Influence of sulfide (S2−) on preservation and speciation of inorganic arsenic in drinking water

Chemosphere 65 (2006) 847–853 www.elsevier.com/locate/chemosphere

Influence of sulfide (S2) on preservation and speciation of inorganic arsenic in drinking water Gautam Samanta *, Dennis A. Clifford Department of Civil and Environmental Engineering, University of Houston, N 107 Engineering Building 1 Houston, TX 77204-4003, USA Received 16 November 2005; received in revised form 10 March 2006; accepted 13 March 2006 Available online 27 April 2006

Abstract Generally, H2SO4, HNO3, HCl or the combination of ethylenediaminetetraacetate with acetic acid (EDTA-HAc) have been used to preserve arsenite and arsenate species prior to analysis. When these acidic preservatives are added in sulfidic water, instantaneous precipitation of poorly crystalline orpiment, As2S3(am), occurs, thereby lowering the total arsenic, As(Tot), analysis. A new method for the determination of As(Tot) was developed in which acid-preserved sulfidic water samples were oxidized with NaOCl, converting As2S3(am) and thioarsenic species to arsenate. A new method was also developed for the separation of uncharged arsenite and charged thioarsenic species in fresh, unpreserved sulfidic water by adsorbing the charged thioarsenic species while allowing uncharged arsenite to pass through a strong-base resin unhindered. The adsorbed thioarsenic species could be eluted efficiently with 0.16 M NaOCl solution.  2006 Elsevier Ltd. All rights reserved. Keywords: Arsenite; Sulfide; Thioarsenic species; Preservation and speciation; As2S3(am)

1. Introduction The mobility of arsenic in the environment, and its removal efficiency from drinking water are largely dependent on the oxidation state of arsenic. Generally, in groundwaters arsenic is present as arsenite and/or arsenate depending on the pH and redox conditions. Under anaerobic conditions, sulfates and organic sulfur compounds present in groundwaters are reduced by bacterial activity to appreciable concentrations of sulfide, which is present in the form of H2S or HS depending on the pH. Concentrations of dissolved sulfides in aquatic systems are controlled by the rate of bacterial sulfate reduction and by the nature and abundance of iron in the matrix (Fortin and Beveridge, 1997; O’Day et al., 2004). Dissolved sulfide concentrations increase only after the exhaustion of the available ferrous iron. United States Geological Survey (USGS) investigated sources of arsenic in US

*

Corresponding author. Tel.: +1 713 743 4844; fax: +1 713 743 4260. E-mail address: [email protected] (G. Samanta).

0045-6535/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2006.03.023

groundwaters and reported that dissolution of sulfide minerals is one of several natural sources of groundwater arsenic contamination. Thus, the co-existence of arsenic and sulfide in groundwaters is not unexpected and has been reported in Michigan, Wisconsin, Maine, and New Hampshire (). In the presence of sulfide at near-neutral pH, the chemistry of As–S is both complicated and controversial. Arsenate is not stable in sulfidic water, and is rapidly reduced to arsenite (Rochette et al., 2000), which forms soluble negatively charged thioarsenite complexes depending on pH and the concentration of S2. The presence of thioarsenite species in the As–S system has been reported by several research groups (Mironova and Zotov, 1980; Mironova et al., 1984; Webster, 1990; Wood et al., 2002; Wilkin et al., 2003). Furthermore, thermodynamic data (Helz et al., 1995; Pokrovski et al., 1996), and theoretical modeling (Helz et al., 1995; Tossell, 2000; Cleverley et al., 2003) have also predicted the presence of thioarsenite species in As–S systems. According to Rader et al. (2004), when the molar ratio of total sulfide to arsenite was equal

