Photocatalytic defluorination of ciprofloxacin by nitrogen-defected g-C3N4 under simulated sunlight: Mechanistic insight and pathway analysis

Abstract

The photocatalytic degradation of ciprofloxacin (CIP) has been widely reported to be induced by ring cleavage and defluorination. However, the role of photo-generated reactive radicals, especially electrons, in defluorination remains controversial. This study investigates the effects of reactive radicals such as hydroxyl radicals (·OH) and electrons on CIP degradation and defluorination in photocatalytic processes by nitrogen-defected g-C3N4. Almost complete CIP degradation and 46.5 % defluorination were obtained under the best conditions tested. Scavenging tests, degradation intermediate product identification, and theoretical calculations were performed to explore the reaction routes driven by ·OH and electrons in depth. Scavenging tests revealed that ·OH and electrons were predominantly responsible for direct CIP defluorination, whereas electrons played an indirect role as critical intermediates for the formation of ·OH. Time-dependent intermediate evolution analysis demonstrated that CIP degradation mainly occurred by piperazine ring cleavage and defluorination. Density functional theory (DFT) calculations based on the Hirschfeld charge, Fukui index and electrostatic potential confirmed that zwitterionic CIP+- is more reactive than CIP+ and that the piperazine ring in CIP is susceptible to nucleophilic and radical attacks by ·O2– and ·OH. Finally, electron-assisted ·OH substitution and C-F bond cleavage were proposed as the main mechanisms for CIP defluorination. This study provides a thorough understanding of CIP defluorination mechanisms by nitrogen-defected g-C3N4-based photocatalysis.