Combined effects of anions on arsenic removal by iron hydroxides Xiaoguang Meng

Toxicology Letters 133 (2002) 103– 111
Combined effects of anions on arsenic removal by iron
Xiaoguang Meng a,*, George P. Korfiatis, Sunbaek Bang, Ki Woong Bang b
Center for En6ironmental Engineering, Ste6ens Institute of Technology, Hoboken, NJ 07030, USA
Department of En6ironmental Engineering, Hanbat National Uni6ersity, Taejon, South Korea
Batch experiments were conducted to investigate the combined effects of phosphate, silicate, and bicarbonate on
the removal of arsenic from Bangladesh groundwater (BGW) and simulated groundwater by iron hydroxides. The
apparent adsorption constants indicated that the affinity of the anions for iron hydroxide sites decreased in the
following order arsenate \phosphate\ arsenite\silicate\bicarbonate. Phosphate, silicate, and bicarbonate decreased the removal of As(III) even at relatively low concentrations and low surface site coverage. Phosphate (0 – 0.08
mM), silicate (0–0.8 mM), and bicarbonate (0–14 mM) in separate solutions had none to moderate effects on As(V)
removal in a solution containing 6.7 mg/l Fe and 0.3 ppm As(V). In the presence of bicarbonate and silicate the
adverse effect of phosphate on As(V) adsorption was magnified. The residual As(V) concentration after iron
hydroxide treatment increased from less than 13 mg/l in separate bicarbonate (2.2 mM) and phosphate (0.062 mM)
solutions to 110 mg/l in the solution containing both anions. The results suggested the combined effects of phosphate,
silicate, and bicarbonate caused the high mobility of arsenic in Bangladesh water. © 2002 Elsevier Science Ireland
Ltd. All rights reserved.
Keywords: Arsenic; Iron hydroxides; Adsorption; Bangladesh; Phosphate; Silicate; Bicarbonate; Anion effects; Selectivity
1. Introduction
Arsenate [As(V)] and arsenite [As(III)] are common arsenic species in naturally contaminated
groundwater and surface water in many countries.
Millions of wells are drilled into Ganges alluvial
deposits for public water supply in Bangladesh
and West Bengal (Nickson et al., 1998; Das et al.,
1996). The release of arsenic from the arsenic* Corresponding author. Tel.: +1-201-216-8014; fax: + 1201-216-8303.
E-mail address: [email protected] (X. Meng).
bearing aquifer sediments may have polluted
more than 3 million of the approximately 5 million existing wells in Bangladesh, affecting upto
70 million people (Lepkowski, 1999). The mobility of arsenic in groundwater sediment and water
treatment sludge is governed by the redox potential (Meng et al., 2001b; Nickson et al., 1998).
Arsenic is typically associated with iron oxides
under oxic environment and with pyrite minerals
under anoxic conditions. Arsenic can be released
as the results of pyrite mineral oxidation or reduction of iron oxides in an intermediate redox range
(i.e. − 4B peB 0) (Meng et al., 2001b).
0378-4274/02/$ - see front matter © 2002 Elsevier Science Ireland Ltd. All rights reserved.
PII: S 0 3 7 8 - 4 2 7 4 ( 0 2 ) 0 0 0 8 0 - 2
X. Meng et al. / Toxicology Letters 133 (2002) 103–111
Ferric chloride and sulfate are commonly used
for the removal of arsenic from water because
iron hydroxides have high removal capacity for
arsenic (Gulledge and O’Conner, 1973; Cheng et
al., 1994; McNeill and Edwards, 1995). The chemical composition of water such as phosphate and
silicate concentrations can significantly affect the
removal of arsenic by iron hydroxides. Silicate
decreases the removal of As(III) and As(V) in
potassium nitrate solutions by coprecipitation
with ferric chloride (Meng et al., 2000). Phosphate
enhances the mobility of As(V) in soils contaminated with lead arsenate (Peryea and Kammereck,
1997). The removal of SO24 − , SeO23 − , PO34 − , and
CrO24 − by iron hydroxides was affected adversely
by silicate (Meng and Letterman, 1996; Goldberg,
1985; Zachara et al., 1987).
