Mechanistic insights into the enhanced degradation of hydroxychloroquine by singlet oxygen in PMS and PMS/Cl₂...

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.