Tuning reactive chlorine species to singlet oxygen via peroxymonosulfate synergy: Ultra-efficient electrooxidation...

Chlorination electrooxidation (EO/Cl^-) systems face challenges in balancing efficient pollutant degradation with hazardous chlorinated byproduct formation and unclear formation mechanisms of active species. Here, we propose an innovative peroxymonosulfate (PMS)-mediated strategy to redirect reactive chlorine species (RCS) toward singlet oxygen (¹O₂) generation, achieving simultaneous ultra-efficient ¹O₂ production and chlorinated byproduct suppression. By adapting a refined quenching protocol, we identified contributions of key species (Cl^•/Cl₂^•-, HOCl, Cl₂, and ¹O₂), and unraveled the selective conversion mechanism of RCS to ¹O₂ in the EO/Cl⁻/PMS system. Mechanistic studies demonstrate that Cl^•, Cl₂^•-, and HOCl react with PMS to form ¹O₂ via thermodynamically favorable pathways. Compared to Cl^•/Cl₂^•- attack, ¹O₂ can bind to tetracycline (TC) through peroxide bond addition and its intermediate exhibits a higher energy barrier in reactions with Cl^-, effectively preventing chlorinated byproduct formation. Consequently, this EO/Cl^-/PMS system can degrade 99.6 % TC in 30 min at an ultra-low energy consumption (0.30 kWh m^-3 order^-1), outperforming most oxidation systems. Furthermore, this system demonstrates wide pH adaptability, long-term stability, pollutant degradation applicability, and compatibility with various water matrices. This work provides a paradigm for modulating RCS-to-¹O₂ conversion to reconcile oxidation efficiency and byproduct control in electrochemical water treatment.