Preparation And Characterization Of HEC/PP Solvent-resistant Composite Nanofiltration Membrane

Nanofiltration technology has become the most promising separation process of chemical,pharmaceutical and food industries for its unique characterics.Solvent-resistant nanofiltration(SRNF)as a branch of nanofiltration technology has been applied to separate industrial processing fluids of organic solvent.The separation performance and efficiency of SRNF process are mainly dependent on the membrane used,and it is of great importance to design and develop novel solvent-resistant nanofiltration membrane with high chemical stability,permeability and solute selectivity.In this study,a novel HEC/PP solvent-resistant composite nanofiltration membrane was prepared by the method of solution coating followed by crosslinking with polypropylene(PP)porous membranes as support,hydroxyethyl cellulose(HEC)as the material of the selective layer,and glutaraldehyde(GA)as cross-linking agent.Membrane properties including surface chemical composition,morphology,charge,hydrophilicity were characterized by ATR-FTIR,SEM,AFM,Zeta potential measurement and surface contact angle measurement.Membrane solute rejection performance was determined through cross-flow permeation tests with the model solutes of inorganic salts,dyes and polyethylene glycol(PEG).Membrane solvent resistance was evaluated through both static immersion and dynamic separation tests.The conclusions were as follows:(1)Composite membrane with the HEC selective layer and PP porous support layer could be manufactured through the method of solution coating followed by crosslinking.The active skin layer of the formed thin-film was smooth,dense and stable,and usually had a thickness of about 0.2 mm.The results of AFM and water contact angle measurements showed that the HEC/PP composite film had a smaller roughness and a higher hydrophilicity compared with the PP support.Furthermore,the surface of the composite membrane was negatively charged under the pH within the scope of 3-9.(2)The permeation property of HEC/PP composite membrane was mainly determined by the preparation parameters of the HEC concentration and coating time,the GA concentration and coating time,and the heat treatment temperature.With the prolongation of HEC coating time,the Na2SO4 rejection of the resultant membrane increased first and then decreased,while the reverse was true for the water flux.On the other hand,with the increase of HEC concentration,GA concentration,GA coating time and/or heat treatment temperature,the Na2SO4 rejection increased and the water flux decreased,showing an obvious trade-off phenomenon.The optimum preparation parameters were as follows: HEC concentration = 0.15 w/v%,HEC coating time = 40 min,GA concentration = 1.0 w/v%,GA coating time = 4 min,heat treatment temperature = 60 ? and reaction time = 5 min.The desired HEC/PP composite membrane showed a pure water flux of 39.1 l/m~2·h and rejection of 91.3% to 500 ppm Na2SO4 aqueous solution under 25 ± 1 ?,5 bar.(3)The molecular weight and pore size of the composite membrane could be modulated through the control of the concentration of cross-linking agent during the preparation process.The desired membrane had a molecular weight cut off of around 1150 Da,corresponding to a mean pore diameter of about 2.21 nm.The salts rejections of the HEC/PP composite membrane to different inorganic salts followed the order of RNa2SO4(91.9%)> RMgSO4(27.5%)> RNaCl(23.3%)> RMgCl2(3.9%)> RCaCl2(2.4%).The HEC/PP composite membrane also exhibited good removal efficiency to anionic dyes of congo red and methyl blue based on the effects of size exclusion and Donnan electrostatic repulsion.The results of soaking tests revealed that the HEC/PP composite membrane possessed high resistances to both acid and alkali.(4)Solvent-resistance of HEC/PP composite nanofiltration membrane was investigated through both static soaking and dynamic filtration tests.After soaking with the solvents of tetrahydrofuran,n-hexane,ethyl acetate,ethanol and carbon tetrachloride for 360 h,only very slightly change of separation performance was observed with the HEC/PP composite membrane,indicating the good solvent-resistance and chemical stability of the studied membrane.Permeation test with pure ethanol indicated that the HEC/PP composite membrane fabricated in this study possessed good resistance to compaction and performance stability,having a stable permeability up to 1.4 l/m~2·h·bar to ethanol.Furthermore,the HEC/PP composite membrane also possessed good performance in separation the mixture of ethanol and methyl blue,showing a flux up to 3.4 l/m~2·h and rejection of 95.4% to methyl blue under 5.0 bar.