Selective reduction of chlorite to chloride in drinking water by UV/sulfite system: pathways, mechanisms, and kinetics

Sulfite radicals-based advanced reduction processes (ARPs) have shown promise for efficiently removing chlorite, a toxic by-product of chlorine dioxide disinfection. However, the role of sulfite radicals in chlorite reduction through UV-mediated ARPs has been underestimated, as overshadowed by the superior reductive abilities of hydrated electrons. In this study, we investigated the contributions of sulfite radicals and hydrated electrons in the UV/sulfite (UV/S(IV)) system for chlorite reduction. UV/S(IV) system achieved a nearly complete reduction of 13.5 mg/L chlorite to chloride within 60 min in deionized water using 160 mg/L S(IV) and UV doses of 6.0 mW/cm². The first-order rate constant for chlorite removal in this system was 0.080 ± 0.006 min⁻¹, up to 27 times faster than in previous systems. In addition, hydrated electrons and sulfite radicals were identified as the dominant reducing species, mainly based on the EPR spectra and results of nitrate/nitrite quenching experiments. Kinetic simulations revealed that short-lived hydrated electrons initiated efficient chlorite reduction, while stable sulfite radicals selectively decomposed chlorite into chloride in complex water matrices throughout the process. Moreover, UV/S(IV) system showed the potential for selectively reducing chlorite to chloride in real water, with > 81.0 % removal of 1.35 mg/L in 60 min. Importantly, this reduction process was hardly influenced by the presence of coexisting water constituents such as HCO₃⁻, SO₄²⁻, Cl⁻, NO₃⁻, and NO₂⁻. This work brightens an overlooked route to selective reduction of chlorite to chloride in UV/S(IV) system.