Direct and Activated Chlorine Dioxide Oxidation for Micropollutant Abatement: A Review on Kinetics, Reactive Sites, and Degradation Pathway.

Recently, ClO₂-based oxidation has attracted increasing attention to micropollutant abatement, due to high oxidation potential, low disinfection byproduct (DBPs) formation, and easy technical implementation. However, the kinetics, reactive sites, activation methods, and degradation pathways involved are not fully understood. Therefore, we reviewed current literature on ClO₂-based oxidation in micropollutant abatement. In direct ClO₂ oxidation, the reactions of micropollutants with ClO₂ followed second-order reaction kinetics (k_app = 10⁻³–10⁶ M⁻¹ s⁻¹ at neutral pH). The k_app depends significantly on the molecular structures of the micropollutant and solution pH. The reactive sites of micropollutants start with certain functional groups with the highest electron densities including piperazine, sulfonyl amido, amino, aniline, pyrazolone, phenol groups, urea group, etc. The one-electron transfer was the dominant micropollutant degradation pathway, followed by indirect oxidation by superoxide anion radical (O₂^•−) or hydroxyl radical (^•OH). In UV-activated ClO₂ oxidation, the reactions of micropollutants followed the pseudo-first-order reaction kinetics with the rates of 1.3 × 10⁻⁴–12.9 s⁻¹ at pH 7.0. Their degradation pathways include direct ClO₂ oxidation, direct UV photolysis, ozonation, ^•OH-involved reaction, and reactive chlorine species (RCS)-involved reaction. Finally, we identified the research gaps and provided recommendations for further research. Therefore, this review gives a critical evaluation of ClO₂-based oxidation in micropollutant abatement, and provides recommendations for further research. [ABSTRACT FROM AUTHOR]