
undergo exchange reactions with the porous media or react to form complexes with
cations that are not of environmental concern. For this reason the dose of a ligand
needed to effectively release adsorbed metals will vary with the composition of the
porous media and ligand addition may not prove feasible in some cases.
Oxidants and Reductants
Changing solution composition by the introduction of oxidizing or reducing agents
may accomplish the release of adsorbed metals. Iron oxides are strong metal binding
Soil Washing to Remove Mixed Wastes
76
agents and may be solubilized by reduction from ferric (Fe III) to ferrous (Fe II) iron.
Many transition metals (e.g. Cd, Co, Cu, Ni, Pb, Zn) will remain as divalent cations
during such a shift in redox status, and may therefore simply re-absorb to another
surface. In some cases, alteration of the media redox conditions may directly
influence metal mobility. For example reducing conditions would favor the presence
of a cationic form of chrome (Cr+3) over the more mobile anionic form (Cr2O7-2).
Addition of oxidants may therefore help to mobilize chrome, however the organic
matter in soils and ferrous minerals will also react with added oxidants.
In a manner similar to the role of iron oxides, the organic matter in porous media
can be responsible for the binding of metal cations. The reaction of an added oxidant
with humic-type organic matter may therefore accomplish solubilization of some
metals (Lion et al., 1982). Strong oxidants will also act to break down organic
contaminants.
Hydroxyl Radicals
One application that has been used for remediation of organic contaminated soils is
the introduction of Fenton’s reagent. Fenton’s reagent is a mixture of hydrogen
peroxide and ferrous iron (Fe+2). These chemicals react to produce hydroxyl radicals3
(OH•) according to the following reaction:
Fe+2 + H2 O2 Fe+3 +OH- +OH
7.9
The hydroxyl radicals produced by Fenton’s reagent are highly reactive and can
effectively degrade recalcitrant aromatic compounds by ring substitution followed by
ring cleavage (Sedlak and Andren, 1991).
Surfactants
Additions of surfactants may aid in the release of sorbed nonionic organic
pollutants. In the case of sorption reactions that are driven by hydrophobic
interactions, surfactant additions can have two beneficial effects: 1) a decrease in the
aqueous activity coefficient for the dissolved nonionic organic compound and 2)
formation of micelles in the aqueous phase.
The effect of the aqueous activity coefficient can be illustrated by examination of
the sorption isotherm for the organic pollutant. If the isotherm is linear, then we may
write:
KSL CL
7.10
S
where is the mass of solute sorbed per mass of solid, K L is the sorptive distribution
coefficient, CL is the aqueous concentration of the sorbate, and is the activity
coefficient of the dissolved sorbate.
Surfactants may act to decrease the activity coefficient, for a nonionic molecule
increasing the concentration in the aqueous phase in equilibrium with a given
adsorbed amount, . Surfactants increase the solubility of nonionic molecules
3
Radicals contain an odd number of electrons.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
77
because the hydrophobic-nonionic molecules adsorb to the long hydrocarbon group
while the ionic sulfonic group provides high solubility
(Figure 1).
However, since surfactants are surface-active, they may
O
also sorb to the porous medium, increase its organic content,
and consequently increase the sorption of a nonionic organic
R O S O
contaminant. High concentrations of water-soluble
O
cosolvents such as methanol and acetone can also act to
decrease the activity coefficient, , and act to solubilize
Figure 1. Molecular
sorbed nonionic organic compounds (Schwarzenbach et al.,
structure of detergent.
1993).
The
hydrocarbon
Surfactant molecules can aggregate into micelles in which
chain (R group) of the
their polar functional groups are oriented towards the
detergent used in this
aqueous solvent and their non-polar tails are oriented inward
lab is C12H25.
toward each other. The space within the micelles therefore
provides a hydrophobic refuge for nonionic contaminants
(Edwards et al., 1991). Surfactants will form micelles at
aqueous concentrations greater than their “critical micelle concentration” (CMC).
Since, as noted above, surfactants will sorb at the surface of the porous media, a high
dose of surfactant may be required in order to maintain an aqueous concentration
greater than the CMC.
Bacterial Polymers
Many of the solution modifications discussed above involve the addition of
synthetic agents to contaminated soil to accomplish the release of sorbed
contaminants. Natural constituents that occur in soils and aquifers may also enhance
contaminant transport (McCarthy and Zachara, 1989). Bacterial polymers naturally
occur in soil solution and have well-documented metal binding properties. The
presence of bacterial polymers may therefore act as a natural process by which metal
mobility is enhanced (Chen et al., 1995). The extracellular polymers produced by
bacteria are hetero-polysaccharides and have high molecular weight. Interestingly,
these large molecules have also been show to be effective at binding nonionic organic
pollutants and at enhancing their transport in aquifer materials (Dohse and Lion,
1994). In principle, bacterial polymers with suitable binding properties could be
produced in engineered reactor systems and be applied to contaminated waste sites to
enhance the mobility of metal and nonionic organic contaminant mixtures. The
efficacy of this type of remediation process has yet to be determined.
Apparatus
Students will apply a range of extractant types (or mixtures of different types) to
remove contaminants (Zn and methylene blue) from a porous medium. Laboratory
extractions will mimic an engineered soil washing system in which the contaminated
soil is actively mixed with the extractant and then separated. A rotator will be used to
provide agitation of samples of the medium with extractants, and a centrifuge will be
used to provide phase separation. A UV/visible spectrophotometer with a diode array
detector will be used to measure the concentration of the extracted organic pollutant.
Soil Washing to Remove Mixed Wastes
78
Extracted metal concentrations will be measured with an atomic absorption (AA)
spectrophotometer.
Experimental Procedures
Each group will develop their own hypothesis and experimental protocol. Different
concentrations of extractants, different organic contaminants, and different washing
techniques could be the investigation subjects. Alternate organic contaminants should
be cleared with the instructor prior to the lab period. Each group should limit the
investigation to approximately 10 samples and should include appropriate controls
and replicates.
The following protocol assumes that a common sand is employed to represent the
porous medium. It is desirable, but not essential, to characterize each medium to be
used (prior to the laboratory exercise) with respect to its carbon content [the “Walkley
Black” method is one common procedure (Allison, 1965)], cation exchange capacity,
and specific surface area [by sorption of ethylene glycol monoethyl ether (EGME)
(Cihacek and Bremmer, 1979)].
I. Creation of a Contaminated Porous Medium
A stock solution containing the soil contaminants will be provided [50 mg/L Zn
and 100 mg/L methylene blue]. For each extractant used in part II below, 2 samples
of contaminated sand and one sample of clean sand will be used. The following
procedure is based on the assumption that each student group will evaluate 3
extractants or 3 concentrations of an extractant.
1) Weigh out 9 aliquots of sand, 2.5±0.05 g each, and pour into 10 mL plastic
centrifuge tube.
2) Record the mass of the centrifuge tube with the sand (see Table 1).
3) Add 5 mL of the contaminant stock solution to 6 of the samples.
4) Add 5 mL distilled water to 3 of the samples (clean controls).
5) Place all of the samples on a rotator to mix the sand and the contaminant/clean
solutions. Agitate for 15 minutes.
6) Centrifuge the suspensions at 3000 x g for 5 minutes.
7) Pour the supernatant from the 6 contaminated sand samples into a 125 mL bottle.
8) Pour the supernatant from the 3 clean sand samples into a separate 125 mL bottle.
9) Weigh the centrifuge tubes with the sand and pore water. Calculate the volume of
pore water by subtracting the centrifuge tube and sand masses.
II. Determination of the Amount of Contaminant Sorbed by the Sand
Methylene blue - UV/Vis Spectroscopy
Nitrate absorbs ultraviolet light and is present in the contaminated samples from
the addition of Zn(NO3)2·6H2O. We could account for this either by preparing a
nitrate standard and using it as a component in spectral analysis or by eliminating the
ultraviolet part of the spectrum from the analysis. We will eliminate the nitrate
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
79
interference by using a wavelength of 660 nm when measuring methylene blue. See
page 160 for instructions on using the UV/Vis Spectrophotometer.
1) Measure the absorbance of 1, 5, and 10 mg/L methylene blue solutions as
“Standards.”
Save
the
file
as
\\Enviro\enviro\Courses\453\soilwash\netid_MBstd.
2) Measure the absorbance of the combined supernatant from the 3 clean sand
samples, the combined supernatant from the 6 contaminated sand samples, and
the contaminating solution (diluted by a factor of 10) as “Samples.” Save the file
as \\Enviro\enviro\Courses\453\soilwash\netid_contamsuper.
3) Record the concentration of methylene blue in the clean supernatant and
contaminated supernatant (see Table 2). You can drag the blue cursor on the
"standard graph" to the wavelength of choice and read the exact absorbance (and
wavelength) in the digital display to the left of the graph and concentration in the
digital display at the bottom of the Spectrophotometer window. If the clean
supernatant has significant absorbance at 660 nm then alternate analytical
techniques may need to be used.
4) The difference between the methylene blue concentration in the contaminant
solution and the concentration in the supernatant may be used to determine the
sorbed contaminant concentration as:
=
(Cinitial C final )(solution volume)
7.11
mass of sand
where Cinitial is the contaminant solution concentration and Cfinal is the
concentration of the supernatant. Solution volume is the volume of
contaminant added initially.
Zinc - Atomic Absorption Spectroscopy
1) Calibrate the AA using the zinc standards (1, 2, and 6 mg/L).
2) Dilute all of the following samples by a factor of 10 to ensure sufficient sample
volume for the analysis and to ensure that the results are in the calibrated range.
3) Measure and record the zinc concentration of the combined supernatant from the
3 clean sand samples.
4) Measure and record the zinc concentration of the combined supernatant from the
6 contaminated sand samples (see Table 4).
5) Measure and record the zinc concentration in the contaminating solution (the zinc
concentration should be close to 50 mg/L).
6) The difference between the zinc concentration in the contaminant stock and the
concentration in the supernatant may be used to determine the sorbed contaminant
concentration using equation 7.11.
III. Soil Washing Solutions
Students may wish to experiment with extractant mixtures. Some combinations of
extractant solutions may react violently! All proposed mixtures of extractants should
Soil Washing to Remove Mixed Wastes
80
be cleared with the course instructor prior to their use. The following combinations
should be avoided: mixtures of oxidants with reductants, mixtures of acids with bases,
and mixtures of oxidants with organic extractants including: surfactants, chelating
agents or cosolvents. The following extractant solutions will be available for testing.
At least one group should measure the extractant capabilities of distilled water
because it is by far the cheapest!
1) Distilled water.
2) Acid: ≈ 1 M solution of HCl prepared by diluting 27.4 mL of the concentrated
acid to 1 L with distilled water.
3) Cosolvent: 1:1 (v/v) mixture of acetone and distilled water.
4) Non-ionic surfactant: 10% (v/v) solution of Triton X-100 prepared by diluting
100 mL to 1 L with distilled water. Note: Triton X-100 is a non-ionic surfactant
with a CMC of 2x10-5 M (Edwards et al., 1991). The chemical formula for Triton
X-100
is:
CH3
CH3
CH3 -C-CH2 -C
CH3
5)
6)
7)
8)
(OCH 2CH 2)xOH
CH3
where: x = 9 to 10, giving the surfactant a molecular weight of ≈ 607g/mole.
Anionic surfactant: 100 mM solution of dodecyl sulfate, sodium salt
(C12H25SO4Na with MW of 288.4 so 28.84 g/L). This extractant works very well
at full strength; lower concentrations could be investigated.
Chelating agent: ≈ 0.1 M solution of ethylenediamine-tetraacetate (EDTA)
prepared by dissolving 37.22 g of the disodium salt in distilled water and diluting
to 1 L.
Base: ≈ 1 M NaOH solution prepared by dissolving 40 g of NaOH distilled water
and diluting to 1 L.
Oxidant: 1:1 (v/v) mixture of 30% H2O2 and distilled water.
9) Reductant: ≈ 1 M solution of Na2S2O3.5H2O prepared by dissolving 248 g in
distilled water and diluting to 1 L.
IV. Soil Washing Protocol
The ability of each extractant to remove the zinc and methylene blue from the
contaminated soil will be measured by exposing the contaminated soil to the
extractant and then measuring the concentration of the zinc and methylene blue in the
extractant.
1) Add 5 mL of each extractant to be tested to 2 contaminated sand samples and 1
clean sand sample.
2) Place the sand extractant mixtures on a rotator to mix for 15 minutes.
3) Centrifuge the suspensions at 3000 x g for 5 minutes.
4) Decant the supernatant from each centrifuge tube into labeled 15 mL bottles. A
small air line can be used to help force the supernatant from the centrifuge tubes.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
81
V. Analysis of Extracted Metal and Organic Pollutants
Methylene blue - UV/Vis Spectroscopy
Note that it is unnecessary to measure the methylene blue concentration in samples
that do not appear to have any blue. Samples that are visually free of methylene blue
can be recorded as 0 mg/L methylene blue.
1) Measure the absorbance of each of the clean sand extracts as “Samples.” All of
the clean extracts can be analyzed together if desired. Save as
\\Enviro\enviro\Courses\453\soilwash\netid_cleanext.
2) Measure concentration of methylene blue in each of the clean extracts based on
the absorbance at 660 nm (see Table 2).
3) Measure the absorbance of each of the extracts of the contaminated sand as
“unknowns.” There will be 2 replicates for each extract. All of the contaminated
extracts can be analyzed as a group so that their results are saved in a single file.
Save as \\Enviro\enviro\Courses\453\soilwash\netid_contamext.
4) Measure concentration of methylene blue in each of the contaminated extracts
based on the absorbance at 660 nm (see Table 2). Note that it may be necessary to
choose a different analytical wavelength or to dilute the sample if the absorbance
exceeds ≈2.5 at 660 nm.
5) Calculate the mass extracted per mass of sand as:
Cextracted solution volume
7.12
mass of sand
where solution volume is the sum of residual pore water volume after decanting
the contaminant plus the extractant volume.
Zinc - Atomic Absorption Spectroscopy
1) Dilute all samples by a factor of 10 prior to analysis.
2) Measure and record the absorbance of the supernatant from the 3 clean sand
samples (see Table 4).
3) Measure and record the absorbance of the supernatant from the 6 contaminated
sand samples. Dilute the supernatant with distilled water if the absorbance is not
less than the absorbance of the 6-mg/L standard.
4) Calculate the concentration of zinc in each of the sand extracts.
5) Calculate the mass of Zn removed per mass of sand using equation 7.12.
The results for any extractant must justify the cost of its use. The results obtained
by extraction of contaminated soil using distilled water serve as a basis for
comparison to which results obtained with extractants should be compared. Results
from all extractants or extractant combinations evaluated should be compared to
provide an inter-comparison of their relative effects of the removal of metal and
organic pollutants.
Soil Washing to Remove Mixed Wastes
82
Prelab Questions
1) The point of zero charge for SiO2 is approximately at pH = 2.5. Is the charge of
SiO2 positive or negative at a pH of 7?
2) Do cations or anions generally bind most strongly to soil?
3) Develop a hypothesis concerning soil washing, and write an experiment protocol
to test your hypothesis that you can do in a lab period. Include detail of
concentrations of extractants and contaminants for each vial. You may want to
work with your lab partner(s) because this will be your experiment! Design your
experiment to use no more than 9 vials.
Data Analysis
1) Report the contaminated sand concentration (grams of contaminant/gram of sand)
for zinc and methylene blue.
2) Calculate the fractional removal of zinc and methylene blue for each extractant or
extractant concentration. The fractional removal based on the amount of
contaminant initially sorbed is f =
where is defined in equation 7.11 and
∆ is defined in equation 7.12. Present this using an appropriate graph.
3) Discuss which extractant performed best at removal of the zinc. Which was best
at removing the test organic? Which extractant worked best at removing both
contaminants? Discuss these results in terms of the chemical change that the
extractant was designed to accomplish. (Note that these questions may need to be
modified based on the samples you analyzed.)
4) Discuss any difficulties in evaluating extractant effectiveness and propose
improved analytical techniques.
5) Discuss your results and their implications for the hypothesis that you developed.
6) Analyze your results including the reproducibility of replicate analyses in terms of
possible sources of error.
7) Suggest options for additional research.
References
Allison, L. E., “Organic carbon”, in: Soil Analysis Part 2: Chemical and
Microbiological Properties, C. A. Black (ed.), Amer. Soc. Agronomy, Madison,
WI, p 1367, 1965.
Benjamin, M.M. and J.O. Leckie, “Effects of complexation by Cl, SO4 and S2O3 on
adsorption behavior of Cd on oxide surfaces”, Env. Sci. & Tech. 16(3), pp.
162-170, 1982.
Chen, J.-H., L.W. Lion, W.C. Ghiorse, and M.L. Shuler, “Trace metal mobilization in
soil by bacterial extracellular polymers”; Water Research, (1995, in press).
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
83
Cihacek, L.J. and J.M. Bremmer, “A simplified ethylene glycol monoethyl ether
procedure for assessment of soil surface area”, J. Soil Sci. Soc. Am. 43 pp.
821-822, 1979.
Dohse, D.M., L.W. Lion, “The effect of microbial polymers on the partitioning and
transport of phenanthrene in a low-carbon sand”; Environmental Sci. & Tech.
28(4), 541-547 (1994).
Edwards, D.A., R.G. Luthy, and Z. Liu, “Solubilization of polycyclic aromatic
hydrocarbons in micellar nonionic surfactant solutions”, Env. Sci. & Tech. 25, pp.
127-133, 1991.
Honeyman, B.D. and P.H. Santschi, “Metals in aquatic systems”, Env. Sci. & Tech.
22(8), pp. 862-871.
Karickhoff, S.W., "Organic Pollutant Sorption in Aquatic Systems", J. Hydraulic
Engrg., 110(6), p. 707, 1984.
Kurbatov, M.H., G.B. Wood, and J.D. Kurbatov, “Isothermal adsorption of cobalt
from dilute solutions”, J. Phys. Chem. 55, pp. 1170-1182, 1951.
Lion, L.W., R.S. Altmann, and J.O. Leckie, "Trace metal adsorption characteristics of
estuarine particulate matter: Evaluation of contributions of Fe/Mn oxide and
organic surface coatings," Env. Sci. and Tech. 16(10), pp. 660-666 (1982).
McCarthy, J. F. and J. M. Zachara, “Subsurface transport of contaminants”, Env. Sci.
& Tech. 23, pp. 496-502, 1989.
Parks, G.A. and P.L. DeBruyn, “The zero point of charge of oxides”, J. Phys. Chem.
66, pp. 967-973, 1962.
Sedlak, D.L., and A.W. Andren, “Oxidation of chlorobenzene with Fenton’s reagent”,
Env. Sci Tech. 25(4), pp. 777-782, 1991.
Schwarzenbach, R.P., P.M. Gschwend, and D.M. Imboden, Environmental Organic
Chemistry, Wiley Interscience Publ., New York, NY; 681 pp., 1993.
Standard Methods for the Examination of Water and Wastewater, L.S. Clesceri, A.E.
Greenberg and R.R. Trussell (eds.) 17th edition, Am. Public Health
Assoc.(Publisher), Washington, DC; 1989.
Westall, J. and H. Hohl, “A comparison of electrostatic models for the oxide/solution
interface”, Adv. Colloid Interface Sci. 12 p. 265, 1980.
Soil Washing to Remove Mixed Wastes
84
Table 1. Data table.
bottle # contaminated mass
or clean
sand (g)
mass mass bottle +
bottle + sand + pore
sand (g)
water (g)
clean
cont.
cont.
clean
cont.
cont.
clean
cont.
cont.
Table 2. Methylene blue data table.
concentration
clean sand supernatant
contaminated sand supernatant
extractant 1 clean sand
extractant 1 cont. sand rep 1
extractant 1 cont. sand rep 2
extractant 2 clean sand
extractant 2 cont. sand rep 1
extractant 2 cont. sand rep 2
extractant 3 clean sand
extractant 3 cont. sand rep 1
extractant 3 cont. sand rep 2
Table 3. Zinc concentration measurements data table.
dilution concentration
clean sand supernatant
contaminated sand supernatant
extractant 1 clean sand
extractant 1 cont. sand rep 1
extractant 1 cont. sand rep 2
extractant 2 clean sand
extractant 2 cont. sand rep 1
extractant 2 cont. sand rep 2
extractant 3 clean sand
extractant 3 cont. sand rep 1
extractant 3 cont. sand rep 2
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
85
Lab Prep Notes
Table 4. Reagent list.
