Interfaces between liquid water and other phases (minerals, air, carbon-rich fluids, living organisms) are ubiquitous in terrestrial natural environments. Surface chemistry and mass fluxes at these interfaces play key roles in influencing a broad range of environmental phenomena including contaminant fate and transport, metal biogeochemical cycling, multiphase flow in porous media, cloud nucleation, and gas transfer between surface water and the atmosphere. Our research is designed to gain molecular-scale insight into the properties of liquid water at interfaces using state-of-the-art atomistic-level simulation techniques, macroscopic scale models, and laboratory experiments. Current projects are focused on understanding water and solute mass fluxes in clayey media (soils, sediments, engineered clay barriers, clay-shales) in conditions relevant to geologic carbon sequestration, waste isolation, and contaminant migration in soils.
Molecular simulation techniques are emerging as powerful complements to experimental geochemistry methods. A key strength of these techniques lies in their ability to probe small systems (up to hundreds of thousands of atoms) on a broader range of time scales than any experimental method (from femtoseconds to microseconds). Molecular simulations can reveal the behavior of individual atoms in complex geochemical systems (where spectroscopic and other experimental methods would probe the average behavior of large numbers of atoms or molecules) and the manner in which macroscopic-scale properties (adsorption and partitioning coefficients, interfacial tension, mass transfer kinetics, diffusion coefficients) arise from atomistic-level phenomena. Finally, molecular simulation techniques can probe geochemical systems under constraints that would be difficult or impossible to impose in the laboratory, such as fixed pore sizes, hypothetical isotopic masses, or imposed non-equilibrium conditions.
Natural and Engineered Clay Barriers
Clay minerals--natural nanoparticles with a high surface area and cation exchange capacity--are ubiquitous in soils, sediments, and sedimentary rocks where they strongly influence groundwater hydrology and contaminant fate and transport. Engineered clay barriers are widely used for the isolation of landfills and contaminated sites. Clay-rich rock formations (shales, mudstones) are emerging as key entities in the world's energy future (and, also, in the management of freshwater resources) through their role in geologic carbon sequestration, high-level radioactive waste storage, and unconventional hydrocarbon extraction. Our research examines the impact of clay-water interfaces on the barrier properties of these materials.
Our current focus in this area is on elucidating the relationships between microstructure and aqueous geochemistry in clayey media. For example, the feedback between pore size and cation adsorption in swelling clays is well documented (in some conditions, it gives rise to a spontaneous "de-mixing" of heteroionic swelling clays into mixtures of homoionic domains), but it remains incompletely characterized at the macroscale and essentially unexamined at the nanoscale. Our current projects in this area use MD simulations and surface complexation models to examine the impact of pH and salinity on clay reactivity and mechanics, on the fate and transport of cesium in soils of the Fukushima region, and on the nano-scale details of ion adsorption on clay and mica basal surfaces.
Geologic Carbon Sequestration
Carbon capture and storage (CCS) has the potential to contribute significantly to CO2 abatement efforts required to stabilize global temperatures over the next century. The "storage" component of CCS, known as geologic carbon sequestration, involves injecting large quantities of supercritical CO2 in suitable geologic formations (such as brine aquifers and depleted hydrocarbon reservoirs) where it will eventually react with host rocks to form carbonate minerals. A key requirement of CCS is the demonstration that fine-grained geologic formations (seals or "caprocks") can trap CO2 in the subsurface over time-scales of hundreds of years. Our research in this area uses MD simulations and laboratory experiments to probe the fundamental properties that underlie CO2 trapping mechanisms. A significant portion of this research is carried under the auspices of an Energy Frontiers Research Center, the Center for Nanoscale Control of Geologic CO2.
Our major focus in this area is on developing an improved understanding of stress-porosity-permeability relations in porous media. These relations are key parameters in predicting the impacts of geochemistry and geomechanics on seal integrity and fracture permeability. The stress-porosity-permeability relations of porous media are known to be highly sensitive to clay content (differences in clay content can result in permeability differences of more than 6 orders of magnitude at a fixed porosity), clay mineralogy, and aqueous geochemistry. The relations used in existing GCS models, however, bear little resemblance with the experimental database. A second focus of our research in this area is on understanding the fundamental basis of mineral-brine-CO2 wetting angles and the impact of wettability on colloidal transport associated with multiphase flow in porous media.
Kinetic Isotope Effects at Water Surfaces
Kinetic isotope effects (KIEs)--whereby two isotopes of the same species undergo a transformation at slightly different rates--are powerful probes of interfacial mass fluxes and the mechanisms that control them (for example, during water evaporation, gas transfer between water and air, metal biogeochemical cycling, and solute diffusion in groundwater). Our group has pioneered the use of MD simulations to examine KIEs in liquid water. In particular, we showed that molecular diffusion of solutes in liquid water causes a KIE that significantly impacts hydrologic reconstructions of mass fluxes in fine-grained rocks and noble gas geochemistry reconstructions of continental paleoclimate. We also showed that the ligand-exchange rates of aquated metals are sensitive to isotopic mass in a manner that is consistent with experimental data on Ca isotope fractionation during calcite precipitation (an important paleo-proxy).
Our ongoing research in this area is focused on quantifying and understanding the KIEs associated with water evaporation and condensation at water-air interfaces. Isotopic fractionation at these interfaces is determined primarily by the diffusion coefficient of water in air and by the accommodation coefficient of water at the water-air interface. The influence of isotopic mass and of the site of isotopic substitution (H vs. O) on these coefficients remains incompletely understood despite its importance in reconstructing ice records and evaporative water fluxes between the Earth's surface and the atmosphere. For example, the best available data on water isotope fractionation by diffusion in air suggest that the ratio of 1/2H to 16/18O KIEs is on the order of 0.84, whereas the kinetic theory of gases predicts a much smaller ratio of 0.52; the origin of this discrepancy is currently unknown.