Clay minerals are essential components of soil due to their nanoparticulate nature, their very high surface area, and in some cases their structural charge. These properties enable clay to immobilize or adsorb nutrients and contaminants from the aqueous phase and can provide a useful catalytic substrate.
We are using classical molecular dynamics simulations (MD) to investigate the adsorption of the Cs-137 radionuclide to surface and interlayer sites of the clay mineral illite. These simulations could provide information on the mechanisms by which illite strongly retains Cs deep within the clay interlayer structure, giving us insights into the transport of Cs in contaminated soil.
Clay minerals are essential components of soils and geologic reservoir rocks due to their impact on bulk mechanical properties and permeability as well as their incredible capacity to adsorb ions and organic species. The exchange capacity of clays arises from their high reactive surface area, and in some cases their structural charge, which enable clays to immobilize large amounts of nutrients, contaminants, and carbon.
We are using a variety of simulation techniques including classical and ab initio molecular simulations as well as coarse-grained MD and kinetic Monte Carlo simulations to investigate the kinetics and thermodynamics of ion exchange and radiocesium adsorption to exterior and interlayer surfaces of clay minerals. These simulations provide information on the mechanisms by which illite clay strongly retains Cs over long timescales, giving us insights into the transport of Cs through engineered barriers for nuclear waste disposal and in contaminated soils such as those in the vicinity of the Fukushima nuclear disaster. More fundamentally, our findings elucidate the key role of chemical-mechanical coupling in facilitating ion exchange in clay interlayers, which bear the majority of potential exchange capacity.
Four layer nanoparticle of K-illite in aqueous solution (water molecules not shown).
The isotopic and trace element compositions of carbonate minerals are widely used to reconstruct paleoenvironment. Carbonate precipitation is also used to scavenge contaminants such as radioactive Sr-90 from groundwater. Growth rate affects the partitioning of isotopes and trace elements into minerals, but the mechanisms controlling the rate dependence of tracer fractionation are not well understood.
We investigate the interfacial processes governing carbonate mineral growth and recrystallization in soils and marine carbonate sediments, as well as the basic science of crystal growth, using chemostat experiments, stable isotope tracers, and modeling.
(At right) Schematic of the calcite surface identifying reactive kink sites, which are highly under-coordinated at the surface. Calcium (A) and carbonate (B) ions in solution exchange with the surface primarily at these sites.
Carbon dioxide sequestration in geologic repositories is a promising means to limit our greenhouse gas emissions. The cost of this technology is currently on par with or less than many other technologies that avoid CO2 emissions.
To keep stored carbon dioxide underground, the pressure of the compressed (supercritical) fluid must not exceed the capillary breakthrough pressure (Pc). This pressure depends strongly on interfacial properties including interfacial tension (IFT) and wetting angles.
We use Molecular dynamics (MD) simulations to model and understand the physiochemical controls over IFT and wettability.
(At left) Snapshot of an MD simulation (top) with the corresponding profile of CO2 and water phase densities (bottom). Carbon dioxide density is enhanced at the water surface, and the dependence of this surface excess CO2 on pressure is related to interfacial tension by the Gibbs adsorption equation.
Apatite, nominally Ca5(PO4)3(OH, F,Cl), is a phosphate mineral that forms a vast array of compositions and serves equally many functions in the environment as a bone-former in vertebrates, a phosphate nutrient reservoir in soils, and as a contaminant sink (and excess P sink) in engineered systems. Growth and dissolution mechanisms of apatites at the nano-scale are poorly understood, and increasingly, non-classical pathways involving crystallization by particle attachment (CPA) are being invoked to explain apatite growth. We are performing kinetic experiments coupled with nanoscale imaging (AFM and TEM) and stable isotopes to understand apatite mineralization.