Description
Supplier
Catalog
number
Zn(NO3)2·6H2O
Methylene Blue
HCl
NaOH
H 2O 2
Na2S2O3
nitrilotriacetic
acid
Triton X-100
acetone
FeSO4·7H2O
alginic acid,
sodium salt
Fe(NO3)3·9H2O
Humic acid
Nitric Acid 6 N
Dodecyl sulfate,
sodium salt
(C12H25SO4Na)
Zinc reference
solution
Fisher Scientific
Fisher Scientific
Fisher Scientific
Fisher Scientific
Fisher Scientific
Fisher Scientific
Aldrich
S318-500
H325-500
S445-500
N840-7
Aldrich
Fisher Scientific
Aldrich
Aldrich
23,472-9
A929-1
31,007-7
18,094-7
Aldrich
Aldrich
Fisher Scientific
Aldrich
21,682-8
H1,675-2
LC17-70-2
85,192-2
Fisher Scientific
SZ13-100
M291-25
Table 5. Stock Solutions (100 mL each).
Description
C16H18N3SCl
Zn(NO3)2·6H2O
MW (g/M) conc. (g/L)
319.87
297.4
10
227.4048
100 mL
1g
22.74 g
5 g as Zn
Table 6. Contaminating Solution (1 L)
Description
MW
(g/M)
319.87
C16H18N3SCl
Zn(NO3)2·6H2O 297.48
conc.
(mg/L)
100
50 (as Zn)
1L
10 mL stock
10 mL stock
Soil Washing to Remove Mixed Wastes
86
Table 7. Equipment list
Description
Supplier
refrigerated
centrifuge MP4R
4-place rotor 4B
Diode array
spectrophotometer
rototorque rotator
10 mL centrifuge
vials
repipet II Dispensor
PP bottles 15 mL
Fisher Scientific
Catalog
number
05-006-4
Fisher Scientific
Hewlett-Packard
05-006-9
8452A
Cole Parmer
Fisher Scientific
H-07637-00
05-529-1A
Fisher Scientific
Fisher Scientific
13-687-62B
02-923-8G
Zinc Disposal Guidelines
The amount of zinc that can be disposed to the sanitary sewer is limited. The
wastewater treatment plant has a limit on the concentration of zinc that can be in the
sludge. The Zinc stock solution should not be disposed to the sanitary sewer. Zinc
that is sorbed to the sand can be dried and sent to the landfill in the trash.
Setup
1) Use repipet dispensors for contaminating solution, distilled water and possibly for
additives.
2) Prepare calibration standards for the AA and for the UV-Vis spectrophotometers.
3) Each group needs 9 centrifuge vials, 20 15-mL bottles, and 2 125-mL bottles.
4) Connect a very fine tube to an air line at each island to be used to help empty the
supernatant from the centrifuge vials.
5) Devise technique to filter samples prior to AA analysis!
Class Plan
1) Demonstrate use of AA when samples contain particulate matter. (Keep sipper
tube off of the bottom of the vial!)
2) EDTA works well for Zn at 0.1 M
3) Dodecyl sulfate works well for MB at 0.01 M
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
87
Oxygen Demand Concepts and Dissolved Oxygen Sag
in Streams
Introduction
In recent years “biodegradable” has become a popular word. Often it is assumed
that if something is biodegradable, then disposal is not a problem. We know that
throwing non-biodegradable substances into our environment leads to degradation of
our planet. But disposal of biodegradable compounds also can be detrimental to the
environment.
The effects of improper disposal of biodegradable substances became a source of
public outrage in the early 1800's. The flush toilet was becoming popular and sewage
was discharged directly into the nearest waterway. The receiving waters were quickly
polluted. Fish in the receiving waters died and the water had a very offensive odor.
Although there are many reasons why we no longer discharge untreated sewage into
the environment, (including disease transmission, sediment buildup...) one of the
reasons is directly related to the fact that sewage contains much that is biodegradable.
Theory
Biodegradable means that a substance can be converted into simpler compounds by
biologically mediated reactions. The second law of thermodynamics predicts that
oxidation of high energy level organics (relative to low energy level CO 2) is favored.
Oxygen is one of the strongest oxidizing agents found in natural aquatic systems.
Oxidation reactions are thermodynamically favored, but kinetically slow unless
microbially mediated. The end products of complete aerobic biodegradation are CO2
and H2O. Production of CO2 has recently come under fire as a potential cause of
global warming, but that is not the subject of this lab. The problem is not with the
products of biodegradation, the problem is that aerobic biodegradation of a compound
requires another reactant. Let's look at the biodegradation of a simple organic
compound, glucose.
C6 H12 O6 +? 6CO2 +6H 2 O
8.1
To balance the equation oxygen is needed.
C6 H12 O6 +6O2 6CO2 +6H2 O
8.2
The consumption of oxygen needed for biodegradation can be a problem. Oxygen is
not very soluble in water. The equilibrium concentration of oxygen in water is
approximately 10 mg/L (see Figure 1). That means that the degradation of a few
mg/L of a biodegradable compound in a river could result in the depletion of
dissolved oxygen. Fish have a bad day when oxygen is depleted from their
environment. Some species of fish such as trout begin to suffer when the dissolved
oxygen concentration drops below 5 mg/L.
Oxygen Demand Concepts and Dissolved Oxygen Sag in Streams
88
Oxygen in water is consumed during
aerobic biodegradation of organic
compounds and is replenished from the
atmosphere. The two processes have
different kinetics, but are coupled. As
the
oxygen
is
depleted
by
biodegradation the rate at which
oxygen is transferred into the water
increases because the transfer driving
force increases. The rate at which
oxygen is dissolved into water from the
atmosphere is proportional to the deficit
of oxygen in the water. The oxygen
deficit is simply the difference between
Figure 1. Equilibrium dissolved oxygen
the equilibrium oxygen concentration
concentration as a function of water
and the actual oxygen concentration.
temperature.
These two reactions (reaeration, and
biochemical utilization) are modeled by
the Streeter-Phelps equation. In order to
increase the rate at which the biodegradation occurs, the concentration of bacteria was
increased for use in this laboratory experiment. Bacteria respire and thus consume
oxygen even when no substrate is present. Thus an additional term for bacterial
respiration will be needed to model the oxygen sag results obtained in the laboratory.
Streeter Phelps Equation Development
We’ll begin by developing the oxygen deficit as a function of time in a completely
mixed batch reactor (no inflow and no outflow) with initial concentrations of
Biochemical Oxygen Demand (BODL) and dissolved oxygen. We will include
oxidation of BODL and reaeration from the atmosphere. These effects are coupled in
equation 8.3 where C represents oxygen concentration. The first two terms on the
right are negative since oxidation of BOD and respiration consume oxygen while the
third term is usually positive since reaeration increases the concentration of oxygen
(except in the rare instance where the dissolved oxygen concentration is greater than
the equilibrium dissolved oxygen concentration). Eventually we will make a
comparison between time in a reactor and distance down a river.
Crespiration
Coxidation
Creaeration
C
t
t
t
t
8.3
Oxidation of BOD
We must first develop a relationship for the change in oxygen concentration due to
oxidation of organics. The rate that oxygen is used will be proportional to the rate that
substrate (or biochemical oxygen demand) is oxidized. The rate of substrate
utilization by bacteria is given by the Monod relationship
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
89
dL
kLX
dt
Ks L
8.4
where L is substrate concentration expressed as oxygen demand or BOD L [mg/L], k is
the maximum specific substrate utilization rate, Ks is the half velocity constant, and X
is the concentration of bacteria. However, the concentration of bacteria is a function
of the substrate concentration and thus application of the Monod equation to a
polluted river is not trivial. Often the bacterial concentration remains relatively
constant. If the half velocity concentration is large relative to the concentration of
substrate we obtain
dL
kXL kX
8.5
L kox L
dt
Ks L Ks
where kox is a first order oxidation rate constant that includes both the approximation
that the bacteria concentration is roughly constant and that the substrate concentration
is smaller than the half velocity constant.
Separate variables and integrate
L
t
dL
L L = 0 (kox )dt
o
8.6
L Lo e koxt
8.7
to obtain
The rate of oxygen utilization is equal to the rate of substrate utilization (when
measured as oxygen demand) and thus we have
Coxidation dL
8.8
=
= -k ox L
t
dt
where C is the dissolved oxygen concentration [mg/L]. Now we can substitute for L
in equation 8.8 using equation 8.7 to obtain
Coxidation
= -k ox Lo e koxt
t
8.9
Respiration
Bacteria utilize oxygen for respiration and for cell synthesis. When no substrate is
present the bacteria cease synthesis, but must continue respiration. This continual use
of oxygen is termed "endogenous respiration." Bacteria use stored reserves for
endogenous respiration. We can model this oxygen demand as a constant that is
added to the demand for oxygen caused by substrate utilization. As a first
approximation, we can assume that this oxygen demand is proportional to the
concentration of bacteria. In addition, we will assume that the population of bacteria
is relatively constant throughout the experiment.
Oxygen Demand Concepts and Dissolved Oxygen Sag in Streams
90
Crespiration
8.10
=-bX =-k e
t
where b is the specific endogenous oxygen consumption rate and ke is the endogenous
oxygen consumption rate.
Oxygen Transfer Coefficient
The rate of oxygen transfer is directly proportional to the difference between the
actual dissolved oxygen concentration and the equilibrium dissolved oxygen
concentration.
Creaeration ˆ
8.11
= kv,l C* - C
t
where C* is the equilibrium oxygen concentration, C is the actual dissolved oxygen
concentration, and kˆv ,l is the is the overall volumetric oxygen transfer coefficient. If
reaeration is the only process affecting the oxygen concentration then equation 8.11
can be integrated to obtain
C* C
ln *
kˆv ,l (t t0 )
C C0
8.12
Oxygen Deficit
We now have equations for the reaction of oxygen with BODL, endogenous
respiration, and for reaeration. Substituting into equation 8.3 we get
C
= -k ox Lo e koxt k e + kˆv ,l C* - C
t
We can simplify the equation by defining oxygen deficit (D) as:
8.13
D=C* -C
8.14
and noting that the rate of change of the deficit must be equal and opposite to the rate
of change of oxygen concentration
dC
dD
=
8.15
dt
dt
We must remember that the deficit can never be greater than the equilibrium
concentration (D must always be less than C*)! In addition, the BOD model breaks
down if the dissolved oxygen concentration is less than about 2 mg/L because the
dL
lack of oxygen will limit microbial kinetics and
will no longer equal -kox L . If we
dt
stick to conditions under which our assumptions are valid then we can substitute
equations 8.14 and 8.15 into equation 8.3 to obtain
D
ke kox Lo e koxt - kˆv ,l D
t
CEE 453: Laboratory Research in Environmental Engineering
8.16
Spring 2001
91
This is a first order linear differential equation. Integration with initial oxygen deficit
= Do @ t = 0 gives:
D
ke
k kˆ t
k L
- kˆ t
Do e e v ,l ox o e koxt - e v ,l
kˆv ,l kˆv ,l
kˆv ,l kox
8.17
Application to a River
We are interested in the oxygen deficit as a function of distance down a stream. As
an approximation we can think of a cross section of a river as a completely stirred
reactor that is slowly moving downstream. The relation between time in a batch
reactor and distance down the river is simply
t=
x
u
8.18
where u is the stream velocity and x is distance. The Streeter-Phelps model assumes a
constant input of biodegradable substrate, Lo, at x = 0 and the model is valid under
steady-state conditions.
Of particular concern is the maximum deficit, Dc. We want to know the value of Dc
x
and where (or when) it will occur ( t c = c ). This will be the "critical point." If there
u
are going to be adverse effects (like dead fish) this will be the place.
The maximum oxygen deficit occurs when
D
=0
t
We can substitute this into the first order differential equation 8.6 to get
0 ke kox Lo e koxtc - kˆv ,l Dc
8.19
8.20
and solve for Dc to get
Dc
ke kox Lo e koxtc
kˆ
8.21
v ,l
an equation with unknowns tc and Dc. The Streeter-Phelps equation still holds at the
critical point so we also have
ke
k kˆ t
k L
- kˆ t
8.22
Do e e v ,l c ox o e koxtc - e v ,l c
kˆv ,l
kˆv ,l
kˆv ,l kox
also with unknowns xc and Dc. So now we have two equations in two unknowns. We
can solve for tc by eliminating Dc.
Dc =
Oxygen Demand Concepts and Dissolved Oxygen Sag in Streams
92
ke -kˆv ,l Do kˆv ,l -kox
kˆv ,l
8.23
tc =
ln
kox
Lo kox2
kˆv ,l -kox
To find Dc given the kinetic coefficients and the initial oxygen deficit, first find tc
using equation 8.23. Then use equation 8.21 to solve for Dc.
1
Zero Order Kinetics
An alternate model can be derived based on the assumptions that the concentration
of bacteria is relatively constant and that the rate of substrate utilization is zero order,
i.e., the concentration of the substrate is greater than the half velocity constant Ks.
dL kLX
=
-kX =-k0
dt K s L
8.24
The change of the deficit of dissolved oxygen is equal to the change caused by
microbial degradation (k0) plus change due to endogenous respiration minus the
reaeration ( kˆ D ).
v ,l
dD
= k0 + ke kˆv ,l D
8.25
dt
This equation is only valid when the substrate concentration is greater than Ks. To
simplify derivation, assume that Ks is very small relative to the initial BOD added to
the system and apply the zero-order model until the substrate is completely oxidized.
When the substrate concentration reaches zero a discontinuity will occur as substrate
oxidation stops. Separating variables and integrating
D
k
Do
t
dD
0
ke kˆv ,l D
= dt
8.26
0
1 k0 ke kˆv ,l D
ln
= t
kˆv ,l k0 ke kˆv ,l Do
and solving for the dissolved oxygen deficit
tkˆ
k0 ke (k0 ke kˆv ,l Do )e v ,l
D=
kˆ
8.27
8.28
v ,l
Lo
. Substituting into equation 8.28
k0
to get the maximum dissolved oxygen deficit yields
The substrate concentration is depleted when t =
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
93
Dt =
k0 ke (k0 ke kˆv ,l Do )e
kˆ
Lo kˆv , l
k0
8.29
v ,l
where Dt is the dissolved oxygen deficit at the transition when the substrate is all
L
utilized. For times greater than t = o there is no longer any substrate and thus k0 = 0
k
and equation 8.25 becomes
dD
= ke kˆv ,l D
dt
Separating variables and integrating
D
Dt
8.30
t
dD
= dt
ke kˆv ,l D tt
8.31
-1 ke kˆv ,l D
ln
= t - tt
kˆv ,l ke kˆv ,l Dt
D=
ke ke kˆv ,l Dt
e
8.32
kˆv , l tt t
8.33
kˆv ,l
Equation 8.33 is valid for all times greater than
Lo
. The general shapes of the two
k
types of sag curves are shown in
Figure 2.
Experimental Objectives
0.0
1.0
The objectives of this lab are to:
firs t order
DO 2.0
sag
1) Illustrate the effects of adding (mg/L) 3.0
4.0
biodegradable compounds to
zero order
5.0
natural waters.
6.0
2) Evaluate the Streeter-Phelps
0.0
200 .0
400 .0
600 .0
800 .0
dissolved oxygen sag model
Time (s)
and a zero order substrate
utilization model and compare Figure 2. Dissolved oxygen sag curves obtained
from zero and first order models for substrate
with laboratory data.
utilization.
3) Explain the theory and use of
dissolved oxygen probes.
Oxygen Demand Concepts and Dissolved Oxygen Sag in Streams
94
Experimental Methods
In this lab we will examine the
effects of adding a small amount of a
biodegradable compound to a small
batch reactor. We will measure the
dissolved oxygen concentration over
Hypodermic diffuser
time using a dissolved oxygen probe.
DO probe
The apparatus is shown in Figure 3.
The BOD measured using this
100 mL beaker
technique will be lower than the BOD
Water surface
measured using the standard BOD test
because a significant fraction of the
Stirbar
glucose will be converted into cell
material (i.e. used for synthesis
instead of for respiration). This
technique can be used to obtain
kinetic parameters for yield, half
velocity constant and maximum Figure 3. Apparatus used to measure
substrate utilization rate (Ellis et al., dissolved oxygen consumption rates.
1996).
Probe Calibration
Calibrate
the
dissolved
oxygen
http://www.cee.cornell.edu/mws/Software/DOcal.htm).
probe
(see
Oxygen Transfer Coefficient
1)
2)
3)
4)
5)
6)
7)
Prepare to monitor dissolved oxygen.
Place the dissolved oxygen probe in the reactor.
Pour 50 mL of deoxygenated distilled water into the batch reactor.
Set the stirrer speed to 5.
Set the airflow rate to 50 mL/min.
Monitor the dissolved oxygen for 3 minutes (or longer).
Save the data as \\Enviro\enviro\Courses\453\oxygen\netid_O2trans. The data will
be used later to estimate the oxygen transfer coefficient.
Endogenous respiration oxygen requirements
1)
2)
3)
4)
5)
Pour 50 mL of a bacterial suspension into the batch reactor.
Place the dissolved oxygen probe in the reactor.
Set the stirrer speed to 5.
Set the airflow rate to 250 mL/min to aerate the reactor contents.
Prepare to monitor dissolved oxygen.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
95
6) After the dissolved oxygen concentration is close to saturation turn off the air and
monitor the dissolved oxygen for 3 minutes (or longer).
7) Save the data as \\Enviro\enviro\Courses\453\oxygen\netid_endog. The data will
be used later to estimate the endogenous respiration rate.
BOD of glucose solution
1) Set the airflow rate to 250 mL/min and aerate the bacterial suspension used
previously.
2) Prepare pipette to add 75 µL of glucose solution (this will provide a BOD of 15
mg/L when diluted by 50 mL bacterial suspension).
3) Prepare to monitor dissolved oxygen.
4) Turn off the airflow.
5) As quickly as possible, add glucose through the port in the bottle and begin
monitoring the dissolved oxygen concentration.
6) Monitor the dissolved oxygen until the dissolved oxygen concentration reaches
approximately 0 mg/L.
7) Save the data as \\Enviro\enviro\Courses\453\oxygen\netid_BOD. The data will
be used later to estimate the BOD of the glucose solution.
DO Sag Curves
1) Set the airflow rate to 250 mL/min and aerate the bacterial suspension used
previously.
2) Prepare to monitor dissolved oxygen.
3) Reduce the airflow rate to 50 mL/min.
4) Begin monitoring the dissolved oxygen in the reactor. Use 5 second data intervals.
5) After ≈300 seconds of monitoring add 10 mg glucose BOD/L (50 L stock) to the
reactor.
6) Observe the oxygen depletion in the reactor.
7) Continue monitoring until the dissolved oxygen concentration returns to within
90% of the original DO concentration.
8) Save the data as \\Enviro\enviro\Courses\453\oxygen\netid_sag.
Prelab Questions
1) A dissolved oxygen probe was placed in a small vial in such a way that the vial
was sealed. The water in the vial was sterile. Over a period of several hours the
dissolved oxygen concentration gradually decreased to zero. Why? (You need to
know how dissolved oxygen probes work to answer this!)
2) Which assumption is different between the Streeter-Phelps and the zero order
model?
Oxygen Demand Concepts and Dissolved Oxygen Sag in Streams
96
Data Analysis
The rate constants can be estimated using Excel. A sample spreadsheet is available
at the course web site.
Oxygen Transfer Coefficient
1) Estimate the gas transfer coefficient from equation 8.12 or by using the
spreadsheet model.
2) Graph the dissolved oxygen concentration vs. time along with the theoretical
curve.
Endogenous Decay
1) Estimate the endogenous oxygen consumption rate from the slope of the graph or
by using the spreadsheet model.
2) Graph the dissolved oxygen concentration for the bacteria culture in the BOD
bottle without any added BOD vs. time along with the theoretical curve.
BOD of Glucose
1) Use the "DO sag" Excel spreadsheet to estimate the first or zero order oxygen
utilization coefficients, and the BOD exerted by the glucose. Which model fits the
data best?
2) How long did it take for the biodegradation of the glucose to occur?
3) What was the change in dissolved oxygen concentration during that time?
4) How much BOD did the glucose solution exert expressed as a fraction of the
BOD of the glucose added.
5) Graph the dissolved oxygen concentration vs. time for the glucose solutions along
with the theoretical curves. Identify the regions where biodegradation of the
glucose was occurring.
Dissolved Oxygen Sag
1) Use the previous estimates of the oxygen transfer coefficient, endogenous
respiration rate, and fraction of BOD exerted (note that a different amount of
BOD was added for the sag curve than for the BOD measurement!) to plot zero
and first order model predictions of the dissolved oxygen sag curve. Discuss any
discrepancies.
2) Estimate the first and zero order oxygen utilization coefficients and the BOD
exerted using the spreadsheet models by minimizing the RMSE using both
models with your data. Use the endogenous respiration rate and the reaeration rate
estimated previously. Which model (zero or first order) fits the data best? Are the
fit parameters significantly different than those obtained in the BOD of glucose
analysis? Include the estimated parameters in your report.
