## 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.

Original language | English (US) |
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Pages (from-to) | 363-374 |

Number of pages | 12 |

Journal | Clays and Clay Minerals |

Volume | 54 |

Issue number | 3 |

DOIs | |

State | Published - Jun 1 2006 |

## All Science Journal Classification (ASJC) codes

- Water Science and Technology
- Soil Science
- Geochemistry and Petrology
- Earth and Planetary Sciences (miscellaneous)

## Keywords

- Bentonite
- Diffusion
- Geometric factor
- Interlayer
- Montmorillonite
- Nanopore
- Smectite
- Waste containment