Study On Preparation And Water Purification Efficiency Of Gravity Driven Ultrafiltration Membrane Based On Block Copolymer

Gravity-driven ultrafiltration membrane（GDUM）filtration is one of the most promising separation methods due to its energy-saving,eco-friendly and auto-oper-ating features.However,the low separation efficiency is still the main reason that limits its wide application.In GDUM filtration process,various foulants can adsorb or accumulate on/in membrane surface and pore,cause membrane fouling,resulting in flux decline.In addition,since GDUM usually have more porous structure,even at lower pressures,the GDUM will deform,resulting in decreased porosity,in-creased filtration resistance and eventual water flux decline.Therefore,improving the anti-fouling properties and anti-deformation performance of GDUM is of great significance to enhancing its separation efficiency.In this study,in order to inhibit membrane fouling caused by organic non-migrating foulant,membrane pore defor-mation caused by trans-membrane pressure and“cover pore membrane fouling”caused by coverage of large-size foulant,three kinds of block copolymers were pre-pared as modifiers of PVDF membranes.Subsequently,three kinds of modifi ed membranes were prepared by simple non-solvent induced phase separation method.This study realized the integration of anti-fouling and anti-deformation of the PVDF ultrafiltration membrane,and enhanced the water flux and separation efficiency of PVDF ultrafiltration membrane under gravity-driven filtration.In order to reduce membrane fouling caused by non-migrating organic foulants in water,based on the self-assembly mechanism of amphiphilic block copolymer during phase separation,a new structure of amphiphilic multi-arms block copolymer poly（propylene glycol）-silane-poly（ethylene glycol）（PPG-Si-PEG）was designed and prepared.The fourier transform infrared spectroscopy（FTIR）and nuclear mag-netic resonance spectroscopy（NMR）confirmed that the copolymer PPG-Si-PEG contained 4 hydrophilic PEG arms and a hydrophobic PPG segment in each mole-cule.Subsequently,the copolymer PPG-Si-PEG was blended into the membrane casting solution with PVDF as the base material.The concentration of PPG-Si-PEG in the membrane casting solution was adjusted.The PVDF@PPG-Si-PEG mem-branes were fabricated via nonsolvent induced phase separation method.The effect of PPG-Si-PEG addition on the surface morphology,pore size,roughness and chem-ical composition of PVDF membranes was studied by means of characterization.The separation and antifouling properties of the PVDF@PPG-Si-PEG membranes were evaluated by static adsorption experiments and dynamic gravity-driven ultra-filtration experiments,employing bovine serum albumin（BSA）as the model foulant.When the addition amount of PPG-Si-PEG was 5 wt%,the water flux of the PVDF membrane was increased to 12.1 L m^-2 h^-1.The flux recovery ratio was up to 99.4%.Finally,the anti-fouling stability of the PVDF@PPG-Si-PEG membrane was inves-tigated through 3 cycles of filtration experiments.The results confirmed that the modifier PPG-Si-PEG could stably exist in the PVDF membrane and provided con-tinuous anti-fouling performance due to the immobilization of hydrophobic PPG.In the pure water filtration of PVDF@PPG-Si-PEG membrane,the water flux showed a decline trend.Considering the pure water filtration（without membrane fouling）,the water flux decline could only be attributed to the pore deformation（membrane compaction）.Therefore,in order to solve the problem of the pore defor-mation of the organic membrane in gravity-driven filtration,based on the principle of modifier"structure-activity relationship",a rigid block copolymer four-arm star polystyrene（FAS-PS）with a compact structure was prepared as anti-deformation modifier of PVDF membrane.Subsequently,the copolymer FAS-PS was blended into the membrane casting solution with PVDF and PPG-Si-PEG.The concentration of FAS-PS in the membrane casting solution was adjusted.The PVDF@FAS-PS membranes were fabricated by in-situ formed four-arms star polystyrene（FAS-PS）microspheres during an easy one-step phase separation process.The effect of FAS-PS addition on the surface morphology,pore size,roughness and surface chemical composition of PVDF membranes was studied by means of characterization.The filtration performance and anti-deformation ability of PVDF@FAS-PS membranes in gravity-driven filtration were studied by dynamic gravity-driven BSA filtration experiment,pure water filtration experiment,fouling model analysis and laser con-focal test（CLSM）.The relationship between pore deformation and membrane pore blockage was analyzed by measuring the TOC concentration of the backwash solu-tion.These results showed that the formation of FAS-PS microspheres enhanced the anti-deformation capability of the PVDF membrane.The stable water flux of PVDF@FAS-PS membrane increased to 19.7 L m^-2 h^-1,and the flux decline rate was only 55.3%.Furthermore,the anti-deformation character of PVDF@FAS-PS membrane was beneficial to reduce pore blockage.Finally,the anti-deformation mechanism of PVDF@FAS-PS membrane was studied by porosity measurement,depth-sensitive indentation（DSI）and dynamic mechanical performance analysis（DMA）.The results confirmed that the"molecular tightening effect"of FAS-PS microspheres could effectively resist the plastic deformation and elastic defor-mation of the membrane under applied pressure.In order to solve the problem of“cover pore membrane fouling”caused by the coverage of large-scale foulants on the membrane surface in gravity-driven filtra-tion,according to the character of FAS-PS forming microspheres in the membrane,the amphiphilic four-arm star copolymer four-arm star poly（styrene）-block poly（ethylene oxide）monomethacrylate（FAS-PS-b-PEGMA）was designed,prepared and used as the surface morphology modifier of the PVDF membrane.Subsequently,the copolymer was blended into PVDF membrane casting solution.The concentration of FAS-PS-b-PEGMA in the membrane casting solution was ad-justed.The PVDF@FAS-PS-b-PEGMA membranes were fabricated by in-situ formed FAS-PS-b-PEGMA micropillars on membrane surface via nonsolvent in-duced phase separation technique.The influence of the form ed micropillars on the surface morphology,pore size,roughness and chemical composition of PVDF mem-branes was studied by means of characterization.The separation and antifouling properties of the PVDF@AS-PS-b-PEGMA membranes under gravity were evalu-ated by dynamic gravity-driven ultrafiltration experiments,employing microcystis aeruginosa solution as the model foulant.When the addition amount of FAS-PS-b-PEGMA was 5 wt%,the stable water flux of PVDF membrane increased from 1.4L m^-2 h^-1 to 7.1 L m^-2 h^-1.The flux decline ratio was only 51.3%,which was much lower than the control membrane（80.1%）.Finally,the analyses of fouling layer further confirmed that the hydrophilicity,electronegativity and supportability of mi-cropillars could inhibit“cover pore membrane fouling”caused by the coverage of algae cells on the membrane surface.