3) Graph the dissolved oxygen concentration vs. time for the dissolved oxygen sag
curve along with the theoretical curves.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
97
4) On the graph indicate maximum dissolved oxygen sag and compare with the BOD
added.
5) Why is the dissolved oxygen sag less than the BOD added?
References
Ellis, T. G.; D. S. Barbeau; B. F. Smets and C. P. L. J. Grady. 1996. “Respirometric
technique for determination of extant kinetic parameters describing
biodegradation” Water Environment Research 68(5): 917-926.
Oxygen Demand Concepts and Dissolved Oxygen Sag in Streams
98
Lab Prep Notes
Bacterial stock preparation using
20% PTYG
Grow 4 liter culture of Ps. putida
1) Heat 1 L of distilled water and
dissolve media for 4 L of 20%
PTYG.
2) Dilute to 4 L in 6 L container
containing aeration stone and
stirrer.
3) Thaw one cryovial containing Ps.
putida and transfer into PTYG
media.
4) Stir and aerate for 24 hours.
Wash/enumerate Ps. putida culture
1) Centrifuge 4 L culture in 250 mL
bottles to obtain concentrated
stock (5000 rpm for 10 minutes).
2) Resuspend total culture in 500
mL using 10x BOD dilution
water (pH control is essential for
bacterial growth and trace
nutrients are required).
3) Refrigerate at 4C.
Table 3. 20%
PTYG
culture
media.
(Prepare 4 L)
compound
peptone
tryptone
yeast extract
glucose
MgSO4
CaCl2·2H2O
Setup
mg/L
1000
1000
2000
1000
470
70
g/4L
4
4
8
4
1.9
0.28
Table 1. Reagent list
Description
Supplier
peptone
tryptone
glucose
yeast extract
MgSO4·7H2O
CaCl2·2H2O
KH2PO4
K2HPO4
Na2HPO4 ·
7H2O
NH4Cl
FeCl3 · 6H2O
Fisher Scientific
Fisher Scientific
Aldrich
Fisher Scientific
Fisher Scientific
Fisher Scientific
Fisher Scientific
Fisher Scientific
Fisher Scientific
Catalog
number
BP1420-100
BP1421-100
15,896-8
BP1422-100
Fisher Scientific
Fisher Scientific
Table 2. Equipment list
Description
Supplier
magnetic stirrer
Accumet™ 50
pH meter
ATI Orion DO
probe
6 L container
250 mL PP
bottle
15 mL PP
bottles
variable flow
digital drive
Easy-Load
pump head
PharMed tubing
size 18
4 prong
hypodermic
tubing diffuser
1/4” plug
1/4” union
stainless steel
hypodermic
tubing
gas diffusing
stone
Fisher Scientific
Fisher Scientific
Catalog
number
11-500-7S
13-635-50
Fisher Scientific
13-299-85
Fisher Scientific
Fisher Scientific
03-484-22
02-925D
Fisher Scientific
02-923-8G
Cole Parmer
H-07523-30
Cole Parmer
H-07518-00
Cole Parmer
H-06485-18
CEE 453: Laboratory Research in Environmental Engineering
CEE shop
Cole Parmer
Cole Parmer
McMaster Carr
H-06372-50
H-06372-50
Fisher Scientific
11-139B
Spring 2001
99
1) Prepare the Ps. putida culture
starting 48 hours before lab.
2) Prepare 100 mL glucose stock Table 4. BOD dilution water stock solutions.
Use 10 mL per liter of each of the 4
solution.
solutions to prepare 10x BOD dilution
3) Attach one Easy-Load pump
water.
head to the pump drives and
plumb with size 18 tubing
µM
connected to the hypodermic phosphate buffer M.W. g/L mg/100
mL
diffuser.
136.09 8.5
850
62.46
KH2PO4
174.18 21.7
2175
124.87
4) Verify that DO probes are
K2HPO4
5
operational, stable, and can be
268.07
33.4
3340
124.60
Na
HPO
·
7H
O
2
4
2
calibrated.
53.49
1.7
170
31.78
NH4Cl
5) Mount DO probes on magnetic
stirrers. (Use large stirbars.)
Magnesium
sulfate
6) Use 100 mL plastic beakers
120.39 11
1100
91.37
MgSO4
containing 50 mL of bacteria
suspension. The open tops will
Calcium chloride
result in negligible oxygen
110.99 27.5
2750
247.77
CaCl2
transfer during the course of
the experiments.
Ferric chloride
270.3 0.25
25
0.925
FeCl3 · 6H2O
7) Prepare 1 L of deoxygenated
distilled water right before class using the techniques outlined in the gas transfer
lab (see page 154).
Glucose Stock Solution
C6H12O6 + 6O2 6CO2 + 6H2O
10gO2
L
10 gO2 moleO2 1moleC6 H12 O6 180 gC6 H12 O6
0.1 L = 0.9375 g C6 H12 O6 in 100 mL
L
32 gO2
6moleO2
moleC6 H12 O6
Glucose Dilutions
10 mg BOD
L
100 mL 100 L
L
10000 mg O2
100 µL in 100 mL will provide 10 mg/L BOD
10 µL of stock solution diluted into 100 mL provides 1 mg BOD/L.
Oxygen Demand Concepts and Dissolved Oxygen Sag in Streams
100
Methane Production from Municipal Solid Waste
Introduction
Archaeological investigations of landfills have revealed that biodegradable wastes
can be found — virtually intact — 25 years after burial. We know that landfills
contain bacteria with the metabolic capability to degrade many of the materials that
are common components of municipal refuse. The persistence for decades of
degradable materials in the presence of such organisms appears somewhat
paradoxical. In this experiment students will explore the factors that influence
biodegradation of waste materials in landfills. Although recycling has significantly
reduced the amount of landfill space dedicated to paper and other lignocellulosics,
paper products are still a significant fraction of the solid waste stream. In this
laboratory students will measure the rate and extent of anaerobic degradation of
newsprint, Kraft paper, coated paper, and food scraps.
Theory
Over 150 million tons of municipal solid waste (MSW) are generated every year in
the United States, and more than 70% of the MSW is deposited in landfills (Gurijala
and Suflita 1993). Paper constitutes the major weight fraction of MSW, and this
laboratory will focus on the biodegradation of that component. Anaerobic
biodegradation of paper produces methane and carbon dioxide. Methane is a fuel and
is the major component of natural gas. Methane produced in sanitary landfills
represents a usable but underutilized source of energy. Energy recovery projects are
frequently rejected because the onset
of
methane
production
is Table 1. Typical physical composition of
unpredictable and methane yields
residential MSW in 1990 excluding
vary from 1-30% of potential yields
recycled materials and food wastes
based on refuse biodegradability data
discharged
with
wastewater
(Barlaz, Ham et al. 1992). The low
(Tchobanoglous, Theisen et al.
methane yields are the result of
1993)
several factors that conspire to inhibit
Component
Range
Typical
anaerobic biodegradation including
(% by weight)
(% by weight)
Organic
low moisture levels, resistance to
food wastes
6-18
9.0
biodegradation, conditions that favor
paper
25-40
34.0
bacterial degradation pathways that
cardboard
3-10
6.0
plastics
4-10
7.0
do not result in methane as an end
textiles
0-4
2.0
product, and poor contact between
rubber
0-2
0.5
bacteria and the organic matter.
leather
0-2
0.5
Characteristics of municipal solid
waste
The physical composition of
residential municipal solid waste
(MSW) in the United States is given
in
Table
1.
The
fractional
yard wastes
wood
Inorganic
glass
tin cans
aluminum
other metal
dirt, ash, etc.
CEE 453: Laboratory Research in Environmental Engineering
5-20
1-4
Organic total
18.5
2.0
79.5
4-12
2-8
0-1
1-4
0-6
Inorganic total
8.0
6.0
0.5
3.0
3.0
20.5
Spring 2001
101
contribution of the listed categories has evolved over time, with a trend toward a
decrease in food wastes because of
increased use of kitchen food waste
Table 2. Percentage distribution by weight of
grinders, an increase in plastics
paper
types
in
MSW
through the growth of their use for
(Tchobanoglous, Theisen et al.
packaging, and an increase in yard
1993)
wastes as burning has ceased to be
Range
Typical
allowed by most communities Type of paper
newspaper
10-20
17.7
(Tchobanoglous, Theisen et al. 1993).
books and
5-10
8.7
Excluding plastic, rubber, and
magazines
leather, the organic components listed
commercial
4-8
6.4
printing
in Table 1 are, given sufficient time,
office paper
8-12
10.1
biodegradable.
Although recycling efforts divert a
significant fraction of paper away
from landfills, paper continues to be a
major component of landfilled waste.
The types of paper found in MSW are
listed in Table 2.
other
paperboard
paper packaging
other
nonpackaging
paper
tissue paper and
towels
corrugated
materials
The elemental composition of
newsprint and office paper are listed
in Table 3.
The major elements in paper are
carbon, hydrogen, and oxygen that
together constitute 93.5% of the total
solids. The approximate molecular
ratios for newspaper and office paper
are
C6H9O4
and
C6H9.5O4.5
respectively.
Biodegradation of cellulose,
hemicellulose, and lignin
Cellulose and hemicellulose are the
principal biodegradable constituents
of refuse accounting for 91% of the
total methane potential. Cellulose
forms the structural fiber of many
plants. Mammals, including humans,
lack the enzymes to degrade
cellulose. However, bacteria that can
break cellulose down into its subunits
are widely distributed in natural
systems, and ruminants, such as
8-12
10.1
6-10
8-12
7.8
10.6
4-8
5.9
20-25
22.7
Total
100.0
Table 3. Elemental composition of two paper
types on a dry weight basis
(Tchobanoglous, Theisen et al.
1993).
Constituent
C
H
O
NH4-N
NO3-N
P
PO4-P
K
SO4-S
Ca
Mg
Na
B
Zn
Mn
Fe
Cu
Newsprint
49.1%
6.1%
43.0%
4 ppm
4 ppm
44 ppm
20 ppm
0.35%
159 ppm
0.01%
0.02%
0.74%
14 ppm
22 ppm
49 ppm
57 ppm
12 ppm
Methane Production from Municipal Solid Waste
Office Paper
43.4%
5.8%
44.3%
61 ppm
218 ppm
295 ppm
164 ppm
0.29%
324 ppm
0.10%
0.04%
1.05%
28 ppm
177 ppm
15 ppm
396 ppm
14 ppm
102
cows, have these microorganisms in their digestive tract. Cellulose is a
polysaccharide that is composed of
glucose subunits (see Figure 1).
Another component of the walls of
plants is hemicellulose, which sounds
similar to cellulose but is unrelated
other that that it is another type of
polysaccharide. Hemicelluloses made
up of five carbon sugars (primarily Figure 1. Cellulose (two glucose subunits are
xylose) are the most abundant in shown).
nature.
Lignin is an important structural
component in plant materials and constitutes roughly 30% of wood. Significant
components of lignin include coniferyl alcohol and syringyl alcohol subunits (Figure
2).
The exact chemical structure of
lignin is not known but its CH O
CH O
3
3
reactivity, breakdown products, and
-C-C-CHO-C-C-Cthe results of spectroscopic studies HOreveal it to be a polymeric material
CH O
3
containing aromatic rings with
methoxy
groups
(-OCH3) Figure 2. Coniferyl (left) and syringyl (right)
(Tchobanoglous, Theisen et al. subunits of lignin.
1993). One of the many proposed
structures for lignin is shown in
Figure 3.
Degradation of lignin requires the
presence of moisture and oxygen
and is carried out by filamentous
fungi (Prescot, Harley et al. 1993).
The
biodegradability
of
lignocellulosic materials can be
increased by an
array of
physical/chemical
processes
including pretreatment to increase
surface area (size reduction), heat
treatment, and treatment with acids
or bases. Such treatments are useful
when wood and plant materials are
to be anaerobically degraded to Figure 3. A postulated formulation for spruce
produce methane. Research on this lignin (by (Brauns 1962), as cited by (Pearl
topic has been performed by 1967)). This structure is suggested by
Cornell Prof. James Gossett spectroscopic studies and the chemical
(Gossett and McCarty 1976; reactions of lignin.
Chandler, Jewell et al. 1980;
Gossett, Stuckey et al. 1982;
H
H
H
C
C H
O H
H
CH O
3
C
C
H
O
C
OH
C
C
C
OCH
3
-OH
H
H
C
H C
HO
H
C
H
H
OCH
3
-OH
H
H
O
C H
HO
H
-OH
H
C
O
O
H
C
OCH
3
C
O
H
C
H
O
H
H
C
H
OH
C
H
C
O
H
H
HO
C C
H
OCH
C
3
-OH
OCH
3
O
H
C
OCH
3
H
O
C
H
O
O
H
OCH
H
3
C
C
O
C
H
OCH
3
O
H
CH O
3
H
C
O
H
H
H
C
C
O
H
O
H
H
H
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
103
Pavlostathis and Gossett 1985a; Pavlostathis and Gossett 1985b).
Three major groups of bacteria are involved in the conversion of cellulosic material
to methane (Zehnder 1978): (1) the hydrolytic and fermentative bacteria that break
down biological polymers such as cellulose and hemicellulose to sugars that are then
fermented to carboxylic acids, alcohols, carbon dioxide and hydrogen gas, (2) the
obligate hydrogen reducing acetogenic bacteria that convert carboxylic acids and
alcohols to acetate and hydrogen, and (3) the methanogenic bacteria that convert
primarily acetate and hydrogen plus carbon dioxide to methane. Sulfate reducing
bacteria (SRB) may also play a role in the anaerobic mineralization of cellulosic
material. In the presence of sulfate, the degradation process may be directed towards
sulfate reduction by SRB with the production of hydrogen sulfide and carbon dioxide
(Barlaz, Ham et al. 1992).
Cellular requirements for growth
The availability of oxygen is a prime determinant in the type of microbial
metabolism that will occur. Microbial respiration of organic carbon is a combustion
process, in which the carbon is oxidized (i.e., is the electron donor) in tandem with
the reduction of an electron acceptor. The energy available to microorganisms is
greatest when oxygen is used as the electron acceptor and therefore aerobic metabolic
processes will dominate when oxygen is available. Some microorganisms require
oxygen to obtain their energy and are termed “obligate aerobes.” In the absence of
oxygen, other electron acceptors such as nitrate (NO3-), sulfate (SO4-2) and carbon
dioxide (CO2) can by used. Organisms that can only exist in an environment that
contains no oxygen are termed “obligate anaerobes.” Organisms that have the ability
to grow in both the presence and the absence of oxygen are said to be “facultative.”
The availability of nutrients can limit the ability of cells to grow and consequently
the extent of biodegradation. Nitrogen and/or phosphorous constitute important
nutrients required for cell synthesis. Inorganic bacterial nutritional requirements also
include sulfur, potassium, magnesium, calcium, iron, sodium and chloride. In
addition, inorganic nutrients needed in small amounts (minor or trace nutrients)
include zinc, manganese, molybdenum, selenium, cobalt, copper, nickel, vanadium
and tungsten. Organic nutrients (termed “growth factors”) are also sometimes needed
(depending on the microorganism) and include certain amino acids, and vitamins
(Metcalf & Eddy 1991).
Environmental conditions such as pH, temperature, moisture content, and salt
concentration can have a great influence on the ability of bacteria to grow and
survive. Most bacteria grow in the pH range from 4.0 to 9.5 (although some
organisms can tolerate more extreme pH values), and typically grow best in the
relatively narrow range from 6.5 to 7.5 (Metcalf & Eddy, 1991). Microorganisms
have a temperature range over which they function best, and are loosely characterized
as phychrophilic (ability to grow at 0°C), mesophilic (optimal growth at 25-40°C) or
thermophilic (optimal growth above 45-50°C) (Brock 1970). Many common
methanogens are mesophilic. Elevated temperatures also favor faster reaction rates.
While some microorganisms are very tolerant of low moisture conditions, active
microbial growth and degradation of organic matter necessitates that water not be a
scarce resource. Cells take water in through their semi-permeable membrane surface
Methane Production from Municipal Solid Waste
104
by osmosis. This uptake mechanism requires that the solute concentration inside the
cell be higher than that of the outside media. Organisms that grow in dilute solutions
can not tolerate high salt concentrations because their normal osmotic gradient is
reversed and they can not take in water. Some cell strains, termed “halophiles” are
adapted for growth at very high salt concentrations.
The above factors suggest that bacterial degradation of MSW to produce methane
will occur optimally at circumneutral pH, low ionic strength, in the absence of
oxygen, nitrate and sulfate, in the presence of moisture and nutrients, and under
mesophilic conditions.
Estimates of paper biodegradability
Volatile solids (VS) content (determined by weight loss on ignition at 550°C) has
been used to estimate the biodegradability of MSW components, but this measure
overestimates the biodegradability of paper. Paper products have a very high volatile
solids content. Newsprint, office paper, and cardboard have VS of 94%, 96.4%, and
94% respectively (Tchobanoglous, Theisen et al. 1993). Paper products also can have
a high content of lignocellulosic components that are only slowly degradable. Lignin
constitutes approximately 21.9%, 0.4% and 12.9% respectively of the VS in
newsprint, office paper, and cardboard. Lignin content and biodegradability are
strongly correlated and thus lignin content can be used to estimate biodegradability
and potential methane production. Chandler et al. (1980) found a relationship
between lignin content and biodegradable volatile solids using a wide variety of waste
materials. The empirical relationship suggests that not only is lignin not easily
biodegraded, but that lignin also reduces the biodegradability of the nonlignin
components. This reduction in biodegradability may be caused by lignin polymeric
material physically preventing enzymatic access to the nonlignin components. The
relationship is
VSbiodegradable 2.8VSlignin 0.83
9.1
where VSbiodegradable is the biodegradable fraction of the volatile solids and VSlignin is
the fraction of volatile solids that are lignin. From equation 9.1 the maximum
destruction of VS is limited to
about 83%, a limitation due to the
of
selected
production
of
bacterial Table 4. Biodegradability
components of MSW (Tchobanoglous,
by-products.
The
high
Theisen et al. 1993)
concentration
of
lignin
in
VS/TS Lignin/VS VSbiodegradable*
newsprint makes it much less
%
%
%
biodegradable than more highly Type of waste
mixed food
7-15
0.4
82
processed office paper (Table 4).
newsprint
94
21.9
22
office paper
96.4
0.4
82
Energy recovery from MSW
cardboard
94.0
12.9
47
Energy could be recovered from
MSW by direct combustion in an * Obtained by using equation 9.1
incinerator or by anaerobic biodegradation and production of methane. Proximate
analysis is used to measure moisture content, volatile matter, fixed carbon
(combustible but not volatile), and ash. Proximate analysis can be used to predict ash
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
105
production from incineration. The energy content is measured in a bomb calorimeter.
Proximate analysis results and energy content of MSW are given in Table 5.
Gas
production Table 5. Proximate analysis and energy content of selected
from
anaerobic
components of MSW (Tchobanoglous, Theisen et al.
digestion is typically
1993).
30% CO2 and 70%
moisture volatile fixed
ash energy energy
CH4. The methane is
matter carbon
as
dry
a valuable fuel and
collected
%
%
%
%
(MJ/kg) (MJ/kg)
has
an
energy Type of waste
fats
2
95.3
2.5
0.2
37.5
38.3
content of 802.3
mixed food
70
21.4
3.6
5
4.2
13.9
kJ/mol or 50 MJ/kg.
fruit
waste
78.7
16.6
4
0.7
4.0
18.6
The combustion of
meat
waste
38.8
56.4
1.8
3.1
17.7
29.0
methane produces
cardboard
5.2
77.5
123
5
16.4
17.3
only carbon dioxide
magazines
4.1
66.4
7
22.5
12.2
12.7
and water.
newsprint
6
81.1
11.5
1.4
18.6
19.7
Because
paper
mixed paper
10.2
75.9
8.4
5.4
15.8
17.6
products are a major waxed cartons
3.4
90.9
4.5
1.2
26.3
27.3
fraction of MSW
and paper energy content is significant, the majority of energy in MSW is contained
in paper products. Incineration or methane production can be used to capture some of
this available energy.
CH4 +2O2 CO2 +2H2 O
9.2
Effect of MSW particle size
The large size of pieces of MSW is suspected to decrease the ability of microbes to
degrade the material. Landfill gas production has been correlated with refuse particle
size (Ferguson 1993). The effect of particle size reduction was initially explained by
the resultant increase in surface area available for microbial attach. Laboratory studies
under saturated conditions, however, suggest that size reduction, even down to a few
microns or tens of microns has little effect on the rate of degradation. According to
Ferguson (1993), surface area increases only slightly with decreasing particle size for
platey and fibrous particles such as paper. Thus the effect of size reduction on the
methane production in landfills may be that relatively large pieces of plastic, paper, or
other material shield the materials beneath them from infiltrating water. The shielded
material may remain too dry for biodegradation. Pulverization breaks down the
impermeable barriers and more of the waste is exposed to water (Ferguson 1993).
