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Tracer Diffusion in Compacted, Water-Saturated Bentonite

Published online by Cambridge University Press:  01 January 2024

Ian C. Bourg*
Affiliation:
Civil and Environmental Engineering, Davis Hall #1710, University of California, Berkeley, CA 94720-1710, USAEnvironmental HydroGeochemistry (LHGE-JE2397), Université de Pau et des Pays de l’Adour, BP 1155, 64013 Pau Cedex, FranceGeochemistry Department, Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USAANDRA, Parc de la Croix Blanche, 1/7 rue Jean Monnet, 92298 Châtenay-Malabry cedex, France
Garrison Sposito
Affiliation:
Civil and Environmental Engineering, Davis Hall #1710, University of California, Berkeley, CA 94720-1710, USAGeochemistry Department, Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
Alain C. M. Bourg
Affiliation:
Environmental HydroGeochemistry (LHGE-JE2397), Université de Pau et des Pays de l’Adour, BP 1155, 64013 Pau Cedex, France
*
*E-mail address of corresponding author: ibourg@nature.berkeley.edu
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Abstract

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Compacted Na-bentonite clay barriers, widely used in the isolation of solid-waste landfills and other contaminated sites, have been proposed for a similar use in the disposal of high-level radioactive waste. Molecular diffusion through the pore space in these barriers plays a key role in their performance, thus motivating recent measurements of the apparent diffusion coefficient tensor of water tracers in compacted, water-saturated Na-bentonites. In the present study, we introduce a conceptual model in which the pore space of water-saturated bentonite is divided into ‘macropore’ and ‘interlayer nanopore’ compartments. With this model we determine quantitatively the relative contributions of pore-network geometry (expressed as a geometric factor) and of the diffusive behavior of water molecules near montmorillonite basal surfaces (expressed as a constrictivity factor) to the apparent diffusion coefficient tensor. Our model predicts, in agreement with experiment, that the mean principal value of the apparent diffusion coefficient tensor follows a single relationship when plotted against the partial montmorillonite dry density (mass of montmorillonite per combined volume of montmorillonite and pore space). Using a single fitted parameter, the mean principal geometric factor, our model successfully describes this relationship for a broad range of bentonite-water systems, from dilute gel to highly-compacted bentonite with 80% of its pore water in interlayer nanopores.

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Type
Research Article
Copyright
Copyright © 2006, The Clay Minerals Society