TiO2-Photocatalytic Reduction of Pentavalent and Trivalent Arsenic: Production of Elemental Arsenic and Arsine

Heterogeneous photocatalytic reduction of As(V) and As(III) at different concentrations over TiO2 under UV light in deoxygenated aqueous suspensions is described. For the first time, As(0) was unambiguously identified together with arsine (AsH3) as reaction products. As(V) reduction requires the presence of an electron donor (methanol in the present case) and takes place through the hydroxymethyl radical formed from methanol oxidation by holes or hydroxyl radicals. On the contrary, As(III) reduction takes place through direct reduction by the TiO2-conduction band electrons. Detailed mechanisms for the photocatalytic processes are proposed. Although reduction to solid As(0) is convenient for purposes of As removal from water as a deposit on TiO2, attention must be paid to formation of AsH3, one of the most toxic forms of As, and strategies for AsH3 treatment should be considered.