Thioarsenite Detection and Implications for Arsenic Transport in Groundwater

Abstract

Arsenic toxicity and mobility in groundwater depend on its aqueous speciation. Uncertainty about the methods used for measuring arsenic speciation in sulfate-reducing environments hampers transport and fate analyses and the development of in-situ remediation approaches for treating impacted aquifers. New anion-exchange chromatography methods linked to inductively coupled plasma mass spectrometry (ICP-MS) are presented that allow for sample/eluent pH matching. Sample/eluent pH matching is advantageous to prevent thioarsenic species transformation during chromatographic separation because: species protonation states remain unaffected, hydroxyl-for-bisulfide ligand substitution is avoided, and oxidation of reduced arsenic species is minimized. We characterized model and natural solutions containing mixtures of arsenic oxyanions with dissolved sulfide and solutions derived from the dissolution of thioarsenite and thioarsenate solids. In sulfidic solutions containing arsenite, two thioarsenic species with S/As ratios of 2:1 and 3:1 were important over the pH range from 5.5 to 8.5. The 3:1 thioarsenic species dominated when disordered As₂S₃ dissolved into sulfide-containing solution at pH 5.4. Together with the preferential formation of arsenite following sample dilution, these data provide evidence for the formation and detection of thioarsenite species. This study helps resolve inconsistencies between spectroscopic and chromatographic evidence regarding the nature of arsenic in sulfidic waters.

Graphical Abstract

Introduction

The transport of arsenic in groundwater is governed by its chemical speciation. For the oxyanion forms of arsenic, pH and redox status in groundwater control their protonation and oxidation state. Generally, arsenate oxyanions (e.g., H₂AsO₄^-, HAsO₄^2-) are dominant in oxic systems in the presence of dissolved oxygen, while the fully protonated arsenite oxyanion (H₃AsO₃⁰) dominates in suboxic-anoxic systems in the presence of electron donors.^1–3 For systems in which free (dissolved) sulfide is present in solution, sulfur may displace oxygen in the immediate coordination sphere around arsenic, forming thioarsenic species.^4–10 This substitution reaction can go to completion for both arsenic oxyanions, resulting in displacement of all oxygen ligands and formation of thioarsenite (H₃As^IIIS₃⁰) or thioarsenate (H₃As^VS₄⁰).

A continuum between the oxyanion and thioanion endpoints exists, resulting in a progressive distribution of coordinated hydroxyl and bisulfide ligands around arsenic.¹⁰ The formation of thioarsenic species may be tied to arsenic mobilization in reduced aquifers^11–16; thus, the development of analytical approaches that reliably characterize in-situ arsenic speciation in groundwater is critical to design management or remediation strategies and to maintain or return groundwater for potable use. While it is acknowledged that all forms of oxyanions and thioanions can exist in low temperature systems, there is limited knowledge about the specific conditions that dictate the predominant forms in groundwater. This uncertainty is compounded by the observation that each of these forms of arsenic have varying rates of transformation when removed from their native environment.^17–19

Several analytical approaches have been employed to better understand the distribution of aqueous arsenic species that may exist in water. Spectroscopic methods are employed to tease apart the matrix of co-existing species under various solution conditions.^20–25 These techniques benefit from minimal manipulation of the native water chemistry, but their power to differentiate between co-existing species has limitations. These techniques also generally require analysis at relatively high arsenic concentrations and may not be readily extrapolated to concentrations that are more common in groundwater systems.

Analytical approaches based on separation of the individual arsenic species prior to quantitation have proven more beneficial for groundwater analyses, with the use of on-line chromatographic separation coupled to mass spectrometry or atomic fluorescence spectrometry showing greatest promise.^{13,17,26–32} A potential weak link for this technique is the chromatographic separation step, during which alterations to the native arsenic speciation may occur due to the imposed eluent chemistry or interactions with the immobile phase in the chromatography column. The most common chromatographic columns used for arsenic species separation were designed for separation of hard acid anionic species such as chloride, nitrate, and sulfate.^33–35 The inherent properties of the arsenic oxyanion and thioanion species make them soft acids that tend to bind to the immobile phase in the chromatographic column. Thus, a chemically aggressive eluent is needed to ensure that sufficient mass of each arsenic species can be eluted from the column for detection.^27,28,32 The eluent chemistry can modify the species distribution during the chromatographic separation process. Specifically, high pH eluents in common use can alter species protonation states, force hydroxyl-for-bisulfide ligand substitution, and accelerate oxidation of reduced arsenic species. We posit that this analytical shortcoming is one of the causes for the unresolved debate on which thioarsenic species should be expected to occur in reducing aqueous systems.^{10,17,23,24,26,28}

The purpose of this current work was to investigate alternative eluent-chromatography column combinations that avoid use of high pH conditions. The logic behind this investigation was consistent with that employed by Vorlicek et al.³⁶, in which the chromatography development included use of an immobile phase with weaker binding for soft acid species. Objectives of this research included: 1) use of controlled synthesis methods at circumneutral pH (5.4 to 8.5) and conditions undersaturated to saturated with respect to arsenic-sulfide precipitation to evaluate elution performance for a range of chromatography immobile phases; 2) demonstrate the optimized chromatographic separation to ascertain thioarsenite speciation trends for groundwater chemistries in which free aqueous sulfide is present; and, 3) examine the environmental significance of thioarsenite speciation in relation to the potential for arsenic mobilization/attenuation in reducing aquifers.