Potential methane production from municipal solid waste
Under anaerobic conditions microorganisms can produce both CO2 and CH4
(methane) without consuming any oxygen. Other significant end products include
odorous gases such as ammonia (NH3), and hydrogen sulfide (H2S) (see Figure 4).
Because anaerobic biodegradation produces gas it is possible to monitor the extent
and rate of anaerobic biodegradation by measuring gas production (Suflita and
Concannon 1995).
Methane Production from Municipal Solid Waste
106
Gas production
Because anaerobes get relatively
little energy from the organic
matter their conversion of carbon
to cell material (synthesis) is much
lower than for aerobes. Typically
10% of the organic matter may be
converted to anaerobe cell mass.
Thus the majority of the
biodegraded organic matter is
converted to gas and the gas
production can be used as a
measure of biodegradation. The
ideal gas law is used to determine
volume, and temperature.
Nutrients
NH3
H2O
H2S
Cells
CH4
CO2
New Cells
Heat
Organic Matter
Refractory organic matter
Figure 4. Reactants and products for anaerobic
degradation of organic matter.
the moles of gas produced from the pressure,
PV
9.3
RT
The pressure in the sealed test bottles that will be used in this laboratory is initially
atmospheric. Because the number of moles is a linear function of the pressure we can
write
n
PV
9.4
RT
where ∆P is the change in pressure relative to the initial pressure in the bottle.
In these experiments the bottle volume is 120 mL and the maximum recommended
pressure increase is 80 kPa (12 psi). The volume of liquid in the bottles is 20 mL and
the volume contributed by solids is expected to be negligible. Thus the nominal
volume of gas in the bottles will be 100 mL. Solving for the number of moles of gas
(CH4 and CO2) produced by anaerobic digestion
n
80 x10 Pa 100 x10
n
3
6
m3
Pa m3
8.31
308K
mol K
3.13 mmole C
9.5
The molecular formula of cellulose is C6H10O5 and thus 27 g of cellulose has 1
mole of carbon. The relation obtained in equation 9.5 is used to determine the
maximum amount of cellulose that can be anaerobicly degraded without exceeding 80
kPa in the bottles.
27 mg cellulose
84 mg cellulose
9.6
mmole C
The mass of paper containing 84 mg of biodegradable cellulose can be obtained
using Table 4 and the results of equation 9.6. The mass of dry newspaper that will
produce a pressure increase of 80 kPa is
3.13 mmole C
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
107
84 mg VSbiodegradable
80 kPa
VS
TS
400 mg dry newspaper
0.22 VSbiodegradable 0.94 VS
80 kPa
9.7
Similar calculations can be performed for other types of waste.
The maximum mass of glucose (CH2O has 30 g of glucose per mole of carbon) is
30 mg glucose
94 mg glucose
9.8
mmole C
Although glucose is expected to be completely biodegradable, a small amount of
glucose will be converted into refractory cell byproducts.
The above calculations are based on the assumption that all of the gas produced is
volatile and is not dissolved. Carbon dioxide is soluble and thus some of the CO2
produced will be dissolved and will not result in increased pressure.
3.13 mmole C
Acid neutralizing capacity requirements
The high partial pressure of CO2 resulting from anaerobic biodegradation causes a
high concentration of carbonic acid H 2 CO3* and thus would result in a reduced pH
if there were insufficient Acid Neutralizing Capacity (ANC). The amount of ANC
required to counteract the high partial pressure of CO2 can be obtained from the
Henry’s constant for dissolution of CO2, and from the dissociation constant for
carbonic acid.
H 2 CO3*
KH
PCO2
9.9
where KH has a value of 3.12 x 10-4 moles/J. The first dissociation constant for
carbonic acid is
H HCO3
K1
H 2 CO3*
9.10
where K1 has a value of 10-6.3. The definition of ANC for a carbonate system in
equilibrium with the gas phase is
ANC
PCO2 K H
0
1 2 2
Kw
H
H
9.11
Where 0, 1, 2 are the fractions of total carbonate present as carbonic acid
H 2 CO3* , bicarbonate HCO3 , and carbonate CO32 respectively and Kw is the
dissociation constant for water. At circumneutral pH the hydrogen ion, hydroxide ion,
and carbonate ion concentrations are negligible and equation 9.11 simplifies to
ANC
PCO2 K H 1
0
Methane Production from Municipal Solid Waste
9.12
108
The ratio of bicarbonate to carbonic acid may be determined from equation 9.10.
Solving for the ratio of bicarbonate to carbonic acid:
HCO3
K1
1
*
H 2 CO3 0 H
9.13
Equation 9.13 can be substituted into equation 9.12 to obtain
ANC
PCO2 K H K1
H
9.14
An estimate of the ANC required to maintain a neutral pH under a pressure of 30 kPa
of CO2 can be obtained by substituting appropriate values into equation 9.14.
moles 6.3
Pa 3.12 x104
10 M
N m
ANC
107 M
ANC 47meq / L
3x10
4
9.15
The basal medium that will be used in this laboratory contains 71 meq/L ANC
from sodium bicarbonate. If the pressure of CO2 reaches 60 kPa (30 kPa initial
pressure plus 30 kPa from the production of CO2 during an experiment) then solving
equation 9.14 for pH shows that (given the 71 meq ANC in the basal medium and a
CO2 pressure of 60 kPa) the pH will drop to 6.88. Thus, the basal medium is
sufficiently buffered to protect against significant pH changes.
Carbon dioxide solubility
At pH less than ≈9 the inorganic carbon will partition into three species, gaseous
CO2 , aqueous H 2 CO3* , and aqueous HCO3 .
nCO2( total ) nCO2( g ) nH CO* nHCO
2
3
9.16
3
The number of moles of the inorganic carbon species can be determined based on the
partial pressure of CO2 , the ANC of the liquid and the gas and liquid volumes. The
moles of gaseous CO2 is obtained from the ideal gas law
nCO2( g )
PCO2 Vg
RT
9.17
The number of moles of H 2 CO3* is obtained from the Henry’s law constant.
nH CO* PCO2 K H Vl
2
9.18
3
The concentration of bicarbonate, HCO3 , is equal to the ANC (for pH < 9).
nHCO ANC Vl
9.19
3
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
109
The total number of moles of inorganic carbon is the sum of the three species.
PCO2 Vg
9.20
PCO2 K H ANC Vl
RT
Therefore, the number of moles of inorganic carbon in an enclosed volume is a linear
function of the partial pressure of CO2 (Figure 5).
The pH will change as the partial pressure of CO2 changes as shown in equation
9.14. Solving for the concentration of hydrogen ions equation 9.14 becomes
nCO2( total )
H
PCO2 K H K1
ANC
9.21
inorganic carbon (mmoles)
pH
The relationship between pH and
partial pressure of CO2 is shown
in Figure 5.
7.5
9.0
The basal medium to be used
pH
7.4
8.5
in this experiment will be purged
7.3
8.0
mmoles
with a 30:70 mixture of carbon
inorganic carbon
7.2
7.5
dioxide and nitrogen prior to use.
7.1
7.0
As shown in Figure 5 the pH of
7.0
6.5
the basal medium is expected to
6.9
6.0
6.8
5.5
rise to approximately 7.17. The
6.7
5.0
headspace will also be purged
6.6
4.5
with the same gas mixture and
6.5
4.0
thus there will be 5.6 mmoles of
0
20
40
60
80
100
inorganic carbon in the bottles
Partial pressure of CO2 (kPa)
initially. After the anaerobic
biodegradation has gone to Figure 5. Number of moles of inorganic carbon
completion, the carbon dioxide and pH as a function of the partial pressure of
concentration will be measured carbon dioxide given gas and liquid volumes of
by gas chromatography and the 60 mL and ANC of 71 mmoles/L.
gas pressure by pressure sensors
and thus the partial pressure of
carbon dioxide will be known.
Figure 5 or equation 9.20 can be used to determine the final mass of inorganic carbon
in the bottles. The difference between the initial and final inorganic carbon
concentration can be used to determine the amount of organic carbon converted to
carbon dioxide.
Methane solubility
The Henry’s constant for methane ( K H CH 4 ) at 25°C is 1.48 x 10-5 mol/J (Mackay
and Shiu 1981). Methane is significantly less soluble than carbon dioxide and does
not form other soluble aqueous species. The mass of gaseous and dissolved methane
is given by equations 9.22 and 9.23.
Methane Production from Municipal Solid Waste
110
nCH 4 ( g )
PCH 4 Vg
RT
nCH4 ( aq ) PCH4 K H CH4 Vl
9.22
9.23
The ratio of the mass of gaseous to dissolved methane gives an indication of the
significance of dissolved methane.
nCH 4 ( g )
nCH 4 ( aq )
Vg
Vl K H CH 4 RT
9.24
Substituting appropriate values into equation 9.24
nCH 4 ( g )
nCH 4 ( aq )
Vg
mol
J
Vl 1.48 x105
8.31
308 K
J
mol K
9.25
If the gas and liquid volumes are approximately equal there will be approximately 26
times as much methane in the gaseous phase as in the dissolved phase. This ratio is
independent of the methane partial pressure. The total number of moles of methane
can be obtained from the partial pressure of methane in the gaseous phase.
Vg
9.26
nCH4 (total ) PCH 4
K H CH4 Vl
RT
The partial pressure of methane will be determined from the pressure in the bottle and
mass of methane as measured by the gas chromatograph.
Temperature effects
The temperature of the bottles directly affects the pressure of gas as well as
influences the rate of gas production by the microbes. Cummings and Stewart found
that methane production was sharply inhibited by temperatures in excess of the
optimum (37°C) and was undetectable at 20°C (1995). However, Suflita and
Concannon (1995) reported anaerobic digestion at “room temperature” over a period
of 2 months. If desired a constant temperature water bath can be used to keep all of
the digesters at a constant and optimal temperature (35°C) for anaerobic degradation.
Experiment description
The experimental setup is a flexible system for obtaining data on the anaerobic
decomposition of various organic materials by measuring the pressure of the gas
produced. A schematic of the experimental setup is shown in Figure 6.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
111
Bacterial
degradation
of
35º C
RJ 11 plug
selected materials will be
incubator
Connector panel
assayed by placing known Serum
Pressure
sensor
quantities in 120 mL bottles, bottle
Multiplexer
inoculating with an active
Crimp cap
analog
anaerobic mixed culture, sealing
with septa
to
Power Supply (12 V)
digital
the bottles with rubber septa and
Hypodermic
needle
aluminum crimp caps, and
monitoring gas pressure and
Anaerobic
solution
composition
over
time.
Anaerobic digester supernatant
from the Ithaca Wastewater Figure 6. Experimental setup (not to scale).
Treatment Plant will be used as Pressure generated by microbial gas production is
a source of microbes. The monitored by pressure sensors.
bottles will be monitored for
biogas production with pressure
sensors connected with a needle
through the septa (see Figure 6). Gas composition will be determined by periodic
analysis of methane (CH4) and carbon dioxide (CO2) via gas chromatography with a
thermal-conductivity detector.
Gas production measurements will be automated by using pressure sensors in a
procedure comparable to that described by Suflita and Concannon (1995). With this
technique, a large number of bottles can be monitored with automated data
acquisition by a single computer, allowing a wide variety of chemical and
environmental parameters to be explored. The automated acquisition of gas data is
necessary due to the numbers of bottles and length of incubation (ca. 4 weeks)
anticipated. This experiment will be set-up and left virtually unattended while other
laboratory exercises continue in intervening weeks.
Each sample type should be cut
into small enough pieces to easily Table 6. Suggested sample preparation. Each
insert into the bottle. Students may
group does 2 sample types with 2
also be interested in exploring
replicates. Each section does 2 water
biodegradation of other organic
and 2 inoculum controls.
components of municipal solid
Sample Inoculum Replicates
waste (banana peels, rags, plastic
Size
bags, etc.). Table 6 suggests one
Sample Type
mg
mL
#
configuration of several sample
environmental
none
0
2
types. Each sample will receive 15
control
bacterial control
none
5
2
mL of basal medium.
positive
control
90
5
2
One
control
should
be
(glucose)
“unamended” (i.e., with no waste)
filter paper
50
5
2
and contain the microbial inoculum
cardboard
?
5
2
and the O2-free water to monitor any
office paper
?
5
2
newsprint
?
5
2
gas production attributable just to
Student selected
?
5
up to 4
the added sludge, one “positive” organic materials
vials
Methane Production from Municipal Solid Waste
112
control should be the microbial inoculum, plus 90 milligrams of glucose and the
O2-free water (to verify that the microbial population is active in the added sludge),
and one control should be plain O2-free water to control for variations in temperature,
and air pressure. The sample sizes of the various samples should be determined so
that the bottles will not generate pressure greater than 80 kPa.
Experimental methods
Safety concerns
1) Municipal wastewater sludge will be used as a source of microbes. The sludge
may contain biological and/or chemical hazards and should be handled
accordingly.
2) Biological production of gas will generate pressure in a closed container. Testing
has shown that this system is safe up to at least 200 kPa (30 psi). At
approximately this pressure the needle is typically forced out of the septa. If the
bottle is not vented the pressure can increase until the crimp cap is forced off.
Bottles should not be capped for very long before the needles are inserted and
pressure monitoring begins. The pressure trends should be monitored and, if
excessive pressures are produced, the bottles must be vented and/or the
temperature of the bath may be reduced.
3) Sharp needles are used in the experimental setup and precautions should be taken
to avoid puncturing unintended objects (including students).
Analysis of moisture content and volatile solids
The fraction of volatile solids in the paper samples is the maximum that could
possibly be degraded. Note that paper products cannot be ashed accurately because
the strong flames easily carry some of the ashes away. Steps for sample moisture
content and volatile solids fraction follow:
1) Weigh an aluminum boat
2) Weigh the aluminum boat with an organic sample (boat + water + VS + ash)
3) Dry in the 105°C oven
4) Cool in a desiccator
5) Weigh (boat + VS + ash)
6) Ash in the 550°C muffle furnace
7) Cool in a desiccator
8) Weigh (boat + ash)
Sample preparation
The bottles will be purged initially with a mixture of 30% CO2 and 70% N2 to
remove any O2 and to establish an initial carbon dioxide concentration so that the
initial pH is not excessively high. If no carbon dioxide were present in the purging
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
113
gases the carbonic acid would be stripped out of solution and the pH would rise in the
basal medium.
Acquisition of pressure data
The biogas pressure will be measured indirectly by the pressure sensors. The
specified sensors work from zero to 100 kPa (6.89476 kPa/psi). They will withstand
2.5 times rated pressure (i.e., 250 kPa) but their output may be erroneous above the
upper limit of the working range. The output of the pressure sensors is zero to 0.100
volts with 0.100 volts indicating approximately 100 kPa. The outputs of the sensors
are fed through a 32-channel multiplexer/signal conditioner, an A/D converter board
and are monitored using LabVIEW software.
Gas chromatograph analysis for separation of CO2, CH4 and N2 (Optional)
Permanent gases can be analyzed using a thermal conductivity detector (TCD) on a
gas chromatograph. The thermal conductivity detector measures the rate at which heat
is transported from the detector. If a gas with a different thermal conductivity than the
carrier gas passes through the detector a peak is detected. A flame ionization detector
could be used to measure methane, but would not be able to detect carbon dioxide or
nitrogen since they do not burn. A micropacked column containing packing designed
for analyses of permanent gases and light hydrocarbons (Supelco Carboxen 1004) is
used to separate the gases.
The TCD must be calibrated with known masses of the gases of interest. Nitrogen,
carbon dioxide and methane are available as compressed gases and can be sampled at
atmospheric pressure by opening a valve in a compressed gas line slightly and
sampling the discharge with a gas tight syringe. The ideal gas law is used to calculate
the moles of gas. The current atmospheric pressure in Ithaca is available through the
World Wide Web at http://cuinfo.cornell.edu/Ithaca/Weather/. If the atmospheric
pressure is reported in inches of mercury it can be converted to Pascals by
multiplying by 3386 Pascals/Inch of Hg. The temperature of the laboratory is
available from the pH meters equipped with temperature probes. If a 100 µL gas
sample is used, the atmospheric pressure is 100 kPa, and the temperature is 22°C then
the number of moles of gas are calculated as:
n
100,000 Pa 100x10-9 m3
Pa m3
8.31
295K
mol K
4.08 mol
9.27
The number of moles of gas is independent of the type of gas. The relationship
between peak area and moles of gas is calculated by analyzing a known number of
moles of each gas. The TCD response will be different for each gas since the thermal
conductivity of each gas is different.
Experimental method (short version)
1) Dry 2 – 2 g samples for each sample type in the 105C oven.
2) Take dried organic sample from the oven.
Methane Production from Municipal Solid Waste
114
3)
4)
5)
6)
7)
8)
Keep 1 dried sample and determine the VS of the other sample.
Weigh appropriate amounts of the various dried samples for methane production.
Load bottles with organic samples (cut to smaller size as needed).
Add 15 mL of basal medium to each of the bottles.
Add 5 mL of inoculum to each of the bottles.
Purge the headspace of the bottles with an oxygen-free gas stream that is 30%
CO2 and 70% N2.
9) Seal the bottles.
10) Insert the pressure sensor hypodermic needle into the bottle.
11) Sample the bottle pressures using the data acquisition software (take samples
every hour and save the data as \\Enviro\enviro\Courses\453\methane\pressure).
Gas Analysis Method
1) Calibrate the gas chromatograph using methane and carbon dioxide and using 20
L samples
2) Take an initial headspace gas sample and analyze it using the gas chromatograph.
3) Sample gas composition after gas production has ceased using the gas
chromatograph.
Prelab questions
1) Estimate the mass of cardboard and the mass of office paper that will produce a
pressure rise of 80 kPa in the sample bottles at 35°C if the headspace volume is
100 mL. Use the predicted biodegradability based on the lignin content of the
paper.
Data analysis
Perform the analysis on the data from your lab section.
1) Calculate total gas production in moles. For each sample use the record of
pressure vs. time to determine if the reaction appears to have gone to completion.
2) For your samples, compare volatile solids (VS) and gas production by converting
the mass of volatile solids to moles of carbon using an approximate molecular
formula for the sample. The molecular formula for the volatile fraction of paper
can be approximated by C6H10O5. Calculate and plot the fraction of VS degraded
as a function of time for each sample.
3) Compare the fuel value of the methane produced with the fuel value of the
original sample for each of the samples. Use the estimates of the original fuel
value (Table 5) and the measured methane production. The fuel value of glucose
is 424.7 KJ/mole C. If you don’t have the gas composition of your samples, then
assume 70% of the gas produced was methane.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
115
Optional Analysis Requiring Gas Composition
1) Calculate the moles of CO2 and CH4 produced by your samples based on the gas
chromatograph analysis. Include the effect of carbon dioxide solubility. Use the
basal medium control to subtract the initial headspace as well as any gas
production by the inoculum.
2) Calculate the final pressure based on the GC measurements and compare with the
pressure transducer measurements. Remember that the pressure transducer
measured gage pressure.
References
Barlaz, M. A., R. K. Ham, et al. (1992). “Microbial chemical and methane production
characteristics of anaerobically decomposed refuse with and without leachate
recycling.” Waste Management & Research 10(3): 257-267.
Brauns, F. E. (1962). “Soluble native lignin, milled wood lignin, synthetic lignin and
the structure of lignin.” Holzforschung 16: 97-102.
Brock, T. D. (1970). Biology of Microorganisms. London, Prentice-Hall.
Chandler, J. A., W. J. Jewell, et al. (1980). Predicting Methane Fermentation
Biodegradability. Biotechnology and Bioengineering Symposium No. 10, John
Wiley & Sons, Inc.
Cummings, S. P. and C. S. Stewart (1995). “Methanogenic interactions in model
landfill co-cultures with paper as the carbon source.” Letters in Applied
Microbiology 20(5): 286-289.
DiStefano, T. D. (1992). Biological dechlorination of tetrachloroethene under
anaerobic conditions, Cornell University.
Ferguson, C. C. (1993). “A hydraulic model for estimating specific surface area in
landfill.” Waste Management & Research 11(3): 227-248.
Gossett, J. M. and P. L. McCarty (1976). “Heat Treatment of Refuse for Increasing
Anaerobic Biodegradability.” AIChE Symposium Series 158(72): 64-71.
Gossett, J. M., D. C. Stuckey, et al. (1982). “Heat Treatment and Anaerobic Digestion
of Refuse.” Journal of the Environmental Engineering Division 108: 437-454.
Gurijala, K. R. and J. M. Suflita (1993). “Environmental factors influencing
methanogenesis from refuse in landfill samples.” Environmental Science and
Technology 27(6): 1176-1181.
