Adsorption of monothioarsenate on amorphous aluminum hydroxide under anaerobic conditions

• Adsorption of monothioarsenate on amorphous Al(OH)3 was firstly investigated.
• We found less sorption of mtAsV compared to AsV on am-Al(OH)3.
• The interaction with am-Al(OH)3 enhanced the reduction of mtAsV to AsIII.
• MtAsV was sorbed to am-Al(OH)3 via bidentate binuclear complexation.

Thioarsenates have been found to be major arsenic species in some sulfidic waters and their interactions with minerals play an important role in the geochemical cycling of arsenic. This work investigated the sorption behavior of monothioarsenate (mtAsV) on amorphous aluminum hydroxide (am-Al(OH)3) at various pHs and characterized the adsorbed arsenic by using X-ray absorption spectroscopy (XAS) and Fourier transform infrared spectroscopy (FTIR). The uptake of arsenic by am-Al(OH)3 from mtAsV solution (86–16 mg·g− 1) was found to be much less than that from arsenate (AsV) solution (112–50.5 mg·g− 1) at all pHs applied (4–10). MtAsV was partly reduced to arsenite with concurrent production of elemental sulfur, and the interaction with am-Al(OH)3 at acidic pH enhanced the reduction, which is attributed to acid-catalysis at the am-Al(OH)3 surface. Transformation of mtAsV to arsenate also occurred during the adsorption process. The linear combination fitting (LCF) of As K-edge X-ray absorption near edge structure (XANES) spectra indicated that the arsenic adsorbed on am-Al(OH)3 was present dominantly as mtAsV, with minor amounts as arsenite and arsenate. The As K-edge extended X-ray absorption fine structure (EXAFS) and FTIR results suggested that bidentate binuclear complexation was involved in the adsorption of mtAsV on am-Al(OH)3. This study is the first documentation of mtAsV sorption on aluminum hydroxides and is necessary to assess geochemical cycling of arsenic in sulfidic natural or mining-impacted environments.