UV-induced reactive species dynamics and product formation by chlorite

Chlorite (ClO2- ) is a regulated byproduct of chlorine dioxide water treatment processes. The transformation of chlorite under UV irradiation into chloride (Cl-) and chlorate (ClO3- ) involves reactive species chain reactions that could enhance chlorine dioxide water treatment efficiency while reducing residual chlorite levels. This study conducted a mechanistic investigation of chlorite phototransformation by analyzing reaction intermediates and stable end products, including chlorine dioxide (ClO2), free chlorine (HOCl/OCl-), hydroxyl-radical (center dot OH), Cl-, and ClO3- through combined experimental and modeling approaches. Experiments were performed at UV254 irradiation in pure buffered water within the pH range of 6 to 8. Results indicated that the apparent quantum yields for chlorite phototransformation increased from 0.86 to 1.45, and steady-state center dot OH concentrations at 1 mM initial chlorite concentration rose from 8.16 x 10-14 M - 16.1 x 10-14 M with decreasing pH values. It was observed that under UV irradiation, chlorite acts as both a significant producer and consumer of reactive species through three distinct reaction pathways. The developed kinetic model, which incorporates optimized intrinsic chlorite quantum yields Phi inchlorite ranging from 0.33 to 0.39, effectively simulated the loss of oxidants and the formation of major products. It also accurately predicted steady-state concentrations of various species, including center dot OH, center dot ClO, Cl center dot and O3. For the first time, this study provides a comprehensive transformation pathway scheme for chlorite phototransformation. The findings offer important insights into the mechanistic aspects of product and oxidizing species formation during chlorite phototransformation.