PNNL-13501 - Pacific Northwest National Laboratory

Thermodynamics and Kinetics of Cation Exchange at Defects in Micas
Study Control Number: PN99071/1399
Kevin M. Rosso, Eric J. Bylaska
Understanding the fate and transport of radionuclides like cesium-137 in the subsurface is a key element of characterizing
the environmental status and risks associated with contaminated DOE sites. This project uses molecular level modeling
to analyze a key element for understanding the interactions and binding of these elements by layered silicate minerals.
Project Description
This project involves development of a molecular
modeling capability for radionuclide-layered silicate
mineral interactions. The focus of this work is calculation
of thermochemical cycles for important cation exchange
reactions, such as cesium (Cs) → potassium (K), in
interlayer sites in the mineral muscovite. Such reactions
limit the transport of the otherwise highly mobile
contaminant 137 Cs in natural sediments. Density
functional theory methods were used to calculate
optimized structural parameters and energies for
muscovite with cesium or potassium in the interlayer site.
The independent effects of layer charge and structure
have been separated out and their influence on the
exchange energetics has been evaluated. The project has
greatly improved our understanding of the factors
affecting the uptake and fixation of cesium in the layered
silicate family of minerals.
Results and Accomplishments
Accomplishments include the successful implementation
and benchmarking of planewave density functional theory
codes on model mineral systems, calculation of optimized
structural parameters for muscovite and cesiumexchanged
muscovite, and calculation of Cs/K cation
exchange thermodynamics. The results are discussed in
more detail below.
Benchmarking Density Functional Theory Methods
A variety of planewave density functional theory methods
and calculation parameters are potentially applicable to
the problem of cation exchange. The relative accuracies
of the particular methods can be difficult to gauge against
experiment on complicated systems. Therefore, the
various methods were applied to simpler but related
model mineral systems to benchmark their performance.
Using the mineral diaspore, we evaluated the ability of
local-density and generalized gradient approximation
density functional theory methods to mimic the
compressibility of an anisotropic mineral structure as a
function of pressure up to 25 GPa (Figure 1). Calculated
structures, bulk moduli, and anisotropic compressibilities
for diaspore using the generalized gradient approximation
density functional theory method demonstrated the best
agreement with experiment. The test implies that
generalized gradient approximation density functional
theory is sufficiently accurate to describe changes in the
total energy of a mineral phase as a function of subtle
structural changes.
Figure 1. The structure of the mineral diaspore. The
structure is known to compress unequally along the three
unit cell axes. Generalized gradient approximation density
functional theory methods accurately reproduce this
anisotrophy in the compressibility.
Density functional theory methods were also
benchmarked to evaluate their ability to calculate relative
energies for different structures of equivalent chemical
composition, or polymorphs. Phase relationships for
Earth System Science 245