Preparation Of Photocatalytic Membrane Loaded With ZnIn₂S₄ And Its Degradation Performance Of Fluvastatin

Pharmaceuticals and personal care products（PPCPs）,as new micro-organic pollutants,have been detected in surface water,groundwater,sewage plant effluent,drinking water and even soil all over the world.The stable existence of PPCPs in water environment will do harm to the ecosystem and human health,which has aroused wide concern among researchers in the past decade.Statins are a class of common lipid regulators,which are mainly used to reduce cholesterol levels in serum,liver and aorta.Among them,fluvastatin is a kind of statins favored by patients.After oral administration,only 24%of fluvastatin is absorbed and utilized by humans,and the unused portion is excreted into the environment through urine or feces in the form of prototype or metabolites.The aqueous solution has poor biodegradability and is persistent,bioaccumulative and difficult to degrade.Therefore,it is necessary to develop technologies that can effectively remove statins so as to prevent them from entering the environmental water.Photocatalytic membrane technology combines photocatalytic technology with membrane filtration technology,which can not only completely mineralize organic pollutants into H₂O and CO₂,but also overcome the secondary pollution of water caused by the difficulty in catalyst separation caused by traditional suspension photocatalytic technology,so it has a broad application prospect.Zn In₂S₄ is a new type of visible light catalyst,with a small band gap width,easy to be excited by visible light,and simple preparation method,mild reaction conditions,so it is widely used in the degradation of organic pollutants and other fields.In this study,Zn In₂S₄ was prepared by hydrothermal method,ultrafiltration membrane was prepared by phase transformation method,the photocatalytic membrane was prepared by filtrating suspended Zn In₂S₄ on the surface of ultrafiltration membrane and was characterized by SEM,UV-Vis,XPS,XRD,and the separation and ultrafiltration performance were also analyzed.The optimal catalyst capacity was determined through the degradation performance of fluvastatin by photocatalytic membrane with different loading capacity and the stability of the photocatalytic membrane was investigated.The influence of different water quality conditions and reaction conditions on the degradation of fluvastatin by photocatalytic membrane were also investigated,as well as the photocatalytic reaction intermediates and the degradation pathway.The main research conclusions are as follows:（1）With the increase of the photocatalyst loading,the absorbance of photocatalytic membrane in ultraviolet and visible light region increased,the characteristic peak response of Zn,In and S elements increased,the diffraction peak of Zn In₂S₄ with hexagonal phase increased.The photocatalytic functional layer was formed on the membrane surface,the hydrophility of the membrane surface was enhanced,the attenuation of pure water flux was alleviated,and the degradation rate of fluvastatin was significantly improved.However,agglomeration of excess Zn In₂S₄ on the surface of the membrane leaded to reducing the utilization rate of the photocatalyst and affecting the photocatalytic efficiency.（2）When the catalyst load increased from 0 g to 0.2 g,the removal rate of fluvastatin by photocatalytic membrane was significantly increased.However,when the load increased to 0.3 g,the adsorption performance and degradation rate of fluvastatin did not increase significantly due to the decrease of catalyst utilization.The photocatalytic membrane with a load of 0.2 g had good stability,and the degradation rate of fluvastatin remained above 85%after the end of six experimental cycles,which could be reused for many times.（3）As the initial concentration of fluvastatin increased,more molecules of fluvastatin and intermediate products of the light reaction were adsorbed by the photocatalytic membrane,and the active sites of the catalysts were occupied,thereby affecting the photocatalytic efficiency.The p H of solution affected the charge of Zn In₂S₄catalyst,leading to changes in the adsorption effect of the photocatalytic membrane on the molecule of fluvastatin,but its influence on the photocatalytic effect was relatively small.With the increase of ion concentration in solution,the inhibition of Cl^-on photocatalytic reaction became more and more obvious,while the mass transfer between fluvastatin and photocatalytic membrane became stronger and stronger.The combination of the two effects inhibited the degradation of fluvastatin.（4）Dead-end filtration and cross-flow filtration had little effect on the removal rate of fluvastatin,and filtrate backflow could promote the degradation effect under the condition of dead-end filtration.In the range of medium and low light intensity,the stronger the light,the higher the removal rate of fluvastatin.Zn In₂S₄ could be excited by visible light and ultraviolet light at the same time,and the removal effect of simultaneous irradiation of both bands was better than that of visible light or ultraviolet light alone.（5）When the photocatalytic membrane was loaded with 0.2g Zn In₂S₄,the initial concentration of fluvastatin was 10 mg/L,the p H of the solution was its original p H,the dead-end and filtrate reflux filtration methods were adopted,and the light intensity was230 W/m² under the light of xenon lamp,the degradation effect of fluvastatin was the best,and the degradation rate could reach 94.75%.（6）The intermediate products of photocatalytic membrane degradation of fluvastatin was determined by LCMS-IT-TOF,The degradation path of fluvastatin is as follows:superoxide radicals attack the C—N bond and the C—C bond to break the chains;hydroxyl radicals attack the C=C double bond to add one or two hydroxyl functional groups;some hydroxyl functional groups can be oxidized by superoxide radicals to form carbonyl functional groups.The generated small molecules of cyclic organics,chain organics and hydroxylated derivatives are ultimately oxidized to CO₂ and H₂O.