Singlet oxygen-mediated fluconazole degradation during the activation of chlorine dioxide with sulfite

Singlet oxygen (¹O₂)-mediated advanced oxidations have received considerable attention due to their strong capacity to resist the water matrix and high selectivity for organic pollutants. In this study, the activation of chlorine dioxide with sulfite (sulfite/ClO₂ process) to effectively produce ¹O₂ was proposed to degrade fluconazole (FLC) and simultaneously control the formation of disinfection byproducts (DBPs). The results revealed that FLC could be rapidly degraded by 78.6 % within 10 s by the sulfite/ClO₂ process. Radical quenching tests and electron paramagnetic resonance (EPR) measurements confirm that ¹O₂ produced by the cleavage of epoxides formed by the combination of triazole electron-rich groups in FLC with peroxymonosulfate (PMS) was the main active species in the sulfite/ClO₂ process. The degradation of FLC was favored under alkaline conditions because of the fast electron transfer rate at higher pH values. The presence of chloride (Cl^-), bicarbonate (HCO₃^-), and humic acid (HA) hindered the degradation of FLC mainly because they compete with PMS for the electron-rich groups produced by the reaction. The degradation intermediates of FLC were identified by UPLC‒MS/MS, and their transformation pathways were deduced by the condensed Fukui function (CFF) theory. Using sulfite/ClO₂ as a pretreatment process to treat real potable water, aldehydes, ketones, carboxylic acids and other intermediates may be produced via the carboxylation and carbonylation reactions mediated by ¹O₂, therefore promoting the formation of DBPs during the following chlorination. This study provided a new perspective that while ¹O₂ is effectively produced in the sulfite/ClO₂ process for contaminant degradation, the formation of DBPs during subsequent chlorination should be cautioned.