United States
Environmental Protection
Agency
Water Engineering
Research Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/S2-86/021 Apr. 1986
&EPA Project Summary
Arsenic (III) Oxidation and
Removal from Drinking Water
Phyllis Frank and Dennis Clifford
The oxidative pretreatment of As(lll)
using chlorine and oxygen was studied
following quantification of As(lll) and
As(V) removals by activated alumina
columns.
Activated alumina removed 100 uglL
As(V) from a typical groundwater at pH
6.0 much more effectively than it did
As(lll). Approximately 23,500 bed
volumes of water could be treated by ac-
tivated alumina columns before As(V)
reached the 0.05-mg/L maximum con-
taminant level (MCL), whereas only 300
bed volumes could be treated before As(lll)
reached that level.
Variables affecting the oxidation of
As(lll) by chlorine include the pH, chloride
concentration, other ions, chloramine for-
mation, and total organic carbon (TOG). In
artificial groundwater containing no
ammonia or TOG with 100 ng/L As(lll)
present initially and 1.0 mg/L chlorine
dosage, the reaction reached 95 percent
completion in less than our shortest possi-
ble observation time of 5 sea Thus, with
1.0 mg/L chlorine dosage, the As(lll) oxi-
dation rate was greater than 20 pg/L per
sec. The extent of oxidation at 30 sec was
insensitive to pH in the range of 6.5 to 9.5,
with decreasing reaction outside this
range. Increasing chloride concentration
slowed the reaction slightly, but not sig-
nificantly for water treatment. The
counterion (sodium or calcium) did not ap-
pear to affect the extent of reaction in the
artificial groundwater or in chloride solu-
tions up to 0.010 M. Monochloramine is
capable of oxidizing 40 percent of the in-
itial 100 ng/L As(IH) in the pH range of 6.5
to 10.5. The presence of 5 mg/L TOG
substantially slowed the oxidation kinetics
of 100 fjg/L As(lll) by 1.0 mg/L chlorine
dosage. Although the reaction reached 50
percent completion in less than 30 sec, it
did not reach 80 percent completion until
approximately 30 min.
In artificial groundwater, sparging 1 hr
with oxygen did not oxidize 100 j*g/L
As(lll); in deionized water, however, 14
percent was oxidized. However, capped
samples of As(lll) in deionized water and
artificial groundwater were completely
oxidized after 61 days on the shelf with
air in the headspace.
This Project Summary was developed
by EPA's Water Engineering 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 order-
ing information at back).
Introduction
Arsenic is a well-known toxic element
and not an uncommon contaminant of
groundwaters used for potable water
supply. Worldwide, the concentration of
arsenic in contaminated groundwaters
ranges from a few j*g/L to 8 mg/L. Con-
taminated wells included in a 1982
American Water Works Association survey
of inorganic contaminants in the United
States had a modal total arsenic concen-
tration within the 0.075- to 0.100-mg/L
concentration range. Data on the speci-
ation of arsenic-bearing groundwaters is
relatively scarce; however, it is known that
the valence state of arsenic in ground-
water varies with location. Inorganic
arsenic may occur totally as As(lll), as
As(V), or any mixture of these two valence
states. Water drawn from an infiltration
gallery 4 m (13 ft) deep in San Ysidro, New
Mexico, had a total arsenic concentration
of 80 ng/L with 40 percent of the total
occurring as As(lll). Nineteen wells sam-
pled in Hanford, California, had total
-------
arsenic concentrations ranging from 30 to
90 fjg/L, all of which was trivalent arsenic.
The speciation of arsenic is important
because trivalent arsenic is considered to
be 3 to 20 times as toxic as pentavalent
arsenic. The exact role of arsenic in human
metabolism remains obscure with some
researchers claiming that it is carcinogenic
and others believing that it may be an
essential trace element. Despite the lack
of knowledge about the consequences of
ingesting very small amounts of arsenic,
the ultimate toxicity of this element is well
recognized. The National Interim Primary
Drinking Water Regulations set a max-
imum contaminant limit (MCL) for arsenic
at 0.05 mg/L.
