| The applications of traditional polymer nanofiltration(NF)membranes are still restricted by several challenges including the trade-off effect between permeability and selectivity,low resistance to fouling and chemical stability.In recent years,the rapid development of nanomaterials plays an important role in promoting the R&D of the new-generation polymer-matrix membranes.Especially,the appearance of two-dimensional carbon nanomaterials such as graphene oxide(GO)is very appreciated.The introduction of the two-dimensional graphene nanomaterials is expected to overcome the trade-off effect between permeability and selectivity,as well as develop high-throughput,anti-fouling,chemical-resistant membrane products.In this study,three kinds of graphene derivatives with different hydrophilic groups were incorporated into the polymer-matrix NF membranes by blending or chemical crosslinking.The structure-activity and dose-effect relationships between the graphene derivatives and polymer-matrix nanocomposite NF membranes were revealed,and the formation mechanisms of polymer-matrix nanocomposite NF membranes were elaborated.This study mainly includes the following four parts:(1)Preparation and characterization of three kinds of graphene derivatives with different hydrophilic groups:(I)The improved Hummers method was employed to prepare GO having many oxygen-containing groups such as-OH,C-O-C,-COOH,the average diameter and thickness of GO nanosheets were 400～800 nm and 1.1 nm,respectively.(II)More-COOH was introduced into GO nanosheets by the ring opening reaction between maleic anhydride and the hydroxyl of GO.The oxygen content of maleic anhydride functionalized graphene oxide(MAH-GO)significantly increased from 12,88%to 25.95%.The average diameter and thickness of MAH-GO were very similar to those of MAH-GO,which suggested that the chemical modification didn’t affect the main structure and morphology of GO.(Ⅲ)Sulfonated graphene(SG)is enriched with-SO3H and the S content reaches to 9.14 wt%.(2)The conventional PA composite NF membrane is unable to deal with the strong alkali solution due to its narrow pH tolerance.Therefore,PES was selected as the main polymer-matrix because of its excellent chemical resistance,and SG and GO were used as nanofillers.The alkali-resistant mixed matrix NF membranes were prepared via immersion precipitation phase formation.The results showed that SG and GO nanosheets could permanently improve the hydrophilicity,membrane surface charge,permeaselectivity and antifouling property of the mixed-matrix NF membrane.Because SG had the stronger hydrophilicity than GO,and SG was easier to migrate and enrich on the membrane surface,the overall performance of PES-SG series membranes were superior to PES-GO series membranes.When SG concentration in the casting solution was O.lwt%,the water flux of PES-0.10-SG reached to 190.5 L·m-2·h-1,which corresponded to 219.7%of pure PES NF membrane.Furthermore,PES-0.10-SG exhibited the long-term stability during a seven-day test in the alkaline solution.(3)In order to enhance the permeaselectivity,antifouling property and chlorine resistance of the conventional PA composite NF membrane,novel thin-film nanocomposite NF membranes were developed through modification with MAH-GO via interfacial polymerization(IP).The results showed that MAH-GO and GO were horizontally embedded in the active layers via the chemical bonds.The active layer became looser and thinner and smoother because the diffusions of PIP monomers to the organic phase were hindered by MAH-GO or GO nanosheets.Because MAH-GO having more carboxyl was more hydrophilic and negatively charged compared with GO,TFN-MG series nanocomposite membranes demonstrated the superior water permeability,antifouling capability and chlorine resistance over TFN-GO series nanocomposite membrane.When MAH-GO concentration in aqueous phase was 60 ppm,the water flux of TFN-MG-60 was found to be 49.3 L·m-2·h-1,corresponding to 176.7%of TFC-blank.Meanwhile,TFN-MG-60 retained the high salt rejection rate of 97.6%to Na2S04 comparable to that of TFC-blank,also showed the excellent antifouling property and chlorine resistance.(4)In order to maximize the performance of composite NF membrane,the dual modification strategy was carried out.Firstly,the structure and property of the sublayer was adjusted and optimized by blending sulfonated polysulfone(SPSU)and SG while PSU used as the main material.Secondly,the structure and property of the active layer was adjusted and optimized by incorporating an appropriate amount of MAH-GO or GO via IP.The results showed that the IP reaction and the formation of active layer were closely related to the physicochemical properties of the sublayers including the pore size,porosity,hydrophilicity and surface roughness.When the ratio of SPSU/(PSU+SPSU)was 15%,SG concentration was 0.10 wt%,and MAH-GO concentration in aqueous phase was 20 ppm,TFN-MG20-USG10 nanocomposite membrane demonstrated the high water flux up to 85.8 L·m-2·h-1 while maintaining the salt rejection of 98%,and it also showed the excellent antifouling property and chlorine resistance.Based on the influence mechanisms of the porous sublayers(pore size of 15～35 nm,porosity of 50～85%,water contact angle of 64～82° and surface roughness of 1-6 nm)on the PPA active layers of TFC and TFN NF membranes,PSU-SPSU-SG blending support membrane with medium pore size,high porosity and strong hydrophilicity can improve the separation performance of TFC and TFN NF membranes at the same time.Therefore,two-dimensional graphene derivatives are applied to adjust the structure and performance of the porous support layer and the active layer at the same time,which can maximize the overall performance of composite NF membrane and provide an effective strategy for fabricating composite NF membranes with a high flux as well a reasonable rejection. |
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