Modeling ClO₂–NOM Reactions for Predicting Byproduct Formation and Micropollutant Degradation in Surface Water

Chlorine dioxide (ClO₂) is a promising alternative disinfectant/oxidant to free chlorine in drinking water treatment, while it reacts with natural organic matter (NOM) to form free chlorine, chlorite ions (ClO₂^-), and chlorate ions (ClO₃^-) as byproducts. Predicting the ClO₂ consumption and the formation of these byproducts using a kinetic model helps to balance the trade-off between disinfection/oxidation efficiency and byproduct formation. This study establishes a summative equation to describe the reaction between ClO₂ and ClO₂-reactive moieties in the NOM (CRNOM). The average molar yields of ClO₂^-, free chlorine, Cl^-, and ClO₃^- from the reactions between ClO₂ and nine NOM isolates are determined to be 0.576 ± 0.017, 0.258 ± 0.022, 0.141 ± 0.010, and 0.039 ± 0.002 per consumed ClO₂, respectively. The bimolecular rate constants of CRNOM toward ClO₂ (<i>k</i>_CRNOM-ClO₂) are comparable among nine NOM isolates (683 ± 57 M^-1·s^-1 at pH 7.0). The CRNOM concentrations and <i>k</i>_CRNOM-ClO₂ increase by 2-fold and 1.3-fold, respectively, as pH increases from 6.0 to 9.0, while pH barely affects the molar yields of inorganic products. A kinetic model is established and enables the accurate prediction of ClO₂^- and ClO₃^- formation and ofloxacin degradation during ClO₂ oxidation in surface water.