Recent experimental results have shown that
arsenic in Bangladesh well groundwater is difficult
to remove by iron hydroxides because of elevated
phosphate and silicate concentrations (Meng et
al., 2001a). An iron to arsenic mass ratio of
greater than 40 is required to remove arsenic to
less than 50 mg/l which is the current drinking
water standard in Bangladesh. On the other hand,
a Fe/As ratio of less than 12 is sufficient to
remove greater than 99% of arsenic from groundwater collected from New Hampshire (NH), USA.
In the present study, batch coagulation tests were
conducted with Bangladesh well water and simulated groundwater to evaluate the combined effects of phosphate, silicate, and bicarbonate on
the removal of As(V) and As(III) by iron hydroxides. The results reported in the present work will
benefit the design of more effective arsenic treatment processes and the development of more accurate models for predicting the transport of
arsenic in aquifers.
2. Experimental methods
All chemicals used in the experiments were
reagent grade. A Fe(III) stock solution containing
2000 mg/l Fe(III) and 0.1% HCl was prepared
from FeCl3 · 6H2O (Fisher, Pittsburgh, PA) and
trace metal grade HCl (Fisher). As2O5 · 3H2O
(Aldrich, Milwaukee, WI) was dissolved in dis-
tilled-deionized (DI) water to prepare a primary
stock solution containing 1000 mg/l As(V). A
secondary stock solution of 10 mg/l As(V) was
prepared every week by dilution of the primary
stock solution with DI water. NaAsO2 (Fisher)
stock solution containing 1000 mg/l As(III) was
prepared every 2 weeks. Simulated Bangladesh
groundwater (BGW) containing 0.82 mM MgCl2,
2.5 mM CaCl2, and different amounts of
Na2SiO3 · 5H2O (0– 0.8 mM), NaH2PO4 (0–0.08
mM), NaHCO3 (0–14 mM), and NaCl (0 or 2.2
mM) were used to test the effects of phosphate,
silicate, and bicarbonate on arsenic removal.
The As(III) or As(V) stock solution was added
to the water in 1 l beakers to reach an arsenic
concentration of 300 mg/l in the batch experiments. Then ferric chloride solution was added to
the beakers to reach an iron concentration of 6.7
mg/l. The solution pH was controlled at 6.8 by
addition of NaOH or HCl. After 30 min of mixing the suspension was filtered through a 0.45 mm
membrane filter for the analysis of soluble arsenic,
iron, phosphate, and silicate.
Removal of arsenic from Bangladesh well waters was tested by oxidizing ferrous ion and
As(III) through aeration or addition of sodium
hypochlorite solution and subsequent filtration.
Water samples were collected from four tube wells
in Kishoreganj and Munshiganj districts of
Bangladesh. The samples were aerated and stored
in 125 ml polypropylene bottles without acidification. After the samples were oxidized by exposure
to air for 10 days, they were passed through a
0.45 mm membrane filter to remove the ferric
hydroxide precipitate. In another set of field experiments, sodium hypochlorite solution was
added to the water samples to oxidize As(III) and
Fe(II). A residual chlorine content of approximately 1 mg/l was maintained by the addition of
the hypochlorite solution during approximately 5
min of mixing. After the samples were mixed for
30 min, they were filtered through 0.4 mm membrane filter. Similar arsenic removal was obtained
when As(III) and Fe(II) were completely oxidized
by air and hypochlorite.
Groundwater collected from a well in NH was
used to represent groundwater with low phosphate concentration. The water sample contained
X. Meng et al. / Toxicology Letters 133 (2002) 103–111
Table 1
Chemical composition of the waters used in coprecipitation tests
As (mg/l)
Fe (mg/l)
P (mg/l)
Si (mg/l)
Na (mg/l)
Ca (mg/l)
Mg (mg/l)
BGW, Bangladesh well water; NH, New Hampshire well water; SB, Simulated Bangladesh water.
approximately 70 mg/l As(V) and 0.7 mg/l iron
(Table 1), respectively. As(III) and Fe(II) were
added into the NH samples to reach similar total
As and Fe concentrations as in the Bangladesh
water samples (BGW). The water samples was
treated with sodium hypochlorite and then filtered
to removal particulate arsenic with the same procedures as those used for the treatment of BGW.