A survey of the literature and our own
experimental studies have shown that
arsenic may be removed by iron or alum
coagulation, filtration, reverse osmosis,
ion-exchange, electrodialysis, and adsorp-
tion onto activated alumina. With all of
these processes, however, it was found
that As(V) (arsenate) is much more effec-
tively removed than is As(lll) (arsenite)
because the former exists in natural water
as a monovalent or divalent anion of
arsenic acid (H3AsO4, pK2 = 7.0). Arsen-
ite, on the other hand, exists predominant-
ly as a neutral species—arsenious acid
(H3AsO3, pK, = 9.2). And it is well known
that coprecipitation, sorption, ion ex-
change, electrodialysis membrane trans-
port, and reverse osmosis membrane
rejection are much more effective with
ions like the arsenates than with neutral
species like arsenious acid. However, the
removal of As(lll), particularly by activated
alumina columns, has not been quantified.
One purpose of this study was, there-
fore, to quantify and compare the colum-
nar removal of both As(lll) and As(V) at the
known optimum pH of 6.0. This study was
performed in a typical high-sulfate, high
total-dissolved-solids water containing 3.0
mg/L fluoride.
Following this short alumina adsorption
study, a more comprehensive study was
performed on the oxidation of As(lll) to
As(V). The kinetics of oxidation by chlor-
ine were studied as a function of pH and
ionic composition. The oxidation of As(lll)
by oxygen was studied because of its
potential low cost and because the oxida-
tion of As(lll) in stored water samples was
inadvertently observed.
This study was undertaken at the
University of Houston as part of a com-
prehensive research effort on arsenic
removal from drinking water. Related field
studies using the University of Houston/
U.S. Environmental Protection Agency
(EPA) Mobile Drinking Water Treatment
Research Facility have been performed in
San Ysidro, New Mexico, and are contin-
uing in Hanford, California. Related
laboratory studies have been done on
establishing the As(V) capacity of acti-
vated alumina, regenerating arsenic-spent
alumina, establishing the fundamentals of
As(V) uptake by ion-exchange resins, and
developing an analytical method for the
separation and determination of As(lll) and
As(V).
Experimental Procedures
Column Studies
The objective of the column studies was
to verify the reportedly efficient removal
of pentavalent arsenic and the nonremoval
of trivalent arsenic on activated alumina.
Since some groundwaters in the United
States contain both excess arsenic and
fluoride, and since activated alumina is
known as a successful adsorbent for
fluoride, these column studies were con-
ducted on water that contained typical
concentrations of both fluoride and
arsenic.
Two column runs were conducted
simultaneously. Each column was con-
structed from 1/4-in-ID Plexiglas* tubing
with stainless steel Swagelock fittings at
each end and was loaded with 5 ml of
conditioned Alcoa F-1 activated alumina
(U.S. standard mesh size 24 X 48). Milton
Roy metering pumps were used to control
the flowrate at 1.7 mL/min for an empty
bed contact time (EBCT) of 2.9 min. These
pumps provided a steady flow rate over
the entire length of the longest column run
(58 days).
The column feedwater was prepared to
resemble groundwater that had been
acidified to pH 6.0 with sulfuric acid. This
pretreatment creates a sulfate-enriched,
bicarbonate-free water. An arsenic con-
centration of 0.100 mg/L was chosen
because it is twice the MCL and not un-
common in arsenic-contaminated ground-
waters (e.g., Hanford, California, and San
Ysidro, New Mexico). The same line of
reasoning was used in choosing 3 mg/L as
the fluoride concentration. The composi-
tion of the pH 6.0 column feedwater was
100 fjg/L arsenic, 3.0 mg/L fluoride, 71
mg/L chloride, 384 mg/L sulfate, 210 mg/L
sodium, and 20 mg/L calcium. The influent
to each column differed only in the valence
state of the arsenic; one column received
100 f^g/L As(V), and the other received
100 pig/L As(lll), which was prepared fresh
daily to avoid oxidation.
* Mention of trade names or commercial products does
not constitute endorsement or recommendation for
A fraction collector was used to collect,
25.5-mL samples every 15 min. Selected
samples were analyzed for total arsenic
content with a Perkin-Elmer Model 5000
atomic absorption spectrophotometer
with a graphite furnace (GFAAS), Zeeman
background correction, and an HGA-400
furnace temperature programmer. An elec-
trodeless discharge arsenic lamp was used
as the source lamp, and nickel nitrate was
used as a matrix modifier. Fluoride
analyses were conducted using a f luoride-
ion-selective electrode and a digital pH/ion
analyzer.