Materials and Methods

Analytical Methods

Arsenic species concentrations were measured by anion-exchange chromatography coupled to inductively coupled plasma mass spectrometry (AEC-ICP-MS; Thermo X-Series II) in collision-cell mode (m/z 75) using 7% H₂ gas in He to eliminate spectral interferences. Simultaneous arsenic-sulfur speciation analysis was performed in selected experiments by using 15% O₂ gas in He as the collision gas and monitoring m/z 48 (³²S¹⁶O⁺) and m/z 91 (⁷⁵As¹⁶O⁺) ²⁸ (see Supporting Information).

We tested an array of chromatography columns for arsenic thioanions and oxyanions, including the IonPac AS4, AS7, AS11, AS15, AS16, and AS22 analytical columns (250 mm x 4 mm; Dionex); PRP-X110 and X100 anion exchange columns (250 mm x 4.1 mm; Hamilton); and, a C18 reverse-phase column (150 mm x 4.6 mm, 5 μm; Atlantis). Here we focus on results obtained using the AS22 column with isocratic elution (60 mM NH₄NO₃; Sigma-Aldrich, SigmaUltra) and the AS7 column eluted with a NH₄NO₃ gradient (30–60 mM) at 25° C and using a flow rate of 1 mL/min. Use of these columns to achieve arsenic species separation is documented in research examining natural water samples^37–39 and biological fluids.^40–42 Selection of NH₄NO₃ as the mobile phase was based on its pH flexibility, column stripping potential, and plasma compatibility.³³ To circumvent pH-related methodological artifacts, sample/eluent pH was matched within ±0.15 pH units by adding NH₄OH (Sigma-Aldrich, ACS reagent) to the eluent (pH range: 5.5 to 9.0). By using pH-adjusted NH₄NO₃ as the mobile phase we avoided the high pH (pH > 12) conditions imposed by the standard 20–100 mM NaOH gradient eluent method for measuring thioarsenic species.^26–29 For ⁷⁵As⁺ detection, methanol (Sigma-Aldrich) was added to the mobile phase (0.5% w/w) to improve column elution³³ and for signal enhancement.⁴³ Chromatography details and mass spectrometer tuning are described in the Supporting Information.

Experiments

Laboratory test solutions were prepared in an anaerobic chamber (Coy Laboratory Products) under a 95% N₂: 5% H₂ atmosphere. All solutions were prepared with deionized water (Millipore, 18.2 MΩ cm) that was deoxygenated using a gas scrubber and ultra-high purity N₂. Stock solutions were prepared from NaAsO₂ (J.T. Baker, reagent) and Na₂HAsO₄·7H₂O (Sigma-Aldrich, 99.995%); and, NH₄HS solution was prepared by adding NH₄OH (Sigma-Aldrich, ACS reagent) to deoxygenated water (0.01 M NH₄OH) and purging with 1.0% H₂S gas/balance N₂ (Praxair). Test solutions (pH range: 5.5 to 8.5) were mixed by spiking aliquots of the arsenic and sulfide stock solutions into deoxygenated water, typically to prepare 0.01 mM (750 μg/L) arsenite, 0.002 mM (150 μg/L) arsenate, and dissolved sulfide concentrations from 0.03 to 0.13 mM (1 to 4 mg/L; Supporting Information). The solutions were poured into 43 mL amber glass volatile organic analysis vials (ESS, certified) and capped with no headspace. The vials were stored inverted in an anaerobic chamber. Samples were periodically collected from separate test vials prepared from the same mixture. In the glovebox, samples were transferred into 2 mL amber glass vials (Waters) without preservation and immediately transported to the AEC-ICP-MS autosampler for speciation analysis (see Supporting Information).

To examine the nature of the thioanions produced during reactions between As(III) solids and dissolved sulfide, experiments were conducted to measure arsenic species formed during the anaerobic dissolution of disordered As₂S₃ at pH 5.4 and variable ΣH₂S (0.003 to 1.25 mM). Disordered As₂S₃ was prepared by acidifying 0.067 M NaAsO₂ + 0.100 M NaHS to pH 3 with 1 M HCl.⁸ The synthetic As₂S₃ was characterized using powder X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS; see Supporting Information; Figure S1 and Table S1).