Arsenic and iron concentrations were determined using a Furnace Atomic Absorption Spectrometer (FAAS) (Varian SpectrAA-400) and
inductively coupled plasma (ICP) emission spectrometer (Varian Liberty-200). As(V) and As(III)
in water samples were separated using arsenic
speciation cartridges (Meng et al., 2001b) immediately after the samples were collected. Soluble
phosphate and silicate concentrations were determined by the ascorbic acid and heteropoly blue
methods, respectively (Clesceri et al., 1989).
much higher than the current maximum contaminant level of 50 mg/l arsenic for drinking water in
Bangladesh. Speciation analysis of the soluble arsenic showed that As(III) was completely oxidized
to As(V). The removal of iron was greater than
99%, indicating a complete conversion of ferrous
ions to ferric hydroxide.
Arsenic removal by coprecipitation with iron
hydroxide from spiked NH water samples was
more efficient than from Bangladesh water (Fig.
1). When As(III) and Fe(II) was added into the
NH water to reach the same total arsenic and iron
concentrations as in Bangladesh well 1 water,
arsenic concentration was reduced from 587 to 15
mg/l by oxidation and filtration. In contrast, the
arsenic in the treated well 1 water sample was
only reduced to 187 mg/l. The final pH of the NH
water samples was controlled at similar values to
that of the well water samples. The results suggested that some coexisting solutes in Bangladesh
waters adversely affected the removal of arsenic.
3. Results and discussion
3.1. Effects of anions on As remo6al from
Bangladesh water
The removal of arsenic from BGW and spiked
NH groundwater is shown diagrammatically in
Fig. 1. Total arsenic and iron concentrations in
the well waters ranged from 280 to 587 mg/l and
5.5 to 7.8 mg/l, respectively. Approximately, 86%
of the arsenic in the well waters was As(III).
Greater than 95% of the Fe was in soluble form.
The pH of the well water was approximately 6.9.
After the water samples were oxidized and passed
through the membrane filter, 51– 75% of the arsenic was removed. The residual arsenic concentrations ranged from 100 to 187 mg/l, which was
Fig. 1. Removal of arsenic from BGW and spiked NH water
samples by ferric hydroxides at equilibrium pH 7.0 90.2.
Total Fe: 7.7 mg/l in well 1 water; 6.7 mg/l in well 2; 5.5 mg/l
for well 3; and 7.8 mg/l for well 4.
X. Meng et al. / Toxicology Letters 133 (2002) 103–111
Fig. 2. Effects of anions on As(V) and As(III) removal from
SBW (2.5 mM CaCl2, 0.82 mM MgCl2, and 2.2 mM NaCl)
containing single anions. Initial As = 300 mg/l; total Fe(III) =
6.7 mg/l; equilibrium pH 6.9 9 0.1.
The concentrations of selected anions and
cations are listed in Table 1 for Bangladesh and
NH water samples. Both cation and anion concentrations in BGW were obviously higher than in
Bangladesh water was approximately two orders
of magnitude higher than in NH water. Silicate
concentration in Bangladesh water was two to
three times higher than that in NH water.
The effects of the anions on the removal of
Bangladesh water (SB) containing the same arsenic, iron, and cation concentrations as
Bangladesh well 2 water (Table 1). Bicarbonate,
silicate, and phosphate were added into the SB
water samples separately. The effect of sulfate was
not investigated since its concentration was less
than 3 mg/l in most of Bangladesh well waters (2).
As(V) and As(III) (in separate solutions) were
effectively removed to 2 and 16 mg/l, respectively,
by iron hydroxides from the NaCl solution (Fig.
2). In the bicarbonate, silicate, and phosphate
solutions the residual As(III) concentration was
79, 156, and 186 mg/l, respectively. The adverse
effects of the anions on As(III) removal decreased
in the following order: phosphate\ silicate\ bicarbonate\ chloride. The chloride and bicarbonate concentrations were much higher than silicate
concentration in the water. The phosphate concentration was lower than silicate concentration
by an order of magnitude. It should be noted that
phosphate had only a slight effect on the removal
of As(V). Bicarbonate and silicate had none and
moderate effects on As(V) removal, respectively.
None of the anions inhibited As(V) removal to
the extent observed in Bangladesh well waters
(Fig. 1).
