Citation:

Wallschlager, D., R T. Wilkin*, AND R Ford*. T05 DETERMINATION OF REDUCED ARSENIC-THIO SPECIES IN WATERS BY ION CHROMATOGRAPHY-INDUCTIVELY-COUPLED PLASMA-MASS SSPECTROMETRY (IC-ICP-MS). Presented at Winter Plasma Conference 2002, Phoenix, AZ, 01/10-12/2002.

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Elevated arsenic concentrations in ground water are a significant concern for human health, because they may lead to increased arsenic exposure via drinking water. As the inorganic arsenic species arsenite (As(III)) and arsenate (As(V)) are known carcinogens, it is desirable to reduce their concentration in drinking water to the lowest achievable level. As a compromise between human health risk, technical possibilities and economic considerations, the WHO has set a drinking water standard of 10 _g L-1 total arsenic. The main source of arsenic in ground water is usually mobilization of arsenic from the parent rock material, which can occur by natural processes or through human influence. Contrary to surface drinking water sources, ground waters are often anoxic, and recent reports indicate that the mobilization mechanisms of arsenic in reducing environments may differ from those in oxic milieus.
Although it has long been known in the synthetic literature that arsenic forms two groups of (mixed) arsenic-(oxo)thiospecies, namely thioarsenites and thioarsenates, in which one or more of the oxygen atoms of arsenite or arsenate are replaced by sulfur atoms, these groups of compounds have only received marginal attention in analytical method development and environmental studies to date. Recent studies, however, suggest that they may play an important role in the reduction of As(V) in sulfidic waters and the subsequent formation of orpiment (As2S3). To investigate these processes properly, analytical methods are required that allow the determination of thioarsenites and thioarsenates in the presence of the corresponding oxyanions. This task is complicated by the fact that the arsenic-thio species are not available as standards.
In this study, we developed a method for the simultaneous determination of anionic arsenic and sulfur species by ion chromatography-anion self-regenerating suppression-inductively-coupled plasma-mass spectrometry (IC-ASRS-ICP-MS). The anions are separated by anion-exchange chromatography using gradient elution with an NaOH eluant. The eluant is then converted to water on-line by the continuously-regenerated cation-exchanger in the proton form, which improves detection as well as instrumental stability and robustness without affecting the analytes. The neutralized eluant is then mixed with Sb(V) as an internal standard, and introduced into the nebulizer. Arsenic is measured uncorrected at m/z = 75, while sulfur is measured as SO+ at m/z = 48. When the nebulization and plasma conditions are optimized for arsenic, detection limits around 1 ng/L are achieved for a 1 mL sample; likewise, sulfur detection limits reach down to 1 _g L-1 under conditions favoring SO+ formation and detection.
With the developed method, up to four mixed As-S species were detected besides arsenite and arsenate in synthetic and real-world water samples, as verified by the simultaneous presence of arsenic and sulfur in each of the chromatographic peaks. Under isocratic conditions, the As-S species elute in equidistant intervals after arsenate, indicating that they form a homologue series with AsO43-. Quantitation of the signals resulted in sulfur/arsenic ratios of approximately 1, 2, 3 and 4 for the four observed peaks (denoted as As-S1, As-S2, As-S3 and As-S4 in the following). Based on these observations and the chromatographic behavior, we propose that the four compounds are deprotonated forms of the thioarsenic acids H3AsSO3, H3AsS2O2, H3AsS3O and H3AsS4, depending on the pH of the studied system. Other possible species identities that cannot be ruled out at this point are the corresponding thioarsenites (for the first three species), or polymeric species that have been proposed as precursors in the precipitation of As-sulfide minerals.
Reaction of arsenite with sulfide in model solutions quantitatively produced a mixture of the four As-S species within 24 hours, with As-S3 usually encountered as the predominant species. The formed As-S species are relatively stable in unpreserved circumneutral samples, but the arsenic speciation pattern changes slowly on the time scale of weeks to months, with species with higher S/As ratio converting to species with lower S/As ratio, until finally arsenate is formed as the end product.
The four As-S species were detected and quantified in several real-world waters. Although the relative distribution pattern of the four species varied with time and between sites, the As-S species always constituted a major fraction of the total As present. Examples of the relevance of these species are presented and discussed for alkaline ground waters, landfill leachates and a meromictic (permanently stratified) lake.

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