Determination of (Oxy) thioarsenates in Sulfidic Waters

Apr 17, 2007 - Environmental & Resource Sciences Program, Trent University, 1600 West Bank Drive,. Peterborough, ON K9J 7B8, Canada. Although it has ...
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Anal. Chem. 2007, 79, 3873-3880

Determination of (Oxy)thioarsenates in Sulfidic Waters Dirk Wallschla 1 ger*,† and Christopher J. Stadey‡

Environmental & Resource Sciences Program, Trent University, 1600 West Bank Drive, Peterborough, ON K9J 7B8, Canada

Although it has long been known that soluble arsenicsulfur (As-S) compounds exist in sulfidic waters and may play significant roles in several important processes in the biogeochemical arsenic cycle, no suitable analytical methods exist for their determination. We provide evidence that the four homologue (oxy)thioarsenates, mono-, di-, tri-, and tetrathioarsenate (AsO3S3-, AsO2S23-, AsOS33and AsS43-), can be formed in geochemical model reactions between arsenite and sulfide under anoxic conditions (through currently unknown reaction mechanisms) and that these compounds appear to be major As species in natural sulfidic waters. These As-S species are quantified by anion-exchange chromatography-inductively coupled plasma mass spectrometry (AEC-ICPMS) with instrumental detection limits of ∼0.1 nmol of As L-1 in undiluted samples; arsenite, arsenate, and monomethylarsenate are quantified as well, but dimethylarsenate cannot be analyzed by this technique. Sulfur in the eluting peaks can be measured as SO+ with detection limits of ∼0.1 µmol of S L-1. The (oxy)thioarsenates were synthesized in solution and characterized by electrospraytandem mass spectrometry (ES-MS-MS). In geochemical model solutions, we confirmed that both the AEC-ICPMS retention times and the ES-MS-MS spectra of the reaction products of sulfide and arsenite matched the synthesized (oxy)thioarsenate standards; for natural waters, the mass spectrometric confirmation was unsuccessful, due to matrix interferences. Arsenic contamination in groundwaters and drinking waters, particularly in Southeast Asia, currently generates high interest in the biogeochemical mechanisms determining As solubility in such environments.1 Although As mobility is elevated under reducing conditions, and reduced sulfur compounds are one predominant reaction partner for As in such environments, the geochemical interaction between As and reduced sulfur species on a molecular level and its impact on As mobilization are currently insufficiently characterized. * Corresponding author. E-mail: [email protected]. Phone: (705) 7481011 x7378. Fax: (705) 748-1569. † Previously at: Frontier Geosciences, Inc., 414 Pontius Ave. N, Seattle, WA 98109. ‡ Now at: Waters Canada, Ltd., 6427 Northam Dr., Mississauga, ON L4V 1H9, Canada. E-mail [email protected]. (1) Smedley, P. L.; Kinniburgh, D. G. Appl. Geochem. 2002, 17, 517-568. 10.1021/ac070061g CCC: $37.00 Published on Web 04/17/2007