Sulfidic groundwater springs were sampled from the Chickasaw National Recreation Area (Sulphur, OK) and amended in the field with arsenic oxyanions to evaluate the chromatographic method(s) applied to natural water matrices containing dissolved sulfide. Details on the preparation, collection, and analysis of the test solutions are described in the Supporting Information.

Results and Discussion

Behavior of Model Solutions

Information about the compositions of solutions examined in this study is provided in Table 1, including: initial arsenite and arsenate concentrations, pH, sulfide concentrations (ΣH₂S = H₂S_(aq) + HS^-), and the solution saturation state with respect to disordered As₂S₃. The first set of solutions were mixtures of arsenite, arsenate, and dissolved sulfide at circumneutral pH (Table 1). Arsenate was included in these synthetic samples to help clarify which arsenic oxyanion species participated in reactions leading to formation of thioarsenic species. The concentrations of arsenite and sulfide were maintained below saturation with respect to precipitation of disordered As₂S₃ and are similar to what has been observed in natural, sulfate-reducing groundwater systems.^{13,16,44–46}

Table 1.

Arsenic- and sulfide-containing test solutions prepared in this study.

	As(III)-O μg/L 1	As(V)-O μg/L 1	pH	ΣH2S mg/L2	[S/As]3	[HS-/OH-]4	As2S3 SI5
Arsenic oxyanion mixtures	750	150	5.5	1.0	3	310	−0.28
	750	150	5.5	4.2	13	1250	0.05
	750	150	7.0	1.0	3	140	−1.44
	750	150	7.0	4.2	13	560	−1.58
	750	150	8.5	1.0	3	9	−5.15
	750	150	8.5	4.2	13	34	−4.12
	As(III)-O μg/L1	As(V)-O μg/L1	pH	ΣH2S mg/L2	[S/As]3	[HS-/OH-]4	As2S3 SI5
As-S chromatography tests	8000	250	7.0	10	2.8	710	−0.02
	15000	1000	8.5	10	1.6	35	−1.53
	8000	250	8.5	20	5.6	130	−3.34
	ΣAs μg/L6	Ratio7	pH	ΣH2S mg/L2	[S/As]	[HS-/OH-]4	As2S3 SI5
As2S3 solubility tests	11527	11526	5.4	0.10	0.02	30	−0.88
	8326	8325	5.4	0.18	0.05	60	−0.10
	4339	4338	5.4	0.30	0.16	90	0.07
	3478	347	5.4	0.50	0.34	160	0.15
	316	2.95	5.4	2.4	18	750	0.19
	365	0.14	5.4	9.5	61	2930	0.23
	427	0.10	5.4	12	66	3700	0.08
	575	0.05	5.4	16	65	4940	−0.01
	577	0.02	5.4	16	65	4930	−0.03
	1251	0.0004	5.4	24	45	7310	−0.15
	1653	0.0018	5.4	40	57	12300	−0.37
	As(III)-O μg/L1	As(V)-O μg/L1	pH	ΣH2S mg/L2	[S/As]3	[HS-/OH-]4	As2S3 SI5
Natural matrices	500 8	500	7.02	0.99	4.6	140	−1.77
	500 9	500	7.15	1.48	6.9	175	−1.58

¹Initial concentration of arsenite and arsenate prior to adding dissolved sulfide.

²Initial total dissolved sulfide concentration.

³Initial molar ratio of sulfide to arsenite; final [S/As] ratio is given for the As₂S₃ solubility experiments.

⁴Initial molar ratio of bisulfide to hydroxide.

⁵Calculated saturation index (SI) of disordered As₂S₃ [using Geochemist’s Workbench, v. 12 and the “thermo.com.V8.R6+” database updated to include solubility constants from ref [8]; SI values near 0 correspond to equilibrium with disordered As₂S₃, negative and positive values correspond to undersaturated and oversaturated conditions, respectively].

⁶Measured total concentration of arsenic determined in As₂S₃ solubility experiments.

⁷Measured ratio of [arsenite/thioarsenic] species in As₂S₃ solubility experiments.

⁸Chickasaw National Recreation Area – Site 1.

⁹Chickasaw National Recreation Area – Site 3.

