Ion adsorption on mineral surfaces plays a critical role in controlling the fate of dissolved heavy metals in natural waters. Determining the adsorption structure can provide insight into the adsorption mechanism and, thereby, adsorption strength of ions at the interfaces. Here, we show how competing monovalent Rb+ and Na+ can influence the distribution and adsorption free energy of divalent Sr2+ at the muscovite (001)–water interface. Our previous study [1] revealed that Sr2+ adsorbs on the muscovite surface in approximately equal proportions of inner-sphere (IS) and outer-sphere (OS) surface complexes from a 10 mM Sr(NO3)2 solution without any other background electrolyte. A drastic change in adsorbed Sr2+ speciation was observed in the presence of 3 mM RbCl, where Sr2+ adsorbed on the surface predominantly (~90%) as OS complexes from solutions containing 0.1 to 20 mM SrCl2. This change in adsorbed Sr2+ speciation appears to influence its adsorption strength: the magnitude of the adsorption free energy for Sr2+ was reduced by 7 ± 1 kJ/mol compared to that without Rb+ when determined by the Frumkin isotherm at Γ(cSr) = ½Γmax (where Γ is the surface coverage of Sr2+ at a dissolved Sr2+ concentration, cSr, and Γmax is the maximum surface coverage). In comparison, such a background electrolyte effect was less pronounced with NaCl even with a higher background electrolyte concentration (75 mM), resulting in 70% OS surface complex and 5 ± 1 kJ/mol change in adsorption free energy for Sr2+ . These results clearly demonstrate the complexity and variability of adsorbed ion speciation at charged mineral–water interfaces, indicating a need for incorporation of this ion-specific interaction in transport models for better prediction of ion mobility in aqueous environments.

Effect of monovalent cations on the adsorption energetics and structure of Sr2+ at the muscovite (001)–water interface

2019 Fall National Meeting

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