Performance and mechanism of simultaneous removal of chromium and arsenate by Fe(II) from contaminated groundwater

Performance and mechanism of simultaneous removal of chromium and arsenate by Fe(II) from contaminated groundwater

Separation and Purification Technology 80 (2011) 179–185 Contents lists available at ScienceDirect Separation and Purification Technology journal home...

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Separation and Purification Technology 80 (2011) 179–185

Contents lists available at ScienceDirect

Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur

Performance and mechanism of simultaneous removal of chromium and arsenate by Fe(II) from contaminated groundwater Xiaohong Guan a,⇑, Haoran Dong a, Jun Ma a, Irene M.C. Lo b, Xiaomin Dou c a

State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, 150090 Harbin, PR China Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Hong Kong, PR China c College of Environmental Science and Engineering, Beijing Forestry University, 100083 Beijing, PR China b

a r t i c l e

i n f o

Article history: Received 19 November 2010 Received in revised form 17 April 2011 Accepted 19 April 2011 Available online 5 May 2011 Keywords: Chromate Arsenate Ferrous iron Reduction Co-precipitation

a b s t r a c t The feasibility and the mechanisms of simultaneous removal of chromium and arsenate by Fe(II) were investigated. In the absence of arsenate, chromium removal by Fe(II) increased to the maximum with increasing pH from 4 to 7 and then decreased with further increase in pH. Chromium removal by Fe(II) was controlled by the rate of Cr(VI) reduction by Fe(II) and the solubility of Fe0.75Cr0.25(OH)3 at pH 6 7, but by the extent of Cr(VI) reduction under alkaline conditions. The presence of arsenate resulted in a decrease in chromium removal by Fe(II) under neutral and alkaline conditions as a result of the depression in the magnitude of Cr(VI) reduction by Fe(II) and sequestration of the Fe0.75Cr0.25(OH)3 and FeOOH precipitation by HAsO2 4 . Arsenate removal by Fe(II) alone was trivial but was improved significantly at pH 4–9 due to the presence of 10–30 lmol L1 chromate. It was the oxidative property of chromate that resulted in the oxidization of Fe(II) to Fe(III) concomitantly facilitating the removal of arsenate. Arsenate was removed by both adsorption and co-precipitation with Fe0.75Cr0.25(OH)3 and FeOOH precipitates. EXAFS results revealed that arsenate mainly coordinated with Fe(III) rather than Cr(III) and arsenate formed bidentate-binuclear complexes with FeO(OH) octahydra as evidenced by an average Fe–As(V) bond distance of 3.25–3.26 Å. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Chromated copper arsenate (CCA), the most commonly used wood preservative in North America in recent decades, poses a risk to groundwater due to the potential leaching of copper, chromate and arsenate [1–3]. The toxicity of copper, chromium and arsenic to aquatic organisms has been well recorded with these elements listed as priority pollutants by the United States Environmental Protection Agency [4]. Among these three metals, Cu is relatively benign and most easily retained by soils under common environmental conditions (pH 6–8) and thus least likely to contaminate groundwater [5,6]. In contrast to copper, chromate and arsenate are much more mobile in soil and hence pose serious risks to groundwater quality [5,7,8]. In view of the numerous health problems arising from groundwater contaminated by these two pollutants [9,10], the World Health Organization (WHO) and the Ministry of Health of P.R. China have established a provisional guideline of 10 lg/L for As and 50 lg/L for Cr(VI) in drinking water [11,12]. Recent public concern regarding As and Cr(VI) in drinking ⇑ Corresponding author. Tel.: +86 (451) 8628 3010, fax: +86 (451) 82368074. E-mail addresses: [email protected] (X. Guan), [email protected] (H. Dong), [email protected] (J. Ma), [email protected] (I.M.C. Lo), [email protected] (X. Dou). 1383-5866/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2011.04.034

