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

Abstract

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 (Sr 2+ ) 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 Sr 2+ 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 Sr 2+ 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 2+ 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 Ba 2+ , implying fast exchange among adsorbed Sr 2+ species. The Sr 2+ uptake measured by RAXR followed a Frumkin isotherm defined with an apparent free energy of sorption, ΔG Sr ≈ − 22 kJ/mol, and an effective attractive interaction constant, γ ≈ − 4.5 kJ/mol, between sorbed Sr 2+ . While the observed free energy can be mostly explained by the (CMD) Helmholtz free energy of adsorption for Sr 2+ , ΔF 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 Sr 2+ sorption behavior at the barite (001) surface.

Studies of Mineral Nucleation and Growth Across Multiple Scales: Review of the Current State of Research Using the Example of Barite (BaSO4)

“…Formation of the barite (001) surface breaks three bonds to the topmost barium ion (high position, Ba high ) and one bond to the second topmost barium ion (low position, Ba low ), leaving these two ions undercoordinated with respect to their bulk coordination environments (Figure ). This leads to minor displacements of barium and sulfate groups, adsorption of water molecules, and adsorption of ions. − Water molecules adsorb directly to one or two of the surface sites, fully or partially completing the coordination shell of surface bariums (Figure ). Adsorbed waters form a primary hydration layer that is split into two distinct sublayers with a 2D density of 12.0 H 2 O/nm 2 , similar to that of other minerals such as calcite (10.9 H 2 O/nm 2 ) and muscovite (10 H 2 O/nm 2 ) .…”

“…Sr 2+ and Pb 2+ , which can substitute for Ba 2+ in the barite crystal lattice, are known to specifically sorb to sites at the barite (001) surface , and precipitate as epitaxial sulfate mineral films. , The two ions have similarities in how they interact with the barite (001) surface (Figure ). Both adsorb inner-sphere, presumably to oxygens on the surface sulfate groups.…”

“…Comparison of the Sr and Pb coverage and positions at 225 μM. Significantly more Pb is adsorbed and incorporated than Sr. Data from Bracco et al, 2019 and Bracco et al, 2020 …”

The Effect of Ionic Strength and Sraq upon the Uptake of Ra during the Recrystallization of Barite

“…The free energy landscape for metal adsorption of Sr 2+ indicated distinct inner-sphere sites with additional outer-sphere sites which are energetically less favourable. In particular, the energy barrier for Sr 2+ was observed to be lower than for Ba 2+ which indicates a fast exchange among the adsorbed Sr 2+ species [44]. Based on these results, it is most likely that the presence of high Sr aq concentrations is unfavourable for Ra-uptake because here the competition between Ra and Sr in solution for sulphate binding sites is won by Sr.…”

“…Recent studies using in situ resonant anomalous X-ray reflectivity (RAXR) in combination with density functional theory (DFT) and classical molecular dynamics (CMD) showed the adsorption of Sr as inner-sphere complexes and the incorporation into the outermost barite layers [44]. The free energy landscape for metal adsorption of Sr 2+ indicated distinct inner-sphere sites with additional outer-sphere sites which are energetically less favourable.…”