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 (^•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 ^•OH concentrations at 1 mM initial chlorite concentration rose from 8.16 × 10^-14 M - 16.1 × 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 Φ_chloriteⁱⁿ 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 ^•OH, ^•ClO, Cl^• 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.