Sorption of Monothioarsenate to the Natural Sediments and Its Competition with Arsenite and Arsenate

Monothioarsenate (MTAs(V)) is one of the major arsenic species in sulfur- or iron-rich groundwater, and the sediment adsorption of MTAs(V) plays an important role in arsenic cycling in the subsurface environment. In this study, batch experiments and characterization are conducted to investigate the sorption characteristic and mechanism of MTAs(V) on natural sediments and the influences of arsenite and arsenate. Results show that MTAs(V) adsorption on natural sediments is similar to arsenate and arsenite, manifested by a rapid early increasing stage, a slowly increasing stage at an intermediate time until 8 h, before finally approaching an asymptote. The sediment sorption for MTAs(V) mainly occurs on localized sites with high contents of Fe and Al, where MTAs(V) forms a monolayer on the surface of natural sediments via a chemisorption mechanism and meanwhile the adsorbed MTAs(V) mainly transforms into other As species, such as AlAs, Al-As-O, and Fe-As-O compounds. At low concentration, MTAs(V) sorption isotherm by natural sediments becomes the Freundlich isotherm model, while at high concentration of MTAs(V), its sorption isotherm becomes the Langmuir isotherm model. The best-fitted maximum adsorption capacity for MTAs(V) adsorption is about 362.22 μg/g. Furthermore, there is a competitive effect between MTAs(V) and arsenate adsorption, and MTAs(V) and arsenite adsorption on natural sediments. More specifically, the presence of arsenite greatly decreases MTAs(V) sorption, while the presence of MTAs(V) causes a certain degree of reduction of arsenite adsorption on the sediments before 4 h, and this effect becomes weaker when approaching the equilibrium state. The presence of arsenate greatly decreases MTAs(V) sorption and the presence of MTAs(V) also greatly decreases arsenate sorption. These competitive effects may greatly affect MTAs(V) transport in groundwater systems and need more attention in the future.