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] (]]]]) ]]]– ]]]
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/watres
Role of Fe(II), phosphate, silicate, sulfate, and carbonate
in arsenic uptake by coprecipitation in synthetic and
natural groundwater
Mark C. Ciardellia,, Huifang Xua, Nita Sahaia,b,
a
Department of Geology & Geophysics, 1215 W. Dayton Street, University of Wisconsin, Madison, WI 53706, USA
Environmental Chemistry and Technology Program, 1215 W. Dayton Street, University of Wisconsin, Madison, WI 53706, USA
b
art i cle info
ab st rac t
Article history:
Competitive effects of phosphate, silicate, sulfate, and carbonate on As(III) and As(V)
Received 23 April 2007
removal at pH 7.2 have been investigated to test the feasibility of Fe(II)aq and
Received in revised form
hydroxylapatite crystals as inexpensive and potentially efficient agents for remediation
27 July 2007
of contaminated well-water, using Bangladesh as a type study. Arsenic(III) removal
Accepted 10 August 2007
50–55% is achieved, when Fe(II)aq oxidizes to Fe(III) and precipitates as Fe(OH)3 at 25 1C
and 3 h reaction time, in the presence of all the oxyanion. Similar results were obtained for
Keywords:
Oxyanions
Competition
Phosphate
Silicate
Carbonate
Bicarbonate
Calcium
Iron oxyhydroxides
Hydroxylapatite
Mixed phases
well-water samples from two sites in Bangladesh. Heating at 95 1C for 24 h results in 70%
As(III) uptake due to precipitation of magnesian calcite. A two-step process, Fe(II) oxidation
and Fe(OH)3 precipitation at 25 1C for 2 h, followed by magnesian calcite precipitation at
95 1C for 3 h, yields 65% arsenic removal while reducing the expensive heating period. In
the absence of silicate, up to 70% As(III) uptake occurs at 25 1C. In all cases, As(III) was
oxidized to As(V) in solution by dissolved oxygen and the reaction rate was probably
promoted by intermediates formed during Fe(II) oxidation. Iron-catalyzed oxidation of
As(III) by oxygen and hydrogen peroxide is pH-dependent with formation of oxidants in the
Fenton reaction. Buffering pH at near-neutral values by dissolved carbonate and
hydroxylapatite seeds is important for faster Fe(II) oxidation kinetics ensuring rapid
coprecipitation of As as As(V) in the ferric phases.
Oxidation
& 2007 Elsevier Ltd. All rights reserved.
Fenton
Sorption
Arsenate
Arsenite
Seed crystals
Heating
Induced precipitation
1.
Introduction
Naturally occurring arsenic contamination in groundwater
has become a major environmental concern on national and
global levels, affecting large human populations in Nepal,
Vietnam, Bangladesh, Taiwan, Eastern India, Chile, Argentina,
Mexico, the western United States and even in mid western
USA (e.g. Berg et al., 2001; Cheng et al., 2005; Dixit and Hering,
Corresponding authors. Department of Geology & Geophysics, 1215 W. Dayton St., University of Wisconsin, Madison, WI 53706, USA.
Tel.: +1 608 262 4972; fax: +1 608 262 0693.
E-mail addresses: [email protected] (M.C. Ciardelli), [email protected] (N. Sahai).
0043-1354/$ - see front matter & 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.watres.2007.08.011
Please cite this article as: Ciardelli, M.C., et al., Role of Fe(II), phosphate, silicate, sulfate, and carbonate in arsenic uptake by
coprecipitation in synthetic and natural groundwater. Water Res. (2007), doi:10.1016/j.watres.2007.08.011
ARTICLE IN PRESS
2
WAT E R R E S E A R C H
2003; Nickson et al., 2000; Roberts et al., 2004; Thornburg and
Sahai, 2004). As(III) is the more mobile form and is 60 times
more toxic than As(V) (Ferguson and Gavis, 1972; Jain and Ali,
2000). In Bangladesh, 30–40 million people (Roberts et al.,
2004) are estimated to be consuming water with arsenic
concentrations greater than 50 mg L1, which is 5 times more
than is allowed by the US Environmental Protection Agency
and World Heath Organization guidelines. Additionally,
thousands to tens of thousands of people already show the
effects of long-term exposure. The average (3534 tubewells
sampled) chemical composition of Bangladesh groundwater
including arsenic levels is shown in Table 1.