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to or greater than two, virtually all arsenite was in the dithoarsenite form. But Stauder et al. (2005) reported controversial results. When the molar ratio of As:S was 3 1:1, the formation of mono-ðAsSO3 Þ and dithoarsenate ðAsS2 O3 2 Þ was favored, and, at a higher ratio of sulfide (As:S = 1:50), arsenite almost quantitatively converted to 3 tri-ðAsS3 3 Þ and tetrathioarsenate ðAsS4 Þ. To avoid the confusion of thioarsenite and thioarsenate, we have used a generalized term ‘thioarsenic’ species in this paper. The bioavailability and toxicity of arsenite and thioarsenic species vary significantly (Tossell, 2000; Rader et al., 2004; Stauder et al., 2005). So, it is important to know the actual concentrations of arsenite and thioarsenic species in sulfidic water samples. Generally, acid has been used without regard to the presence of sulfide for the preservation of inorganic arsenic species (arsenite and arsenate). However, in sulfidic water, Smieja and Wilkin (2003) reported that upon acidification to pH < 2 with HCl or HNO3, instantaneous precipitation of poorly crystalline As2S3(am) occurred, which resulted in the loss of dissolved arsenic. It must be noted that all of their experiments were conducted in the range of 0.35– 10 mg l1 of arsenite, which is generally much higher than the arsenic levels in drinking water supplies and, up to 1000 times higher than the maximum contamination level (MCL) of 10 lg l1 for arsenic in US drinking water. Thus, it is important to study the effect of acid-preservatives in sulfidic water when arsenic concentrations are near 10 lg l1. Field speciation methods for separation of arsenite and arsenate have also typically used HCl, H2SO4, or acetic acid for acidification prior to speciation (Ficklin, 1983; Edwards et al., 1998; Karori et al., in press). However, the influence of S2 on samples acidified with HAc or H2SO4 has not been studied. To determine the true arsenic concentration in acidified sulfidic waters, Clarke and Helz (2000) and Smieja and Wilkin (2003) proposed a three-step method involving base addition, alkaline H2O2 oxidation, and re-acidification. The H2O2 method is complicated and time consuming, and a simpler method is needed to measure As(Tot) in acidified sulfidic waters. Thus, the objectives of the present work were to (1) determine how much uncharged arsenite remained under different preservation conditions, (2) establish a new method for the determination of As(Tot) in acidified sulfidic water, and (3) develop a new method to determine arsenite and thioarsenic species in drinking water supplies containing sulfide.

MO) in HCl solution was used to reduce arsenate to arsenite. Sodium tetrahydroborate (EM Science, Germany) solutions supplemented with NaOH were prepared fresh daily. Sulfide solution was prepared using Na2S Æ 9H2O crystals washed with Milli-Q water that had been sparged with pre-purified N2 (<5 ppmv O2) for more than 30 min to remove dissolved oxygen (DO). The oxidizing agents, hydrogen peroxide (H2O2), sodium hypochlorite (NaOCl), potassium peroxymonopersulfate (Oxone), peracetic acid (CH3COOOH) (Sigma–Aldrich Co., MO), were used for oxidative dissolution of As2S3(am) and thioarsenic species. The ion-exchange resin columns used for speciation were 1-cm i.d. · 10-cm length Kontes flex glass columns with porous polyethylene frits containing 7-ml of chloride- or acetate-form strong-base anion resin (50–100 mesh Dowex 1X8-100, Supelco, PA). Acetate-form resin was prepared for use in columns according to the procedure of Edwards et al. (1998).

