Investigating the effect of Cu$^{2+}$ sorption in montmorillonite using density functional theory and molecular dynamics simulations

Montmorillonite (MMT) is the main mineral component of bentonite, which is currently proposed as a sealing material in deep geological repositories (DGRs) for used nuclear fuel. In the Canadian program, which will utilize copper-cladded used fuel containers, safety analysis considers the effect of copper corrosion, during which Cu$^{2+}$ ions could potentially be adsorbed by the surrounding MMT. In such a scenario, ion exchange between Na$^+$ and Cu$^{2+}$ is expected. In this study, a multiscale approach that combines electronic density functional theory (DFT) and force-field-based molecular dynamics (MD) simulations was employed to study the effect of introducing Cu$^{2+}$ ions to MMT. An extension to the ClayFF force field is parametrized and validated using DFT to model how Cu$^{2+}$ interacts with clay systems. MD simulations were performed to calculate the interaction free energies between MMT platelets containing Cu$^{2+}$ ions (Cu-MMT) and compared them to inter-platelet interaction energies in Na-MMT and Ca-MMT. Our calculations suggest Cu-MMT develops swelling pressures between those of Ca-MMT and Na-MMT. Furthermore, our MD simulations suggest that Cu$^{2+}$ has MMT interlayer mobility that is significantly slower than that of Ca$^{2+}$.