The results in Fig. 3 illustrate the combined
effects of the anions on arsenic removal. The
concentrations of arsenic, iron, and the anions
were the same as in the solutions shown in Fig. 2
except that combinations of the three types of
anions were used in the solutions. The residual
As(III) concentrations in the multi-anion solutions (Fig. 3) were slightly higher than in the
single anion solutions (Fig. 2). For instance, the
residual As(III) concentration was increased from
156 mg/l in silicate solution (Fig. 2) to 165 mg/l in
a solution containing both silicate and bicarbonate (Si+HCO3, Fig. 3). The residual As(III) concentration was increased from 186 mg/l in the
phosphate solution to 221 mg/l in the phosphate–
silicate–bicarbonate solution.
As seen from Fig. 3, the residual As(V) was
dramatically increased in the multi-anion systems
except for the bicarbonate–silicate solution. The
residual As(V) in the bicarbonate–silicate solution
was 13 mg/l, which was lower than that in the
silicate solution (Fig. 2). This behavior was not
expected since the addition of bicarbonate to the
Fig. 3. Combined effects of anions on As(V) and As(III)
removal from SBW (2.5 mM CaCl2, 0.82 mM MgCl2, and 2.2
mM NaCl). Initial As =300 mg/l; total Fe(III) =6.7 mg/l;
equilibrium pH 6.9 90.1.
X. Meng et al. / Toxicology Letters 133 (2002) 103–111
Fig. 4. Removal of As(V) as a function of bicarbonate and
silicate concentrations. Initial As(V) = 300 mg/l, Fe(III) = 6.7
mg/l, MgCl2 = 0.82 mM, CaCl2 = 2.5 mM, 2.2 mM NaCl,
equilibrium pH 6.8.
silicate system should further reduce the amount
of surface sites available for As(V) adsorption. All
the experimental results were verified through at
least three repeated tests. The residual As(V) concentration was increased from less than 13 mg/l in
the single phosphate and bicarbonate solutions
(Fig. 2) to 111 mg/l in the phosphate– bicarbonate
solution (Fig. 3). When silicate and phosphate
coexisted, As(V) concentration was also dramatically increased. The residual As(V) concentration
in the phosphate– silicate – bicarbonate solution
was 124 mg/l.
The efficiency of As(V) removal from the multianion solutions (Fig. 3) and from well 2 water
(Fig. 1) was similar. The results in Fig. 3 demonstrated that the coexistence of phosphate with
silicate and bicarbonate hindered the removal of
As(V) from the well waters. Phosphate was a key
competing anion affecting the removal of As(V).
The effect of phosphate was magnified in the
presence of bicarbonate and silicate.
affected when silicate concentration was in a
range 0–0.6 mM. At high silicate concentration
rang, the removal of As(V) was obviously reduced. When silicate concentration increased to
approximately 0.7 mM, arsenic removal was reduced from greater than 99% (i.e. residual As: 2
mg/l) to approximately 85% (i.e. residual As: 44
mg/l). The results were consistent with the previously reported data which showed that silicate
decreased As(V) removal in KNO3 solution especially at pH greater than 7 (Meng et al., 2000).
When silicate and bicarbonate coexisted in the
solution, As(V) removal was high than in single
silicate solution. The observation agreed with the
results shown in Figs. 2 and 3. Additional experiments are required to understand the effects of
bicarbonate on As(V) removal in silicate solution.
Phosphate had very little effect on the removal
of As(V) when its concentration increased from 0
to 70 mM (i.e. 0.07 mM) in the single phosphate
solution (Fig. 5). At a phosphate concentration of
80 mM, the removal of As(V) was decreased from
99.4% (residual arsenic As= 1.9 mg/l) to 90.7%
(residual As=28 mg/l). In the presence of 2.2 mM
3 , As(V) removal decreased linearly with the
increase of phosphate concentration from 20 to 80
mM. Only 56% of As(V) was removed at a phosphate concentration of 80 mM in the combined
solution. When phosphate concentration increased from 0 to 80 mM in a 0.64 mM silicate
3.2. Remo6al of As as a function of anion
The effects of the anions on the arsenic removal
were further investigated over a wide concentration range of the anions. When bicarbonate concentration increased from 0 to 13 mM, the
removal of As(V) by iron hydroxides was not
affected (Fig. 4). The removal of As(V) was not
Fig. 5. Removal of As(V) as a function of phosphate concentration in different anion solutions. Initial As(V) = 300 mg/l,
Fe(III)= 6.7 mg/l, MgCl2 =0.82 mM, CaCl2 =2.5 mM, 2.2
mM NaCl, equilibrium pH 6.8.
