Insight into photocatalytic degradation of ciprofloxacin over CeO2/ZnO nanocomposites: Unravelling the synergy between the metal oxides and analysis of reaction pathways

Abstract

The aim of this study was to provide deep insight into photocatalytic degradation of ciprofloxacin (CIP) over CeO2/ZnO nanocomposites. Catalysts used in this work were synthesized by a simple co-precipitation method and fully characterized by means of XRD, nitrogen physisorption, TEM, UV–vis, PL and XPS. Degradation of CIP was tracked by UV–vis and LC-MS techniques and the mineralization efficiency was determined by TOC analyses. It was documented that CeO2/ZnO nanocomposites were ca. twice more active than undoped ZnO and ca. 10 times more active than pristine CeO2. The improved activity of the nanocomposites resulted from the formation of a Z-scheme heterojunction in which photo-excited electrons from the conduction band of ZnO migrate to the valence band of CeO2. Photocatalytic degradation of CIP over the nanocomposites was found to be a surface reaction in which CIP molecules adsorbed onto ZnO are directly oxidized by photogenerated holes (h+). Degradation of the antibiotic proceeded through two parallel pathways: i) pathway I (dominant) in which piperazine moiety of CIP molecules was selectively oxidized by h+, and ii) pathway II in which both fluoroquinolone and piperazine moieties of the antibiotic were degraded. The latter pathway required participation of both h+ and hydroxyl radicals (HO•).