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H2O self-diffusion coefficient of water-rich MX-80 bentonite gels

Published online by Cambridge University Press:  09 July 2018

Y. Nakashima
Y. Nakashima*
Affiliation:
Exploration Geophysics Research Group, National Institute of Advanced Industrial Science and Technology (AIST), Higashi 1-1-1 Central 7, Tsukuba, Ibaraki 305-8567
*
*E-mail: [email protected]
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Abstract

MX-80 is a bentonite clay from the Clay Spur bed, Wyoming, USA, consisting of ~80 wt.% Na-rich montmorillonite and is a leading candidate material for use in engineered barriers within underground nuclear waste disposal sites in Switzerland and Sweden. The diffusive invasion of groundwater into the engineered barriers is likely to cause the dry high-density bentonite to swell and evolve into a less-dense, water-rich gel during re-submergence of the shafts and galleries. Data on H20 diffusivity within water-rich bentonite gels are therefore essential to the safe design of the engineered barriers. To address this need, the self-diffusion coefficient, D, of 1H2O molecules in water-rich MX-80 bentonite gels was measured using the pulsed-gradient spin-echo method of 1H nuclear magnetic resonance spectroscopy at 0.47 T; D was measured over a wide range of temperatures (20.6–70.1°C) and of bentonite volume fractions (0 to 17.2 vol.%). The results showed that D increased markedly to values of bulk water self-diffusivity as the bentonite volume fraction decreased towards 0 vol.%. The activation energy of the diffusion process for the gels was approximately equal to that for bulk water. As a result, the normalized H20 self-diffusivity, D/D0, was approximately independent of temperature, where D0 is D in bulk water. The D/D0 data for ≤6.39 vol.% bentonite fraction were successfully fitted to a porous clay gel model that involves unbound water molecules that diffuse in the pore space by avoiding randomly placed discoidal obstacles (clay particles) of a high aspect ratio. The D/D0 data for MX-80 were similar to data for SWy-2 (Wyoming bentonite from the Newcastle formation) and Kunigel-Vl (bentonite from Yamagata, Japan), suggesting that all three bentonites are equally suitable candidate materials, in terms of water diffusivity performance, for use within engineered barriers.

Keywords

back-filling materialbuffer materialColony mineMX80PGSE NMRporous mediarandom walksurface relaxivitytortuosity.
Type
Research Article
Information
Clay Minerals , Volume 41 , Issue 2 , June 2006 , pp. 659 - 668
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2006

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