Adsorption behavior of NH4+ and Mg2+ at kaolinite surfaces: Effect of the ion concentration, NH4+/Mg2+ mixing ratio, and layer charge

The adsorption behavior of NH₄⁺ and Mg²⁺ at kaolinite surfaces was investigated by using molecular dynamics (MD) simulations, considering the factors such as ion concentration, NH₄⁺/Mg²⁺ mixing ratio, and layer charge of kaolinite. The results showed that the increase in ion concentration did not affect the adsorption modes of NH₄⁺ and Mg²⁺ ions but promote the increase in the adsorption capacity. The total adsorption capacities of Mg²⁺ and NH₄⁺ were 3.25 × 10⁻⁶ and 2.85 × 10⁻⁶ μmol·mm⁻² at the ion concentration of 1.5 mol·L⁻¹, respectively. When NH₄⁺ and Mg²⁺ were co-adsorbed, they could inhibit the adsorption of each other at the surface of kaolinite, except that the inner-sphere (IS) adsorption of NH₄⁺ at aluminum hydroxyl (Al–OH) surface could be enhanced by the presence of Mg²⁺. Both NH₄⁺ and Mg²⁺ tended to adsorb at the siloxane (Si–O) surface of kaolinite rather than Al–OH surface. When layer charge occurred in kaolinite, a small number of Mg²⁺ began to adsorb in the IS complexes at 1.7 and 2.3 Å above the Al and O atoms of the lattice-substituted tetrahedra of the Si–O surface, and at 1.7 Å above the hexahedra of the Al–OH surface. However, most of NH₄⁺ were adsorbed in IS complexes at 1.7 Å above the center of the oxygen six-membered ring of the Si–O surface and above the hexahedron of the Al–OH surface. The adsorption capacity of Mg²⁺ changed little with the increase of layer charge density, while the IS and total adsorption capacity of NH₄⁺ increased significantly.