X. Meng et al. / Toxicology Letters 133 (2002) 103–111
3.3. Binding affinity of anions and adsorption
The binding affinity of the anions for the surface sites and the adsorption density were determined in order to understand the effects of the
competing anions on As(V) and As(III) removal.
The binding affinity was determined using adsorption equilibrium constants. The adsorption of anions on iron hydroxide surface can be described
by the equation:
SOH + Ln − + mH+USHq L(n − 1 − q) − + H2O
Fig. 6. Removal of As(III) as a function of bicarbonate,
silicate, and phosphate concentrations. Initial As(III) =300
mg/l, Fe(III)=6.7 mg/l, MgCl2 = 0.82 mM, CaCl2 = 2.5 mM,
2.2 mM NaCl, equilibrium pH 6.8.
solution, As(V) removal decreased from 85 to
65%. It is obvious that bicarbonate and silicate
magnified the effects of phosphate on As(V) removal by ferric hydroxides.
Iron hydroxides were less effective in removing
As(III) than As(V) at a neutral pH. In the chloride solution, approximately 95% of the As(III)
was removed (Fig. 6). The removal of As(III)
decreased significantly as bicarbonate and silicate
concentrations increased in the separate solutions.
When bicarbonate and silicate concentrations
were 13 and 0.8 mM, the removal of As(III) was
reduced to 64 and 42%, respectively. The effect of
phosphate on As(III) removal was more dramatic
than bicarbonate and silicate. When phosphate
concentration was 0.08 mM, the removal of
As(III) was only 24%.
(m= 1 to 4; q= m−1)
where SOH denotes the hydroxyl sites on iron
hydroxide surface, Ln − indicates the anions, and
SHq L(n − 1 − q) − represents the adsorbed anion as
surface complexes. The equilibrium constant for
reaction Eq. (1) can be expressed as
[SHq L(n − 1 − q) − ]
[SOH][Ln − ][H+]m
where [ ] indicates the concentration of the
aqueous and surface species. The equilibrium constant is affected by electrostatic or Coulombic
interaction between the surface potential and the
adsorbed ions (Stumm, 1992). At constant pH, an
apparent equilibrium constant can be used to
describe the adsorption of the anions.
K app =
[SHq L(n − 1 − q) − ]
= [H+]mK
[SOH][Ln − ]
K app values for the adsorption of the anions by
iron hydroxides were determined from the initial
and equilibrium anion concentrations and the surface site concentration (Table 2). The initial con-
Table 2
Binding affinity of anions for Fe(OH)3 in SB
Total SOH sites (mM)
Initial anion concentration (mM)
Equilibrium anion concentration (mM)
K app
High Equilibrium pH 6.8, 2.5 mM CaCl2, 0.82 mM MgCl2, 2.2 mM NaCl.
X. Meng et al. / Toxicology Letters 133 (2002) 103–111
Table 3
Surface site coverage by adsorbed anions
Total concentration (mM)
Anion/SOH molar ratio
As(V)+P system
Site coverage by adsorbed anions (%)
3.619 0.03
3.2 90.1
2.429 0.13
45.2 90.03
28.4 91.3
Total coverage (%)
60 922
42 929
Total Fe(III) concentration = 6.7 mg/l (0.108 mM SOH sites); equilibrium pH 6.8; solution composition: 2.5 mM CaCl2, 0.82 mM
MgCl2, 2.2 mM NaCl.
centration of the anions, except bicarbonate, was
0.010 mM in separate anion solutions. Total
Fe(III) concentration and equilibrium pH was 6.7
mg/l (i.e. 0.12 mM) and 6.8, respectively. Based
on a surface site density of 0.9 mmol SOH per
mmol Fe (Meng and Letterman, 1993), the total
surface site concentration in the suspension was
0.108 mM. As(V) adsorption isotherms also
showed that the adsorption capacity of ferric hydroxides was greater than 0.6 mmol As(V) per
mmol Fe (Meng et al., 2000).
