United States
Environmental Protection
Agency
Municipal Environmental Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-83-107 Feb. 1984
Project Summary
The Equilibrium Arsenic Capacity
of Activated Alumina
Eric Rosenblum and Dennis Clifford
Arsenic(V) can be effectively re-
moved from water by adsorption onto
activated alumina. Variables affecting
the extent of adsorption include pH,
temperature, and the presence of other
ions in solution. Adsorption isotherms
establishing a relation between solid
and liquid phase arsenic concentrations
at equilibrium can be developed by
batch tests of 7 days or more. Despite
the fact that batch tests exposed alu-
mina to uncharacteristically high initial
arsenic concentrations, the resulting
capacities were close to those obtained
by minicolumn tests that exposed alu-
mina to low arsenic concentrations for
21 days.
Batch tests performed with granular
Alcoa* F-1 type activated alumina (28 x
48 mesh) required as many as 7 days to
reach an equilibrium solid phase loading
of 13.5 mg As(V)/g at a liquid phase
concentration of 1.0 mg As(V)/L in an
artificial ground water containing 266
mg CIVL and 367 mg SOl'/L (pH 6,
25°C). Minicolumn tests performed
under similar conditions obtained a
solid phase loading of 16.1 mg As(V)/g
ajumina at 1.0 mg As(V)/L, but they
yielded results identical to the batch
tests at equilibrium liquid phase con-
centrations less than 0.50 mg As(V)/L.
These results verified the feasibility of
batch tests for modeling column capa-
cities in that range.
Minicolumn tests showed As(V)
adsorption to be extremely dependent
on pH, as expected, since the latter
determines both the surface charge on
the alumina and the valence of the
arsenate. Maximum adsorption oc-
curred at pH 6. Increased temperature
resulted in increased adsorption, as
batch tests performed at 40°C yielded
As(V) solid phase loadings 33 percent
higher (at 1.0 mg As(V)/L) than tests
'Mention of trade names or commercial products
does not contitute endorsement or recommendation
for use.
performed at 25°C. This increased
adsorption was attributed to kinetic
rather than energetic considerations.
Arsenic adsorption from pH 6.0 deion-
ized water reached a maximum loading
of 25 mg As(V)/g alumina at 1.0 mg
As(V)/L, which decreased by over 50
percent in the presence of 720 mg
S04 /L and 16 percent in the presence
of 532 mg CIVL. These decreases were
greater than expected on the basis of
previous experiments with fluoride
adsorption onto the same type of
alumina.
This Project Summary was developed
by EPA's Municipal Environmental
Research Laboratory, Cincinnati, OH,
to announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
Arsenic is a natural constituent of a
significant percent of the world's surface
and ground water. In several areas,
arsenic contamination of water supplies
has been reported at levels as high as 10
mg/L far in excess of the 0.05 mg/L
maximum contaminant level (MCL)setby
the U.S. Environmental Protection Agen-
cy (EPA). When arsenic is present in
surface water, it is usually removed by
conventional water treatment methods,
including lime softening and filtration, or
coagulation by alum or ferric sulfate. But
where communities rely on untreated
well water containing arsenic for their
water supply, treatment for arsenic
contamination removal must be consi-
dered. For the treatment of well water, a
packed bed process is generally preferred;
and one of the most promising column
media for arsenic removal is activated
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alumina, which has been used success-
fully in this manner to remove fluoride.
This study of arsenic removal by
adsorption onto activated alumina was
designed to accomplish the following
objectives:
1. To determine the capacity of activated
alumina for As(V) by both batch and
column methods, and to compare the
isotherms thus obtained.
2. To determine the optimum pH for
adsorption of As(V) onto activated alumi-
na.
3. To determine the effect of competi-
tion by chloride and sulfate ions on the
adsorption of As(V) onto activated alu-
mina.
The work was undertaken as laboratory
support for future pilot-scale studies to be
performed in the University of Houston/
EPA Mobile Drinking Water Treatment
Research Facility.
Materials and Methods
Alcoa F-1 type granular activated
alumina used in the experiments was
manually screened to a mesh size of 28 x
48 and conditioned with two cycles of 1
percent NaOH and 1 percent H2S04, with
80 bed volumes of each solution passed
through the conditioning column for each
cycle. Conditioned alumina was exhaus-
tively rinsed with deionized water to pH
5.0. The As(V) solutions were prepared
with reagent grade KH2As04. Except for
tests of adsorption from deionized water,
all arsenic solutions contained total anion
concentrations of 15 meq/L made up of
chloride, bicarbonate, and sulfate ions at
the concentrations specified in Table 1.
Graphite furnace atomic absorption
spectroscopy was used to determine
arsenic, with Ni(N03>2 added to prevent
premature volatilization. Three different
arsenic sources As203, KH2As04, and
a purchased standard were used for
standardization and cross-checking.
