Coagulation of organic color with ferric chloride
by C Hunter Nolen, jr
A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in
Environmental Health Engineering
Montana State University
© Copyright by C Hunter Nolen, jr (1984)
Abstract:
This study investigates the conditions of pH and ferric chloride dosage required for the removal of
organic color from water.
The results of this laboratory study were obtained with the use of a jar testing apparatus. Extensive data
were collected and it was found that ferric chloride can be a very effective coagulant for color removal.
The data presented in this thesis should provide a useful tool for the operation and design of water
treatment facilities when coagulation with ferric chloride is considered. COAGULATION OF ORGANIC COLOR WITH FERRIC CHLORIDE
■ by
C. Hunter Nolen, Jr.
A thesis submitted in partial fulfillment
of the requirements for the degree
of
Master of Science
in
Environmental Health Engineering
MONTANA STATE UNIVERSITY
Bozeman, Montana
August 1984
APPROVAL
of a thesis submitted by
C. Hunter Nolen, Jr.
This thesis has been read by each member of the thesis committee and has been found
to be satisfactory regarding content, English usage, format, citation, bibliographic style,
and consistency, and is ready for submission to the College of Graduate Studies.
________ 3 J y
Date
Z+ ,
_______
QitAMuiAZj y
Chairperson, Graduate Committee
Approved for the Major Department
Approved for the College of Graduate Studies
Date
Graduate Dean
STATEMENT OF PERMISSION TO USE
In presenting this thesis in partial fulfillment of the requirements for a master’s degree
at Montana State University, I agree that the Library shall make it available to borrowers
under rules of the Library. Brief quotations from this thesis are allowable without special
permission, provided that accurate acknowledgment of source is made.
Permission for extensive quotation from or reproduction of this thesis may be granted
by my major professor, or in his absence, by the Dean of Libraries when, in the opinion of
either, the proposed use of the material is for scholarly purposes. Any copying or use of
the material in this thesis for financial gain shall not be allowed without my permission.
Signature
Date
7 -
^
iv
ACKNOWLEDGMENTS
My sincere thanks to all. the persons and institutions for the support they have given
me during this program.
Special thanks to the Department of Civil Engineering and Engineering Mechanics and
the Engineering Experiment Station at Montana State University for support for this study;
and to Dr. A. Amirtharajah for his advice and assistance.
To my parents and to my sisters, for their moral support.
V
TABLE OF CONTENTS
Page
APPROVAL........................................................................................................................
ii
STATEMENT OF PERMISSION TO USE........................................................................
iii
ACKNOWLEDGMENTS....................................................................................................
iv
TABLE OF CONTENTS....................................................................................................
v
LIST OF TABLES...............................................................................................................
vi.
LIST OF FIGURES.............................................................................................................
vii
ABSTRACT........................................................................................................................
ix
INTRODUCTION...............................................................................................................
I
NATURE OF ORGANIC COLOR.....................................................................................
2
THEORY OF COAGULATION.......................................................................................
3
Coagulation of Color..................................................................................................
4
AQUEOUS CHEMISTRY OF F e (III).......................................................................
5
MATERIALS AND METHODS.........................................................................................
9
RESULTS AND DISCUSSION.........................................................................................
14
Color R em oval...........................................................................................................
Effect of Turbidity....................................................................................................
Removal Mechanisms................................................................................................
Apparent Color Removal...........................................................................................
Turbidity Rem oval....................................................................................................
14
16
24
36
44
CONCLUSIONS........................................................................
48
REFERENCES
49
vi
LIST OF TABLES
Table
I.
Page
Color Solutions....................................................................................................
9
vii
LIST OF FIGURES
Figures
Page
1.
Fe(III) Log Concentration Diagram......................................... ; .......................
7
2.
Iron (III) Coagulation Diagram..........................................................................
8
3.
Color Removal Domain—Initial Color = 100 CU,
Turbidity = 27-30 T U .........................................................................................
11
Data Points and Corresponding Percent Color Removal Values Initial Color - 100 CU, Turbidity = 27-30 TU..................................................
13
Data Points and Corresponding Percent Color Removal Values Initial Color = 100 CU, Turbidity = 0 ...............................................................
15
Data Points and Corresponding Percent Color Removal Values Initial Color = 500 CU, Turbidity = 0 .................................................... .. . . .
17
Data Points and Corresponding Percent Color Removal Values Initial Color = 1000 CU, Turbidity = 0 .............................................................
18
Common Color Removal Domain - Initial Color. 100-1000 CU,
Turbidity = 0.........................................................................................................
19
Data Points and Corresponding Percent Color Removal Values Initial Color = 225 CU, Turbidity = 27-30 TU..................................................
20
Data Points and Corresponding Percent Color Removal Values Initial Color = 500 CU, Turbidity = 27-30 TU..................................................
21
Data Points and Corresponding Percent Color Removal Values Initial Color = 700 CU, Turbidity = 27-30 TU..................................................
22
Data Points and Corresponding Percent Color Removal Values Initial Color = 1000 CU, Turbidity = 27-30 TU................................................
23
Common Color Removal Domain - Initial Color 100-1000 CU,
Turbidity 27-30 TU...................................'..........................................................
25
14.
Color Removal Domain - Initial Color = 100 CU, Turbidity = 0 ...................
26
15.
Color Removal Domain - Initial Color = 500 CU, Turbidity = 0 ....................
27
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
viii
Figures
16.
Page
Color Removal Domain - Initial Color = 1000 CU, Turbidity
= 0..........................................................................................................................
