| Recently, there are many kinds of antibiotics that have been detected in surface and ground water. Among them, tetracycline hydrochloride(TCH) is one of the most common antibiotics, its residues in the environment can induce antibiotic-resistant pathogens and consequently cause a great threat to human health and the safety of organism. In addition, it also limits traditional biological treatment process to some extent. Photocatalytic technology has drawn high attention due to its own potential applications in environmental cleaning and energy conversion. Among a multitude of photocatalysts, the novel bismuth tungstate(Bi2WO6) semiconductor catalyst has gained extensive attention owing to its own a wide range of visible light absorption, excellent catalytic performance and enhanceed stability. Nowadays, futher improving the photocatalytic activity of Bi2WO6 have become a hot issue. However, there are less researches on the application of photocatalytic technology with Bi2WO6 in the practical wastewater treatment, in addition, it is urgent to be studied on improving the economy-cost efficiency during the process of application, such as powder catalyst recycling, photocatalytic efficiency of the reactor.Therefore, preparation and immobilization of Bi2WO6 and application in the relavent continuous flow photocatalytic reactor were carried out aiming at these problems. The conditions of preparation and performance of Bi2WO6 were optimized with different dispersants. Then the magnetic composite catalysts Bi2WO6/Fe3O4 and Bi2WO6/Zn Fe2O4, Bi2WO6 sponge-load catalyst and Bi2WO6/Zn Fe2O4 sponge-load composite catalyst were prepared. And the structure and composition of the composite catalysts were analyzed by using the series of characterization methods. After that, the composite catalysts were applied to the continuous flow photocatalytic reactor for degradation of tetracycline wastewater, which provided technical support for practical application of photocatalytic technology in antibiotics wastewater.The research was carried out and the relevant results were achieved as follows:(1) The optimization of Bi2WO6 preparation: The Bi2WO6 catalyst was prepared by the solvothermal method with dispersing agent(PVP, SDBS) adding, and then the preparation conditions and parameters were optimized. The optimal preparation conditions of PVP-Bi2WO6 catalyst were proposed: 1.0wt% PVP, 24 h reaction time. The optimum preparation conditions of SDBS-Bi2WO6 catalyst were 1.0wt% SDBS and 24 h reaction time. The addition of dispersing agent improved the morphology of catalyst, expanded the spectral response range, and increased the utilization ratio of visible light to some extent. SDBS-Bi2WO6 catalyst exhibited the best morphology and photocatalytic activity, and the photocatalytic degradation efficiency on degradation of tetracycline hydrochloride that could be stabilized up to 90.39% after 60 min photocatalytic reaction. Based on the method of UPLC-MSMS, the intermediate products that generated in the process of degrading tetracycline by Bi2WO6 were identified, meanwhile the possible degradation pathways were proposed.(2) Preparation of sponge-load Bi2WO6 catalyst and application in continuous tank type photocatalytic reactor: sponge-load Bi2WO6 was prepared by taking sponge as the carrier material, based on the special features that were light and easy to be fluidized. The optimum preparation conditions of composite catalyst were confirmed: 0.04 mmol PVA, 1:1 the mass ratio of the sponge and catalyst, 8(g/L)the volume ratio of sponge and catalyst, and 100°C load temperature. Sponge-load Bi2WO6 composite catalyst had a stable photocatalytic activity under visible light irradiation. Sponge-load Bi2WO6 composite catalyst was applied in the continuous tank type photocatalytic reactor, the optimal operation parameters of reactor under the condition of 50mg/L inlet pollutant concentration were confirmed: the dosing amount of catalyst was 4g/L, the light intensity was 140000 lux, and the hydraulic retention time was over 6h, meanwhile the removal rate of tetracycline in the reactor was more than 90%. Some of the powder catalyst had fallen off by the continuous aeration, and the loss rate was about 9.16%.(3) Preparation of magnetic composite catalysts and exploration of performance: To improve the recovery rate of falling-off powder catalyst, the magnetic composite catalyst Bi2WO6/Fe3O4 and Bi2WO6/Zn Fe2O4 were successfully prepared by the two-step solvothermal method. The optimal doping ratio of Fe3O4 was 0.16wt%, and the optimal doping ratio of Zn Fe2O4 was 0.15wt%. Upon applying an external magnetic field, the powder catalysts were recovered readily within 2 min, which better improved the separation between catalyst and water. The removal rate of three kinds of catalysts Bi2WO6, Bi2WO6/Fe3O4 and Bi2WO6/Zn Fe2O4 on tetracycline were 90.39%, 90.39% and 96.85%, respectively; the rate constants of first-order kinetics were 0.0313, 0.0239, and 0.0366 min-1; it could be seen that the doping of Zn Fe2O4 not only improved the adsorption performance and photocatalytic activity, but also increased the magnetic of the composite catalyst and improved the recovery efficiency, which had great value in practical application. Both of the composite catalysts had better stability, which had laid a foundation for the further application of Bi2WO6 based magnetic composite catalyst.(4) Preparation of sponge-load Bi2WO6/Zn Fe2O4 catalysts and application in the continuous tank type photocatalytic reactor: By taking PVB as the immobilized agent, sponge-load Bi2WO6/Zn Fe2O4 catalyst was prepared by a simple physical method. The optimum preparation conditions were confirmed: 1:1 mass ratio of the sponge and catalyst, 80°C load temperature, 0.10wt% of the amount of PVB, and 8(g/L) the volume ratio of sponge and catalyst. The loading rate of the composite catalyst under the best preparation conditions was 56.18%, and the removal rate of tetracycline was 98.35%. The composite catalyst had a stable photocatalytic activity, and the loss rate of catalyst in 4 time runs was only 5.73%. Sponge-load Bi2WO6/Zn Fe2O4 magnetic composite catalyst was applied in the continuous tank type photocatalytic reactor. The removal efficiency of tetracycline was over 90% under the best operating conditions that was confirmed above. While the loss rate of falling-off catalyst decreased up to 2.53%, the falling-off powder catalyst still could be recycled by magnet adsorption, and the recovery efficiency was over 95%.(5) Application of Bi2WO6/Zn Fe2O4 catalysts in magnetic fixed film reactor: Based on the magnetism of magnetic balls, Bi2WO6/Zn Fe2O4 was applied into tube-type magnetic fixed film reactor. The influence of the intensity of light source, the inlet concentration of tetracycline and the hydraulic retention time of the reactor performance were investigated. The results showed that the influence of hydraulic retention time of the reactor performance was the greatest, followed by the inlet concentration and the light intensity. Under the conditions that the concentration of inlet water pollutant was 51.58 mg/L, the intensity of light source was 3.20×105 lux, the hydraulic retention time was 94 min, the removal rate of reactor on tetracycline was over 90%. For the different initial concentration of tetracycline, the requirements of different inlet water indexes could be achieved by the regulation time for hydraulic retention within a certain range, so as to achieve the maximum treatment effect of the reactor. Compared with the traditional fixed film reactor, the magnetic fixed film reactor was simpler and more stable, which significantly improved the defects of the falling-off catalyst and had a certain application prospect. |
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