Quantification of Exchangeable Cations in Interlayer of Tsukinuno Sodium-Montmorillonite
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Quantification of Exchangeable Cations in Interlayer of Tsukinuno Sodium-Montmorillonite Haruo Sato1 1 Japan Atomic Energy Agency, 432-2 Hokushin, Horonobe-cho, Hokkaido 098-3224, Japan ABSTRACT In this study, the cation exchange capacity (CEC) and leached exchangeable cations (LC) of montmorillonite purified from bentonite produced in the Tsukinuno bentonite mine, Yamagata, Japan, were measured, and the exchangeable cations in the interlayer of the montmorillonite were discussed. A montmorillonite, in which the soluble minerals were completely removed, was prepared. Kunipia-F and Kunipia-P, for which both bentonites originally contain approximately 100 wt.% montmorillonite, were used as the initial material. All of the measurements were carried out in a N2 atmosphere-controlled glove-box. The CEC values of montmorillonites for both bentonites (100-110 meq/100g) were similar to data conventionally reported, and the sum of LC was also approximately in good agreement with the CEC values. The share of Na+ in the interlayer of montmorillonite calculated from the LC was about 3/4 of the sum of the LC (≈ CEC), and Mg2+ and Ca2+ occupied about 7 and 19 %, respectively. Although montmorillonite in bentonite produced in the Tsukinuno bentonite mine is known as a Na type, the sum of Ca2+ and Mg2+ occupied about 26 % of all exchangeable cations in the interlayer. Based on these data , the ion exchange reaction constant between Na+ and H+ in the interlayer of montmorillonite was calculated to be -0.07. This is nearly 2 orders of magnitude lower than data that are usually adopted. INTRODUCTION Bentonite is used as backfilling material and buffer material composing engineered barriers of the geological disposal for high-level radioactive waste, and many functions such as self-sealing by swelling, chemical buffering by the dissolution of soluble salts, retardation of nuclide migration by ion exchange, low-permeability and so on, are expected. Data concerning fundamental properties, nuclide migration properties and so on, have been obtained under various conditions, taking into account effect for long-term behaviour, in addition to effects for variations of the geological environmental conditions and designing condition of engineering barriers. The type of exchangeable cation and its share in the interlayer of montmorillonite, which is the major clay mineral constituent of bentonite, are known to affect various properties such as porewater chemistry in addition to the above functions, and are the important fundamental data for quantitatively analyzing the effect on the above properties and for enhancing the reliability of the related models. In this study, the cation exchange capacity (CEC) and leached exchangeable cations (LC: Na+, K+, Ca2+, Mg2+) of montmorillonite in bentonite produced in the Tsukinuno bentonite mine, Yamagata, Japan, are precisely measured, and the exchangeable cations in the interlayer of the montmorillonite are discussed. The CEC and LC of bentonite produced in the Tsukinuno bentonite mine have been reported
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