2. Materials and methods

Cations

2.2. Experimental 2.2.1. Preparation of synthetic ground water For all laboratory experiments, synthetic groundwater (hereafter referred to as synthetic water) was prepared from the following salts: NaNO3, NaHCO3, Na2HPO4 Æ H2O, NaF, Na2SiO3 Æ 9H2O, MgSO4 Æ 7H2O, and CaCl2 Æ 2H2O. The composition of the synthetic water is given in Table 1. Detailed description of the preparation of the arsenic containing synthetic water is given in our earlier publications (Samanta and Clifford, 2005; Karori et al., in press). Sulfide in water gradually oxidizes in the presence of dissolved oxygen (Wilmot et al., 1988). Thus, to prevent sulfide oxidation in the synthetic water prior to adding a preservative, the synthetic water was sparged with pre-purified N2 (O2 content <5.0 ppmv) at 1000 ml min1 for 1.5 h to reduce the DO to <50 lg l1. Then arsenite and freshly prepared S2 solution were spiked during continued N2 sparging. About 2–3 min after S2 spiking, the N2 flow rate was reduced to 300 ml min1 to maintain the N2 atmosphere in the bottle. The S2 concentration was measured after sparging using the methylene blue method (APHA et al., 1998). During the experiments, samples of the As–S-containing synthetic water were taken at predetermined times in 250-ml pre-cleaned polypropylene bottles without preservatives, or with one of the following preserTable 1 Composition of synthetic groundwater without arsenite and sulfide +

2.1. Materials All reagents used were of analytical reagent grade. MilliQ water (18 MX cm) was used to prepare all solutions. A standard of 100 mg As l1 of arsenite was prepared from arsenic oxide (As2O3) (Sigma Chemical Co., MO). A 4 mg ml1 solution of L-cysteine (Sigma Chemical Co.,

meq l1

mg l1

Anions

meq l1

mg l1

HCO 3 SO2 4 

2.0 1.0 0.36 0.143 0.053 0.0013 0.66 4.224

122.0 48.0 12.8 2.0 1.0 0.04 20.0 205.8

Na Ca2+ Mg2+

3.604 0.36 0.26

82.9 7.21 3.16

Total

4.224

93.27

Cl NO 3N F PO3 4 –P Silicate as SiO2

TDS by evaporation = 287 mg l1.

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vatives (final concentrations in parentheses): EDTA and HAc (1.34 and 8.7 mM, respectively), or H2SO4 (0.05% v/v), or HNO3 (0.15% v/v).

(22–24 C), and then diluted to 10 ml. As(Tot) was measured in diluted samples the using FI-HG-AAS technique (Samanta and Clifford, 2005).

2.2.2. Separation of arsenite and thioarsenic species Unpreserved sulfidic water was rapidly forced through a 7-ml Cl-form anion-exchange resin column using a 60-ml syringe to separate charged thioarsenic species from uncharged arsenite. The initial 15-ml displacement volume of the effluent was discarded, and the remaining 45 ml was collected in a 50-ml centrifuge tube with HNO3 or EDTAHAc as a preservative. A portion of the original sample was preserved with HNO3 or EDTA-HAc for subsequent As(Tot) measurement after treatment with NaOCl. The concentration of charged thioarsenic species sorbed onto the resin was calculated from the difference of As(Tot) and the concentration of arsenite in the ion-exchange minicolumn effluent.

2.3. Analysis of As(Tot) by FI-HG-AAS

2.2.3. Elution of thioarsenic species from anion-exchange resin column Thirty milliliter of 20 lg l1 arsenite in the presence of 3.75 mg l1 S2 at pH 7.5 ± 0.1 was passed through a 7ml chloride-form resin column. The first 15 ml of the eluent was discarded and the remaining 15 ml was collected to measure arsenite concentration. After separation of arsenite and thioarsenic species, the column was washed with 10-ml Milli-Q water and then the sorbed thioarsenic species were eluted with 0.16 M NaOCl solution. 2.2.4. Pre-treatment of acid-preserved sulfidic water for the determination of As(Tot) For the determination of As(Tot) in acidified sulfidic waters, 3.2 mM NaOCl (1 ml of 1.6 · 102 M NaOCl solution in 5 ml sample) was used. Samples containing the oxidizer were held for 2–3 min at room temperature

