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

Abstract

The residual and release of ammonium in ion-adsorption rare earth tailings and soil are related to the interaction of NH4+ with clay minerals. Molecular dynamics simulations were used to study the adsorption and diffusion of NH4+ 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 NH4+ concentration (from 0.64 to 1.60 mol⋅L−1), and anion types (Cl−, NO3− and SO42−) were considered. The results of NH4+ adsorption on the external surface were also compared with that in the interlayer. The layer charge of smectite significantly affected the interaction between NH4+ and smectite. With increasing charge density, the major NH4+ 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 NH4+ 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. NO3− promoted the adsorption of NH4+ as inner-sphere complexes relative to Cl− and SO42−. As the water content in the interlayer increased, the confinement effect weakened and the mobility of NH4+ increased, and the NH4+ adsorption in the interlayer exhibited similar characteristics to that on the external surface at a water content of 10 H2O⋅uc−1.