Remediation strategies for arsenic have focused on sorption
onto aluminum and iron (oxyhydr)oxides, and oxidation
(primarily photocatalytic) of As(III) to As(V) followed by
sorption (Beaulieu and Savage, 2005; Dutta et al., 2005; Gupta
et al., 2005; Hug et al., 2001; Kanel et al., 2005; Meng et al.,
2001; Su and Puls, 2003; Zhang and Itoh, 2005). A complicating
factor in arsenic remediation strategies involving adsorption
is competition by other oxyacids such as H4SiO4, HCO
3,
2
H2PO
4 , and HPO4 that commonly occur in groundwater (Jain
and Loppert, 2000; Meng et al., 2000, 2002; Goldberg, 2002).
These ions also inhibit precipitation of crystalline ferric
oxyhydroxide phases (Rose et al., 1996; Waychunas et al.,
1996; Masion et al., 1997, 2001; Doelsch et al., 2000). Using a
different strategy, synthetic amorphous and crystalline FePO4
phases were studied for arsenic uptake by adsorption and
coprecipitation (Lenoble et al., 2005). When iron is added to
synthetic Bangladesh groundwater, ferric (oxyhydr)oxides
precipitate, taking up arsenic simultaneously. Fe(II) is more
effective than Fe(III), and repetitive additions of Fe(II) are
more effective than a single, large dose (Roberts et al., 2004). It
was proposed that Fe(II) promotes rapid oxidation of As(III) to
As(V) by molecular oxygen via the involvement of reactive
oxygen species in the Fenton reaction and of putative Fe(IV)
and As(IV) reactive intermediates (Hug and Leupin, 2003).
Repeated additions of Fe(II) and lemon juice have shown to be
quite effective (up to 90% arsenic removal with an initial
] (]]]]) ] ]]– ]]]
arsenic concentration of 500 mg L1), because they promote
faster oxidation of Fe(II) to Fe(III) and, apparently, of As(III) to
As(V) (Hug et al., 2001; Roberts et al., 2004).
Preliminary aqueous speciation calculations indicated that
the average Bangladesh groundwater composition is supersaturated with respect to hydroxylapatite (Ca5(PO4)3OH, HAP)
(Ciardelli, 2006), although we recognize that variations in Ca
and P concentrations greatly impact the degree of supersaturation. The motivation for the present research is to
investigate the efficacy of a strategy involving rapid coprecipitation of arsenic along with HAP such that competition
from other oxyacids would be minimal.
Because of their ability for multiple ionic substitutions
(Pan and Fleet, 2002), apatites have received attention for
their potential to be used as a soil amendment, and as an
additive to water for decreasing toxic and radioactive metal
concentrations including lead, zinc, and uranium (Gomez
de Rio et al., 2004; Lee et al., 2005; Mavropoulos et al.,
2002; Ohnuki et al., 2004). The effectiveness of uptake
depends on specific metal and on the conditions of the
system. The anticipated ability to coprecipitate arsenic with
HAP was based on the existence of the mineral, johnbaumite
(Ca5(AsO4)3OH).
Our study is unique in several ways. First, we characterized
not only the solution phase but, importantly, also the solid
phases formed from nano- to micron-sized by high-resolution
transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), energy-dispersive spectra (EDS), and
X-ray diffraction (XRD). Equally significant, instead of the
highly simplified 1:1 electrolyte solutions and very high As
concentrations typically employed in experimental studies,
we used complex synthetic groundwater solutions of compositions similar to average Bangladesh groundwater solutions.
We used Bangladesh as a type-case because the As contamination problem has been extensively characterized, represents a typical groundwater composition, and affects a huge
human population. Further, we compared our results to
actual Bangladesh well-waters from two locations.
