The arsenic species in the sulfidic environments: Determination, transformation, and geochemical implications

The species and fate of arsenic (As) are closely related to sulfide (S^-II) in the anaerobic and sulfidic environment. In this work, the mechanisms and kinetics of arsenate (As^V) reduction by S^-II at different pHs, S^-II/As^V molar ratios, and initial As^V concentrations in the absence (or presence) of Al-hydroxide were studied, where the concentrations of various kinds of As species, namely As^V, arsenite (As^III), and thioarsenics (ThioAs) were qualitatively and quantitatively determined by liquid chromatography with atomic fluorescence spectrophotometry. The results showed that under acidic or neutral conditions, ThioAs may act as intermediate(s), where amorphous As₂S₃ precipitate was observed at pH 5 in high S^-II condition. By comparison, at pH 9, As^V was probably directly reduced to As^III with polysulfide as the byproduct. The reaction rate was faster at mildly acidic pH than that of neutral or alkaline pH, as well as in the presence of Al-hydroxide. The findings may give further insights about the role of ThioAs in the biogeochemical cycle of As.