| The salty waste water is produced from the processes of metallurgy, medicine, food and other fields. If discharged directly, the waste water will cause water pollution and result in hardening of the land, burning of plants, deterioration of water quality and other hazards. Therefore, the recovery and removal of salts is an important task. At present, crystallization, nanofiltration, reverse osmosis and other ways are usually utilized for desalination in the industry, but there are some defects, such as large energy consumption, complex equipment and serious membrane pollution. The diffusion dialysis (DD) process is of green nature with low energy consumption and low membrane pollution and hence can be considered for recovery and separation of salts. However, only the concentration difference is the driving force and there is no external electric field or pressure, so the permeability is low. Besides, the positive and negative ions of the salts are larger and the migration activities are lower as compared with H+ and OH- ions. Hence the permeation flux of salts is insufficient with use of traditional ion exchange membranes. The polyelectrolyte complexes (PECs)membrane is a special ion exchange membrane containing both anionic and cation exchange groups. This thesis will explore its stability and permeability to salts.Sometimes, higher desalination rate is needed in certain pharmaceutical or food fields. For instance, a small amount of salts is present in the fermentation mother liquor of threonine, which needs to be removed before other follow-up treatments in order to get the desired threonine products. The desalination rate of the DD process is not high. As comparison, the electrodialysis (ED) can not only get higher desalination rate, but also higher efficiency under the electric field, which can meet the demands of the industrial process for efficiency and speed. Therefore, another part of this thesis is the use of ED for the desalination of simulated threonine mother liquor, and the exploration of the process' feasibility.In addition, conventional organic polymer membranes have defects in the anti-swelling behavior, strength, stability and other aspects, while the organic-inorganic hybrid membranes can show advantage in the stability. Accordingly,another focus of this thesis is preparation of membranes with organic-inorganic hybrid ion exchange materials, and their application to DD and ED experiments. The full thesis consists of five parts, and each part is as follows:The first chapter is the introduction. A brief introduction is presented about the concepts of ion exchange membranes including organic-inorganic hybrid ion exchange membranes. Then the structure, excellent characteristics and unique advantages of polyelectrolyte complexes (PECs) membrane in salt dialysis process are shown, followed by introduction of the principle and application characteristics of DD and ED, the significance of desalination and the application background of this thesis.Finally, the origin, significance and main contents of the thesis are summarized.The second chapter is about the preparation and characterizations of PECs membranes. Two polyanionic solutions and one polycationic solution were prepared with commercial polyelectrolyte or multi-silicon copolymers as raw material, and polyvinyl alcohol (PVA) was added as the matrix. Then the polyanionic and polycationic solutions were mixed in different proportions and underwent sol-gel process to yield two series of PECs membranes. The membranes contain not only PVA and other organic components, but also Si-O-Si inorganic network structure and hence they are organic-inorganic hybrid membranes. The prepared PECs membranes were characterized, and the anion exchange membrane (including self-made membrane and commercial membrane DF-9010) and cation exchange membrane(including self-made membrane and commercial membrane CJMCDD-1) were used as the reference at the same time. The results showed that the hydrophilicity and the water content of the PECs membranes were decreased, the transport number of ions was reduced,while the area resistance was increased,and the area expansion and the mass loss rate were lowered in the salt solution (NaCl or NaAc). Therefore, the membrane structure was more compact and stable. The polymer chains could form interaction through coulomb force between the anion and cationic groups in the membrane, which can make the membrane more compact. Hence, the water content decreased. In addition, the amount of net charge in the membrane was reduced, so that the transport number of ions decreased and the area resistance increased. The membranes were applied to the diffusion dialysis of NaCl aqueous solution and the results showed that the permeability of the PECs membrane was superior to that of the commercial membranes.The third chapter is the DD desalination by PECs membranes. Firstly, the DD performances of NaCl or NaAc were investigated and compared with the self-made cationic membrane and commercial ion exchange membranes (DF-9010 and CJMCDD-1). The results showed that the dialysis coefficient of PECs membranes were significantly higher than those of the commercial membranes. The membranes were then applied to a mixed solution of threonine and NaCl, showing high permeability and selectivity. Finally, the feasibility of the DD for sodium acetate residue solution was explored. The dialysis coefficient of salt after 5 h running was 0.000602 ?0.00120 m/h,which was 13 ?26 times the value of commercial CJMCDD-1 membrane. And the leakage rate of impurities was not high, the TOC value of the impurity in the water chamber was 0.78 ?1.02 g/L at the end of the experiment. It can be seen that PECs membranes have high permeability and selectivity to salt, and can be potentially used in salt recovery of industrial effluent.The fourth chapter is the desalination of simulated threonine mother liquor by ED.Self-made organic-inorganic hybrid membranes (CEM-4/12 and AM-QP-30) were utilized, and the commercial membranes (CJMC-2 and CJMA-2) were used as the references. The results showed that lower current density could lead to higher desalination rate. When the current density was 10 mA/cm2 for threonine - NaCl mixed solution, the desalination rate was up to 98.6%. With the increase of current density, desalination rate decreased, but the time of the experiment was shortened greatly, so high current density could be utilized to realize rapid desalination. When the current density was 60 mA/cm2, the time of the experiment was only 44 min, and the desalination rate was 91.3%. With different operating conditions,the recovery rate of threonine had no significant difference. As compared with the commercial membranes, the desalination rate of the hybrid membranes was similar, the recovery rate of threonine was not significantly different, and the anion exchange membranes'surface contamination was smaller. It can be seen that the organic-inorganic hybrid ion-exchange membranes can be applied to the ED process to show good effect on desalting the simulated threonine mother liquor, and good stability for reusing.The chapter 5 summarizes the full thesis. Based on the above chapters, it is concluded that the PECs membranes by organic-inorganic hybrid material have better stability and can be used in DD to recover salt, and the permeation flux is higher than that of anion or cation exchange membranes. The membranes can also be used in industrial waste liquid desalination treatment. The anion and cation exchange membrane by organic-inorganic hybrid materials are used in ED treatment of threonine-salt solution, and the results show that the desalination performances are excellent and stable,and membrane pollution is low. Overall,the organic-inorganic hybrid membranes have good application prospects in industrial fields. |
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