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

PRIORITY RESEARCH DIRECTION:NANOPARTICULATE AND COLLOID CHEMISTRY AND PHYSICSrange correlations (< 5 Å) or long-range ordering (> 40 Å), so colloid-scale correlations occur atlength scales currently inaccessible for study. This historically unavailable window ofinformation is crucial to the development of a predictive understanding of colloid structures insolution and their aging and precipitation reactions as well as providing input for modelingdissolution energetics and migration phenomena. The current probe of choice for short rangecorrelations in non-crystalline systems is XAS (X-ray absorption spectroscopy), which includesboth XANES (X-ray absorption near-edge structure) and EXAFS (extended X-ray absorptionfine structure). XAS is a single-ion probe that can be used to target metal-ion speciation, atconcentrations down to submillimolar, in a complex system, through selectivity provided bycharacteristic X-ray absorption lines. The technique provides information about speciationincluding oxidation state and near-neighbor coordination environments out to distances of about5 Å.For long-range correlations (> 40 Å) that exhibit periodicity (order), X-ray diffraction is themethod of choice. The challenge, within the context of colloid characterization, is finding waysto synthesize or isolate molecular crystals suitable for structural characterization. Rarelyavailable, this information constitutes the ultimate structural characterization.Small angle and high-energy X-ray scattering has been applied to the study of solute correlationsin aqueous solution, providing the missing bridge for structural characterization of intrinsiccolloids. Although not extensively employed to date, small angle scattering (SAXS (Small AngleX-ray) and ASAX (Anomalous Small Angle X-ray)) potentially provides important informationabout colloidal morphologies, particularly for heavy-metal containing nanophases, where thecontrast with the aqueous solution is optimal. Under favorable conditions, high-energy(> 60 keV) X-ray scattering (HEXS) experiments can be used to probe solute correlations atlength scales to distances of 20 Å or longer, precisely the size-range important to anunderstanding of aggregate formation. Information about interactions over this crucial lengthscale is not currently available from any other technique. Recent advances in X-ray imaging areextending the spatial resolution down to 30 nm or less, well within the length scale of importanceto colloid characterization by diffraction, fluorescence or transmission imaging (Nilsson et al.2005).Theory, modeling and simulationModeling colloids at the molecular level is a challenge because of the broad range of sizes,structures, compositions, and heterogeneous atomic positions. There are specific issues due todifferences between intrinsic colloids and pseudocolloids in the area of radionuclide transport. Acolloidal particle can react and effectively adsorb contaminants, but these may be only a verysmall fraction of the number of atoms in the actual colloid. This leads to significant questions interms of the size of the aggregate that should be used to model colloids important in naturalsystems. Furthermore, colloids relevant to geochemistry and radionuclide activation andtransport are essentially metastable “phases” that exist in aqueous solution. Modeling of colloids,their formation, and their reactivity requires the inclusion of the solvent. Besides size, there areconsiderable problems in dealing with the evolution of the colloid from its formation andreactivity through its aging to its transport, all of which span many orders of magnitude of timescale, making computations difficult. Time is inherently a linear process and the number of timesteps scales linearly with processor speed. Current and planned computer architectures showBasic Research Needs for Geosciences: Facilitating 21 st Century Energy Systems 117