Ab Initio Calculations of Partial Charges at Kaolinite Edge Sites and Molecular Dynamics Simulations of Cation Adsorption in Saline Solutions at and above the pH of Zero Charge

Molecular dynamics simulations are used to study adsorption of cations on the (010) kaolinite edge surface at and above the pH of zero charge. The cation solutions are highly concentrated and include alkali and alkaline-earth metals. It is known that the pH-dependent edge surface of kaolinite is more reactive than the basal surfaces and more eager to adsorb metal ions; however, knowledge of the atomic scale is scarce regarding the structure of the surface edge, charge distribution, solvation, and structure of layers of adsorbed cations. First, ab initio calculations are used to determine the energetically most favorable surface terminations and the distribution of partial atomic charges on both neutral (protonated) and negatively charged (deprotonated) edge surfaces of kaolinite. Then, molecular dynamics simulations are used to study the solvation of kaolinite and the adsorption of cations. Results include density profiles of adsorbed cations, orientation profiles of water molecules close to the mineral surface for different cations, and the distance at which such surfaces become neutral or reverse their charges. Results compare well with available experimental and simulation data. Findings are expected to contribute to the selection or design of organic compounds that effectively adhere to kaolinite in aqueous electrolyte solutions in water recovery processes.