Investigating the effect of Na⁺, Ca²⁺, and Cu²⁺ sorption in montmorillonite using density functional theory and molecular dynamics simulations

Abstract:

Bentonite is considered as a sealing and adsorbent material in deep geological repositories (DGRs) for used nuclear fuels. Proposed DGRs include multiple protective barriers including radioactive waste form, a copper-coated steel container encased in bentonite clay, and bedrock. Bentonite clay typically contains 80% montmorillonite (MMT) minerals, making the fundamental properties of bentonite largely reliant on MMT. MMT is comprised of two silicon tetrahedral layers (T) and one aluminum octahedral layer (O) to form TOT layers at nanoscales. Key to MMT’s behavior is its net negative charge, attributed to the isomorphic metal substitution. Within the T layers, Si⁴⁺ can be substituted by Al³⁺ or Fe³⁺, while in the O layers, Al³⁺ can be replaced by Mg²⁺ or Li⁺. These charges are balanced by hydrated cations such as Na⁺ and Ca²⁺, leading to the expansion of MMT layers.

In deep DGRs, copper-coated canisters are expected to encounter different environments, transitioning from an oxic environment for up to a few months to anoxic conditions thereafter. During the initial phase of the geologic repository, oxygen atoms will be consumed due to their reaction with copper, resulting in the formation of a surface film of Cu₂O or CuO/Cu(OH)₂. Once the oxygen atoms are consumed, the potential for copper corrosion emerges as a consequence of exposure to sulfide-containing groundwater. This exposure can lead to the release of Cu²⁺ ions into the surrounding MMT. Consequently, ion exchange may occur between Na+ within the MMT and Cu²⁺, which could in turn affect the structure of the MMT. Similarly, there may be exchanges between the Na⁺ and the Ca²⁺ present in the groundwater.

This study employs multiscale simulations, combining density functional theory (DFT) and molecular dynamics (MD) simulations, to investigate these phenomena. All-atom MD simulations provide insights into nanoscale interactions between MMT platelets containing different cations, revealing perspectives unattainable through traditional characterization techniques like electron microscopy and X-ray diffraction (XRD). MD relies on interatomic force-fields; to perform the research, existing force-fields need to be extended to account for Cu²⁺ on MMT surfaces. Our study follows a two-fold approach. Firstly, we parametrized and validated a novel interatomic interaction force-field for Cu²⁺ in clay systems, using DFT to extend the ClayFF model. Secondly, MD simulations are employed to calculate interaction energies between MMT platelets with Cu²⁺ and Ca²⁺ counter-ions. These results are compared with interaction energies between MMT platelets with Na⁺ counter-ions. The study also explores other atomistic properties, including cation diffusion between platelets and swelling pressure.

Short bio:

I am a PhD student in the Mechanical and Materials Engineering department at Queen’s University in Canada, working within the Computational Materials Physics research group under the supervision of Dr. Laurent Karim Béland. Currently, I am undertaking an internship at École des Ponts ParisTech. My research utilizes multi-scale modeling and simulation techniques to understand the interactions and behaviors of materials under various conditions. This approach helps bridge the gap between theoretical predictions and experimental findings, ultimately contributing to the advancement of materials science and engineering.