Chlorite formation during ClO₂ oxidation of model compounds having various functional groups and humic substances

Chlorine dioxide (ClO₂) has been used as an alternative to chlorine in water purification to reduce the formation of halogenated by-products and give superior inactivation of microorganisms. However, the formation of chlorite (ClO₂^-) is a major consideration in the application of ClO₂. In order to improve understanding in ClO₂^- formation kinetics and mechanisms, this study investigated the reactions of ClO₂ with 30 model compounds, 10 humic substances and 2 surface waters. ClO₂^- yields were found to be dependent on the distribution of functional groups. ClO₂ oxidation of amines, di- and tri-hydroxybenzenes at pH 7.0 had ClO₂^- yields >50%, while oxidation of olefins, thiols and benzoquinones had ClO₂^- yields <50%. ClO₂^- yields from humic substances depended on the ClO₂ dose, pH and varied with different reaction intervals, which mirrored the behavior of the model compounds. Phenolic moieties served as dominant fast-reacting precursors (during the first 5 min of disinfection). Aromatic precursors (e.g., non-phenolic lignins or benzoquinones) contributed to ClO₂^- formation over longer reaction time (up to 24  h). The total antioxidant capacity (indication of the amount of electron-donating moieties) determined by the Folin-Ciocalteu method was a good indicator of ClO₂-reactive precursors in waters, which correlated with the ClO₂ demand of waters. Waters bearing high total antioxidant capacity tended to generate more ClO₂^- at equivalent ClO₂ exposure, but the prediction in natural water should be conservative.