Basic Research Needs for Geosciences - Energetics Meetings and ...

PRIORITY RESEARCH DIRECTION:NANOPARTICULATE AND COLLOID CHEMISTRY AND PHYSICSlarge numbers of processors, but no single processor will have high performance. One can takeadvantage of massively parallel computer architectures in terms of system size, but not in termsof making predictions of the system over the long time scales necessary for modelinggeochemical processes involving colloids. Also, there is need for improved accuracy insimulations from the perspectives of the data needed for models at larger scales and the ability topredict electronic structures as system size increases.Computational methods for describing particles greater than 2 nm in size (500 to 1000 atoms) donot currently have the accuracy that can be obtained for molecular systems at or below this size.This is especially true for molecular systems containing radionuclides, notably the actinides,which require relativistic treatments due to the high atomic number (Vallet et al. 2006). Methodsmust be developed to predict the electronic structure of large systems, including relativisticeffects, with the accuracy necessary for developing predictive models at the molecular tonanoscale and at higher scales. These could include new exchange-correlation functionals forDensity Functional Theory and basis set-free methods as well as improvements in ab initiomolecular dynamics and solvent models to deal with long-range interactions. New approacheswill be needed for force-field-based methods to predict the behavior of much larger systems atlonger times, including improved formulations for long-term dynamics, sampling of phase space,force fields for all atoms in the Periodic Table, and mixed electronic structure/force-fieldmethods based on theoretical or empirical parameters. Reactions within colloids and on theirsurfaces will require novel ways to predict solvation/desolvation and reactivity in complexsystems. Innovation is essential for the modeling of reaction kinetics, including transition statesand other rare events, and thermodynamics in solution, which must include appropriate samplingof phase space for reliable predictions of entropy in complex, heterogeneous systems. Newstatistical physical approaches will be needed to predict the chemical behavior and motion oflarge colloids that may or may not be charged in solution.At the next set of scales, approaches suitable for predicting the movement of colloid-associatedcontaminants through the vadose zone and into groundwater environments rely on advances inour understanding of colloid mobilization, transport, and deposition within water-saturated andunsaturated porous media. Mobilization, which describes the release of colloids into pore water,is driven by fluctuations in flow and perturbations in pore water chemistry. Quantitativeinferences on mobilization rates are limited by too few observations of the phenomenon.Laboratory measurements made at the Darcy (column) scale are needed to elucidate howmobilization rates are linked to measurable properties of the porous media, pore water, andcolloids. Pore-scale observations that identify mechanisms that govern mobilization are neededto guide the development of a theory for colloid release in unsaturated and saturated geologicsolids. Once mobilized, colloids move by advection and dispersion and are susceptible toremoval from the pore water (deposition) by reactions that take place near solid-water and, in thecase of unsaturated media, air-water interfaces. Knowledge of colloid transport and deposition inideal systems must be extended to account for complexities associated with real subsurfaceenvironments. In particular, the presence of organic coatings on colloid surfaces, andheterogeneity in the physical and chemical properties of the porous media, should be evaluatedfor their roles in influencing colloid transport and deposition. Also, parameters and models fromsmaller scales must be incorporated into the scale describing colloidal transport in naturalsystems. Thus, new and improved reactive transport approaches need to be developed to dealwith the transport of these nanoscale particles. Collectively, developments from research at the118 Basic Research Needs for Geosciences: Facilitating 21 st Century Energy Systems