Chromatographic traces of arsenite, arsenate, plus dissolved sulfide mixtures after 2 d of aging showed four arsenic species at pH 5.5 to 8.5 using the AS22 column with isocratic elution and sample/eluent pH matching (Figure 1). Arsenite eluted consistently across the pH range after 150 s, followed by arsenate (230 s; pH 5.5), and two additional species at 270 s and 440 s, respectively (pH 5.5). With increasing sample/eluent pH, the retention time of arsenate shifted to 340 s at pH 7.0 and back to 310 s at pH 8.5. Retention times for the two additional species increased with pH, to a maximum of 710 s for the second species at pH 8.5 (Figure 1; see Supporting Information, Table S4). Based on prior work using controlled synthetic conditions, we anticipated that the additional species were thioarsenites and this was confirmed using simultaneous As-S chromatography (Figure 2). The pH-dependent shifts in arsenic species retention times are related to a combination of interactions between the target analytes and the stationary phase and mobile phase. Consistency of arsenite elution time reflects its uncharged character below pH 9.2 and non-retention to the stationary phase. The variable analyte-specific retention times represent a necessary analytical tradeoff that comes with the benefit of minimizing the potential for pH-related transformation of thioarsenite species during chromatographic separation.

Figure. 1.

AEC-ICP-MS chromatograms of model solutions obtained using the AS22 column with isocratic elution of deoxygenated 60 mM NH₄NO₃. The traces for 0.012 mM total arsenic plus 0.03 mM ΣH₂S (blue-colored) and 0.012 mM total arsenic plus 0.13 mM ΣH₂S (orange-colored) are offset along the x-axis by 1 min and 2 min, respectively, to show relationships in peak intensity for the various arsenic species. Sample aging time was 48 h.

Figure. 2.

Simultaneous traces of m/z 48 (³²S¹⁶O⁺) and m/z 91 (⁷⁵As¹⁶O⁺) signals in model solution mixtures of arsenite, arsenate, plus dissolved sulfide (see Table 1; AS22 column with isocratic elution). Based on peak area analysis, two thioarsenic species were identified with S/As ratios of 2:1 and 3:1.

Other key observations from the chromatographic traces in Figure 1 include: i) arsenite consistently decreased in concentration with increasing ΣH₂S; ii) thioarsenite species developed progressively with ΣH₂S at the expense of arsenite; iii) arsenate concentrations only decreased at pH 5.5, which has been shown to result from reduction to arsenite in the presence of H₂S⁴⁷; and, iv) although absolute elution times changed as a function of pH for arsenate and thioarsenite species, the relative elution times and order remained consistent as a function of H₂S concentration. The same arsenic speciation observations were obtained using the AS7 column with gradient elution of NH₃NO₃ and sample/eluent pH matching (Figure S2, Table S4), demonstrating consistency of speciation results with different column types.

Arsenate reduction at pH 5.5 was expected based on previous experimental observations that showed arsenate transformation to As(III) by dissolved sulfide at pH < 7.⁴⁷ Thus, the pH 5.5 results in Figure 1 reflect arsenate reduction to arsenite and possible subsequent transformation to thioarsenite species. After 2 d (pH 5.5), arsenate was reduced to 22% of the initial value in 0.13 mM ΣH₂S and 30% of the initial arsenate value in 0.03 mM ΣH₂S. Redox status of the model solutions based on measured pH and Eh is discussed in the Supporting Information (Figure S3).

The presence of thioarsenic species was confirmed by simultaneously tracing the m/z 48 (³²S¹⁶O⁺) and m/z 91 (⁷⁵As¹⁶O⁺) signals in collision cell mode using a He-O₂ gas mix (Figure 2). Experimental samples for simultaneous As-S chromatography were prepared at higher initial arsenite and dissolved sulfide concentrations at pH ≥ 7 to obtain sufficient signal for sulfur detection²⁸ and to avoid saturating solutions with disordered As₂S₃ at lower pH²⁶ (Table 1). Based on peak area analysis, the S/As ratio in the 2^nd thioarsenic species was 1.54 ± 0.08 (n = 6) times greater than the 1^st thioarsenic species, indicating two distinct thioarsenic species in the model solutions with S/As ratios of 2:1 and 3:1. Additional arsenic- and sulfur-containing species were not detected in long-term sample runs (up to 45 min). Sulfur peaks without simultaneous arsenic elution were attributed to bisulfide, sulfate, and thiosulfate and their retention times were confirmed using standard reagents (Supporting Information).

Dissolution of As₂S₃

To examine the thioanions produced during reaction between an As(III) sulfide solid and dissolved sulfide, batch solubility experiments with disordered As₂S₃ were conducted at pH 5.4 and variable ΣH₂S. The oxidation state and bonding environment of arsenic in the As₂S₃ used in batch solubility experiments was confirmed with X-ray absorption spectroscopy (Supporting Information; Figure S1). Solution conditions were established to span the range from sulfide-deficient to sulfide-excessive as described by Eary ⁸. At low ΣH₂S, disordered As₂S₃ (solid thioarsenite) is expected to dissolve and form arsenite:

$$0.5{\text{As}}_2{\text{S}}_3+3{\text{H}}_2\text{O}={\text{H}}_3{\text{AsO}}_{3(\text{aq})}+1.5{\text{H}}_2{\text{S}}_{(\text{aq})}\text{log}{K}_1=-11.9\pm 0.3\left(25°\text{C}\right)$$ [1]