As(Tot) analyses were performed using a Perkin–Elmer Model 5000 atomic absorption spectrometer (AAS) coupled with a Perkin–Elmer FIAS-100 unit for hydride generation. Detailed description of the arsenic analysis has been given in our earlier publication (Samanta and Clifford, 2005) related to arsenic preservation. 3. Results and discussion 3.1. Formation of thioarsenic species with time in sulfidic water Experiments were conducted to observe the stability of arsenite (19.3 lg l1) in low DO (<50 lg l1) sulfidic water (0.89 mg l1 S2) at pH 7.5. The unpreserved samples (100 ml) were immediately filtered through a 0.2 lm membrane filter and speciated by Cl-form anion-exchange resin column. The concentration of arsenite in the effluent was measured after treatment with NaOCl to avoid As2S3 precipitation during the acid (HCl) addition step of the analytical procedure for the determination of As(Tot) by HGAAS. The experimental results are shown in Fig. 1, which indicates after 15 min of contact with sulfide (0.89 mg l1 S2), about 30% of the initial arsenite was adsorbed. After 240 min and 1440 min of arsenite-S2 equilibration, about 53% and 80%, respectively, of the initial arsenite were adsorbed onto the column. These results show that adsorbed negatively charged thioarsenic species, present in sulfidic groundwater would be incorrectly reported as arsenate. According to Wilkin et al. (2003), Helz et al. (1995),

Fig. 1. Soluble arsenite as a function of arsenite–S2 equilibration time in the synthetic water at pH 7.5 (arsenite concentration = 19.3 lg l1 and the S2 concentration = 0.89 mg l1).

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and Stauder et al. (2005) under anaerobic conditions and near neutral to alkaline pH, in the presence of appreciable concentrations of sulfide, arsenite formed several water soluble anionic thioarsenic species. No precipitation was observed under these conditions, as our filtered and unfiltered results showed no loss of arsenic during filtration. So in the typical sampling situation, groundwaters containing sulfide that had been in equilibrium with arsenite for a long time at near neutral pH, negatively charged thioarsenic species should be predominant. The sulfide concentrations after 240 min and 1440 min were 0.83 and 0.56 mg l1, respectively. The loss of sulfide was probably due to slow volatilization of H2S during the minimal N2 sparging. 3.2. Effect of acid-preservatives on preservation and speciation of arsenic in sulfidic water Among many arsenic preservation and field speciation methods, the EDTA-HAc method at reduced pH 3.2 (Samanta and Clifford, 2005; Karori et al., in press) and the H2SO4 method at pH < 2 (Edwards et al., 1998) have been well researched. Experiments were carried out to

study the effects of those acidic preservatives at pH 2.0– 3.2 on arsenic speciation in sulfidic water samples using low-DO-synthetic water spiked with approximately 20 lg l1 arsenite and 0.5–2 mg l1 S2. Within 15 min of S2 addition, the synthetic water was preserved with EDTA-HAc or H2SO4. After mixing, the EDTA-HAc preserved samples were immediately filtered and speciated with Cl-form resin while the H2SO4 preserved samples were filtered and speciated using acetate-form resin. Experiments were done in triplicate on two different days. The results are shown in Table 2, which indicates that up to 1 mg l1 S2, in the presence of EDTA-HAc, the loss of arsenite was not significant. However, in the presence of H2SO4 at pH 2.0, the loss of arsenite was about 10% at 0.5 mg l1 S2 and about 19% at 1.0 mg l1 S2. Initially, in near-neutral pH sulfidic water, arsenic was present as thioarsenic species and arsenite. When acid was added, depending upon the pH and S2 concentrations, slow conversion of thioarsenic species to arsenite or precipitation of As2S3(am) occurred. At pH 2.0, a significant portion of the arsenite precipitated as As2S3(am), whereas little if any precipitate formed at pH 3.2.

