Adsorption of Mg2+ and K+ on the kaolinite (0 0 1) surface in aqueous system: A combined DFT and AIMD study with an experimental verification

Microscopic adsorption mechanism of Mg2+ and K+ on kaolinite (0 0 1) surface in aqueous system was investigated by density functional theory (DFT) calculations and ab-initio molecular dynamics (AIMD) simulations. A verification experiment has also been done to compare the adsorption capacity of kaolinite to the two ions. The structure for both hydrated complexes and mono-/bi-dentate adsorption complexes of Mg2+ and K+ in aqueous environment was examined, with radial distribution function and binding energies calculated. [Mg(H2O)6]2+ and [K(H2O)6]+ are the dominant complexes of hydrated Mg2+ and K+, respectively. Bidentate complexes of Mg2+ and monodentate complexes of K+ are their respective dominant complexes, and the binding energies of all the adsorption complexes of Mg2+ are higher than those of K+. Partial density of states projections combining with Mulliken bond charge and populations show the strong ionic characters and weak bonding and anti-bonding states filling of Mg − Os and K − Os in the adsorption complexes. The adsorption experiment verified that Mg2+ is easier to adsorb on kaolinite surface than K+.