Molecular Simulations of Clay Mineral Surface Geochemistry

G. Sposito, S.-H. Park, and R. Sutton, Lawrence Berkeley National Laboratory

Research Objectives

This project involves basic research directed toward an accurate model of molecular structure at the surface of hydrated 2:1 clay minerals (smectites). These minerals are of great importance in petroleum production, nuclear waste containment, and contaminant attenuation by designed clay liners.

The specific objective addressed was Monte Carlo (MC) simulations of molecular structure on smectite minerals adsorbing water and either Na⁺ or K⁺ cations. The results obtained were compared to recent neutron diffraction data for these systems, based on H/D isotopic-difference techniques.

Computational Approach

Computational algorithms for the MC simulations, based on the code MONTE developed by our collaborators N. T. Skipper (University College London) and K. Refson (Oxford University), were optimized and run on Cray C90 supercomputers at NERSC.

Accomplishments

Monte Carlo simulations based on tested intermolecular potential functions were used to calculate radial distribution functions for O-O, O-H, and H-H spatial correlations in the interlayer region of two-layer hydrates of Na- and K-montmorillonite (see figure). The simulated radial distribution functions then were used to compute the total radial distribution function for interlayer water , a physical quantity that can be determined experimentally by H/D isotopic-difference neutron diffraction. The simulated total radial distribution functions were compared to that for bulk liquid water and to a total radial distribution function recently determined experimentally for the two-layer hydrate of Na-montmorillonite.

The shape of the simulated for either montmorillonite hydrate was similar to that of measured by neutron diffraction. For example, a well-defined peak in the simulated was predicted near r 1.9 Å, reflecting O-H spatial correlations, and this peak appeared as well in the measured , as did a peak predicted to arise near r 3.3 Å. Our simulation results suggest that water molecules in the two-layer hydrates of montmorillonite have nearest-neighbor configurations, which differ significantly from the tetrahedral ordering of nearest neighbors that characterizes bulk liquid water.

Visualization of adsorbed Na⁺ bound in an outer-sphere surface complex to an octahedral charge site of the clay mineral montmorillonite. The MC output indicated individual Na-H₂O separations varying from 2.2 to 2.5 Å, with a most-probable Na-H₂O distance of 2.3 Å and a solvation shell confined to within 3.2 Å. The water molecules form a distorted octahedron in agreement with Na-H₂O distances and coordination numbers determined for concentrated NaCl solutions.

Significance

The results obtained in this project support the essential correctness of our MC simulation model as a predictor of clay-mineral hydration behavior. A fundamental understanding of this behavior is critical to the effective use of clay minerals in designed containment scenarios at DOE waste sites, as well as to improved control of waste plume attenuation in natural porous formations.

Publications

F.-R. C. Chang, N. T. Skipper, and G. Sposito, "Monte Carlo and molecular dynamics simulations of electrical double-layer structure in potassium-montmorillonite hydrates," Langmuir 14, 1201 (1998).

G. Sposito, S.-H. Park, and R. Sutton, "Monte Carlo simulation of the total radial distribution function for interlayer water in sodium and potassium montmorillonites," Clays and Clay Minerals (in press, 1998).