water has promoted the investigation of treatment technologies with the potential to remove them simultaneously to levels well below the drinking water maximum contaminant level. Previous studies revealed that co-removal of chromate and arsenate by ion exchange [13] or adsorption with iron-oxide coated sand [14] suffered from low efficiency. Additionally, adsorptive retention by application of zero-valent iron was also ineffective for arsenate removal as a result of the competition between chromate and arsenate for active sites [6]. Therefore, more effective methods for simultaneous removal of chromate and arsenate from contaminated groundwater should be explored. Since Cr(III) is relatively innocuous and immobile, the reduction of Cr(VI) to Cr(III) and the formation of insoluble chromium precipitates are essential steps in remediating the Cr(VI)-contaminated groundwater. A number of chemical reductants for converting Cr(VI) to Cr(III) have been described in the literature including Fe(II), FeS2, iron electrodes and reduced sulfur compounds [15– 18]. Of these reductants, Fe(II) is the most widely reagent utilized for chromium removal from solution [15,19–22]. In the reaction between Cr(VI) and Fe(II), Cr(VI) is reduced to Cr(III) by Fe(II) with Fe(II) concomitantly oxidized to Fe(III). The Fe(III) so produced will hydrolyze and precipitate with subsequent adsorptive removal of arsenate from solution [23]. Moreover, the reduced Cr(III) can be easily sorbed and/or coprecipitated with ferric hydroxide and

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forms FexCr1x(OH)3(s) precipitates, which also has the capacity to adsorb arsenate [24]. Although many studies have investigated chromate reduction by Fe(II), almost all studies focused on Cr(VI) removal and few studies paid attention to the removal of soluble Cr(III). The soluble Cr(III) is much more mobile than the precipitated Cr(III) and more probably re-oxidized by the naturally occurring manganese oxides in groundwater [22]. For drinking water treatment, removal of both Cr(VI) and Cr(III) is vital to a successful Cr treatment technology since Cr(III) can be re-oxidized to Cr(VI) by chlorine or other strong oxidizing disinfectants [21]. Therefore, this study proposed to employ Fe(II) to remove chromium and arsenate simultaneously from the contaminated groundwater with the following objectives: (i) to examine the performance of this process; (ii) to determine the residual chromium species in the solution; and (iii) to investigate the mechanisms of chromium removal by Fe(II) at various pH values, chromium removal in the presence of arsenate, and arsenate removal by Fe(II) in the presence of chromate.

practical retention time in groundwater treatment, a reaction time of 2 h was employed in this study. Each experiment was carried out at least in duplicate with reproducible results. All points in the figures are the mean of the results in duplicate and error bars represent standard deviation of the means. After each test, an aliquot of the supernatant was filtered immediately through a cellulose acetate membrane (MFS) of 0.45 lm pore size, acidified with one drop of 65% HNO3 and analyzed for total As, Cr and Fe by an PerkinElmer Optima 5300 DV inductively coupled plasma optical emission spectrometer (ICP-OES), whose detection limits for As, Cr and Fe are about 7 lg L1, 1 lg L1, 0.1 lg L1, respectively. The amount of iron in the precipitate was calculated by subtracting the residual Fe concentration in aqueous solution from the initial Fe(II) concentration. The removal percentage of chromium or arsenate, defined as the ratio of difference in initial Cr/As(V) concentration before and after reaction (Ci  Ce) to the initial Cr/As(V) concentration in the aqueous solution (Ci), was calculated using the following equation:

2. Materials and methods

% Removal ¼

2.1. Chemicals and reagents

To analyze Cr(VI), 50 lL of 1 M acetate buffer was dosed into the samples collected at various pH and the resulting pH was 4.3. The Cr(VI) concentration of the mixture was determined within 3– 5 min after filtration by measuring the absorbance at 370 nm using 1 cm quartz cells in a Cary 300 UV/visible spectrophotometer with detection level of 10–20 lg L1. Fe(II) concentration were examined by the modified ferrozine method described by Stookey [27] using a T6-New Happy UV/visible spectrophotometer at a wavelength of 562 nm. The detection limit of Fe(II) measurement method was 33 lg L1. The Cr(III) concentration was obtained by subtracting the Cr(VI) concentration from the total Cr concentration and the Fe(III) concentration was determined by subtracting Fe(II) from the total Fe. A high performance pH meter with a saturated KCl solution as electrolyte (Corning 350) was used to measure solution pH. Daily calibration with pH 4.00, 6.86 and 9.18 buffer solutions was performed to ensure its accuracy. The detailed procedure of spectroscopic investigation was presented in the supporting information.