Table 1 – Chemical composition of average Bangladesh groundwatera compared with the concentrations of natural
Bangladesh groundwaterb and synthetic Bangaladesh groundwater (SBGN)c
Species
[Ca2+]
[Mg2+]
[HCO3]
[As]
[Fe]
[Si]
[S]
[P]
pH
a
b
c
Average BGW, Mean7Std. dev
BGW, Mean7Std. dev
SBGW
1.9071.4 mM (76 mg L1)
1.3070.8 mM (30.9 mg L1)
7.872.7 mM (488 mg L1)
2.6572.2 mM (199 mg L1)
0.09570.085 mM (5.3 mg L1)
0.7070.17 mM (19.2 mg L1)
0.07870.031 mM (2.5 mg L1)
0.04770.048 mM (1.47 mg L1)
7.270.2
1.9970.5 mM (80 mg L1)
0.9970.25 mM (24 mg L1)
Not measured
6.2170.87 mM (465 mg L1)
0.18070.054 mM (10 mg L1)
0.870.18 mM (22.5 mg L1)
0.03770.031mM (1.2 mg L1)
0.05270.019 mM (1.6 mg L1)
Initial: 7.270.2
Final: 7.570.2
2.5 mM (100 mg L1)
1.6 mM (38 mg L1)
8.2 mM (500 mg L1)
6.67 mM (500 mg L1)
0.180 mM (10 mg L1)
1.1 mM (30 mg L1)
0.160 mM (5 mg L1)
0.065 mM (2 mg L1)
Initial: 7.170.1
Final: 7.670.2
Average BGW, surveyed from 3534 tubewells (Kinniburgh and Smedley, 2001; Roberts et al., 2004).
BGW, Srinagar village, Munshiganj district.
SBGW.
Please cite this article as: Ciardelli, M.C., et al., Role of Fe(II), phosphate, silicate, sulfate, and carbonate in arsenic uptake by
coprecipitation in synthetic and natural groundwater. Water Res. (2007), doi:10.1016/j.watres.2007.08.011
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2.
Methods
2.1.
Simplified synthetic Bangladesh groundwater
(SSBGW)
SSBGW, containing only 2.5 mM Ca2+, 1.6 mM Mg2+, and
8.2 mM HCO
3 was prepared fresh for each set of experiments
by dissolving CaCO3 (Fisher) and MgCO3 (Fisher) in deionized
water (18.2 MO-cm). To aid in the dissolution of the carbonate
species, CO2(g) (99% pure, Linde Gas) was bubbled through the
suspension with vigorous stirring for approximately 8 h or
until all of the CaCO3 and MgCO3 was dissolved (Roberts et al.,
2004). The resulting solution had a pH of 5.6–6.0. Complete
synthetic Bangladesh groundwater (SBGW) is listed in Table 1.
2.2.
Batch experiments: 1–24 h experiments with and
without HAP at 25 and 95 1C
Experiments at 25 1C were conducted in triplicate with
increasing complexity, as summarized in Supporting Information Table 1. For each experiment, 200 mL of SSBGW was
dispensed into acid (HNO3)-washed low-density polyethylene
(LDPE) bottles. Compressed air was bubbled through each
solution to displace the excess CO2 and raise the pH to
7.170.05. Depending on the experiment, Si, As, S, and P
(concentrations given in Table 1) were added, in this order,
from stock solutions of 0.1 M Na2SiO4 (Aldrich), 0.01 M NaAsO2
or Na2HAsO4 7H2O (Aldrich), 0.1 M Na2SO4 (Fisher), and
0.01 M Na2HPO4 (Fisher). Experiments were initiated by adding
synthetic 0.5 g HAP L1 HAP (J.T. Baker) seeds with an average
particle size of 100 nm and a BET surface area of 57 m2 g1,
followed by the addition of 180 mM of Fe(II) from a stock
solution of 0.1 M FeCl2 4H2O (Fisher). A fresh Fe(II) stock
solution was prepared for each experiment and used within
5 min of preparation.
All solutions were reacted open to atmosphere for 1, 3, 6,
and 24 h. Over 24 h, the pH increased to 7.670.2, due to
degassing of CO2 from the solutions. This is also observed
when natural Bangladesh groundwater is reacted open to
atmosphere (Roberts et al., 2004). At the end of each reaction
time, the suspensions were centrifuged for 30 min at
15,000 rpm and filtered through a 0.1 mm nylon filter. These
solutions were then acidified (2% HNO3, ultra-high purity
(Fisher)), and stored at 4 1C until ICP-OES analysis.