© 2007 American Chemical Society

Based on geochemical considerations, the reaction between arsenite and sulfide should produce reduced (oxy)thioarsenite species. Older Eh-pH (Pourbaix) diagrams propose that metathioarsenite AsS2- exists in neutral to alkaline reducing solutions with high sulfur concentration (1 mmol L-1),2,3 but this hypothesis was not based on adequate analytical evidence. Consequently, the occurrence of this monomeric meta form has been questioned,4 and more recent Pourbaix diagrams omit this species.5 Raman spectroscopic studies of model solutions representative of certain hydrothermal systems (arsenite plus sulfide) indicate the presence of monomeric trithioarsenite in various protonation states when the solution is undersaturated with respect to orpiment precipitation,6 but suggest that polymeric thioarsenite species may dominate at higher concentrations; increasing sulfide concentration, pH, or both are shown to suppress this polymerization. A similar study7 provides evidence that at least six different As-thio species (including various protonation states of the same compound) exist in the range of pH 7-14 and sulfide/arsenite ) 0.1-10. An interesting reaction path modeling experiment concluded that the dimeric thioarsenite species As2S3(aq), HAs2S4-, and As2S42contribute to arsenic transport and mineralization in a geothermal field.8 The homologue series of (oxy)thioarsenates AsOxS4-x3- (x ) 0-3) can be formed either by reaction of arsenite (AsO33-) with elemental sulfur under alkaline conditions9 or by alkaline dissolution of As-sulfide minerals, e.g., orpiment As4S6.10 However, to date, these compounds have received very little attention in environmental studies, despite the fact that their formation reactions mimic the conditions of highly relevant processes in the hydrogeochemical As cycle. A pioneering study developed an anion-exchange chromatography (AEC) separation for the four (oxy)thioarsenates9 and indicated the presence of small amounts of monothioarsenate in historic silver mining tailings,11 but unspecific conductivity detection was used, leaving some doubt (2) Ferguson, J. F.; Gavis, J. Water Res. 1972, 6, 1259-1274. (3) Cherry, J. A.; Shaikh, A. U.; Tallman, D. E.; Nicholson, R. V. J. Hydrol. 1979, 43, 373-392. (4) Cullen, W. R.; Reimer, K. J. Chem. Rev. 1989, 89, 713-764. (5) Vink, B. W. Chem. Geol. 1996, 130, 21-30. (6) Helz, G. R.; Tossell, J. A.; Charnock, J. M.; Pattrick, R. A. D.; Vaughan, D. J.; Garner, C. D. Geochim. Cosmochim. Acta 1995, 59, 4591-4604. (7) Wood, S. A.; Tait, C. D.; Janecky, D. R. Geochem. Trans. 2002, 3, 31-39. (8) Cleverly, J. S.; Benning, L. G.; Mountain, B. W. Appl. Geochem. 2003, 18, 1325-1345. (9) Schwedt, G.; Rieckhoff, M. J. Chromatogr., A 1996, 736, 341-350. (10) Berzelius, J. J. Ann. Chim. Phys. 1826, 32, 265-286. (11) Schwedt, G.; Rieckhoff, M. J. Prakt. Chem. 1996, 338, 55-59.

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whether the peak observed in the tailings samples was really an arsenic compound. Extractable arsenic in lake sediments impacted by historic gold mining was present mostly in the form of one or more unidentified anionic species;12 liquid chromatographyinductively coupled plasma mass spectrometry (LC-ICPMS) evidence presented shows that this species (labeled as “As complex”) contains both sulfur and arsenic, and the authors speculated that it is involved in As mobilization from sediments into the overlying water column. A laboratory study on the reduction of arsenate by sulfide under acidic conditions13 observed one arsenic-containing peak eluting after arsenate in AEC chromatograms. The compound(s) appeared to be formed as intermediate(s) before orpiment precipitation occurred and were proposed to represent a (chromatographically unresolved) succession of monomeric and polymeric (oxy)thioarsenates and -arsenites based on previous literature and modeling considerations, but no further evidence regarding the identity of the species was presented. Finally, our own laboratory experiments14 on the interaction between arsenite and sulfide at concentrations well below orpiment saturation demonstrated that up to four soluble arsenic-sulfur (As-S) compounds are formed under such conditions; the same compounds were found in a field study in meromictic Mono Lake, CA.15 In these studies, the encountered As-S species remained unidentified: while the geochemical (reducing and anoxic) conditions suggested (oxy)thioarsenites, the analytical evidence (four compounds eluting in a homologue series with arsenate) favored (oxy)thioarsenates. In this report, we provide evidence that (oxy)thioarsenates are formed by the reaction of sulfide with arsenite and that they occur and may play important roles in environmental sulfidic waters.

