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.

Comment on “Thioarsenite Detection and Implications for Arsenic Transport in Groundwater”

“…Apart from the oxidation argument which already excludes thioarsenite presence, also the presented data does not support thioarsenite species assignment. A lot of emphasis is put on S:As ratios (“On the basis of prior work using controlled synthetic conditions, we anticipated that the additional species were thioarsenites and this was confirmed using simultaneous As–S chromatography”).…”

“…There are no commercial standards for thioarsenites or thioarsenates, but synthesis protocols are published, also for thioarsenites . Interestingly, the authors have presented nice retention-time matches, but with thioarsenates. Unfortunately, this data is hiding in the Supporting Information.…”

“…In their recent publication, Wilkin et al present a novel method for detection of thioarsenites (As III S n 3– ; n = 1–3) by anion exchange chromatography linked to inductively coupled plasma-mass spectrometry (AEC-ICP-MS). Already in 2003, Wilkin et al had presented a method for detection of “thioarsenites” which were later clearly identified as thioarsenates (As V S n 3– ; n = 1–4). − This time, the 20–100 mM NaOH eluent was replaced by a 30–60 mM NH 4 NO 3 eluent of sample-matched pH. The rationale given in the publication is that high-pH eluents “...force hydroxyl-for-bisulfide ligand substitution, and accelerate oxidation of reduced arsenic species”.…”

“…Considering this rapid oxidation of thioarsenites, it is unclear why Wilkin et al did not install their AEC in the glovebox and investigate if (with oxidation excluded) their novel method with pH-matched eluents yields peaks other than the previously observed arsenite . Instead, samples were removed from the glovebox and analyzed 1–2 h later.…”

DC Sputtered Ultralow Loading Gold Nanofilm Electrodes for Detection of As (III) in Water

“…3,4 The solubilization of As (III) is commonly performed through microbial respiration in anoxic soils and is dependent on a multitude of variables including temperature and presence of elements such as iron, sulfide, manganese and carbon. [5][6][7][8][9][10] Many naturally occurring minerals contain As which can be mobilized into ground or surface waters by either reducing or oxidizing conditions to form oxyanions primarily containing arsenite, As (III), and arsenate, As (V), respectively. 11 Arsenic can be found in valence states of −3, −1, 0, +3 and +5, however +3 and +5 are the most common forms in natural waters and present the most concern for toxicity from ingestion.…”