Preparation And Separation Performances Of High-performance Nanofiltration Membranes And Their Ultrafiltration Substrates

Ultrafiltration membranes and nanofiltration membranes have been widely used in water treatment recently.However,both kinds of membranes have their own drawbacks.For example,ultrafiltration membranes are widely used in industry application but their anti-fouling ability is normally unsatisfactory.Nanofiltration membranes have better separation performance but their stability needs further improvement.In this paper,to solve these two problems,novel double-layers composite ultrafiltration membranes with improved anti-fouling ability were designed and prepared.Then,novel triple-layers composite nanofiltration membranes with enhanced stability were also designed and prepared by using the ultrafiltration membranes as substrates.The chemical composition,morphological structure and separation performances of the membranes were investigated systematically.The detail contents and results are described as following:Firstly,modifiable polymer named tetra-methyl polyethersulfone(TMPES)was designed and synthesized.Then,brominated polyethersulfone(BPES)was gotten via bromination.After that,asymmetric BPES ultrafiltration membranes were prepared by traditional non-solvent induced phase separation method.Finally,hyperbranched polyethyleneimine(PEI)was grafted on BPES membranes via quaternarization of benzyl bromide and tertiary amine to fabricate the double-layers composite ultrafiltration membranes(PEI-PES).The influence of bromide degree of BPES polyer and concentration of PEI solution was investigated systematically.The prepared ultrafiltration membranes have stable chemical and physical structure along with wonderful separation performances.For example,when bromide degree of BPES polyer is 73%and concentration of PEI solution is 10%,the water flux of the membrane is as high as 71.7 L m-2 h-1 bar-1.The rejection of ferritin and 10 nm Au is 96.3%and 95.5%,respectively.The molecular weight cut off is 64 kDa.Moreover,the ultrafiltration membrane has long-term stability with wonderful anti-fouling ability and renewability.The flux recovery ratio is up to 0.9 after protein fouling.Meanwhile,because of abundant amino groups on the surface,these ultrafiltration membranes can be used as substrates for interfacial polymerization nanofiltration membrane in the next step.Secondly,on basis of former work,the triple-layers composite nanofiltration membranes(PA-PEI-PES)were fabricated on the ultrafiltration membranes by interfacial polymerization.The nanofiltration membranes have three layers.The lower one is PES substrate.The middle one is PEI layer.The upper one is PA layer.There are quaternary ammonium bonds between lower layer and middle layer while there are amido bonds between middle layer and upper layer,which contributes to the improvement of chemical and physical stability.The prepared nano filtration membranes also have wonderful separation performances.For example,when interfacial polymerization time is 2 minutes,the nanofiltration membrane has the water flux of 12.3 Lm-2h-1bar-1.The rejection of MgSO4,Na2SO4,MgCl2 and NaCl is 86.3%,80.1%,74.7%and 64.3%,respectively.Moreover,the nanofiltration membrane has better immersion stability,anti-pressure stability and long-term stability than traditional interfacial polymerization membranes,which means a good application prospect.Overall,to improve anti-fouling ability of ultrafiltration membranes and stability of nanofiltration membranes,a facile strategy of surface grafting was applied to fabricate double-layers composite ultrafiltration membranes with improved anti-fouling ability.Then,triple-layers composite nanofiltration membranes with enhanced stability were prepared on the ultrafiltration membranes.These membranes have wonderful separation performances and good application prospects.Furthermore,this paper will also provide ideas and references for the unified construction of ultrafiltration membrane and nanofiltration membrane in the future.