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高エネルギー重イオンビームによる海水濃縮用カチオン, アニオン交換膜の開発

Development of cation and anion exchange membranes for saline water concentration using high-energy heavy-ion beams

澤田 真一*; 安川 政宏*; 越川 博*; 喜多村 茜; 比嘉 充*; 八巻 徹也*

Sawada, Shinichi*; Yasukawa, Masahiro*; Koshikawa, Hiroshi*; Kitamura, Akane; Higa, Mitsuru*; Yamaki, Tetsuya*

日本における食塩生産は、カチオン交換膜(Cation Exchange Membrane: CEM)とアニオン交換膜(Anion Exchange Membrane: AEM)を用いた電気透析による海水濃縮工程を経る。この海水濃縮工程の性能はCEMとAEMの輸送特性に大きく依存する。高濃度の濃縮海水を得るためには、より低い電力での電気透析を実現する膜の抵抗が低い方が好ましい。しかし一般的に、低抵抗の膜では含水による膨潤が起こりやすく、輸率は下がって水透過率は上がるため濃縮海水濃度が低下する。このように電解電力と濃縮海水濃度はトレードオフの関係にあり、この関係を打破するためには、低い抵抗、高い輸率、低い水透過率を兼ね備えたCEMとAEMが必要である。そこで我々は、重イオンビームを利用したイオン飛跡グラフト重合法により、現行のCEMとAEMよりも抵抗が低く、高輸率と低水透過率を兼ね備えた新規膜の開発を進めている。本稿では、イオン飛跡グラフト重合法を概説し、本手法によるCEM及びAEMの作製方法、輸送特性についてそれぞれ説明し、海水濃縮試験の結果を報告する。

For applications to saline water concentration by electrodialysis, we prepared nano-structure-controlled cation and anion exchange membranes (CEMs and AEMs) by a so-called ion-track grafting technique. This new technique involves irradiation of a polymer substrate with an MeV-GeV heavy-ion beam to form the nano-sized cylindrical ion tracks and the graft polymerization only into the ion tracks for the creation of one-dimensional transport pathways. A 25-$$mu$$m-thick poly(ethylene-co-tetrafluoroethylene) film was irradiated with 560 MeV $$^{129}$$Xe or 310 MeV $$^{84}$$Kr. The irradiated films were immersed in grafting solutions of ethyl p-styrenesulfonate (EtSS) and chloromethylstyrene (CMS),and then subjected to the hydrolysis of EtSS units and quaternization of CMS units to prepare CEMs and AEMs, respectively. These CEMs and AEMs showed lower resistance than the commercially-available membranes even at the very low water uptake. This would be due to the signifcantly-effcient transport of ions through the unique one-dimensional highly-connected transport pathways. In the saline water concentration experiment, a pair of our CEM and a commercial AEM or vice versa led to a higher salt concentration in the concentration chamber than did a pair of the commercial membranes. This result demonstrated great applicability of our ion-track-grafted CEMs and AEMs for saline water concentration.

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