Theoretical studies of arsenite adsorption and its oxidation mechanism on a perfect TiO ₂ anatase (1 0 1) surface

There are many areas in the world where the ground water has been contaminated by arsenic. TiO ₂ is one of the most promising materials that can remove arsenic from groundwater supplies by the adsorption-based processes. The TiO ₂ surface is capable of photo-catalytic oxidation (PCO) changing the arsenite [As(III)] to arsenate [As(V)] which is more easily absorbed by the surface, increasing the efficiency of the process. In this paper, a density functional theory calculation has been performed to investigate the adsorption of As(III) on a perfect TiO ₂ anatase (1 0 1) surface. All the As(III) solution species such as H ₃ AsO ₃ , H ₂ AsO ₃ ^- , HAsO ₃ ^2- and AsO ₃ ^3- are put onto the surface with many different possible attitudes to obtain the adsorption energy. Based on the adsorption energy and the concentration of H ₃ AsO ₃ , H ₂ AsO ₃ ^- , HAsO ₃ ^2- and AsO ₃ ^3- in an aqueous solution, the bidentate binuclear (BB) adsorption configurations of H ₂ AsO ₃ ^- on the surface are more favorable at low As(III) concentrations, whereas BB form and monodentate mononuclear (MM) form may coexist at higher concentrations. By calculating H ₂ AsO ₃ ^- co-adsorption with water and oxygen, we can confirm the deep acceptor character of an adsorbed O ₂ molecule which implies that surface superoxide (or hydroperoxyl radical) plays an important role during the PCO process of As(III) on TiO ₂ surface.