In Situ Photoelectrochemical Chloride Activation Using a WO₃ Electrode for Oxidative Treatment with Simultaneous H₂ Evolution under Visible Light

Abstract

Reactive chlorine species (RCS) such as HOCl and chlorine radical species is a strong oxidant and has been widely used for water disinfection. This study investigated a photoelectrochemical (PEC) method of RCS production from ubiquitous chloride ions using a WO₃ film electrode and visible light. The degradation of organic substrates coupled with H₂ evolution using a WO₃ electrode was compared among electrochemical (EC), photocatalytic (PC), and PEC conditions (potential bias: +0.5 V vs Ag/AgCl; λ > 420 nm). The degradation of 4-chlorophenol, bisphenol A, acetaminophen, carbamazepine, humic acid, and fulvic acid and the inactivation of <i>E. coli</i> were remarkably enhanced by in situ RCS generated in PEC conditions, whereas the activities of the PC and EC processes were negligible. The activities of the WO₃ film were limited by rapid charge recombination in the PC condition, and the potential bias of +0.5 V did not induce any significant reactions in the EC condition. The PEC activities of WO₃ were limited in the absence of Cl^- but significantly enhanced in the presence of Cl^-, which confirmed the essential role of RCS in this PEC system. The PEC mineralization of organic compounds was also markedly enhanced in the presence of Cl^- where dark chemical chlorination by NaOCl addition induced a negligible mineralization. The H₂ generation was observed only at the PEC condition and was negligible at PC and EC conditions. On the other hand, the oxidation of chloride on a WO₃ photoanode produced chlorate (ClO₃^-) as a toxic byproduct under UV irradiation, but the visible light-irradiated PEC system generated no chlorate.