All chemicals used in the experiments were reagent grade and all solutions were prepared with distilled water. The stock solutions of As(V) and Cr(VI) were prepared weekly from Na3AsO47H2O and K2Cr2O7, respectively. FeSO4 solution was prepared freshly for each set of experiments by dissolving FeSO47H2O in distilled water and then acidified (to avoid Fe(II) oxygenation) by adding drops of concentrated HCl to the solution. Background electrolyte solutions were prepared from the reagent-grade salts NaCl and NaHCO3. Samples containing As(V) and/or Cr(VI) were prepared by diluting stock solutions to target concentrations with a constant ionic strength of 0.01 mol L1 NaCl with 0.001 mol L1 NaHCO3 added to provide alkalinity. All glassware was cleaned by soaking in 10% HNO3 and rinsed three times with distilled water. 2.2. Batch experiments and chemical analysis A series of 100 mL samples containing As(V) and/or Cr(VI) of predetermined concentration were prepared and their pH values were adjusted to 4–10 with HCl or NaOH. The reaction was initiated by dosing acidified ferrous iron and the flasks were immediately capped and placed in a reciprocating shaker. The concentration of Fe(II) was checked just before use, using the analytical techniques described below, to ensure that Fe(II) had not been oxidized before the experiments. During the reaction, the pH of the mixture was not adjusted and final pH was recorded at the end of each experiment. The concentrations of both Cr(VI) and As(V) employed in this study were typical of those in contaminated groundwater with Cr(VI) concentrations in the range of 0–22 mg L1 and As concentrations in the range of 0.0005–5 mg L1 [6,25,26]. A shaking rate of 130 rpm was used with all experiments being carried out at 22 °C in a water bath. Given that it is very difficult to purge oxygen from the process stream during full scale drinking water treatment, the experiments were carried out without any attempt to control dissolved oxygen (DO) concentrations. The initial DO concentration was in the range of 5.46–5.82 mg/L, determined by an oxygen microprobe (YSI Pro20). The preliminary study indicated that both chromium and arsenate removal by Fe(II) could approach equilibrium in 2 h under common environmental conditions (pH 6–8) when they coexisted in solution. Although prolonged reaction time could improve chromium and arsenate removal significantly at pH < 6, it had limited influence on chromium and arsenate removal at pH > 6 (data not shown). Considering the

Ci  Ce  100 Ci

ð1Þ

3. Results and discussion 3.1. Chromium removal by Fe(II) at various pH levels Chromium removal by Fe(II) as a function of pH in the absence of arsenate (i.e. in the presence of 0 lmol L1 arsenate) is presented in Fig. 1(a). Chromium removal increased from 11.3% to 97.5% as pH increased from 3.9 to 6.9 then decreased gradually to 29.1% with pH further increase to 9.8. The species of residual chromium and iron in the process of Cr(VI) removal by Fe(II) were determined and are shown in Fig. 2. The concentration of residual Cr(III) in the supernatant was in the range of 0.83–1.11 lmol L1 at pH 3.9–4.7 and below 0.43 lmol L1 at pH > 4.7. The predominant residual chromium species present in solution was Cr(VI) over the pH range of 3.9–9.8. The concentration of residual Cr(VI) at pH 3.9 was as high as 7.43 lmol L1 but dropped to 0.22 lmol L1 at pH 7. However, the concentration of soluble Cr(VI) rose markedly with further increase in pH and was 5.9 lmol L1 at pH 9.8. The concentration of residual Fe(II) decreased from 23.8 to 4.1 lmol L1 as pH increased from 3.9 to 6.0 while it was negligible at pH > 6.0, indicating the nearly complete oxidation of Fe(II) under neutral and alkaline conditions. The concentration of soluble Fe(III) varied from 0.18 to 1.53 lmol L1 throughout the pH range of 3.9–9.8. The Cr 2p and Fe 2p line XPS spectra of the precipitates collected in the process of Cr(VI) removal by Fe(II) at various pH levels

181

Chromium removal (%)

100

As(V)=0 μmol L-1 As(V)=10 μmol L-1 As(V)=20 μmol L-1

80

60

40

20

0

b

30 25

-1

a

Iron in the precipitate (μmol L )

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20 15 10 As(V)=0 μmol L-1 As(V)=10 μmol L-1 As(V)=20 μmol L-1

5 0

3

4

5

6

7

8

9

10

11

3

4

5

6

7

8

9

10

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Final pH

Final pH

Fig. 1. (a) Chromium removal by Fe(II) in the presence of arsenate of various concentrations at different pH levels; (b) the amount of iron entrapped in the precipitate generated in this process (Cr(VI) = 10 lmol L1, Fe(II) = 30 lmol L1).

