Understanding the chemistry of cation leaching in illite/water interfacial system using reactive molecular dynamics simulations and hydrothermal experiments

Despite long standing pursuit, fundamental questions concerning the chemical pathways of leaching of ions in minerals, a phenomenon crucial to energy extraction, hydrometallurgy, metal recovery, and agriculture remain unanswered. Here we use large-scale ReaxFF reactive molecular dynamics (MD) simulations in combination with hydrothermal experiments to understand the chemistry of leaching in illite in contact with water. Our simulations under equilibrium conditions show that 12.58% of potassium cations leach out to the solution. On the other hand, aluminum and silicon, which form the structural network of illite, are far less leachable as only 0.51% and 2.86% respectively of Al and Si leach out. Upon analyzing the chemical pathway from the trajectory of MD simulations, water molecules supply protons near the illite surface that binds with the non-bridging oxygen (NBO) of the Al-O-Si linkage forming an [Al-O-Si]---H transition state that later converts to silanol group upon Al-O bond dissociation. Proton addition also weakens the interlayer K-O bonds, resulting in the diffusion of K+ ions to illite surface, where they combine with the hydroxyl group formed from water dissociation, to form KOH molecules. KOH molecules diffuse out reactively to bulk water via proton exchange mechanism. Furthermore, we also find that continued protonation results in the formation of Al(OH)3 and Si(OH)4 groups predominantly at the surface, which diffuse out into water resulting in the leaching of Al and Si cations. We observe excellent agreement in the equilibrium cation concentrations of ReaxFF simulations and hydrothermal experiments. Reactive MD methodology with reliable, ab-initio based forcefields is, therefore, a promising approach to understand chemistry of water dynamics and ion transport and can be extended to similar mineral/water interfacial systems.