Heated batch experiments performed at 95 1C were prepared as described above in the presence of all oxyacids
including As(III). Following Fe(II) addition, the solutions were
placed in a water bath and heated to 95 1C. The time it took for
the solutions to reach 95 1C was recorded (thermal equilibration time 15 min). Once the solutions reached 95 1C, they
were reacted open to the atmosphere for 1, 3, 6, and 24 h, and
sampled as previously described.
2.3.
Batch experiments: two-step process
The two-step process combined the methods used for the
batch experiments at 25 and 95 1C. The SBGW solution was
prepared as previously described. Following Fe(II) addition,
the solutions were reacted open to atmosphere at 25 1C for
] (]]] ]) ]]]– ]]]
3
2 h, and filtered with a 0.1 mm nylon filter to remove the
precipitates. Each filtered solution was sampled, stored, and
analyzed as described above. HAP seed crystals (0.5 g HAP L1)
were added to the filtered solutions, heated to 95 1C for 3 h,
filtered, sampled, and analyzed as described above.
2.4.
Batch experiments: natural Bangladesh groundwater
Two sets of experiments were preformed using natural
Bangladesh groundwater taken from two different rural wells.
Both wells were located in the Munshiganj district, close to
the village of Srinagar, which is 30 km south of Dhaka. At each
well, two 250 mL LDPE bottles were filled with 200 mL of well
water. About 100 mg of HAP seed crystals were added to one
of the solutions (200 mL). Both solutions were equilibrated
open to atmosphere for 24 h, filtered, sampled, and stored at
25 1C without centrifugation. A week after the samples were
taken, they were mailed back to the University of Wisconsin—Madison. During the week-long mailing period, the
samples were not kept cold.
2.5.
Solution analysis
Solutions were analyzed for aqueous concentrations of
[As]total, [Ca]total, [Fe]total, [K]total, [Mg]total, [Na]total, [P]total,
[S]total, and [Si]total by using ICP-OES on a Perkin–Elmer
Optima 4300 DV instrument. All solutions were analyzed
within 2 weeks of being sampled. In order to determine the
concentration of As(V) in solutions produced by oxidation of
As(III), [As]total measured from solutions, passed through As
speciation cartridges (MetalSoft, Inc.) was subtracted from
[As]total measured before the solution was passed through the
cartridge. Thus,
½Astotal;precartridge ½Astotal;postcartridge ¼ ½AsðVÞ.
These cartridges are reported to have an As(V) retention
efficiency of 91–95% (Meng and Wang, 1998; Dr. Linda Roberts,
EAWAG, Switzerland, pers. comm.).
2.6.
Analysis of precipitates
Precipitates were filtered out of solution with a 0.1 mm nylon
filter and characterized by powder XRD using a Scintag PAD V
Diffractometer with a Cu Ka X-ray tube rated at 2000 W. All
samples were analyzed with the X-ray diffractometer set at
40 mV and 35 keV, and with a step size of 0.02 2y and 4 s
dwelling time.
HRTEM and SEM were also used to analyze precipitated
solids in the nanometer size range. TEM/SEM samples were
prepared by dropping precipitate-bearing suspensions onto
holey-carbon-coated copper grids. All the TEM and EDS
analyses were obtained by using a JEOL 2010F FASTEM
equipped with Oxford Instruments EDS system and Gatan
GIF system at the University of New Mexico, and a Tecnai F30
equipped with a EM Vision 4.0 EDS system at the University of
Chicago.
SEM was preformed on precipitated solids in the micrometer size range using a Hitachi S-3400 N SEM and EDS
system at the University of Wisconsin—Madison.
Please cite this article as: Ciardelli, M.C., et al., Role of Fe(II), phosphate, silicate, sulfate, and carbonate in arsenic uptake by
coprecipitation in synthetic and natural groundwater. Water Res. (2007), doi:10.1016/j.watres.2007.08.011
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2.7.
Thermodynamic modeling
Arsenic speciation, mineral stability, and saturation indices
were calculated using the Act 2 and React modules of
Geochemist’s Workbenchs 3.1 (Bethke, 1996), for solutions
having SBGW composition. The program uses the DATA0
thermodynamic database that is also used in the SUPCRT and
EQ3NR/EQ6NR programs (Johnson et al., 1992; Wolery,
1992a, b).
