Study On The Hydrophilic Modification Of Ultrafiltration Membrane By Grafting PVA-amino Acid Copolymer With Polydopamine

Membrane fouling is the main obstacle to the further development of membrane technology in the wastewater treatment.Improving the membrane hydrophilicity is considered as an effective way to reduce the membrane fouling.With the characteristics of dopamine such as self-assemble and chemical activity,this paper presents the preparation the polyvinyl alcohol(PVA)-amino acid copolymer by the esterification between PVA and amino acid,and the grafting process by the covalent bonds between the-NH2 group and coated polydopamine layer.In general,our work focus on the preparation of PVA-amino acid copolymer and the characterization of the modified membrane.Firstly,the PVA-amino acid copolymer was prepared by the esterification of PVA and amino acid,and the synthesized product was detected by fourier transform infrared spectroscopy(FTIR)and nuclear magnetic resonance spectroscopy(NMR).The results indicated that the synthesized product was PVA-amino acid copolymer.The economic analysis illustrates that the cost of the PVA-amino copolymer account for 19%of the mPEG-NH2 cost.Secondly,the modified ultrafiltration membranes were characterized with some detecting methods,including composition,morphology analysis,hydrophilicity and wettability.The results showed that pore size was reduced and porosity decreased by the grafting modification.Effect of the modification on the membrane surface roughness was relative to the pore size.For the membrane with the larger pore size,the roughness of the modified membrane decreased significantly,and for the membrane with the smaller pore size,just the reverse.The surface hydrophilicity improved after modification,which was indicated by fact that the contact angle of the PES100K unmodified membrane of 91°,decreased to 71° by dopamine coating,53° by PVA grafting,and the PVDF100K unmodified membrane of 75°,decreased to 63° by dopamine coating,53° by PVA grafting.The reason of the improvement of the modifed membrane surface hydrophilicity was the hydrophilic feature of PVA and the change of the surface roughness.After grafting,the pure water flux of PES100K membrane reduced from 222.7L/(m2·h)of the coated membrane to 145.0L/(m2·h)of the grafted membrane,the pure water flux of PVDF100K membrane reduced from 195.6L/(m2·h)of the coated membrane to 143.7L/(m2·h)of the grafted membrane.Finally,the membrane performance was investigated,as well as the stability and anti-fouling ability of the modified membrane.The results showed that the hydrophilicity and wettability of the membrane surface increased with the increase of the solvent concentration and duration time of the grafing reaction,while the pure water flux decreased with the increase of the solvent concentration and duration time of the grafing reaction.The stability experiments showed that the modified membrane had a preferable resistance to acid,but a poor performance in the alkali and oxidation solution.PVA layer played a role as a defender to prevent the polydopamine layer to erosion.With the oil emulsion as the pollutants system,the separation experiments showed that the modified membrane had a better anti-fouling ability than the pristine membrane.After the contaminated PES100K membranes cleaned by 1%surfactant solution at 50℃,the flux recovery rate of coated membrane and grafted membrane were 72.5%and 93.4%,respectively.While the unmodified membrane was only 53%.In circulating filtration,the modified membranes exhibited a better stability and reproducibility.