Mackay, D. and W. Y. Shiu (1981). “A critical review of Henry's law constants for
chemicals of environmental interest.” Journal of Physical Chemical Reference
Data 10(4): 1175-1199.
Metcalf & Eddy, I. (1991). Wastewater Engineering. Treatment, Disposal, and Reuse.
New York, McGraw-Hill, Inc.
Pavlostathis, S. G. and J. M. Gossett (1985). “Alkaline Treatment of Wheat Straw for
Increasing Anaerobic Biodegradability.” Biotechnology and Bioengineering 27:
334-344.
Methane Production from Municipal Solid Waste
116
Pavlostathis, S. G. and J. M. Gossett (1985). “Modeling Alkali Consumption and
Digestibility Improvement From Alkaline Treatment of Wheat Straw.”
Biotechnology and Bioengineering 27: 345-354.
Pearl, I. A. (1967). The Chemistry of Lignin. New York, NY, Marcel Dekker, Inc.
Prescot, L. M., J. P. Harley, et al. (1993). Microbiology. Dubuque, IA, Wm. C.
Brown, Publ.
Suflita, J. M. and F. Concannon (1995). “Screening tests for assessing the anaerobic
biodegradation of pollutant chemicals in subsurface environments.” Journal of
Microbiological Methods 21: 267-281.
Tchobanoglous, G., H. Theisen, et al. (1993). Integrated Solid Waste Management.
Engineering Principles and Management Issues. New York, McGraw-Hill, Inc.
Zehnder, A. J. B. (1978). Ecology of methane formation. Water Pollution
Microbiology. R. Mitchell. New York, John Wiley & Sons. 2: 349-376.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
117
Lab Prep Notes
Setup
Table 7. Equipment list
1) Use
anaerobic
digester
supernatant as inoculum
source. Place supernatant
under fume hood. Use 5 mL
per sample.
2) Setup 10 port purger with
CO2 and N2 gas metered
through rotometers. The top
ball should be at 24 mm for
CO2 and at 84 mm for N2.
3) Set the GC with 300 Kpa
column pressure, 180ºC oven,
250ºC injector and detectors,
and 1.2 minute run time. Use
20 L sample. The gases
should come out in the order
N2, CH4, and CO2 at 0.44,
0.72,
and
1
minute
respectively. (Only if you are
doing the optional GC
analysis)
4) 4 samples/group plus 2
inoculum blanks and 2 water
blanks.
Class Plan
1) Sign up for samples
2) Each group chooses 2 types
of samples
3) Dry samples in oven
4) Ash 1 of the 2 samples
Description
500 µl syringe w/
valve
side port needle
Carboxen 1004
micropacked
column
Hp 5890 Series II
GC
TCD kit
1/8" column
adapter
pressure regulators
RS232C board
Helium
Wrist action Shaker
Vials
Aluminum crimp
tops
Butyl stopper
Crimping tool
EPDM
stoppers-13x20 mm
luer lock needles
21 gauge
Pressure
transducer, 0 to 15
psig
12 V DC Power
supply
Incubator
Multiplexer
Vender
Supelco
Catalog
2-2272
Supelco
Supelco
2-2289
1-2846
Hewlett-Packard
5890A
Hewlett-Packard
Hewlett-Packard
19232E
option 095
Hewlett-Packard
Hewlett-Packard
Cornell Stores
Fisher Scientific
Supelco
Fisher
L43
option 560
14-260
3-3111
03-375-23C
Fisher
Supelco
Sigma
03-375-22AA
3-3280
Z16607-3
Fisher
14-826-5B
Omega
PX136-015GV
Omega
PSS-12
Fisher
National
Instruments
11-690-650D
4 slot chassis
SCXI-1000
32 channel
SCXI-1100
SCXI-1200 parallel
port
Methane Production from Municipal Solid Waste
776570-01
776572-00
776783-00
118
Table 8. Reagents
Reagent
basal medium
glucose
paper (various
types)
Whatman Filter
Paper (No. 1)
Table 11.
Vender
NA
Aldrich
NA
Fisher Scientific
Catalog
15,896-8
Compound
09-805-1A
Gas chromatograph conditions
gas
carrier (He)
Ref
temperatures
oven (isothermal)
Injector
TCD
pressure
kPa
Column
Supplier
Carboxen 1004
micropacked
column
Supelco
Table 9. Basal medium for
anaerobic
growth
(DiStefano 1992).
flow
5 mL/min
15 mL/min
°C
250
250
Catalog
number
1-2846
NH4Cl
K2HPO4·3H2O
KH2PO4
MgCl2·6H2O
Resazurin
FeCl2·4H2O
Trace Metals
Solution
500 mg
Na2S·9H2O
6g
NaHCO3
The first six compounds are added
to distilled-deionized water, then
purged with N2 until solution turns
from blue to pink. The remaining
components are added, followed
by a 15-minute purge with the 70%
N2/30% CO2 gas mixture.
Table 10.
Trace metals
for
anaerobic
growth (DiStefano
1992).
Compound
MnCL2·4H2O
CoCl2·6H2O
ZnCl2
CaCl2·2H2O
H3BO3
NiCl2·6H2O
Na2MoO4·2H2O
CEE 453: Laboratory Research in Environmental Engineering
Quantity (per
liter)
200 mg
100 mg
55 mg
200 mg
1 mg
100 mg
10 mL
Quantity
(mg/L)
100
170
100
251
19
50
20
Spring 2001
119
Volatile Organic Carbon Contaminated Site
Assessment
Introduction
Roughly 75 percent of the major cities in the U.S. depend, at least in part, on
groundwater for their water supply. Various estimates of the nationwide extent of
groundwater contamination are stated to range from one to over two percent of the
nation's usable groundwater (Council on Environmental Quality, 1981). Volatile
organic compounds (VOCs) are the most frequently detected organic pollutants of
groundwater in the United States. In fact, the VOCs are so ubiquitous that their
analysis has been considered by the U.S. Environmental Protection Agency as a
screening procedure to establish the need for more extensive characterization of
groundwater samples from hazardous waste disposal sites. In the upstate region of
New York (excluding Long Island), of approximately 570 groundwater contamination
incidents reported by 1985, 98% involved either the volatile components of gasoline
and petroleum or solvents and degreasers (NY State DEC, 1985).
Volatile organics may be transported in the subsurface as dissolved components in
groundwater. However, by virtue of their volatility, VOCs will also exist in the gas
phase of unsaturated porous media. As a result, volatile contaminants can be
transported by advection and diffusion in the vapor phase. VOC transport processes
are illustrated in Figure 1.
Experiment
Description
Students will
use soil gas
sampling
to
prospect for a
VOC that has
leaked from a
subsurface
source into an
unsaturated soil
system.
A
rectangular “soil
box”
contaminated
with
a
combination of
liquid acetone,
octane
and Figure 1. Subsurface VOC transport processes. The vadose zone
toluene will be is the region of the soil profile in which some pores contain gas
used. A soil with and are therefore, unsaturated.
high
organic
Volatile Organic Carbon Contaminated Site Assessment
120
content (potting soil) or low organic content (sand) may be used as the box filling
material (porous medium). The VOCs will be identified and measured using a gas
chromatograph (GC).
Experimental Procedures
Calibration (Peak Times)
Each compound will have a unique retention time in the gas chromatograph. The
time for each of the 3 VOC peaks can be obtained by injection of 100 µl head space
samples from crimp cap sealed vials containing a small volume liquid acetone,
octane, and toluene. Use the syringe technique described below. Analyze each
compound 4 times (12 samples) using a gas chromatograph (see page 160 for
information on using the gas chromatograph). These analyses will also serve to
“calibrate” the GC by generating the peak area that corresponds to the saturated vapor
concentration. Gas chromatogram peak areas may be assumed to be directly
proportional to the mass of vapor injected.
Syringe technique for sampling vial headspace
The purpose of this syringe technique is to minimize the effects of sorption to the
Teflon and glass surfaces in the syringe and to eliminate carryover of sample in the
needle. Using separate needles to collect samples and to inject into the GC eliminates
needle carryover of sample.
1) Remove GC needle.
2) Purge syringe 5 times with room air to remove any residual VOCs.
3) Put on sample needle.
4) Insert into sample bottle (with syringe at zero volume)
5) Fill syringe fully with gas, wait 4 seconds, and purge syringe contents back into
the source bottle (repeat 3 times).
6) Fill syringe and adjust to 100 µL.
7) Close syringe valve and remove syringe from sample vial and remove sample
needle.
8) Put on GC needle.
9) Instruct GC to measure sample (see page 160 for information on using GC
software).
10) Insert needle in injection port, open syringe valve, inject sample, hit start button
all as quickly as possible.
11) Remove syringe from the GC injection port.
Soil Moisture Content
The dry weight of moist soil may be readily determined by placing ≈ 5 g moist soil
into a tarred aluminum weighing pan. Weigh the pan and its contents to obtain the
soil’s wet weight, and place the pan into a 105oC oven for ≥ 1 hour. Remove the soil
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
121
from the oven and place in a desiccator and allow it 5 minutes to cool. Weigh the
cooled soil to obtain the dry weight. The moisture content is
Moisture Content =
wet weight - dry weight
wet weight
10.1
Soil Density
To determine the approximate density of the soil, soil, place a weighed quantity of
dry soil (≈ 30 g) into a 100 mL graduated cylinder containing 60 mL water. Record
the volume occupied by the water plus the soil.
soil =
M soil
Vtotal VH 2O
10.2
where soil = b/, b= bulk density of the soil and = fraction of void volume.
Soil Gas Sampling
See Table 1 for physical properties of the VOCs. See Tables 2, 3, and 4 (in the Lab
Prep Notes) for necessary reagents, equipment and GC method. Prior to the
laboratory the instructor will create a “spill” of a VOC by injecting 10 mL of liquid of
two or more NAPLs into
the “soil box” to be
Table 1. Physical data for octane, acetone, and
sampled by students.
toluene.
During the lab students
Octane
Acetone
Toluene
Aqueous
0.6
very
515
will
analyze
solubility
approximately 50 soil gas
3
(g/m )
samples from the “soil
Vapor
1.88 (1.47)
24
3.8 (2.9)
box” using the syringe
Pressure (kPa)
technique outlined below.
H (kPa
300
0.0159
0.67
3
Results from the soil box
m /mol)
analyses may be mapped
123
0.0065
0.275
HGL
using
units
of
(g/L)/(g/L)
Molecular
CH3(CH2)6CH3 CH3COCH3
C6H5CH3
concentration (g/m3).
Formula
Molecular
weight
density
(g/mL)
114.23
58.08
92.14
Syringe technique for soil
gas sampling
0.71
0.7857
0.8669
1) Remove GC needle.
2) Purge syringe 10 times with room air to remove any residual VOCs.
3) Put on sample needle.
4) Insert into soil bed (with syringe at zero volume).
5) Fill syringe and adjust to 100 µl.
6) Close syringe valve, remove syringe from soil bed and remove sample needle.
7) Put on GC needle.
8) Instruct GC to measure sample (using software).
Volatile Organic Carbon Contaminated Site Assessment
122
9) Insert needle in injection port, open syringe valve, inject sample, hit the enter key
(or OK) all as quickly as possible.
10) Remove syringe from the GC injection port.
Analysis of Soil Gas Sampling
Students will use their analysis of VOC standards to obtain the corresponding GC
retention times and use this information to identify the unknown VOCs in the
contaminated soil box. The vapor pressure and ideal gas law are used to estimate the
mass of each compound present in the samples used to calibrate the GC.
PV
10.3
RT
where n is the number of moles of the compound, P is the vapor pressure of the
compound [kPa], V is the syringe volume [L], R is the Gas Constant (8.31
[ L kPa] [mol K ] ), and T is the temperature of the gas in the syringe [K]. The
relationship between peak area (as measured by the GC) and mass of the compound is
determined from the calibration.
Soil gas concentrations should be reported and plotted as contour lines on a map of
the soil box (see Figure 2 for an
illustration).
n
Procedure (short version)
1) Instructor will demonstrate
syringe technique (be careful
not to pull plunger out of barrel)
and
Gas
Chromatograph
technique.
2) Place ≈5 g of accurately
weighed soil in oven to
determine moisture content.
(Weigh both the empty dish and
the soil + dish.)
Figure 2. Students will prepare a map of the
3) “Calibrate” GC by analyzing 4 surface of their soil box. The map will show
isoconcentration lines for each VOC.
samples for each VOC.
4) Take soil gas samples.
5) Convert the soil gas peak areas
to concentrations (g/m3). This data will be shared between groups.
6) Finish soil moisture content measurement (cool dry soil in desiccator and then
weigh).
7) Measure soil density using dry soil.
8) Pour waste potting soil into designated waste container.
9) Clean plasticware.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
123
Prelab Questions
1) How are the identities of the chromatogram peaks determined when using a gas
chromatograph?
2) Explain why different needles are used for sampling from source vials and
injecting the sample into the GC. Consider the temperature of the injection port
(see Table 4) and the fact that these compounds absorb to most surfaces.
Data Analysis
1) Calculate the mass of each VOC in 100 L of headspace.
2) Calculate the concentration of saturated vapor for each compound in g/m3.
3) Plot isoconcentration lines of the identified VOCs (expressed as gas concentration
in g/m3) on maps of the contaminated site (see figure 2 for example). Prepare a
map for each compound showing isoconcentration lines. (The Excel 3-D surface
plot with contour lines can be used. Note that the grid needs to have uniform
distance between samples for the Excel 3-D surface plot to work correctly.)
4) Compare the saturated vapor concentration with the peak concentration observed
in the “sand box.”
5) Calculate the soil moisture content and density.
References
Ashworth, R. A., G. B. Howe, and T. N. Rogers. “Air-Water Partitioning Coefficients
of Organics in Dilute Aqueous Solutions.” J. Hazard. Mater. 18, p. 25-36, 1988.
Council on Environmental Quality, "Contamination of Groundwater by Toxic
Organic Chemicals", 1981.
Hwang, Y., J. D. Olson, and G. E. Keller, II, “Steam Stripping for Removal of
Organic Pollutants from Water. 2. Vapor-Liquid Equilibrium Data.” Ind. Eng.
Chem. Res. 31, p. 1759-1768, 1992.
Mackay, D. and W. Y. Shiu, “A Critical Review of Henry’s Law Constants for
Chemicals of Environmental Interest”, J. Phys. Chem. Ref. Data. 10, p.
1175-1199, 1981.
New York State Department of Environmental Conservation, "Draft Upstate New
York Groundwater Management Program", N.Y.S.D.E.C., Division of Water,
Draft Report WM P-94, January, 1985.
Volatile Organic Carbon Contaminated Site Assessment
124
Lab Prep Notes
Setup
Table 2. Reagents list
1) Prepare 1 soil box under fume
Description
Source
Catalog
number
hood.
Octane
Fisher Scientific
03008-1
2) Moisten the sand but not so much
Acetone
Fisher Scientific
O299-1
that there is standing water.
toluene
Fisher Scientific
T324-500
Potting soil
Agway (remove
3) Pipette 10 mL of liquid acetone,
large particles
octane, and toluene in sand box
by screening to
and record injection locations.
2 mm)
This should be done in the
morning before the lab
Table 3. Equipment list
exercise.
4) Dry approximately 100 g of
Description
Supplier
Catalog number
potting soil for each group
500 µl syringe w/
Supelco
2-2272
that will be used for density
valve
determinations.
side port needle
Supelco
2-2289
1 mL syringe w/
Supelco
2-2273
5) Replace injection port septa
valve
on both GC’s.
Hp 5890 Series II Hewlett-Packard
5890A
GC
6) Verify that GC’s are working
Sep
Hewlett-Packard
option 600
properly by injecting gas
purge-packed/FID
samples from each VOC
1/8" column
Hewlett-Packard
option 095
source bottle. If the baseline
adapter
L43
is above 30 (as read on the pressure regulators Hewlett-Packard
RS232C board
Hewlett-Packard
option 560
computer display) then heat
the oven to 200°C to clean the Nitrogen, Air, and General Stores
Hydrogen gas
column.
Wrist action Shaker Fisher Scientific
14-260
Desiccator
Fisher
Scientific
08-642-15
7) Verify that sufficient gas is in
Vials
Supelco
3-3111
the gas cylinders (hydrogen,
Aluminum crimp
Supelco
3-3220
air, nitrogen).
tops
Septa
Supelco
3-3200
8) Prepare VOC source vials that
Crimping tool
Supelco
3-3280
contain
liquid
acetone,
octane, and toluene (they can be shared by two groups of students).
Class Plan
1) Setup uniform spreadsheets for data entry
2) Make sure spreadsheet is completely filled out by end of lab
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
125
Table 4. Gas chromatograph conditions
gas
carrier (N2)
Air
Hydrogen
pressure
320 kPa
230 kPa
130 kPa
temperatures
oven (isothermal)
Injector
FID
°C
100
250
250
Column
Supplier
Supelcowax 10
30 meters
Supelco
flow
15 mL/min
300 mL/min
45 mL/min
Catalog
number
2-5301
Run length of 66 seconds with octane, acetone, and toluene at 0.57, 0.63, 0.96
minutes respectively. Maximum sample volume is about 100 µl. Larger samples can
lead to a significant broadening of the peak.
Syringe clean up
Disassemble and heat syringes to 45°C overnight to remove residual VOCs. Place
syringe barrels upside down on top of openings above fan in oven to facilitate mass
transfer.
Volatile Organic Carbon Contaminated Site Assessment
126
Volatile Organic Carbon Sorption to Soil
Introduction
Volatile organic carbon compounds (VOCs) can exist as vapors, non-aqueous
phase liquids, dissolved in water, or sorbed to surfaces. Sorption is the term used to
refer to the binding reactions between organic pollutants and the subsurface medium.
Sorption slows the rate of transport of both dissolved and volatile organic pollutants.
Laboratory experiments will be performed to evaluate the sorption of acetone,
hexane, and octane vapors to a soil and to estimate the extent to which VOC transport
is slowed by binding to the soil media.
Theory
Gas-Liquid Partitioning
The equilibrium between gas and solution is described by the following reaction:
CLiquid CGas
11.1
The equilibrium constant ( H LG ) for the reaction is given by:
H LG
CG
CL
11.2
where CG is the concentration in the gas phase (Example units: g/m3) and CL is the
concentration in the aqueous phase (g/m3)
Alternately, the liquid-vapor equilibrium is sometimes expressed as:
H
PG
CL
11.3
where PG is the partial pressure of the gas (atm), and
H LG
H
RT
11.4
m3 atm
where R is the universal gas constant 8.21x105
and T is the absolute
mol K
temperature (oK).
Both H and H LG are referred to as “Henry's law constants” and may be viewed
conceptually as distribution coefficients for gases between the aqueous solution phase
and the vapor phase. All other factors being equal, VOCs with higher Henry's law
constants will have a greater fraction of their total mass in the gas phase of an
unsaturated porous medium.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
127
Liquid-Solid Partitioning
The sorption of organic pollutants that are dissolved in water onto soils and aquifer
materials also may be described in many cases by a distribution coefficient ( K LS ):
K LS
CL
11.5
where is the mass of solute sorbed per mass of solid.
Equation 11.5 predicts that the amount of pollutant bound to the soil () will
increase linearly with the concentration in the aqueous phase (C L). Any relationship
between the amount bound to the soil and the concentration in solution applies at a
constant temperature and is referred to as an “isotherm.” Equation 11.5 is an example
of a “linear isotherm.” The success of the linear isotherm in describing sorption of
nonionic organic pollutants in saturated soils has been remarkable. Linear isotherms
have been found to describe sorption of a wide array of nonionic compounds onto
sediments and soils (Karickhoff et al. 1979, Chiou et al. 1979).
Many investigations have demonstrated that the distribution coefficient ( K LS ) for
sorption of a single organic contaminant onto a variety of soil materials can be related
to the organic content of the sorbent. This observation permits the definition of an
organic normalized partitioning coefficient (Koc):
K oc
K LS
f oc
11.6
where foc is the weight fraction of organic carbon in the soil.
Koc values for a range of different organic compounds have been shown to be
related to their octanol-water partitioning coefficients (Kow) and also to their aqueous
solubilities (Karickhoff, 1984). An important implication of these results is that
sorptive distribution coefficients of organic pollutants in water saturated aquifers
( K LS ) may be predicted given knowledge of the organic content of the soil (foc) and
the octanol-water partitioning coefficient (Kow) of the contaminant or a related
parameter such as its aqueous solubility.
Gas-Solid Partitioning
Sorption of gases is frequently described using the classic equation developed by
Brunauer, Emmett and Teller (1938), i.e., the BET equation:
B
M P0 P P0 P
B
P0 P0
11.7
where is the amount of sorbed gas per unit of surface (with units such as moles/m 2
or µg/g if the surface area is not known), M is the amount of sorbed gas
corresponding to monolayer surface coverage, P is the partial pressure of the sorbed
Volatile Organic Carbon Sorption to Soil
128
gas, Po is its saturated vapor pressure, and B is a parameter related to solute binding
intensity; more specifically:
v
kT
B e
11.8
where is the energy of the adsorbate vapor-adsorbent surface interaction (cal/mole),
v is the vaporization energy of the organic (cal/mole), k is the Boltzmann constant,
and T is the absolute temperature (oK).
