Organic-copper coordination drives the formation of Cu(III) intermediates: a novel pathway for peroxymonosulfate activation and quinolone antibiotics degradation

Abstract

Copper ions, particularly Cu(II), were generally regarded as poor activators for peroxymonosulfate (PMS) in advanced oxidation processes. However, this study demonstrated that quinolone antibiotics (QNs), a class of widely used antibiotics, can effectively promote PMS in the presence of Cu(II), resulting in the rapid degradation of QNs themselves. High-valent copper species, namely, Cu(III), were identified as the primary reactive intermediates. Mechanistic analysis revealed that enhanced PMS generation Cu(III) formation was highly associated with organic-copper coordination. Notably, the degradation kinetics of 10 different QNs exhibited a strong linear dependence on their coordination constants with Cu(II), quantitatively demonstrating the ligand-specific kinetic enhancement effect. A redox cycle mechanism involving different copper species was proposed, wherein the Cu(II)–QNs complex facilitates electron transfer from PMS to Cu(II), forming monovalent Cu(I), which is subsequently oxidized by PMS through a coupled electron-proton transfer (CEPT) process to generate Cu(III). Results obtained from this study highlight the critical role of contaminant–metal coordination in mediating valence transitions and provide mechanistic insights for optimizing transition metal-catalyzed advanced oxidation processes in water treatment.