Role of Fe(II), phosphate, silicate, sulfate, and carbonate … Masion et al., 1997, 2001; Doelsch et al., 2000). Using a different strategy, synthetic amorphous and crystalline FePO

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Available at www.sciencedirect.com

WAT E R R E S E A R C H ] ( ] ] ] ] ) ] ] ] – ] ] ]

0043-1354/$ - see frodoi:10.1016/j.watres

�Corresponding aTel.: +1 608 262 4972

E-mail addresses

Please cite this acoprecipitation in

journal homepage: www.elsevier.com/locate/watres

Role of Fe(II), phosphate, silicate, sulfate, and carbonatein arsenic uptake by coprecipitation in synthetic andnatural groundwater

Mark C. Ciardellia,�, Huifang Xua, Nita Sahaia,b,�

aDepartment of Geology & Geophysics, 1215 W. Dayton Street, University of Wisconsin, Madison, WI 53706, USAbEnvironmental Chemistry and Technology Program, 1215 W. Dayton Street, University of Wisconsin, Madison, WI 53706, USA

a r t i c l e i n f o

Article history:

Received 23 April 2007

Received in revised form

27 July 2007

Accepted 10 August 2007

Keywords:

Oxyanions

Competition

Phosphate

Silicate

Carbonate

Bicarbonate

Calcium

Iron oxyhydroxides

Hydroxylapatite

Mixed phases

Oxidation

Fenton

Sorption

Arsenate

Arsenite

Seed crystals

Heating

Induced precipitation

nt matter & 2007 Elsevie.2007.08.011

uthors. Department of G; fax: +1 608 262 0693.: [email protected]

rticle as: Ciardelli, M.C.synthetic and natural

a b s t r a c t

Competitive effects of phosphate, silicate, sulfate, and carbonate on As(III) and As(V)

removal at pH �7.2 have been investigated to test the feasibility of Fe(II)aq and

hydroxylapatite crystals as inexpensive and potentially efficient agents for remediation

of contaminated well-water, using Bangladesh as a type study. Arsenic(III) removal

�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

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.

& 2007 Elsevier Ltd. All rights reserved.

1. Introduction

Naturally occurring arsenic contamination in groundwater

has become a major environmental concern on national and

r Ltd. All rights reserved.

eology & Geophysics, 1215

om (M.C. Ciardelli), sahai

, et al., Role of Fe(II), phogroundwater. Water Re

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,

W. Dayton St., University of Wisconsin, Madison, WI 53706, USA.

@geology.wisc.edu (N. Sahai).

sphate, silicate, sulfate, and carbonate in arsenic uptake bys. (2007), doi:10.1016/j.watres.2007.08.011

dx.doi.org/10.1016/j.watres.2007.08.011

mailto:[email protected]

mailto:[email protected]


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WAT E R R E S E A R C H ] ( ] ] ] ] ) ] ] ] – ] ] ]2

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 50mg L�1, 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, HCO3�,

H2PO4�, and HPO4

2� 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

Table 1 – Chemical composition of average Bangladesh grounBangladesh groundwaterb and synthetic Bangaladesh groundw

Species Average BGW, Mean7Std. dev

[Ca2+] 1.9071.4 mM (76 mg L�1)

[Mg2+] 1.3070.8 mM (30.9 mg L�1)

[HCO3] 7.872.7 mM (488 mg L�1)

[As] 2.6572.2 mM (199 mg L�1)

[Fe] 0.09570.085 mM (5.3 mg L�1)

[Si] 0.7070.17 mM (19.2 mg L�1)

[S] 0.07870.031 mM (2.5 mg L�1)

[P] 0.04770.048 mM (1.47 mg L�1)

pH 7.270.2

a Average BGW, surveyed from 3534 tubewells (Kinniburgh and Smedleyb BGW, Srinagar village, Munshiganj district.c SBGW.

Please cite this article as: Ciardelli, M.C., et al., Role of Fe(II), phocoprecipitation in synthetic and natural groundwater. Water Re

arsenic concentration of 500mg L�1), 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 super-

saturated 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 super-

saturation. The motivation for the present research is to

investigate the efficacy of a strategy involving rapid copreci-

pitation 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 elec-

tron 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 compo-

sitions similar to average Bangladesh groundwater solutions.

