Chromium In the Aquatic
Environment
Polina Liberman
Sarah Schmidt
June 7, 2002
Outline
 Introduction
 Chemistry of Chromium
• Cr(III)
• Cr(VI)
• Precipitation and Dissolution
 Speciation Analysis
• On-line Methods
• Off-line Methods
 Chromium in the Environment
• Sources of Chromium
• Environmental/Health Impacts
• Example: The Great Lakes
 Conclusion
 Questions?
Introduction
In fresh waters, trace metals may exist in
various physicochemical forms. This phenomenon,
also called speciation, refers to the partitioning of
trace metals among solids, colloids, surfaces,
dissolved free ions, complexed with inorganic
ligands in the dissolved phase, and complexed with
organics in the dissolved phase.
Chromium speciation is of particular interest in
the environment due to the existence of two major
chromium species that have significantly different
environmental implications.
Chemistry of Chromium

Two common, stable oxidation states: Cr(III) & Cr(VI)
 Factors that control interconversion between species:
• concentration of Cr species
• oxidizing or reducing species
• electrochemical potentials of redox reactions
• ambient temperature
• light
• acid-base reactions
• complexing agents
• precipitation reactions
 Don’t exist as free ions Cr6+ or Cr3+
Cr(III)
 Cr(III) characteristics
• harmless trace element essential for life
• micronutrient in an organic form
• most thermodynamically stable Cr oxidation state
• hard acid
 In absence of complexing agents Cr(III) exists as hexaaquachromium(3+) Cr(H2O)63+, a moderately strong acid, and
its deprotonated forms
 CrOH2+ and Cr(OH)3(aq) are dominant forms in environment
 Forms complexes with water, ammonia, urea, ethylenediamine,
and other organic ligands containing oxygen, nitrogen or
sulphur donor atoms
Cr(III)

Cr(VI)/Cr(III) redox potential is high so oxidation of Cr(III) is
negligible without mediate species
 Sources of Oxygen needed for oxidation of Cr(III) to Cr(VI).
Most of these are not present in high enough concentrations in
natural waters to accomplish the transition.
• water (most important)
• ozone
• hydrogen Peroxide
• manganese dioxide
• lead dioxide
 Inverse relationship between Eh and pH thus Cr(III) is more
easily oxidized at higher pH
Cr(VI)

Cr(VI) characteristics
• powerful epithelial irritant
• confirmed human carcinogen
• toxic to many plants, aquatic animals, and bacteria
 Exists as chromate(CrO42-) (pH>7), HCrO4-(1<pH<7),
dichromate(Cr2O72-), or chromium trioxide(CrO3)
 In acidic solution it has a very high positive redox potential,
therefore strongly oxidizing and unstable in presence of e- donors
HCrO4- + 7H+ + 3e- = Cr3+ + 4H2O
 In basic solution reduction of CrO42- occurs
CrO42- + 4H2O + 3e- = Cr(OH)3 + 5OH-
Precipitation and Dissolution

Solubility of Cr(III) and Cr(VI) vary over many orders of
magnitude
 Cr(VI) ions are soluble at all pHs but chromate (CrO42-) can exist
as insoluble salt of a variety of divalent cations such as Ba2+, Sr2+,
Pb2+, Zn2+, and Cu2+ whose rates of precipitation vary and are pH
dependent
 Most Cr(III) water soluble species don’t occur naturally and are
unstable in the environment
• hydroxylation, which is pH dependent, is the principle
reaction of Cr(III) with the trihydroxide, Cr(OH)3, being the
least soluble.
Cr3+ + 3OH- = Cr(OH)3 logK=30
• also precipitates as (Cr,Fe)(OH)3 which has lower solubility
than Cr(OH)3 and rapid precipitation/dissolution kinetics
Eh-pH diagram
Speciation Analysis





Speciation is an analytical process consisting of identification
and quantification of various forms of a given element present
in analyzed samples
Typically includes
• sampling
• sample storage
• sample pre-treatment
• instrumental analysis
Distinction between determination of total Cr, which is less
complex and determination of Cr(VI)
There is a lack of reliable analytical procedures to extract
Cr(VI) from environmental samples without altering its
oxidation state
Cr is present in the environment at trace or ultra trace levels and
is hard to detect
Speciation Analysis

Off-line methods
• Separation and pre-concentration of Cr species are carried
out before the insertion into the detection instrument
• Spectroscopic methods are generally used for detection
 UV-Vis spectrometry
 Atomic Absorption Spectrometry (AAS)
 Electrothermal atomic absorption spectrometry (ETAAS)
 Inductively coupled plasma atomic emission
spectrometry (ICP-AES)
• Have many disadvantages
 complicated
 time consuming
 affects Cr speciation
 results often in losses of the analyte
Speciation Analysis

