IngentaConnect Tracer diffusion in compacted, water-saturated bentonite

Authors: Bourg, Ian C.; Sposito, Garrison; Bourg, Alain C.M.

Source: Clays and Clay Minerals, Volume 54, Number 3, June 2006 , pp. 363-374(12)

Publisher: The Clay Minerals Society

Abstract:

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.

Keywords: BENTONITE; DIFFUSION; GEOMETRIC FACTOR; INTERLAYER; MONTMORILLONITE; NANOPORE; SMECTITE; WASTE CONTAINMENT

Document Type: Research article

DOI: 10.1346/CCMN.2006.0540307

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