Table 2 Ion-exchange speciation of uncharged arsenite in the S2 spiked synthetic groundwater using the EDTA-HAc (Clifford et al., 2004) and H2SO4 (Edwards et al., 1998) at two different days Conc. of S2 mg l1

Day 1 Initial As(III) lg l1

EDTA-HAc Arsenite conc. (mean ± S.D) lg l1 (n = 3)

H2SO4 Arsenite conc. (mean ± SD) lg l1 (n = 3)

Initial As(III) lg l1

EDTA-HAc Arsenite conc. (mean ± SD) lg l1 (n = 3)

H2SO4 Arsenite conc. (mean ± SD) lg l1 (n = 3)

0.5 1.0 2.0

20.6 20.5 NA

20.1 ± 0.4 19.5 ± 0.5 NA

18.6 ± 0.4 16.7 ± 1.3 NA

20.1 20.5 20.0

19.9 ± 0.2 19.1 ± 0.2 16.4 ± 0.2

18.3 ± 0.2 16.7 ± 0.6 9.6 ± 0.9

Day 2

NA = not available. Preservatives were added after 15 min of addition of S2 in the synthetic water.

Fig. 2. Concentrations of arsenite and As(Tot) measured at different time up to 45 days in the EDTA-HAc preserved synthetic water with 20 lg l1 As(III) and 3.96 mg l1 sulfide. Arsenite was speciated by anion-exchange resin column. Samples were preserved 15 min after sulfide addition and speciated at the indicated times.

As evident from literature reports and the experiments just discussed, the analysis of As(Tot) in sulfide containing water samples preserved with HNO3 or H2SO4, or a mixture of EDTA and HAc would likely give low results due to the precipitation of As2S3(am). In the present work, attempts were made to oxidatively dissolve As2S3(am) in the preserved samples using oxidants, which were expected to react with As2S3(am) in the acid medium. To study the oxidative dissolution of As2S3(am), 3.2 mM NaOCl (1 ml of 1.6 · 102 M NaOCl solution in

18.9 ± 0.3 NA 19.4 ± 0.3 18.4 ± 0.5 19.6 ± 0.2 18.7 ± 0.3 18.7 ± 0.2 NA 18.5 ± 0.1 17.9 ± 0.3 19.4 ± 0.2 18.0 ± 0.4 18.1 ± 0.2 NA 19.5 ± 0.5 18.5 ± 0.1

HNO3 (n = 3) EDTA-HAc (n = 3) H2SO4 (n = 3) HNO3 (n = 3)

Conc. of As after treatment with H2O2 (lg l1) Conc. of As after treatment with peracetic acid (lg l1)

EDTA-HAc (n = 3) H2SO4 (n = 3) HNO3 (n = 3) EDTA-HAc (n = 3)

18.9 ± 0.5 18.1 ± 0.3 21.2 ± 0.3 NA 22.1 ± 0.2 19.7 ± 0.3 22.0 ± 0.3 19.8 ± 0.2

NA = Not available.

3.3. Determination of As(Tot) in sulfidic water

17.6 ± 0.3 NA

In acidic, sulfide-deficient solutions, Reaction (3) dominates, whereas in acidic solutions with excess sulfide, Reaction (4) plays a greater role. Thus, the preservation of arsenic species in sulfidic water in the presence of any acid was not possible due to several complicated reactions of arsenic and sulfides.

17.8 ± 0.2 16.1 ± 0.2

ð4Þ

18.0 ± 0.3 16.4 ± 0.1



() AsSðOHÞðSHÞ þ Hþ ðClarke and Helz; 2000Þ

22.2 ± 0.1 19.9 ± 0.2

0:5 As2 S3 ðamÞ þ H2 O þ 0:5H2 S

H2SO4 (n = 3)

ð3Þ

HNO3 (n = 3)

() H3 AsO3 þ 1:5H2 S ðClarke and Helz; 2000Þ

EDTA-HAc (n = 3)

0:5 As2 S3 ðamÞ þ 3H2 O

H2SO4 (n = 3)

ð2Þ

HNO3 (n = 3)

() H3 AsO3 ðaqÞ þ S ðStauder et al:; 2005Þ

EDTA-HAc (n = 3)

