Bulletin of the Society of Sea Water Science, Japan

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Anion-exchange membranes with permselectivity for mono-valent ions were prepared using the following four steps: （1） electron beam irradiation of an ultrahigh-molecular-weight polyethylene （UHMWPE） film, （2） graft polymerization using chloromethylstyrene （CMS） as a monomer, （3） reaction between three diamine crosslinkers, namely N,N,N’,N’-tetramethyl-1,2-ethylenediamin （TMEDA）, N,N,N’,N’-tetramethyl-1,4-buthanediamine （TMBDA） and N,N,N’,N’- tetramethyl-1,6-hexanediamine （TMHDA）, and CMS graft chains by specific crosslinking near both sides of the CMS-grafted UHMWPE film, （4） introduction of trimethylammonium groups into the remaining CMS graft chains by reaction in a trimethylamine aqueous solution. One of the three types of anion-exchange membranes obtained by treatment with TMEDA, TMBDA or TMHDA and the currently used cation-exchange membrane, SELEMION^®CSO, were installed in an electrodialyzer to evaluate chloride ion selectivity and the resulting brine concentration in the concentrating chamber. The anion-exchange membrane using TMHDA exhibited the highest chloride ion selectivity of the three kinds of anion-exchange membranes. The anion-exchange membrane using TMEDA showed the lowest selectivity. In particular, permselectivity for mono-valent ions of an anion-exchange membrane using TMHDA was equivalent to that using SELEMION^®ASA. In addition, regardless of the diamine crosslinker used, all anion-exchange membranes produced a brine concentration equivalent to that of SELEMION^®ASA.

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As applications of oxygen evolution anodes for seawater electrolysis outside of hydrogen production industry, we have proposed an application for electrodialysis method in salt production. This process causes anode chamber acidification and cathode chamber alkalization during electrolysis. This results in deposition of magnesium hydroxide on electrodialysis membrane in cathode chamber, which is degraded. In addition, the difference in pH between anode chamber and cathode chamber causes an increase in cell voltage theoretically. We have studied oxygen evolution anodes which do not make chlorine at all during seawater electrolysis, and also researched a seawater feeding method which consists of feeding from acidic solution in anode chamber to cathode chamber. In this paper, the oxygen evolution anode and seawater feeding system have been introduced into a conventional electrodialysis method. As a result, it has become possible to keep pH low in cathode chamber and electrolyze in the same pH solution for both chambers.

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