8

20 6 15 4 10 2

5

0

-1

25

Residual iron in the solution (μmol L )

30 Cr(VI) Cr(III) Fe(II) Fe(III)

-1

Residual chromium in the solution (μmol L )

10

0 3

4

5

6 7 Final pH

8

9

10

Fig. 2. Speciation of residual chromium and iron in the process of chromate removal by Fe(II) (Cr(VI) = 10 lmol L1, Fe(II) = 30 lmol L1).

are illustrated in Fig. S1 and Fig. S2 in Supporting information, respectively. The Cr 2p1/2 and Cr 2p3/2 lines appear at 587.0 ± 0.2 eV and 577.0 ± 0.3 eV, respectively, indicating that chromium in the precipitates is present as Cr(III), confirming that the reduction of Cr(VI) to Cr(III) occurs during reaction with Fe(II) [20]. The single and smooth Gaussian-shaped peak indicates the formation of Cr(OH)3 precipitate, FexCr1x(OH)3 coprecipitate, or possibly a hydrous Cr(III) oxide (CrOOH) instead of Cr2O3 [28]. The Fe 2p1/2 and Fe 2p3/2 lines appear at 724.8 ± 0.1 eV and 711.8 ± 0.1 eV, respectively, suggesting that the iron in the precipitate is present as Fe(III) and is typical of iron oxy-hydroxides (FeOOH) [29,30]. The XPS results also revealed that the Fe/Cr molar ratios of the precipitates collected in the process of Cr(VI) reduction by Fe(II) was 2.74–3.23 at pH < 7.0. In addition, the speciation analysis of chromium and iron in the solution revealed that the molar ratio of residual Fe(II) and Cr(VI) in solution at pH 3.9–6.0 is 3.0 ± 0.20. Thus, it was concluded that the reaction between Cr(VI) and Fe(II) at pH < 7.0 follows the equation proposed by other investigators [31] with 3.0 mol of aqueous Fe(II) consumed in the reduction of 1.0 mol of aqueous Cr(VI) with the formation of solid Fe0.75Cr0.25(OH)3; i.e.

1 3 CrðVIÞ þ FeðIIÞ þ 3H2 O ! Fe0:75 Cr0:25 ðOHÞ3 þ 3Hþ 4 4

ð2Þ

At pH 3.9–4.7, less than 40% of the original chromate was reduced to Cr(III) by Fe(II). The presence of high concentrations of both Cr(VI) and Fe(II) in the solution under acidic conditions after

2 h indicated that the reaction of Cr(VI) with Fe(II) under these conditions was slow. The reduction rates of Cr(VI) by Fe(II) at pH 4–5 were examined and presented in Fig. S3, which confirmed the slow reaction rate of Cr(VI) with Fe(II) under acidic conditions. Many other researchers, including Buerge and Hug [15] and Pettine et al. [19] also reported that the reduction rate of Cr(VI) by Fe(II) was very slow under acidic conditions. Moreover, the Fe0.75Cr0.25(OH)3 precipitates are more soluble at pH 3.9–4.7 than those at higher pH, which also contributed to the low removal rate of chromium under acidic conditions. Therefore, the variation in removal efficiency of chromium by Fe(II) over the pH range 3.9–6.9 was mainly ascribed to the different rates of Cr(VI) reduction by Fe(II) and solubility of Fe0.75Cr0.25(OH)3 precipitates occurring at various pH levels. Fig. 1(b) shows that, in the presence of 0 lmol L1 arsenate, the amount of iron in the precipitates rose gradually with increase in pH and then reached a plateau. Fig. 1(b) and Fig. S2 indicate that at pH above 7, over 98% of Fe(II) was oxidized to Fe(III) and precipitated. The molar ratio of Fe(II) oxidized to Cr(VI) reduced was larger than the expected stoichiometric value of 3 under alkaline condition and this ratio increased significantly with increasing pH, implying that the competition from oxygen resulted in a more significant decrease in the amount of chromate that could be reduced by Fe(II) at higher pH [20]. The reduction rate of Cr(VI) by Fe(II) in the presence of oxygen can be expressed by Eq. (3), adapted from Pettine et al. [19].