Oxidation Studies
Objectives
The oxidation studies were undertaken
to establish the kinetics of As(lll) oxida-
tion by chlorine and monochloramine as
influenced by pH, anions, cations, and
TOC. The oxidation of As(lll) by pure ox-
ygen and by air was also studied in the
hope that the use of chlorine could be
avoided.
Procedure Development for
Chlorine Oxidation
No method was available for instan-
taneous arsenic speciation and quantifica-
tion. The methods available for analytical
As(lll) separation from As(V) include ion j
exchange, pH-controlled arsine generation, '
and selective extraction. Each of these
methods requires subsequent analysis for
total arsenic in various fractions. An ion-
exchange method previously developed at
the University of Houston was chosen for
arsenic speciation. Five milliliters of
chloride-form IRA-458 anion-exchange
resin contained in a mini-column was ex-
posed to a sample at a flow rate of 10
mL/min and was used to separate As(lll)
from As(V) in a 100-mL sample. Under
these conditions, the fastest possible
separation time was about 10 min.
An oxidation quenching agent was
found to be necessary because the reac-
tion time of the As(lll) oxidation was
discovered in screening tests to be much
faster than the minimum separation time
of 10 min. Several criteria are required of
such a quenching agent: (1) the quench-
ing agent must react with chlorine instan-
taneously in a manner to prevent further
oxidation of As(lll), (2) it cannot oxidize
As(lll) or reduce As(V), (3) it must not
deleteriously affect the separation pro-
cedure, and (4) it should not substantially
interfere with arsenic analysis by GFAAS.
Several quenching agents were tried.
Ammonia, which had been used previous-
ly in Se(IV) oxidation studies, appeared to
-------
be a good candidate. However, ammonia
could not be used as a quenching agent
because the resulting monochloramine
was found to oxidize As(lll).
Thiosulfate, a common dechlorinating
agent, was also considered but was aban-
doned without testing because it is known
to react with chlorine in a somewhat slow,
stepwise manner. Sodium sulfite, another
common dechlorinating agent, is reported
to react instantaneously with chlorine.
However, experiments indicated that
As(V) was reduced to As(lll) when a
threefold excess of sodium sulfite was
used to quench the 1.0 mg/L CI2 dosage.
N,N-diethyl-p-phenylene diamine oxalate
(DPD), which is used in the colorimetric
test for chlorine, is reported to react in-
stanteously with free chlorine under the
proper conditions. This reaction is one-to-
one, follows Beer's Law, and results in the
destruction of free chlorine. A 4.3-fold
excess of DPD added during turbulent mix-
ing proved to be an excellent quenching
agent. It immediately consumed the free
chlorine without oxidizing As(lll) or reduc-
ing As(V). The pH was adjusted into the
6.2 to 6.5 range by adding a predeter-
mined amount of HCI or NaOH followed
by one-tenth the usual amount of phos-
phate buffer. The amount of phosphate
buffer had to be reduced because phos-
phate interferes with the GFAAS deter-
mination of total arsenic
Chlorine Oxidant Experiments
The composition of the background
water used for the As(lll) oxidation tests,
unless otherwise indicated, was 10 millie-
quivalents (meq)/L comprising 96 mg/L
sulf ate, 71 mg/L chloride, 366 mg/L bicar-
bonate, and 230 mg/L sodium. The pH
was adjusted, if needed, by dropwise ad-
dition of either 0.1 N HCI or 1.0 N NaOH
solution. An initial As(lll) concentration of
0.100 mg/L was chosen because it is twice
the MCL and is not an uncommon concen-
tration in arsenic contaminated ground-
waters. A chlorine dosage of 1.0 mg/L was
considered to be sufficient for the batch
tests because, with the exception of
As(lll), there was no chlorine demand in
the artificial groundwater. The batch
chlorine oxidation tests were conducted
in 2-L polyethylene beakers containing 1-L
samples. A plastic-coated propeller mixer
provided rapid mixing for the addition of
stock chlorine solution and quenching
agent. All time increments were measured
with a laboratory timer. A 100-mL pipette
was used to collect the sample, which was
immediately separated. An initial sample
and the As(lll) fraction were stored in a
60-mL or 125-mL polyethylene bottle for
subsequent GFAAS analysis. An occa-
sional As(V) fraction was eluted and col-
lected as a spot check of separation
recovery; otherwise, As(V) was deter-
mined by difference
Oxygen Oxidant Tests
Oxygen oxidation was tried using two
methods—stagnant and bubbling. The
bubbling oxygen oxidation tests were con-
ducted using different background waters
containing 100 pig/L As(lll). For all these
experiments, a 500-mL sample in a Pyrex
gas washing bottle was bubbled with O2
at a flow rate of 300 to 400 mL/min for
60 min. A control sample to serve as a
blank was bubbled with nitrogen under
these same conditions.
