Molecular Models of Cesium and Rubidium Adsorption on Weathered Micaceous Minerals

Abstract

Understanding the adsorption mechanisms of metal cations onto soils and sediments is of critical importance in the protection of the environment, especially for the case of radioactive materials including the fission product (137)Cs. Mechanism-based adsorption models for the long-term interaction of chemical and radionuclide species with clay minerals are needed to improve the accuracy of groundwater reaction and flow models, as well as related simulations for performance assessment of waste sites and repositories. Toward this goal, molecular simulation using geometry optimization and molecular dynamics methods have been used to investigate the adsorption behavior of Cs(+) and Rb(+) cations at frayed edge wedges (a proxy for frayed edge sites, FES) and in the interlayer region formed as a result of the transformation of muscovite to Al-hydroxy interlayered vermiculite (HIV) during weathering and pedogenesis. Frayed edge wedges, formed both on individual smectite and illite phases and on the mica-HIV intergrade, have previously been recognized as significant sinks for the strong adsorption of Cs(+) and Rb(+). Atomic density profiles, interlayer adsorption site maps, radial distribution functions, and adsorption enthalpies derived from the equilibrated structural models are used to evaluate the optimal adsorption configurations and thermodynamics for Cs- and Rb-endmembers, a 50:50 Cs-Rb composition for the aqueous interlayer of vermiculite, and for the interlayer wedge zone as mica is transformed to HIV (i.e., HIV-mica wedge). Adsorption enthalpies for both cations are significantly larger for the frayed edge wedges (as represented by the HIV-mica wedge model) compared to values for the vermiculite and mica interlayers. Cesium cation binds more strongly than Rb(+) in the vermiculite interlayer, while Rb(+) binds more strongly than Cs(+) in the HIV-mica wedge. In all cases, the derived adsorption enthalpies for both cations indicate a preference for the wedge environment where electrostatic interaction is enhanced due to the presence of layer charge and the increased size of interlayer at the wedge accommodating cations larger than K(+).

Molecular dynamics simulations of cesium adsorption on illite nanoparticles

“…The difference is likely due to the isomorphic substitutions being located in the tetrahedral sheet (closer to the basal surface than in studies of smectite clay minerals) and, also, to the simulations presented here being performed using a flexible clay structure. The CLAYFF model, when used to model flexible phyllosilicate sheets, is known to predict an unphysical relaxation of the siloxane surface that transforms the ditrigonal cavities to larger hexagonal cavities [107]. The deep entry of Na in the hexagonal cavities is an artifact of this unphysical relaxation.…”

“…Recent efforts have deployed a combination of wet chemistry experiments [102,4,26], high-resolution imaging [47,63,95,34], synchrotron Xray spectroscopy [29,42], and atomistic-level simulations [68,93,44,107] to gain detailed insight into Cs adsorption mechanisms, selectivity, and kinetics. The emerging view from these studies is that Cs adsorption involves at least three types of surface sites: basal sites located on the external basal surfaces of illite particles, slow sites located in anhydrous illite interlayers, and high affinity sites of unclear nature.…”

“…First, the structure of the high affinity sites is a continuing topic of investigation [46,62,93,68]. The predominant hypothesis is that they occur in ''wedge" regions where illite or vermiculite interlayers transition from a collapsed state to an expanded state [18,102,68,107]. The existence of these regions is supported by electron microscopy observations showing that weathering induces a discernible ''fraying" at the edges of illite and mica crystals [62,34], perhaps associated with the replacement of K by solvated Ca near the extremities of the anhydrous interlayers [34].…”

“…Atomistic simulation techniques such as molecular dynamics (MD) simulations and density functional theory (DFT) calculations can, in principle, circumvent these difficulties by allowing a direct observation of individual Cs ions on different surface sites. Existing simulation studies have probed cesium adsorption in the interlayer nanopores of swelling clay minerals [64,97,78,79,10,44], in anhydrous clay interlayers [79,93], and at illite and vermiculite wedge sites [68,107]. However, no existing atomistic simulation study has examined cesium adsorption selectivity on different surfaces of a single illite particle, because of two methodological challenges.…”

Adsorption/Desorption Characteristics of Cesium Ions on Natural and Synthetic Minerals

“…For example, zeolites have three-dimensional network structures; also, some soil minerals categorized as silicates have lamellar structures [1][2][3][4][5][6][7][8][9] . These minerals show differences in their adsorption of exchangeable cations on the surfaces of their mineral skeletons; they also have different adsorption sites.…”