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

PRIORITY RESEARCH DIRECTION:MINERAL-WATER INTERFACE COMPLEXITY AND DYNAMICSMINERAL-WATER INTERFACE COMPLEXITY AND DYNAMICSABSTRACTOver the past two decades, molecular level information on the structure and reactivity ofidealized mineral-water interfaces has become available in unprecedented detail. However,natural subsurface materials do not have ideal mineral surfaces, but rather surfaces that are oftenstructurally complex with variable composition, topographic roughness, defect content, andmineralogic and organic coatings. This complexity is usually accompanied by spatially variablereactivity operating over a wide distribution of time scales. These aspects of mineral-waterinterfaces make it difficult or impossible to extrapolate information, such as the adsorptionbehavior of highly dispersed radionuclides, gained using idealized interfaces in laboratorysystems to the field environment. The issue of time dependence is especially important given theneed to extrapolate such data thousands of years into the future in performance assessment andother disposal safety analyses. This Priority Research Direction has the objective of determininghow the spatial and dynamic complexity of mineral-water interface reactions influence thecycling of elements and the ultimate chemical disposition of radionuclides in subsurfaceenvironments.EXECUTIVE SUMMARYNatural mineral-water interfaces are invariably complex both in terms of structure and reactivity.Complexity exists at natural mineral-water interfaces across a range of length scales, fromatomic-scale structure at crystallographic mineral terminations to multiscale variabilityassociated with imperfections such as defects, pores, or even fractal topography. This complexityis accompanied by spatially variable reactivity operating over a wide distribution of time scalesleading to overall dynamic behavior that is difficult to predict. Furthermore, mineral surfaces canhave partial or complete coverage by various kinds of coatings. Inorganic coatings can evolveeither by precipitation or sorption of material from solution to the surface, or via dissolution ofmore soluble parts of the material, leaving behind more insoluble coatings that impact theadsorption of ionic species. A significant fraction of mineral surfaces in natural environments arecoated by organics or extensively colonized by microbial organisms, which creates complexinterfaces with the surrounding aqueous solution. Biofilms may obscure the mineral surface fromsolution, or create microenvironments in which the local solution conditions are different fromthose in the bulk solution. In addition, bacterial activity may catalyze the transformation of toxicmetals into less or more toxic species, affect their mobility through redox chemistry andbiomineralization, or enhance the dissolution of the underlying mineral substrate. Therefore,most processes affecting the cycling of elements and radionuclide mobility, such as adsorption,diffusion, growth, dissolution, precipitation and interfacial redox reactions, operate on a verycomplex microscopic landscape with a high degree of spatial and temporal heterogeneity (see thesidebar on Intrinsic structural complexity at mineral-water interfaces).Investigation of idealized mineral surfaces, such as those using prepared single crystalterminations, has provided unprecedented insight into the structure and reactivity at themolecular level. There is a pressing need to drive these kinds of investigations towardsBasic Research Needs for Geosciences: Facilitating 21 st Century Energy Systems 107