In the stagnant oxygen tests, As(lll)
samples were allowed to sit in 125-mL
polyethylene bottles with air in the
headspace for 61 days. Samples were 200
fjg/L As(lll) in deionized water at pH 6.0,
7.0, and 8.0; 50 u.gll As(V) and 50 u.g/L
As(lll) in deionized water at pH 6.0, 7.0,
and 8.0; and 50 ng/L As(lll) and 50 u.g/L
As(V) in artificial groundwater described
Run Number 1
Influent Composition
previously except with 1 meq Ca2* at pH
7.3 and 8.3.
Reagents
All stock As(V) solutions were prepared
according to Standard Methods using
reagent grade KH2AsO4. All stock As(lll)
solutions were prepared immediately
preceding their use according to Standard
Methods using reagent grade As2O3. All
other solutions, with the exception of
chlorine oxidant, were prepared using the
appropriate reagent-grade chemicals.
Stock chlorine oxidant was prepared from
a commercial bleach (Clorox) containing
sodium hypochlorita The titer of this solu-
tion was checked immediately preceding
use. GFAAS analysis showed no arsenic to
be present in the bleach.
Results
Column Studies
The breakthrough curves for arsenic and
fluoride from the As(lll) and As(V) columns
are shown in Figures 1 and 2. As(lll)
reached the MCL almost immediately
cr
sor
r
As(lll)
Na*
Ca**
pH
EBCT
= 70.9 mg/L. 2.0 meq/L
= 384 mg/L, 8.0 meq/L
= 3.0 mg/L, 0.16 meq/L
= lOOug/L
= 211 mg/L, 9.16 meq/L
= 20.0 mg/L, 1.0 meq/L
= 6.0
= 2.9 min.
1234
Time, Days
8 9 W 11 12 13 14 15 16 17 18
Figure 1.
3456789
1000 Bed Volumes
As(lll)(arsenite). and fluoride breakthrough curves from a minicolumn containing 28
X 48 mesh, F-1 activated alumina.
-------
(Figure 1) at 300 bed volumes (0.6 days),
whereas As(V) did not reach the MCL until
23,400 bed volumes (48 days) (Figure 2).
Thus the presence of pentavalent arsenic
results in column runs nearly 80 times
longer than trivalent arsenic (Figure 3).
Therefore, preoxidation of As(lll)-contain-
ing waters is essential for efficient treat-
ment using activated alumina.
Even though trivalent arsenic appeared
in the effluent very quickly, some As(lll)
was removed. In fact, when the effluent
As(lll) concentration reached 90 percent
of the influent value, a mass balance
showed that 0.344 mg As(lll) had been
removed, yielding an average mass loading
of 0.078 mg As(lll)/g alumina. Speciation
of both the influent and the effluent
showed the arsenic to be totally As(lll).
Thus it appears that it is the trivalent
species that is actually absorbed onto the
alumina, since no oxidation of As(lll)
occurred.
Fluoride adsorption was little affected
by the difference in adsorption of As(lll)
and As(V). The effluent concentration of
fluoride reached the MCL at 1600 bed
volumes, for the column with As(lll) and at
1500 bed volumes for the column with
As(V). Although As(V) is more preferred
than fluoride and As(lll) is less preferred,
the difference in arsenic adsorption is
unlikely to affect fluoride adsorption
because the molar ratio of F- to arsenic is
117.
