Photocatalytic Oxidation of Arsenite on TiO₂:  Understanding the Controversial Oxidation Mechanism Involving Superoxides and the Effect of Alternative Electron Acceptors

Previously we have reported that superoxide plays the primary role as oxidant of As(III) in the UV/TiO₂ system, however, since then there has been a controversy over the true identity of the major As(III) oxidant. This study aims to establish a comprehensive understanding of the oxidative mechanism which satisfactorily explains all of the observed results during the photocatalytic oxidation (PCO) of As(III). The key step that has masked the true oxidative mechanism is related to the fact that the adsorbed As(III) on TiO₂ serves as an external charge-recombination center where the reaction of As(III) with an OH radical (or hole) is immediately followed by an electron transfer to make a null cycle. This was confirmed by the observation that the photoanodic current obtained with a TiO₂ electrode immediately decreased upon spiking with As(III), portraying the superoxide-mediated PCO as the dominant pathway. The degradation of competitive substrates (benzoic acid and formic acid) was delayed until As(III) was fully converted into As(V) since the normal PCO mechanism that is based on the action of adsorbed OH radicals (or holes) is not working as long as As(III) is present on the TiO₂ surface. However, the As(III) PCO mechanism is entirely altered when alternative electron acceptors (Ag⁺, Cu²⁺, polyoxometalate) are present. When these alternative electron acceptors are more efficient than O₂ they are able to intercept the CB electron, impeding the recombination pathway and enabling an anoxic oxidation mechanism in which OH radicals and holes play the role of main As(III) oxidant. In the presence of polyoxometalate or Cu²⁺, the above-mentioned photoanodic current immediately increases upon spiking As(III), indicating that the PCO mechanism has changed in the presence of more efficient electron acceptors. Comprehensive mechanisms of As(III) PCO and experimental factors that alter the mechanism are discussed.