To the extent that it is valid in soil, the BET equation predicts vapor sorption
isotherms to be nonlinear. Nonlinear isotherms have been observed for sorption of
organic vapors onto dry soils at high vapor concentrations by several investigators
(Chiou, 1990; Rhue, et al., 1988; and Ong and Lion, 1991c). Under conditions of low
vapor pressure, P << Po, the BET equation reduces to:
B
M P0
P B
11.9
which is the “Langmuir adsorption equation” that applies to monolayer limited
adsorption.
The BET equation further reduces to a linear isotherm when B << (Po/P).
P
B
M
P0
11.10
Figure 1 illustrates
linear, Langmuir and
A
Linear
BET isotherms that
BET
share a common set of B
and m values. Linear
Langmuir
sorption isotherms for
vapors are a reasonable
expectation at low vapor
P/Po
concentrations (Ong and
B
Lion, 1991a).
At
higher
vapor
concentrations, surface
BET
site limitations and the
Langmuir
phenomenon of vapor
condensation at the
surface (for which the
P/Po
BET model attempts to
account) will result in Figure 1. Linear, Langmuir and BET sorption isotherms
nonlinear VOC sorption for the case where m = 10,000 and B = 1,000. A)
isotherms.
Results Isotherms for P/Po < 0.3. B) Isotherms for P/Po up to 1.0.
obtained at Cornell (Ong
200 00
160 00
120 00
800 0
B = 1 ,000
max = 10,000
400 0
0
0.0
0.1
0.2
0.3
300 000
200 000
100 000
0
0.0
0.2
CEE 453: Laboratory Research in Environmental Engineering
0.4
0.6
0.8
1.0
Spring 2001
129
et al., 1991; Ong and Lion, 1991c) have confirmed that condensation of organic
vapors will occur at high vapor pressures in moist porous media. Condensed water
and organic vapors compete for the available pore space. Since soils are generally wet
prior to the introduction of organic contaminants, vapor condensation is expected to
be limited to the pore volume not occupied by water. The extent to which organic
vapor condensation is significant will, therefore, be a function of the soil moisture
content, and the relative vapor concentration (P/P0).
Since the unsaturated zone in an aquifer will typically contain condensed water,
description of the sorption of organic vapors in the unsaturated zone must, at a
minimum, consider a binary mixture of the organic and water vapor. The organic
vapor of concern may accumulate in the unsaturated zone through at least three
processes: (a) by direct sorption from the vapor phase, including vapor sorption to dry
mineral surfaces (if present), vapor sorption at the gas-water interface, and vapor
condensation, (b) by solubilization in condensed pore water as governed by Henry's
Law (equation 11.2 or 11.3), and (c) by sorption from condensed pore-water solution
onto the soil (as governed by equation 11.5). Since, at very low vapor pressures, a
linear isotherm is expected to govern vapor sorption, we may write a linear isotherm
in terms of the gas concentration:
KGS CG
11.11
where the magnitude of K GS depends on the sorbent’s moisture content.
The relative contributions to of processes such as vapor dissolution into soil
moisture and sorption at the liquid-air interface can be assessed through the use of a
mass balance. The total mass of vapor sorbed onto the soil, under equilibrium
conditions, can be viewed as being distributed between:
(Mass sorbed at the solid-liquid interface)
+ (Mass dissolved in the liquid phase)
+ (Mass sorbed at liquid-air interface + condensation)
= M s =X +CLVL +
11.12
where Ms is the mass of the sorbent (soil or other porous media) and is a lumped
parameter that includes the effects of water surface sorption and condensation.
From equation 11.5 for liquid-phase sorption: X K LS M S CL where = X/Ms and
Ms is the mass of the sorbent. Also, from Henry's Law: CG H Lg CL . Substituting
these two relationships and equation 11.11 into equation 11.12 gives:
K LS
VL
K G
G
H L M S H L CG M S
S
G
11.13
If the density of water on the soil surface is expressed as H 2O M H 2O / VL , the
M H 2O
term VL/Ms becomes
. Assuming water surface sorption and condensation
M s H O
2
are negligible ( ≈ 0), then for high moisture content equation 11.13 will plot as a
Volatile Organic Carbon Sorption to Soil
130
straight line, with the ordinate intercept equal to the contribution of aqueous phase
sorption to vapor-phase partitioning ( K LS / H LG ).
K LS Moisture Content %
K G
HL
100 H LG
S
G
11.14
Equation 11.14 indicates that the linear vapor distribution coefficient, K GS , can be
predicted from the saturated distribution coefficient and the Henry’s law constant for
a given VOC. Experiments by Ong and Lion (1991a) have shown that such
predictions are reasonable as long as the moisture content of the soil is high enough to
ensure that the VOC that is dissolved in sorbent-bound water behaves as if the liquid
were comparable to the water in a bulk aqueous phase. In general, a moisture content
equivalent to an average surface coverage of ≈ 5 layers of water molecules is
adequate for this assumption to be obeyed (Ong and Lion, 1991a). Many soil ambient
moisture contents are in excess of this limiting value.
Pollutant Transport in Porous Media
The advective dispersion equation is used to describe pollutant movement in
porous media. For one-dimensional (ex., horizontal) transport of a conservative
( K LS 0 ) pollutant:
C
C
2C
11.15
-U
E 2
t
x
x
where t is time, x is distance, U is the groundwater pore velocity and E is the
macroscopic dispersion coefficient (Freeze and Cherry, 1979).
Since volatile organic pollutants react with the surfaces of the porous media
through which the contaminant flows, equation 11.15 must be modified to account for
b
the sorption reaction by addition of the term:
where b is the bulk density of
t
the porous medium (g/cm3), and is the volumetric moisture content (volume of
liquid per unit bulk volume of the porous medium). is equal to the porosity, , in a
saturated porous medium.
b C
K LS .
From the chain rule, b
and for a linear isotherm,
C
t C t
Therefore, the advection-dispersion equation for a compound that experiences
sorptive binding to the soil matrix becomes:
C
C
2 C K S C
= -u
+E 2 b L
t
x
x
t
F
G
H
IJ
K
C
KS
C
2 C
1+ b L = -u
+E 2
t
x
x
11.16
11.17
The retardation factor, R, for a pollutant in soil is defined as:
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
131
KS
11.18
R 1+ b L
Therefore the advective dispersion equation modified for sorptive binding
becomes:
C -u C E 2 C
11.19
=
+
t R x R x 2
From equation 11.19 it is apparent that the velocity and dispersion of a sorbed
compound will be reduced by the magnitude of retardation factor, R. So,
R=
pore water velocity
1
contaminant velocity
11.20
Note that R may be determined directly from knowledge of the medium properties
( b and ) and from the distribution coefficient for sorption ( K LS for water saturated
media or K GS for a vapor).
Interestingly, equation 11.19 may also be applied to describe vapor movement in a
gas chromatograph (GC). GC columns are selected to ensure that the components of a
vapor mixture will be separated (by virtue of their different retardation factors) by the
time they arrive at the GC detector (situated at the end of the column).
In the absence of pressure gradients, transport of vapors will occur primarily
through the process of diffusion and equation 11.19 reduces to:
E 2C
C
s
t Ebulk x 2
11.21
where Es is the effective diffusion coefficient of the VOC in the porous media.
Vapor diffusion coefficients in unsaturated porous media are different from those
in a bulk gas phase because the vapor must follow a tortuous path to move through
the open pores. The relationship proposed by Millington and Quirk (1961) is
commonly used to correct vapor diffusion coefficients for the conditions in the soil
media.
Es
a10 / 3
2
Ebulk
11.22
where Ebulk is the vapor diffusion coefficient in air, a is the volumetric air content of
the porous medium (volume of gas per unit bulk volume of medium), and is the
porosity (a + = ).
Analysis of the Unsaturated Distribution Coefficient ( K GS )
A mass balance calculation will be used to determine the unsaturated vapor
distribution coefficient ( K GS ). After equilibration the VOC mass will be distributed
between the vapor phase and the solid phase (sorbed VOC).
Volatile Organic Carbon Sorption to Soil
132
MVOC =CGVG +M s
11.23
where M s is the mass of sorbed VOC and equals K GS M S CG (from equation 11.11).
In control bottles there is no sorbent so the VOC mass must reside entirely in the
vapor phase:
M VOCUC = CGUC VGUC
11.24
where the subscript uc indicates that volumes and concentrations are those measured
in the unsaturated control bottles.
Setting equations 11.23 and 11.24 equal to one another (the mass of VOC was the
same for all vials) and rearranging gives:
MVOCUC = CGVG KGS M S CG
11.25
Solving for K GS
KGS
M VOCUC CGVG
M S CG
11.26
Using the measured soil moisture contents and values of K GS , students may check
the validity of equation 11.14.
Unsaturated Mass Fraction Distribution
The total mass of the VOC is distributed between the gas and sorbed phases
(equation 11.23).
1=ƒG +ƒ S
11.27
where ƒG is the fraction of the VOC mass in the gas phase and ƒ S is the fraction of
the VOC mass sorbed to the soil. The relationship between the fraction of VOC in
each phase is obtained from the definition of the unsaturated distribution coefficient
(equation 11.11).
ƒS
KGS M S
=
ƒG
VG
11.28
Thus we have two equations in two unknowns. Solving we obtain
ƒG =
1
KSM
1 G S
VG
ƒS =
1
V
1 S G
KG M S
11.29
11.30
where
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
133
VG =Vvial
MS
soil
11.31
Determination of K GS requires a measurement of the fraction sorbed given
measurements of the total mass added and the mass in the gas phase. This analysis
becomes inaccurate as the magnitude of ƒ S decreases and approaches the coefficient
of variation of M VOCUC .
Analysis of the Saturated Distribution Coefficient ( K LS )
A mass balance calculation will be used to determine the saturated vapor
distribution coefficient ( K LS ). The calculation assumes that students can reproduce the
introduction of the mass of VOC (MVOC) into each sample bottle. After equilibration
the VOC mass can be distributed between the vapor phase the aqueous phase and the
solid phase (sorbed VOC).
MVOC = CGVG +CLVL + M S
11.32
where Ms is the mass of sorbed VOC and equals K LS CL M S (from equation 11.5).
In saturated control bottles there is no sorbent so the VOC mass must just be
distributed between the vapor phase and the aqueous phase:
M VOCSC =CGSC VGSC +CLSC VLSC
11.33
where the subscript sc indicates that volumes and concentrations are those measured
in the control bottles. Henry’s law (equation 11.2) can be used to eliminate C Lsc .
VL
MVOCSC =CGSC VGSC SCG
11.34
HL
Equation 11.34 can be used to obtain an estimate of Henry’s law constant by
assuming that the mass of VOC added is the same for the vials with and without
water. Solving for H LG and substituting M VOCUC for M VOCSC we obtain:
H LG =
VLSC CGSC
MVOCUC VGSC CGSC
11.35
Equation 11.32 can now be used to obtain an estimate of K LS . The mass of VOC
added to the bottles was the same for all vials. Thus ( M VOC ) in equation 11.32 is
equal to M VOCSC . In addition, CL and CG are interrelated through Henry’s law
(equation 11.2). Substituting into equation 11.32 gives:
MVOCsc =CGVG +
CG
C
V + M S K SL GG
G L
HL
HL
Solving for K LS
Volatile Organic Carbon Sorption to Soil
11.36
134
CG
11.37
MVOCsc CGVG G VL
HL
Measured VOC headspace concentrations are substituted into equation 11.37.
Values for Henry’s law constants are given in Table 1. M VOCSC is obtained from
equation 11.34. In vials containing soil, VG is determined by subtracting both VL (50
mL) and the volume of the sorbent from the total vial volume. The volume of the
sorbent may be calculated from the sorbent weight and density as:
K LS
H LG
M S CG
Vsoil
Ms
11.38
soil
It is instructive to calculate the phase distribution of each VOC in bottles that
contain no soil. The fraction (ƒ) of VOC mass in the gas phase is given by
f=
CGVG
VG
=
CGVG CLVL V VL
G
11.39
H LG
Assuming the total volume of the bottle is 120 mL and 50 mL of liquid are added,
then
f=
70
70 50
11.40
H
G
L
ƒ values for octane, acetone, and toluene are therefore 0.994, 0.009, and 0.278
respectively indicating that only toluene has a significant mass fraction in both the
gas and aqueous phases. In the absence of strong sorption by soil, octane will reside
primarily in the gas phase and acetone will reside primarily in the aqueous phase.
Determinations of K LS for these two compounds may therefore not be feasible using
the headspace technique.
Saturated Mass Fraction Distribution
The total mass of the VOC is distributed between the gas, sorbed, and liquid phases
(equation 11.32).
1=fG +f S +f L
11.41
where f L is the fraction of the VOC mass in the liquid phase. The relationship
between the fractions of VOC in the solid and liquid phases is obtained from the
definition of the saturated distribution coefficient (equation 11.5).
fS
KSM
= L S
fL
VL
11.42
The relationship between the fractions of VOC in the gas and liquid phases is
obtained from the definition of the Henry’s law constant (equation 11.2).
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
135
fG H LG VG
=
fL
VL
11.43
Thus we have three equations in three unknowns. Solving we obtain
VL
VL K M S +H LGVG
11.44
fS =
K LS M S
VL +K LS M S +H LGVG
11.45
fG =
H LGVG
VL +K LS M S +H LGVG
11.46
fL =
S
L
where
VG = Vvial VL
MS
S
11.47
Determination of K LS requires a measurement of the fraction sorbed given
measurements of the total mass added, the mass in the gas phase, and knowledge of
the Henry’s law constant. This analysis becomes inaccurate when ƒS decreases and
approaches the coefficient of variation of M VOCSC .
Experimental procedures
VOC distribution coefficients will be determined using crimp top vials sealed with
Teflon backed septa and aluminum crimp caps. VOCs will be analyzed by students
using a gas chromatograph (GC).
A data table for preparation of the necessary vials is shown in Table 4. The data
table is also available in spreadsheet form as “isotherm calculator.”
Analysis of the Unsaturated Distribution Coefficient ( K GS )
The distribution coefficient for three VOCs (acetone, octane, and toluene) with the
unsaturated soil medium ( K GS ) will be determined using the headspace method of
Peterson et al. (1988). Students will prepare 120 mL vials (actually 121.5 ± 0.5 mL)
in triplicate containing 0 and 20 g of moist soil (6 vials total). The weight of
replicates need not be precisely matched, but the weight of soil should be measured
accurately. Seal the vials using Teflon-backed septa (the shiny Teflon side faces the
vial contents) and aluminum crimp cap.
The 3 VOCs are added to the vials by introduction of 1 mL of the saturated vapor
taken from the headspace of “source vials” (crimp capped vials containing liquid
octane, toluene, and acetone). Use a dedicated syringe for each VOC and leave
dedicated needles in the source bottles.
The vials should be placed on a wrist action shaker to continue agitation for ≥ 30
minutes to permit the VOC time to sorb to the soil medium. After equilibration, vials
Volatile Organic Carbon Sorption to Soil
136
are removed from the wrist action shaker and the head space sampled using a 500 µL
gas tight syringe. Use the syringe technique as described previously for sampling vial
headspace.
Analysis of the Saturated Distribution Coefficient ( K LS )
The saturated distribution coefficient ( K LS ) for three VOCs (acetone, octane, and
toluene) with the soil medium will be determined using the headspace method of
Garbarini and Lion (1985) as modified by Peterson et al. (1988). These vapors are
selected to demonstrate the effect of VOC Henry's law constant on VOC phase
distribution (see Table 1).
Students will prepare 120 mL vials (actually 121.5 ± 0.5 mL) in triplicate
containing 0 and 20 g of potting soil (6 vials total). The weight of replicates need not
be precisely matched, but the weight of soil should be measured accurately. Soil
density should be separately measured as described below. If moist soil is used
students should also determine the dry weight unless the instructor provides this
information (see below). Into each vial students will add 50 mL of tap water.
Seal the vials using a Teflon-backed septa (the shiny Teflon side faces the vial
contents) and aluminum crimp cap. Octane and toluene are then added to the vials by
introduction of 1 mL of the saturated vapor taken from the headspace of “source
vials” (crimp capped vials containing liquid octane and toluene). Use different
needles for collecting and delivering the VOC to reduce the transfer of VOC on the
needles. The needle used to collect the VOC from the source bottle can be left in the
source bottle and simply attached to the syringe when needed. Acetone is added by
introduction of 200 µl of the liquid compound using a gas tight syringe.
The vials should be placed on a wrist action shaker to continue agitation for ≥ 30
minutes to permit the VOC time to sorb to the soil medium. After equilibration, vials
are removed from the wrist action shaker and the head space sampled using a 500 µL
gas tight syringe. To sample the vial headspace, use a 100 L sample and the syringe
technique described previously.
Procedure (short version)
1)
2)
3)
4)
Weigh soil and add to isotherm vials (6 vials with 20 g).
Add 50 mL tap water to 6 vials (3 each with 0 and 20 g soil).
Seal 12 vials.
Add octane, toluene, and acetone to all vials (1 mL gas from source vials for all
VOC’s except 200 µL liquid acetone for vials with tap water).
5) Place vials on shaker for 30 minutes.
“Calibrate” GC by analyzing 4 100-L samples for each VOC.
Take vials off of shaker.
Measure VOC concentrations for each vial and record peak areas in spreadsheet.
Reanalyze the VOC concentrations for any vials for which anomalous data was
obtained.
10) Remove vial caps.
6)
7)
8)
9)
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
137
11) Pour waste potting soil into designated container.
12) Wash vials.
Prelab Questions
1) Why does the determination of K GS become inaccurate as the magnitude of ƒS
decreases and approaches the coefficient of variation of M VOCUC ?
2) What conditions are necessary to obtain linear isotherms for gas/solid partitioning
of organic vapors?
3) When can equation 11.14 be used to predict the relationship between liquid/solid
partitioning and gas/solid partitioning based on soil moisture content and Henry’s
law constant?
Data Analysis
A Note on Units
Express mass of VOC in grams (as measured by the GC). Express concentrations
in g/mL. Remember to account for the fact that the syringe volume for GC analysis is
100 µL Express all volumes in mL.
1) Estimate the mass of each VOC added to the unsaturated sample vials based on
M VOCUC (from equation 11.24). Report mean and coefficient of variation (standard
deviation/mean) for each VOC.
2) Calculate the unsaturated vapor distribution coefficient ( K GS ) using equation
11.26. Report a single mean and coefficient of variation for each VOC.
3) Calculate the mass fraction associated with the soil and gas phases under
unsaturated conditions for each of the VOCs. Use equations 11.29, 11.30, and
11.31. Assume Ms = 20 g and bottle volume is 121.5 mL. Use the average K GS
calculated in 2 above. Compare the coefficient of variation of M VOCUC to the mass
fractions to evaluate which K GS determinations are potentially accurate. Create a
stacked bar graph of the mass fractions for each VOC.
4) Estimate the Henry’s law constant ( H LG ) for octane and toluene using equation
11.35 (different masses of acetone were used for the saturated and unsaturated
vials and thus equation 11.35 can not be used for acetone). Report mean and
coefficient of variation. Compare your results to the Henry’s law constants
reported in the table on page 121.
5) Estimate the mass of each VOC added to the saturated sample vials based on
M VOCSC (from equation 11.34) using tabulated Henry’s constants (see table on
page 121). Report mean and coefficient of variation for each VOC.
6) Calculate the saturated vapor distribution coefficient ( K LS ) using equation 11.37.
Report a single mean and coefficient of variation for each VOC. Use tabulated
Henry’s constants reported in the table on page 121.
Volatile Organic Carbon Sorption to Soil
138
7) Calculate the mass fraction associated with the soil, gas, and water phases under
saturated conditions using equations 11.44 - 11.47. Assume Ms = 20 g, VL = 50
mL, and bottle volume is 121.5 mL. Use the average calculated K LS and the
Henry’s law constants reported in the table on page 121. Compare the coefficient
of variation of M VOCSC to the mass fractions to evaluate which K LS determinations
are potentially accurate. Create a stacked bar graph of the mass fractions for each
VOC.
8) Calculate K GS for toluene using equation 11.14 and compare with the measured
value of K GS .
9) Assuming a typical soil porosity () of 0.33 and bulk density (b) of 1.7 g/cm3,
equation 11.18 may be used to calculate the retardation of dissolved VOCs and
VOC vapors. Assuming a pore water velocity of 1 m/day, how long will it take
for the dissolved toluene to be transported a distance of 100 m? Include a range of
the estimate based on 1 standard deviation.
