Preparation And Antifouling Modification Of PTFE Composite Nanofiltration Membranes

The function of nanofiltration（NF）membrane ranges from reverse osmosis（RO）membrane and ultrafiltration（UF）membrane.The NF membrane has a high removal rate for divalent and multivalent ions and organic substances with a relative molecular mass greater than 200,and is widely used in water treatment.But they can be easily fouled by pollutants,which will weaken the performance and life.Thus,it is meaningful for improving the anti-fouling performance of NF membranes.A polytetrafluoroethylene（PTFE）flat film was used as the base membrane of this experiment.M-phenylenediamine（MPD）was used as the water phase monomer while trimesic acid chloride（TMC）was used as the oil phase monomer,then the MPD/TMC NF membranes were synthesized by interfacial polymerization.The surface of the prepared MPD/TMC nanofiltration membrane was modified by using two substances,silica and zwitterionic polymer,respectively,and a composite nanofiltration membrane with excellent anti-fouling performance was obtained.（1）Silane coupling agent（KH560）was used as grafting monomer and ethyl orthosilicate（TEOS）was used as the modifying monomer.The silica modified composite nanofiltration membrane was prepared by hydrolysis reaction on the MPD/TMC membrane.The surface chemical structure,morphology,hydrophily and the charge of the membrane were studied by Fourier transform infrared spectroscopy（FTIR）,field emission scanning electron microscopy（FE-SEM）,water contact angle tester and Zeta potential analyzer and other test methods.The separation performance and molecular weight cut-off（MWCO）were tested by a self-made cross-flow filtration device.The anti-fouling properties of the modified membrane was tested by bovine serum albumin（BSA）and humic acid（HA）.The experimental conclusions are as follows:By testing the anti-BSA fouling performance of the silica-modified composite nanofiltration membrane under different modification conditions,the optimal membrane preparation conditions for the modified membrane were obtained as follows:the concentration of grafting monomer KH560 was 3 wt%,the grafting The time was 0.5 h,the concentration of modified monomer TEOS was TEOS:n-butanol=1:3（volume ratio）,the modification time was 1 h,the heat treatment temperature was 40℃and the heat treatment time was 15 min.ATR-FTIR spectra ascribed that the modified film had two characteristic peaks of Si-O-Si groups at 1090 cm^-1 and 786 cm^-1 and the characteristic of Si-C group appeared at 875 cm^-1.The peaks indicate that hydrophilic Si O₂ was generated on the surface of the original film.SEM test showed that the modified film generated a Si O₂ functional layer on the surface of the original film.The static water contact angle test results show that the water contact angle of the modified membrane was 15.16°,which was significantly smaller than 42.32°belonged to the original membrane.The surface hydrophilic performance of the modified membrane was greatly improved.The separation performance test shows that the molecular weight cut-off of the modified membrane is 468 Da and the Mg SO₄ rejection rate was 92.56%,which showed a good nanofiltration performance.The anti-fouling performance test shows that the modified membrane’s ability was greatly improved contrast to the original membrane.When using BSA,the flux recovery rates of the modified membrane and the original membrane were 95.76%and 83.06%respectively.The flux decay rates were40.24%and 54.10%respectively;when using HA,the flux recovery rates of the modified membrane and the original membrane were 95.45%and 76.72%,and the flux decay rates were 40.76%and 58.73%,respectively.（2）In order to further improve the performance of the nanofiltration membrane,polyacetimide（PEI）and sulfobetaine（SBMA）were used as the main reaction monomers,and the zwitterionic polymers were synthesized by polymerization reaction.The zwitterionic polymer modified composite nanofiltration membrane was prepared by interfacial polymerization with the water phase（zwitterionic polymer）and oil phase（TMC）on the MPD/TMC membrane.The structure and properties of the zwitterionic polymer-modified composite nanofiltration membrane were characterized using the above analysis methods,and the conclusions are as follows:By exploring the effect to the anti-BSA fouling performance of the zwitterionic polymer-modified composite nanofiltration membrane by changing different modification conditions,the optimal membrane fabrication conditions were obtained:the concentration of the catalyst,zwitterionic polymer and oil phase TMC were 10 wt%,0.6wt%and 0.3 wt%,respectively.Interfacial polymerization reaction time was 10 min.Heat treatment temperature was 70°C and heat treatment time was 15 min.The ATR-FTIR spectrum analysis shows that a new-SO₃^-groups formed on modified membrane surface,indicating that the zwitterionic polymer was successfully grafted to the surface of the original film through interfacial polymerization;SEM test showed that the modified film was modified.The surface morphology of the membrane is smoother than that of the original membrane.The Zeta potential test results show that the surface of the modified membrane is negatively charged when the p H is neutral.The static water contact angle test shows that the modified membrane is better than the original membrane.The membrane contact angle was reduced from 42.32°to 10.51°,and the surface hydrophilicity was greatly improved.The separation performance of the zwitterionic polymer-modified composite nanofiltration membrane was tested.The molecular weight cut-off of the membrane was579 Da,and the rejection ratio order for Mg SO₄ solutions was 96.48%.Compared with the silica-modified composite nanofiltration membrane,the zwitterionic polymer-modified composite nanofiltration membrane has better nanofiltration performance.The anti-fouling ability of the modified membrane was greatly improved compared with the original membrane.After BSA solution pollution,the flux recovery rate increased from83.06%to 97.51%,and after HA solution pollution,the flux recovery rate increased from 76.72%to 97.61%.The modified membrane can still maintain good anti-fouling performance in the process of multiple cycles of BSA pollutants.The properties of the modified composite nanofiltration membranes prepared by the above two modification methods were compared.The conclusions are as follow:Compared with the MPD/TMC nanofiltration membrane,the salt rejection performance and anti-fouling performance of the composite nanofiltration membrane modified by the two methods were greatly improved.The nanofiltration membrane with excellent anti-fouling performance was successfully obtained in the experiment.The interception performance and anti-fouling performance of the zwitterionic polymer-modified composite nanofiltration membrane are better than those of the silica-modified composite nanofiltration membrane,and the performance in the process of sewage treatment is even better.