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PRIORITY RESEARCH DIRECTION:MINERAL-WATER INTERFACE COMPLEXITY AND DYNAMICSIntrinsic structural complexity at mineral-water interfacesThe ability to observe molecular-scale processes at mineral-water interfaces is deriveddirectly from the availability of advanced national scientific user facilities. Highbrilliance hard X-ray sources provide unique opportunities to probe mineral-waterinterface processes in situ and in real-time, making use of the highly penetrating nature ofX-rays. X-rays can probe interfacial processes through various approaches, ranging fromspectroscopy and diffraction to microscopy-based techniques.Recent advances provide new opportunities by combining the benefits of these differentapproaches. Resonant anomalous X-ray reflectivity (RAXR) has the elemental andchemical sensitivities of X-ray spectroscopy combined with the structural sensitivity andinterface specificity of X-ray scattering techniques. Applied to the case of ion adsorptionat the mica-water interface (e.g., as a model clay mineral), a comprehensiveunderstanding of ion adsorption was obtained (Park et al. 2006a). These results revealedthat the interfacial binding of even simple ions (e.g., Sr 2+ ) can be complicated by thebalance between the electrostatic attraction and the energy cost of ion dehydration,resulting in coexisting adsorption configurations. These results also directly revealed thatthe hydration structure immediately adjacent to the interface was disrupted by adsorptionof the ion, thereby demonstrating the inherent coupling and interplay between molecularscalestructure and the associated processes.Interfacial electron density within 0.01 M Sr(NO 3 ) 2 in contact with muscovite (black solidline) compared with that of deionized water (blue dashed line) is shown as a function ofheight above the surface (left panel). The element-specific Sr profile (red dashed line)reveals two distinct ion heights corresponding to the coexistence of “inner-sphere” and“outer-sphere” adsorption complexes, shown schematically in the right panel. (Park etal. 2006a)incorporation of increasing structural, compositional and physical complexity to strengthen theirlink to conditions expected in natural settings. Research needs to take advantage of advances insurface analytical spectroscopies and microscopies, the emerging improvements in synchrotronand neutron capabilities, and the continually increasing power of computational facilities toenable next-generation insights.108 Basic Research Needs for Geosciences: Facilitating 21 st Century Energy Systems
SUMMARY OF RESEARCH DIRECTIONPRIORITY RESEARCH DIRECTION:MINERAL-WATER INTERFACE COMPLEXITY AND DYNAMICSThere is an overarching need to interrogate the complex structure and dynamics at mineral-waterinterfaces with increasing spatial and temporal resolution using existing and emergingexperimental and computational advances. Scanning probe microscopic methods, andsynchrotron (Fenter et al. 2002) and neutron sources (Wenk 2006) allow mineral-water interfacesto be investigated in situ with unprecedented detail. Such studies have recently shown, forexample, that the atomic structure of surfaces is sensitive to the presence of water and aqueousspecies at the interface, and to temperature/pressure, that a complex interplay of surfacehydration/hydroxylation processes and ion desolvation/ligand exchange reactions can dominatenominally simple sorption reactions of ions on well-defined mineral surfaces, that electronicfeedback between surface electron transfer processes and bulk electronic structure properties ofsemiconducting minerals supports spatially coupled site reactivity, and that leached layerscommonly develop on mineral surfaces during dissolution (Brown et al. 1999). It is crucial toimprove the spatial, energetic and temporal resolution capabilities of in situ spectroscopies andmicroscopies to facilitate distinctions between various possible sorption structures andcompositions at mineral-water interfaces. Next-generation sources and computational hardwareand methods will help to develop an understanding of this complexity and these dynamics at arobust quantum mechanical level. Fundamental studies on idealized mineral-water interfaces arecritical for providing a means to probe molecular-level structure, reaction mechanisms, anddynamics of elementary processes.To strengthen the link between these studies and the complex conditions expected for naturalmineral-water interfaces, a great deal of research is required to understand the role of nonidealitiesand the time-dependent behavior of the interface. For example, the incorporation oftrace impurities alters growth kinetics through the modification of elementary step dynamics bymechanisms such as the pinning of step motion. Changes in interfacial composition duringgrowth due to, for example, differential ion dehydration kinetics as the rate-limiting step forincorporation or by incorporation of vacancies, lead to the development of lattice strain that canchange both the conformation of the growth (island vs. film) and self-limiting interfacialreactions. Furthermore, defects can dominate the thermodynamic stability of surfaces and thekinetics of growth and dissolution, affecting the incorporation or release of minor constituents.Ion and charge transport processes both on the solution and mineral sides of the interface cancouple in ways that makes their separation impossible or irrelevant (see the sidebar on Solid-statecharge transport at mineral-water interfaces).Extension and development of new molecular probes using next-generation sources forunderstanding the structure and dynamics of complex mineral-water interfaces are needed tobetter understand mechanisms and kinetics of element cycling and radionuclide mobility in thesubsurface. Advances in computational methods are also needed to provide feedback in supportof experiments and to gain insights into processes that will remain difficult to experimentallyprobe. Traditional computational methods involving ab initio optimization of surface structureshave been useful but yield only a static description of the interface; techniques andcomputational resources are needed that allow a better understanding of surface dynamics(Cygan and Kubicki 2001). Large-scale computer hardware and efficient algorithms for ab initiomolecular dynamics at the density functional theory level need to be advanced to the stage ofBasic Research Needs for Geosciences: Facilitating 21 st Century Energy Systems 109
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TABLE OF CONTENTSBasic Science Chal
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TABLE OF CONTENTSTechnology Impacts
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EXECUTIVE SUMMARYpurpose of linking
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INTRODUCTIONINTRODUCTIONWorldwide e
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INTRODUCTIONway into the rock forma
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PANEL REPORTSMULTIPHASE FLUID TRANS
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PANEL REPORT: MULTIPHASE FLUID TRAN
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PANEL REPORT: CHEMICAL MIGRATION PR
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PANEL REPORT: SUBSURFACE CHARACTERI
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PANEL REPORT: MODELING AND SIMULATI
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Page 72 and 73: PANEL REPORT: MODELING AND SIMULATI
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CROSSCUTTING ISSUE:THE MICROSOPIC B
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CROSSCUTTING ISSUE:HIGHLY REACTIVE
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CROSSCUTTING ISSUE:THERMODYNAMICS O
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CROSSCUTTING ISSUE:THERMODYNAMICS O
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REFERENCESBenner, S.G., Hansel, C.M
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REFERENCESChen, J., Hubbard, S., Pe
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REFERENCESEnnis-King, J., Preston,
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REFERENCESHolman, H.-Y., Perry, D.L
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REFERENCESLi, L., and Tchelepi, H.A
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REFERENCESNeal, A.L., Techkarnjanar
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REFERENCESReeves, P.C., and Celia,
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REFERENCESSteefel, C.I., DePaolo, D
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REFERENCESZachara, J.M., Ainsworth,
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AppendicesBasic Research Needs for
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APPENDIX 1: TECHNICAL PERSPECTIVES
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APPENDIX 2: WORKSHOP PROGRAMThursda
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Basic Research Needs for Geoscience
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APPENDIX 4: ACRONYMS AND ABBREVIATI
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