10) If toluene is removed by withdrawing vapor at a velocity of 100 m/day, how long
will it take toluene vapor to travel 100 m? [Note in this case K GS replaces K LS and
a replaces in equation 11.18. Assume the soil voids are filled with gas so a =
= 0.33.] Include a range of the estimate based on 1 standard deviation.
11) Discuss how the results of this experiment would guide you in remediating a site
contaminated with toluene, acetone, and, octane.
References
Brunauer, S., P.H. Emmett and E. Teller, "Adsorption of Gases in Multimolecular
Layers", J. Amer. Chem. Soc., 60, p. 309, 1938.
Chiou, C.T., L.J. Peters and J.H. Freed, "A Physical Concept of Soil-Water Equilibria
For Nonionic Organic Compounds", Science, 206, p. 831, 1979.
Chiou, C.T., "Roles of Organic Matter, Minerals and Moisture in Sorption of
Nonionic Compounds and Pesticides by Soils", in Humic Substances in Soil and
Crop Sciences; Selected Readings, P. MacCarthy, C.E. Clapp, R.L. Malcolm and
R.R. Bloom (Eds.), Am. Soc. of Agron. & Soil Sci. Soc. of Am, Madison, WI, pp.
111-160, 1990.
Freeze, R.A. and J.A. Cherry; Groundwater, Prentice Hall, Inc.; Englewood Cliffs,
NJ, 604 pp., 1979.
Karickhoff, S.W., D.S. Brown and T.A. Scott, "Sorption of Hydrophobic Pollutants
on Natural Sediments", Water Res., 13, p. 241-248, 1979.
Karickhoff, S.W., "Organic Pollutant Sorption in Aquatic Systems", J. Hydraulic
Engrg., 110(6), p. 707, 1984.
Millington, R.J., J.M. Quirk, “Permeability of porous solids”, Trans. Faraday Soc.
57, p. 1200-1207, 1961.
Ong, S.K., and L.W. Lion, "Sorption Equilibria and Mechanisms for
Trichloroethylene onto Soil Minerals", J. Env. Qual., 20(1), p. 180-188, 1991a.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
139
Ong, S.K., and L.W. Lion, "Effects of Soil Properties and Moisture on the Sorption of
TCE Vapor", Water Resources Research, 25(1), p. 29-36, 1991b.
Ong, S.K., and L.W. Lion, "Trichloroethylene Vapor Sorption onto Soil Minerals at
High Relative Vapor Pressure", Soil Sci. Soc. Am. J., 55(6), p. 1559-1568, 1991c.
Ong, S.K., S.R. Lindner and L.W. Lion, "Applicability of Linear Partitioning
Relationships for Organic Vapors onto Soil Minerals", in: Organic Substances and
Sediments in Water, R.A. Baker (Ed.), Lewis Publ., Chelsea, MI, pp. 275-289,
1991.
Rhue, R.D., P.S.C. Rao and R.E. Smith, "Vapor Phase Adsorption of Alkylbenzenes
and Water on Soils and Clays", Chemosphere, 17, p. 727-741, 1988.
Additional References Relevant to Data Reduction
Garbarini, D.R. and L.W. Lion, “Evaluation of sorptive partitioning of nonionic
organic pollutants in closed systems by headspace analysis,” Env. Sci. & Tech.,
19(11), 1122-1128 (1985).
Gossett, J.M. “Measurement of Henry's law constants for C1 and C2 chlorinated
hydrocarbons,” Env. Sci. & Tech., 21(2), 202 (1987).
Peterson, M.S., L.W. Lion, and C.A. Shoemaker, “Influence of vapor phase sorption
and diffusion on the fate of trichloroethylene in an unsaturated aquifer system.”
Env. Sci. & Tech. 22(5), 571-578, (1988).
Volatile Organic Carbon Sorption to Soil
140
Symbol List
Symbol
H LG
Definition
Henry’s Constant
K LS
Liquid/solid partitioning coefficient
K GS
Gas/solid partitioning coefficient
B
C
E
f
k
Koc
Solute binding intensity
Concentration
Dispersion coefficient
Mass fraction
Boltzmann constant
Liquid/organic carbon partitioning
coefficient
octanol-water partitioning coefficients
Mass
Partial pressure
Saturated vapor pressure
Universal gas constant
Retardation factor
o
Absolute Temperature( K).
Temperature
Time
Groundwater pore velocity
Volume
Mass sorbed at the solid-liquid interface
Distance
Mass of solute sorbed per mass of solid
vapor-adsorbent surface interaction
vaporization energy of the organic
Kow
M
P
P0
R
R
T
T
t
U
V
X
x
v
Porosity
Volumetric moisture content
Density of water
Mass sorbed at liquid-air interface +
condensation
Subscript
b
G
L
M
oc
S
sc
uc
VOC
Definition
Bulk
Gas phase
Aqueous phase
Monolayer
Organic carbon
Soil phase or sorbent
Saturated control
Unsaturated control
Volatile organic carbon
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
141
Lab Prep Notes
For equipment list and gas
chromatograph method see page 124.
Table 5. Reagents list
Description
Setup
Source
Catalog
number
03008-1
O299-1
T324-500
Octane
Fisher Scientific
1) Replace injection port septa on all
Acetone
Fisher Scientific
GC’s.
toluene
Fisher Scientific
2) Verify that GC’s are working
Potting soil
Agway (remove
large particles
properly by injecting gas samples
by
screening to
from each VOC source bottle. If
2 mm)
the baseline is above 30 (as read
on the computer display) then heat the oven to 200°C to clean the column.
3) Verify that sufficient gas is in the gas cylinders (hydrogen, air, nitrogen).
4) Verify that sufficient vials/aluminum crimp tops and Teflon seals are available.
5) Prepare VOC source vials that contain liquid acetone, octane, and toluene (they
can be shared by two groups of students).
6) Fill repipet dispensors with tap water and place on each lab bench.
Table 6. Each group of students requires the following syringes
Octane (gas)
Toluene (gas)
Acetone (gas)
Acetone (liquid)
Source vial (gas)
Isotherm (gas)
1 mL
1 mL
1 mL
0.5 mL
0.5 mL
0.5 mL
Syringe clean up
Disassemble and heat syringes to 45°C overnight to remove residual VOCs. Place
syringe barrels upside down on top of openings above fan in oven to facilitate mass
transfer.
Volatile Organic Carbon Sorption to Soil
142
Enhanced Filtration
Introduction
Slow sand filters have been used to remove particles from drinking water since the
early 1800's. Although slow sand filtration is an old technology, the mechanisms
responsible for particle removal are not well understood. Because slow sand filter
performance gradually increases with time, it has often been assumed that the growth
of biofilms is responsible for the gradual improvement in filter performance.
Research conducted at Cornell suggests that biofilms are not responsible for
significant particle removal and that most particles are removed by physical-chemical
mechanisms.
The particles that are captured on slow sand filters have been shown to
significantly improve filter performance (Weber-Shirk and Dick, 1997). More recent
research has shown that a filter aid can be extracted under acid conditions from
particles harvested from Cayuga Lake or from Cayuga Lake sediment. The filter aid
has been shown to greatly enhance bacteria removal. The filter aid is soluble at very
low pH, and forms floc at neutral pH. This naturally occurring filter aid may be able
to improve rapid sand filter performance.
Theory
In new slow sand filters with clean filter media, particles are initially removed by
attaching to the filter media. However, as the filter media begins to be covered with
removed particles, particles begin to attach to previously removed particles. If
particle-particle interaction is more favorable than particle-media interaction then
particle removal efficiency increases as the media becomes covered with particles.
This improvement in filter performance with time is commonly observed in slow sand
filters and is referred to as filter “ripening.” Filter ripening often takes several weeks
to several months for new slow sand filters. Slow sand filters that operate with
pristine water sources may never achieve efficient particle removal because the lack
of particles in the source water results in a sparse coating of the filter media.
Potential mechanisms of particle removal by slow sand filters are summarized in
Figure 1. Physical-chemical removal mechanisms are responsible for most of the
particle removal that occurs in slow sand filters. The one exception is that suspensionfeeding nanoflagellates attached to the filter media can capture a significant fraction
of bacteria (Weber-Shirk and Dick, 1999). Thus, bacteria removal by suspension
feeding predators is significant provided the influent bacteria concentration is
sufficient to maintain a large predator population. Biofilms on the filter media have
not been shown to significantly increase particle removal.
Straining of bacteria-sized particles by the filter media and attachment of
bacteria-sized particles to the filter media were shown to not be significant because
the removal of bacteria by a clean filter column was negligible (Weber-Shirk and
Dick, 1997). It is possible that straining becomes significant as filters clog and pores
become smaller. Attachment of particles to previously removed particles is
considered likely.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
143
Physicalchemical
filter
by medium
Straining
ripening may be the
(fluid and
gravitational
by
result
of
the
previously
forces)
removed
changes in pore
particles
Physical-Chemical
geometry
that
to medium
enhance straining or
Attachment
the modification of
(electrochemical
Particle
to previously
forces)
filter media surfaces
Removal
removed
particles
Mechanisms
that enhance the
Attachment to
ability of particles
biofilms
to
attach.
Biological
Suspension
feeders
Decreasing the pore
Capture by
predators
size to enhance
Grazers
straining is not a
reasonable way to Figure 1. Particle removal mechanisms that potentially could
improve
particle be operative in slow sand filters.
removal because the
head loss through
the filter increases rapidly as the pore size decreases. Thus, the best way to enhance
physical-chemical ripening is to modify filter media surfaces. The filter aid may act
by coating the surface and providing more favorable attachment sites.
Filtration theory suggests that particle removal will be first order with respect to
depth if the filter media is homogeneous (Iwasaki, 1937). In equation form the
relationship between particle concentration, C, and depth is given by
dC
C
dz
12.1
where is the filter coefficient with units of [1/L]. Setting appropriate integration
limits
C0
C
0
dC
dz
C
L
12.2
where L is the depth of the filter bed and Co is the influent particle concentration and
integrating gives:
ln
C
L
C0
Enhanced Filtration
12.3
144
Equation 12.3 can be used to evaluate the filter coefficient, . A list of previously
measured filtration constants
is given in Table 1.
Filtration theory suggests Table 1. Typical values of filter coefficients adapted
that filter performance would
from (Tien and Payatakes, 1979).
be optimal if the filter aid
Filter
Grain
particle
particle approach
were
applied
uniformly
medium
size
type
size
velocity
(1/cm)
throughout the filter. Uniform
(mm)
(µm)
(cm/hr)
Calcium
??
Ferric
10
500
0.1
application
is
difficult,
floc
however, because the filter carbonate
Calcium
??
Ferric
10
1000
0.044
aid will be captured first order
carbonate
floc
with respect to depth if the Anthracite 0.77
Quartz
2-22
500
0.064
filter aid is applied using
powder
normal down flow operation.
Sand
0.54
Chlorell
5
500
0.34
a
It may be possible to apply
Sand
0.647
Fuller’s
6
470
0.363
the filter aid during a gentle
earth
backwash thus enabling the
Granular
0.594
Clay
4-40
500
0.102
filter aid to distribute more
carbon
uniformly.
Application
techniques that optimize the filter aid distribution require further study.
Previous Research Results
Previous research (Weber-Shirk and Dick, 1997) has shown that Cayuga Lake
water particles can enhance filter performance and thus Cayuga Lake particles (CLP)
from the Bolton Point Water Treatment Plant sedimentation basin were tested. Three
filters were treated with 30 mL of concentrated CLP suspension from the Bolton
Point Water Treatment Plant. One filter had the CLP mixed throughout the filter bed,
one filter had the CLP mixed throughout the top 2 cm of the filter bed, and one filter
had the CLP applied only to the top of the filter bed. The three application techniques
were used because particles may improve filtration efficiency by providing surfaces
to which bacteria attach more readily or because the pores within the sediment are
smaller and thus more effective at straining particles. The filter with the particles
distributed throughout the filter bed performed the best with approximately 99%
removal of kaolin compared with 96% removal for the filter with the CLP on top of
the filter bed. This result suggested that kaolin was being removed by attaching to
CLP rather than by straining. CLP from the Bolton Point facility contain alum and
possibly other polymers used in the water treatment process.
Previous research also indicated that an acid treatment of Cayuga Lake sediment
dissolves species that flocculate and attach to filter media at neutral pH. This Cayuga
Lake Sediment Extract (CLSE) has been shown to rapidly ripen slow sand filters and
achieve up to 6 log (99.9999%) removal of E. coli. The CLSE has also been shown to
enhance E. coli removal at rapid sand filtration rates.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
145
Filter Performance Evaluation
Several measurement techniques could be used to characterize filter performance.
Particle concentrations could be measured using a particle counter, or measured
indirectly using a turbidimeter. If the particle suspension absorbed a significant
amount of light, a spectrophotometer could be used. A microscope could be used to
count particles. If microorganisms are used as the source particles, they could be
enumerated using standard microbiological techniques such as membrane filtration
followed by growth on selective media.
Turbidimeters measure the amount of light scatter caused by a suspension of
particles. Because absorption and scattering of light are influenced by both size and
surface characteristics of the
suspended material, turbidity is not
a direct quantitative measurement of
90° Detector
Transmitted
the concentration of suspended
light detector
solids. In a turbidimeter the
scattered light (measured at a right
angle to the incident light) and the
transmitted light intensities are
Sample
Lamp
cell
measured (Figure 2). The ratio of
Lens
scattered light to transmitted light is
proportional to the turbidity of the Figure 2. Light path in a turbidimeter.
sample.
The
constant
of
proportionality is determined by
measuring a known standard.
Experimental Objectives
The purpose of this research is to evaluate the ability of the CLSE filter aid to
enhance particle removal in a filter operating at rapid sand filtration rates. We will
use tap water amended with kaolin, 2.5 cm diameter filter columns, and turbidimeters.
Students will assembly the apparatus.
Experimental Methods
1) Setup 2.5 cm diameter filter column plumbing (Make all connections firmly and
verify that the connections can’t be pulled apart) including 1 L of clay suspension
on a stirrer, peristaltic pump for metering in clay suspension and filter aid, flow
meter, pressure reducing valve, and pressure sensor for head loss.
2) Add 8 cm of sand to the filter column (by mass).
3) Carefully observe the sand surface as you gradually increase the flow rate from
zero in backwash mode. Measure the pressure required to begin to lift the bed.
Continue backwashing the filter to clean the sand until the effluent turbidity is
less than 0.5 NTU
Enhanced Filtration
146
4) Obtain head loss (in cm) as a function of flow rate (down flow mode) over a range
of 1 to 25 m/hr (8.2 to 204 mL/min) using at least 5 data points. Use the rotometer
to measure the flow rate.
5) Challenge the filter with a kaolin suspension (approximately 5 NTU) for 30
minutes to determine baseline filter performance.
6) Backwash the filter
7) Add the filter aid (the amount and method of application will be discussed during
lab)
8) Set the down flow rate to 5 m/hr.
9) Measure the head loss to see if the filter aid increased the head loss
10) Pump a clay suspension into the filter influent so that the influent concentration is
10-mg/L kaolin. Measure effluent turbidity and head loss as a function of time for
30 minutes. Take
turbidity
measurements every 5
minutes and measure
the
head
loss
continuously
using
the Signal Monitor
software.
11) Backwash the filter.
12) If you have time test
the filter again to see
if the filter aid
improved
filter
performance
even
after backwashing.
Figure 3. Picture of experiment setup.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
147
Table 2. Filtration parameters.
parameter
approach velocity
column diameter
symbol
Va
d
value units
5.0 m/hr
2.5 cm
2
column area
A
4.9 cm
Column length
Lcolumn
20.0 cm
Media depth
L
8.0 cm
3
bulk density of media
bulkdensity
1650 kg/m
mass of media
sandmass
64.8 g
Backwash velocity
Vb
50.0 m/hr
flowrate (forward)
Qd
40.9 mL/min
flowrate (backwash)
Qb
409.1 mL/min
Influent clay concentration C0
10.0 mg/L
dilution factor
dilution
100
clay stock concentration Cconcentrate
1000 mg/L
clay stock flowrate
Qc
0.41 mL/min
media residence time
thetam
0.96 min
total residence time
thetac
2.4 min
Prelab Questions
1) How much water is required to operate one of the laboratory filters for 2 hours?
Don’t include the water required to fill the filter initially.
2) Given the dimensions for the filter column, a glass density of 2.65 g/cm3, and
filter porosity of 0.4, estimate the mass of glass beads in one filter column. (Show
your calculations.)
3) Draw a plumbing schematic of a filter column that allows you to do the following:
Measure the pressure drop across the column using a pressure sensor,
reverse the flow of water for backwash,
and maintain a high pressure in the filter column to avoid dissolution of gasses.
4) Explain how you will switch the filter from down flow to back wash mode.
Data Analysis
1) Compare the pressure required to begin to lift the bed with the calculated value
based on fluid statics.
2) Plot head loss vs. flow rate for a clean bed and estimate the hydraulic conductivity
of the sand. Is the flow laminar or turbulent? What technique did you use to
determine the flow regime?
3) Plot the fraction of influent particles remaining in the effluent vs. time for each
run on a single graph.
4) Plot head loss as a function of time for each run on a single graph.
5) Calculate the filter coefficient (equation 12.3) for the filter with and without the
filter aid.
Enhanced Filtration
148
Questions for Discussion
1) Did the filter aid make a significant difference in filter performance?
2) How was the head loss affected by the addition of the filter aid?
3) The laboratory filter columns were 8 cm deep. Rapid sand filters have 60 cm of
media. Estimate the fractional bacteria removal for a 60 cm deep filter of media.
What assumptions did you make to predict the performance of a 60 cm column?
4) What further experimentation do you recommend?
References
Iwasaki, T. 1937. “Some Notes on Sand Filtration” Journal American Water Works
Association 29: 1591.
Liljestrand, H. M.; I. M. C. Lo and Y. Shimizu. 1992. “Sorption of humic materials
onto inorganic surfaces for the mitigation of facilitated pollutant transport
processes” Proceedings Of The Sixteenth Biennial Conference Of The
International Association On Water Pollution Research And Control, Washington,
D.C., USA, May 26(1-11): 1221-1228.
Tien, C. and A. Payatakes. 1979. “Advances in Deep Bed Filtration” AIChE Journal
25(5): 737.
Weber-Shirk, M., and R. I. Dick. 1997. Physical-Chemical Mechanisms in Slow Sand
Filters. Jour. AWWA. 89:87-100.
Weber-Shirk, M. L. and R. I. Dick (1999). “Bacterivory by a Chrysophyte in Slow
Sand Filters.” Water Research 33(3): 631-638.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
149
Lab Prep Notes
Setup
Table 3. Equipment list
1) Attach two Easy-Load pump
heads to the pump drives.
2) Setup turbidimeters and verify
that the vials are clean.
Description
Supplier
magnetic stirrer
variable flow
digital drive
Easy-Load
pump head
Filter columns
100-1095 µl
pipette
10-109.5 µl
pipette
2100P
Turbidimeter
2100N
Turbidimeter
high pressure
flow cell
20 liter HDPE
Jerrican
Fisher Scientific
Cole Parmer
Catalog
number
11-500-7S
H-07523-30
Cole Parmer
H-07518-02
Fisher Scientific
13-707-5
Fisher Scientific
13-707-3
Hach Company
46500-00
Hach Company
47000-00
Hach Company
47451-0
Fisher Scientific
02-961-50C
Enhanced Filtration
150
Gas Transfer
Introduction
Exchange of gases between aqueous and gaseous phases is an essential element of
many environmental processes. Wastewater treatment plants require enhanced
transfer of oxygen into activated sludge tanks to maintain aerobic degradation. Water
treatment plants require gas transfer to dissolve chlorine gas or ozone. Gas transfer
can also be used to remove unwanted volatile chemicals such as carbon tetrachloride,
tetrachloroethylene, trichloroethylene, chloroform, bromdichloromethane, and
bromoform from water (Zander et al., 1989). Exchange of a dissolved compound with
the atmosphere is controlled by the extent of mixing in the aqueous and gaseous
phase, the surface area of the interface, the concentration of the compound in the two
phases, and the equilibrium distribution of the compound. Technologies that have
been developed to enhance gas transfer include: aeration diffusers, packed-tower air
stripping, and membrane stripping. Each of these technologies creates a high interface
surface area to enhance gas transfer.
Theory
Oxygen transfer is important in many environmental systems. Oxygen transfer is
controlled by the partial pressure of oxygen in the atmosphere (0.21 atm) and the
corresponding equilibrium concentration in water (approximately 10 mg/L).
According to Henry’s Law, the equilibrium concentration of oxygen in water is
proportional to the partial pressure of oxygen in the atmosphere.
Natural bodies of water may be either supersaturated or undersaturated with
oxygen depending on the relative magnitude of the sources and sinks of oxygen.
