Water transport pathway in clay interlayer upon dehydration

Abstract:

Smectite dioctahedral clays such as montmorillonite (MMT) are swelling phyllosilicate minerals found in bentonites. The latter have been considered as key backfill barrier materials in deep geological nuclear waste repository concepts. Many X-ray diffraction (XRD) investigations of smectite swelling indicate that the interlayer can accommodate zero (0W), one (1W), two (2W) or three (3W) hydration layers, depending on the relative humidity (RH). However, how water molecules move into and from the interlayer (i.e., water transport mechanism) during smectite hydration and dehydration remains elusive. Using molecular dynamics (MD) simulations and thermal analysis experiments, we demonstrate that the clay dehydration process from 2W to 0W structure can be divided into two stages. In the first stage, the mass loss is linear with time because water transport is fast from hydrophobic regions in the interlayers. The number of water molecules in the first hydration shell of the interlayer ion (Na⁺) remains rather constant. The second stage is associated with water movement from hydrophilic regions through the interlayer via diffusive mechanism (mass loss is not linearly proportional to time). The water coordination number of Na⁺ steeply decreases, followed by a gradual reduction. The collapse of the hydrated MMT interlayer structure from 2W to 1W, and to 0W states is strongly correlated with the decrease in the coordination number of the interlayer ion. Our results provide molecular insights into the effect of water transport mechanisms on the transformation of the MMT hydrated structure.

Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. This work was supported by the DOE Spent Fuel Waste Science & Technology (SFWST) Program.