Study On Anti-fouling Performance Of Composite Nanofiltration Membranes Modified By Zwitterionic Polymers

Nanofiltration membranes had the advantages of high efficiency and energy saving during the use process,but membranes fouling was unavoidable,which will cause certain influences and restrictions on the use of nanofiltration membranes.Therefore,it was of great significance to improve the anti-fouling performance of nanofiltration membranes through a series of methods and means.Firstly by using the atom transfer radical polymerization（ATRP）method,polydimethylsiloxane（PDMS）and sulfobetaine（SBMA）were used as the main reactive monomers to polymerize to produce zwitterionic polymers with secondary amine functional groups.Secondly,the MPD/TMC composite nanofiltration membranes were prepared by interfacial polymerization using the activated polytetrafluoroethylene（PTFE）flat membranes as the base membranes,m-phenylenediamine（MPD）as the water phase monomer and trimesoyl chloride（TMC）as the oil phase monomerand.Then using the synthesized zwitterionic polymer as the water phase monomer and TMC as the oil phase monomer,the MPD/TMC composite nanofiltration membranes were modified to resist pollution through interfacial polymerization.At last,the modified composite nanofiltration membranes were prepared.With the help of a cross-flow filtration experimental device made in the laboratory,the main influencing factors on the anti-fouling performance of modified composite nanofiltration membranes,such as:the concentration of zwitterionic polymer,the amount of4-dimethylaminopyridine（DMAP）catalyst in the water phase,the concentration of TMC,the time of interfacial polymerization,the temperature of heat treatment and the time of heat treatment were studied.The best modification conditions of the modified composite nanofiltration membranes were obtained.With the help of Fourier infrared spectroscopy（FTIR）,field emission scanning electron microscope（FE-SEM）and solid surface Zeta potentiometer,etc.,the surface chemical structure,surface morphology and surface charge performance of the modified composite nanofiltration membranes were characterized.With the help of a cross-flow filtration experimental device made in the laboratory,the separation performance and molecular weight cut-off of the modified composite nanofiltration membrane（MWCO）and the anti-fouling properties of different pollutants（bovine serum albumin（BSA）and humic acid（HA））were studied.The specific research results were as follows:（1）By using Fourier Infrared Spectroscopy（FTIR）,it was found that the synthesized zwitterionic polymer had a characteristic absorption peak at 1043 cm^-1,which was the characteristic peak of-SO₃,the characteristic absorption at 3388 cm^-1 and 1429 cm^-1,which was the characteristic peak of-NH-.Finally the zwitterionic polymer with secondary amine functional group was successfully synthesized.（2）Discovered in the research of the anti-fouling performance of the modified composite nanofiltration membranes on BSA,the fouling flux recovery rate（FRR）first increased and then stabilized,and the flux decay rate（DRt）first decreased and then stabilized with the increase of zwitterion concentration.This was the result of the gradual completion of the reaction between zwitterionic polymer and TMC.As the amount of DMAP increased,FRR first increased and then stabilized,DRt first decreased and then stabilized.This was the result of the gradual catalysis of the reaction between catalyzed zwitterionic polymer and TMC.As the TMC concentration increased,FRR first increased and then slowly increased,DRt first decreased and then slowly decreased.This was the result of the hydrolysis of acid chloride groups in TMC to carboxyl groups.As the interfacial polymerization time prolonged,FRR first increased and then stabilized,DRt first decreased and then stabilized.This was the result of the gradual completion of the reaction between zwitterionic polymer and TMC.As the heat treatment temperature increased and heat treatment time prolonged,FRR first increased and then decreased,DRt first decreased and then increased.This was the result of film surface defects caused by too high temperature or too long heat treatment time.The best preparation process conditions for preparing the modified composite nanofiltration membranes were as follows:The amount of the catalyst DMAP was 15%of the mass of the zwitterionic polymer,the concentration of the zwitterionic polymer was 0.4wt%,the concentration of trimesoyl chloride（TMC）was 0.2 wt%,the interfacial polymerization time was 2 min,the heat treatment temperature was 70 ℃,and the heat treatment time was 10min.（3）The surface chemical structure of the modified composite nanofiltration membranes was studied with the help of FTIR spectroscopy,and it was found that there were amide groups and-SO₃ groups on the surface of the modified composite nanofiltration membranes.The results showed that the modified composite nanofiltration membrane was successfully prepared.The surface morphology of the modified composite nanofiltration membranes were studied with the help of SEM,and the results showed that the surface of the modified composite nanofiltration membranee was dense and the introduction of zwitterions would not affect the original nanofiltration structure.The surface charge performance of the modified composite nanofiltration membranes was studied with the help of a Zeta potential analyzer,and the results showed that the surface of the modified composite nanofiltration membranes was negatively charged at p H=7.（4）The molecular weight cut-off of the modified composite nanofiltration membranes were607 Da.The order of the rejection rate of the modified composite nanofiltration membranes for four different salt solutions was:Mg SO₄>Na₂SO₄>Na Cl>Mg Cl₂.This was the common result of Donnan effect and aperture sieve effect.（5）The modified composite nanofiltration membranes compared with MPD/TMC composite nanofiltration membranes had good anti-fouling performance:When the pollutant was BSA solution,the flux recovery rate of the modified composite nanofiltration membranes and the MPD/TMC composite nanofiltration membranes were respectively 96.71%and 83.22%,and the flux attenuation rates were respectively 40%and 53.56%.When the pollutant was HA solution,the flux recovery rates of the modified composite nanofiltration membranes and MPD/TMC composite nanofiltration membranes were respectively 97.52%and 75.96%,and the flux attenuation rates were respectively 50%and 58.67%.（6）The modified composite nanofiltration membranes were subjected to a 100 h interception test on the Mg SO₄ solution,and it was concluded that the modified composite nanofiltration membranes had good operational stability.