Photocatalytic oxidation of arsenic(III): Evidence of hydroxyl radicals

Arsenic contamination has been found in the groundwater of several countries. Photocatalysis can rapidly oxidize arsenite (As(III))to less labile and less toxic arsenate (As(V)), which then can be removed by adsorption onto photocatalyst surfaces. This study investigates the photocatalytic oxidation of As(III) to As(V) as a function of As(III) concentration, pH, catalyst loading, light intensity, dissolved oxygen concentration, type of TiO2 surfaces, and ferric ions to understand the kinetics and the mechanism of As(III) oxidation in the UV/TiO2 system. Photocatalytic oxidation of As(III) to As(V) takes place in minutes and follows zero-order kinetics. Benzoic acid (BA) was used as a hydroxyl radical (center dot OH) scavenger to provide evidence for the center dot OH as the main oxidant for oxidation of As(III). The center dot OH radical was independently generated by nitrate photolysis, and kinetics of As(III) oxidation by the center dot OH radical was determined. Formation of salicylic acid (SA)from the oxidation of BA by center dot OH also demonstrates the involvement of center dot OH in the mechanism of As(Ill) oxidation. The effect of Fe(III) on As(Ill) oxidation at different pH values with and without TiO2 under UV light was examined. The results suggest that center dot OH is the dominant oxidant for As(III) oxidation. Two commercially available TiO2 suspensions, Degussa P25 and Hombikat UV100, were tested for the removal of arsenic through oxidation of As(Ill) to As(V) followed by adsorption of As(V) onto TiO2 surfaces. Results showed that complete removal of arsenic below the World Health Organization drinking water limit of 10 mu g/L could be achieved.