Study On Preparation And Performance Of Surface Ionic Nanofiltration Membranes

Nanofiltration membranes with pore size around 1 nm and operation pressure among 0.3-1.5 MPa are emerging as promising resolution to water shortage and water pollution problems.Owing to their low cost,energy efficiency and environment-friendly characteristics,they have been applied in a wide spectrum of fields,such as freshwater production,wastewater treatment,products separation in foodstuff and leechdom,and so on.There is an intense requirement for advanced membranes that have excellent perm-selectivity,strong resistance to various foulants and/or stimuli responsibility,to deal with a growing array of practical applications.In this work,a series of surface ionic nanofiltration membranes were prepared via interfacial polymerization,solution coating and surface-initiated atom transfer radical polymerization methods.Effects of ion content on membrane surface on chemical structures,surface properties,stimuli responsibility,antifouling property and separation performance of nanofiltration membranes were systematically investigated.(1)pH-responsive nanofiltration membranes containing carboxybetaine.Poly(carboxybetaine methacrylamide-co-N-(Hydroxymethyl)acrylamide)(PCHs)was synthesized from CBMA and HAM by aqueous radical copolymerization.The pH-responsive nanofiltration membranes(PCHMs)were prepared with PCHs through surface coating and glutaraldehyde cross-linking method.By varying the monomer ratio in copolymerization,CBMA content on membrane surface can be conveniently adjusted,which can tune the separation performance and pH responsibility of PCHMs.At pH 3.0,the retention of PCHMs to MgCl2 is much higher than that to NaCl.And by adjusting the feed pH value to pH 10.0,the retention to Na2SO4 is much higher than that to NaCl.While at neutral environment,PCHMs allow the transport of the inorganic salts but block the transport of PEG800.It indicates that PCHMs have great potential to deal with the divalent/monovalent ions separation and organics/inorganic salts separation using the same membrane by simply altering the pH value of the feed.(2)Zwitterionic polyamide nanofiltration membranes containing sulfobetaine.Zwitterionic polyamide thin film composite nanofiltration membranes(ZTFCMs)were prepared with N-aminoethyl piperazine propane sulfonate(AEPPS)and trimesoyl chloride(TMC)by interfacial polymerization.By varying the AEPPS aqueous concentration in interfacial polymerization,AEPPS content,microstructures and hydrophilicity of membrane surface can be conveniently adjusted,which can tune the separation performance and antifouling property of ZTFCMs.The water flux,retention to ERY and NaCl of the ZTFCM(AEPPS aqueous concentration is 3.0 wt%)are 50.4 L m-2 h-1,96.5%and 14.4%,respectively.The ERY concentration of ERY/NaCl mixture increases exponentially from 100.0 to 350.7 mg L-1 with a treatment capacity of 61.6 L m-2 h-1 during 7.25 h continuous concentration operation.In addition,ZTFCMs show exceptional antibacterial property due to the high hydrophilicity and smooth membrane surface.(3)Sulfobetaine-grafted zwitterionic polyamide nanofiltration membranes.2-bromoisobutyryl bromide(BIBB)-immobilized polyamide nanofiltration membranes(PA-Brs)were prepared with piperazine(PIP),trimesoyl chloride(TMC)and BIBB by interfacial polymerization.Subsequently,sulfobetaine-grafted zwitterionic polyamide nanofiltration membranes(PA-Brs-PSBMAt)were prepared from PA-Brs through surface-initiated atom transfer radical polymerization(SI-ATRP)of sulfobetaine methacrylate(SBMA).By varying the BIBB concentration in interfacial polymerization or SI-ATRP time,the surface density and chain length of PSBMA,thickness and hydrophilicity of the membranes can be adjusted,which can tune the separation performance and antifouling property of PA-Brs-PSBMAt.PA-Brs-PSBMAt with low surface density and moderate chain length has higher flux.When BIBB concentration is 1.00 wt%and SI-ATRP time is 0.2 h,the flux of as-prepared PA-Brs-PSBMAt is 63.9 L m-2 h-1.PA-Brs-PSBMAt with high surface density or long chain length exhibits an exceptional antifouling property.When BIBB concentration is higher than 1.00 wt%or SI-ATRP time is longer than 1.0 h,the amount of protein adsorbed onto the as-prepared PA-Brs-PSBMAt is less than 20 μg cm-2.Furthermore,when tested in humic acid filtration experiments,the flux decline ratio and flux recovery ratio of the membranes are less than 5%and more than 98%,respectively.(4)Positively charged polyamide nanofiltration membranes.Positively charged polyamide nanofiltration membranes(PA-PDMCt)were prepared from PA-Brs through SI-ATRP of methacryloxyethyltrimethyl ammonium chloride(DMC).By varying SI-ATRP time,the degree of polymerization of PDMC,thickness,hydrophilicity and surface charge of the membranes can be adjusted,which can tune the separation performance of PA-PDMCt.When BIBB concentration is 1.00 wt%and SI-ATRP time is 3.0 h,the flux and retention to MgCl2,CaCl2,CuCl2 and ZnCl2 of as-prepared PA-PDMCt is 82.5 L m-2 h-1,92.8%,90.8%,93.5%and 96.8%,respectively.It indicates that PA-PDMCt have great potential to deal with softening and heavy metal removal.In conclusion,a series of pH-responsive nanofiltration membranes,zwitterionic polyamide nanofiltration membranes and positively charged polyamide nanofiltration membranes containing ions on their surface were successfully prepared.The relationship among ion content on membrane surface,chemical structures and surface properties,and stimuli responsibility,antifouling property and separation performance of nanofiltration membranes was established.It is expected that this work would be helpful to explore advanced nanofiltration membranes with excellent perm-selectivity,nonfouling property and/or stimuli responsibility.