Molecular Simulation Study of Montmorillonite in Contact with Water

Abstract

Grand canonical Monte Carlo and molecular dynamics simulations were applied to understand the molecular mechanism of ion and water transport in montmorillonite clays as a function of relative humidity (RH). The variation of basal spacings of montmorillonite as a function of RH predicted based on the swelling free energy profiles was consistent with X-ray data. The hydration of the montmorillonite shows the following well-known order: Mg 2+ > Ca 2+ > Sr 2+ > Li + > Na + > K +. The relative contribution of water on external surfaces only becomes significant close to the saturation pressure. However, this behavior for K-montmorillonite starts to occur well below the saturation pressure due to the clay-swelling inhibition by potassium ions. The diffusion of water and ions generally increases with RH in all samples. However, for samples with weakly hydrated ions, the water mobility in thin films adsorbed on external basal surfaces of clay can be higher than that in the water-saturated mesopores. For a given RH, mobility of interlayer species is typically lower than that from the external surfaces. The results of the simulations were applied to interpret recent laboratory measurements of ion mobility with changing RH. We also assess the effect of layer charge distribution on such sorption and diffusion processes.

Layer Charge Effects on Adsorption and Diffusion of Water and Ions in Interlayers and on External Surfaces of Montmorillonite

“…For a given layer charge, the swelling process with respect to the basal distance and the H 2 O uptake was not much influenced by the layer charge distributions. 31 Factors such as the steric effects and the enhanced electrostatic interactions may play important roles in the diffusion process. We see that the self-diffusion coefficients of water and ions in the high-charge montmorillonite are generally lower than those of the corresponding low-charge system (see Tables S2-S5 and S7-S10, and Fig.…”

“…45,46 Recently, molecular simulations have been applied to study the influence of RH on the adsorption and dynamics of water and ions in low-charge (Wyoming-type) montmorillonite. 26,29,31 However, a detailed knowledge of the effect of factors such as layer charge on adsorption and diffusion properties of H 2 O and ions in smectites is still lacking.…”

“…Here we compare simulation data of low-charge (M 0.75/n (Si 7.75 Al 0.25 )(Al 3.5 Mg 0.5 )O 20 (OH) 4 )and high-charge (see above) montmorillonite clays. GCMC simulations of this lowcharge montmorillonite (Wyoming-type) in contact with water can be found elsewhere 31. Note that a rigid framework of clay was employed in all our previous work 31.…”

“…GCMC simulations of this lowcharge montmorillonite (Wyoming-type) in contact with water can be found elsewhere 31. Note that a rigid framework of clay was employed in all our previous work 31. Furthermore, as in the case of the high-charge system, flexible models of water and clay are used here for new MD simulations of the low-charge system.The comparison shows that the high-charge clay generally shifts swelling transitions toward lower RH values consistent with experiments6,13 (see, for example,Tables 2 and S6).…”

Solvometallurgical process for extraction of copper from chalcopyrite and other sulfidic ore minerals

“…It is complementary to hydrometallurgy and pyrometallurgy. The examples of solvometallurgy are solvoleaching, 19,20 nonaqueous solvent extraction [21][22][23][24] and non-aqueous electrodeposition. 25 If ionic liquids or deep-eutectic solvents are employed, one also speaks of ionometallurgy.…”