28
Color Removal Domain - Initial Color = 100 CU, Turbidity
= 27-30 T U ...........................................................................................................
29
18. . Color Removal Domain - Initial Color = 225 CU, Turbidity
= 27-30 T U ...........................................................................................................
30
17.
19.
Color Removal Domain - Initial Color = 500 CU, Turbidity
= 27-30 T U ...........................................................................................................
31
Color Removal Domain - Initial Color = 700 CU, Turbidity
= 27-30 T U ...........................................................................................................
32
Color Removal Domain - Initial Color = 1000 CU, Turbidity
= 27-30 T U ...........................................................................................................
33
22.
Stoichiometry for the Color Removal Domains pH - 4 . 5 ...............................
34
23.
Stoichiometry for the Color Removal Domains pH - 5.5,
pH - 6 . 3 ...........................................................................................
35
Particle Charge and EM Values - Initial Color =100 CU, Turbidity
= 27-30 T U ...........................................................................................................
37
25.
Particle Charge - Initial Color =100 CU, Turbidity = 27-30 T U ....................
38
26.
Particle Charge - Initial Color = 500 CU, Turbidity = 27-30 T U ....................
39
27.
Particle Charge - Initial Color =1000 CU, Turbidity = 27-30 T U ....................
40
28.
Apparent Color Removal Domain - Initial Color =100 CU,
Turbidity = 27-30 T U ........................................................................
41
Apparent Color Removal Domain - Initial Color = 500 CU,
Turbidity = 27-30 T U .........................................................................................
42
Apparent Color Removal Domain - Initial Color = 1000 CU,
Turbidity = 27-30 T U .........................................................................................
43
Turbidity Removal Domain - Initial Color = 100 CU, Turbidity
' = 27-30 T U ...........................................................................................................
45
Turbidity Removal Domain - Initial Color = 500 CU, Turbidity
= 27-30 T U ...........................................................................................................
46
Turbidity Removal Domain - Initial Color = 1000 CU, Turbidity
= 27-30 T U ...........................................................................................................
47
20.
21.
24.
29.
30.
3 1.
32.
33.
ABSTRACT
This study investigates the conditions of pH and ferric chloride dosage required for
the removal of organic color from water.
The results of this laboratory study were obtained with the use of a jar testing appa­
ratus. Extensive data were collected and it was found that ferric chloride can be a very
effective coagulant for color removal. The data presented in this thesis should provide a
useful tool for the operation and design of water treatment facilities when coagulation
with ferric chloride is considered.
I
INTRODUCTION
Organic color, in natural water supplies, is due primarily to the presence of humic
substances, i.e., humic acid, fulvic acid, and humins. Humic substances are undesirable in a
potential water supply for a number of reasons including aesthetics, the formation of trihalomethanes and other chlorinated organic substances, utilization of adsorption capacity
of activated carbon beds, extension of chlorine demand, and possible transport of organic
and inorganic pollutants through water treatment facilities and into the water supply [1].
Coagulation with ferric and aluminum salts is standard treatment for the removal of
suspended colloids from water supplies. It has been shown that aluminum sulfate, the most
commonly used coagulant for turbidity removal is also effective in color removal [1-6].
The coagulant ferric chloride, in contrast, has been the focus of very few color removal
studies. Tlie objective of this research is to determine the effectiveness of ferric chloride in
the removal of organic color from water.
This paper deals with the delineation of optimum color removal areas with respect to
the operational diagram for ferric chloride coagulation [7]. The independent variables con­
sidered are: humic acid (color) concentration, coagulant dosage, and pH. Results for true
color removal, apparent color removal, turbidity removal, and electrophoretic mobility are
presented.
2
NATURE OF ORGANIC COLOR
Humic substances are of natural origin, derived from organic material in soil or pro­
duced within surface waters and sediments by biological and chemical processes [ I ].
Schnitzer and Khan [8] characterized them as “amorphous, brown or black hydrophilic,
acidic, polydisperse substances of molecular weight ranging from several hundred to tens
of thousands.” The primary definition of humic substances, however, is operational. They
are classified into the three main groups based on their solubilities: (I) humic acid, which
is soluble in base, but insoluble in acid; (2) fulvic acid, which is soluble in both base and
acid; and (3) humins, which are insoluble in both [9]. Operational definitions are not abso­
lute, however, and humic substances from any single source have a continuous range of
solubility, absorptivity, molecular size, and chemical reactivity [ I ].
The individual molecules of humic substances are complex as well. The molecules
appear to be part aromatic and part aliphatic [10]. There are acidic hydrophilic sites on
the molecules which are reactive with cations such as trace metals and the hydrolysis species
of aluminum and iron. Also, there are hydrophobic sites which may be responsible for the
association between humic substances and certain nonionic, organic compounds [ I ] .
Humic substances contain the following functional groups which affect their stability
in water; carboxyl, phenolic, alcoholic, ketonic, quinoniod, and methoxyl. Both the charge
density and configuration of the humic macromolecules in solution are affected by pH.
Under the pH conditions of most natural waters, humic materials are negatively charged
macromolecules (anionic polymers). As the pH is increased„increasing stability results due
to dissociation of the functional groups and a resulting increase in the magnitude of the
negative charge on the color molecules [3].
3
THEORY OF COAGULATION
Colloidal particles range in size from about I to 100 nanometers and are characterized
by large surface area and very slow settling rates [11]. The surface of a colloidal particle
tends to acquire an electrostatic charge due to the ionization of surface groups and the
adsorption of ions from the aqueous phase. Most colloids in water develop a negative pri­
mary charge but a colloidal dispersion does not have a net charge. Charges on the particles
are counterbalanced by charges in the solution phase. This need for electroneutrality results
in an electrical double layer which consists of the charges on the particle and an equivalent
excess of counter ions which accumulate in the water near the surface of the particle.