EXPERIMENTAL SECTION Synthesis of (Oxy)thioarsenate Standards. Note: All As species are shown in their completely deprotonated state here for simplicity. Monothioarsenate was synthesized from arsenite and elemental sulfur under alkaline conditions.9 The same procedure also yielded AsO2S23- and AsOS33- using higher S/As ratios and reaction times, but no AsS43-, which was obtained by dissolving As4S10 (produced by bubbling H2S through cold concentrated HCl containing arsenate) in alkaline sulfide solution.16 AEC-ICPMS analyses of the reaction mixtures showed that only AsO3S3- was obtained with high purity (>95%), while the other three species were always present in mixtures containing major amounts of other species. Attempts to purify the (oxy)thioarsenates further by crystallization failed, so no pure solid standard substances were obtained. Samples. Geochemical model samples were prepared by mixing arsenite and sulfide in degassed water at varying ratios and pH conditions in a glove box.14 The Mono Lake water sample (12) Zheng, J.; Hintelmann, H.; Dimock, B.; Dzurko, M. S. Anal. Bioanal. Chem. 2003, 377, 14-24. (13) Rochette, E. A.; Bostick, B. C.; Li, G.; Fendorf, S. Environ. Sci. Technol. 2000, 34, 4714-4720. (14) Wilkin, R. T.; Wallschla¨ger, D.; Ford, R. G. Geochem. Trans. 2003, 4, 1-7. (15) Hollibaugh, J. T.; Carini, S.; Gu ¨ rleyu ¨ k, H.; Jellison, R.; Joye, S. B.; Lecleir, G.; Meile, C.; Vasquez, L.; Wallschla¨ger, D. Geochim. Cosmochim. Acta 2005, 69, 1925-1937. (16) Palazzi, M.; Jaulmes, S.; Laruelle, P. Acta Crystallogr., B 1974, 30, 23782381.

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Table 1. Instrumental Parameters

columns eluant gradient

sample volume typical retention times suppression

instrument plasma rf power nebulizer gas flow rate nebulizer spray chamber dwell times

cone voltage capillary voltage extractor voltage source temperature desolvation gas temp sample flow rate MS-MS collision energy

AEC Separation IonPac AS-16/AG-16 4-mm (10-32) (Dionex, Sunnyvale, CA) NaOH (0.1 mol L-1) at 1.2 mL min-1 0-7 min 20 mmol L-1 7 f 17 min 20 f 100 mmol L-1 17-25 min 100 mmol L-1 25-28 min 20 mmol L-1 1 mL AsO33-, 217 ( 9 s; AsO43-, 840 ( 27 s; AsO3S3-, 935 ( 23 s AsO2S23-, 1036 ( 20 s; AsOS33-, 1141 ( 25 s; AsS43-, 1201 ( 22 s ASRS-Ultra 4-mm (Dionex), 300 mA current, 5 mL min-1 water (external mode) ICPMS Detection Elan 6000 (PerkinElmer, Shelton, CT) 1,400 W (1,550 W for As only) 1.1 L min-1 (1.0 L min-1 for As only) TR-30-C3 (Meinhard, Santa Ana, CA) cyclonic (Glass Expansion, Hawthorn East, Victoria, Australia) m/z ) 75 (As) 200 ms (900 ms for As only) m/z ) 48 (SO) 800 ms (100 ms for As only) m/z ) 77 (ArCl) 100 ms m/z ) 121 (Sb) 100 ms (optional as internal standard) ES-MS-MS 30 V 3 kV 1V 80 °C 150 °C 10 µL min-1 2-20 eV

was collected in March 2003 at 28-m depth.15 The sulfidic groundwater sample stems from the site receiving industrial inorganic arsenic emissions. To maintain anonymity of sample site and owner, we cannot provide further hydrochemical or technical details. Pore water from a sediment core (10-15 cm depth layer) collected in August 2002 at Moira Lake, Madoc, ON, Canada, impacted by arsenic from an abandoned gold mine,12 was extracted anoxically by centrifugation in a glove box. All samples were collected under minimal exposure to the atmosphere, placed unpreserved in sampling containers, and immediately flash-frozen in liquid N2. They were stored frozen at - 20 °C, thawed in a glove box immediately prior to analysis, and diluted as required to eliminate chromatographic matrix effects and stay within the As calibration range. Analytical Methods and Instrumentation. Total arsenic concentrations were determined by ICPMS using the dynamic reaction cell (DRC) technology (Elan DRC 6100+, Perkin-Elmer, Shelton, CT) with NH3 as the reaction gas. The AEC-ICPMS method for measuring As speciation (Table 1) was modified from our previously published selenium speciation analysis technique.17 ICPMS detection was optimized for maximum sulfur signal (as SO+ at m/z ) 48) to allow confirmation of sulfur presence in the eluting (oxy)thioarsenate peaks and determination of S/As ratios. Once the retention behavior of (oxy)thioarsenates is established, plasma conditions optimized for As (shown in parentheses in Table 1)sat the expense of reduced S sensitivityscan be used for (17) Wallschla¨ger, D.; Roehl, R. J. Anal. At. Spectrom. 2001, 16, 922-925.

