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.