Mechanism Of Photocatalytic Degradation Of Typical Aromatic Derivatives Over ZnIn₂S₄ And Performance Of Its Coating Membrane

Aromatic compounds are important materials for synthesis of organics.Dyes and statins are typical synthetic aromatic derivatives,that widely used in industry and daily life.Due to their stable structures and strong toxicity,they are difficult to be degraded in the traditional wastewater treatment system and will pose potential risk to humans after being released into aquatic environment.As an advanced oxidation technology,photocatalytic degradation of pollutants is achieved by the reactive oxygen species(ROS)generated through excition of photocatalyst under irradiation.The strong oxidation ability of ROS are nonselective,which could oxidation of various organic pollutants in system.However,the degradation efficiency and mechanism of different pollutants in the same photocatalytic system are significantly different.In this paper,the photocatalytic degradation mechanisms of dyes and statins which are typical aromatic derivatives were investigated based on the excellent photocatalytic performance of ZnIn₂S₄.On the basis of understanding the mechanisms of photocatalytic degradation of dyes and statins by ZnIn₂S₄,the photocatalytic membranes coated with photocatalysts were fabricated to solve the problem such as separation and recovery of dispersed photocatalysts in suspended reactor.The structure and performance of the coated photocatalytic membranes were systematically studied,which provides a technical approach for the efficient utilization of photocatalytic technology.The main achievements are summarized as follows.1.ZnIn₂S₄ was prepared by the hydrothermal method and its structure was characterized.Rhodamine B(Rh B)and methyl organge(MO)were selected as typical dyes which were aromatic derivatives.The mechanisms of photocatalytic degradation of Rh B and MO by ZnIn₂S₄ were investigated,which were combined to analyze the reasons of achieving the different photocatalytic performance for degradation of different dyes by ZnIn₂S₄.(1)Photolytic degradation of Rh B was negligible,while ZnIn₂S₄ had excellent adsorption and photocatalytic performance for Rh B.According to the results of identification of ROS and transformation products by EPR and LCMS-IT-TOF,Rh B was degraded into small molecular products with simple structure through N-deethylation and cleavage of chromophores under synergistic oxidation of superoxide radicals(·O₂^–)and hydroxyl radicals(·OH).(2)However,the removal efficiency of MO by photolysis,adsorption and photocatalysis in ZnIn₂S₄ system was lower than that of Rh B.MO was oxidized into degradable products when methyl and azo bond of MO were destroyed under·O₂^–and·OH attack.(3)The results of identification of ROS showed that ROS generated in ZnIn₂S₄ photocatalytic system were same when degradation of different dyes.The difference in photocatalytic performance of ZnIn₂S₄ for degradation of different dyes were mainly due to the different adsorptivity of hexagonal ZnIn₂S₄ for different functional groups of Rh B and MO,and different chemical bonds in the presence of dye molecules.2.Fluvastatin was selected as the target pollutant to study the removal of fluvastatin by photocatalysis and results showed that photolytic degradation of fluvastatin was obvious.The ROS were qualitatively and quantitatively analyzed by in situ capture experiments and probe molecule transformation experiments.The transformation products were identified by LCMS-IT-TOF,which toxicity was evaluated by acute toxicity tests.The mechanisms of photolysis and photocatalysis of fluvastatin were analyzed.(1)Photolytic degradation of fluvastatin was achieved through direct photolysis with cleavage of chain structure by adsorbing photon energy in absence of ROS.But the photolytic products were stable carbonyl functional products and cyclization products which were more toxic than fluvastatin.(2)In ZnIn₂S₄photocatalytic system,removal of fluvastatin and COD were significantly enhanced because fluvastatin was degraded through direct cleavage of the chemical bonds and hydroxylation reaction under synergistic oxidation of·O₂^–and·OH.The photocatalytic products were slightly less toxic than fluvastatin and significantly less toxic than the photolytic products.(3)Fluvastatin was degraded by photolysis when discharged into natural water,which resulted in the increased toxicity of water.Photocatalysis is a more effective way for mineralization of statins and mitigation of harm of statins to the environment.3.The dynamic photocatalytic membranes were fabricated by deposition of ZnIn₂S₄on the surface of polyvinylidene fluoride(PVDF)membrane by dead-end filtration to solve the problem of separation and recovery of dispersed photocatalysts in suspended reactor.Meanwhile,the membrane fouling was alleviated by photocatalysis,the integrated process of photocatalysis and separation was realized.The effect of photocatalyst loading on structure and performance of dynamic photocatalytic membrane were investigated.(1)The optimal loading of ZnIn₂S₄ was 2.6 mg/cm²,the uniform and dense functional layer was formed on the surface of PVDF membrane under this condition.The roughness and the adsorption performance for UV-Vis of dynamic photocatalytic membranes increased because of formation of photocatalytic functional layer.The dynamic photocatalytic membranes possessed optical property while retained the characteristics of PVDF membrane.(2)The dynamic photocatalytic membranes exhibited excellent photocatalytic performance and high stability.The removal efficiency of fluvastatin and TOC by dynamic photocatalytic membrane with loading of 2.6 mg/cm²were 97.19%and 53.29%,respectively.91.53%of fluvastatin was removed after 18 h of photocatalytic reaction.(3)When continuous filtration of Rh B,the dynamic photocatalytic membranes exhibited excellent antifouling property and self-cleaning performance.For the dynamic photocatalytic membrane with load of 2.6 mg/cm²,the fouling ratio was lowest and the flux recovery ratio was highest.4.On the basis of research of dynamic photocatalytic membrane,the surface composite photocatalytic membranes were fabricated by surface hybridization based on gel property of PVDF polymer casting solution.The effect of photocatalyst loading on structure and performance of surface composite photocatalytic membrane were investigated.The problem of loss of photocatalyst caused by weak interaction between membrane surface and catalyst in dynamic photocatalytic membrane was furehter solved.Meanwhile,the permeation performance of membrane was enhanced based on hydrophilicity of inorganic photocatalyst.(1)The optimal loading of ZnIn₂S₄ was 1.2mg/cm².The formation of the functional layer not only provided the guarantee for the photocatalytic reaction,but also effectively improved the hydrophilicity and the pure water flux of surface composite photocatalytic membranes.The pure water flux of the surface composite photocatalytic membrane with loading of 1.2 mg/cm² was 628L/(m²·h).(2)Compared with the dynamic photocatalytic membranes,the surface composite photocatalytic membranes achieved highly efficient photocatalytic process with low photocatalyst loading.92.76%of fluvastatin and 41.53%of TOC were removed by surface composite photocatalytic membrane with loading of 1.2 mg/cm²,which also showed high photocatalytic stability in the process of cyclic operation.(3)The permeation flux and flux recovery ratio of surface composite photocatalytic membrane were significantly improved compared with PVDF membrane during filtration of Rh B,indicated that excellent antifouling performance of surface composite photocatalytic membrane.The fabrication of surface composite photocatalytic membrane with excellent photocatalytic and antifouling performance provides an effective way to combine the advantages of photocatalytic technology and membrane separation.