Study On Preparation And Properties Of Polytetrafluoroethylene Composite Nanofiltration Membrane Based On Tannic Acid

At present,the most important applications of nanofiltration membranes are the removal of organics and the softening of drinking water.In the field of concentration and desalination,nanofiltration membranes have gradually replaced the traditional separation methods such as heating evaporation and centrifugal separation.In China,the market scale of nanofiltration membrane is only 1/10 of that of reverse osmosis membrane,and there are very few enterprises that can produce nanofiltration membranes in China.Therefore,there is a great room for expansion in the scale and application of nanofiltration membranes.In this paper,a hydrophilic modified polytetrafluoroethylene?PTFE?flat microporous membrane was used as the substrate,the natural material tannic acid?TA?was used as the water phase monomer,and the trimesoyl chloride?TMC?was the oil phase monomer.TA-TMC composite nanofiltration membrane was prepared on the surface of hydrophilic PTFE substrate by interfacial polymerization.The effects of the preparation conditions including TA concentration,TMC concentration,immersion time in TA,interfacial polymerization time,heat treatment temperature and heat treatment time on the separation performance of TA-TMC composite nanofiltration membrane were studied?cross-flow filtration of MgSO₄ solution and Congo red dye solution?,thus the optimum conditions for the preparation of TA-TMC composite nanofiltration membrane were obtained.Surface chemical structure,surface morphology,hydrophilicity and charge of the TA-TMC composite nanofiltration membrane were characterized by attenuated total reflectance-Fourier transform infrared?ATR-FTIR?,scanning electron microscopy?SEM?,contact angle measurement and Zeta potential analyzer.The molecular weight cut-off?MWCO?,the pure water fluxes of TA-TMC composite nanofiltration membranes and the separation performance of TA-TMC composite nanofiltration membranes for three dyes?Congo red,methyl blue and crystal violet?were tested by cross-flow filter.Finally,the following results are obtained:?1?During the preparation of TA-TMC composite nanofiltration membrane,with the increase of TA concentration,the rejection rate of MgSO₄ and Congo red dyes increase,while the permeate flux decrease,finally the rejection rate and flux will gradually tend to a stable value.With the increase of TMC concentration,the prolongation of immersion time in TA,the increase of heat treatment temperature or the prolongation of heat treatment time,the rejection rate of MgSO₄ and Congo red dyes increase firstly and then decrease,while the permeate flux of TA-TMC composite nanofiltration membrane decrease firstly and then increase.With the prolongation of interfacial polymerization time,the rejection rate of MgSO₄ and Congo red dyes increase firstly and then decrease slowly,while the permeate flux decrease firstly and then increase slowly,finally the rejection and flux will be gradually stable.Considering the influence of rejection rate and permeate flux together,the optimum conditions for preparing TA-TMC composite nanofiltration membrane are as follows:the concentration of TA,the concentration of TMC,immersion time in TA,interfacial polymerization time,heat treatment temperature and heat treatment time were 2.5wt%,0.7wt%,15min,5min,60?and 15min,respectively?the concentration of aqueous phase additives including SDS and NaHCO₃ were 0.05wt%,1.5wt%,respectively?.The composite nanofiltration membrane prepared under this condition has a rejection rate of MgSO₄ under 40%,but that of the Congo red dye is 97.98%and the flux is 2.17 L·m^-2·h^-1?test pressure 0.2 MPa,test temperature 25??.?2?According to the results of ATR-FTIR measurement,polyester have been successfully formed through the interfacial polymerization of TA and TMC on the surface of the hydrophilic PTFE substrate.A thin polyester functional layer with loose structure can be seen on the surface of PTFE substrate by FE-SEM.The water contact angle of the TA-TMC composite nanofiltration membrane is 48.34°,indicating that it has good hydrophilicity.The zeta potential of the TA-TMC composite nanofiltration membrane is-48.74 mV,indicating that it is a negatively charged film.?3?The molecular weight cut-off of TA-TMC composite nanofiltration membrane is 520Da.With the pressure rising from 0.2MPa to 0.6MPa,the pure water flux of the composite membrane increases from 6.15 L·m^-2·h^-1 to 18.73 L·m^-2·h^-1.The rejection rate of three dyes follow the orders:Crystal violet?90.34%?<Methyl blue?96.85%?<Congo red?97.98%?,which is the result of Donnan ion repulsion effect and pore size screening effect.