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

Abstract

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.