These laboratory results were in general
agreement with studies on arsenic and
fluoride removal conducted in San Ysidro,
New Mexico, in 1984 using the University
of Houston/EPA Mobile Drinking Water
Treatment Research Facility. The San
Ysidro water contained 48 u.g/L As(V) and
32 u.QlL As(lll), and based on the
laboratory results just presented, it
performed generally as expected with
regard to the bed volumes required to
reach the arsenic MCL. A comparison of
results from the field and laboratory data
for arsenic and fluoride removal are
presented in Table 1. Although more bed
volumes were required to exceed the
fluoride MCL in the San Ysidro water,
which contained only 2.0 mg/L fluoride,
the capacity to the fluoride MCL was
almost exactly the same as that obtained
in laboratory tests. The shape of the
arsenic breakthrough curve in the field
study was surprisingly sharp; a much
earlier breakthrough of trivalent arsenic
was expected. The explanation may be
that some oxidation of As(lll) to As(V) oc-
curred in the field column; also, the
trivalent arsenic concentration of the in-
fluent in the field study was only 0.032
Run Number 2
Influent Composition
Cr
As(V)
/Va+
Ca2*
pH
EBCT
70.9 mg/L. 2.0 meq/L
384 mg/L. 8.0 meq/L
3.0 mg/L. 0. 1 6 meq/L
lOOug/L
211 mg/L. 9. 1 6 meq/L
20.0 mg/L. 1.0 meq/L
6.0
2.9 min.
10
Days
20
100
90
80
70
60
50
40
30
20
10
OJ-
Figure 2.
30
40
50
Fluoride
2 4 6 8 10 12 14 16 18 20 22 24 26
1000 Bed Volumes
As (V) (arsenate), and fluoride breakthrough curves from a minicolumn containing
28 X 48 mesh, F-1 activated alumina.
mg/L—one-third the concentration in the
laboratory study. Furthermore, the pen-
tavalent arsenic breakthrough curve in-
dicated early breakthrough of As(V). This
effect was presumably because of the
shorter EBCT of the lab column, its
shallow bed depth, and the fact that the
adsorption zone was a large fraction of the
bed depth.
Chlorine Oxidant Studies
The data in Figure 4 indicate the results
of the kinetic experiments on the oxida-
tion of arsenic(lll) by chlorine in artificial
groundwater with sodium as the cation.
This figure indicates that the reaction
reached a stable 95 percent completion
within the fastest possible measuring
time—5 sec. This result is not in agree-
ment with equilibrium calculations, which
predict essentially 100 percent comple-
tion. The residual As(lll) concentration
(Figure 4) may, however, be attributable to
leakage of As( V) during the speciation pro-
cedure. In any event, the apparent 95 per-
cent complete reaction is surely sufficient
for water treatment practice.
The experimentally observed effect of
pH on the oxidation of As(lll) by chlorine
is shown in Figure 5. In the neutral pH
range of 6.5 to 9.5, the pH does not
significantly affect the extent of reaction.
Not until pH values are greater than 10.5
is the oxidation reaction affected adverse-
ly. The relative insensitivity of the oxida-
tion reaction of As(lll) to As(V) by chlorine
in the neutral pH range may be attributed
to the fact that As(lll) exists predominantly
as arsenious acid in this pH range and that
the changes in pH do not significantly af-
fect the activation of the species. The
slight decrease in reaction extent at pH 5.5
may be explained by the fact that the reac-
tion produces acid, H+, in the 2.2 to 7.0
pH range. As the pH decreases, the H+
concentration increases, representing an
accumulation of products. Another con-
tributing factor may be the increase in C|-
as a result of acidification from pH 8.3
with HCI.
Oxidation tests performed using 1.0
mg/L chlorine at pH 6.0 in 0.00 to 0.002
M chloride solutions (0 to 710 mg/L
chloride) made from sodium chloride or
calcium chloride indicates that the extent
of As(lll) oxidation was slightly diminished
with increasing chloride concentration.
