Adsorption of ammonium ions onto the external surface of smectite: Effects of layer charge, concentration, anion and comparison with interlayer adsorption

The residual and release of ammonium in ion-adsorption rare earth tailings and soil are related to the interaction of NH ₄ ⁺ with clay minerals. Molecular dynamics simulations were used to study the adsorption and diffusion of NH ₄ ⁺ on the external surface of smectite. The factors, such as layer charge position (octahedral and tetrahedral substitution) and charge density (0.5, 1.0 and 1.5 e⋅uc ⁻¹, where uc denotes unit cell), initial NH ₄ ⁺ concentration (from 0.64 to 1.60 mol⋅L ⁻¹), and anion types (Cl ⁻, NO ₃ ⁻ and SO ₄ ²⁻) were considered. The results of NH ₄ ⁺ adsorption on the external surface were also compared with that in the interlayer. The layer charge of smectite significantly affected the interaction between NH ₄ ⁺ and smectite. With increasing charge density, the major NH ₄ ⁺ surface complexes were changed from outer-sphere to inner-sphere, resulting in increased adsorption density and stability, which was verified by calculated hydration structure and adsorption free energies. The inner-sphere NH ₄ ⁺ complexes were located above the centers of the hexagonal rings and the Si tetrahedras for montmorillonite at a charge density of 1.0 e⋅uc ⁻¹, while only located in the vicinity of tetrahedral substitutions for beidellite. NO ₃ ⁻ promoted the adsorption of NH ₄ ⁺ as inner-sphere complexes relative to Cl ⁻ and SO ₄ ²⁻. As the water content in the interlayer increased, the confinement effect weakened and the mobility of NH ₄ ⁺ increased, and the NH ₄ ⁺ adsorption in the interlayer exhibited similar characteristics to that on the external surface at a water content of 10 H ₂O⋅uc ⁻¹.