Table 2. Arsenic Speciation and Ancillary Parameters in Natural Sulfidic Watersa samples

parameter AsO3 (mg of As L-1) AsO43- (mg of As L-1) AsO3S3- (mg of As L-1) AsO2S23- (mg of As L-1) AsOS33- (mg of As L-1) AsS43- (mg of As L-1) Σ species (mg of As L-1) total As (mg L-1)* fraction of As-S species (%) pH redox potential (mV) total inorganic carbon (mg of C L-1) 3-

Mono Lake bottom water

sulfidic groundwater

sediment pore water

2.5 5.0 0.69 1.5 4.5 nd 14.2 15 47.1 9.6 nm 4,800

0.0165 0.0102 0.0027 0.0068 0.0009 nd 0.0370 0.0418 28.1 7.1 -216 52.1

0.509 0.177 0.222 0.069 0.022 0.015 1.01 1.11 32.5 8.3 nm 22.6

a nd, not detected; nm, not measured; *, measured by DRC-IPCMS (see Experimental Section).

routine As speciation measurements. Interferences of ArCl+ on m/z ) 75 are controlled by monitoring m/z ) 77, but only the overlap of chloride with arsenite was observed, which is generally insignificant in freshwaters (the molar cross-sensitivity for Cl- as 40Ar35Cl+ on 75As+ is ∼10-6 under the chosen plasma conditions), so using the DRC is unnecessary. To monitor and compensate for instrument sensitivity drift, 121Sb is used as an internal standard. It has to be added after the separation, because antimony also forms stable thio species.18 No significant short-term sensitivity changes were observed during individual chromatographic runs (ignoring the actual Sb species, which can be identified by monitoring 123Sb+ simultaneously), and the long-term signal stability for As, S, and Sb was very good (10). After excluding sulfur signals that were either too small or had overlapping sulfur species, the average S/As ratios and their variation (expressed as RPD for n ) 2 and relative standard deviation σn-1 for n ) 9) determined in the geochemical model samples were 1.05 ( 0.36 (n ) 2), 2.13 ( 0.31 (n ) 2), 3.07 ( 0.68 (n ) 9), and 4.19 ( 0.26 (n ) 2), respectively, for the four (oxy)thioarsenates. Despite the significant intersample variability,

Figure 2. ES-MS-MS spectra of mono- (a), di- (b), tri- (c), and tetrathioarsenate (d).

these averages are reasonably close to the expected values (1, 2, 3, and 4). We were unable to replicate these S/As ratio determinations in the natural waters, even though those samples contained up to 15 mg L-1 total arsenic. This was primarily due to the fact that all natural waters had to be diluted significantly, which decreased the measured concentrations of the As-S species, and consequently often made the sulfur signals undetectable. Additionally, those samples contained overlapping sulfur species in higher number and concentration than the model solutions. Therefore, matching retention times are the only evidence at this point proving that the As-S species in the natural waters are (oxy)thioarsenates, so additional structural information (either S/As ratios after refined separation or direct molecular MS evidence) needs to be generated in analogy to the model experiments presented here to confirm this hypothesis. Mass Spectrometric Characterization of (Oxy)thioarsenates. All four (oxy)thioarsenates and AsO43- itself show the same typical signals in their ES-MS spectra. The molecular ion is encountered in the form H2AsO4-xSx- (x ) 0-4), regardless of solution pH, demonstrating that ES-MS is not directly suitable for distinguishing between different protonation states of these compounds. The main fragments encountered (from in-source fragmentation) are AsO3-xSx- (x ) 0-3) and AsO4-xSx-1- (x ) 1-4), formed by elimination of H2O or H2S (where possible), respectively, from the molecular ion; their relative intensities naturally depend strongly on source conditions. Additionally, two clusters of higher mass than the molecular ion were encountered consistently at M + 22 and M + 62, possibly corresponding to the ions NaHAsO4-xSx- and Na2AsO4-xSx-‚H2O, respectively (not verified here). The relative abundance of these clusters depends strongly on the sample matrix, as demonstrated previously for

