Simultaneous Adsorption and Incorporation of Sr2+ at the Barite (001)-Water Interface

Ionically bonded minerals are ubiquitous and play a determinative role in controlling the mobility of toxic metals in natural environments. However, little is known about the mechanism of ion uptake by these mineral surfaces. Here, the sorption of strontium ions (Sr2+) to the barite (001) water interface was studied using a combination of synchrotron X-ray scattering and three types of computational simulations (density functional theory, classical molecular dynamics (CMD), and CMD-metadynamics). In situ resonant anomalous X-ray reflectivity (RAXR) revealed that Sr2+ adsorbed on the barite surface as inner-sphere surface complexes and was incorporated within the outermost barite atomic layers. Density functional theory combined with CMD simulations confirmed the thermodynamic stability of these species, demonstrating almost equal magnitudes in the free energy of sorption between these species. Metadynamics simulations showed a more detailed feature in the free energy landscape for metal adsorption where adsorbed Sr2+ are stabilized in as many as four distinct inner-sphere sites and additional outer-sphere sites that are more diffuse and less energetically favorable than the inner-sphere sites. All three techniques confirmed Sr' adsorbs inner-sphere and binds to oxygens in the top two surface sulfate groups. The energy barriers among the inner-sphere sites were significantly lower compared with those for constituent cation Ba2+, implying fast exchange among adsorbed Sr2+ species. The Sr2+ uptake measured by RAXR followed a Frumkin isotherm defined with an apparent free energy of sorption, Delta G(sr) approximate to -22 kJ/mob and an effective attractive interaction constant, gamma approximate to -4.5 kJ/mol, between sorbed Sr2+. While the observed free energy can be mostly explained by the (CMD) Helmholtz free energy of adsorption for Sr2+, Delta F-sr = -15.3 kJ/mol, the origin of the sorbate sorbate correlation could not be fully described by our computational work. Together, these experimental and computational results demonstrate the complexity of Sr2+ sorption behavior at the barite (001) surface.