Arsenic is a carcinogenic toxin that has a maximum standard of 10 ug/L in drinking water, as established by the World Health Organization. In sulfidic environments, particularly sulfate-reducing groundwaters, arsenic readily forms a mixture of thioarsenite species and thioarsenate species (mono-, di-, tri-, and tetra- thio). Quantification of the thioarsenite species is difficult for aqueous samples at low arsenic concentration. Knowledge of groundwater arsenic speciation is important because the species vary both in levels of toxicity and difficulty in removal. The specific formation mechanisms are poorly understood. Formation of dithioarsenate has been proposed to be by reaction of sulfide with monothioarsenate, however contradictory evidence exists for the importance of this mechanism. This reaction was investigated through the use of ion chromatography interfaced to inductively coupled plasma mass spectrometry (IC-ICP-MS). Concentrations of thioarsenic species following reaction were determined and compared against a 350 ppb standard of monothioarsenate, prepared from solid form. This standard also contained about 12% each of arsenite and arsenate. Following reaction arsenite decreased by approximately 75 % while monothioarsenate increased by 30%. The percent of dithioarsenate was slightly decreased. The decrease in arsenite suggests that the monothioarsenate was not the reactant, but rather, the arsenite combined with the sodium sulfide to produce more monothioarsenate. These results suggest a redox reaction mechanism to produce the monthioarsenate, as opposed to previous assumptions that monothioarsenate would undergo further substitution of sulfur to generate dithioarsenate. Results provide insights into arsenic behavior in reduced groundwaters as well as into treatment and impacts of arsenic toxicity.

Investigating the formation of thioarsenic species under sulfidic conditions

249th National Meeting (2015)

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