3.
Results and discussion
3.1.
Arsenic removal at 25 1C
Fig. (1a–c) shows data for As removal at 25 1C and uptake at
95 1C is shown in Fig. (1d) discussed later. In control
experiments with SSBGW containing As(III) or As(V), no
arsenic uptake was detected, and concentrations of Ca2+
and Mg2+ also stayed constant. Visually, the solutions
appeared clear. In contrast, all systems with Fe(II) turned
reddish-brown in color within the first 15–30 min of reaction
73
time, and significant arsenic uptake was measured. The
equilibration period (1, 3, 6, and 24 h) had little effect on the
amount of arsenic uptake after the first hour. As(III) removal
achieved was 73% for the 24 h time period (Fig. 1a), and
greater than 90% removal of As(V) was observed (not shown).
Maximum arsenic removal occurred in the presence of Fe(II),
and where no other oxyacids were present.
Oxyacids greatly affected the amount of arsenic uptake in
the As(III) systems. Fig. (1a) shows that silicate has the worst
effect. Arsenic(III) removal in the presence of silicate or
phosphate was 67% and 63%, respectively, which are less by
5% and 9% than the systems without competing oxyacids.
The uptake at 24 h was further reduced to 52% when all
oxyacids were present (Fig. 1b). For all systems with As(V), the
oxyacids had no significant effect (490% As removed; not
shown). In all systems with As(III) and As(V), HAP seed
crystals only minimally affected arsenic uptake (Fig. 1).
Greater than 99% removal of iron was observed in all
experiments.
Samples of natural Bangladesh groundwater turned reddish-brown in color within 15–30 min of being exposed to the
atmosphere. The initial Fe(II) concentration in wells number 1
77 76
76
67
63
62
80
70
60
% As Removed
% As Removed
80
] (]]]]) ] ]]– ]]]
40
20
68
Fe
69
60
58
52
40
20
Fe+Si
Fe+S+P
Fe+Si+S
Fe+P
Fe+S
Fe+Si+S+P
Solution Composition
Solution Composition
100
60
53
44
37
36
% As Removed
80
% As Removed
62
0
0
40
64
20
80
60
70
45
53
60
56
46
49
47
40
20
0
0
Well 1
1
Well 2
Well Number
Without HAP
With HAP
3
6
24
Time (hours)
Fig. 1 – Amount of arsenic removed at 24 hours from systems with SSBGW, As(III) and Fe(II) with (a) individual oxyacids, and
(b) multiple oxyacids. (c) Amount of arsenic removed from natural Bangladesh groundwater. (d) Amount of arsenic removed at
95 1C from SBGW. Arsenic was added as As(III) in all experiments.
Please cite this article as: Ciardelli, M.C., et al., Role of Fe(II), phosphate, silicate, sulfate, and carbonate in arsenic uptake by
coprecipitation in synthetic and natural groundwater. Water Res. (2007), doi:10.1016/j.watres.2007.08.011
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and 2 was 7 and 12 mg L1, respectively. Almost 100% iron
was removed during aeration, along with 37–44% and 44–53%
arsenic removal, respectively, in the absence and presence of
HAP seed crystals (Fig. 1c).
In studies where very similar synthetic groundwater solutions were used, precipitation of ferric oxyhydroxides was
observed with efficient As removal (Hug and Leupin, 2003;
Roberts et al., 2004). The oxidation of Fe(II) to Fe(III) involves
reactive oxygen intermediates via the Fenton Reaction. The
rate of As(III) oxidation to As(V) by molecular oxygen is
usually very slow. It was proposed that in the presence of
Fe(II), reactive intermediate species such as Fe(IV) and As(IV)
may be involved in accelerating the rate of As(III) oxidation to
As(V), followed by uptake in the Fe(III) oxyhydroxides formed.
A similar process is likely responsible for As uptake in our
systems that initially contained As(III).