Forces of diffusion and electrostatic attraction spread the charge in the water around each
particle in a diffuse layer [12].
The stability of colloids in water is due primarily to their electrostatic characteristics.
When two colloidal particles with similar charge approach each other, their diffuse layers
interact and produce a repulsive force. The repulsive force is counterbalanced to some
degree by an attractive van der Waals force, and the resulting potential of both the repul­
sive and attractive forces often forms an energy barrier which prevents aggregation [13].
The process of overcoming the energy barrier and allowing aggregation to occur is called
coagulation.
0 1Melia [12] presents four distinct mechanisms of coagulation. These are: (I) com­
pression of the diffuse layer (DLVO theory); (2) adsorption to produce charge neutraliza­
tion (adsorption-destabilization); (3) enmeshment in a precipitate (sweep coagulation); and
(4) adsorption to permit interparticle bridging. In water treatment, coagulation of colloids
4
is probably accomplished by adsorption-destabilization, sweep coagulation, or both simul­
taneously.
In the adsorption-destabilization process, metal cations such as Fe(III) and Al(III) are
hydrated in water and are present as aquocomplexes. Hydroxo-metal complexes are
formed and are readily adsorbed at interfaces. Because of the adsorption of oppositely
charged species, the colloids become destabilized or neutralized, allowing aggregation to
occur. Stoichiometry between the colloidal surface area and coagulant dosage exists, and
overdosing results in charge reversal, i.e., restabilization [12].
In sweep coagulation, the dosage of the aluminum or iron salt is large enough that a
sufficient degree of oversaturation occurs to produce a rapid precipitation of the metal
hydroxide. As the precipitate settles, it enmeshes the colloidal particles and sweeps them
from the solution.
Coagulation of Color
The mechanisms that are responsible for the removal of humic substances when salts
of aluminum or ferric iron are used as coagulants are often uncertain but may include
direct precipitation of humic substances by dissolved iron species (sometimes described as
a ferric or iron humate precipitate), adsorption of humic substances on solid ferric
hydroxide [Fe(OH)3 ^ ] , or a combination of the two. In this paper, “precipitation”
describes the formation of Fe(OH)3 ^
as well as the formation of a solid phase that is
composed of previously soluble species of iron and humic acid. “Adsorption” is used to
describe the reaction between solid phase ferric hydroxide and the soluble humic acid
molecules [I ].
5
AQUEOUS CHEMISTRY OF Fe(III)
To understand the ability of ferric chloride to destabilize colloidal particles, it is
necessary to consider the chemistry of Fe(III). 0 1Melia [12] stated that all metal cations
are hydrated in water and that simple species such as Fe3+ do not exist. Rather, Fe(III) is
present as part of aquocomplexes such as Fe(H2O)6 3+. Ions or molecules, such as water
molecules, which are bonded to a central metal atom are termed ligands.
The addition of an Fe(III) salt to water in concentrations less than the solubility limit
of the metal hydroxide leads to the formation of soluble monomeric, dimeric, and small
polymeric hydroxo-metal complexes, in addition to the free aquo-metal ion. The hydroly­
sis of Fe(III) can be described as a stepwise consecutive replacement of H2O molecules in
the hydration shell by OH" ions. For example:
Fe(H2O)6 3t -+ Fe(H2O)6OH++ -> Fe(H2O)4 (OH)+ -+ Fe(H2O)3 (OH)3 -> Fe(H2O)2 (OH)4
The OH" ions are also ligands.
When writing the hydrolysis reactions of a metal ion, it is common to omit the H2O
ligands for brevity. For example, the monomeric hydroxo-metal complex Fe(H2O)6 (OH)2+
is written as Fe(OH)2+. For Fe3+ in pure water, Snoeyink and Jenkins [9] present the fol­
lowing equilibria:
Fe3+ + H2O
-
Fe OH2+ + H+
; IogK
Fe3++ 2H20
*
Fe(OH)+ + 2H+
; log K - : -6:74
Fe(OH)3
-
F e3+ + 30H"
F e3+ + 4H20
*
Fe(OH)4 + 4H+
; IogK
Fe2(OH)S+ + 2H+
; log K - -2.85
2Fe3+ + 2H20
= -2.16
; i°gK ,o=
= -23
6
The effect of these hydroxo-complexes on solubility is dramatic and is most con­
veniently illustrated by using a log concentration diagram or pC-pH diagram (Figure I).
Figure I is the most commonly encountered stability diagram when ferric salts are used
in coagulation and is developed from purely thermodynamic equilibria. The interior por­
tion of the diagram indicates the area in which precipitation of Fe(OH)3 ^ may theoreti­
cally occur. Figure 2 is a superposition of the areas of sweep coagulation, adsorptiondestabilization, and restabilization, with charge reversed, on the operational diagram. The
locations of these zones have been developed in previous work on coagulation of turbidity
with ferric chloride [7]. The boundaries and locations of color removal areas are slightly
different as will be shown later in this paper.
