Distribution and formation of thioarsenate in high arsenic groundwater from the Datong Basin, northern China

Abstract

Understanding the formation of thioarsenate, an important arsenic species, is of great significance with respect to explaining the mobilization of arsenic in fresh groundwater systems. Monothioarsenate (MTA), dithioarsenate (DTA), and trithioarsenate (TriTA) are three main forms of thioarsenate in fresh groundwater. A total of 30 groundwater samples were collected from the Datong basin to conduct hydrochemistry and arsenic speciation analysis. Geochemical simulations were established to verify the occurrence of thioarsenate. The results show thioarsenate was detected in 40% of the groundwater samples at concentrations ranging from 2.36 to 209.9 μg/L; the highest concentration accounted for 68% of total arsenic. Thioarsenate mainly occurs in the discharge area of the Datong basin, where the groundwater was characterized by strongly reducing conditions (Eh up to −175.3 mV) and higher concentrations of arsenic (up to 3450 μg/L) and sulfide (up to 8480 μg/L). A strong positive correlation (r = 0.81, p < 0.05) was observed between thioarsenate and sulfide. Strong reducibility and high sulfide concentration are beneficial for the formation of DTA vs. MTA. Geochemical modelling results confirm that suitable S/As mass ratio (<2.2) favors the formation of thioarsenate and that compared with MTA, DTA is easier to form under strong reduction environment and DTA is directly affected by the concentrate of sulfides, while MTA may be related to elemental sulfur. This understanding of the different formation conditions of thioarsenate species helps to explain the migration and enrichment of As in sulfide-rich groundwater.