Membrane separation is an energy-conservative and space-saving technology with small environmental loading, and it has been widely used as an electrodialysis and reverse osmosis. It will come into widespread use further if the selective permeability and active transport can be realized as biomembranes. Ion exchange is one of the most fundamental factors in the membrane permeation and our researches are overviewed on ion exchange in membrane chemistry.
Some solutes are selectively transported across an ion-exchange membrane fixing the ion which reacts specifically with some solute. We have already reported the selective transport of aldehydes, sugars, amino acids, and heavy metal ions across the ion exchange membranes based on the above permeation mechanism. These solutes are transported against the concentration gradient under some driving forces, such as hydrogen ion concentration gradient. This method is simple, and it can be widely used for the separation of many solutes.
Solvent extraction is an effective separation method but it is not a continuous process. We have investigated the separation with hydrophobic membranes. The permeability of solutes across the membrane depends on their hydrophobicity and metal ions are concentrated by forming their chelates under a pressure difference. In addition, organic solutes are effectively separated by hydrophobic membranes and the separation factor of a hydrophobic solute, thymol, to a hydrophilic solute, glucose, was over 200.
Ions are effectively transported and separated by using ion-exchange membrane under a concentration gradient of hydrogen or hydroxide ion. The neutralization dialysis is the method based on this mechanism, and salt solution is effectively desalinated with this method. The salts are effectively separated from organic non-electrolytes in the neutralization dialysis, and the application of this method to some industrial processes was examined.
The ion exchange is one of the major controlling factors of membrane permeation. In reverse osmosis, the charge of the membrane functions to reject ions from the membrane. We have investigated environmental science other than membrane chemistry, that is, the fog chemistry and the plant physiology of the forest decline caused by acid fog. The ion exchange of the metal ions in the leafage organizations with the proton in the acid fog is the first step of the declining process. Ion exchange is also important in the soil chemistry, and ion exchange should be further investigated in many fields including the environmental science.

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A variety of layered transition metal oxides with cation-exchange properties were delaminated into their unilamellar nanosheets by reacting their protonated form with bulky organic cations. Exfoliation of anion-exchangeable hosts of layered double hydroxides was attained by the action of formamide upon their NO₃^-or ClO₄^- form. Ionexchange reactivity played an important role in these delamination processes. The resulting two-dimensional nanosheets can be used as a building block to fabricate a range of nanostructured materials. Addition of appropriate electrolytes into a colloidal suspension of nanosheets induced flocculation to yield a restacked structure. Sequential adsorption of nanosheets with polyelectrolytes could produce layer-by-layer assembled multilayer films having precisely controlled nanostructures. These processes could be applied to design a range of functional materials, e.g. fluorescent materials, photocatalysts, dielectrics, electrochromic films.

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Charged interfaces are used for separation and recognition such as ion-exchange chromatography, micellar partition and so on. X-ray absorption fine structure (XAFS) method has been applied to analyze the local structure of solvated ions at charged interfaces. Counter-anions at ion-exchange resins soaked in water or methanol have strongly solvated, while resins soaked in aprotic solvents such as acetone, acetonitrile, etc. almost give the same local structure as dry resins.
Surface monolayers have advantages as models of micellar surfaces because these intrinsic properties can easily be varied. The amount of anions attracted by cationic surface monolayers has been estimated by the total-reflection total-conversion-helium ion yield XAFS method. The surface absorption states of anions gradually change depending on the surface charge density as the surface area was changed by membrane compression.

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Adsorption of cadmium ions from aqueous solutions onto two kinds of activated carbon, one with an enhanced amount of acidic functional groups prepared by oxidation with nitric acid treatment (oxidized carbon) and the other with no acidic functional group prepared by heating in a helium flow (out-gassed carbon), was investigated to clarify the influence of acidic functional groups on cadmium ion adsorption. The uptake of cadmium ions on the oxidized carbon was greater than that on the out-gassed carbon. Protons were released nearly stoichiometrically accompanying cadmium ion adsorption for the former, while no such phenomenon was observed for the latter. In desorption, the exchange of adsorbed cadmium ions with protons was observed only for the oxidized carbon. The amount of adsorbed cadmium ions increased with increasing solution pH for both the activated carbons, although adsorption started at a lower pH value for the oxidized carbon.

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An immobilized metal affinity porous sheet was prepared by radiation-induced graft polymerization and subsequent chemical modifications. A 2-mm thick porous sheet, made of polyethylene, with a pore size and porosity of 1.3μm and 75%, respectively, was irradiated with electron beam. An epoxy-group-containing vinyl monomer was graft-polymerized onto the irradiated porous sheet, followed by the conversion of the epoxy group into an iminodiacetate group. The resultant chelating porous sheet was cut into disks 13 mm in diameter suitable for the packing into the cylindrical cartridge. Nickel ions were immobilized before the permeation of a histidine (His) -tagged fusion protein solution through the porous sheet. Purification of His-tagged fusion green fluorescent proteins was demonstrated in a permeation mode using the nickel-ion-immobilized metal affinity porous sheet.

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Huge amounts of coal fly ash and heated seawater are being discharged from thermal power plants. For the effective utilization of these wastes, zeolitic materials were prepared from coal fly ash with alkali fusion. In distilled water as an activated solution, zeolite-X, zeolite-A, and hydroxysodalite were prepared at 100°C. In seawater, zeolite-X and hydroxysodalite were formed, while zeolite-A was not. Zeolite-X and herschelite were prepared in the half-diluted seawater. The cation exchange capacity of the product prepared in the diluted seawater reached 400 cmol/kg, which was equivalent to that of commercial synthetic zeolite-X.

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The electrochemical methods have recently applied to the detection of ion and molecule recognitions based on supramolecular assembly, although many methods have been used to the detection. The electrochemical methods have many advantages, such as simple instrument and operation, and high sensitivity. Moreover, the electrochemical methods can apply to the detection even in the case of the absence of the electrochemical activity in both host and guest molecule, if the electroactive species, such as ferrocene, were introduced to the host or guest molecule, or the electrochemical impedance spectroscopy was used. The electrochemical methods were widely used in detection of bio-logical substance, such as DNA and enzyme, by means of modified electrode based on the self-assembly. In this paper, first, four electrochemical methods and modification methods of the electrode were briefly introduced, and then ion and molecular recognition by means of modified electrode was reviewed. Especially, we focused on and reviewed the reports mainly published in 2007 for the electrochemical detection of the biological substances, such as DNA and en-zyme, by means of modified electrode based on the self-assembly.

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The advancement of reprocessing and radioactive waste treatment becomes very important subject in the backend chemistry of nuclear fuel cycle. In particular, the development of selective separation and recovery method is expected for reducing a burden to storage facilities, environmental remediation and nuclide utilization. Ion-exchange, solvent extraction and precipitation methods so far have been used for the separation of heat-generating elements, while the ion-exchange is considered to be the most promising method for the effective separation using compact processes, and the advanced separation methods have been developed in various countries.
This special issue deals with the explanation on (1) Objective and impact for the separation of heat-generating ele-ments, (2) Separation characteristics and durability of promising ion exchangers, (3) Solidification and utilization of heat-generating elements.

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