The reaction was essentially 100 percent
-------
Days
100
\
Aslllli. Arsenite. Hun No. 1
Aslv), Arsenate, Run No. 2
20
25
1000 Bed Volumes
Figure 3.
Comparison ofAs(lll) andAs(V) breakthrough curves from minicolumns of activated
alumina. CT= 700 ug/L. £BCT=2.9 min, andpH = 6.0.
complete in deionized water, but only 90
to 95 percent complete with 350 mg/L
chloride present. This trend may be ex-
pected from the oxidation reaction in
which chloride is a product. Regarding the
effect of calcium as compared with
sodium as the chloride counterion, calcium
seemed to have a slightly greater effect on
diminishing the extent of reaction. These
data indicate, however, that neither
chloride nor the presence of sodium as op-
posed to calcium should be considered
important during As(lll) oxidation by
chlorine in water treatment practice.
Effect of Chloramines
Figure 6 shows the kinetics of oxidation
of 100 ug/L As(lll) in artificial groundwater
containing a dosage of 1.0 mg/L chlorine
with a 10-fold stoichiometric excess of
ammonium chloride at pH 8.3. As ex-
pected, analysis of the chloramine species
showed only the presence of mono-
chloramine—NH2CI. Figure 6 has two
salient points. First, the reaction appears
to reach completion at an As(V)/As(lll)
ratio of approximately two-thirds. And,
secondly, the reaction appears to have
slower kinetics than the oxidation of As(lll)
by free chlorine The following reaction can
be written:
NH2CI
NH4+ +
H3As03 +
HAsCU2- +
H20 =
Cl- + 2
(1)
This reaction has a standard free energy
of —82.4 kcal/mol and an equilibrium
constant of 2.57 X 1060. Under the exper-
imental conditions, thermodynamic calcu-
lations predict an As(V)/As(lll) ratio of 5.66
X 1078 at equilibrium. The observed data
do not match the predicted value. Al-
though the thermodynamic data available
may be somewhat in error, these errors
cannot account for the total difference
between observed and predicted values.
One possible explanation for the in-
complete oxidation is that although the
above reaction is feasible, it does not ac-
tually occur because the energy barrier of
activation is too high. In this case, oxida-
tion may be accomplished by analytically
undetectable amounts (less than 0.1 mg/L)
of free chlorine remaining in solution in
equilibrium with monochloramine. The
sequence of reactions, beginning with the
known-to-be-slow hydrolysis of mono-
chloramine followed by the observed fast
oxidation of As(lll), for pH 6.0, makes the
overall reaction appear as if monochlor-
amine were the oxidant, as shown in
Equation 1. However, HOCI may be the
true oxidant. Note, however, that the ex-
perimentally observed 40 percent oxida-
tion of As(lll) does not quantitatively agree
with the expected oxidation based upon
the most recent studies of monochlor-
amine hydrolysis. Significantly less As(lll)
oxidation is predicted based on the
amount of free chlorine computed from
the published equilibrium constant and the
slow rate of reaction. More work is re-
quired to elucidate the monochloramine
oxidation mechanism.
As Figure 7 shows, the solution pH does
not appear to substantially affect the ex-
tent of reaction in the pH range of 6.5 to
10.5. This pH sensitivity of the chloramine
reaction is very similar to that obtained for
oxidation of As(lll) by free chlorine where
a somewhat decreased rate is observed at
pH 5.5. Although pH-dependent, the reac-
tion of monochloramine to dichloramine is
not considered here because the reaction
occurs slowly and only at a pH of less than
6.0. Calculations from kinetic data show
that after 1 hr, only 7 percent of the
monochloramine should be converted, and
at the 1 min quenching time used in this
experiment, less than 0.1 percent of the
monochloramine should be converted to
dichloramine at the lowest pH examined.
Effect of TOC
The data in Figure 8 show the kinetics
of As(lll) oxidation in aged Houston, Texas,
tap water with 1.0 mg/L chlorine dosage.
Although the oxidation reaction was ap-
preciably slowed in this water, it reached
the same 95 percent completion as did the
oxidation of 100 u.glL As(lll) in artificial
groundwater. Presumably, the chlorine de-
Table 1.
