Copper complexation inhibits fluoroquinolone photodegradation: An overlooked factor enhancing environmental...

Fluoroquinolones (FQs) and copper ions (Cu²⁺) frequently coexist in aquaculture and livestock wastewater, yet their photochemical interactions remain poorly understood. This study demonstrates that Cu²⁺ exhibits significant inhibition on the photodegradation of 12 different FQs, with FQ-copper complexation as the dominating driver. Crucially, the inhibition efficiency exhibits a robust linear correlation (R²=0.95) with the complexation constants. By integrating steady-state quenching experiments, transient absorption spectroscopy, and density functional theory calculations, a static quenching mechanism is further proposed. Upon complexation, excited FQ-Cu complexes preferentially undergo non-radiative decay to the ground state, significantly reducing the fluorescence quantum yield and intersystem crossing efficiency. This energy dissipation pathway effectively suppresses the formation of triplet excited states and singlet oxygen, thereby inhibiting subsequent photodegradation processes. These findings highlight the overlooked role of metal complexation in prolonging the photochemical activity and environmental persistence of antibiotics, providing critical insights into their environmental fates.