d½CrðVIÞ=dt ¼ K Cr ½CrðVIÞ½FeðIIÞ  expK O2 ½O2 ½OH



t



ð3Þ

where KCr and K O2 are the overall constants for the reaction of Fe(II) with Cr(VI) and O2, respectively. Eq. (3) includes a decaying term for Fe(II) due to the presence of oxygen and clearly shows that the competitive effect from oxygen is more obvious at higher pH. Fig. S4 showed that chromate in the deoxygenated solution could be removed almost completely by Fe(II) under alkaline conditions, suggesting that Cr(VI) reduction by Fe(II) can complete in 2 h and the incomplete reduction of Cr(VI) by Fe(II) under oxic condition under alkaline conditions be not resulted from the slow rate of reduction of Cr(VI) by Fe(II). Therefore, Fig. S4 confirmed that a fraction of Fe(II) was oxidized by oxygen instead of chromate in the oxic systems under alkaline conditions and formed FeOOH according to the information provided in Fig. S2. Some previous studies also reported that dissolved oxygen (DO) competed with Cr(VI) in the oxidation of Fe(II) and that chromate reacted very rapidly with Fe(II) under alkaline conditions [15,19,32]. However, the DO in our system exhibited stronger capability in competing Fe(II) with Cr(VI)

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compared to those reported in the literature, which might result from enhanced oxidation rate of Fe(II) by oxygen caused by the presence of HCO 3 [33]. Thus, the minute concentration of Cr(III), the high concentration of Cr(VI) remaining in the solution at pH 7.5–9.8 and the XPS spectra collected at pH > 7 suggest that chromium removal under alkaline conditions was mainly controlled by the magnitude of chromate reduction by Fe(II). 3.2. Effect of arsenate on chromium removal

X

Me½OHn þ m

X ½Mem Hk Ln ½OHi 

ð4Þ

2 1

3.3. Effect of chromate on arsenate removal by Fe(II)

a

As(V)=0 μmol L -1 As(V)=10 μmol L -1 As(V)=20 μmol L -1

8

6

4

2

0 b 5 -1

Soluble Cr(III) (μmol L )

MeT ¼ ½Mefree þ

where L stands for the ligand other than OH. Eq. (4) reveals that the solubility of (hydr)oxides is determined by both OH and L while OH should play a more important role with increasing pH. HAsO2 4 is the dominate arsenate species at pH 7.3 when the maximum concentration of soluble Cr(III) and Fe(III) is observed, indicating that the complexation of HAsO2 4 with Fe0.75Cr0.25(OH)3 and FeOOH resulted in the high concentration of soluble Cr(III) and Fe(III). The significant reduction in the concentration of soluble Cr(III) from pH 7.3 to pH 9.8 should be associated with the much higher concentration and stronger complexation ability of OH at pH 9.8. Arsenate decreased the concentration of soluble Fe(III) but had dual influences on the concentration of soluble Cr(III) at pH < 6.0. Arsenate of lower concentration coordinated with Fe(III) species preferentially under acidic conditions compared with Cr(III), therefore, arsenate of lower concentration resulted in more soluble Cr(III). As shown in Fig. S6, Cr(III) and Fe(III) are present in solution as CrOH2+ and FeðOHÞþ 2 at pH 3.9–5.8, respectively, while arsenate 1 exists as the anion H2 AsO 4 [40]. Thus, the presence of 20 lmol L arsenate may facilitate the precipitation of Fe0.75Cr0.25(OH)3 or the adsorption of CrOH2+ and FeðOHÞþ 2 on the Fe0.75Cr0.25(OH)3 precipitates at pH 4.6–6.0 through ternary surface complex formation in a similar manner to that proposed to account for the arsenate enhancement of uranium sorption on aluminum oxide [41].

10

-1

Residual Cr(VI) in the solution (μmol L )

