Study On In Situ Repair And Anti-Fouling Functionalization Of Polyamide Reverse Osmosis Membrane

Reverse osmosis（RO）is a membrane separation process based on polymer semi-permeable membrane to achieve molecular separation.It has become an efficient and economical process for water desalination,ultra-pure water production and wastewater recycling.Aromatic polyamide（PA）thin film composite membrane（TFC）is the most advanced reverse osmosis membrane at present.It has high desalination and water permeability,as well as excellent chemical,thermal and mechanical stability.However,the main obstacle restricting the large-scale application of reverse osmosis membrane is membrane fouling,which will lead to frequent cleaning,performance degradation and shorter service life of the membrane.Therefore,it is of great significance to repair the deteriorated reverse osmosis membrane and give the repaired membrane anti-fouling and other functions to prolong the service life of the membrane and reduce the operating cost of reverse osmosis.In this paper,the performance repair and functionalization of hydrolyzed polyamide reverse osmosis membranes were studied.Firstly,the carboxyl group on the surface of the degraded membrane was activated by1-（3-dimethylaminopropyl）-3-ethylcarbodiimidehydrochloride（EDC）/N-hydroxysuccinimide（NHS）,and polyethyleneimine（PEI）was grafted on the surface of the degraded polyamide reverse osmosis membrane to repair the separation performance of the degraded polyamide reverse osmosis membrane in situ.Then,dimethyl aminopropyl methylacrylamide（DMAPMA）was grafted on the surface of PEI repair membrane by Michael addition reaction to regulate the chemical composition and tertiary amine content of the membrane surface.Finally,the zwitterionic layer was constructed by using 3-chloro-2-hydroxypropanesulfonic acid sodium to functionalize the membrane surface.The effects of PEI concentration,PEI reaction time,PEI reaction p H,DMAPMA concentration and sodium3-chloro-2-hydroxypropanesulfonate concentration on the separation performance of the repair membrane were systematically studied.The physicochemical properties of the membrane surface before and after reverse osmosis membrane repair were studied by ATR-FTIR,XPS,Zeta potential instrument,contact angle measuring instrument,SEM and AFM.The antifouling performance and chlorine resistance of the deteriorated membrane and the repaired membrane were evaluated by cross-flow permeation experiments.The results are as follows:（1）Using EDC/NHS to activate and degrade the carboxyl group on the surface of polyamide reverse osmosis membrane,polyethyleneimine can be successfully grafted on the surface of the membrane by amidation reaction to obtain PA-PEI repair membrane.ATR-FTIR analysis showed that the peak intensity ratio of the separation layer amide II（N-H）1541cm^-1absorption peak to the characteristic absorption peak at1488cm^-1of the polysulfone membrane was enhanced after grafting polyethyleneimine.XPS analysis showed that the nitrogen content on the surface of the PEI repair film was significantly higher than that of the degraded film.After grafting PEI,the carboxyl group on the membrane surface was transformed into amino group,the positive charge of the membrane surface was enhanced,the isoelectric point was increased,and the hydrophilicity of the membrane surface was enhanced.The contact angle was reduced from 44.4°of the degraded membrane to23.8°.Morphology analysis showed that PEI grafting hardly affected the surface morphology of the membrane,but the roughness decreased.The optimal conditions for chemical grafting of PEI on the membrane surface were:PEI concentration of1.0wt%,grafting time of 8h,grafting p H=11.The rejection of sodium chloride increased from 94.08%to 98.10%,and the pure water flux decreased from 74.04L/（m²·h）to 31.57 L/（m²·h）.（2）The primary and secondary amines in PEI can be successfully converted into tertiary amines by Michael addition reaction between dimethyl aminopropyl methacrylamide（DMAPMA）and PEI to obtain PA-PEI-DMAPMA membrane.XPS analysis showed that the content of tertiary amine on the surface of PA-PEI-DMAPMA membrane increased with the increase of DMAPMA concentration.When the concentration of DMAPMA was 6wt%,the content of tertiary amine on the membrane surface reached the maximum.The separation performance evaluation showed that the surface grafting of dimethyl aminopropyl methacrylamide（DMAPMA）had little effect on the salt water separation performance of the repair membrane.（3）Through the quaternization reaction of tertiary amine with sodium3-chloro-2-hydroxypropanesulfonate,the zwitterionization of the membrane surface can be successfully realized,and the zwitterion layer is constructed on the membrane surface to obtain PA-Z membrane.Infrared analysis showed that the PA-Z membrane had a characteristic absorption peak of the sulfonic acid group at 1040cm^-1.XPS analysis showed that the S element belonging to the sulfonic acid group appeared on themembranesurface,indicatingthatthesodium3-chloro-2-hydroxypropanesulfonate had been successfully grafted on the membrane surface.Compared with PA-PEI membrane,the Zeta potential of PA-Z membrane decreased significantly,the hydrophilicity increased,the contact angle decreased from23.8°to 18.9°,and the surface roughness increased slightly.The optimal conditions for membrane surface zwitterionization were as follows:the concentration of sodium 3-chloro-2-hydroxypropanesulfonate was 0.1 g/ml,and the reaction was carried out at 30°C for 12h.After zwitterionic modification,the sodium chloride rejection of the PA-Z membrane remained basically unchanged,and the pure water flux increased from 31.57 L/（m²·h）to 42.99 L/（m²·h）.（4）The anti-fouling properties of polyamide membrane（PA-V）,PEI membrane（PA-PEI）and zwitterionic membrane（PA-Z）were studied.The results showed that the order of anti-fouling ability of PA-Z membrane>PA-PEI membrane>PA-V membrane.For BSA aqueous solution,the flux loss rates（FDR）of PA-V membrane,PA-PEI membrane and PA-Z membrane were 33.5%,11.3%and 3.6%,respectively,and the flux recovery rates（FRR）were 71.4%,90.7%and 97.7%,respectively.For DTAB aqueous solution,the FDR of PA-V membrane,PA-PEI membrane and PA-Z membrane were 78.53%,24.91%and 18.71%,respectively,and the FRR were24.91%,78.27%and 84.88%,respectively.For SA aqueous solution,the FDR of PA-V membrane,PA-PEI membrane and PA-Z membrane were 44.1%,14.4%and2.6%,respectively,and the FRR were 62.2%,88.24%and 98.62%,respectively.The study of chlorine resistance showed that under the same chlorination intensity,the order of chlorine resistance was PA-Z membrane>PA-PEI membrane>PA-V membrane.The Na Cl rejection of PA-Z membrane only decreased from 97.90%to96.45%after 78000 ppmh free chlorine treatment at p H=9.