Algae can be a significant source of oxygen during active photosynthesis and can
produce supersaturation. Algae also deplete oxygen levels during the night.
At high levels of supersaturation dissolved gas will form microbubbles that
eventually coalesce, rise, and burst at the water surface. The bubbles provide a very
efficient transfer of supersaturated dissolved gas to the gaseous phase, a process that
can be observed when the partial pressure of carbon dioxide is decreased by opening
a carbonated beverage. Bubble formation by supersaturated gasses also occurs in the
environment when cold water in equilibrium with the atmosphere is warmed rapidly.
The equilibrium dissolved oxygen concentration is a function of temperature and as
the water is warmed the equilibrium concentration decreases (Figure 1).
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
151
Oxygen (ppm)
Supersaturation of dissolved gases
can also occur when water carrying gas
bubbles from a water fall or spillway
12
plunges into a deep pool. The pressure
11
increases with depth in the pool and
10
gasses carried deep into the pool
9
dissolve in the water. When the water
8
eventually approaches the surface the
7
pressure decreases and the dissolved
6
gases come out of solution and form
10
20
30
40
bubbles.
Bubble
formation
by
supersaturated gases can kill fish
Temperature (°C)
(similar to the “bends” in humans) as
the bubbles form in the bloodstream.
Figure 1. Dissolved oxygen concentrations
Gas transfer rate can be modeled as
in equilibrium with the atmosphere as a
the product of a driving force (the
function of water temperature.
difference between the equilibrium
concentration
and
the
actual
concentration)
and
an
overall
volumetric gas transfer coefficient (a function of the geometry, mixing levels of the
system and the solubility of the compound). In equation form
dC ˆ
k v ,l C * C
dt
13.1
where C is the dissolved gas concentration, C* is the equilibrium dissolved gas
concentration and kˆv ,l is the overall volumetric gas transfer coefficient . Although kˆv ,l
has dimensions of 1/T, it is a function of the interface surface area (A), the liquid
volume (V), the oxygen diffusion coefficient in water (D), and the thickness of the
laminar boundary layer () through which the gas must diffuse before the much faster
turbulent mixing process can disperse the dissolved gas throughout the reactor.
kˆv,l f ( D, , A,V )
13.2
The overall volumetric gas transfer
coefficient is system specific and thus
must be evaluated separately for each
oxygen gas
system of interest (Weber and
Digiano, 1996).
A schematic of the gas transfer
dissolv ed
process is shown in Figure 2. Fickian
oxygen
V
/
A
diffusion controls the gas transfer in
the laminar boundary layer. The
C
oxygen concentration in the bulk of
C*
the fluid is assumed to be
homogeneous due to turbulent mixing Figure 2. Single film model of interphase
and the oxygen concentration above mass transfer of oxygen.
Gas Transfer
152
the liquid is assumed to be that of the atmosphere.
The gas transfer coefficient will increase with the interface area and the diffusion
coefficient and will decrease with the reactor volume and the thickness of the
boundary layer. The functional form of the relationship is given by
AD
kˆv ,l
V
13.3
Equation 13.1 can be integrated with appropriate initial conditions to obtain the
concentration of oxygen as a function of time. However, care must be taken to ensure
that the overall volumetric gas transfer coefficient is not a function of the dissolved
oxygen concentration. This dependency can occur where air is pumped through
diffusers on the bottom of activated sludge tanks. Rising air bubbles are significantly
depleted of oxygen as they rise through the activated sludge tank and the extent of
oxygen depletion is a function of the concentration of oxygen in the activated sludge.
Integrating equation 13.1 with initial conditions of C = C0 at t = t0
C
ln
t
dC
C C* C t kˆv,l dt
0
0
13.4
C* C
kˆv ,l (t t0 )
*
C C0
13.5
This equation can be linearized so that kˆv ,l is the slope of the line.
ln C * C kˆv,l t ln C * C0 kˆv,l t0
13.6
The simple gas transfer model given in equation 13.5 is appropriate when the gas
transfer coefficient is independent of the dissolved gas concentration. This
requirement can be met in systems where the gas bubbles do not change
concentration significantly as they rise through the water column. This condition is
met when the water column is shallow, the bubbles have large diameters, or the
difference between the concentration of dissolved gas and the equilibrium
concentration is small.
Experimental Objectives
The objectives of this lab are to:
1) illustrate the dependence of gas transfer on gas flow rate.
2) develop a functional relationship between gas flow rate and gas transfer.
3) Explain the theory and use of dissolved oxygen probes. See the appendix of this
manual (page 158) for information on how the dissolved oxygen probe works and
how to calibrate it.
A small reactor that meets the conditions of a constant gas transfer coefficient will
be used to characterize the dependence of the gas transfer coefficient on the gas flow
rate through a simple diffuser. The gas transfer coefficient is a function of the gas
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
153
flow rate because the interface surface area (i.e. the surface area of the air bubbles)
increases as the gas flow rate increases.
In order to measure the reaeration rate it is necessary to first remove the oxygen
from the reactor. This can be accomplished by bubbling the solution with a gas that
contains no oxygen. Nitrogen gas is typically used to remove oxygen from laboratory
reactors. Alternately, a reductant can be used. Sulfite is a strong reductant that will
reduce dissolved oxygen in the presence of a catalyst.
cobalt
O 2 2SO32
2SO 42
13.7
If complete deoxygenation is desired a 10% excess of sulfite can be added. The
sulfite will continue to react with oxygen as oxygen is transferred into the solution.
The oxygen concentration can be measured with a dissolved oxygen probe or can be
estimated if the temperature is known and equilibrium with the atmosphere assumed
(Figure 1).
Experimental Methods
The reactor is a 250 mL
polypropylene narrow mouth bottle
with an additional port drilled near the
neck to allow a hypodermic tubing
diffuser to extend into the bottle
Hypodermic diffuser
(Figure 3). The hypodermic tubing
DO probe
diffuser consists of 4 stainless steel
100 mL beaker
hypodermic tubes that are glued to a
plug that mates to 1/4” tubing
Water surface
connectors. The hypodermic tubes are
Stirbar
bent so that they are spaced at 90°
intervals around the perimeter of the
bottle. The DO probe funnel/mixer
assembly is inserted into the bottle
opening. The bottle is placed on a
magnetic stirrer and the DO probe is
inserted
into
the
funnel/mixer Figure 3. Apparatus used to measure
assembly. The spacing of the reaeration rate.
hypodermic tubing at the perimeter of
the bottle is designed to prevent rising
bubbles from touching the DO probe membrane. The hypodermic diffuser is
connected to a peristaltic pump (or other source of regulated air flow).
1) Calibrate the DO probe (See page 159).
2) Prepare to monitor the dissolved oxygen concentration using the Compumet™
software. Use 5 second data intervals and monitor pH on channel A. The
Accumet™ meter and the Compumet™ software think the dissolved oxygen
probe is a pH probe. Although the output of the meter is pH, it should be
interpreted as mg/L of dissolved oxygen. See page 157 for instructions on using
Compumet™ software.
Gas Transfer
154
3) Add 50 mL of distilled water to a 100 mL beaker.
4) Set the stirrer speed to 5.
5) Add ≈10 mg CoCl2· 6H2O (note this only needs to be added once because it is the
catalyst). A stock solution of CoCl2· 6H2O (100 mg/mL – thus add 100 L) has
been prepared to facilitate measurement of small cobalt doses.
6) Set the air flowrate to 50 mL/min (or to the desired flow rate).
7) Turn the air off.
8) Add enough sodium sulfite to deoxygenate the solution. A stock solution of
sodium sulfite (100 mg/mL) has been prepared to facilitate measurement of small
sulfite doses. (50 mL of water at 10 mg O2/L = 0.5 mg O2, therefore add 5 mg
sodium sulfate or 20 L of stock solution.)
9) Turn the air on and start collecting data using the Compumet™ software.
10) Monitor the dissolved oxygen concentration until it reaches 80% of saturation
value.
11) Save the data as \\Enviro\enviro\Courses\453\gastran\netid_100 for later analysis.
Repeat steps 6-11 using flow rates of 100, 200, 300, and 500 mL/min.
Prelab Questions
1) Calculate the mass of sodium sulfite needed to reduce all the dissolved oxygen in
50 mL of pure water in equilibrium with the atmosphere and at 30°C.
2) Sketch your expectations for dissolved oxygen concentration as a function of time
for the flow rates used on a single graph. The graph can be done by hand and
doesn’t need to have any numbers on the time scale.
3) Sketch your expectations for kˆ as a function of gas flow rate. Do you expect a
v ,l
perfectly straight line or do you expect some nonlinearities? Why? What do you
expect kˆv ,l to be when the gas flow rate is zero?
Data Analysis
1) Eliminate the data from each data set when the dissolved oxygen concentration
was less than 0.5 mg/L. This will ensure that all of the sulfite has reacted.
2) Set t0 to the time at the beginning of the remaining data. Subtract t0 from each of
the times so the remaining data now starts at zero.
3) Plot the 5 data sets with the corrected times on a single graph.
4) Estimate kˆ using linear regression and equation 13.6 for each data set. Show a
v ,l
graph with the linearized data and the best fit lines.
5) Graph kˆv ,l as a function of gas flow rate.
6) Comment on results and compare with your expectations and with theory.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
155
References
Weber, W. J. J. and F. A. Digiano. 1996. Process Dynamics in Environmental
Systems. New York, John Wiley & Sons, Inc.
Zander, A. K.; M. J. Semmens and R. M. Narbaitz. 1989. “Removing VOCs by
membrane stripping” American Water Works Association Journal 81(11): 76-81.
Gas Transfer
156
Lab Prep Notes
Setup
1) Prepare the sodium sulfite
immediately before class and
distribute to groups in 15 mL PP
bottles to minimize oxygen
dissolution and reaction with the
sulfite.
2) The cobalt solution can be
prepared anytime and stored long
term. Distribute to student
stations in 15 mL PP bottles.
3) Attach two Easy-Load pump
heads to the pump drives and
plumb with size 18 tubing joined
and connected to the hypodermic
diffuser.
4) Verify that DO probes are
operational, stable, and can be
calibrated. The solution behind
the membranes should be clear
and free of bubbles. If necessary
replace membranes.
5) Mount DO probes on magnetic
stirrers.
Table 1. Reagent list
Description
Supplier
Na2SO3
CoCl2· 6H2O
Fisher Scientific
Fisher Scientific
Catalog
number
S430-500
C371-100
Table 2. Stock solutions list
reagent
Na2SO3
CoCl2·
6H2O
M.W.
g/100
mL
126.04 10 g
237.92 10 g
mg/
mL
100
100
mL/
group
10
1
solubility
g/L
125
770
Table 3. Equipment list
Description
Supplier
magnetic stirrer
100-1095 µL
pipette
10-109.5 µL
pipette
15 mL PP
bottles
variable flow
digital drive
Easy-Load
pump head
PharMed tubing
# 18
4 prong
hypodermic
tubing diffuser
1/4” plug
1/4” union
stainless steel
hypodermic
tubing
Fisher Scientific
Fisher Scientific
Catalog
number
11-500-7S
13-707-5
Fisher Scientific
13-707-3
Fisher Scientific
02-923-8G
Cole Parmer
H-07523-30
Cole Parmer
H-07518-00
Cole Parmer
H-06485-18
CEE 453: Laboratory Research in Environmental Engineering
CEE shop
Cole Parmer
Cole Parmer
McMaster Carr
H-06372-50
H-06372-50
Spring 2001
157
Instrument Instructions
Compumet software
Information on use of the Compumet software
http://www.cee.cornell.edu/mws/Software/Compumet.htm.
is
available
at
pH Probe Calibration
Select U.S. Standard Buffers
From the main screen, press the setup key. Setup 1 menu is shown.
1) Press the 1 key to select Buffer Recognition. Observe display of Setup Buffer
Recognition.
2) Press the 1 key again to select U.S. standard buffers.
3) Press the enter key to implement the selection.
4) Press the clear key to return to the main screen.
Full Calibration (done once a day)
This will calibrate both the slope and the null point. Use either 2 or 3 buffers.
Clear Existing Buffers
Press the pH key to select pH measurement. Observe display of the current pH
standardization points.
1) Press the standardization key. A menu of standardization options appears.
2) Press the 2 key to select Clear Existing Standards.
3) The meter returns to the main screen, but with all pH standardization points (for
the current channel) cleared from memory. The electrode slope is reset to 100% or
59.16 mV/pH.
Add Buffers (can use buffers at pH of 4, 7, and 10)
1) Press the standardization key. A menu of standardization options is displayed.
2) Press the 1 key to select Update or Add a Standard.
3) The Prepare Buffer/Standard screen appears. Make sure the electrode is in the
buffer solution. Press the enter key.
4) The meter will wait until an electrode stability criterion is reached. Then it will
automatically read the signal and calibrate.
5) The meter returns to the main screen with the added buffer point shown.
Repeat these steps for each of the buffers.
Updating the Standardization (done every hour)
Use a single buffer to adjust the null point of the calibration curve. The calibration
slope will remain the same as from the last full calibration.
Press the standardization key. A menu of standardization options is displayed.
Instrument Instructions
158
1) Press the 1 key to select Update or Add a Standard.
2) The Prepare Buffer/Standard screen appears. Make sure the electrode(s) is in the
buffer solution. Press enter.
3) The meter will wait until a pre-determined electrode stability is reached, then it
will automatically read the signal and calibrate.
pH Probe Storage
Preparation for storage
1) Rinse with distilled water
2) Air dry
3) Store for months as needed
Preparation for use
1.
2.
3.
4.
Heat in 60°C water and stir for 15 minutes
Place in pH buffer 4 or 7 at room temperature for 15 minutes
Standardize
Probes that fail to standardize may require cleaning
Procedure for Cleaning pH Gel-Filled Polymer Electrode
1. Warm distilled water to 40 - 60 C
2. Suspend the electrode in the warm water for about 15 min while stirring with a
magnetic stirrer. This will loosen any material attached to the probe
3. Add 1/2 tsp of detergent Terg-A-Zyme to the water. Keep stirring for 15 min
4. Rinse well with distilled water
5. Store probe in pH 4 buffer for at least 3 hr (the longer the better). Although less
preferable, the probe can also be stored in pH 7 buffer
6. Standardize
7. Probes that fail to standardize may need to be discarded
Note: Terg-A-Zyme - Fischer cat. no. 04-322--11A.
Dissolved Oxygen Probe
Theory
The probe makes use of the fact that an applied potential of 0.8 V can reduce O2 to
H2O:
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
159
4e- + 4H+ + O2 2H2 O
13.8
The cell is separated from solution by a gas permeable membrane that allows O 2 to
pass through (Figure 1). The concentration of O2 in the cell is kept very low by
reduction to H2O. The rate at which oxygen diffuses through the gas permeable
membrane is proportional to the difference in oxygen concentration across the
membrane. The concentration of oxygen in the cell is ≈0 and thus the rate at which
oxygen diffuses through the membrane is proportional to the oxygen concentration in
the solution.
Oxygen is reduced to water at a silver (Ag) cathode of the probe. Oxygen reduction
produces a current that is measured by the meter.
The probe is calibrated using a two-point calibration and can be temperature
compensated
if
desired.
Directions
can
be
obtained
at
http://www.cee.cornell.edu/mws/Software/DOcal.htm.
Monitoring
The dissolved oxygen probe output is in
the Pico amp (10-12 A) range. This current
must be converted to a voltage in order to
be measured by the laboratory data
acquisition system. An analog circuit is
used to convert the dissolved oxygen
probe into a voltage. The voltage is
monitored using the Signal Monitor
software. Information on using the Signal
Monitor software is available at
http://www.cee.cornell.edu/mws/Software/
signal_monitor.htm.
The 8-pin plug from the signalconditioning box that is attached to the
dissolved oxygen probe needs to be
plugged into the middle row of ports on
the lab bench.
Figure 1. Schematic
oxygen probe.
Calibration
of
dissolved
1) A calibration routine is available in the
Signal Monitor software. Follow the instructions in the software and use the help
as needed. http://www.cee.cornell.edu/mws/Software/DOcal.htm.
Dissolved Oxygen Probe Storage
1) Remove membrane from probe.
2) Rinse electrode with distilled water.
3) Store electrode covered with electrode cap (dry).
Instrument Instructions
160
Gas Chromatograph
Information on the spectrophotometer analysis software is available at
http://www.cee.cornell.edu/mws/Software/gas_chromatograph.htm.
UV-Vis Spectrophotometer
Information on the spectrophotometer analysis software is available at
http://www.cee.cornell.edu/mws/Software/Spectrophotometer.htm.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
161
Index
gasoline, 119
global warming, 87
Gran plot, 53, 58, 59, 61, 62
Gran Plot, 53, 58
groundwater, 33, 119, 130
A
absorption, 20, 71, 72, 78, 145
acid neutralizing capacity, 43, 45, 46
acid precipitation, 43, 47, 50
adsorption, 24, 70, 71, 72, 74, 75, 82, 83, 128
advective dispersion, 33, 34, 130, 131
alkalinity, 45, 50, 61
ANC, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 57,
58, 59, 60, 61, 62, 107, 108, 109
atomic absorption, 78
H
hazardous waste site, 70
Henry's Law, 46, 129
hydraulic residence time, 32, 34, 40, 44, 46, 50, 53
I
B
ion exchange, 71, 74
isotherms, 74, 127, 128, 137
BET, 127, 128
Biochemical Oxygen Demand, 88
Biodegradable, 87
Boltzmann constant, 128, 140
L
landfill, 86, 100, 115
Langmuir, 128
ligand, 75
C
carbonate system, 44, 45, 107
chelating agent, 70, 80
coefficient of variation, 23, 133, 135, 137, 138
completely mixed flow, 30, 39, 53
Compumet, 37, 38, 39, 49, 51, 62, 153, 154, 157
conductivity, 35, 36, 37, 38, 39, 41, 111, 113, 147
correlation coefficient, 21, 24, 28
M
mass balance, 33, 40, 50, 129, 131, 133
methane, 100, 101, 102, 103, 104, 105, 109, 110,
111, 113, 114, 115, 116
methylene blue, 22, 23, 24, 25, 28, 29, 77, 78, 79,
80, 81, 82
Monod, 88, 89
D
density of sodium chloride solution, 25
density of water, 129
deoxygenate, 154
diffusion, 34, 119, 131, 139, 151, 152
diode array, 21, 65, 77
dispersion coefficient, 30, 31, 32, 33, 37, 40, 130
dissolved oxygen, 87, 88, 89, 90, 92, 93, 94, 95, 96,
97, 150, 151, 152, 153, 154, 159
O
oxidant, 76
Oxidation, 83, 87, 88
oxygen deficit, 88, 90, 91, 92, 93
oxygen probe, 93, 94, 95, 152, 153, 159
oxygen sag, 88, 93, 96, 97
P
E
Peclet number, 32, 33, 40
petroleum, 119
pH probe, 48, 49, 51, 56, 60, 63, 153
pipette, 11, 23, 55, 61, 66, 69, 95, 149, 156
plug flow, 30, 32, 34, 39
Plug flow, 34
plume, 40
porous media, 33, 37, 40, 41, 55, 56, 70, 71, 73, 74,
75, 76, 77, 119, 129, 130, 131
pressure sensor, 109, 111, 113, 114, 145, 147
pump and treat, 70
endogenous respiration, 88, 89, 90, 92, 95, 96
extracellular polymer, 77, 82
extractant, 77, 78, 79, 80, 81, 82, 84
F
flow with dispersion, 30, 33
G
gas chromatograph, 9, 109, 110, 111, 113, 114, 115,
120, 123, 131, 135, 141
gas transfer, 96, 99, 150, 151, 152
Gas transfer, 150, 151
Index
162
R
reactor, 9, 30, 31, 32, 33, 34, 35, 37, 38, 39, 40, 41,
50, 53, 77, 88, 91, 94, 95, 151, 152, 153
reductant, 153
remediation, 47, 48, 70, 71, 76, 77
Remediation, 43, 47, 49, 53
retardation factor, 130, 131
S
Signal Monitor, 146, 159
slow sand filtration, 142
soil density, 122
soil gas sampling, 119, 121
soil moisture content, 122, 123, 129, 132, 137
soil porosity, 138
solvents, 12, 16, 17, 119
sorption, 71, 72, 74, 76, 77, 78, 120, 126, 127, 128,
129, 130, 131, 134, 139
spectrophotometer, 21, 22, 24, 28, 29, 65, 66, 67,
69, 77, 86, 145, 160
spectrum, 21, 66, 78
Streeter Phelps, 88
Superfund, 70
T
titration, 52, 57, 58, 59, 60, 61, 62
tracer, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41,
42
turbidimeter, 145
turbidity, 145, 146
W
watershed, 43, 46, 47, 48
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
