Preparation And Performance Of Nanofiltration Membranes Incorporating Graphene Derivatives

The conventional polymer nanofiltration(NF)membranes still face many challenges including the“trade-off" between permeability and selectivity,and the properties of anti-fouling and chlorine resistance.The rapid development of nanomaterials is significantly promoting the R&D of the polymer membranes.Especially the two-dimensional graphene nanomaterials are attracting more attentions due to its unique properties in recent years.However,graphene is difficult to disperse in water and polar organic solvents,which restricts its applications in the membrane materials.In order to improve the water fluxes,anti-fouling property and chlorine resistance of conventional polymer NF membranes,this first selected or prepared graphene derivatives with the excellent dispersity in water and polar solvents,and was used to improve the structure and performance of mixed-martix NF membranes and nanocomposite NF membranes.The main research contents and results of this work are listed as follows:(1)Using polyether sulfone(PES),sulfonated polyether sulfone(SPES)as the main membrane materials,N-methylpyrrolidone(NMP)as the solvent,and sulfonated graphene(SG)as the two-dimensional nano-modifier,the mixed-matrix NF membranes were prepared by L-S phase inversion method.The effects of SG concentrations(0?0.3wt%)on the viscosities of casting solutions,membrane microstructures,hydrophobicities,chemical compositions,water fluxes,rejection rates,and aiti-fouling abilities were investigated.the results show that:?The viscosity of the casting solution gradually decreased,and the number of hydrophilic sites increased,with the increasing SG concentration,which corresponding accelerated the exchange rate between solvent and non-solvent,and it resulted in the instantaneous phase separation and generated the more and longer finger-like pores;?Compared to the blank mixed NF membrane,when the SG concentration was 0.15 wt%,SPG-15 mixed-matrix NF membrane had the rejection rate of evans blue above 98%,while its water flux significantly increased from 130.5 L·m-2·h-1 to 202.6 L,m-2·h-1,improved by 55.2%;?The introduction of SG could enhance the hydrophilicity and negative charge properties of the mixed-matrix NF membranes,thus improving its anti-fouling property.The static anti-fouling test showed that the adsorption of BSA for SPG-15 mixed-matrix NF membrane was only 1/2 of the blank mixed NF membrane.(2)Polysulfone(PSf)was used as the main membrane material of supporting membrane,firstly the effects of three kinds of solvents including NMP,N,N-dimethylacetamide(DMAc)and N,N-dimethylformamide(DMF)on the structures and properties of PSf supporting membrane were investigated.Then,graphene oxide(GO)was used as nano-modifier dispersed in aqueou solution and GO composite NF membranes on different supporting membranes were prepared by interfacial polymerization method.The results showed that:?When NMP was used as the solvent,the pore size of the supporting membrane surface was small;and the porous sublayer demonstrated the finger-like structure.When DMF or DMAc was used as the solvent,the pore size of the supporting membrane surface was large and the porous sublayer demonstrated sponge-like structure.The sequences of the average pore sizes and the water fluxes were:SM-DMF>SM-DMAc>SM-NMP,and the order of the BSA rejection rates was reversed;?The order of water fluxes of three kinds of the blank composite NF membranes were:NF-NMP-GO-0>NF-DMF-GO-0>NF-DMAc-GO-O.Because NF-NMP-GO-0 and NF-DMAc-GO had the higher rejection rate of Na2SO4(>98%),while NF-DMAc-GO-0 had the lower rejection rate,therefore,NMP was selected as the most suitable solvent for preparing the supporting membrane;?SM-NMP was used as supporting membrane and the GO concentration in the aqueous phase was 40 ppm,the prepared NF-NMP-GO-40 composite membrane showed the best permselectivity.Its pure water flux reached to 46.9 L·m-2·h-1 and improved by 35.3%compared with the blank composite NF membrane,while maintaining the high Na2SO4 rejection(98.6%).At the same time,the GO-embedding active layer could effectively enhance the hydrophilicity and negative charge properties and reduce the surface roughness,as a result,thus the anti-fouling property of NF-NMP-GO-40 was improve,and BSA was used as a source of pollution in the dynamic anti-fouling property test,its recovery rate reached to 88.5%.In addition,the introduction of GO also effectively its chlorine resistance,After being immersed in sodium hypochlorite(NaCIO)for 72 h,the rejection rate decreased to 69.3%.(3)A novel nanocomposite membrane was prepared by interfacial polymerization,while using the carboxylated hyperbranched polyester graphene oxide(GO-HBPE-COOH)as a novel nanofiller and using SM-NMP as the supporting membrane.The results indicated:?The hyperbranched polyester(HBPE)was successfully grafted onto the GO surface and the chemical modification didn't affect the main structure and morphology of GO;?When the GO-HBPE-COOH concentration in the aqueous phase was 60 ppm,the prepared NF-GHC-60 composite NF membrane had the best permselectivity,When the Na2SO4 rejection rate was maintained at about 98%,and the water flux reached to 48.0 L·m-2·h-1,improved by 43.7%compared with the blank composite NF membrane;?The GO-HBPE-COOH embedded in the active layer could effectively enhance the anti-fouling property and chlorine resistance of the nanocomposite NF membrane,and BSA was used as a source of pollution in the dynamic anti-fouling property test,recovery rate of NF-GHC-60 reached to 97.5%.After being immersed in NaClO for 72 h,the rejection rate decreased to 72.1%.