This review paper is devoted to two topics, i.e., fluoropolymer-based porous and ion-exchange membranes, both of which include the creation of nanostructure-controlled functional membranes with high-energy ion beams. Latent tracks of the MeV−GeV heavy ions in a polymer foil can sometimes be chemically etched out to form a membrane with micro- and nano-sized through-pores, the so-called ion-track membrane. Our focus is on ion-track membranes of poly（vinylidene fluoride）（PVDF）, which have been considered as a porous matrix for nanotechnological applications. Although the PVDF-based ion-track membranes have already been reported, their preparation methods have never been optimized. The etching behavior mainly depended on the energy deposition of the ion beams, and thus its depth distribution, estimated by a theoretical simulation, was successfully applied to control the shapes of the etched pores. The oxidation of the tracks by means of heating in air and ozone exposure was effective as a pre-etching treatment. The cation- and anion-exchange membranes（CEMs and AEMs, respectively）were prepared with two methods, which are based on the chemical etching and/or modification of each track, in other words, the radiation grafting into the track-etched pores in PVDF and ion-track direct grafting into poly（ethylene-co-tetrafluoroethylene）（with no etching process）. The resulting “nano-structure controlled” CEMs and AEMs were found to have one-dimensional ion-conductive pathways parallel to their thickness direction, while, in contrast, most of the existing membranes exhibited ion transport in the three-dimensional random media. This is probably because the nearly columnar ionic phase with a width of tens-to-hundreds of nanometers extended through the membrane. Other excellent membrane properties for applications to fuel cells and seawater electrodialysis, e.g., high dimensional stability, should also be due to such a controlled structure. Finally, worldwide and TIARA’s recent progress in irradiation technologies are presented to pursue the possibilities for industrial applications of our new functional membranes.
Nanofibrous materials, which are one–dimensional nanomaterials, have unique properties based on their nanoscaled-size, highly specific surface area, and highly molecular orientation. They easily form three–dimensional network structures composed of nanofiber frameworks and the interconnected pores formed between nanofibers. This network structure enables efficient ion or water transport through the network backbone and/or the network interface, and an efficient chemical reaction at the network surface. This report describes applications of the nanofibrous materials and their composites for membranes and electrolytes, including high-performance filters, separation membranes, ion-exchange membranes, and solid and quasi-solid electrolytes.
The accident at Fukushima Dai-ichi Nuclear Power Plant in 2011 contaminated a large area of Fukushima, and the waste resulting from the decontamination of the area has been increasing. Its volume reduction is a very important subject. The radioactive material deposited on the area is cesium, and the application of both electrokinetic remediation（EK）and a specific adsorbent for cesium can clean up the cesium contaminated soil for reuse. The authors tried an EK process to remove natural cesium in the black soil. The desorption of the cesium from the soil is the process dominating the efficiency. EK efficiency for cesium removal was low, but it was improved for the cesium added to the soil because of its dependence on the interaction between the cesium and the soil. The desorption step should be improved in order to apply EK process to the volume reduction of the decontamination waste.
Anion-exchange membranes for application to electrodialysis were prepared from commercially available polyethylene films using electron-beam-induced（EB-induced）graft polymerization. Two kinds of polyethylene, namely, high-density（HDPE）and ultra-high molecular weight polyethylene（UHMWPE）, were adopted as the starting films. Chloromethylstyrene（CMS）was graft-polymerized onto the film by immersing EB-preirradiated PE film in a CMS/xylene solution. After that, the CMS-grafted film was immersed in an aqueous solution of trimethylamine to introduce the anion-exchange group. The ion-exchange capacity, water content, membrane resistance, and tensile strength were measured, along with the evaluation of performance of electrodialysis of seawater. The tensile strength of an anion-exchange membrane prepared using a HDPE film（3000 ～4000 N/cm 2 ）and a UHMPE film（4500 ～ 6000 N/cm 2 ）were found to be comparable to that of a commonly used anion-exchange membrane. An electrodialyzer fabricated using a membrane pair consisting of the developed anion-exchange membrane and the commercial cation-exchange membrane, Selemion ® CSO, exhibited a higher concentration of chloride in the concentration chamber than a pair of Selemion CSO and ASA, where 0.5 mol/L NaCl aqueous solution was used as a seawater model. The chloride concentration of the developed anion-exchange membrane was about 5 ～ 10 ％ higher than that of commercially available membranes.
We investigated a novel technique for the production of pure-carbonated water using bipolar membrane electrodialysis accompanied by a chemical reaction. The electrodialyzer was composed of Feed, Strip, and Electrode compartments, which were divided by bipolar and cation-exchange membranes. A sodium carbonate aqueous solution was injected into those compartments. When a voltage was applied to the electrodialyzer, hydrogen ions were generated by water splitting in the bipolar membrane, and produced carbon dioxide by means of chemical reaction with the carbonate ion. The sodium ion also simultaneously moved from the Feed to Strip compartments. Consequently, pure carbonated water was produced. However, generation of carbon dioxide bubbles was barely observed during electrodialysis under the condition of 10 A/m2. These results suggest that generation and absorption of carbon dioxide gas occur mainly in the bipolar membrane.
A multiple-effect diffusion still whose upstream surface was heated at 100 ℃ and downstream surface was insulated, was proposed to improve performance, and a steady-state two-dimensional analytical model was constructed for the still. The numerical analyses showed that solution and concentrate flowing down each effect increase in temperature and store heat in the heated region, and then decrease in temperature and release heat, causing additional distillation in the insulated region. The proposed 10-effect still increases the heat ratio（HR）of total evaporation to the supply for the still by 24 ％, and the total evaporation rate per the length of the heated region by 50 ％. However, the total evaporation rate per total length of the heated and insulated regions is decreased by 25 ％. With an increase in the concentration ratio of the influent to effluent from 2 to 4, or in temperature of the influent from 30 ℃ to 60 ℃, the proposed still improves HR greatly.
Reverse electrodialysis（RED）is a process that can generate electric power from salinity gradients. For practical operation of a RED system, the influence of multivalent ions in the feed solutions for the system should be examined. In this study, in order to evaluate the effect of divalent ions on the RED performance more quantitatively, we have evaluated the relationship between the cation concentration in a cation exchange membrane（CEM）and molar fraction of divalent ions in a mixed solution of NaCl and MgSO 4 . As predicted from the Donnan equilibrium theory, the partition coefficient of Mg ions in the CEM was higher than that of Na ions. It indicates that the increase of membrane resistance in the mixed solution will be mainly due to the increase in the concentration of Mg ions with low ionic mobility in the CEM.