þ 3H

Conc. of As after treatment with Oxone (lg l1)

AsO3 S

þ

Conc. of As after treatment with NaOCl (lg l1)

ð1Þ 3

Conc. of As after preservation, (lg l1) (without treatment)

() H3 AsO3 ðaqÞ þ 2H2 SðaqÞ ðRochette et al:; 2000Þ

As conc. (lg l)

þ H2 AsOS 2 þ H þ 2H2 O

Table 3 Comparison of oxidizing efficiency of different oxidizers in acid-preserved arsenic-spiked unfiltered synthetic groundwater containing 3.86 mg l1 S2 at room temperature

The EDTA-HAc preserved sulfidic synthetic water at pH 3.2 was studied over time to observe the effect of the preservative on arsenite speciation. N2-sparged challenge water samples containing 20 lg l1 arsenite and spiked with 3.96 mg l1 S2 were preserved with EDTA-HAc 15 min after S2 addition during N2 sparging. Preserved samples were shaken and then filtered with 0.2 lm membrane filters at predetermined times up to 45 days. The filtrate was analyzed for arsenite and As(Tot) (without any oxidizing agent). The results are shown in Fig. 2. Fifteen minutes after S2 addition in the arsenite-containing synthetic water, the addition of EDTA-HAc reduced the concentrations of arsenite and As(Tot) about 30% and 25%, respectively. But, with time (5 min to 45 d), the concentrations of arsenite and As(Tot) in the preserved sample steadily increased. After 45 d, the concentration of arsenic was very close to the initially spiked arsenic concentration (20 lg l1). The observed increase of arsenite in the preserved sample with time was governed by two factors: (1) slow conversion of thioarsenic species with time as shown in reactions (1) and (2), and (2) spontaneous solubilization of As2S3(am) with time as shown in reactions (3) and (4), below.

851

H2SO4 (n = 3)

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5 ml sample), 0.02 M potassium peroxymonopersulfate (oxone) (1 ml of 0.1 M oxone in 5 ml sample), 0.2% peracetic acid (1 ml of 1% peracetic acid solution in 5 ml sample), and 0.6% H2O2 (1 ml 3% H2O2 in 5 ml sample) were used. After mixing, samples containing the oxidizers were held for 6 h before analysis of As(Tot). The experimental results are given in Table 3, which shows that oxone, peracetic acid, and H2O2 were not effective oxidizers in acid-preserved sulfidic water samples. Even when concentrated solutions of 0.2 M oxone, 6% H2O2, and 2% peracetic acid were used, the recovery of arsenic was not improved. This is because of the oxidation of L-cysteine, which is used in the pre-reduction step of As(Tot) analysis by the excess amount of oxone or peracetic acid. Satisfactory results for the recovery of As(Tot), even in the presence of 3.86 mg l1 of sulfide, were obtained using NaOCl as the oxidizer, which was present as HOCl (pKa 7.5) in the acidic solutions, and reacted with As2S3(am), thioarsenic species, and arsenite to produce arsenate. The kinetics of the HOCl-sulfide reaction was also studied in an effort to establish an appropriate contact time to solubilize the As2S3(am) after the addition of NaOCl. The experimental results indicated that the oxidation of As–S solution by HOCl was very fast. Within 2 min after the addition of NaOCl, recovery of As(Tot) was 98%. So, the advantages of using NaOCl are (1) no need to make alkaline the acid-preserved sample before addition of oxidizer, (2) no need to re-acidify after oxidation, and (3) rapid oxidation in comparison with H2O2. 3.4. Separation of arsenite and thioarsenic species Due to the variation in toxicity and the bioavailability of arsenite and thioarsenic species, the knowledge of the actual concentrations of arsenite and thioarsenic species in water