FeCI -GH O - m g /I
LOG [ F e ] -moles/lite r
-2
pH
FIGURE
I : Fe(III) Log Concentration Diagram
ESTAE5/LIZA
V A
\
;
y
y%
Sw e e p zCOagulaJION
///'////// • ' * / / / (/'Z// Z///
/z"
/ / // //Z/ S'
' ZZ
ZZ
Z
Z
>
Z ZZZz Z^/
/ 's / *
270
IOO
ZVZ/^
Z/^
27
IO
LOG [Fe] - mole5/ I iter
M
-BESJABIL ZATlONX
t e
ZONE(CHANGE S I _W| TH COLLOID/5
FACE ARE A)
X^FeOl H2 +
~7
/
Fe(OH)"
4
FefC
/
_
4
6
8
IQ
12
RH
FIGURE 2 :
IRON (III) COAGULATION DIAGRAM
2.7
1.0
.27
| / 6 uj - oCH9' ID3J
ADS
- 4 -
Z
O
\
-------------- H-
Q
2
________ CL
I—
O.
o:
,------------ ©1
F = \
9
MATERIALS AND METHODS
—. The most critical factors influencing the efficiency of coagulation for color removal
are pH, coagulant dosage, and concentration of color [14]. The pH controls both the speciation of the inorganic coagulant and the charge on the organic color molecule.
Synthetic waters were prepared containing the desired concentrations of humic acid,
turbidity, and alkalinity. Five solutions of different color concentrations plus turbidity and
three varied color solutions without turbidity were used in the color removal tests. Tech
Grade humic acid* was used for color, and turbidity, in the range of 27-30 turbidity units
(TH), was obtained through the addition of the colloid, Min-U-Sil-30.** Alkalinity was
added to the distilled water in the form OfNaHCO3. Table I shows the constituents of the
raw water solutions.
Table I. Color Solutions.
Color
Designation
Low
Med. Low '
Medium
Med. High
High
Humic Acid
Concentration
True-Color
(CU)
4 mg/1
8.2 mg/1
20 mg/1
28.8 mg/1
37.5 mg/1
100
225
500
700
1000
Min-U-Sil
I
I
I
I
I
NaHCO3
.2N
g/1
g/1
g/1
g/1
g/1
5 ml/1
5 ml/1
5 ml/1
5 ml/1
5 ml/1
Standard color solutions without turbidity are similar to those for Low, Medium, and High
concentrations, without Min-U-Sil.
Extensive jar testing was conducted so that areas of optimum color removal could be
defined. A Phipps-Bird jar testing apparatus with a six unit mixer was used for the tests.
Ferric chloride was added during a rapid mix period of one minute at 100 rpm. This was
*Aldrich Chemical Company, Inc., Milwaukee, WI 53233.
^Pennsylvania Glass Sand Corporation, 3 Tenn Center, Pittsburgh, PA 15235.
10
followed by a slow mix, flocculation stage of 20 minutes at 24 rpm. Sedimentation was
then allowed to occur over a 2 hour period.
True color, apparent color, pH, and turbidity values were obtained for all samples. pH
values were taken during the slow mix period. True color, apparent color, and turbidity
readings were taken after the slow mix and at the end of the 2 hour settling period. Color
readings were made with an Hitachi-Perkin Elmer, Colemen 139, Spectrophotometer. The
wave length used was 422.2 nm with a slit size of 2.36 mm. Percent transmittance was con­
verted to color units with the use of a standardization curve. Turbidity readings were made
with a HACH model 2100 turbidimeter.
In disagreement with the procedure for color measurement presented in Standard
Methods [15], the pH of the sample was not adjusted to a common value prior to spectrophotometric analysis. In coagulation experimentation, it is impossible to adjust the pH for
color measurement without possibly inducing further coagulation thus interfering with the
accuracy of the results.
After turbidity readings and apparent color readings were taken, the samples were
centrifuged for 5 minutes at 7500 rpm to remove excess turbidity which could interfere
with true color readings. A decrease in color of 90% of the original measurement within
the 2 hour settling period was considered to indicate effective color removal. The electro­
phoretic mobility (EM), which is proportional to the zeta potential or particle charge, was
determined with the use of a “Zeta-Meter.” Electrophoretic mobility is a measure of the
velocity at which particles traverse a measured path in a DC current at a specific voltage.
Because of the variation in initial color concentrations of the different waters tested,
removal areas on the operational diagram are defined by percent removal, not the magni­
tude of the residual color.
For each o f the raw waters tested, approximately 50-80 jars were run. This adds up to
a total of approximately 500 jars in a series of 83 jar tests. Figure 3 shows the operational
FeCI • G H O - m g /I
LOG [ F e ] - m o l e s / n t e r
Removal
PH
FIGURE 3 : Color Removal Domain — Initial Color = 100 CU, Turbidity = 27-30 TU.
12
diagram for ferric chloride with the 90% color removal area for a water containing a color
concentration of 100 color units (CU) and turbidity in the range of 27-30 TU. Figure 4 is
an expanded form of part of the operational diagram. Shown are percent removal values
used to delineate the optimum color removal area. This gives an example of the number of
points used to define the color removal domains. Similar data were obtained for all of the
suspensions tested. Because the boundaries and the extent of the color removal areas can
be better seen on the expanded diagram, it will be used in the following presentation of
results.
The delineation of color removal areas are restricted to pH values not less than 4 and
ferric chloride dosages not greater than 100 mg/1. Data outside of these ranges would be of
little use in practice.
13
FeCI3- 6 H2O - mg/I
F I G U R E 4: D a t a P o i n t s a n d C o r r e s p o n d i n g
Percent
Color Removal
Val ue s - initial
Color = 100
CU, T u r b i d i t y = 2 7 - 3 0
TU
14
RESULTS AND DISCUSSION
Color Removal
In this research, ferric chloride was found to be very effective in the removal of color
from water. In cases where the dosage of the coagulant was either too low or too high for
color removal, however, the color of the water was actually increased as a result of the
addition of Fe(III). It is therefore important that correct dosages of ferric chloride be
used in order to achieve color removal and to prevent an actual increase in the color of the
water.
