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

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^[rad]) and chlorine atom (Cl^[rad]) 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^[rad]. 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^[rad] 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⁻.