Batch Tests
In the batch tests, identical volumes
and concentrations of arsenic in solution
were exposed to different quantities of
activated alumina. The control contained
only arsenic solution, without alumina.
final arsenic concentrations were deter-
mined, and the difference between
batch-test and control final concentra-
tions was attributed to adsorption of
arsenic onto activated alumina.
The procedure for batch tests was as
follows:
1. Aliquots (150-ml) of 5.0-mg As(V)/
L solution were pipetted into 200-ml
glass bottles along with measured
quantities (10 to 500 mg) of granular
activated alumina (28x48 mesh).
2. Bottles were capped and placed on
an Eberback shaker operating at 140 5-
cm excursions/min.
3. After shaking," the bottles were
removed and their contents were decant-
ed and filtered through a 0.45-yum Milli-
pore filter.
4. Each concentration of alumina was
tested in duplicate. All samples were
analyzed for arsenic concentration and
pH, and the results were recorded.
Column Tests
Column tests to determine the equili-
brium loading of As(V) onto activated
alumina consisted of passing an As(V)
solution through glass minicolumns
Table 1.
Make-Up of Artificial Ground Water for Use in Arsenic (III) and Arsenic (V) Adsorption
Experiments
pH
4 to 6
pH
7 to 10
Ions
Cations:
Na*
An ions:
cr
HCOl
scfr
mg/L
345
266.2
367.5
meq/L
15
7.5
7.5
mg/L
345
177.5
305
240
meq/L
15
5
5
5
Total Dissolved Solids
978.7
10675
Total concentration of ions: Cr = 15 meq/L = 0.015 N
Ionic strength of pH 4 to 6 solutions: I = 0.0263 M
Ionic strength of pH 7 to 10 solutions: I = 0.0225 M
containing fixed amounts of activated
alumina until the effluent concentration
reached 90 percent of the influent
concentration that is, until As(V)
removal was reduced to < 10 percent.
Control columns containing no alumina
were included for each arsenic concen-
tration and pH tested. Arsenic removed
was determined as the difference be-
tween the arsenic in the influent and that
in the total effluent collected. The arsenic
loading on the alumina was calculated by
dividing the total weight of As(V) removed
by the weight of alumina in the column. A
diagram of the column apparatus appears
in Figure 1.
Operation of the columns included
regular sampling, repeated preparation of
influent solution, and measurement and
sampling of collected effluent. In addition,
alumina in the minicolumns was periodi-
cally reclassified (broken up) by inversion
to prevent the channeling and cementa-
tion that occurred in the upper layers of
the column.
Results and Discussion
Equilibrium
Figure 2 compares As(V) isotherms
developed from batch tests conducted
under similar conditions for different
lengths of time. As can be seen, the
isotherm representing results of the 14-
day batch tests indicates significantly
greater adsorption at each liquid phase
concentration than does that for either
the 1- or 2-day tests. On the other hand,
isotherms obtained by 7- and 14-day
tests appear to be in close agreement,
indicating that most of the various con-
centrations of alumina had reached
equilibrium with the liquid phase ar-
senic within 7 days at 40°C.
Furthermore, a comparison of equili-
brium isotherms derived under similar
conditions from batch tests and column
tests (Figure 3) indicates that the results
obained by these two methods agree very
well at liquid phase concentrations of less
than 0.50 mg As(V)/L. Above that
concentration, however, the column
isotherm predicts solid phase loadings
progressively higher than those obtained
by the 7-day batch tests.
One explanation for this divergence is
suggested by Figure 2, which indicates
greater deviation between the 7- and 14-
day isotherms at higher liquid phase
arsenic concentrations. This result
implies that the higher loadings achieved
by the column method might be obtained
by batch tests performed for some period of
time longer than 7 days. Nevertheless,
the batch method seems adequate for
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20-L Nalgene Carboy
"T'-Connection
Nalgene Tubing -
Gum Rubber
Connection
15.0
1.0 g Alumina _
(5 cm depth)
Glass Wool Plug
Peristaltic Pumps -
"T"-Connection
w/Gum Rubber
Hose and
Hoseclamp
'Minicolumns
(Duplicates + Blank)
- Blank (no alumina)
Glass Wood Plug
"T"-Connection
w/Gum Rubber Hose
(shown open for sampling)
5-gallon Polycarbonate
Bottles
Figure 1.
Minicolumn apparatus for arsenic(V) adsorption onto alumina by the column
method.
predicting column capacity in the low
range « 0.50 mg/L) of arsenic concen-
trations likely to be encountered in
contaminated ground water.