The area of 90% color removal on the operational diagram varies significantly depend­
ing on the initial concentration of color. First, samples which contained color but no tur­
bidity are considered. Three different color concentrations were used, and the optimum
removal, areas for each were delineated.
Numerous studies [1,2,3] have shown that when alum is used in coagulation, opti­
mum color removal occurs in the range of pH 5 to 6.5. Ferric chloride, on the other hand,
can be effective over a wider pH range [16]. Figure 5 shows the color removal domain for
a solution containing a color concentration of 100 CU. Ninety percent color removal
occurred over a range from pH 4 to pH 8.8. The area of highest color removal occurred in
the range of pH 6.1 to 8.8 with ferric chloride dosages greater than approximately 10 mg/1.
The dominant removal, mechanism here is probably the adsorption of color molecules on
ferric hydroxide floe. The presence of an upper boundary for color removal probably
results from particle restabilization, i.e., charge reversal. This can be viewed as an excess of
positively charged ferric hydroxide interacting with the negatively charged color molecules.
The lower boundary for coagulation is probably a function of the magnitude of.charge on
15
IQ)
100
loo
IOO
0
O O
IOO IOO 8
9 2 .: IOO IOCKO
Removal
9 3 . 0 9 6.1/0
KO IOO
O O
pH
FIGURE 5: Da t a Po i n t s and C o r r e s p o n d i n g
Percent
Color Removal
V a l u e s - Initial
C o l o r= 100
CU, T u r b l d I t y = O
FeCI3 1GH2O - mg/l
O SG.4
16
the color molecules. As the pH is increased, the dosage of ferric chloride required for
removal also increases. This results from the fact that, with increasing pH, color molecules
become more stable due to dissociation of functional groups, thus increasing the magni­
tude of the negative charge [2].
Color removal in the pH range of 4.0 to 6.1 at ferric chloride dosages of 2-100 mg/1 is
slightly lower than in the area where adsorption of color molecules to ferric hydroxide floe
seems dominant. This is believed to be a function of the concentration of humic acid mole­
cules present in solution. With low concentration, the chances of collisions following
destabilization, and ensuing floe growth, are reduced.
In general, the 90% removal areas follow a trend toward lower pH levels and higher
ferric chloride dosages as the color concentration increases. With an initial color concen­
tration of 500 CU, the area o f 90% color removal occurred in the range of pH 4 to 6.7 with
ferric chloride dosages of 10 mg/1 and greater (Figure 6). Color removal, for the water con­
taining a color concentration of 1000 CU, occurred in the pH range of 4 to 6.3 with ferric
chloride dosages not less than 18 mg/1 (Figure 7). Tire only common color removal area for
all three color concentrations occurred from pH 5.8 to pH 6.3 at ferric chloride dosages
from 50 to 100 mg/1 (Figure 8).
Effect of Turbidity
The presence of turbidity in the range of 27-30 TU has little effect on the removal of
color. Studies on coagulation with alum [2,4] have also shown this to be the case. Figures
4 and 9-12 show the optimum color removal areas obtained for the suspensions which con­
tained color concentrations of 100, 225, 500, 700, and 1000 CU with turbidity. The color
removal domains show nearly identical ranges for pH and coagulant dosages as for those
waters containing no turbidity. This probably results from the fact that the addition of
negative charge due to the presence of turbidity, in the range of 27-30 TU, is negligible in
17
0 /9 8 .4
0
9 9 .5 /0
9 8.4
9 0 % Removal
D o o
7
PH
FIGURE 6 : Dat a Poi nt s a n d C o r r e s p o n d i n g
Percent
Color Removal
Va l u e s - Initial
C o l o r = 5 0 0 CU, T u r b I d I t y = O
FeCI3 • 6 H2O - m g /|
9 2 .3
7 /6
18
0*0
98.7
! 5 .3 /0
0
0 0
E 9.3
9 9 .6
9 9.7
9 6 .3
9 0 % Removal
PH
FIGURE 7 : Dat a Poi nt s and C o r r e s p o n d i n g
Percent
Color Removal
Va l u e s - Initial
C o l o r = IOOO C U , T u r b I d I t y = O
Fe CI 3 • 6 H2O - m g / |
9 4 . 5 / 3 0 .0
19
• 6 HgO - m g / I
FI GURE
8
: Common
-
Initial
Color
Color
Turbidity = 0
Removal
1 00
-
Domai n
1000
CU,
|/ 6 i u
- O 2H S - I D S - !
20
FI GURE 9 : D a t a P o i n t s a n d C o r r e s p o n d i n g
Percent
Col or R e mo v a l
V a l u e s - Initial
C o l o r = 2 2 5 CU, T u r b i d i t y = 2 7 - 3 0
TU
21
0
0
9 9.7
100
100
0
0
IOO
.5
10 0
9X 7 9 9 .3
0
0
0 9Sr8 IOO
38.2
6 .7
IOC I 0 0
50
100
9 6 .0
30
97.3
20
99.4 99.1!
