Adsorption and transformation of thioarsenite at hematite/water interface under anaerobic condition in the presence of sulfide.

The adsorption behavior of thioarsenite (TAs) on the surface of hematite (α-FeO) is unknown at present. In the present study, we have investigated the transformation and reactions of TAs [monothioarsenite (MTAs) and dithioarsneite (DTAs)] on the surface of α-FeO in the presence of sulfide at S/As = 1 and 3 by X-ray absorption spectroscopy (XAS) and Raman spectroscopy. The adsorption envelopes reveal that the adsorption of TAs on α-FeO is significantly less than that of arsenite (As) in the pH range from 7 to 11 with the initial As concentration of 25 mg L. However, at the initial As concentration of 135 mg L, the uptake of TAs by α-FeO is higher at pH 7 but lower at pH 8-11 than that of As. The adsorption isotherms show that the adsorption of As on α-FeO is largely inhibited by the presence of aqueous sulfide at pH 7 with low As equilibrium concentration (<40 mg L). Whereas the uptake of As by α-FeO is highly elevated compared with the value predicted by Langmuir model at pH 7 with high As equilibrium concentration (>40 mg L), implying the formation of As-bearing (surface) precipitate. The As and S K-edge XAS as well as Raman spectroscopy confirm the formation of As sulfide precipitate on the surface of α-FeO in MTAs system. It is worth to note that the oxidation of (thio)As occurs on the surface of α-FeO in DTAs system under strictly anaerobic conditions. These results shed new light on the understanding of the interfacial behavior of As and point to the potential implication in immobilization and removal of arsenic in sulfidic environment.