Comparison of Laboratory and Field Data for Fluoride and Arsenic Removal
UH Laboratory Test San Ysidro
Item
As(lll)
As(V>
40% Asdllt
Bed volumes to As MCL
Arsenic capacity, g/m3
Bed volumes to F~ MCL
Fluoride capacity, g/m3
300
18
1600
4190
23400
1610
1550
4280
8760
575
2520
4160
-------
-J
"•
1
.£
<8
-------
Initial Conditions
Qi
3
rjL
•1
.5
5
5
Q)
* 40}-
30 •
20 •
10 •
Initial Conditions
8
pH
10
11
Figure 7.
The effect ofpH on the oxidation of 100 ng/L A sflll) using 1.0 mg/L monochloramine
and excess ammonia.
4. the type of background water.
Adsorption of arsenic onto the
polyethylene bottle surfaces was not
observed during this experiment, as deter-
mined by before-and-after total arsenic
analyses.
Conclusions
Preoxidation of As(lll) to As(V) is essen-
tial to achieve efficient arsenic removal
using activated alumina.
1.0 mg/L free chlorine dosage readily
oxidizes As(lll) to As(V) in the pH range
of 6 to 10, although the reaction is slowed
when TOC is present.
1.0 mg/L combined chlorine (mono-
chloramine) oxidizes about 45 percent of
the As(lll) to As(V) in the pH range of 6
to 10.
Fresh As(lll) solutions should be
prepared as needed because during
storage dilute As(lll) solutions may be
completely oxidized to As(V).
Recommendations
Laboratory Studies
Further studies of the chloramine oxida-
tion of As(lll) should prove useful in
elucidating both the mechanism of As(lll)
oxidation and the means by which
monochloramine acts as an oxidant in
general. The extent of As(lll) oxidation by
monochloramine may vary with the
monochloramine concentration and the
amount of excess ammonia present. Ex-
periments under the conditions used in
this work with different monochloramine
dosages (i.e., 0.5, 2.0, 5.0, and 10.0 mg/L)
and different amounts of excess ammonia
should help determine whether free
chlorine or combined chlorine is the actual
oxidant. Also, further study of longer reac-
tion times may indicate which is the true
oxidant.
Pilot Studies
Pilot studies of As(lll) oxidation by
chlorination of actual arsenic-
contaminated waters will be beneficial.
The presence of reduced species such as
sulfides, Fe(ll), and Mn(ll) may adversely
affect the ease of As(lll) oxidation.
Furthermore, species such as copper are
known to catalyze the destruction of
HOC), which may adversely affect the ex-
tent of reaction at a given chlorine dosaga
The full report was submitted in ful-
fillment of Cooperative Agreement No.
CR 807939 by the University of Houston
under the sponsorship of the U.S. Environ-
mental Protection Agency.
-------
Figure 8.
100 ( •
90
80
70
A Aged Tap Water
TOO = 5 mg/L
pH = 7.5 mg/L
• Synthetic Groundwater
pH = 8.0
5 10 15 20 25 30 35 40 45 50 55 60
Time, Minutes
Comparison of the kinetics of oxidation of 100 fjg/L As(lll) by 1.0 mg/L chlorine
dosage in aged tap water and artificial groundwater.
Table 2. The Results of O2 Oxidation Batch Tests*
Arsenic (III) Oxidized in:
Conditions
Synthetic
Groundwater+
Deionized Water
pH6.0
pH 7.5
pH8.3
1.0 mg/L Fe(ll)
1.0 mg/L Fellll)
3%
5%
5%
8% (pH 7.31
14%
16%
28% (pH 6)
"All tests were of 60 min duration with 100 \ig/L Asllll) initially.
+ Composition: 2 meq/L Cl~, 6 meq/L HCO3~, 2 meq/L SO42~,
1 meq/L Ca2+, 9 meq/L Na +
-------
Phyllis Frank and Dennis Clifford are with the University of Houston, Houston. TX
77004.
Thomas J. Sorg is the EPA Project Officer fsee below).
The complete report, entitled "Arsenicflll) Oxidation and Removal from Drinking
Water," {Order No. PB86-158 607/AS; Cost: $11.95, 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:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
9
U. S. GOVONMBff PRINTING OFFICE:1986/646-] 16/20807
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