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.