Modelling the reaction kinetics between NOM and ClO2 for predicting chlorite/chlorate formation in water

Chlorine dioxide (ClO₂) is increasingly used as an alternative to free chlorine for water disinfection/oxidation due to the reduced formation of halogenated disinfection byproducts (DBPs), while the formation of chlorite and chlorate as byproducts remains a concern. The reaction kinetics between natural organic matter (NOM) and ClO₂ are poorly understood, making it challenging to model and predict the formation of chlorite and chlorate in real water matrix under varied environmental conditions. In this study, we investigated the reaction kinetics of ClO₂ with 11 NOM isolates with a timescale of minutes. We quantified the contents of ClO₂-reactive moieties in NOM and determined the bimolecular rate constants between ClO₂ and the ClO₂-reactive moieties using the least square fitting method. We found a strong correlation between the contents of ClO₂-reactive moieties and the electron-donating capacity (EDC) of NOM (R² = 0.97), which can be applied to estimate the ClO₂-reactive moiety concentrations in water using the EDC of NOM without tedious experiments and fitting. The bimolecular rate constants between ClO₂ and the ClO₂-reactive moieties in the 11 NOM isolates fell in a narrow range of 1390–1640 M^–1 s^–1 at pH 7.0. We then determined the yields of chlorite, free chlorine, chloride, and chlorate from NOM-ClO₂ reactions, which were 0.64–0.71, 0.20–0.26, 0.04–0.10, and 0.02–0.03 per consumed ClO₂ at pH 7.0, respectively. The concentration of ClO₂-reactive moieties estimated from the quantitative relationship with EDC, together with the obtained average biomolecular rate constant and inorganic product yields, were incorporated into a kinetic model to predict the formation of chlorite and chlorate from the NOM-ClO₂ reactions in real surface water. The investigation on the effects of pH on the reaction kinetics and byproduct formation is on-going, and the results will also be discussed during the presentation.