9 0 % Removal
vt o
10
5
3
I
PH
F I GURE 10: D a t a P o i n t s a n d C o r r e s p o n d i n g
P e r c e n t Col or Re mo v a l Va l u e s - I ni t i al
C o l o r = 5 0 0 CU, T u r b i d i t y = 2 7 - 3 0 TU
•6 H O - mg/1
9 8 .5
heCI
100 9 5 .8
22
9 9 .2
9Z I 9 9 .4
9 9 .4
O O
PH
F I G U R E 11 : D a t a P o i n t s a n d C o r r e s p o n d i n g
Percent
Color Removal
V a l u e s - Initial
C o l o r = 7 0 0 C U , T u r b i d i t y = 2 7 - 3 0 TU
^ e CI3 ' 6 HgO - m g / 1
9 0 % Removal
23
O 98.6
99.3 99.7 99.4
9 9.7 IOO
9 8 .4 9 9 .8 100
9 8 .7
7.0 9 9.5
9 9 .4
9 9 .3 /0 0
100
99.1 / , 2 . 2
0
i.5 0
10 0
9 0 % Removal
PH
F I G U R E 12: D a t a P o i n t s a n d C o r r e s p o n d i n g
Percent
Color Removal
V a l u e s - Initial
C o l o r =s 1 0 0 0 C U , T u r b i d i t y = 2 7 - 3 0
TU
f'CCIg • 6 H O - m g / |
0 0 0
24
comparison to the magnitude of negative charge due to the presence of color in the range
of concentration tested.
A composition of the areas for color removal again defines a common region of color
removal for color concentrations in the range of 100 to 1000 CU (Figure 13). The pH and
ferric chloride dosage ranges are near those defined in the composite diagram (Figure 7) for
waters containing no turbidity.
The color removal domains for all the solutions tested are shown in Figures 14-21.
This data should provide a useful tool in the design and operation of water treatment facil­
ities.
Removal Mechanisms
Color removal may be accomplished by charge neutralization resulting from specific
chemical interactions between positively charged ferric species and the negatively charged
groups on the humic colloids. The fixation of multivalent cations onto ionized groups on
hydrophilic colloids may be due to electrostatic or chemical interactions causing a reduc­
tion in the charge on the particle [16]. Precipitation by this mechanism probably occurs at
all the pH values tested but seems to be dominant at pH values ranging from less than pH
6.1 to values less than about 5.0 (Figures 4 and 9-12) depending on the concentration of
humic acid in the water.
Figure 22 illustrates that color removal and charge restabilization are dependent on
the color concentration. In Figure 22, the upper and lower boundaries of the removal areas
shown in Figures 4 and 9-12 are graphed as a function of coagulant dosage versus color
concentration. Results at pH 4.5 show a straight line or stoichiometric relationship. Similar
stoichiometric relationships are observed at pH values of 5.5 and 6.3 as well (Figure 23).
The range of ferric chloride concentration within which coagulation is observed widens
considerably with increasing concentration of surface. This is probably due to improved
25
HeCI3 • 6 H2O - mg/I
FIGURE
13
Common
Initial
Color
Color
Turbidity
27
Removal
100
-
30
-
Domai n
1000
TU
CU,
26
I
p p * ' '
m
W
■
^ ^ 9 0 %
M
FeCI • 6 H O - mg/1
W
Rem oval
*
—
■ I f
lliSISIl
HFr
—
4
5
6
7
8
pH
F I G U R E 14:
-Initial
Color
C o l o r = 100
Removal
CU,
Domain
Turbidity = 0
9
27
Mm m p -
I OO
r
■
P
90 %
Removal
—
--
—
------ --------- ----I
PH
F I G U R E 15:
-Initial
Color
Color=
Removal
500
CU,
Domain
TurbIdity = O
FeCI 3 • 6 H O - mg/1
i
28
IOO
r ™
! ■ k
J jP ^
30
"^90 %
Rem oval
—
—
PH
F I G U R E 16 : C o l o r
Initial
Removal
Domain
C o l o r = 1000 C U, T u r b i d i t y = O
20
FeCI3 ' 6 H O - mg/1
—
29
Wf*
18
F
50
30
10
—
5
3
—
1
5
6
F I G U R E 17:
Initial
Color
Color =
7
8
Removal
Domain
pH
100 C U , T u r b i d l t y = 2 7 - 3 0
9
TU
f-eCI • 6 H2O - mg/1
— 20
9 0 % Removal
/
ill W
IOO
30
■11
FeCI • 6 FLO - mg/1
f
—
^
9 0
%
Removal
UP
—
—
5
6
7
Q
9
pH
F I G U R E 18:
Initial
Color
Removal
C o l o r = 2 2 5 CU,
Domain
T u r b I d I t y = Z 7 - 3 0 TU
31
/■
i
^
^
9 0 %
30
—
20
Removal
—
----------------------
pH
F I G U R E 19:
Initial
Color
Removal
Domain
C o l o r = 5 0 0 C U , T u r b i d i t y = 2 7 - 3 0 TU
FeCI • 6 H2O - mg/1
/ W
f
p
IOO
32
W
III W
0
k
PeOI ' 6 H O - mg/1
—
a
^^9 0 %
Removal
—
—
4
5
6
7
8
R e m o v a l
Domain
9
pH
F I G U R E 20:
Initial
C olo r
C o l o r = 7 0 0 C U , T u r b I d 11 y= 2 7 - 3 0 T U
33
F
j
a m
I p i ^
^ 9 0 %
Removal
—
----------- --------- -
pH
F I G U R E 21 : C o l o r
Initial
Removal
Domain
C o l o r = IOOO C U , T u r b I d 11 y = 2 7 - 3 0 TU
FeCI 3 • 6 H O - m g / |
—
f/
ioo90H
- upper boundary
Jov/er boundary
COLOR CONCENTRATION- CU
FIGURE 2 2 : Stoichiometry for the Color Removal Domains
pH - 4.5
ioo-
on
COLOR CONCENTRATION- CU
FIGURE 2 3 - ‘ Stoichiometry for the Color Removal Domains
• p H - 5.5 , © pH- 6.3
36
■coagulation kinetics (improved contact opportunities) with increasing color concentration
[17].