phosphate.26 While the nominal masses of fragments for several (oxy)thioarsenates are identical, the molecular ions are unique within this homologue series and can therefore be used for identification of each species in a mixture. Additional confirmation is obtained via the more characteristic ES-MS-MS fragment spectra of the molecular ions (Figure 2); due to the extra collision energy transferred to the molecules, further fragments beyond the H2O and H2S eliminations are observed here. Direct ES-MS-MS quantification of (oxy)thioarsenates from bulk solution was impossible due to lack of pure standards, as well as substantial matrix effects changing signal intensities and creating interferences of the same nominal mass as the molecular ions, leading to altered (superimposed) fragmentation patterns that prevented even identity confirmation. Confirmation of (Oxy)Thioarsenate Formation from Arsenite and Sulfide. The reaction of AsO33- with sulfide under controlled oxygen-free conditions yields products that coelute with the synthesized (oxy)thioarsenate standards and have the matching S/As ratios (see above). While this strongly suggests that the reaction products are indeed (oxy)thioarsenates, simple coelution is generally not accepted as proof of identity in speciation analyses. Additionally, strictly geochemical considerations mandate that the reaction between arsenite and sulfide must yield reduced As(III) species, i.e., the three (oxy)thioarsenites AsOxS3-x3- (x ) 0-2), unless an oxidation mechanism can be invoked and proven. Therefore, we require additional analytical evidence to confirm that (oxy)thioarsenates can be formed from arsenite and sulfide, aside from our previous observation of four reaction products, while the (oxy)thioarsenite homologue series only has three members.14 (26) Hao, C.; March, R. E. J. Mass Spectrom. 2001, 36, 509-521.

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Table 3. Exact Mass Determination for the Main ES-Q-TOF Signals in the Reaction Solution of Arsenite and Sulfide after 18 Daysa measured mass

ion for (oxy)thioarsenate series

theoretical mass for (oxy)thioarsenate series

mass deviation (ppm)

174.865a 172.868 158.890a 156.891 154.859 140.914 138.882 122.905

H2AsO2S2H2AsO2S2H2AsO3SH2AsO3SAsOS2H2AsO4AsO2SAsO3-

174.8670 172.8712 158.8899 156.8941 154.8607 140.9169 138.8835 122.9063

-11.4 -18.5 +0.6 -19.8 -11.0 -20.6 -10.8 -10.6

a

ion for (oxy)thioarsenite series

theoretical mass for (oxy)thioarsenite series

mass deviation (ppm)

H2AsS3174.8493 +89.8 H2AsS3172.8535 +83.9 H2AsOS2158.8721 +112.7 H2AsOS2156.8763 +93.7 this fragment ion is unique to the (oxy)thioarsenate series H2AsO2S140.8991 +105.7 AsS2138.8657 +117.4 AsOS122.8886 +133.5

Isotopic satellite signal of next lower measured mass.

