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

Montmorillonite (MMT) is the main mineral component of bentonite clay, which is currently proposed as a sealing material for deep geological repositories (DGRs) of used nuclear fuel. If copper-cladded used fuel containers corrode, 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$^{2+}$ interact 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 possesses swelling pressures intermediate between those of Ca-MMT and Na-MMT. Furthermore, our MD simulations suggest Cu$^{2+}$ possesses MMT interlayer mobility similar to that of Ca$^{2+}$.