Preparation And Characterization Of Ion Exchange Membranes Containing Hydrophilic Auxiliary Groups For Diffusion Dialysis

Due to the people’s continuous pursuit of life quality,the increasingly serious environmental pollution has also attracted more and more attention from the society.In particular,a large amount of wastewater is producing in some industrial processes,such as hydrometallurgy,electroplating,papermaking,printing,dyeing,etc.,contains a considerable amount of acids or bases and heavy metal salts.A direct discharge will not only pose a great threat to the ecological environment and people’s health but also cause a severe waste of resources and enormous economic loss.Until now,among many methods of treating these effluents,the diffusion dialysis(DD)based on ion-exchange membranes has gradually developed into one of the most potential technologies for acid or base recovery due to its characteristics of energy-saving,low energy consumption,easy operation,and industrialization.However,the lack of highly stable membranes with high flux and selectivity greatly limits the application and development of the DD process.Therefore,it is particularly important to develop and prepare ion exchange membranes with the outstanding performance.Different from the traditional method of inhibiting metal or acid radical ion transfer in the preparation of DD membranes,a series of high-performance DD ion exchange membranes with ion transport channels were prepared based on reducing H+/OH’ transfer resistance by introducing the hydrophilic auxiliary group,adjusting the polymer microphase separation as well as the appropriate cross-linking strategy.In this thesis,the relationship among the auxiliary mass transfer groups,ion transport channels and the performance of diffusion dialysis has been explored in detail,so as to simultaneously realize the improvement of the flux,selectivity and stability even under a lower IEC level,and to solve the key problems encountered in the preparation of DD membranes.The concrete research content is as follows:(1)Firstly,a strategy of self-catalyzed in-situ crosslinked membrane preparation was elaborately designed and proposed.During the membrane preparation,the delocalized H+of sulfonated poly(2,6-dimethyl-1,4-phenylene oxide)(H+-SPPO)in-situ inducted the acetal catalyzed reaction between the hydroxyl groups of the hydrophilic poly(vinyl alcohol)(PVA)and glutaraldehyde(GA)without any additional initiator.Among them,PVA acted as an auxiliary mass transfer phase was introduced to construct a semi-interpenetrating cross-linked network containing multi-hydroxy groups in SPPO polymers.By this strategy,a series of high-performance SPPO/PVA composite membranes were prepared for the alkali recovery.ATR-FTIR and the crosslinking contrast experiments demonstrated the successful in-situ crosslinking of the PVA and GA.Thermogravimetric analysis,mechanical properties,and long-term stability tests showed that the prepared membranes have excellent thermal stability,mechanical properties,and alkali resistance.Through the diffusion dialysis(NaOH/Na2WO4 system)experiments,it was found the prepared composite films had the diffusion dialysis coefficient(UOH)as high as 0.0107 m h-1 and high selectivity(S=45)even with a low IEC level,showing more excellent DD performance as compared with recently reported cation exchange membranes for the DD process.(2)In order to further verify the influence of the auxiliary hydrophilic groups on DD flux,a series of anion-exchange membranes with different IECs were prepared by introducing the tris(2-(2-methoxyethoxy)ethyl)amine with long hydrophilic side chains(ethoxy groups)into the hydrophobic poly(2,6-dimethyl-1,4-phenylene oxide)main chains through Menschutkin reaction and the subsequently thermal crosslinking,where the orderly self-organized nanochannels for ion fast transport were constructed through hydrophilic and hydrophobic microphase separation.The preparation process,physicochemical properties,microstructure and DD performance of the membranes were characterized and analyzed in detail.Water uptake,in-situ heating-FTIR and contact angle tests effectively demonstrated the hydrophilicity of the ethoxy side chain,as well as the hydrogen bonding interaction among ethoxy groups and hydrated ions.AFM showed that the prepared anion exchange membranes have highly efficient self-assembled ion transport nano-channels.From the DD results,the introduction of hydrophilic side chain is beneficial to the improvement of H+flux,and the subsequent thermal crosslinking can effectively improve the selectivity of the membranes.At 25℃,H+　dialysis coefficient UH of prepared anion exchange membranes was 0.013～0.059 m h-1,and separation factor S was 19～197.By comparing with the commercial membrane DF-120(UH=0.009 m h-1,S=18.5)and other anion exchange membranes reported in the literature,the as-prepared membranes in this work have higher flux and selectivity even under a lower IEC level.(3)Based on the work in the previous chapter,a series of anion exchange membranes with highly ordered self-organized nanochannels were prepared by severally grafting the tris(2-(2-methoxyethoxy)ethyl)amine(TDA)with long hydrophilic side chains and dimethyl-aminoethyl methacrylate(DMAEMA)with the terminal vinyl groups into poly(2,6-dimethyl-1,4-phenylene oxide)main chains through Menschutkin reaction and subsequent thermal crosslinking.The introduced hydrophilic side chain(TDA)can effectively constructed the ion transport channels through hydrophilic and hydrophobic microphase separation,which is conducive to the improvement of the H+diffusion coefficient,while the introduction of the DMAEMA and subsequent in-situ crosslinking further promoted the formation of microphase separation in the membrane structure,which is beneficial to the improvement of density and selectivity.The synthesis and preparation of polymer films,as well as the construction of nanoscale ion channels were characterized and analyzed in detail by 1H NMR,XPS,in-situ FTIR and AFM during the experiment.From the DD tests,it was found that in-situ cross-linked anion exchange membranes with highly ordered,self-assembled hydrophilic ion channels had better performance than the commercial DF-120 membrane and the membranes reported in the literature showing a good application prospect.The H+dialysis coefficient of the membranes was as high as 0.0325 m h-1 at 25℃,and possessed high selectivity and operational stability.