To decide this argument, the product mixture of the reaction between arsenite and sulfide (both starting concentrations 36 µmol L-1) at pH of ∼8 was analyzed after 18 days of reaction time. AECICPMS analysis of this reaction mixture detected ∼22 µmol L-1 of unreacted AsO33- and ∼5 µmol L-1 each of AsO43-, AsO3S3and AsO2S23-, traces of AsOS33-, but no AsS43-. Also, analyses by the methylene blue method27 showed that, at this point of the reaction, only 35% of the starting sulfide amount remained in solution as free sulfide, while 50% was bound up in the (oxy)thioarsenates (calculated based on stochiometry) and, thus, unreactive toward methylene blue; 15% of the initial sulfide was lost from solution, possibly via oxidation, precipitation, or volatilization. The fact that there was still a significant amount of free sulfide left in solution suggests that either the (oxy)thioarsenates are not particularly thermodynamically stable or that their formation has slow kinetics. The direct bulk solution ES-MS showed signals at m/z 141, 157, and 173, consistent with the presence of AsO43-, AsO3S3-, and AsO2S23-; there were no measurable signals at m/z 121 (because arsenite is mostly a neutral molecule under these conditions) and at m/z 189 and 205 (below instrumental detection limit). The analytical challenge, however, is now to prove that the first As-S species (S/As ) 1) observed in the AECICPMS analysis is AsO3S3- and not AsO2S3- and that the second observed As-S species (S/As ) 2) is AsO2S23- and not AsOS23-. To distinguish between these two alternatives, three of pieces of analytical evidence were combined. First, we determined the exact mass for each m/z signal in this sample corresponding to either a molecular or fragment ion of AsO43-, AsO3S3-, or AsO2S23-, as well as their isotopic satellites, by direct ES-MS analysis using the Q-TOF configuration. Although the fragmentation behavior of (oxy)thioarsenites is unknown at this point, it seems reasonable to assume that they would eliminate H2O, H2S, or both, like the (oxy)thioarsenates. Consequently, for each observed m/z ratio, we can assign one possible candidate ion resulting from the (oxy)thioarsenate series and an alternative one from the (hypothetical) (oxy)thioarsenite series. The measurement yielded consistently much better agreement between the measured and theoretical exact m/z ratios (∆m ) -21 to +1 ppm), when (oxy)thioarsenate species were postulated, than for the corresponding ions of the (oxy)thioarsenite series (∆m ) +134 to +84 ppm) (Table 3). (27) Cline, J. D. Limnol. Oceanogr. 1969, 14, 454-458.

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Second, we performed ES-MS-MS measurements on the ions m/z ) 141, 157, and 173 in the sample. The obtained fragmentation patterns matched those of AsO43-, AsO3S3-, or AsO2S23-, respectively, in terms of the observed fragment m/z ratios and intensities. Specifically, m/z ) 173 eliminated H2O, so it cannot be H2AsS3-, but must be H2AsO2S2-, which is consistent with S/As ) 2 measured for this compound by AEC-ICPMS. The analogue argument does not differentiate between H2AsO3S- and H2AsOS2-, and for H2AsO4-, we observe successive elimination of H2O and O, which produces a fragmentation pattern that matches the one expected for H2AsO2S-. Third, we collected the AEC fractions corresponding to the first- and second-eluting As-S species and subjected them to ESMS analysis; this had to be done with the ES-Q-Q instrument due to lower absolute analyte amounts, so no exact mass information was available. Since we know from the S/As ratio determinations by AEC-ICPMS that the first-eluting compound has an S/As ratio of 1, we only have to distinguish between AsO3S3- and AsO2S3- in this experiment. The main m/z signal in this MS was 157 (and not m/z 141), so this clearly confirms, in combination with the other evidence, that AsO3S3- was present in the geochemical model solutions. By analogy, the secondeluting As-S compound (S/As ) 2) showed m/z ) 173 (and not m/z 157) as the main signal, which confirms the presence of AsO2S23- in the sample. Finally, the AEC-ICPMS signal for arsenate had no detectable simultaneous S signal, ruling out that this chromatographic signal stems from AsO2S3-. While these experiments seem to show that (oxy)thioarsenates (here AsO3S3- and AsO2S23-) are indeed products of the reaction between arsenite and sulfide under anoxic conditions, they leave us with the geochemical dilemma to explain how the oxidation of As(III) to As(V) occurs under apparently completely reducing hydrochemical conditions. If, as expected geochemically, (oxy)thioarsenites are the primary products of this reaction, then they would have to be oxidized rapidly to (oxy)thioarsenates (by unknown reaction partners), because we have never seen other As-S species eluting at different retention times. We can also rule outsat least for the geochemical model samplessthe hypothesis that potentially present (oxy)thioarsenites are oxidized during the alkaline AEC separation, because direct ES-MS analyses confirm the presence of (oxy)thioarsenates in the bulk sample (this confirmation was not possible in natural samples,