Data from the column tests used to
determine the equilibrium isotherm in
Figure 3 are plotted on a log-log scale in
Figure 4 according to the linearized form
of the Freundlich equation with a
correlation coefficient (r2) of 0.9982,
Constants derived from this plot by the
slope-intercept method yield a Freundlich
equation such that
1 c o r* *0 43)
qe = ID.£ Ce
where qe and Ce are the solid (mg/g) and
liquid phase (mg/L equilibrium As(V)
concentrations, respectively.
The Freundlich model, which fits the
data better than the Langmuir model,
allows for both heterogeneous adsorp-
tion site energies and multilayer adsorp-
tion. Although the isotherms obtained at
pH 8 (Figure 2) have a shape more
characteristic of the BET isotherm model,
saturation concentration was not ob-
tained at pH 6 in the column test results
shown in Figure 3. Determination of
such a saturation concentration at
optimum pH may be considered an area
for further study.
Oo '«>
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0.05
C,(mg/L)
0.2 0.5
log C, (mg/L)
Figure 4. Linearized Freundlich isotherm
for adsorption ofarsenic(V) onto
activated alumina by column
method at 25 °C.
6.0,
the alumina becomes progressively less
positively charged; at pH <6.0, the anions
from the acids added to lower the pH
compete with the arsenate anion for
adsorption sites.
Arsenic Adsorption and
Temperature
Isotherms for adsorption of As(V) onto
activated alumina at two different
temperatures are presented in Figure 6.
As can be seen arsenic adsorption
increased when the temperature of the
batch tests was raised from 25° to 40°C.
This result appears to contradict the
prevailing assumption that adsorption is
an exothermic reaction, since an increase
in temperature would then tend to drive
the reaction in the direction of desorption
from the solid phase.
One explanation for this apparent
anomaly, however, is that a higher
temperature could increase the rate of
diffusion into the solid phase and thus
increase the solid phase loading at a
given pH and time to equilibrium. This
hypothesis agrees with the findings of
both batch and minicolumn tests that
the adsorption reaction was largely
controlled by kinetic constraints.
75.0
70.0
5.0
0.0
-1.200
.134
&
.06V
5.0 6.0 7.0
pH
8.0 9.0
Figure 5. Effect of pH on adsorption of
arsenic(V) onto activated alu-
mina by column method.
(Ca = 1 mg As(V)/L; TDS = 979
\pH 5.6], and 1067 mg/L \pH
7-9])
0.0 1.0 2.0 3.0 4.0 5.0
C. (mg/L)
Figure 6. Effect or temperature on adsorp-
tion of arsenic(V) onto activated
alumina by batch method.
(Co = 5mgA S(V)/L; pH = 6; TDS
= 979 mg/L: Cl" = 7.5 meq/L;
SOl" =7.5 meq/L; *. = 7 days)
Arsenic Adsorption and Ion
Competition
Figure 7 compares equilibrium iso-
therms for adsorption of As(V) onto
activated alumina with those of solutions
containing different concentrations of
competing anions. It indicates that
arsenic adsorption is significantly reduced
in the presence of chloride and sulfate. At
an equilibrium concentration of 1.0 mg
As(V)/L in the liquid phase, solid phase
loading was reduced by 16 percent in the
presence of 532 mg CIYL (15 meq/L),
and by more than 50 percent in the
presence of 720 mg SO?~/L (15 meq/L),
as compared with adsorption from
deionized water. Similarly, in an artificial
ground water composed of 7.5 meq/L
each of chloride and sulfate, adsorption
was reduced by more than 40 percent.
These results agree with the proposed
selectivity sequence for competitive
adsorption onto activated alumina, which
predicts a greater reduction from sulfate
competition than from chloride competi-
tion. However, the sensitivity of As(V)
adsorption to competition from chloride
ions was greater than that expected on
the basis of previous work with fluoride
adsorption onto the same type of alumina.
The full report was submitted in
fulfillment of Cooperative Agreement No.
CR-807939-02 by the University of
Houston under the sponsorship of the
U.S. Environmental Protection Agency.
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35.0
' O
' .>£
Deionized Water
15meq/LNaCI
D~~Q
1.0
7.5meq/LNaCI
7.5 meq/L Na^SOt
.467
.400
.334
.267
.200
.734
.067
.10 1.00 2.00 3.00 4.00 5.00
Ct(mg/L)
Figure 7. Effect of competition by chloride and sulfate ions on adsorption of arsenic(V) onto
activated alumina.
(T = 25°C, C0 = Smg AS(V)/L; pH = 6; f.q = 7 days)
Eric Rosenblum and Dennis Clifford are with the University of Houston, Houston,
TX 77004.
Tom Sorg is the EPA Project Officer (see below).
The complete report, entitled "The Equilibrium Arsenic Capacity of Activated
Alumina," (Order No. PB8411O 527; Cost: $11.50, subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
0000329
U.S. GOVERNMENT PRINTING OFFICE: 1984-759-102/855
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