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 exposed to aqueous environment. Our simulations show that potassium counterions leach out into the solution much earlier and in higher concentration when compared with aluminum and silicon, which form the structural network of illite. 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 also estimated the kinetics of surface reactivity from the MD simulations and explored its effectiveness as a surrogate model for leaching kinetics. However, surface reaction kinetics and experimentally measured leaching kinetics seemed to be off by several orders of magnitude. We also analyzed the effects of leaching on structural distortion and found that more than 20% leaching is required for a notable structural distortion in illite crystal.