due to matrix interferences or insufficient concentrations of (oxy)thioarsenates). It is possible, however, that (oxy)thioarsenites are present in the geochemical model solutions but are oxidized during the electrospray process. We will only be able to test this hypothesis after (oxy)thioarsenite standards have been synthesized (if that is possible, and assuming those compounds are stable in water), but one preliminary argument against this suspicion is that we saw no significant AsO43- signal when AsO33- was analyzed by ES-MS. Finally, we havesstrictly speakingsnot proven that (oxy)thioarsenates exist in natural waters, although the geochemical model experiments simulate the real environmental conditions under which we encounter those compounds quite well. Regardless, the described instrumental limitations need to be resolved, and then the same line of argumentation presented here for the model samples could be generated for natural waters. If, on the other hand, (oxy)thioarsenates are formed directly from arsenite and sulfide (as the results suggest), then we need to explain what the redox partner in this reaction is. A recent report28 suggests that (oxy)thioarsenates are formed via a disproportionation reaction, in which As(III) converts stochiometrically into equal amounts of As(V) and elemental As0, so that no net oxidation of As occurs, but still half of it is available for the formation of As(V)-thio species (via ligand exchange of OH- for SH-). Our experimental data suggest that this hypothesis is false, because we observed quantitative recovery of the starting As(III) amount in the form of soluble As species, as opposed to a 50% loss expected if insoluble As0 were formed. Similarly, other reduced forms of arsenic, including volatile arsine (AsH3), containing As(-III), or insoluble realgar (As4S4), containing As(II), can be excluded as major reaction products, because neither would be expected to yield any signal in our AEC-ICPMS procedure, let alone mimic one of the encountered species. Clearly, future studies need to elucidate the mechanism of (oxy)thioarsenate formation from sulfide and arsenite. They will have to include detailed sulfur speciation analyses, because it is possible that sulfide is initially oxidized (by a currently unknown oxidant), and the resulting partially oxidized sulfur species (e.g., polysulfides or thiosulfate) then act as S0-donors for arsenite, in analogy to some of the synthetic procedures used here.9 Arsenic Speciation in Natural Sulfidic Waters. To illustrate the types of environments where (oxy)thioarsenates are encountered and matter, the results obtained for three environmental waters are presented in Table 2. These samples are only a very small selection of the suite of environments in which we have observed these As-S species, spanning the entire range of environmental As concentrations from background to extreme pollution. In fact, we find these species in every water sample that has neutral or alkaline pH and contains enough free sulfide to be detected by smell. We have yet to observe these compounds in any acidic sample, supporting the hypothesis that they precipitate in the form of As-sulfide minerals at acidic pH.29 In all three samples, (oxy)thioarsenates constitute a significant fraction of the total As concentration; often, we have seen these As-S species accounting for the vast majority of the dissolved (28) Stauder, S.; Raue, B.; Sacher, F. Environ. Sci. Technol. 2005, 39, 59335939. (29) Thilo, E.; Hertzog, K.; Winkler, A. Z. Anorg. Allg. Chem. 1970, 373, 111121.

