Reactive Halogen Species-Mediated Electrocatalytic Oxidation of Arsenite(III)

Electrocatalytic advanced oxidation processes have long been considered among of the most viable ways to remediate aquatic contaminants, including As(III). Although direct electrochemical oxidation of As(III) is thermochemically facile, a high reaction rate is not easily achieved because of the competitive oxygen evolution reaction (OER), particularly at high potentials. This study examines the effect of three halides (Cl^-, Br^-, and I^-) on the electrochemical oxidation of As(III) with nanoparticulate TiO₂ electrodes in an aqueous bicarbonate solution at pH 8.7. The halides significantly enhance As(III) oxidation kinetics by >4, >8, and >20 times, respectively, under optimal conditions. Faradaic efficiencies of As(V) production (As^V-FEs) are also enhanced by a maximum of 10 times by the halides, even at high potentials at which the OER occurs. Pre-electrolysis of each halide solution produces reactive halogen species (ClO^-, BrO^-, and I₃^-). As(III)-spiking of the pre-electrolyzed halide solutions allows simultaneous concentration changes at near-stoichiometric ratios (R² > 0.98) between each halogen species and As(V). Among the three halides, iodide imparts the strongest effect on As(III) oxidation owing to its lowest redox potential. Finally, technical considerations of reactive-halogen-species-mediated As(III) oxidation are discussed.