(253x) Molecular Dynamics Simulation of  Cations (K+, Na+, Rb+, Mg2+ and Sr2+) Adsorption at the Muscovite Mica - Aqueous Solution Interface

Structure, hydration and distribution of ions at mineral-aqueous solution interface varies significantly over the dilute to higher concentration of the aqueous solution. Ions emerges from dissociated electrolytes, competes with the hydrated ions of minerals at the interface. A molecular level understanding of the competitive adsorption between these ions at the interface provides valuable inputs to the fields like soil enrichment, chemical weathering and remediation of heavy metals. In order to understand the properties of these ions we studied adsorption isotherms of pure solutions of chlorides (NaCl, RbCl, MgCl₂ and SrCl₂) on muscovite mica surface at various concentrations. The surface K⁺-ions are exchanged by the cations of aqueous solution depending upon their free energy. We observed that the adsorption position of cations K⁺, Na⁺, Rb⁺, Mg²⁺ and Sr²⁺ on the muscovite mica surface are at 1.8, 1.76, 1.87, 2.35 and 2.82Å respectively, in good agreement with experimental results ( Lee et al., 2013 and Park et al., 2006). The competitive adsorption between surface K⁺- ions and respective cations of the aqueous solutions was reflected at the different regions of the isotherm profiles. Our free energy calculations are able to explain the domination of K⁺- ions adsorption at the dilute concentrations and salting out phenomena as further increase in the concentration of the solutions. In addition to the adsorption isotherms and free energy calculations we evaluated the density distributions and the pair correlations to obtain better understanding.