Kinetic and mechanistic understanding of chlorite oxidation during chlorination: Optimization of ClO pre-oxidation for disinfection byproduct control.

Chlorine dioxide (ClO) applications to drinking water are limited by the formation of chlorite (ClO) which is regulated in many countries. However, when ClO is used as a pre-oxidant, ClO can be oxidized by chlorine during subsequent disinfection. In this study, a kinetic model for the reaction of chlorine with ClO was developed to predict the fate of ClO during chlorine disinfection. The reaction of ClO with chlorine was found to be highly pH-dependent with formation of ClO and ClO in ultrapure water. In presence of dissolved organic matter (DOM), 60-70% of the ClO was transformed to ClO during chlorination, while the in situ regenerated ClO was quickly consumed by reaction with DOM. The remaining 30-40% of the ClO first reacted to ClO which then formed chlorine from the DOM-ClO reaction. Since only part of the ClO was transformed to ClO, the sum of the molar concentrations of oxychlorine species (ClO + ClO) decreased during chlorination. By kinetic modelling, the ClO concentration after 24 h of chlorination was accurately predicted in synthetic waters but was largely overestimated in natural waters, possibly due to a ClO decay enhanced by high concentrations of chloride and in situ formed bromine from bromide. Understanding the chlorine-ClO reaction mechanism and the corresponding kinetics allows to potentially apply higher ClO doses during the pre-oxidation step, thus improving disinfection byproduct mitigation while keeping ClO, and if required, ClO below the regulatory limits. In addition, ClO was demonstrated to efficiently degrade haloacetonitrile precursors, either when used as pre-oxidant or when regenerated in situ during chlorination.