Preparation And Characterization Of Monovalent Cation Selective Membranes

Since the1950s, the earliest research on monovalent cation selective membranes was carried out in Japan in order to achieve the selective separation between monovalent ions (e.g.Na+) and multivalent ions (e.g. Ca2+) in salt production process from seawater, and the first commercial monovalent cation selective membrane was prepared. In the early development of cation selective membranes, the preparation methods mainly include:(1) electrodepositing a layer of polyelectrolyte onto the membrane surface or impregnating membranes directly in the electrolyte solution,(2) the formation of a dense membrane body by means of increasing the cross-linkage. That means the additional crosslinking agent was needed to improve the crosslinking degree of membrane,(3) increase the density of the membrane surface by formation of tight surface layers on the cation exchange membranes,(4)formation a thin functional layer on the surface of membranes through surface grafting using chemical reaction. All the modification methods have their own defects. For example, the long time stability of the electrostatic bonded layer during the ED process is still in debate, the complication in preparation procedure and difficulty in process control, the membrane area resistance increases significantly after surface modification and high permselectivity with low flux, and so on. In order to solve these problems, we propose some new methods to prepare monovalent cation selective membranes.(1) An annealing treatment strategy to enhance the density of PVA-based CEM by tuning crystallinity was attempted. Due to the semi-crystalline nature of PVA, crystallinity of membranes can be deliberately controlled by adjusting annealing temperature. The resultant membranes with the improved crystallinity are expected to exhibit the high monovalent cation selectivity during the ED process due to the pore-size sieving effect. Compared with the earlier methods using additional crosslinking agent to improve the of density of membrane, this method does not need additional crosslinking agent and is simple and easy to control. Because the membranes we prepared are homogeneous, so there is no problem of the long time stability of the modified layer during the ED process. At the same time, it will provide good significance for the preparation of monovalent cation selective membrane using other crystalline polymer.(2) The strategy is polymerizing an alkaline monomer (vinylimidazole) in an acidic polymer(SPPO-H) solution. The electrostatic interaction between acidic groups and alkaline groups is conducive to improve the density of membranes. The formation of acid-base pairs is also conductive to produce the micro phase separation of membranes. This is equivalent to constructing an H+channel based on pore-size sieving effect in the membranes. The H+channel has the effect of both the transfer of monovalent H+and the blockage of divalent Zn2+, respectively. On one hand, the hydrogen bonding network based on acid-base interaction make the structure of membranes more dense and are useful for Zn2+blocking. On the other hand, the sulfonic acid groups electrostatically interacted with imidazole groups can still transfer H+, which guarantees a considerable flux of H+of acid-base pairs membranes. As a result, the final membrane exhibits an extremely low Zn2+leakage and a considerable flux of H+. The increase in PVI content decrease the water uptake and ion exchange capacity. Moreover, the acid-base pairs are demonstrated effective to enhance the thermal stability and mechanical properties. Similarly, there is no problem of the long time stability of the modified layer during the ED process in this series of membranes. This method guarantees that the membranes possess high flux and good permselectivity. Furthermore, this method solves the problem of sharply increase of area resistance after surface modification in the traditional preparation method. It will provide a new train of thought for the preparation of monovalent cation selective membranes.(3) The monovalent cation selective membranes were prepared by polymerizing acrylic acid in polybenzimidazole solution. An acid-base pairs ion channel was constructed in the membranes by the electrostatic interaction between acidic groups and alkaline groups. The idea that monovalent cation selective membranes can be prepared through constructing an acid-base pairs ion channel in membranes was verified universality. Namely, monovalent cation selective membranes can also be prepared through the electrostatic interaction between acidic monomer and alkaline polymer. On the other hand, the different influences on membranes properties between weak acidic carboxylic acid groups and strong acidic sulfonic acid groups were investigated. The relationships between basic physical and chemical properties, thermal stability, area resistance, permselectivity and acrylic acid content were studied.(4) Porous membranes were prepared by the conventional phase inversion method using polyether sulphone (PES) and brominated polyphenylene oxide (BPPO) A thin polymer layer was further formed onto the surface of PES membranes by interfacial polymerization or a direct coating monovalent cation selective membrane solution on the surface of BPPO porous membrane. We investigated the selective separation behavior of the nanofiltration membranes prepared by two methods in electrodialysis process. The results confirmed the feasibility of nanofiltration membrane used for selective separation between monovalent and divalent ions in electrodialysis process. The porous strucrure was found more conductive to the ion transmission, so that the composite membranes have high flux, and dense surface acts as a selective separation effect. At interfacial polymerization method, pore size of porous membranes has great influence. At the same time, impregnation procedure is also very important for a direct coating method and the solvent used in membrane solution can not damage the porous structure. It is necessary to preprocess the porous membranes when the solvent can disolve the porous membranes.In general, three new methods of preparing monovalent cation selective membrane have achieved good results. These methods solve the problems of traditional methods that the long time stability of the electrostatic bonded layer during the ED process, sharply increase of area resistance after modification and good permselectivity with low flux. In this work, we successful innovated to use nanofiltration membrane for selective separation between monovalent and divalent ions in electrodialysis. The new ideas we attempted and the results we achieved in this dissertation will contribute on the progress and future studies in monovalent cation selective membranes.