Study On Preparation And Properties Of PEC/PVA Multi-layer Thin-film Composite Semipermeable Membrane

With the application expension of membrane technology, the separation systembecomes more and more diversification and complexity, and thereby the demands ofseparation performance and chemical stability of the membrane become much higher.Compared with the asymmetric membrane, the thin-film composite membranespossess the advantages of higher permeability, super selectivity as well as bettercompact resistance, and has become the key research object in the filed of membranescience and technology. In consideration of the facts that polyelectrolytes（PECs）have the characterics of higher hydrophilicity, good chemical stability and tunablecharge, and the polyvinyl alcohol (PVA) has the properties of strong solventresistance and good film-forming, this work focused on the study and preparation ofPEC/PVA multi-layer thin-film composite membrane with good separationperformance and chemical stability.In the experiments, the PEC/PVA multi-layer thin-film composite membraneswere prepared through the method of dip-coating followed by insitu crosslinking withthe porous polysulfone membrane as support, the PVA as the first coating layer, thepolyelectrolyte sodium carboxymethylcellulose (CMCNa) as the second coating layerand glutaraldehyde (GA) as the cross-linking agent. ATR-FTIR, SEM, AFM, contactangle measurements and streaming potential measurements were adopted tocharacterize the properties of the resultant composite membrane such as surfacechemical composition, morphology hydrophilicity and charge. The effects ofpreparation conditions on membrane performance were systematically studied byvarying the parameters including the contents of PVA, CMCNa and cross-linkingagent, and the curing temperature, through which the optimal preparation conditionswere obtained. The membrane permeation properties including molecular weightcut-off (MWCO), water permeability, rejections to different inorganic salts andremovals to anionic dyes were evaluated through cross-flow permeation tests. Finally,the anti-fouling property and chemical stability of the obtained CMCNa/PVAcomposite membrane were were also investigated. The following conclusions could be ontained from the experiments:(1) CMCNa/PVA multi-layer thin-film composite semipermeable membranescould be obtained by successive deposition of PVA and CMCNa on the surface ofpolysulfone support through the reaction with GA. The surface of the obtainedCMCNa/PVA composite membrane was smooth, compact and negatively charged atpH within4-10. Compared with the composite membrane of single CMCNa layer, theCMCNa/PVA composite membrane has a more compact active layer and higherretentions to netural solutes. Compared with the composite membrane of single PVAlayer, the CMCNa/PVA composite membrane was negatively charged and possessesthe higher selectivtity to ions of different valents. Compared with the compositemembrane with the active layer of blend CMCNa and PVA, the multi-layerCMCNa/PVA composite membrane has a higher sepeartion resolution.(2) The performance of the CMCNa/PVA composite membrane was largelyaffected by the concentrations of PVA, CMCNa and cross-linking agent, as well asthe curing temperature. With the increasing PVA concentration, CMANaconcentration, GA concentration or curing temperature, the salt rejection of themembrane increased and the permeate flux declined. With the increasingconcentration of the catalyst, the rejections of the CMCNa/PVA to inorganic saltsincreased firstly and then declined. The optimal conditions for the preparation of theCMCNa/PVA composite membrane were as follows: PVA=0.1w/v%, CMCNa=0.1w/v%, GA=0.5w/v%, H2SO4=1.0w/v%, curing temperature=60℃, first curing time=6min, second curing time=10min. The molecular weight cut-off of the desiredcomposite membrane was around1980g/mol. Under the test condition of0.5MPaand25℃, the desired membrane exhibited a pure water flux of47.7l/m2h andrejections of97.8%and52.4%to Na2SO4and NaCl, respectively, when tested with500mg/l electrolyte aqueous solution.(3) The obtained membtane possessed good selectivity to different inorganicslats. At neutral pH, the salt rejections followed the order of MgCl2(25.4%)<NaCl(49.6%)<MgSO4(90.7%)<Na2SO4(97.2%), which is a typical characterics ofthe polymeric positively charged nanofiltration membrane. With the feedconcentration of electrolyte solution increased from0.5to2.5g/l, the rejection ofcomposite membrane to inorganic salt declined rapidly-to-slowly, while the flux decreased slightly. The removals of desired CMCNa/PVA composite membrane toanionic dyes Alizarin yellow R, Sunset yellow and Congo red were46.5%,96.2%and99.6%, respectively. The flux and rejections to Na2SO4and NaCl remained constantafter the chlorination treatment of2000ppm·h NaCl. The fouling experiments withBSA aqueous solution demonstrated that the desiered membrane possessed goodanti-flouing property. Finally, soaking tests also showed that the CMCNa/PVAcomposite membrane possessed excellent chemical stability.