The Journal of Physical Chemistry AAs part of Sandia’s core geochemistry program funded by DOE Office of Basic Energy Sciences (BES), Sandia’s Randall Cygan (in Sandia’s Geoscience Research and Applications Dept.) and his collaborators, W. Crawford Elliott and his PhD student Laura Zaunbrecher from Georgia State University, have been examining the adsorption mechanisms of metal cations onto soils and sediments. This effort is especially important for the environmental treatment of chemical and radioactive contaminants, including the fission product cesium-137—which is a significant contaminant at the Savannah River Site in South Carolina and in the region near the disabled Fukushima reactors in Japan. The article describing their work, “Molecular models of cesium and rubidium adsorption on weathered micaceous minerals,” was chosen by the editors of Journal of Physical Chemistry A to be the cover feature of the June 4, 2015, issue.

Equilibrated molecular structure of interlayer Cs+ (yellow), Rb+ (purple), and K+ (blue) in mica-vermiculite hybrid structure (HIV-mica wedge model) as derived from molecular dynamics. Frayed edges are represented by wedge zone in periodic structure. Wedge, vermiculite, and mica zones are indicated for the lower left section of the simulation cell.

Equilibrated molecular structure of interlayer Cs+ (yellow), Rb+ (purple), and K+ (blue) in mica-vermiculite hybrid structure (HIV-mica wedge model) as derived from molecular dynamics. Frayed edges are represented by wedge zone in periodic structure. Wedge, vermiculite, and mica zones are indicated for the lower left section of the simulation cell.

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, and related simulations for performance assessment of waste sites and repositories. The nanoscale nature of clay minerals often limits their characterization by traditional analytical methods. Therefore, 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) and in the interlayer region formed as a result of the transformation of muscovite to interlayered vermiculite during weathering and soil formation/development. Our results indicate that Cs+ binds more strongly than Rb+ in the vermiculite interlayer, while Rb+ binds more strongly than Cs+ in the vermiculite-mica wedge region. This is the first study to explore the detailed structure and energetics of molecular binding mechanisms at important wedge sites associated with soils and sediments.

Ultimately, these models and results will guide further determination of adsorption capacities associated with complex natural materials, especially those that can impact the attenuation and sequestration of radioactive contaminants in the environment. This work is also critical to performance assessment research activities sponsored by US Nuclear Regulatory Commission in their evaluation of the suitability of nuclear materials storage and disposal.

This research was funded by BES’s GeoSciences program.