However, at higher concentrations of ΣH₂S, according to Eary ⁸, disordered As₂S₃ forms thioarsenite species upon dissolution:

$$1.5{\text{As}}_2{\text{S}}_3+1.5{\text{H}}_2{\text{S}}_{(\text{aq})}={\text{H}}_2{\text{As}}_3{{\text{S}}_6}^{-}+{\text{H}}^{+}\text{log}{K}_2=-5.0\pm 0.3\left(25°\text{C}\right)$$ [2]

Note that reaction [1] indicates a negative slope; decreasing arsenic concentrations are expected as ΣH₂S increases. Whereas, a positive slope is indicated by reaction [2]. These opposing slopes result in a v-shaped arsenic solubility profile with respect to dissolved sulfide at constant pH. Solution of equations [1] and [2] for H₂S_(aq) at pH 5.4 gives a conditional minimum solubility of disordered As₂S₃ at log[H₂S] ≈ log ΣH₂S ≈ 10^-4.1. Helz et al. ²⁰ concluded that monomeric thioarsenite species are favored over polymeric species in typical natural sulfidic solutions that are undersaturated with respect to As₂S₃ solids. Furthermore, Webster ⁷ indicated that polymeric species were avoided by keeping solutions saturated with As₂S₃ at pH < 6; at higher pH, the solubility of As₂S₃ increases and polymerization in solution becomes more probable. Thus, for the systems examined here, we expected the dominance of monomeric thioarsenite species in solution (e.g., 3:1 species H₂AsS₃^-, HAsS₃^2-; 2:1 species H₂AsS₂O^-, HAsS₂O^2-).

Our batch dissolution studies of disordered As₂S₃ replicate Eary’s v-shaped solubility profile and predicted arsenic speciation trend at pH 5.4 (Figure 3; Table 1). At ΣH₂S concentrations below ~10⁻⁴ M, arsenic speciation was dominated by arsenite. A minimum As₂S₃ solubility condition, with both arsenite and thioarsenite species present, was apparent at ΣH₂S near 10⁻⁴ M. At ΣH₂S > 10⁻⁴ M, arsenite was progressively diminished, and arsenic speciation was dominated by thioarsenite species. Increasing thioarsenite concentrations correlated with increasing ΣH₂S (Figure 3). In previous studies, arsenic speciation was inferred based on solubility trends at variable ΣH₂S and pH.^7,8 Thus, our experiments represent the first time in which As₂S₃ solubility analysis as a function of ΣH₂S was coupled to arsenic speciation measurements. Saturation index (SI) calculations using established equilibrium constants indicated that the experimental solutions were near or in equilibrium with respect to disordered As₂S₃ (mean SI = −0.07; Table 1). Note that the thioarsenite species formed during dissolution of As₂S₃ were dominated by the 3:1 S/As stoichiometry; the 2:1 species was only detected in trace amounts (< 0.3 μM; Figure 3). Arsenic speciation measurements using the AS7 column and gradient elution revealed the same trends as the AS22 column with isocratic elution (Figure S4).

Figure 3.

Results of arsenic speciation analyses of solutions equilibrated with As₂S₃ at pH 5.4 and variable ΣH₂S. The left panel shows the combined concentrations of arsenite plus thioarsenite species (log M) versus log H₂S_aq concentration. The solid blue line is the As₂S₃ solubility prediction from ref [8]. The chromatographic traces of four selected data points are color-coded and shown on the middle panel, representing: low-sulfide and arsenite-dominated speciation (gray), near the solubility minimum with both arsenite and 3:1 thioarsenite (purple), and high-sulfide, 3:1 thioarsenite-dominated speciation (blue and orange). The right panel shows species concentrations for arsenite and the 3:1 thioarsenite as a function of sulfide concentration (log M); note that the minimum arsenic solubility is near 10⁻⁴ M ΣH₂S.

Disordered As₂S₃ was adopted as a reference material for producing thioarsenite species assuming that no oxidative transformation occurred during the batch solubility experiments.⁴⁸ Multiple lines of evidence suggest that chromatographic data collected from the As₂S₃ dissolution studies were representative of As(III)-O and As(III)-S species: i) at low ΣH₂S, As(III)-O species were detected without evidence of oxidation to As(V)-O species; ii) the 3:1 thioarsenite is the expected product of As₂S₃ dissolution¹⁰ at ΣH₂S > 10⁻⁴ M in agreement with the S/As ratio in the measured species; iii) the arsenic solubility and speciation trends reasonably match those predicted for systems saturated in As₂S₃ solids ^7,8; iv) the presence of dissolved sulfide at pH 5.4 favors reduction of As(V), not oxidation of As(III), so oxidative transformation to thioarsenates was not expected ⁴⁷; and, v) the chromatographic trends match As-S speciation dominated by thioarsenites for system mixtures of arsenite and dissolved sulfide at circumneutral pH as previously revealed in X-ray absorption spectroscopy experiments.^17,22–24 Further, results obtained from the model solutions (mixtures of arsenite, arsenate, and sulfide) with AS22/AS7 chromotography and matched sample/eluent pH indicate distinctive 2:1 and 3:1 thioarsenite species that form via sulfidation of arsenite across the pH range from 5.5 to 8.5.