Electrophoretic mobility data further substantiates the mechanisms of charge neutral­
ization and restabilization. Figure 24 shows partial charge and EM values (microns/second
per volt/centimeter) for a water containing a color concentration of 100 CUwith turbidity.
Figures 25, 26, and 27 show particle charge results for waters containing color concentra­
tions of 100, 500, and 1000 CU with turbidity. All restabilized dispersions were found to
be positively charged. The production of zero charge on the color particles occurred
infrequently. In agreement with Black and Singley’s [18] data, highest color removal
occurred at slightly negative mobilities.
Highest color removal for the various solutions tested occurred at pH values ranging
from greater than approximately 5.0 to greater than 6.1 (Figures 4 and 9-12). Particle
charge tended to be slightly negative and the dominant removal mechanism was probably
adsorption of color molecules to ferric hydroxide floes. At these higher pH values within
the color removal domains, the OH” formation function ([OH"] bound per [Fe(III)])
increases, thus, the net positive charge on the hydrolysis species decreases. As a result,
complete charge neutralization and restabilization become retarded or eliminated [16].
Apparent Color Removal
Apparent color is true color plus the interference in color measurement due to the
presence of turbidity. The removal of apparent color is a measure of a simultaneous
removal of both color and turbidity.
Figures 28, 29 and 30 show areas of apparent color removal for waters containing
color concentrations of 100, 500, and 1000 CU with turbidity in the range of 27-30 TU.
The boundaries for true color removal (dashed lines) are also depicted on these figures.
The removal areas for apparent color correspond closely to those for true color. For the
,
37
-
+ 1.4,
-
+ 1.9 + 1.7
+ I/. 7
+ 2 .3
0.6
-
-0.5
- 3 .2
0.6
- 0 .7
•
2.6
-U
-
1.8
2.0
- 3 .4
^3.1
-3. i -3.5
-3.7
-1 .6
-3.1-3. L
+ 2 .3
Rem oval
2.1
+ 2 .3 /+ 0 .8
- 3 .2 -3 .1
+ 0.6
3.3
-3 .5
- 3 .2 - 3 .6
PH
F I GURE 2 4 :
-
Particle
Initial
C h a r g e and EM Values
Color
Turbidity =
27
=
-30
I OO
TU
CU
I
FeCI5 • 6 H O - m g /|
+ 2 .4
38
FeCI5 -G H2O - m g /|
pH
FIGURE
25 : P a r t i c l e
-
Initial
Charge
Color
Turbidity =
27
=
-30
I OO
TU
CU
39
pH
figure
26:
-
Particle
Initial
Char ge
Color
Turbidity =
27
=
500
-30
TU
CU,
FeCI3 -G H2O - mg/1
Removal
40
FeCI 3 • 6 H2O - m g /|
FIGURE
27:
-
Particle
Initial
Charge
Color
Turbidity=
27
=
I OOO
-30
TU
CU,
41
IOO
50
30
I
10
pH
FI GURE 28 ■ A p p a r o n t
-
Initial
Color
Col or
Turbidity =
R6 m o v a I Domai n
=
100
2 7 - 3 0 TU
CU,
FeCI 3 • 6 H2O - mg/J
20
42
■
'
/
%
W
r
/
^ ^ 9 0 %
Rem oval
E
—
4
5
6
7
8
pH
FI GURE
29:
Apparent
-
Initial
Color
Col or
Turbidity =
Removal
=
500
2 7 - 3 0 TU
Domai n
CU,
FeCI •6 H O - m g /|
■ij
—
43
Tr-
F
I
—
x
^ ^ 90%
Rem oval
-------------------------- I
5
—
4
5
6
Q------------------- —
7
2
Il
pH
FI GURE
30 • Apparont
-
Initial
Color
Color
Turbidity=
=
Romoval
IOOO
2 7 - 3 O TU
Domai n
CU,
FeCI 3 •6 H2O - m g /|
— 2
44
water containing a color concentration of 100 CU, however, the apparent color removal
domain corresponds only to the upper portion of the true color removal domain. The
lower boundary, which occurs at a higher ferric chloride dose than that for true color
removal, is probably a function of the amount of coagulant involution and related coagu­
lation kinetics. Precipitation does not occur in the time allowed for settling. Centrifuga­
tion for true color analysis, however, resulted in the removal of these small slowly settling
particles.
Turbidity Removal
Although the presence of turbidity in the range of 27-30 TU has little effect on the
removal of color, the presence of color has a dramatic effect on the removal of turbidity.
Figures 31, 32, and 33 show areas to 50% turbidity removal for waters containing color
concentrations of 100, 500, and 1000 CU. The boundaries to true color removal (dashed
lines) are superimposed on these figures as well. Effective turbidity removal occurred in
the ranges of color removal. This is probably due to the fact that the magnitude of the
negative charge resulting from the presence of color far exceeds the magnitude of negative
charge due to the presence of turbidity. Hall and Packham [4] achieved similar results.