The effects of arsenate of 10 or 20 lmol L1 on chromium removal by Fe(II) was strongly dependent on pH, as shown in Fig. 1. The presence of arsenate had minor influence on chromium removal under acidic conditions while it decreased chromium removal to different extents under neutral and alkaline conditions, depending on the concentration of arsenate. Arsenate of 10 lmol L1 decreased chromium removal by 9.0–28.7% at pH 7.5–9.8 while the presence of 20 lmol L1 arsenate had detrimental effects on chromium removal at pH 6.7–9.8. The influence of arsenate on Fe(II)-mediated chromium removal is presumably associated with the effects of arsenate on the reduction of Cr(VI) by Fe(II) and the precipitation of Cr(III). As shown in Fig. 3(a), the presence of 10 lmol L1 arsenate decreased the concentration of residual Cr(VI) in the solution at pH 3.9–4.7 and a higher arsenate concentration resulted in a lower concentration of residual Cr(VI). However, the presence of 10 lmol L1 arsenate decreased the reduction of Cr(VI) with Fe(II) by approximately 3.4–20.0% at pH > 6.9 and arsenate of elevated concentration resulted in stronger inhibition in the reduction of Cr(VI) by Fe(II), which was very similar to the influence of phosphate on Cr(VI) reduction by Fe(II) in the presence of trace amount of oxygen and under alkaline conditions [34]. The rate of the reaction between aqueous Fe(II) and

dissolved oxygen relative to the rate of the reaction between aqueous Fe(II) and Cr(VI) increased with increasing pH [35]. The presence of arsenate caused a more significant enhancement in the rate of Fe(II) oxidation by oxygen than that by Cr(VI) [36]. Therefore the amount of Fe(II) that reacted with aqueous Cr(VI) decreased with increasing pH and arsenate concentration, resulting in an elevation in unreacted Cr(VI) concentration at pH > 6.9. Fig. 3(b) demonstrates that the presence of 10 lmol L1 arsenate elevated the concentration of soluble Cr(III) from 0.02– 1.11 lmol L1 to 0.37–2.07 lmol L1 over the pH range of 4–10. The concentration of soluble Cr(III) was up to 5.00 lmol L1 at pH 7.3 in the presence of 20 lmol L1 arsenate and decreased sharply with increase or decrease in pH. The speciation of soluble iron at the end of reaction was also analyzed, as shown in Fig. S5. The high soluble Cr(III) concentration commonly accompanies elevated concentration of soluble Fe(III) at pH 6.9–9.8. The increase in the concentration of both soluble Cr(III) and Fe(III) at pH 6.9–9.8 caused by the presence of arsenate should be ascribed to the formation of soluble complexes between arsenate and Fe(III)/Cr(III) and inhibition of the precipitation of Fe0.75Cr0.25(OH)3 and FeOOH, as indicated in Fig. 1(b). Rai et al. [37] observed a obvious increase in Cr(OH)3 solubility in the presence of phosphate at pH 4.5–12 and concluded that the increase in solubility resulted from the formation of soluble complexes between Cr(III) and phosphate species. Guan et al. [38] reported that the presence of competing anions decreased the removal of arsenic by reducing the formation of ferric hydroxide precipitate derived from oxidation of Fe(II). Stumm and coworkers [39] quantified the solubility of (hydr)oxides considering the possibility of complex formation with ligand L by the following equation:

4 3

0 3

4

5

6

7

8

9

10

11

Final pH Fig. 3. The concentration of (a) residual Cr(VI); (b) soluble Cr(III) in the process of chromate removal by Fe(II) in the absence or presence of arsenate (Cr(VI) = 10 lmol L1, Fe(II) = 30 lmol L1).

Fe(II) alone was not effective for arsenate removal, as demonstrated in Fig. 4(a), and only 2.6–8.2% of arsenate was removed by Fe(II) at pH 3.9–9.8 in the absence of chromate. Roberts et al. [42] reported a much higher arsenate removal efficiency by Fe(II) than that obtained in this study, which should be due to the high concentration of Ca2+ and CO2 contained in their synthetic 3

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a 100

b

Arsenate removal (%)

25

-1

Iron in the precipitate (μmol L )

Cr(VI)=0 μmol L-1 Cr(VI)=10 μmol L-1 Cr(VI)=20 μmol L-1 Cr(VI)=30 μmol L-1

80

30

60

40

20

20

15

10 Cr(VI)=0 μmol L-1 Cr(VI)=10 μmol L-1 -1 Cr(VI)=20 μmol L Cr(VI)=30 μmol L-1

5

0

0 3

4

5

6

7

8

9

10

11

Final pH

3

4

5

6

7

8

9

10

11

Final pH

Fig. 4. (a) Arsenate removal by Fe(II) in the presence of chromate of various concentrations at different pH values; (b) the amount of iron entrapped in the precipitate generated in this process (As(V) = 10 lmol L1, Fe(II) = 30 lmol L1).