As. The relative distribution between the four species varies strongly between samples, and often not all four (oxy)thioarsenates are encountered simultaneously. We have shown previously in model solutions that the distribution of these species varies systematically with hydrochemical parameters, including pH and S/As ratio in the sample,14 so the different As speciation patterns probably reflect those factors. An intercomparison with As speciation results measured in natural sulfidic waters in other studies seems futile at this point, because the only study that looked specifically at As-S species in environmental samples was conducted on a very different geochemical (oxic) system,11 while all other studies used As speciation methods that may have been unsuitable for (oxy)thioarsenate determination (see below). Although Pourbaix diagrams predict the presence of (only) soluble or insoluble As(III)-thio species in this Eh-pH range, we consistently observe arsenite, and particularly arsenate, coexisting with the (oxy)thioarsenates (Table 2). The coexistence of As oxy and thio anions suggests that other factors besides Eh and pH determine As speciation in sulfidic environments or that the studied systems may be in thermodynamic disequilibrium. Finally, we found no evidence of any non-sulfur-containing As species besides arsenite and arsenate in these samples, although they have hydrochemical compositions for which the existence of stable As-carbonate complexes has been proposed.30 Therefore, we question the existence of As-carbonate complexes and will test it in future research. CONCLUSIONS The occurrence and quantitative importance of (oxy)thioarsenates in sulfidic waters demonstrated here fundamentally questions the existing body of literature on arsenic speciation in sulfidic geochemical zones. Consequently, the current terminology “As(III) and As(V)” should be abandoned when discussing As speciation in sulfidic waters, and the oxyanions should be properly referred to as arsenite and arsenate to distinguish them from the usually present As (oxy)thioanions. Commonly used As speciation preservation strategies and analytical methods need to tested for their suitability for waters containing (oxy)thioarsenates. Particularly, the operationally defined As speciation analysis approach based on selective sequential hydride generation31 is not designed to differentiate between arsenite or arsenate and (oxy)thioarsenates, so their presence would probably go unnoticed. Likewise, the popular preservation of water samples by acidification with HCl appears unsuitable for samples containing (oxy)thioarsenates; preliminary observations in our laboratory indicate that this procedure leads to rapid decomposition of the As-S species discussed here, and a recent study demonstrates the precipitation of orpiment upon acidification from samples containing arsenic and sulfide.32 Although the presence of (oxy)thioarsenates was not confirmed in such studies, recent biogeochemical evidence indicates that they may factor in the moderating effect of sulfide on arsenite toxicity33 and the suppression of arsenite adsorption on FeS and (30) Kim, M.-J.; Nriagu, J.; Haack, S. Environ. Sci. Technol. 2000, 34, 30943100. (31) Braman, R. S.; Foreback, C. C. Science 1973, 182, 1247-1249. (32) Smieja, J. A.; Wilkin, R. T. J. Environ. Monit. 2003, 5, 913-916. (33) Rader, K. J.; Dombrowski, P. M.; Farley, K. J.; Mahony, J. D.; Di Toro, D. M. Environ. Toxicol. Chem. 2004, 23, 1649-1654.

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FeS2 by sulfide.34 As it is likely that (oxy)thioarsenates will have chemical properties different from those of arsenate and, particularly, arsenite, we predict that their presence will significantly affect As mobility in the subsurface environment through altered adsorption behavior. Additionally, the (oxy)thioarsenate formation pathway may affect the distribution of trace elements between mineral and dissolved phase: sulfide has been shown to be very effective at leaching of As from minerals,35 and it has been suggested that soluble As-S species might prevent the precipitation of thiophilic trace metal cations, such as copper, in sulfidic waters by complexation.36 The presented analytical approach will be valuable for future studies investigating the role and significance of (oxy)thioarsenates in these processes, and contribute to a better understanding of As mobility and speciation in sulfidic milieus, possibly including some As-contaminated aquifers in Southeast Asia.

ACKNOWLEDGMENT

(34) Bostick, B. C.; Fendorf, S. Geochim. Cosmochim. Acta 2003, 67, 909-921. (35) Delfini, M.; Ferrini, M.; Manni, A.; Massacci, P.; Piga, L. Miner. Eng. 2003, 16, 45-50. (36) Clarke, M. B.; Helz, G. R. Environ. Sci. Technol. 2000, 34, 1477-1482.

Received for review January 11, 2007. Accepted February 21, 2007.

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We thank Frontier Geosciences for funding the development of the AEC-ICPMS method, Perkin-Elmer (Ruth Wolf) for the loan of the ICPMS and technical support, Dionex (Raimund Roehl and Peter Jackson) for application support, Heather Broadbent (Trent University) for the TIC analyses, and Holger Hintelmann’s group (Trent University) for collecting and supplying the sediment core. Hakan Gu¨rleyu¨k provided D.W. with invaluable assistance regarding the intricate details of day-to-day ICPMS operation. The Mono Lake water sample was collected by staff of the University of Georgia, School of Marine Sciences. George Helz (University of Maryland) contributed valuable comments during the review of the manuscript.

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