45
Z
______
Jfk
\
^
W
Z
L
p
5
—
% Removal
0
—
/
/
CT
—
pH
F I GU R E 3 1 : T u r b i d i t y
-
initial
Removal
Color
Turbidity =
=
27
Do ma i n
I OO
-
30
CU,
TU
FeCI3 • 6 H2O - mg/1
m
/IW
Yk
W
___________________________
46
FrCCI3 ' 6 H2O - mg/1
50 % Removal
FIGURE
32:
Turbidity
-
initial
Removal
Color
Turbidity =
=
2 7 -
Domain
500
30
C U,
TU
47
■ F
—
^ ^ 5 0 %
Removal
—
4
s
----------------- - I
C
PH
F I GU R E 3 3 :
Turbi di ty
-
initial
Removal
Color
Turbidity =
27
=
Domai n
IOOO
-
30
CU,
TU
FeCI3 • 6 H O - m g / |
JP
48
CONCLUSIONS
When the variables involved are properly controlled, ferric chloride is a very effective
coagulant for the removal of organic color. For each colored water there is a specific range
of pH values and ferric chloride dosages within which effective color removal is achieved.
The two variables that must be controlled are pH and ferric chloride dose, both of
which, as a result of the data presented, can be predicted with considerable accuracy from
the raw water color alone. Color removal areas follow a trend toward lower pH levels and
higher ferric chloride dosages as the color concentration is increased.
The presence of turbidity in the range of 27-30 TU has little effect on the removal of
color. Removal areas for both waters with and without turbidity occurred in the same
ranges of pH and coagulant dosage.
The areas of turbidity removal on the operational diagram were dependent on the con­
centration of humic acid in the water.
Effective color removal and charge restabilization are dependent on the color concen­
tration. The range of ferric chloride dosage within which removal occurred widened with
increasing color concentration. Highest color removal tended to occur at higher pH values
where the formation of solid phase ferric hydroxide was probably prevalent.
The results of this paper represent an extensive delineation of optimum color removal
areas with the use of ferric chloride as the coagulant. The diagrams should provide a useful
tool in the design and operation of water treatment facilities.
REFERENCES
50
REFERENCES
1. Dempsey, B. A., Ganho, R. H., and O’Melia, C. R. The Coagulation of Humic Sub­
stances Using Aluminum Szits. Joum alof American Water Works Association, Vol. 76,
No. 4, 141, April 1984.
2. Edwards, G. A., Color Removal Domains on the Alum Coagulation Diagram. Unpub­
lished professional paper submitted in partial fulfillment of the requirements for the
Master of Science degree, Department of Civil Engineering and Engineering Mechanics,
Montana State University, 1982.
3. Edzwald, J. K., Coagulation of Humic Substances, AIChE Symposium Series No. 190,
W ater-1979, Vol. 75, SY, 1979.
4. Hall, E. S. and Packlram, R. F., Coagulation of Organic Color with Hydrolyzing Coag­
ulants, Journal o f American Water Works Association, Vol. 57, 1149, 1965.
5. Semmens, M. I. and Field, T. K., Coagulation: Experiences in Organic Removal, Jour­
nal o f American Water Works Association, Vol. 72, 476, 1980.
6. Mangravite, F. J., Buzzell, T. D., Cassell, E. A., Matijevic, E., and Saxton, G. B.,
Removal of Humic Acid by Coagulation and Microflotation, Journal o f American
Water Works Association, Vol. 67, 88, 1975.
7. Johnson, P. N. and Amirtharajah, A., Ferric Chloride and Alum as Single and Dual
Coagulants, Journal o f American Water Works Association, May, 1983.
8. Schnitzer, M. and Khan, S. V., Humic Substances in the Environment, Dekken, New
York, N.Y., 1972.
9. Snoeyink, V. L. and Jenkins, D., Water Chemistry, John Wiley and Sons, New York,
N.Y. 1980.
10. Siao, W., Christman, R. F., Johnson, J. D., Millington, D. S., and Hass, J. R., Struc­
tural Characterization of Aquatic Humic Material, Environmental Science and Tech­
nology, Vol. 16, No. 7, 403, 1982.
11. Sawyer, C. N. and McCarty, P. L., Chemistry For Environmental Engineering, Third
Ed., McGraw-Hill, New York, 1978.
12. O’Melia, C. R., Coagulation and Flocculation, Physicochemical Processes for Water
Quality Control (W. J. Weber, Jr., ed.), Wiley Interscience, New York, N.Y., 1972.
51
13. Galarragara, E. P., Influence of pH on Dosage Requirements of Ferric Chloride.
Unpublished thesis submitted in partial fulfillment of the requirements for the Master
of Science degree, Department of Civil Engineering, North Carolina State University,
1980.
14. Kavanaugh, M. C., Modified Coagulation for Improved Removal of Trihalomethane
Precursors, Journal o f American Water Works Association, Vol. 70, 613, 1978.
15. Standard M ethodsfor the Examination o f Water and Wastewater, fifteenth ed., Amer­
ican Public Health Association, Washington, D.C., 1981.
16. Hyde, C. H., Practical Aspects of Coagulation with Ferric Chloride, Journal o f Ameri­
can Water Works Association, Vol. 27, No. 5, 631, 1935.
17. O’Melia, C. R. and Stumm, W., Aggregation of Silica Dispersions by Iron (III), Journal
o f Colloid and Interface Science, Vol. 23, 437, 1967.
18. Black, A. P., Singley, J. E., Whittle, G. P., and Maulding, J. S., Stoichiometry of the
Color-Causing Organic Compounds with Ferric Sulfate, Journal o f American Water
Works Association, Vol. 55, 1347, 1963.
MONTANA STATE UNIVERSITY LIBRARIES
K378
N T lT
N o le n , v . n .
c o p .2
Coagulation
c o lo r w ith
fe
date
MAIN LIB.
N378
NTlT
cop. 2