On-line methods
• Separation, identification, and quantification of Cr are carried
out in one-step analytical process
• Separation techniques:
 Flow-Injection Analysis (FIA)
 High performance liquid chromatography (HPLC)
• Detection techniques:
 Flame atomic absorption spectrometry
 ETAAS
 Direct current plasma atomic emission spectrometry
(DCP-AES)
 ICP-AES or ICP-MS (mass spectrometry)
Speciation Analysis

Despite advances in past 25 years much remains to be done
• need for routine Cr species analysis
• need simplification of the speciation schemes
• need to minimize perturbation of the systems
• need for accurate analysis of complexed,
protonated/deprotonated, monomeric/polymeric and
adsorbed/dissolved forms
• need for development of Cr isotope speciation
Sources of Chromium
 Natural Sources
• Weathering of rock constituents
• Wet precipitation
• Dry fallout from the atmosphere
• Runoff from terrestrial systems
 Industrial use – begins with the mining of chromite, typically ferrous
chromite (FeO·Cr3O3)
• Vast majority of the ore is oxidized or reduced and used in other
forms
 Oxidizing agents – Sodium carbonate, Calcium oxide
 Reducing agents – Aluminum, Silicon, Carbon
• Production of metal alloys makes up 70% of U.S. chromium usage
Sources of Chromium
 Examples of chromium chemicals used in industry
• Cr(VI) chemicals – Chromium trichloride (CrCl3),
Chromium nitrate (Cr(NO3)3)
• Cr(II) and Cr(III): small amounts compared with Cr(VI)
 Industrial wastewater discharge from:
• Metallurgical industries
• Electroplating/Tanning industries
• Sanitary landfill leaching
Health Impacts of Chromium

EPA Max. Contaminant Level : 0.1mg/L (total Cr)
 Routes of human exposure
• Dermal absorption
• Ingestion
• Inhalation
 Health effects of exposure include:
• Irritation of the skin
 Dermatosis (skin ulceration)
 Dermatisis (allgeric sensitization)
• Respiratory problems
 Respiratory cancers (caused by CaCrO4)
 Ulceration/perforation of nasal septum
 Irritation of upper airways
Environmental Impact of
Chromium

Chemical speciation greatly affects chromium transport within
land and water systems
• Efficient adsorption of metals by soils limits Cr input to the
atmosphere
• Cr(VI) is the most mobile form of Cr in soil and water
systems
• Redox conversion from Cr(III) to Cr(VI) increases Cr
dislocation from soil to water systems
 Transport of Cr in various types of natural water systems is
controlled by specific conditions pertaining to each system.
Such conditions are temperature, depth, degree of mixing,
amount or organic matter present
Environmental Impact of
Chromium

Differences in transport mechanisms for various natural water
systems
• OCEANS
 Oceans receive Cr from two sources – rivers, atmosphere
 Precipitated and dissolved Cr exist in equilibrium
 Dissolved Cr is lost from oceanic water via incorporation
into biologic material
 Dissolved Cr also lost through adsorption onto sediment
particles
 Dissolution of this incorporated Cr occurs both in the
water column, and the sediment-water interface
Environmental Impact of
Chromium

Differences in transport mechanisms for various natural water systems
• LAKES
 Higher biological activity, greater ratio of sediment-to-water
surface area
 High organic matter supports a reductive and complexing
environment, favoring Cr(VI)
 Very transient mixing/transport features compared to oceans
 Lower dissolved solids; higher particulate loads
 More influenced by river and industrial inputs than oceans
 In anoxic lakes, both concentration and speciation vary with
depth and season. Sunlight affects the redox reactions of
chromium. Specifically, sunlight degrades organic chromium
and releases inorganic chromium
Natural Example: Great Lakes

A 1993 study examined chromium concentrations in Lake
Superior, Lake Erie, and Lake Ontario
 Shows that Cr(VI) is the dominant species (75%-85% of the
total chromium concentration)
 Particulate Cr and Cr(III) concentrations below detection
 Only under strongly reducing conditions was there a significant
formation of Cr(III)
 Concentration of colloidal/organic Cr is approximately 10% of
the total dissolved Cr in the lake water
 As expected, high Cr concentrations in the lakes occur at key
locations where industrial discharge is high (Thunder Bay and
Sault Ste. Marie in Lake Superior, Cleveland and
Detroit/Windsor for Lake Erie)
Conclusion
The chemistry of chromium yields two common forms
stable in the environment, Cr(VI) and Cr(III). Chromium
speciation plays a significant role from an environmental
standpoint, because these two forms have considerably
different environmental impacts. Analyzing chromium
speciation and its behavior in various aquatic systems is
important in order to study the effect the metal has on the
natural environment and on human health. Monitoring any
deleterious effects of industrial discharge is of particular
significance because it is a source of chromium that can be
controlled with regulations. With further technological
developments, Cr species analysis will be improved so that
speciation can be determined with more accuracy.
Chromium Speciation
Questions??