A Thermodynamic Analysis on the Effect of Salinity on Interlayer Space of Na-Montmorillonite
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A Thermodynamic Analysis on the Effect of Salinity on Interlayer Space of Na-Montmorillonite 1
Haruo SATO 1 Graduate School of Natural Science and Technology, Okayama University, 3−1−1, Tsushima-naka, Kita-ku, Okayama-shi, Okayama 700-8530, Japan ABSTRACT Bentonite is used as one of the materials for engineered barrier systems in a radioactive waste repository. Since the major clay mineral constituent of bentonite is montmorillonite, its physico-chemical properties are important. Basal spacing of water-saturated Na-montmorillonite is reported to decrease with increasing Na-montmorillonite density. This paper presents a thermodynamic model to calculate change in the interlayer space of Na-montmorillonite based on the relative partial molar Gibbs free energy (dG) of interlayer water as contacted with a solution of an arbitrary salinity (NaCl concentration). Directly change in montmorillonite density (Udm) against salinity was calculated by the thermodynamic model. The dG of interlayer water as contacted with a solution of an arbitrary salinity can be calculated by dG = dGH2O + dGS (dGH2O: relative partial molar Gibbs free energy of interlayer water, dGS: that of water in a solution of an arbitrary salinity). The author previously reported an empirical correlation of dGH2O vs. water content for Na-montmorillonite. The dependence of Udm on salinity was calculated by replacing dGH2O in the empirical correlation with dG. Udm increased with salinity. Concretely, initially the Udm-values of 0.5 and 1.0 Mg/m3 increased to 1.05 and 1.16 Mg/m3 under 0.5 m-NaCl, respectively. Interlayer space vs. salinity was estimated based on the measured results of basal spacing vs. Udm by XRD and the average density of montmorillonite vs. salinity calculated by this model. INTRODUCTION In the safety assessment of the geological disposal for high-level radioactive waste in Japan, the multi-barrier system controls the release of radionuclides leached from vitrified waste to the surrounding geosphere. Na-bentonite is particularly one of the important engineered barrier materials, and many functions, such as self-sealing, chemical buffering, retardation of nuclide migration, formation of a diffusion controlled, low-permeable barrier, etc., are expected. Since the major clay mineral constituent of bentonite is montmorillonite having the nature of high swelling, understanding the physico-chemical properties of montmorillonite is important. In this study, the effect of salinity (NaCl concentration) on the interlayer space of Na-montmorillonite is discussed. The basal spacing of water-saturated Na-montmorillonite, derived from X-ray diffraction (XRD) measurements [e.g., 1-3] has been reported to be 3.52 (9 water-layer) and 1.88 nm (3 water-layer) in a dry density range of 0.6−1.0 Mg/m3, 1.88 nm in a dry density range of 1.0−1.3 Mg/m3, a mixture of 1.56 (2 water-layer) and 1.88 nm in a dry density range of 1.3−1.6 Mg/m3 and 1.56 nm in a dry density range ≥ 1.6 Mg/m3. The basal spacing of 3.52 nm found in the lower dry density region is considered t
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