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TRACER DIFFUSION IN COMPACTED, WATER-SATURATED BENTONITE

¹ Civil and Environmental Engineering, Davis Hall # 1710, University of California, Berkeley, CA 94720-1710, USA
² Environmental HydroGeochemistry (LHGE-JE2397), Université de Pau et des Pays de l’Adour, BP 1155, 64013 Pau Cedex, France
³ Geochemistry Department, Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
⁴ ANDRA, Parc de la Croix Blanche, 1/7 rue Jean Monnet, 92298 Châtenay-Malabry cedex, France

^* E-mail address of corresponding author: ibourg{at}nature.berkeley.edu

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.

Key Words: Bentonite • Diffusion • Geometric Factor • Interlayer • Montmorillonite • Nanopore • Smectite • Waste Containment

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				G. Kosakowski, S. V. Churakov, and T. Thoenen DIFFUSION OF Na AND Cs IN MONTMORILLONITE Clays and Clay Minerals, April 1, 2008; 56(2): 190 - 206. [Abstract] [Full Text] [PDF]