Catalytic mechanisms for As(III) oxidation by HO over TiO surfaces, and effects of support, vacancy and photoirradiation.

As(III) is much more toxic than As(V) while shows apparently lower affinity at minerals surfaces. Oxidation of As(III) to As(V) by HO over anatase surface provides an attractive avenue for pollution control, and the chemocatalytic and photocatalytic mechanisms are unraveled by means of the DFT + D3 approach. Impacts of anatase as support, O/O vacancy, photoirradiation are addressed as well. As(III) oxidation under various reaction conditions leads to As(V) through dual electron transfers, while energy barriers differ substantially and decline as 1.80 (direct oxidation) > 1.35 (anatase as support) > 1.24 (O vacancy) > 0.50 (chemocatalysis) > 0.28 (photocatalysis) ≥ 0.26 (O vacancy) eV. Anatase as support promotes the reaction through bonding with HO/As(OH) and electron transfers, and its close participation during chemocatalysis produces the TiOOH active site that causes As(III) oxidation to proceed facilely under ambient circumstances. TiOOH exists in two forms (monodentate and bidentate mononuclear) and is critical for chemocatalysis, while its destruction for O vacancy exhibits strongly adverse effects to As(III) oxidation. Photoirradiation readily generates the OH radicals, and corresponding mechanism is plausible while less preferred than the newly posed mechanism based on the Ti(HO) active site. Synergism among a number of surface atoms conduces to the superior activity for O vacancy and photocatalysis. Results provide a comprehensive understanding for As(III) oxidation to As(V) by HO, and facilitate catalysts design for As(III) oxidation that alleviates environmental pollution.