Fabrication And Separation Performance Of Novel Antifouling Dye Desalination Membranes

A large amount of saline dye wastewater is generated in dyeing industry,but the traditional nanofiltration（NF）membranes with high salt rejection renders them difficult to achieve efficacious dye purification and resource recovery.In this thesis,in consideration of low permeance,high salt rejection and membrane pollution of traditional NF membranes when dealing with saline dye wastewater,based on the special properties of two-dimensional materials and porous organic materials,a series of novel loose nanofiltration（LNF）composite membranes featuring high permeance,salt permeation and antifouling properties were constructed via vacuum-assisted filtration and interfacial polymerization for the separation of dyes and salts.A synergistic regulation strategy of mass transfer paths within the membranes and the physicochemical properties on the membranes surface was proposed,and the mechanism for strengthening the membrane permeance and antifouling mechanism was also established.The research contents are as follows:（1）To solve the issues of membrane fouling and low permeance of commercial organic membranes,two-dimensional g-C3N4 nanosheets with photocatalytic property were synthesized by oxidative etching followed by ultrasonic exfoliation.Based on vacuum-assisted filtration technology,the exfoliated g-C3N4 nanosheets were assembled onto polyacrylonitrile（PAN）substrates to fabricate continuous and defect-free g-C3N4 composite membranes.The g-C3N4 composite membranes maintained high dye rejection（MB:99.8%）and low divalent salt rejection（Mg SO4:18.9%）,while the water permeance(11.7 L m^-2 h^-1 bar^-1)significantly improved.Furthermore,under the irradiation of visible light,the pollutants adsorbed on membranes surface could be effectively degraded and the self-cleaning function of the membranes could be realized,thus prolonging the service life of the membranes.The good photocatalytic property endows the composite membranes with the ability to solve fouling issues（organic fouling and biological fouling）,enhancing the efficiency of the membranes.（2）Due to the instability and low permeance of graphene oxide（GO）membranes in water,the synthesized ionic covalent organic nanosheets（i CONs）were inserted into GO nanosheets by a"complementary nanosheets"strategy.Herein,GO/i CONs composite membranes were fabricated based on vacuum-assisted filtration technology.GO and i CONs are physicochemically complementary due to their opposite charges and hydrogen bond acceptors and donors,respectively.Upon mixing,the two nanosheets were assembled into laminar membranes under the action of electrostatic attraction and hydrogen bonding,which suppressed the swelling of the GO-based membranes in water.Moreover,i CONs are not only used as intercalation materials to enhance the stability of GO-based composite membranes,but augment the interlayer spacing of the composite membranes and promote the rapid transport of molecules.The well-defined inner pores of i CONs,resulting in shorter and more direct paths,accelerate the transport of molecules,which is beneficial to higher permeance up to 206.8 L m^-2 h^-1 bar^-1,11 times higher than GO membranes.Meanwhile,the dye rejection rate（direct red 23:97.4%）is not sacrificed,which is conducive to the dye purification.The GO/i CONs membranes also have good molecular sieving ability for dye mixtures with different molecular weights,which is beneficial to the recycling of dye wastewater.In addition,the assembled GO/i CONs composite membranes also exhibited good antibacterial ability,which is related to the synergistic bactericidal effect of GO and i CONs.This work reveals the triple roles of i CONs as nanospacers,stabilizers,and porous fillers,providing high water permeance and stabilization performance of GO-based composite membranes.（3）In view of issues of pollution and chlorine resistance of the traditional NF membranes,a hydroxyl-containing macromolecular porous organic polymer（POP）was synthesized by diazo coupling reaction,and the POP-based loose polyester nanofiltration membranes were constructed by interfacial polymerization with trimesoyl chloride（TMC）.The membranes had a high permeance of 31.3 L m^-2 h^-1bar^-1 while maintaining high dye rejection（RB5:99.1%）and low salt rejection（Na2SO4:25.53%）,allowing that the mixtures of dyes and salts were separated efficiently.Since the average pore size of the POP-based polyester membranes is larger than that of the conventional polyamide membranes,the active layers are relatively loose,which is conducive to the rapid transport of water molecules.The hydrophilic membrane surfaces effectively reduce physically irreversible fouling.The membranes were immersed into Na Cl O aqueous solution（5000 ppm）for 60 h and exhibited stable permeance and dye rejection,indicating that the POP-based polyester membranes had good chlorine resistance.（4）Due to the slightly thicker selective layers of the POP-based polyester composite membranes,the improvement in water permeance is limited.Herein,innovative polyester（PE）membranes were facilely fabricated atop porous PAN substrates via interfacial polymerization,which a novel synthesized hydroxyl-containing guanidinium monomer（Sa TGCl）was employed as an aqueous monomer.Sodium hydroxide（Na OH）as a catalyst was added to the Sa TGCl aqueous to accelerate the ester reaction between hydroxyl groups within Sa TGCl and acyl chloride within trimesoyl chloride（TMC）.The PE membranes exhibited a high water permeance(97.2 L m^-2 h^-1 bar^-1),a superb Eriochrome black T（EBT）retention（99.4%）,and low salt retention（Na2SO4:15.8%,Na Cl:2.3%）.Furthermore,the PE membranes attained an exceptional molecular sieving ability for dye/salts or dye/dye mixtures.Additionally,the PE membranes with 0.05 wt%Sa TGCl were soaked into Na Cl O aqueous solution（5000 ppm）up to 100 h and displayed stable water permeance and dye retention,indicating the prominent chlorine resistance of the PE membranes.Moreover,the membranes showed distinguished antifouling properties（biological fouling and organic fouling）and operational stability.Hence,the newly-developed guanidinium-based PE membranes have enormous application prospect in the domain of dye desalination and purification.