Construction Of High Permeable And Anti-fouling Coating On The Surface Of Polyamide Nanofiltration Membrane

The main obstarcle restricting the application and development of polyamide composite nanofiltration membrane is membrane fouling,which leads to the decrease of membrane permeate flux and effluent quality.Therefore,it is necessary to improve the anti-fouling property of polyamide composite membrane.Poly（vinyl alcohol）（PVA）,a water-soluble biodegradable polymer with inherent hydrophilicity,good film-forming property and excellent thermal,mechanical and chemical stability,has been adopted to form the one of the anti-fouling coating on the surface of polyamide composite membrane.However,the permeability of the PVA anti-fouling coating to water is relatively poor for its high crystallity.Therefore,it is of great significance to improve the water permeability of the PVA anti-flouing coating.In this study,hydroxypropyl methylcellulose（HPMC）was used as functional modification material,and glutaraldehyde was used as crosslinking agent to form a high-permeable anti-fouling coating layer of PVA/HPMC on the surface of a polyamide nanofiltration membrane via the process of surface coating followed by crosslinking.X-ray diffraction（XRD）,Fourier transform infrared spectrum（ATR-FTIR）,field emission scanning electron microscopy（SEM）,atomic force microscope（AFM）,static contact angle measurement and zeta potential measurement were used to investigate the effect of PVA/HPMC coating on the the surface chemical structure,hydrophilicity,charge,microstructure and roughness of the composite membrane.Cross-flow permeation tests were performed to evaluate the effect of PVA/HPMC coating layer on the the separation performance and anti-fouling property.Static soaking tests were also conducted to evaluate the stability of the coating layer.The results obtained are as follows:（1）The results of the analyses of membrane surface physic-chemical properties showed that a PVA/HPMC coating could be formed on the surface of the polyamide composite film through surface coating followed with cross-linking.The new characteristic absorption peak of ether bond（C-O-C）was found at 1026 cm^-1 in the FTIR spectrum,indicating the crosslinking reaction of polyvinyl alcohol and hydroxypropyl methylcellulose with glutaraldehyde.PVA/HPMC coating could effectively reduce the surface roughness and negative charge of polyamide composite membrane.XRD characterization showed that the crystallinity of PVA/HPMC coating decreased significantly compared with that of PVA coating.（2）Permation tests showed that the water permeability resistance of PVA coating could be effectively reduced by blending HPMC.The water permeability resistances of pure PVA and PVA/HPMC coating layers were 1.402×10¹33 m^-1 and 0.810×10¹³m^-1,respectively,showing a reducing ratio of 42.1%.After cross-linking with GA,the water permeability resistances of the PVA and PVA/HPMC coating layer were1.744×10¹33 m^-11 and 1.107×10¹³ m^-1,respectively,showing a reducing ratio of 36.5%.（3）Anti-fouling experiments revealed that PVA/HPMC coating could effectively improve the anti-fouling performance of polyamide nanofiltration membrane to positively charged pollutants,but would make the membrane more prone to be foulded by the negatively charged foulants.Blending HPMC did not affect the anti-fouling performance of PVA coating.The flux decay ratios of the virgin PA membrane,PA-PVA-GA membrane and PA-PVA/HPMC-GA membrane to aqueous dodecyltrimethylammonium bromide（DTAB）solution were 64.5%,46.7%and44.4%,respectively,to aqueous cationic dye Victoria Blue B solution were 59.3%,32.9%and 32.5%,respectively,to aqueous cationic dye neutral red solution were75.7%,49.1%and 49.5%,respectively,and to aqueous bovine serum albumin（BSA）aqueous solution were 25.9%,35.8%and 37.1%,respectively.（4）Stability experiments showed that the PVA/HPMC-GA anti-fouling coating layer possessed good thermal,acid and alkaline stability.After soaking in hot water of90°C for 4 h,or aqueous hydrochloric acid solution of pH=2.0 for 500 h,or aqueous sodium hydroxide solution of pH=12 for 500 h,the water permeability resistance and anti-fouling performance of the coating layer remained unchanged.