Modeling ClO<sub>2</sub>–NOM Reactions for\nPredicting Byproduct Formation and Micropollutant Degradation in Surface\nWater

Chlorine dioxide (ClO<sub>2</sub>) is a promising alternative\ndisinfectant/oxidant\nto free chlorine in drinking water treatment, while it reacts with\nnatural organic matter (NOM) to form free chlorine, chlorite ions\n(ClO<sub>2</sub><sup>–</sup>), and chlorate ions (ClO<sub>3</sub><sup>–</sup>) as byproducts. Predicting the ClO<sub>2</sub> consumption and the formation of these byproducts using a kinetic\nmodel helps to balance the trade-off between disinfection/oxidation\nefficiency and byproduct formation. This study establishes a summative\nequation to describe the reaction between ClO<sub>2</sub> and ClO<sub>2</sub>-reactive moieties in the NOM (CRNOM). The average molar yields\nof ClO<sub>2</sub><sup>–</sup>, free chlorine, Cl<sup>–</sup>, and ClO<sub>3</sub><sup>–</sup> from the reactions between\nClO<sub>2</sub> and nine NOM isolates are determined to be 0.576 ±\n0.017, 0.258 ± 0.022, 0.141 ± 0.010, and 0.039 ± 0.002\nper consumed ClO<sub>2</sub>, respectively. The bimolecular rate constants\nof CRNOM toward ClO<sub>2</sub> (<i>k</i><sub>CRNOM‑ClO<sub>2</sub></sub>) are comparable among nine NOM isolates (683 ±\n57 M<sup>–1</sup>·s<sup>–1</sup> at pH 7.0). The\nCRNOM concentrations and <i>k</i><sub>CRNOM‑ClO<sub>2</sub></sub> increase by 2-fold and 1.3-fold, respectively, as pH\nincreases from 6.0 to 9.0, while pH barely affects the molar yields\nof inorganic products. A kinetic model is established and enables\nthe accurate prediction of ClO<sub>2</sub><sup>–</sup> and\nClO<sub>3</sub><sup>–</sup> formation and ofloxacin degradation\nduring ClO<sub>2</sub> oxidation in surface water.