groundwater. In the presence of 10 lmol L1 chromate, arsenate removal increased from 48.2% to 90.8% as pH increased from 3.9 to 6.0 but it decreased gradually with further increase in pH. Increasing chromate concentration from 10 lmol L1 to 30 lmol L1 resulted in an improvement in arsenate removal by 30.9% and 35.4%, respectively, at pH 3.9 and pH 4.8. However, the increase in chromate concentration had little effect on arsenate removal in the pH range of 6.0–9.8. In the system with initial Cr(VI) and As(V) concentration of 10 lmol L1 each, Fe(II) dosed at 30 lmol L1 was impossible to reduce Cr(VI) and As(V) simultaneously to satisfy the drinking water standard. However, the residual concentration of both Cr(VI) and As(V) at pH 5.9 could meet the drinking water standard when Fe(II) was applied at 60 lmol L1, as illustrated in Fig. S7. Arsenate adsorption on Cr(OH)3 solid was examined and it was found that arsenate could not be removed by Cr(OH)3 (data not shown). Thus, arsenate removal in the process of simultaneous removal of Cr(VI) and As(V) by Fe(II) should be associated with the precipitated iron. The remarkable influence of chromate on arsenate removal by Fe(II) may be correlated with the oxidative property of chromate, which resulted in oxidization of Fe(II) to Fe(III) thus facilitating the removal of arsenate. The amount of iron in the precipitates as a function of pH in the presence of chromate was examined with results shown in Fig. 4(b). The presence of 10 lmol L1 chromate greatly increased the amount of iron in the precipitates over the pH range of 3.9–9.0, which should contribute to the enhancement in arsenate removal as a result of the presence of chromate. The great improvement in arsenate removal at pH 3.9–4.8 caused by increasing chromate concentration from 10 lmol L1 to 30 lmol L1 should be associated with the increase in the amount of precipitated iron in this pH range. As only the precipitated iron could mediate arsenate removal, the amount of arsenate removed per unit of precipitated iron was calculated and presented in Fig. S8. In the presence of 10 lmol L1 chromate, the amount of arsenate removed per unit of precipitated iron elevated slightly as pH increased from 3.9 to 4.8 and then decreased gradually with increasing pH. The maximum arsenate removal per unit of precipitated iron was observed at pH 4.8, which may be associated with the appearance of the highest concentration of CrOH2+ at this pH level, as demonstrated in Fig. S6. The decline of arsenate removal per unit of precipitated iron on increasing pH from 4.8 to 9.8 may be ascribed to the grad2 ual shift of H2 AsO species and the increased 4 species to HAsO4 competition with hydroxide ions at higher pH [23,43]. Fig. S8 also

demonstrates that the presence of chromate drastically enhances the amount of arsenate removed per unit of precipitated iron under acidic conditions. For freshly precipitated ferric hydroxide, the total concentration of surface sites available for sorption is approximately 0.2 mol/mol Fe [44]. However, the amounts of arsenate removed per unit of precipitated iron at pH 3.9–6.9 varied from 0.23 to 0.52 mol As(V)/mol Fe, suggesting that arsenate was removed by both adsorption and co-precipitation with the precipitated Fe0.75Cr0.25(OH)3 and FeOOH. 3.4. Arsenic K-edge EXAFS analysis EXAFS spectra were employed to determine the local coordination environments of arsenate entrapped in the precipitates, as demonstrated in Fig. S9. Fig. 5A and B shows the k3 weighted As K-edge EXAFS spectra and the corresponding radial structure functions (RSF) as Fourier transform (FT) versus radial distance obtained for the arsenate entrapped in the precipitates at various pH levels, respectively. The resolved structural parameters obtained by fitting the theoretical paths to the experimental spectra are shown in Table 1. The FT of the EXAFS spectra isolates the contributions of different coordination shells, in which the peak positions correspond to the interatomic distances. However, these peak positions in Fig. 5B are uncorrected for the phase shift, so they deviate from the true distance by 0.3–0.5 Å [45]. As shown in Fig. 5(B) and Table 1, the first peak in the RSF was the result of backscattering from the nearest neighbor As(V)–O shell. The As–O interatomic distances display a narrow range of variation from 1.68 to 1.69 Å and the coordination number varies from 3.5 to 3.8, which were in agreement with the values previously reported for the tetrahedral arsenate geometry and were diagnostic for the arsenate species [46]. The theoretical paths of As–Fe, As–Cr or a combination of As–Fe and As–Cr were attempted when fitting the raw k3 weighted v(k) function in the data reduction process, and the fits were not successful for As–Cr or a combination of As–Fe and As–Cr. Consequently, As–Fe was finally used and the best fit results are shown in Table 1. Based on these findings, it was inferred that the second shell was primarily attributed to As(V)–Fe bonding and that arsenate mainly coordinated with Fe(III), consistent with the results of arsenate adsorption on Cr(OH)3 solid. Fitting the As–Fe peak was completed in both kspace and R-space using a single As–Fe shell, resulting in a CN of 1.92–2.60. The As–Fe interatomic distances are relatively uniform from 3.25 to 3.26 Å and iron coordination numbers range from

