Photolysis of chlorite by solar light: An overlooked mitigation pathway for chlorite and micropollutants

Chlorite (ClO2−) is an undesirable toxic byproduct commonly produced in the chlorine dioxide and ultraviolet/chlorine dioxide oxidation processes. Various methods have been developed to remove ClO2− but require additional chemicals or energy input. In this study, an overlooked mitigation pathway of ClO2− by solar light photolysis with a bonus for simultaneous removal of micropollutant co-present was reported. ClO2− could be efficiently decomposed to chloride (Cl−) and chlorate by simulated solar light (SSL) at water-relevant pHs with Cl− yield up to 65% at neutral pH. Multiple reactive species including hydroxyl radical (•OH), ozone (O3), chloride radical (Cl•), and chlorine oxide radical (ClO•) were generated in the SSL/ClO2− system with the steady-state concentrations following the order of O3 (≈ 0.8 μΜ) > ClO• (≈ 4.4 × 10−6 μΜ)> •OH (≈ 1.1 × 10−7 μΜ)> Cl• (≈ 6.8 × 10−8 μΜ) at neutral pH under investigated condition. Bezafibrate (BZF) as well as the selected six other micropollutants was efficiently degraded by the SSL/ClO2− system with pseudofirst-order rate constants ranging from 0.057 to 0.21 min−1 at pH 7.0, while most of them were negligibly degraded by SSL or ClO2− treatment alone. Kinetic modeling of BZF degradation by SSL/ClO2− at pHs 6.0 – 8.0 suggested that •OH contributed the most, followed by Cl•, O3, and ClO•. The presence of water background components (i.e., humic acid, bicarbonate, and chloride) exhibited negative effects on BZF degradation by the SSL/ClO2− system, mainly due to their competitive scavenging of reactive species therein. The mitigation of ClO2− and BZF under photolysis by natural solar light or in realistic waters was also confirmed. This study discovered an overlooked natural mitigation pathway for ClO2− and micropollutants, which has significant implications for understanding their fate in natural environments.