HRTEM analysis showed that particles formed in the
absence of HAP in the SBGW system were sub-spherical,
50 nm in size, and lacked any lattice fringes (Fig. 2a). The
particles appeared remarkably similar to the precipitates
obtained from natural Bangladesh groundwater at 25 1C
(Fig. 2b). Fe, O, As, Si, P, and Ca peaks were identified in the
HRTEM-EDS spectra (Fig. 2c). The Fe and O peaks are
consistent with Fe(II) oxidation to Fe(III) and precipitation of
amorphous ferric (oxyhydr)oxide, nominally Fe(OH)3, observed visually as a reddish-brown color in our iron-bearing
] (]]] ]) ]]]– ]]]
5
solutions and as expected from thermodynamic modeling (SI
Fig. 1).
The presence of As peaks in the EDS spectra and the lower
[As]tot measured in Fe-bearing solutions indicate that As is
associated with the Fe(OH)3 phase. In order to infer the mode
of arsenic uptake, as adsorbate or coprecipitate, we considered the EDS spectra. In general, the low sensitivity of the EDS
technique precludes positive identification of adsorption as a
potential uptake mode, because the concentration of sorbed
species would be too low for detection. Thus, the very
presence of As in the EDS spectrum suggests uptake by
coprecipitation. Given the nanometer size of the particles,
however, it is difficult to distinguish clearly between surface
and bulk atoms, so labeling the uptake mode as adsorbate
versus coprecipitate is somewhat semantic.
Similar to As, the presence of Si and P peaks in the EDS
spectra suggests that silicate and phosphate were taken up as
coprecipitates with the Fe(OH)3. Numerous studies have
shown that silicate and phosphate exhibit a high affinity for
adsorption and coprecipitation in the presence of iron
(oxyhydr)oxides (Holm, 2002; Jain and Loeppert, 2000; Meng
et al., 2000, 2002; Roberts et al., 2004; Su and Puls, 2001;
Swedlund and Webster, 1999), and can inhibit formation of
crystalline iron oxyhydroxide phases (Masion et al., 1997,
2001). Furthermore HAP precipitation was not induced by
addition of seed crystals at 25 1C, but the EDS spectra and
Fig. 2 – HRTEM images of precipitates formed without HAP at 25 1C in (a) the SBGW system with As(III) and (b) natural BGW
system. (c) EDS spectra of the solids formed in the SBGW and natural BGW systems.
Please cite this article as: Ciardelli, M.C., et al., Role of Fe(II), phosphate, silicate, sulfate, and carbonate in arsenic uptake by
coprecipitation in synthetic and natural groundwater. Water Res. (2007), doi:10.1016/j.watres.2007.08.011
ARTICLE IN PRESS
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WAT E R R E S E A R C H
solution analyses indicate the presence of Ca in the solid
phase and a slight decrease (o1%) of Ca in solution.
3.2.
Arsenic removal at elevated temperatures
Because HAP did not precipitate at 25 1C, solutions were
heated to 95 1C. As(III) removal increased with longer heating
time, up to 70% at 24 h with HAP (Fig. 1d). The system without
HAP was insensitive to changes in heating time, remaining
constant at 46% arsenic uptake.
White to tan and reddish-brown precipitates of different
particle sizes, were observed for all heating times in all
solution compositions at 95 1C. The larger-sized fraction
(42 mm) of these precipitates showed irregularly shaped,
‘‘amorphous’’ particles, as well as well-crystallized, rhombohedral particles identified as magnesian calcite using SEMEDS and XRD (Fig. 3). The EDS analysis showed Mg and Si but
no P associated with the amorphous particles, and Si plus P in
the magnesian calcite, but no As peaks in either. Yet, solution
analyses indicate that heating increased arsenic uptake at
95 1C compared to 25 1C. This suggests that arsenic does not
] (]]]]) ] ]]– ]]]
coprecipitate with the magnesian calcite phase, but is
probably taken up as an adsorbate. The point of zero charge
of calcite varies from 7 to 10.8 and is a function of
experimental conditions associated with the formation of
the calcite (Romero et al., 2004), so the surface would be
positively charged at our experimental pH 7, allowing for As
adsorption.