Among the anions tested, As(V) had the
strongest binding affinity for iron hydroxides
(Table 2). The K app value of As(V) was seven
times greater than that of phosphate. Therefore,
phosphate had only slight effect on the removal of
As(V) when its concentration was approximately
0.08 mM (Fig. 5). Hingston (1981) also reported
that goethite had higher selectivity for As(V) over
phosphate in a neutral pH range. The binding
constant of As(V) was 800 times greater than
silicate. Silicate had moderate affect on As(V)
removal only at high concentrations (i.e. Si\ 0.6
mM, Fig. 4). The affinity of As(III) for iron
hydroxides was much weaker than As(V) and
phosphate. Therefore, the removal of As(III) was
reduced significantly by phosphate, silicate, and
bicarbonate (Figs. 2 and 6).
The amounts of surface sites occupied by adsorbed anions were determined in suspensions
containing the same concentrations of the anions
and iron as in well 2 water. The molar ratios of
the anions to the surface sites were calculated
based on the total anion concentrations and total
surface sites in the suspensions (Table 3). The
total surface site concentration in the suspension
containing 6.7 mg Fe/l was 0.108 mM. According
to the anion/SOH molar ratios, if all phosphate
(0.05 mM) and As(V) (0.004 mM) were adsorbed
on the iron hydroxide surface, they would have
occupied 46 and 3.7% of the surface sites, respectively. Analysis of the equilibrium phosphate concentration showed that approximately 98% of the
phosphate was removed by iron hydroxides, resulting in a surface site coverage of 45.2%. Therefore, the insignificant effect of phosphate on
As(V) removal in the single anion solution (Figs.
1 and 5) could be attributed to low surface site
coverage by phosphate and high affinity of As(V)
for the surface sites.
Silicate concentration in the water was 5.9 times
higher than the surface site concentration (Table
3). However, only approximately 10% of the total
silicate was removed from the single silicate solution because of weak affinity of silicate for iron
hydroxides. The surface sites covered by the adsorbed silicate were approximately 60%. The standard deviation values for the site coverage were
calculated from the equilibrium silicate concentrations obtained in repeated experiments. Since the
silicate concentrations were much greater than the
surface site concentration, small analytical errors
for the silicate concentration resulted in large
standard deviation in the surface site coverage.
When phosphate and silicate coexisted in the
solution (As(V)+ P+ Si system, Table 3), the percentage of surface sites occupied by each type of
anions decreased due to competitive effects. The
total site coverage increased from 49% in single
phosphate solution to 73% in the solution con-
X. Meng et al. / Toxicology Letters 133 (2002) 103–111
taining both phosphate and silicate. The increased surface site coverage decreased removal
of As(V).
Phosphate significantly decreased the removal
of As(III) even at very low surface site coverage
(Fig. 6). As(III) removal decreased from 95 to
73% when phosphate concentration increased
from 0 to 0.02 mM (Fig. 6). The phosphate to
surface site molar ratio was only 0.18 when
phosphate concentration was 0.02 mM. The results suggested that the surface sites on iron hydroxides were not uniform. Phosphate occupied
the highly active surface sites because of its
higher adsorptive affinity than As(III). Infrared
spectroscopy studies revealed that three types of
surface OH groups existed on goethite (aFeOOH) (Sun and Doner, 1996). The surface
hydroxyl groups of hydrous metal oxides are
heterogeneous due to the structural diversity of
the crystal faces, and the presence of exposed
edges, corners and defects (Kinniburgh and
Jackson, 1981).
4. Conclusions
Since As(V) had the highest affinity for iron
hydroxide surface sites among the anions tested,
phosphate and silicate could significantly reduce
the removal of As(V) only at high surface site
coverage. At normal levels of phosphate in
BGW (PB 70 mM), phosphate alone did not
have a significant effect on As(V) removal by
iron hydroxide. However, the presence of silicate
and bicarbonate magnified the effect of phosphate, thus, inhibiting arsenic removal. Phosphate and silicate could substantially reduce the
removal of As(III) even at low surface coverage
due to low affinity of As(III) for the surface
sites. Bicarbonate had a moderate effect on the
removal of As(III).
The authors would like to thank the Department of Public Health Engineering and the Local Government Engineering Department of
Bangladesh for assistant with the field testing
and sampling. The assistance of Maria Eugenia
Pena with the laboratory experiments is acknowledged.
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