The 1:1 thioarsenite species was not an abundant reaction product in either the As₂S₃ solubility experiments or the model solution mixtures of arsenite and dissolved sulfide. Although additional study is needed, we suspect that the 1:1 thioarsenite species has negligible stability and that solution speciation is dominated by arsenite, the 3:1 thioarsenite species, with some contribution of the 2:1 species at conditions near-saturated with As₂S₃. Close inspection of the chromatograms collected near the As₂S₃ solubility minimum indicated concentrations of the 2:1 thioarsenite that were just greater than the method detection limit (~2 μg As/L or 0.03 μmol As/L; see Supporting Information). An additional low-intensity peak was observed prior to the 2:1 species but separate from the expected elution time of arsenate and we cautiously identify this peak as corresponding to the 1:1 thioarsenite. Follow up experimentation and analysis are needed in order to potentially optimize formation of the 1:1 thioarsenite species and additionally allow simultaneous As-S chromatography for species identification.

The As₂S₃ solubility data can be used to assess the effectiveness of arsenic sulfide precipitation as a mechanism for natural attenuation of arsenic in groundwater systems. The solubility results indicate minimum arsenic concentrations below 500 μg/L when dissolved sulfide concentrations are in the range from about 1 to 10 mg/L at pH 5.4 (Figure 3). Lower dissolved sulfide concentrations favor elevated arsenite levels and higher ΣH₂S drives the formation of thioarsenite species (Figure 3). Previous work showed that arsenic solubility and the rate of As₂S₃ dissolution increased with increasing pH.⁴⁸ Thus, mildly sulfidic environments at pH <6 are most favorable for supporting natural attenuation or in-situ reductive immobilization of arsenic via an As₂S₃ precipitation mechanism. However, other uptake mechanisms for thioarsenite species could involve iron sulfides, such as pyrite (FeS₂)^45,49 and mackinawite (FeS).⁵⁰ A recent study suggested that adsorption of thioarsenite species onto iron oxides is much less significant compared to arsenite adsorption over a pH range encompassing typical natural conditions.⁵¹ In addition, formation of thiolated arsenic species has been shown to correlate with arsenic release in biostimulated aquifers.^13,14 To date, most attention has been given to the role that thioarsenate species play in mobilizing arsenic in reducing environments and comparatively few data are available regarding the environmental behavior of thioarsenite species. While the pH- and ΣH₂S-related controls on As₂S₃ dissolution and precipitation are established, less is known about the interactions between thioarsenite species and mineral surfaces, especially in the presence of dissolved sulfide. Additional study is needed to develop a better understanding of the transport/fate and toxicity characteristics of thioarsenite species.

Dilution Analysis

At constant pH, reduction of dissolved sulfide concentration should result in arsenic oxyanion production and thioarsenic removal via OH^- for HS^- ligand exchange. For example, Fisher et al.⁵² predicted arsenate formation from thioarsenate species following dilution, reduction of bisulfide concentration, and ligand exchange at constant pH. Similarly, arsenite production was predicted to occur following dilution of thioarsenite-containing systems.^17,23 However, there is evidence to indicate that thioarsenates and thioarsenites respond differently to changing ΣH₂S concentrations. For example, Stucker et al.¹⁴ found no evidence for reaction between arsenate- and sulfide-containing solutions at circumneutral pH, which is consistent with results presented here, i.e., thioarsenite species were only detected in the solutions in which arsenite (not arsenate) was mixed with dissolved sulfide at circumneutral pH. As pointed out by Helz and Tossell¹⁰, arsenate is substantially inert requiring ~5 d for 50% exchange of its first-shell O atoms with solvent water. In contrast, much faster (<1 h) oxygen exchange rates are indicated between arsenite and solvent water.⁵³

