Photolysis of Chlorine Dioxide under UVA Irradiation: Radical Formation, Application in Treating Micropollutants, Formation of Disinfection Byproducts, and Toxicity under Scenarios Relevant to Potable Reuse and Drinking Water

Conversion of potable reuse water utilities and drinking water utilities from a low-pressure UV/H₂O₂ (LPUV/H₂O₂) advanced oxidation process (AOP) to alternative AOPs in which oxidants can effectively absorb photons and rapidly generate radicals has attracted great interest. Herein, we propose a novel UVA/ClO₂ AOP for different water treatment scenarios because of reduced photon absorption by the background matrix and high molar absorptivity for ClO₂ at UVA wavelengths. While the photolysis of ClO₂ produces ^•Cl + O₂ or ^•ClO + O(³P) via distinct product channels, we determined the parameters needed to accurately model the loss of oxidants and the formation of byproducts and combined a kinetic model with experimental data to determine quantum yields (Φ). Modeling incorporating the optimized Φ simultaneously predicted oxidant loss and the formation of major products −HOCl, Cl^-, and ClO₃^-. We also systematically investigated the removal of three contaminants exhibiting different radical reactivities, the formation of 35 regulated and unregulated halogenated disinfection byproducts (DBPs), DBP-associated toxicity, and N-acetylcysteine thiol reactivity in synthetic or authentic RO permeates/surface waters treated by different AOPs. The kinetic model developed in this study was used to optimize operating conditions to control undesired products and improve contaminant removal efficiency. The results indicate that UVA/ClO₂ can outperform LPUV/H₂O₂ in terms of electrical energy per order of contaminant degradation, disinfection byproduct formation, and toxicity indices.