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a c

x(k)*k

3

b

a

2

4

6

8

10

12

14

k/(Α-1)

b

Transform Amplitude

c

cipitation of Cr(III) species in the solution under neutral and alkaline conditions, leading to the decrease in chromium removal at pH > 6. However, the presence of chromate significantly increased arsenate removal by Fe(II) at pH 4–9. The remarkable increase was associated with the oxidative property of chromate, which could oxidize Fe(II) to Fe(III) formed in situ and thus facilitated the removal of arsenate. Arsenate was removed by both adsorption and co-precipitation with precipitated Fe0.75Cr0.25(OH)3 and FeOOH. EXAFS results revealed that arsenate mainly coordinated with Fe(III) rather than Cr(III) and arsenate formed bidentate-binuclear complexes with FeO(OH) octahydra as evidenced by an average Fe–As(V) bond distance of 3.25–3.26 Å. Comparing the results of this study with those reported in the literature, Fe(II) is much more efficient for simultaneous removal of chromate and arsenate than single adsorption process and Fe0. Thus, Fe(II) is a feasible reagent for above ground treatment of groundwater contaminated by both chromate and arsenate. However, arsenate of high concentration will lead to the formation of complexed soluble Cr(III), which can create problems if it is subsequently oxidized in the disinfection process. In the case of initial As(V)/Cr(VI) molar ratio P2, the excessive arsenate should be removed by adsorption with iron-based adsorbents, which has much higher affinity for arsenate than chromate [14], before the application of Fe(II). Acknowledgements

b

This work was supported by the National Natural Science Foundation of China (50908060 and 50821002) and the State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (No. 2010TS10). The authors thank beamline BL14W1 (Shanghai Synchrotron Radiation Facility) for providing the beam time.

a

0

1

2

3

4

5

6

r/Α

Appendix A. Supplementary data

Fig. 5. (A) Raw (solid line) and fitted (dotted line) of k3 weighted v(k) spectra and (B) Corresponding radial structure functions (RSFs) for arsenate entrapped in the precipitates (Cr(VI) = 10 lmol L1, As(V) = 10 lmol L1, Fe(II) = 30 lmol L1): (a) pH 4.5, (b) pH 7.0, (c) pH 8.3. The peak positions are uncorrected for phase shift.

Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.seppur.2011.04.034. References

Table 1 Structural parameters for As(V) entrapped in the precipitates collected in the process of simultaneous removal of chromium and arsenate by Fe(II) at various final pH levels (Cr(VI) = 10 lmol L1, As(V) = 10 lmol L1, Fe(II) = 30 lmol L1). As–O

pH 4.5 pH 7 pH 8.3

As–Fe

N

R

a2

E0 (eV)

N

R

a2

E0 (eV)

3.5 3.5 3.8

1.68 1.69 1.69

0.0012 0.0016 0.0015

9.23 9.95 9.90

2.6 1.9 2.4

3.26 3.25 3.25

0.0084 0.0070 0.0081

1.85 2.23 3.78

1.9 to 2.6 for the precipitates collected at different pH levels. These results are consistent with the local structural data of ferric arsenate and the bidentate-binuclear attachment of arsenate to FeO(OH) octahydra [47,48]. 4. Conclusions This study investigated the performance and mechanisms of removal of co-present chromate and arsenate by Fe(II) over the pH range of 4–10. The results showed that Fe(II) was effective for simultaneous removal of chromate and arsenate from contaminated groundwater under neutral conditions. Arsenate had limited effects on Cr(VI) reduction by Fe(II) but markedly inhibited the pre-

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