| Membrane separation technology was widely used in the field of water treatment engineering for its outstanding outlet water quality,high particles separation efficiency,mild process control conditions,contaminants concentration ability,and great potential on resources recycling.Among many membrane materials,polyvinylidene fluoride（PVDF）has attracted extensive attention of researchers due to its good mechanical properties,thermal stability,and chemical resistance.However,the surface of PVDF membrane has typical hydrophobic properties.Therefore,membrane fouling is inevitable in the process of organic wastewater treatmentprocess.In addition,traditional membrane cleaning and recovering process consume a large amount of agentias and energy,which can increase the operating cost.Hence,membrane separation technology encountered obstacles in engineering promotion and application.In order to solve these technical bottleneck problems,it could be utilized that implanting hydrophilic functional catalytic materials in the PVDF membrane.On the one hand,the implanting hydrophilic materials increase the hydrophilic performance of PVDF membrane surface,which can lead the anti-fouling function.The membrane service time could be prolonged and the frequency of membrane cleaning coule be reduced.On the one hand,the PVDF membrane can be endowed with the ability of catalysis,which could degrade the organic pollutants.Therefore,the efficiency of membrane cleaning could be increased,and the feasibility of practical application in engineering could be improved.There are many hydrophilic functional catalysts could be utilized.Among them,titanium dioxide（TiO2）-based photocatalysts was selected for advantages of stable properties,low cost,low toxicity,low risk of secondary pollution,and good hydrophilic properties.However,TiO2efficiency of photocatalytic degradation organic pollutants is limited because the high band gap energy（3.0e V～3.2e V）,low utilization rate of sunlight.Therefore,its photocleaning efficiency of PVDF membrane is not enough either.In recent years,researches showed that nitrogen-doped TiO2（N-TiO2）photocatalysts have the ability to degrade organic pollutants by visible light.This is due to the entry of N into the TiO2lattice,which can reduce the bandgap energy and red-shifts the excitation light wavelength.Therefore,in this study,N-TiO2was used as the implantation catalyst for PVDF membrane,and the N-TiO2@PVDF membrane with anti-pollution and simulated sunlight self-cleaning ability was prepared by the immersion precipitation phase inversion method.This N-TiO2@PVDF membrane has the properties of antifouling and simulated sunlight photocleaing function.The water flux also was improved.The detailed research results are as following:（1）The PVDF microfiltration membrane was successfully synthesised by the immersion precipitation and phase inversion method using N,N-dimethylacetamide（DMAc）as solvent,PVDF as the main frame of the membrane,and polyvinylpyrrolidone（PVP）as a pore-forming agent.The optimal matter quality ratio,which is PVDF:DMAc:PVP=18:76:6,was determined by single factor experiments.The results show that the PVDF microfiltration membrane prepared under optimal quality ratio has a uniform pore size of 0.1μm.The porosity,pure water flux,and water contact angle was 28.5%,223.4 L/m2·h,and 91.7°,respectively.The retention rate of model pollutant bovine serum albumin（BSA,10 mg/L）can reach 86%using this PVDF membrane as filter.However,after BSA solution filtered two cycles for 30 min,the water flux decreased to 121.3 L/m2·h,and the membrane total fouling ratio reached78.7%.（2）The TiO2@PVDF microfiltration membrane was successfully synthesised by added certain TiO2in the PVDF microfiltration membrane casting solution,and the preparation method was same as the PVDF microfiltration membrane which was mentioned above.The optimal mass ratio of TiO2to PVDF（1:10）was obtained.The results showed that TiO2@PVDF has a uniform pore size of 0.18μm and a porosity of 30.9%.The pure water flux can reach 425.8 L/m2·h,which is 1.9 times that of the previous PVDF membrane,and its water contact angle is 65.4°.The hydrophilic performance of TiO2@PVDF is significantly improved compared to PVDF membranes.TiO2@PVDF membrane BSA rejection rate was up to 90%.After BSA solution filtered for 30 min and circulating twice,the membrane flux decreased to 271.5 L/m2·h.Even through,the fouled membrane flux was still 21.5%higher than that of the new PVDF membrane.The BSA adsorption capacity of TiO2@PVDF is 0.11mg/cm2and that of PVDF membrane is up to 1.41 mg/cm2.The BSA adsorption capacity of TiO2@PVDF is only 7.8%of that of PVDF membrane.The TiO2@PVDF membrane fouled by BSA has simulated sunlight self-cleaning function,and the membrane flux recovery rate can reach 66.5%after simulated sunlight irradiation for 2 h.（3）The N-TiO2@PVDF microfiltration membrane was successfully synthesized when N-TiO2was added as an implanted catalyst by the same method mentioned above.The experimental results showed that when the mass ratio of N-TiO2to PVDF is 1:15,the N-TiO2@PVDF microfiltration membrane has excellent anti-pollution and simulated sunlight self-cleaning ability.The N-TiO2@PVDF membrane has a uniform pore size of 0.15μm,a porosity of 34.8%,and a water contact angle of 43.8°.The hydrophilic performance of N-TiO2@PVDF is greatly improved.The pure water flux of N-TiO2@PVDF membrane is1021.2 L/m2·h,which is 4.6 times that of PVDF membrane.The BSA rejection rate of N-TiO2@PVDF membrane reaches 94%.After BSA was filtered twice for 30 min,N-TiO2@PVDF membrane flux decreased to 748.2 L/m2·h.Although there was a total fouling ratio of 69.7%,the fouled flux of N-TiO2@PVDF was still more than 2.4 times that of the clean PVDF membrane.Meanwhile,the BSA adsorption capacity of N-TiO2@PVDF is 0.05mg/cm2,which is lower than 1/28 of that of PVDF and 1/2 of that of TiO2@PVDF.The N-TiO2@PVDF antifouling performance is further improved.In addition,the TiO2@PVDF recovery rate could reach 77.8%after simulated sunlight irradiation for 2 h without any other process.（4）The antifouling performance of PVDF membrane was evaluated by protein adsorption and membrane filtration performance experiments.The 0.15-N-TiO2@PVDF has outstanding anti-fouling performance.The results indicated that the rejection rate of0.15-N-TiO2@PVDF could reach 94%,the minimum protein adsorption capacity was decreased to 0.05 mg/cm2,the highest flux recovery rate was up to 30.3%,and the lowest recovery rate was 69.7%.In order to reveal the simulated sunlight self-cleaning mechanism,the UV-Vis diffuse reflectance measurement and light excitation energy level fitting calculation of the N-TiO2@PVDF membrane was carried out,and the hydroxyl radical（·OH）was also quantitatively detected.The results showed that N-TiO2@PVDF membrane has photocatalytic ability,and its excitation energy level drops to 2.97 e V due to the doping of N element into the TiO2lattice structure.The excitation light has a significant red shift.Visible light can be used to photodegrade organic pollutants.TiO2@PVDF membrane has simulated sunlight self-cleaning function.In addition,after N-TiO2@PVDF membrane reacted for 60min under simulated sunlight irradiation,the concentration of·OH was obtained and it reached to 0.96μmol/L.It can be deduced that·OH is the main oxidizing agent to degrade organic pollutants and the organic pollutants even can be mineralized into CO2and H2O.In summary,a novel antifouling N-TiO2@PVDF membrane with hydrophilic surface properties was obtained.It has the hydrophilic surface and anti-organic pollutant fouling properties.It also has the ability of simulated sunlight self-cleaning,which could be applied into the practical engineering.This study provides a new technical and a new material for Chinese water environmental management and water resource reuse. |
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