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Thioarsenite Detection and Implications for Arsenic Transport in Groundwater
Citation:
Wilkin, Richard T., Robert G. Ford, Lisa M. Costantino, Randall R. Ross, Douglas G. Beak, AND Kirk G. Scheckel. Thioarsenite Detection and Implications for Arsenic Transport in Groundwater. ENVIRONMENTAL SCIENCE & TECHNOLOGY. American Chemical Society, Washington, DC, 53(20):11684-11693, (2019). https://doi.org/10.1021/acs.est.9b04478
Impact/Purpose:
Understanding metal speciation in aqueous systems is fundamental to developing transport and fate models, designing effective remedial systems, and for predicting human and ecological risk. Currently there is uncertainty about the chemical nature of arsenic species present in reducing groundwater systems. This uncertainty causes problems in predicting arsenic behavior and in quantifying the forms of arsenic present in natural waters. New anion-exchange chromatography methods linked to inductively coupled plasma mass spectrometry (ICP-MS) are presented that allow for sample/eluent pH matching and improved arsenic speciation analysis. The work is of interest to the international environmental community, including EPA’s regional, program, and local partners.
Description:
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 serial 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.
URLs/Downloads:
DOI: Thioarsenite Detection and Implications for Arsenic Transport in Groundwater
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