Molecular dynamics simulation of NH4+-smectite interlayer hydration: Influence of layer charge density and location

Classical molecular dynamics (MD) simulations were performed to quantify the possible effects of layer charge density and location on NH4+ and the water structure, energetics and dynamics of hydrated NH4+ smectites. The results showed that the layer spacing decreased with increasing charge density. Hydration occurred more easily in NH4+-smectites with tetrahedral substitution than those with octahedral substitution. Stable monolayer hydrates and hypothetical bi- and trilayer hydrates were found at a water content of 4, 10 and 15 H2O center dot uc(-1) (H2O per unit cell), respectively. The binding affinity of the surface towards NH4+ increased as the charge density of the smectites increased. For low- and medium-charge smectites (sigma = -0.5 and -1.0 e.uc(-1)), the affinity was also dependant on the hydration level: some of the NH4+ in inner-sphere complexes with the surface turned into outer-sphere complexes and even formed bulk water-like structures as hydration levels increased. However, for high-charge smectites (sigma = -1.5 e.uc(-1)), NH4+ was still present in inner-sphere complexes with the oxygens of the surface hexagonal cavities regardless of hydration level. The layer charge also affected the NH4+ diffusion coefficient in the interlayer regions; NH4+ in T-substituted smectites diffused more slowly than in O-substituted ones and increasing the charge density of smectites inhibited the mobility of NH4+. Quantification of these interactions can provide a basis for understanding the swelling process of clay and the release process of NH4+ during the ion-type rare earth mining. (C) 2021 Elsevier B.V. All rights reserved.