1 Zhejiang Key Laboratory of Petrochemical Environmental Pollution, Zhejiang Ocean University, Zhoushan, 316022, PR China; School of Naval Architecture and Maritime, Zhejiang Ocean University, Zhoushan, 316022, PR China.
2 Zhejiang Key Laboratory of Petrochemical Environmental Pollution, Zhejiang Ocean University, Zhoushan, 316022, PR China; National & Local Joint Engineering Research Center of Harbor Oil & Gas Storage and Transportation Technology, Zhejiang Ocean University, Zhoushan, 316022, PR China. Electronic address: qgchen@zjou.edu.cn.
3 Zhejiang Key Laboratory of Petrochemical Environmental Pollution, Zhejiang Ocean University, Zhoushan, 316022, PR China.
4 Department of Building, Civil and Environmental Engineering, Faculty of Engineering & Computer Sciences, Concordia University, Montreal, Quebec, H3G1M8, Canada.
5 Zhejiang Key Laboratory of Petrochemical Environmental Pollution, Zhejiang Ocean University, Zhoushan, 316022, PR China; National & Local Joint Engineering Research Center of Harbor Oil & Gas Storage and Transportation Technology, Zhejiang Ocean University, Zhoushan, 316022, PR China.

In this study, we investigated the doping of Fe-N-C with ZnO (Fe-N-C@ZnO) to enhance its performance in the reduction of biological toxicity and degradation of enrofloxacin (ENR) in seawater. The steady-state/transient fluorescence analysis and free radical quenching test indicated an extremely low electron-hole recombination rate and the generation of reactive oxygen species in Fe-N-C@ZnO, leading to an improvement in the energy efficiency. We compared the ENR degradation efficiencies of Fe-N-C@ZnO and ZnO using both freshwater and seawater. In freshwater, Fe-N-C@ZnO exhibited a slightly higher degradation efficiency (95.00%) than ZnO (90.30%). However, the performance of Fe-N-C@ZnO was significantly improved in seawater compared to that of ZnO. The ENR degradation efficiency of Fe-N-C@ZnO (58.87%) in seawater was 68.39% higher than that of ZnO (34.96%). Furthermore, the reaction rate constant for ENR degradation by Fe-N-C@ZnO in seawater (7.31 × 10^-3 min^-1) was more than twice that of ZnO (3.58 × 10^-3 min^-1). Response surface analysis showed that the optimal reaction conditions were a pH of 7.42, a photocatalyst amount of 1.26 g L^-1, and an initial ENR concentration of 6.56 mg L^-1. Fe-N-C@ZnO prepared at a hydrothermal temperature of 128 °C and heating temperature of 300 °C exhibited the optimal performance for the photocatalytic degradation of ENR. Based on liquid chromatography-mass spectrometry analysis, the degradation processes of ENR were proposed as three pathways: two piperazine routes and one quinolone route.