Metronidazole degradation by Z-scheme Ag2S/BiVO4@α-Al2O3 heterojunction in continuous photo-reactor: response surface methodology optimization, reaction mechanism and the effect of water matrix

The Z-scheme Ag2S/BiVO4@alpha-Al2O3 heterojunction was prepared successfully via hydrothermal method. The as-synthesized catalysts were characterized through different analyses. The photocatalytic activity of the catalysts was examined based on metronidazole degradation under visible light irradiation in batch and continuous photo-reactor (BPR and CPR). The highly effective role of alpha-Al2O3 substrate in the performance of the powdered catalyst was evaluated. Under the optimum operating conditions in BPR the removal efficiency of metronidazole (10 mg/L) was obtained as 11.2, 49.6, 81.6, and 90.5% by Ag2S, BiVO4, 5 wt% Ag2S/BiVO4 and 5 wt% Ag2S/BiVO4@alpha-Al2O3 respectively. The considerable improvement of the photocatalytic performance of pure catalysts could be ascribed to the higher specific area, accelerated interfacial charge transfer, remarkable reduction of PL intensity and prolonged life time of electron-hole pairs in composites. Further, in CPR, the maximum photodegradation of 10 mg/L of MNZ, rhodamine B, tetracycline and sulfamethoxazole were observed as 92.2, 98.5, 93.7 and 82.5% respectively, while the flowrate was kept at 2.8 ml/min. The effects of water matrix as well as different cations and anions were also examined on metronidazole degradation. In the tap water, the degradation efficiency and mineralization of metronidazole were obtained as 78 and 63% respectively after 180 min.