samples is important. HG-AAS cannot be used to measure arsenite directly in the presence of thioarsenic species because acidification of samples to pH 4–6 results in a transformation of thioarsenic species to arsenite (Stauder et al., 2005). In this research anion-exchange resin was used for efficient separation of uncharged arsenite and negatively charged thioarsenic species in unpreserved samples. The uncharged arsenite passed through the column without retention while the negatively charged thioarsenic species sorbed onto the column. Laboratory experiments were carried out spiking known amounts of arsenite into sulfidic synthetic water at pH 7.5. After 15–20 min of addition of S2, As–S-containing synthetic water was speciated by Clform anion-exchange resin column and preserved with HNO3 or EDTA-HAc as a preservative. The separated and original acid-preserved samples were analyzed for arsenic after treatment with NaOCl. The experimental results are shown in Fig. 3, which indicates that the amount of arsenic retained by the resin column increased as the sulfide concentration increased. Correspondingly, the concentrations of arsenite decreased with an increase in sulfide concentration. It is also noted that, as previously shown in Fig. 1, an increase in equilibration time produced more thioarsenic species. Elution of the sorbed thioarsenic species with acid from the anion-exchange is not possible due to precipitation of As2S3(am) within the column (Jay et al., 2004). In this research work, attempts were made to elute the sorbed thioarsenic species from the anion-exchange resin column using NaOCl, which is alkaline. Thirty milliliter of 20 lg l1 As(III) in the presence of 3.75 mg l1 S2 at pH 7.5 ± 0.1 was passed through a 7-ml chloride-form resin column. After separation of non-sorbed arsenite and sorbed thioarsenic species, and washing the column with Milli-Q water, the sorbed thioarsenic species were eluted

Fig. 3. Concentrations of arsenite and thioarsenic species in the synthetic water containing 20.5 lg l1 of As(III) 15 min after adding different amounts of sulfide. Samples were speciated by Cl-form anion-exchange column without any preservative.

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with 0.16 M NaOCl solution. The recovery of arsenic was 101 ± 3% (n = 6) using 100 ml of the eluent. Presumably, NaOCl acted as a combined oxidant and ion exchanger, so that the thioarsenic species were simultaneous decomposed, oxidized to arsenate, and eluted. The effect of NaOCl concentration on thioarsenic species elution from ion-exchange resin after speciation was studied. Greater recovery was obtained by using 0.16 M NaOCl compared with 0.32 M NaOCl while no arsenic recovery was noted at 0.016 M NaOCl. No loss of arsenic was observed during washing of the thioarsenic loaded column with 10 ml Milli-Q water (As concentration in the eluent was below the detection limit). 4. Conclusions The presence of both sulfide and arsenic in groundwater leads to a very complicated thioarsenic chemistry. Acidic preservatives (pH 2.0–3.2) could not preserve the arsenic species in sulfidic water due to precipitation of As2S3(am) or conversion of thioarsenic species to arsenite. It was shown that As(Tot) could only be measured in acidpreserved samples after treatment with NaOCl. Arsenite and thioarsenic species could be separated by anionexchange resin immediately after collection of samples without any preservative. The difference between As(Tot) and arsenite in the effluent gave the value of thioarsenic species. The sorbed thioarsenic species could be eluted from the anion-exchange column efficiently with 0.16 M NaOCl solution. Acknowledgements The authors gratefully acknowledge Awwa Research Foundation for the financial support (Project no 2815) for this work. References APHA, AWWA, WEF (American Public Health Association, American Water Works Association, and Water Environment Federation). 1998. Standards Method for the Examination of Water and Wastewater. 18th ed. Washington, DC. Clarke, M.B., Helz, G.R., 2000. Metal-thiometalate transport of biologically active trace elements in sulfidic environments. 1. Experimental evidence for copper thioarsenite complexing. Environ. Sci. Technol. 34, 1477–1482. Cleverley, J.S., Benning, L.G., Mountain, B.M., 2003. Reaction path modeling in the As–S system: a case study for geothermal As transport. Appl. Geochem. 18, 1325–1345.

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