Membrane permeation characteristics of γ-aminobutyric (GABA), glutamic, and aspartic acids were examined in electrodialysis and/or diffusion dialysis. In the electrodialysis experiments at pH 3, γ-aminobutyric acid permeated through a cation-exchange membrane, but glutamic acid did not. In the experiment of diffusion dialysis of γ-aminobutyric, aspartic, and glutamic acids, the fluxes of those amino acids were affected by electrolytes species and their concentration in the Strip solution. Moreover, the diffusion-dialysis technique was applied to separation of amino acids from the resultant solution of the reaction of glutamic acid using rice bran. The fluxes of amino acids were increased with an increase in HCl concentration of the Strip solution, and these values were in the order of amino acids γ-aminobutyric acid>aspartic acid>glutamic acid. These results suggest that diffusion dialysis with a cation-exchange membrane is applicable to the selective separation of γ-aminobutyric acid from the enzyme reaction solution with glutamic acid and rice bran.

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In this review, we discuss the various facets of ion-exchange phenomena as well as approaches thereto mainly on the basis of our research work. The approaches stated here include ion-exchange chromatography, capillary electrophoresis, X-ray absorption fine structure (XAFS), ion transfer voltammetry at a liquid/liquid interface, a coupled acoustic-gravity filed etc. Ion exchange chromatography has been principally used as a practical tool but is also an efficient tool for evaluating ion-exchange selectivity. However, results have been analyzed in an improper way, which practically gives convenient information but possibly incorrect physical views. This has been partly overcome by introducing electrostatic models into the interpretation of ion-exchange chromatographic retention data. Similar models have proven effective in the discussion of the ionic partition into molecular aggregates such as micelles and surface monolayers. Finally, an acoustic method is discussed as a tool for a dynamic study of ion-exchange. Although an acoustic force is usually a function of the sizes of particles, size effects can be eliminated when a gravity force is incorporated. In this coupled acoustic-gravity field, we can visually resolve the acoustic properties of an ion-exchange resin particle, which are varied by the progress of ion-exchange.

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It is indeed a great honor for me to receive the award of Japan Association of Ion Exchange. On this occasion, several topics about “Ion exchange resins and their technologies” that I have experienced so far are summarized here.

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The advancement of selective separation and recovery method of actinoid is important subject for the advanced reprocessing and radioactive waste treatment. This method is regarded as a first step of the transmutation of long-lived nuclides, and the development of advanced separation techniques has been expected from the standpoint of nonproliferation, reduction of a burden to waste disposal, environmental remediation and effective utilization. Ion-exchange methods have been developed as a pioneer techniques, and the basic evaluation of selectivity and rationalization and compactness of ion-exchange process are extensively studied in various countries. Lately the development of more precise nuclide separation methods are expected for the advanced reprocessing and radioactive waste treatment. The advancement of selective ion-exchange techniques has important significance.
This special issue deals with (1) Purpose and impact of actinoid separation, (2) Separation characteristics and stability of promising ion exchangers and (3) Actinoid separation methods for advanced separation processes.

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