Mechanistic insights into the enhanced degradation of hydroxychloroquine by singlet oxygen in PMS and PMS/Cl2 systems: A comparative study.

Hydroxychloroquine (HCQ), which is a persistent pharmaceutical widely distributed in various environmental compartments, has raised concerns regarding its bioaccumulation potential and ecological risks. This work systematically investigated the feasibility of non-radical peroxymonosulfate (PMS)-driven oxidation for HCQ degradation, focusing on singlet oxygen (1O2)-mediated pathways and comparing among PMS, PMS/Cl2, UV/PMS and UV/PDS systems. In the PMS/Cl2 system, 1O2 generated was the primary contributor to HCQ degradation, exhibiting a second-order rate constant of 1.23 × 108 M-1s-1, which underscored its high reactivity toward HCQ. In PMS-alone treatment, direct PMS oxidation was the primary contributor to HCQ elimination. The removal followed pseudo-first-order kinetics with rates increased under alkaline pH and linearly with PMS dosages. Background anions (Cl- and HCO3-) slightly facilitated HCQ degradation by PMS, whereas natural organic matter (NOM) exerted inhibition likely through competitive scavenging of 1O2. The system was sensitive to NOM and accompanied by low mineralization efficiency. Mechanism analysis identified main HCQ transformation pathways including C-N bond cleavage, deamination, hydroxyl or alkyl group elimination and electrophilic addition to electron-rich moieties. Single PMS can degrade disinfection by-products (DBPs) precursors in HCQ via non-radical oxidation and inhibit DBPs formation. The 1O2 relevance in PMS/Cl2 triggered the enhanced HCQ degradation but also induced toxic chlorinated intermediates, toxicity of which can be reduced by ClO2 disinfection. This study confirmed the effectiveness of non-radical PMS oxidation in HCQ elimination and alleviating DBPs-related health risks while providing some theoretical basis for the application of untriggered PMS systems in water treatment, which offered dual benefits of targeted HCQ removal and improved disinfection safety.