期刊
GEOCHIMICA ET COSMOCHIMICA ACTA
卷 73, 期 14, 页码 4034-4044出版社
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.gca.2009.04.012
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资金
- Agence Nationale pour la Gestion des Dechets Radioactifs (ANDRA)
- GNR PARIS
We study the origin of the ionic exchange enthalpy in montmorillonite clays using microcalorimetry measurements and molecular simulation. We first determine the standard reaction enthalpy for well-defined interlayer water contents. We then show by a detailed analysis based on thermodynamic cycles that replacing Na(+) ions by Cs(+) in the interlayer of montmorillonite clays is an endothermic process, and that the overall exchange is exothermic only because it is dominated by the exothermic replacement of Cs(+) by Na(+) in the aqueous phase. This conclusion from ionic exchange enthalpies supports the one of a recent study of the ionic exchange free energy by Teppen and Miller [Teppen B.J. and Miller D. M. (2006) Hydration energy determines isovalent cation exchange selectivity by clay minerals. Soil Sci. Soc. Am. J. 70(1), 31-40] and contradicts long-held views on the role of ion-clay interactions in determining the ionic exchange thermodynamics. This calls for a paradigm shift for the origin of this exchange: The driving force is the hydrophobicity of Cs(+) compared to Na(+) and not its affinity for clay surfaces. (c) 2009 Elsevier Ltd. All rights reserved.
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