The smaller-sized fraction (75–100 nm) showed particles
with two morphologies, sub-spherical and foil-like (Fig. 4a
and b). For the foil-like particles, a high-energy As peak at
11.7 keV corresponding to Kb is seen in the HRTEM-EDS
spectrum (Fig. 4c). D-spacings from XRD (Fig. 4d) and selected
area electron diffraction (SAED) (Fig. 4b) combined with the
EDS chemical analysis (Fig. 4c) show that these solids do not
correspond to any mineral phase but are closest to the peaks
for ferroan saponite (Ca0.5(Mg, Fe)3(Si, Al)4O10(OH)2 4H2O) (SI
Table 2). The XRD peaks for our precipitate were slightly
shifted from the reference ferroan saponite (Fig. 4d), postulated to be the result of Fe3+ substituting for Al3+ in the
ferroan saponite structure, because there is no Al in our
system. The sub-spherical particles appear similar to the
Fig. 3 – SEM images (a, b) and EDS spectra (c) of coarse-grained precipitates formed from SBGW with As(III), without HAP and
heated for 24 h at 95 1C. Note irregularly shaped, ‘‘amorphous’’ particles (arrows) (a) and rhombohedral crystals (b). Insert in (c)
the XRD spectrum for all the coarse-grained precipitates, where reference peaks and Miller indices for magnesian calcite
(Mg0.1Ca0.9CO3) indicated by C.
Please cite this article as: Ciardelli, M.C., et al., Role of Fe(II), phosphate, silicate, sulfate, and carbonate in arsenic uptake by
coprecipitation in synthetic and natural groundwater. Water Res. (2007), doi:10.1016/j.watres.2007.08.011
ARTICLE IN PRESS
WAT E R R E S E A R C H
] (]]] ]) ]]]– ]]]
7
Fig. 4 – HRTEM images of precipitates formed from the SBGW with As(III), without HAP heated at 95 1C for 24 h (a, b). The
arrow (a) points to amorphous precipitates of morphology similar to precipitates in Fig. 2. Note predominantly foil-like
precipitates. Insert in (b) is SAED pattern for the foil-like precipitates. EDS spectrum for fine-grained, foil-like precipitates (c).
Insert zooms in on As Kb peak. XRD spectrum for fine-grained precipitates (d). Reference peaks for ferroan saponite indicated
by S.
amorphous Fe(OH)3 precipitates obtained for SBGW and
natural BGW.
The presence of amorphous Fe(OH)3 is consistent with
visual observations of a reddish-brown precipitate within the
first 15 min, before the solution reaches thermal equilibrium.
Once the system reaches 95 1C, we infer that the remaining
iron precipitates as the ferroan saponite-like phase; some of
the amorphous Fe(OH)3 may also be converted to ferroan
saponite with retention of the As, as seen in the EDS
spectrum. Thermodynamic modeling results suggest that
neither phase should be present, and even ferroan saponite
is predicted to be metastable only if several other phases are
prevented from precipitating (SI Fig. 2). Thus, the presence of
both phases indicates that the system is metastable even at
95 1C.
3.3.
Two-step process
As expected, 50% arsenic uptake is seen from SBGW in the
As(III) system at 25 1C and 2 h equilibration in air, and up to
62% uptake by subsequent heating to 95 1C for 3 h in the
presence of HAP seed crystals (Fig. 5). Thus, the two-step
process is designed to maximize As(III) uptake by coprecipitation with nanoparticulate, am. Fe(OH)3 in the first step, and
adsorption onto coarse-grained magnesian calcite and coprecipitation with the fine-grained ferroan saponite-like phase in
Please cite this article as: Ciardelli, M.C., et al., Role of Fe(II), phosphate, silicate, sulfate, and carbonate in arsenic uptake by
coprecipitation in synthetic and natural groundwater. Water Res. (2007), doi:10.1016/j.watres.2007.08.011
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Fig. 5 – Amount of arsenic removed in the two-step process.
The back line represents the arsenic removed in one-step
heating at 95 1C for 3 h.
the second step. By breaking up the process into two steps,
roughly 5% more arsenic is removed than during a ‘‘one-step’’
heating experiment for 3 h.
3.4.
