Photocatalytic self-Fenton degradation of ciprofloxacin over S-scheme CuFe2O4/ZnIn2S4 heterojunction: Mechanism insight, degradation pathways and DFT calculations

The photo-Fenton process is a promising technology for wastewater purification, but the extra addition of H2O2 and the limited conversion of Fe2+/Fe3+ greatly hinder its practical application. In this study, we presented S-scheme CuFe2O4/ZnIn2S4 heterojunction photocatalytic self-Fenton system for the degradation of ciprofloxacin (CIP). The formation of an internal electric field in CuFe2O4/ZnIn2S4 heterojunction facilitated the transport and separation of photogenerated carriers, exhibiting high visible light absorption ability and photocatalytic activity. The CuFe2O4/ZnIn2S4 photocatalytic system exhibited the surprisingly high H2O2 selectivity (96.8 %) and H2O2 yield (2545.4 µmol·g−1) through the 2e−-ORR process. DFT calculations found that the O2 molecules were adsorbed on Cu atom of CuFe2O4/ZnIn2S4 in a “Yeager-type” configuration, which facilitated the formation of a key intermediate (*OOH) for the conversion O2 into H2O2. The 25 mg·L−1 of CIP could be completely degraded within 60 min in CuFe2O4/ZnIn2S4 photocatalytic system, while this system also had good cyclic stability and practicality. The photogenerated electrons that accumulated on conduction band of CuFe2O4 facilitated the reduction of Fe3+ and Cu2+ to Fe2+ and Cu+, accompanying by the generation of OH, 1O2 and O2−, while the photogenerated holes that accumulated on the valence band of ZnIn2S4 could directly degrade pollutants. Finally, the vulnerable atomic sites of CIP were successfully predicted by Fukui function, while the CIP degradation pathway and toxicity analysis of degradation products were further clarified. This study provided a new design route for constructing photocatalytic self-Fenton system.