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

PANEL REPORT: CHEMICAL MIGRATION PROCESSES IN GEOLOGIC MEDIAAlthough studies on perfect or near-perfect mineral-water interfaces are valuable for probingmolecular-level structure, reaction mechanisms, and dynamics of elementary processes, the keyscientific challenge is to reconcile this molecular-level information with the behavior of complexmineral-water interfaces expected in nature. There is a basic need to systematically examine thecomponents of this complexity with increasing spatial, energetic and temporal resolution. Forexample, new knowledge of trace-constituent uptake in mineral surfaces can be obtained bydetermining how trace elements modify elementary step dynamics through processes such aschanging the direction of step motion. Growth surfaces can acquire unique structures distinctfrom bulk mineral structures because of the changing composition of the interface that leads tovariable dehydration rates of aqueous ions that attach to the surface and formation of vacant sitesthat are missing atoms. Such interface structures are difficult (if not impossible) to describethermodynamically, and currently cannot be included in macroscopic thermodynamic models.Ion and charge transport in the solution and mineral on either side of the interface can couple inways that makes their separate study irrelevant. Emerging advances in spectroscopic andmicroscopic capabilities, synchrotron X-ray and neutron sources, and computational hardwareand modeling methods will be essential to make significant progress in understanding thesecomplex dynamics at a robust quantum mechanical level. Realization of this science directionwill also drive the development of next-generation capabilities and greatly enhance knowledge ofthe fundamental phenomena underlying element cycling and radionuclide mobility—importantissues that continue to be characterized by orders of magnitude discrepancies in macroscopicreaction rates.Biogeochemistry in extreme subsurface environmentsEstimates of the distribution of life on Earth suggest that over 90% of Earth’s biomass exists inthe form of microorganisms in subsurface environments. These organisms are genetically andmetabolically diverse, and mediate a host of globally significant chemical transformations(biogeochemical reactions) through their interaction with the lithosphere. Subsurface microorganismscan be distinguished (among other attributes) by their required source of carbon forbiosynthesis: heterotropic (requiring organic carbon) and autotrophic (requiring CO 2 ). Microorganismsthat are ubiquitous below ground can have a profound influence on: composition ofsubsurface waters; mineralogy and surface properties of subsurface sediments; valence;geochemical form; and mobility of radionuclide and trace metals. Certain microorganismsconcentrate soluble metals or radionuclides in their periphery through biosorption to exposedbiomolecular complexants, or by enzymatically driven electron transfer reactions that inducebioprecipitation reactions on the organism surface or within the cell periphery.Subsurface microorganisms of many types can nucleate and form unique, high-surface-areabiomineral phases including iron and manganese oxides of single and mixed valence states (e.g.,ferrihydrite, goethite, magnetite, green rust, birnessites), carbonates (siderite, magnesite, calcite,aragonite, dolomite), and phosphates. Bio-precipitated solids can react strongly with traceconstituents through surface and bulk substitution reactions, associating trace elements with thesolid phase and significantly retarding their transport. A more specialized grouping of subsurfacemicroorganisms, termed metal-reducing bacteria, can reduce oxidized, polyvalent radionuclides[e.g., Tc(VII), U(VI), Np(V)] in pore water, groundwater, or nuclear waste streams of differenttypes, causing their precipitation and resulting in significant reduction in their solubility and farfieldmigration potential. Dissolved hydrogen gas and reduced carbon compounds serve as28 Basic Research Needs for Geosciences: Facilitating 21 st Century Energy Systems