We used Bangladesh as a type-case because the As contam-

ination problem has been extensively characterized, repre-

sents a typical groundwater composition, and affects a huge

human population. Further, we compared our results to

actual Bangladesh well-waters from two locations.

dwatera compared with the concentrations of naturalater (SBGN)c

BGW, Mean7Std. dev SBGW

1.9970.5 mM (80 mg L�1) 2.5 mM (100 mg L�1)

0.9970.25 mM (24 mg L�1) 1.6 mM (38 mg L�1)

Not measured 8.2 mM (500 mg L�1)

6.2170.87 mM (465 mg L�1) 6.67 mM (500 mg L�1)

0.18070.054 mM (10 mg L�1) 0.180 mM (10 mg L�1)

0.870.18 mM (22.5 mg L�1) 1.1 mM (30 mg L�1)

0.03770.031mM (1.2 mg L�1) 0.160 mM (5 mg L�1)

0.05270.019 mM (1.6 mg L�1) 0.065 mM (2 mg L�1)

Initial: 7.270.2 Initial: 7.170.1

Final: 7.570.2 Final: 7.670.2

, 2001; Roberts et al., 2004).



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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 HCO3� 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 andwithout HAP at 25 and 95 1C

Experiments at 25 1C were conducted in triplicate with

increasing complexity, as summarized in Supporting Infor-

mation 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 L�1 HAP (J.T. Baker) seeds with an average

particle size of 100 nm and a BET surface area of 57 m2 g�1,

followed by the addition of 180mM 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 0.1mm 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 pre-

pared 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 equilibra-

tion 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


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 L�1)

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 Wiscon-

sin—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,

½As�total;pre�cartridge � ½As�total;post�cartridge 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 0.1mm 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 micro-

meter size range using a Hitachi S-3400 N SEM and EDS

system at the University of Wisconsin—Madison.



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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

7367

77

63

76

62

7670

0

20

40

60

80

Fe+P

Solution Composition

% A

s R

em

oved

37

44

36

53

0

20

40

60

80

Well 1 Well 2

Well Number

% A

s R

em

oved

Without HAP

Fe Fe+Si Fe+S

Fig. 1 – Amount of arsenic removed at 24 hours from systems w

(b) multiple oxyacids. (c) Amount of arsenic removed from natura

95 1C from SBGW. Arsenic was added as As(III) in all experimen


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 red-

dish-brown in color within 15–30 min of being exposed to the

atmosphere. The initial Fe(II) concentration in wells number 1

6862

52

6469

58

0

20

40

60

80

Fe+S+P

Solution Composition

% A

s R

em

oved

% A

s R

em

oved

45 46 47 4953 56

60

70

0

20

40

60

80

100

1

Time (hours)With HAP

3 6 24

Fe+Si+S+PFe+Si+S

ith SSBGW, As(III) and Fe(II) with (a) individual oxyacids, and

l Bangladesh groundwater. (d) Amount of arsenic removed at

ts.



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WAT E R R E S E A R C H ] ( ] ] ] ] ) ] ] ] – ] ] ] 5

and 2 was �7 and �12 mg L�1, 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 solu-

tions 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, ob-

served visually as a reddish-brown color in our iron-bearing

Fig. 2 – HRTEM images of precipitates formed without HAP at 25

system. (c) EDS spectra of the solids formed in the SBGW and n


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 consid-

ered 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

1C in (a) the SBGW system with As(III) and (b) natural BGW

atural BGW systems.



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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

(42mm) of these precipitates showed irregularly shaped,

‘‘amorphous’’ particles, as well as well-crystallized, rhombo-

hedral particles identified as magnesian calcite using SEM-

EDS 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

Fig. 3 – SEM images (a, b) and EDS spectra (c) of coarse-grained p

heated for 24 h at 95 1C. Note irregularly shaped, ‘‘amorphous’’ pa

the XRD spectrum for all the coarse-grained precipitates, where

(Mg0.1Ca0.9CO3) indicated by C.


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), postu-

lated 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

recipitates formed from SBGW with As(III), without HAP and

rticles (arrows) (a) and rhombohedral crystals (b). Insert in (c)

reference peaks and Miller indices for magnesian calcite



ARTICLE IN PRESS

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.

WAT E R R E S E A R C H ] ( ] ] ] ] ) ] ] ] – ] ] ] 7

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 coprecipita-

tion with nanoparticulate, am. Fe(OH)3 in the first step, and

adsorption onto coarse-grained magnesian calcite and copre-

cipitation with the fine-grained ferroan saponite-like phase in



ARTICLE IN PRESS

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.

WAT E R R E S E A R C H ] ( ] ] ] ] ) ] ] ] – ] ] ]8

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 Bangla-

desh, 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 copreci-

pitation 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 Uni-

versity of New Mexico-Albuquerque and Ying-Bing Jiang-

University 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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Role of Fe(II), phosphate, silicate, sulfate, and carbonate … Masion et al., 1997, 2001; Doelsch et al., 2000). Using a different strategy, synthetic amorphous and crystalline FePO