Production of reactive species during UV photolysis of chlorite for the transformation of micropollutants in simulated drinking water

Abstract

Chlorite (ClO2−) is a dominant product during chlorine dioxide (ClO2) disinfection. This study investigated the mechanism for UV photolysis of ClO2− and the production of reactive species. The apparent quantum yield of ClO2− is 1.21 ∼ 1.29 mol einstein−1 by UV photolysis at 254 nm at pH 7. UV photolysis primarily transforms ClO2− to ClO2, and then to HOCl/OCl−, ClO3− and Cl−. Both ClO2 and HOCl/OCl− are major reactive chlorine species, which maximum concentrations are 8.4 and 11.0 μM, respectively, at 50 μM ClO2− and pH 7. Meanwhile, hydroxyl radical (HO) and chlorine atom (Cl) are also formed with the concentrations of 3.1 × 10−14 and 7.8 × 10−15 M, respectively. The abatement of structurally diverse pharmaceuticals and personal care products (PPCPs) by UV/ClO2− is compound specific, depending on their reactivity toward ClO2, HOCl/OCl− and HO. The first-order rate constants of PPCPs primarily relying on ClO2 and HOCl/OCl− range from 0.043 to 1.533 min−1 and from 0.240 to 2.071 min−1, respectively, while those relying on HO range from 0.012 to 0.020 min−1. As for phenolics, phenol and bisphenol A (BPA) are completely removed within 15 min and ClO2 plays a dominant role. Natural organic matter (NOM) inhibits the degradation of phenol and BPA in UV/ClO2−, while chloride and bicarbonate shows slight impacts. The transformation pathways of BPA in the UV/ClO2− system initiate with electron transfer, and then hydroxylation, carbonylation, chlorination, and coupling reactions. The formation of disinfection byproducts (DBPs) and total organic chlorine (TOCl) is not significant during UV/ClO2− treatment, and over 90% TOCl are unknown DBPs. This study improves the understanding of the water chemistry of UV photolysis of ClO2−.