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.

Automated summary

Classical molecular dynamics simulations were used to study the effects of layer charge density and location on NH4+ and water structure, energetics, and dynamics in hydrated NH4+ smectites. Results showed that increasing charge density decreased layer spacing and influenced hydration and binding affinity of NH4+ in smectites. The layer charge also affected NH4+ diffusion coefficient in the interlayer regions, with higher charge density inhibiting NH4+ mobility. This study provides insights for understanding the swelling process of clay and NH4+ release during rare earth mining.