Implications for As remediation
When developing an arsenic remediation strategy for Bangladesh, Eastern India and other developing countries, one must
consider the economic feasibility and simplicity of the system
being developed. We have shown here that employing a
‘‘passive removal system’’ where naturally present Fe(II) is
allowed to oxidize and precipitate as Fe(OH)3 can be effective
at removing arsenic contamination in the absence of silicate
and phosphate and in the presence of carbonate. However,
this effectiveness is decreased to 55% As uptake when initial
concentrations of Fe(II) are low, and when silicate and
phosphate are present.
In a parallel study, we have studied As(III) and As(V) uptake
from solutions of very similar composition to the present
study except that phosphate was present but carbonate and
silicate were absent (Sahai et al., 2007). The HRTEM-EDS and
synchrotron X-ray Absorption Spectroscopy, both EXAFS and
XANES, results showed the formation of a mixed ferric
oxyhydroxide phosphate arsenate phase, FeIII[(OH)3, PO4,
AsVO4,] nH2O. In the absence of carbonate, pH buffering on
addition of Fe(II) was provided by HAP seed crystals but less
efficiently than the carbonate in the present study, so that
solution pH dropped from 7.2 to 6.5, and Fe(II) oxidation
rate was slowed down. As a result, incomplete Fe(III) removal
was obtained with correspondingly reduced As uptake 55%
(Sahai et al., 2007). Thus, we have seen that the presence of
phosphate and silicate and incomplete pH buffering reduces
As(III) uptake by coprecipitation with ferric phases from
groundwater that initially contained Fe(II). Silicate is shown
in our study to have the greatest effect on As uptake. The
inhibitory effect of Si on ferric oxyhydroxide precipitation is
well-studied. Mixed ferric oxyhydroxide phosphate arsenate
phases, in some cases with silicate and calcium, have been
reported in the natural estuarine and lake sediments at
slightly acidic to neutral pHs (Perret et al., 2000; Webb et al.,
2000; Hyacinthe and Van Cappellen, 2004), and mixed ferric
] (]]]]) ] ]]– ]]]
arsenate sulfate phases exist under acid mine drainage
conditions (Craw and Chappell, 2000; Morin et al., 2003).
The results of this research suggest that simply adding HAP
seed crystals to average synthetic and natural Bangladesh
groundwater solutions at 25 and 95 1C is not sufficient to
induce hydroxylapatite precipitation and adsorption and/or
coprecipitation of arsenic. On the other hand, coprecipitation
of As with ferric oxyhydroxide phases, and mixed ferric
oxyhydroxide phosphate arsenate phases does present a
promising strategy, although competitive effects of silicate
and phosphate must be considered. By employing a two-step
remediation strategy, where passive removal is allowed to
occur for 2 h, followed by the addition of HAP seed crystals
and heating at 95 1C, the competitive effects of silicate and
phosphate on arsenic removal are partially mitigated,
although still not to levels below the drinking water limit.
The effectiveness of this proposed strategy is dependent on
the heating time during the second step with greater
efficiency at longer heating times. Although heating water is
potentially expensive for people in developing countries, this
method would also be a means of simultaneously sterilizing
the drinking water against many bacteria. Future work will
focus on developing practical methods for inducing coprecipitation in a more cost-effective manner.
4.
Conclusions
In the presence of competing oxyanions in groundwater
solutions, As(III) uptake efficiency can be increased from 50%
up to 65–70% by coprecipitation with ferric oxyhydroxides and
calcium carbonate, where Fe(II) is oxidized and hydrolyzed to
ferric oxyhydroxides and As(III) is oxidized to As(V) by
molecular oxygen, followed by heating to about 95 1C for 2–3 h.
Acknowledgments
We thank Dr. Stephan Hug, EAWAG, Switzerland for originally
drawing us into the arsenic problem in Bangladesh, and for
performing the field experiments; Drs. Xiao-Zhou Liao University of New Mexico-Albuquerque and Ying-Bing JiangUniversity of Chicago for assistance with HRTEM analysis; Dr.
John Fournelle for SEM analysis; and Dr. Martin Schafer and
Brian Majestic for assistance with ICP-OES analysis. This
project was funded by NSF EAR CAREER Grant 0346689 and
faculty start-up funds from UW-Madison to N.S.
Appendix A.
Supplementary materials
Supplementary data associated with this article can be found
in the online version at doi:10.1016/j.watres.2007.08.011.
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