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Recent studies on selective permeation of specific cations through a cation-exchange membrane in electrodialysis, modification of the membrane and electrodialysis method, are reviewed. The studies are classified (1) to prepare the composite of the cation-exchange membrane with other polymers such as conducting polymers and cationic polyelectrolytes, (2) to change cation-exchange groups from sulfonic acid groups, which are used in conventional membranes, to other groups such as phosphonic acid groups, and (3) to electrodialyze the mixed salt solution in the presence of chelate-forming agent such as poly(ethylene glycol), crown ethers, etc. Though the formation of the layers of the conducting polymers on the surface of the cation-exchange membrane is effective on selective permeation of monovalent cations to divalent cations through the membrane, mechanism of the selective permeation is different depending on species of the conducting polymers: due to sieving of sodium ions from multivalent cations by a tight polypyrrole layer and due to stronger electrostatic repulsion force of the cationic charge in a polyaniline layer to multivalent cations than to the monovalent, which is similar to the membrane having a cationic polyelectrolyte layer on the surface. When cation-exchange groups such as phosphonic acid groups, which are strongly interacted with divalent cations, are introduced in the membrane, monovalent cations selectively permeate through the membrane. However, current efficiency in electrodialysis decreases due to strong binding of divalent cations to the groups—inactivation of cation-exchange groups. Electrodialysis in the presence of chelate-forming agents is effective on the selective permeation of specific cations that have low complex formation constants with the agents through the membrane without any decrease in the current efficiency. |
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