Experimental Study of Cation Diffusivities in Homoionized Montmorillonite by Electrical Conductance Method
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EXPERIMENTAL STUDY OF CATION DIFFUSIVITIES IN HOMOIONIZED MONTMORILLONITE BY ELECTRICAL CONDUCTANCE METHOD MASAHIDE OKAMOTO, TOSHIO TAKASE* and KUNIO HIGASHI Department of Nuclear Engineering, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606, Japan ABSTRACT Montmorillonite purified from bentonite was homoionized by leaching with a chloride solution of sodium, potassium, cobalt or strontium, and then leaching with ethanol until the leachate contained only a negligible amount of the cation. The electrical conductances of thus obtained Na-, K-, Co- and Sr- homoionized montmorillonite were measured. Applying the Nernst- Einstein equation to the electrical conductance, the diffusion coefficient for cations attached to the solid phase (montmorillonite) Ds, and that for cations in the bulk water phase Dw, were evaluated independently. As predicted, Ds was almost independent of the solid/liquid ratio, while Dw increased rapidly with the decrease of the ratio towards the diffusion coefficient in free water. INTRODUCTION Compacted bentonite has been proposed as an effective backfill material in the repository of high level radioactive waste (HLW). Because diffusion is recognized as the most important mode of ion transport in bentonite, it is essential to study on the diffusive behavior of various ions in bentonite. A few methods have been applied to measure the diffusion coefficients of ions in bentonite. Observation of the change of concentration profile with time is the most direct method[1,2,3]. This method has been used successfully to examine the selfdiffusion coefficient of water in bentonite[4]. Usually, however, it takes long time to obtain diffusion coefficients of cations of high sorption. In such cases, the electrical conductance method[5] may be useful. The diffusion coefficients can be determined by applying the Nernst-Einstein equation[6] to the obtained electrical conductance. Bentonite is composed of montmorillonite and minor minerals. Exchangeable cation compositon is variable. So, we performed the experiments of electrical conductance method after preparing homoionic montmorillonite to investigate the fundamental *Present Address: Mitsubishi Materials Corp., 1- 3- 25 Koishigawa, Bunkyo- ku, Tokyo 112, Japan Mat. Res. Soc. Symp. Proc. Vol. 353 @1995 Materials Research Society
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aspects of diffusion in bentonite and to reduce the ambiguities resulting from the composition of minerals and different exchangeable cations. Water- saturated montmorillonite was prepared using a solution of the same cation to the homoionic montmorillonite. Thus, a single kind of cation contributed to the electrical conductance of wet montmorillonite. Using such prepared montmorillonite, we estimated the diffusion coefficients of cations attached to the solid phase (montmorillonite), as well as of those in bulk water phase. EXPERIMENTAL The purified montmorillonite was supplied by Hojun Kogyo Co. Ltd.. Sodium homoionized montmorillonite (Na-HM) was prepared as follows ; (D 100ml of INNaCl solution was added to ig of montmorillonit
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