Seeking to elucidate this ambiguity, dilution analysis was performed on the high-sulfide concentration (0.13 mM ΣH₂S) samples prepared at pH 5.5, 7.0, and 8.5 (Figure 1; Table 1). The samples were diluted 15× using deoxygenated water and aged for 1 d in an anaerobic glovebox prior to AEC-ICP-MS analysis. After dilution, sample pH remained within ±0.2 units. The fractional recovery of arsenite, arsenate, 2:1 and 3:1 thioarsenite species was calculated by comparing peak areas prior to and after sample dilution (Table S6). In all cases, arsenite was preferentially recovered; fractional recoveries of arsenite ranged from 2.0 to 13.7. Arsenate remained constant with fractional recoveries ranging from 0.9 to 1.1. The 2:1 and 3:1 thioarsenite species were incompletely recovered (0.5 to 0.9), indicating that a reaction occurred other than simple dilution which affected their concentration distributions. As an example, at pH 7 the measured arsenic speciation distribution in the 0.13 mM ΣH₂S solution was 8 μg/L arsenite, 160 μg/L arsenate, and 535 μg/L thioarsenite (2:1 plus 3:1). Assuming conservative behavior, a 15× dilution should have yielded <1 μg/L arsenite, 11 μg/L arsenate, and 35 μg/L total thioarsenite. However, measured concentrations following dilution for arsenite, arsenate, and thioarsenites were 10, 10, and 29 μg/L, respectively, showing unchanged arsenate concentrations, loss of thioarsenite, and formation of arsenite (Supporting Information Table S6). Furthermore, dilution of the most sulfide-rich As₂S₃ solubility experiment yielded production of arsenite accompanied by a decreased concentration of the 3:1 thioarsenite species, consistent with a direct ligand-exchange reaction and transformation of thioarsenite to arsenite with decreasing ΣH₂S (see Supporting Information, Figure S4).

The results of the dilution tests indicated exchange reactions between arsenite and the thioarsenite species traced in Figure 1, but not with arsenate. Serial dilution maintains constant S/As; however, at constant pH, the arsenite-to-thioarsenite transformation depends on ΣH₂S and is independent of the S/As ratio. Thus, dilution analysis can be used to constrain the identity of thioarsenic species. Use of an eluent that closely matches sample pH avoids ligand-exchange reactions during column elution, facilitating more conclusive identification of chemical mechanisms that alter arsenic speciation in the native sample. Likewise, analysis at circumneutral pH helps avoid the potential for accelerated oxidation that has been observed for arsenite analysis at more alkaline pH^54–57 and that could likely influence the stability of thioarsenite species.

Natural Sulfidic Matrices/Arsenic Amendments

Sulfidic groundwater springs were sampled from the Chickasaw National Recreation Area (Sulphur, OK) and amended in the field with arsenic oxyanions to evaluate the chromatographic method(s) applied to naturally sulfidic water matrices. The unamended spring waters contained no detectable arsenic when measured using High Resolution-ICP-MS (Supporting Information Table S7). In the field, sulfidic waters were amended with either: 500 μg/L arsenite (10^−5.18 M), 500 μg/L arsenate, or 500 μg/L arsenite plus 500 μg/L arsenate. Unspiked samples were also analyzed to verify absence of arsenic species in the original samples. Unlike the laboratory-prepared samples, these arsenic-amended samples were collected in the field outside of a glovebox. Precautions were taken to minimize oxygen contamination from air in the samples (minimum agitation during sample bottle filling and zero headspace in the capped bottle); however, no other sample preservation method was used other than keeping the samples on ice. Sample analysis was completed within 24 h of the time of sample collection and arsenic amendment.

Major ion and trace metal compositions indicated multiple water types including: Na-Ca-HCO₃, Ca-HCO₃, and Na-Cl (Supporting Information Table S7), with estimated total dissolved solids concentrations ranging from 716 to 1100 mg/L. Sulfide concentrations ranged from about 0.01 to 0.05 mM, with the highest sulfide concentration encountered in a Na-Cl-type water (Table S7; Site 3). The same arsenic speciation patterns observed in the model lab-prepared solutions were encountered in the arsenic-spiked natural sulfidic matrices (Figure 4). Arsenite was consumed with increasing sulfide concentration concurrent with production of the 2:1 and 3:1 thioarsenite species. Arsenate concentration was conserved in the samples spiked only with arsenate or with a mixture of arsenite and arsenate, again confirming direct reaction between arsenite and dissolved sulfide for production of thioarsenite species. For example, the sample from Site 3 spiked with 500 μg/L arsenite plus 500 μg/L arsenate led to decreased arsenite (150 μg/L), unchanged arsenate (510 μg/L), and a total of 335 μg/L of 2:1 plus 3:1 thioarsenite species. The relative abundance of the 3:1 thioarsenite was greater for the groundwater spring containing the higher concentration of sulfide (Figure 4). Comparable results were obtained with the AS7 column and gradient NH₄NO₃ elution (sample/eluent pH matching; Figure S6).

Fig. 4.

Chromatographic (m/z = 75) traces of arsenic speciation in natural sulfidic matrices (see Table S7 for major ion compositions) using the AS22 column and sample pH/eluent pH matching. Site 1 was a Na-Ca-HCO₃-type water (pH 7.02) with 31 μM/L ΣH₂S. Site 3 was a Na-Cl-type water (pH 7.15) with 46 μM/L ΣH₂S. Concentration amendments of arsenite, arsenite, or arsenite + arsenite are noted. Samples were analyzed within 24 h of collection.

