Cu-doped CoCu_xFe_2-xO₄-EG with oxygen vacancies enhances PMS activation for typical antibiotic removal in biologically pretreated landfill leachate: Bidirectional electron transfer mechanism

Landfill leachate is regarded as a reservoir of refractory antibiotics, posing ecological risks and treatment challenges. The heterogeneous peroxymonosulfate-based advanced oxidation processes (PMS-AOPs) offer an effective route for their removal but are limited by sluggish charge transfer and incomplete active metal cycling, with the radical-nonradical interplay often being neglected. In this study, an efficient CoCu_0.4Fe_1.6O₄-EG catalyst with oxygen vacancies (OVs) was developed via Cu doping, simultaneously achieving structural and electronic modulation to optimize active site distribution and enhance radical-nonradical synergy. The catalytic activity was 10 and 3 times higher than that of CoFe₂O₄ and CoFe₂O₄-EG, respectively. DFT calculations with an explicit solvent model revealed a bidirectional electron-transfer mechanism, in which electrons from metal centers reduced PMS to radicals, while PMS was oxidized by nonmetal sites to form nonradicals. Electrochemical analyses coherently substantiated the electron-transfer pathway and the enhanced charge transport, confirming that Cu doping upholds a durable redox cycle. Continuous-flow treatment of landfill leachate over 720 h achieved 95.25% removal of typical antibiotics, surpassing comparable PMS/PDS-based leachate treatments by more than 20%, while fluorescence quenching coupled with kinetic analysis quantitatively confirmed the cooperative contributions of the major radical (30.56%) and singlet oxygen (¹O₂, 69.44%) pathways under practical conditions. Collectively, this work advanced the practical application and mechanistic understanding of multimetallic spinel-based persulfate oxidation for advanced wastewater treatment.