Ion adsorption on charged mineral surfaces influences the mobility of the solutes in natural waters. Understanding the adsorbed ion distribution can provide insight into the mechanisms. In dilute aqueous conditions, the distribution of ions adsorbed at a charged interface can be described by classical electrical double layer (EDL) theories. In brine conditions, however, the interfacial structure is less well understood because of increased ion–ion interactions. To determine the dependence of the EDL structure on the salinity of water, we systematically investigated the distribution of Rb<sup>+</sup> adsorbed at the negatively charged muscovite (001)–water interface as a function of dissolved RbCl or RbI concentration. In situ high-resolution X-ray reflectivity and molecular dynamics simulations revealed non-classical variations in adsorbed Rb coverage as a function of dissolved Rb concentration ([Rb]). In both RbCl and RbI solutions, the adsorbed Rb coverage increased with increasing [Rb] until it reached a plateau at [Rb] ≈ 1 mM, where the saturated coverage matched the amount needed for compensating the muscovite surface charge. For [Rb] ≥ 20 mM, the Rb coverage began to increase again, indicating the overcompensation of the surface charge. The EDL structure associated with this charge overcompensation is characterized by discrete layers of alternating cations and anions, which readily transformed into a crystalline phase in RbI solutions even when they were significantly undersaturated with respect to the secondary crystal. Overall, these results demonstrate intrinsic complexities of ion adsorption structures in brine conditions, which can be considered in geochemical sorption models.

Salinity dependent evolution of the electrical double layer at the muscovite (001) water interface

ACS Spring Meeting 2021

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