Mechanistic insights into hydroxyl radical formation of Cu-doped ZnO/g-C3N4 composite photocatalysis for enhanced degradation of ciprofloxacin under visible light: Efficiency, kinetics, products identification and toxicity evaluation

In this study, a promising composite photocatalyst, Cu-doped ZnO coupled with g-C3N4 (Cu-ZnO/g-C3N4), was synthesized and applied for the enhanced degradation of a fluoroquinolone antibiotic ciprofloxacin (CIP) under visible light irradiation. The nearly complete removal of CIP could be achieved after 6 h, with a rate constant of 0.0105 min−1. As proved by various characterizations, the enhanced photocatalytic activity of Cu-ZnO/g-C3N4 was mainly attributed to the improvements of visible light absorption and electron–hole separation. Using the radical trappers, the electron spin resonance (ESR) and fluorescence measurements confirmed that a Z-scheme charge transfer route was exactly established at the heterojunction interface, thereby increasing the formation of hydroxyl radicals (•OH) from the hole-induced and electron-induced contributions simultaneously. Moreover, the kinetic effects of catalyst dosage, pollutant concentration and solution pH on CIP degradation were studied, and the intermediate products were identified by liquid chromatography–mass spectrometry (LC/MS) to propose the possible degradation pathways. The identified intermediates were basically presented with the decreased toxicities, indicating an effective control of ecological risks during the process. The above results verify the feasibility and applicability of visible light-driven Cu-ZnO/g-C3N4 photocatalysis for antibiotic wastewater treatment.