Design And Preparation Of Thin-Film Composite Nanofiltration Membranes With High Performance

Nanofiltration(NF)membranes are a type of pressure-driven separation membranes with separation properties ranging between ultrafiltration(UF)membranes and reverse osmosis(RO)membranes.These membranes can efficiently remove multivalent ions and organic molecules with molecular weight greater than 200 Da.Hence,NF membranes are widely applied to seawater desalination,wastewater treatment,solvent recovery,drug concentration and purification,etc.At present,NF membranes are mainly based on a thin-film composite(TFC)design which is composed of a ultrathin selective layer and a porous polymer UF support membrane.The selective layer plays a crucial role in the separation performance of TFC NF membrane,and the support membrane provides necessary support and mechanical strength for the selective layer.The selective layer is usually prepared on the surface of support membrane via interfacial polymerization between amine monomers and acyl chloride monomers.Therefore,the pore structure and surface physicochemical properties of support membrane will directly affect the structure of polyamide(PA)layer obtained by interfacial polymerization,and then affect the separation performance of selective layer.Design the structure and chemical properties of polymer UF support membrane,control of interfacial polymerization process,optimization of the structure of the selective layer are the key to obtain high-performance TFC NF membrane.In this thesis,we focus on the design and preparation of high-performance TFC NF membrane:TFC NF membranes with high permeance,organic solvent resistance and acid resistance are prepared through increasing the effective filtration area of selective layer,design of organic solvent resistant UF support membrane and acid resistant selective layer,respectively.Three parts are included in this thesis:(1)Design and preparation of high-permeance TFC NF membraneIn TFC NF membrane,the effective filtration area of selective layer is limited by the strong binding force between selective layer and polymer UF support membrane with low surface porosity,making the permeance of TFC NF membrane limited.In this part,we propose a salt crystal sacrificing template method to optimize the binding state between selective layer and support membrane,which increases the effective filtration area of selective layer and improves the permeance of TFC NF membrane.Inorganic salt crystals with two-dimensional(2D)branching fractal structure(BFS)is introduced between the PA selective layer and the polyethersulfone UF support membrane through adding soluble inorganic salts into the aqueous amine monomer solution in interfacial polymerization.After that,a large number of cavity structures are constructed by removing the crystals between the selective layer and the support membrane,which greatly increases the effective filtration area of the selective layer and improves the permeance of the PA-TFC NF membrane.When sacrificing template is the NaCl 2D BFS crystal,the permeance of the PA-TFC NF membrane is increased from 8 L m-2 h-1 bar-1 to 26 L m-2 h-1 bar-1,and the rejection rate of the PA-TFC NF membrane to inorganic salt remains unchanged(the rejection to Na2SO4 remains above 95%).In addition,the separation performance of the PA-TFC NF membrane keeps stable under the high pressure and high flow rate.(2)Design and preparation of organic solvent resistant TFC NF membraneThe stability of TFC NF membranes based on traditional polymer UF membranes in organic solvents is limited,making TFC NF membranes difficult to be applied in the separation in organic solvent system.The key of the preparation of organic solvent resistant TFC NF membrane is to fabricate novel support membranes which possess excellent organic solvent resistance and can support selective layer effectively.In this part,a novel UF support membrane with excellent organic solvent resistance is constructed by deposition of single-walled carbon nanotube coated by polydopamine(PD/SWCNT)onto the surface of polytetrafluoroethylene(PTFE)microfiltration membrane,to enhance the organic solvent resistance of TFC NF membrane.And the PA-TFC NF membrane based on PD/SWCNT/PTFE support membrane is stable in organic solvents,such as methanol,acetone and acetonitrile,etc.On the other hand,the PA-TFC NF membrane based on the PD/SWCNT/PTFE support membrane remains high rejection(>95%)to methyl orange in methanol during the long-term filtration.These results suggest the TFC NF membrane based on the PD/SWCNT/PTFE support membrane possesses excellent resistance to organic solvents.(3)Design and preparation of acid resistant TFC NF membraneThe acid resistance of PA-TFC NF membrane is poor,which is mainly reflected in the low rejection to negatively charged particles in acidic solution.This is because the surface of PA selective layer is weakly negatively charged or even weakly positively charged in acidic solution.In this part,phenolic monomers are used to prepare poly ary late TFC NF membranes with good acid resistance by interfacial polymerization.The polyary late TFC NF membrane prepared by interfacial polymerization between 5,5’,6,6’-tetrahydroxy-3,3,3’,3’-tetramethylspirobisindane(TTSBI)and isophthaloyl dichloride(IPC),shows negatively charged in the solution of pH 2～9.And the polyarylate TFC NF membrane exhibits a good rejection rate(>99%)to congo red(negatively charged dye)in acidic(pH 2)solution.On the other hand,the permeance of polyarylate TFC NF membrane for acidic(pH 2)solution is up to 180 L m-2h-1 bar-1.