Possible matrix effects and method recovery were further evaluated by comparing triplicate 50 μg/L spikes of arsenite plus arsenate (0.67 μM of each species) into: deionized water, deionized water plus 0.05 mM ΣH₂S, and Site 3 NaCl-type water with 0.05 mM ΣH₂S. Repeatability of the AS22 column/60 mM NH₄NO₃ isocratic elution method with sample/eluent pH matching for the three matrices was demonstrated by low coefficients of variation for the two oxyanions and two thioanions (n = 3; CV range: 1.5 to 9.6%; Table S8). The recovery of arsenic species in sulfidic matrices was estimated by summing the peak area of all arsenic species and determining the % recovery relative to the peak areas of arsenite plus arsenate in deionized water. At 0.05 mM ΣH₂S, the total recovery of arsenic was 94.1 and 93.3%, respectively, for deionized water plus sulfide and natural sulfidic Na-Cl-type matrix (Table S8). Testing at higher dissolved sulfide levels (up to approximately 1 mM) indicated lower method recovery, suggesting that the thioarsenite species may be partially retained on the column at higher sulfide concentrations (data not shown).

Thioarsenate Standards

Thioarsenate species have been reported to form via abiotic reaction of arsenite with zerovalent sulfur.⁵⁸ Two thioarsenate salts were analyzed using AEC-ICP-MS after dissolution into deoxygenated water (see Supporting Information). Analyses used the AS16 column/gradient NaOH eluent method for comparison to the AS22/AS7 column methods developed in this study with sample/eluent pH matching. With the gradient NaOH method, salt TA3 was predominantly composed of the 1:1 thioarsenate species (monothioarsenate, MTA; with minor amounts of arsenate) and salt TA2 was a mixture of arsenite, arsenate, and 4 thioarsenate species (Supporting Information Figure S8). The AS22/AS7 columns yielded similar results for MTA at pH 8.3; MTA eluted after 430 s and 570 s with the AS22 and AS7 column, respectively (Figure S9). Simultaneous S/As chromatography results confirmed the lower S/As ratio of MTA in the synthetic thioarsenate standard compared to the thioarsenite species (Figure S10). These results indicate that the 2:1 thioarsenite and monothioarsenate species have similar chromatographic elution characteristics, but they can be differentiated based on the S/As ratio.

Sample dilution of TA3 (15×) yielded proportional recovery of MTA and arsenate with no production of arsenite (Figure S9). This behavior contrasts with dilution results of the model solutions and solutions resulting from As₂S₃ dissolution which contained thioarsenites. In thioarsenite-containing systems, arsenite was preferentially recovered following sample dilution; whereas, proportional species recovery was observed for thioarsenate-containing systems. For the more complex TA2 salt, the AS22/AS7 columns showed recovery of arsenite, arsenate, and MTA consistent with the AS16 gradient NaOH method, and detection of a thioarsenic species that co-eluted with the 3:1 thioarsenite species. However, the AS22/AS7 columns with NH₄NO₃ eluent did not recover thioarsenate species with S/As ratios ≥ 2 (Figure S9).

Implications for Arsenic Transport in Groundwater

Accurate arsenic speciation data are needed to investigate the in-situ technologies for groundwater remediation that rely on lowering the redox potential to manipulate contaminant behavior. In addition, arsenic speciation data are critical for understanding documented examples of secondary redox-induced mobilization of arsenic in groundwater. Methods provided here for determining arsenic speciation of oxyanions and thioanions are based on the principal of matching sample pH with chromatographic elution pH to minimize species transformation and avoid misidentification of arsenic species. Our results indicate that arsenite reacts with dissolved sulfide to form thioarsenite species with S/As ratios of 2:1 and 3:1 across a wide pH range. Solid-phase As₂S₃ dissolves into sulfidic solution to form a 3:1 thioarsenite as expected from the structural coordination of arsenic in As₂S₃. Thioarsenite species are dominant over arsenite at ΣH₂S ≳ 0.1 mM from pH 5.5 to 8.5. These findings support previous X-ray absorption spectroscopy data that identified thioarsenite species in solution mixtures of arsenite and sulfide and help to resolve contradictory spectroscopic and chromatographic evidence that has impeded progress in understanding arsenic behavior in sulfate-reducing environments. Methods presented here can resolve arsenic oxyanions and thioarsenite species in samples ranging in pH from about 5 to 9. Arsenite production following sample dilution is evidence for the presence of thioarsenite species and simultaneous S/As chromotography can help resolve the 1:1 thioarsenate from the 2:1 thioarsenite, although this approach is challenging at low total arsenic concentrations. The details of aqueous arsenic redox chemistry and interactions of arsenic with solids in reducing aquifers await further study.