Efficient Tetracycline Degradation Under Visible Light Irradiation By Type Ⅱ Heterojunction Photocatalyst Based On Two Spinel Oxides

With the continuous development of economy,the pollution of antibiotics in the environment has gradually attracted worldwide attention.Antibiotics,especially tetracycline,are widely used in the treatment of infectious diseases and as growth promoters in animals,but they are discharged from the body as parent compounds or in the form of metabolites due to incomplete metabolism of animals,leading to their widespread existence in water environment.Tetracycline entering water not only has its own toxicity,but also may enhance the resistance of bacteria in water,thus posing a potential threat to the environment and human health.Traditional wastewater treatment methods are difficult to completely remove tetracycline,while photocatalytic technology is not only environmentally friendly,but also in line with the strategic policy of sustainable development,so the degradation of tetracycline has been studied.In this study,two environment-friendly spinel oxides（CuBi₂O₄and ZnFe₂O₄）were used to construct a type Ⅱ heterojunction to overcome the shortcomings of weak redox capacity of photogenerated electrons and holes and easy recombination of photogenerated carriers,so as to achieve efficient removal of tetracycline in water.CBO-ZFO（CuBi₂O₄/ZnFe₂O₄）composite photocatalyst was synthesized by simple hydrothermal method.The removal rate of tetracycline was detected by dark and light conditions,and the photocatalytic efficiency was evaluated.The results show that when the mass ratio of CuBi₂O₄to ZnFe₂O₄reaches 2:1,the photocatalytic efficiency reaches the highest,and the tetracycline removal rate reaches 80.16%.At this time,the apparent reaction rate constant reaches 0.01246 min^-1,which is 12.98 times of pure CuBi₂O₄and 3.70 times of pure ZnFe₂O₄under the same experimental conditions.At the same time,the removal efficiency of tetracycline by heterojunction is greatly improved compared with monomer within 150 minutes,and the total organic carbon removal rate also has the same result.The photocatalytic activity remained unchanged after five cycles.In addition,the trapping experiment and ESR characterization proved that holes were the main active substances in the photocatalytic system,and hydroxyl radicals also played an important role.The characteristic peaks of X-ray diffraction（XRD）near 28.1°,33.26°and35.24°confirmed the existence of CBO and ZFO phases on CBO-ZFO heterojunction.The morphology of CuBi₂O₄-ZnFe₂O₄heterojunction was characterized by scanning electron microscopy（SEM）,transmission electron microscopy（TEM）and high resolution transmission electron microscopy（HRTEM）.The band gaps of CuBi₂O₄and ZnFe₂O₄were 1.84 e V and 2.06 e V,and the flat band potentials were about 1.07e V and-0.68 e V,respectively,by UV-vis and Mott-Schottky spectra.The conduction band potentials were 0.69 e V and 0.32 e V,and the valence band potentials were 2.53e V and 2.39 e V,respectively.The successful synthesis of CBO-ZFO type Ⅱ heterojunction was proved by the morphology characteristics.The specific surface area of CBO-ZFO-2 was 34.73 m²/g by BET.The reasons for the enhancement of photocatalytic activity of heterojunction were comprehensively analyzed by photoluminescence spectroscopy（PL）and electrochemical testing.The CBO-ZFO type Ⅱ heterojunction significantly accelerates the separation and transfer of photogenerated electron pairs,reduces the loading rate of electron hole pairs,and improves the conductivity of electron transfer.Meanwhile,the specific surface area is no longer the main factor for the enhancement of hypercatalytic activity in the system.