Immobilization Of Graphene Oxide On The Surface Of Anion Exchange Membrane And Its Antifouling Performance

Electrodialysis technology is used for industrial wastewater desalination under normal temperature and pressure,and has the advantages of high salt concentration multiple,low concentrated water discharge,and artificially adjustable desalination rate.However,the organic matter and high-valent ions in industrial wastewater can easily cause membrane fouling,especially the negatively charged organic foulants in industrial wastewater,are easily combined with the positively charged quaternary amine groups in the anion exchange membrane under the driving of an electric field,resulting in serious membrane fouling,thus restraining the wide application of electrodialysis technology in industrial wastewater treatment.In this dissertation,graphene oxide(GO)with two-dimensional sheet structure and rich oxygen-containing functional groups,which can enhance the hydrophilicity and negative charge density of membrane surface,and inhibit the deposition of organic foulants on the membrane surface,is utilized to modified the membrane surface of anion exchange membranes(AEMs).It is proposed as a composed modification strategy that graphene oxide nanosheets are sealed or adhered by the polymer layers generated through the self-polymerization of dopamine,co-polymerization deposition and layer-by-layer interfacial polymerization.The issue of the weak combinations between GO nanosheets and membrane surface or between GO nanosheets is overcome to reduce the detaching of GO nanosheets.The GO-modified AEMs with good stability and antifouling performance are obtained,and their interfacial structures and mechanisms of antifouling are also investigated.The main research content and conclusions are as follows:(1)GO nanosheets were deposited on the surface of AEMs by electrodeposition to obtain GO modified membrane,and the influence of electrodeposition conditions on the surface properties were investigated.The results showed that proper GO concentration,electrolyte concentration,and current density could enhance the hydrophilicity and negative charge density of the membrane surface,but excessive GO and electrolyte concentration and current density affected the uniformity of depositing layer,so the optimum electrodeposition conditions were obtained.In the electrodialysis treatment of sodium dodecylbenzene sulfonate(SDBS)solution,the desalination rate of GO-modified AEMs was 36.2%higher than that of the unmodified AEM,and an excellent antifouling effect was achieved.However,the electrodeposited GO nanosheets was prone to fall off in the aqueous solution.After immersing in the ultrapure water,the desalination rate slowed down,which was only 13.5%higher than that of the unmodified AEM.(2)Using the self-polymerization reaction of dopamine to form an adhesive polydopamine(PDA)layer on the surface of the electrodeposited GO-modified AEMs to improve the stability of electrodeposited GO layer.The results showed that the PDA coating could compress and smooth the electrodeposited GO layer,so that the surface roughness and thickness of the modified layer were significantly reduced,and the GO layer was encapsulated and fixed on the membrane surface.The GO@PDA composite modified AEMs obtained through optimization had excellent antifouling performance and good stability.When the ED fouling experiment ran continuously for more than 20 hours,it could still maintain good antifouling performance.(3)The co-deposition process of polypolyphenol-polypolyamine was utilized to assist the deposition of GO on the surface of AEMs to strengthen the firmness of GO modified layer.In order to increase the deposition amount and overcome the problem of co-deposition coating falling off under alkaline conditions,the co-deposition system with a hydrogel state composed of tannic acid(T A),polyethyleneimine(PEI)and GO with multiple reaction sites was selected.The deposition system enhances the cross-linking degree of the polymer layer.It was found that the TA-PEI hydrogel co-deposition system could increase the adhesion and deposition of the modified layer.The antifouling ability and alkali stability of the GO/TA-PEI modified membrane were significantly improved.When the SDBS solution was treated by electrodialysis,the desalination rate of the modified membrane was 49.2%higher than that of the unmodified AEM.After immersing in the 0.1 M NaCl solution,it was 36.0%higher than that of unmodified AEMs.(4)In order to further enhance the firmness of the modified layer,a layer-by-layer interfacial polymerization method was developed to quickly prepare a polymer-GO composite modified AEMs.The water phase and oil phase solutions were alternately placed on the surface of AEMs.GO,tannic acid in the water phase and trimesoyl chloride(TMC)in the oil phase could undergo multiple interfacial polymerization reactions at the interface of the water and oil phases.The polymer layer produced by this method was very thin,and the GO sheets also participated in the polymerization reaction to form a covalent bond,which could be more stably fixed on the membrane surface.The experimental results showed that layer-by-layer interfacial polymerization could increase the deposition amount of GO.When the number of GOTA/TMC two-phase layers was 1.5,the modified membrane exhibited the best antifouling performance.When the SDBS solution was treated by electrodialysis,the salt removal rate of the modified membrane was 46.4%higher than that of the unmodified AEM.After immersing in the 0.1 M NaCl solution,it was 42.7%higher than that of unmodified AEM.(5)In order to reveal the structure-activity relationship of different modified AEMs,electrochemical impedance spectroscopy was used to characterize different modified AEMs,combined with the properties and structures of different membranes to explain their interfacial structures and antifouling mechanisms.According to the impedance spectroscopy results,compared with the electrodeposited GO-modified AEMs,the GO@PDA modified membrane showed a new capacitive arc,which was due to the increased compactness of the stacked GO sheets by the PDA coating,and forming a new interface between GO layer and the membrane matrix.However,the electrodeposited GO-modified AEMs,GO/TA-PEI modified AEMs and GOTA/TMC modified AEMs didn’t show the new capacitive arc,indicating that there was no obvious change in the interfacial structure.Considering the antifouling performance and the distribution of foulants on the cross-section,the equivalent circuit fitting data was analyzed to explain the antifouling mechanism of the modified AEMs:the loosely stacked electrodeposited GO layer cannot prevent foulants from entering the membrane matrix;for the three polymer-GO composites modified AEMs,strongly negatively-charged and hydrophilic membrane surface can resist the adsorption and deposition of foulants,dense polymer layer could prevent foulants from entering the inside of membrane matrix,effectively preventing serious irreversible fouling;As the degree of covalent cross-linking of the polymer layers were increased,the alkali stabilities of the polymer-GO modified AEMs from weak to strong were as follows:GO@PDA-M<GO/TA-TMC-M<GOTA/TMC-M.