Guo X.,Agriculture and Technology Organized Research Unit |
Guo X.,University of California at Davis |
Guo X.,Los Alamos National Laboratory |
Kukkadapu R.K.,Pacific Northwest National Laboratory |
And 6 more authors.
Inorganic Chemistry | Year: 2015
The garnet structure has been proposed as a potential crystalline nuclear waste form for accommodation of actinide elements, especially uranium (U). In this study, yttrium iron garnet (YIG) as a model garnet host was studied for the incorporation of U analogs, cerium (Ce) and thorium (Th), incorporated by a charge-coupled substitution with calcium (Ca) for yttrium (Y) in YIG, namely, 2Y3+ = Ca2+ + M4+, where M4+ = Ce4+ or Th4+. Single-phase garnets Y3-xCa0.5xM0.5xFe5O12 (x = 0.1-0.7) were synthesized by the citrate-nitrate combustion method. Ce was confirmed to be tetravalent by X-ray absorption spectroscopy and X-ray photoelectron spectroscopy. X-ray diffraction and 57Fe-Mössbauer spectroscopy indicated that M4+ and Ca2+ cations are restricted to the c site, and the local environments of both the tetrahedral and the octahedral Fe3+ are systematically affected by the extent of substitution. The charge-coupled substitution has advantages in incorporating Ce/Th and in stabilizing the substituted phases compared to a single substitution strategy. Enthalpies of formation of garnets were obtained by high temperature oxide melt solution calorimetry, and the enthalpies of substitution of Ce and Th were determined. The thermodynamic analysis demonstrates that the substituted garnets are entropically rather than energetically stabilized. This suggests that such garnets may form and persist in repositories at high temperature but might decompose near room temperature. © 2015 American Chemical Society.
Veeramani H.,Virginia Polytechnic Institute and State University |
Scheinost A.C.,Institute for Radiochemistry |
Monsegue N.,Virginia Polytechnic Institute and State University |
Qafoku N.P.,Pacific Northwest National Laboratory |
And 7 more authors.
Environmental Science and Technology | Year: 2013
During subsurface bioremediation of uranium-contaminated sites, indigenous metal and sulfate-reducing bacteria may utilize a variety of electron acceptors, including ferric iron and sulfate that could lead to the formation of various biogenic minerals in situ. Sulfides, as well as structural and adsorbed Fe(II) associated with biogenic Fe(II)-sulfide phases, can potentially catalyze abiotic U(VI) reduction via direct electron transfer processes. In the present work, the propensity of biogenic mackinawite (Fe1+xS, x = 0 to 0.11) to reduce U(VI) abiotically was investigated. The biogenic mackinawite produced by Shewanella putrefaciens strain CN32 was characterized by employing a suite of analytical techniques including TEM, SEM, XAS, and Mössbauer analyses. Nanoscale and bulk analyses (microscopic and spectroscopic techniques, respectively) of biogenic mackinawite after exposure to U(VI) indicate the formation of nanoparticulate UO2. This study suggests the relevance of sulfide-bearing biogenic minerals in mediating abiotic U(VI) reduction, an alternative pathway in addition to direct enzymatic U(VI) reduction. © 2013 American Chemical Society.
Fu G.,University of Illinois at Urbana - Champaign |
Fu G.,General Electric |
Meng L.-J.,University of Illinois at Urbana - Champaign |
Eng P.,Center for Advanced Radiation Sources |
And 3 more authors.
Medical Physics | Year: 2013
Purpose: X-ray fluorescence computed tomography (XFCT) is an emerging imaging modality that maps the three-dimensional distribution of elements, generally metals, in ex vivo specimens and potentially in living animals and humans. At present, it is generally performed at synchrotrons, taking advantage of the high flux of monochromatic x rays, but recent work has demonstrated the feasibility of using laboratory-based x-ray tube sources. In this paper, the authors report the development and experimental implementation of two novel imaging geometries for mapping of trace metals in biological samples with ∼50-500 μm spatial resolution. Methods: One of the new imaging approaches involves illuminating and scanning a single slice of the object and imaging each slice's x-ray fluorescent emissions using a position-sensitive detector and a pinhole collimator. The other involves illuminating a single line through the object and imaging the emissions using a position-sensitive detector and a slit collimator. They have implemented both of these using synchrotron radiation at the Advanced Photon Source. Results: The authors show that it is possible to achieve 250 eV energy resolution using an electron multiplying CCD operating in a quasiphoton-counting mode. Doing so allowed them to generate elemental images using both of the novel geometries for imaging of phantoms and, for the second geometry, an osmium-stained zebrafish. Conclusions: The authors have demonstrated the feasibility of these two novel approaches to XFCT imaging. While they use synchrotron radiation in this demonstration, the geometries could readily be translated to laboratory systems based on tube sources. © 2013 American Association of Physicists in Medicine.
Cox J.M.,State University of New York at Buffalo |
Walton I.M.,State University of New York at Buffalo |
Benson C.A.,State University of New York at Buffalo |
Chen Y.-S.,Center for Advanced Radiation Sources |
Benedict J.B.,State University of New York at Buffalo
Journal of Applied Crystallography | Year: 2015
In situ single-crystal diffraction experiments provide researchers with the opportunity to study the response of crystalline systems, including metal-organic frameworks and other nanoporous materials, to changing local microenvironments. This paper reports a new environmental control cell that is remarkably easy to use, completely reusable, and capable of delivering static or dynamic vacuum, liquids or gases to a single-crystal sample. Furthermore the device is nearly identical in size to standard single-crystal mounts so a full unrestricted range of motion is expected for most commercial goniometers. In situ single-crystal X-ray diffraction experiments performed under dynamic gas-flow conditions revealed the cell was capable of stabilizing a novel metastable intermediate in the dehydration reaction of a previously reported metal-organic framework. © 2015 International Union of Crystallography.
Fischer R.A.,University of Maryland University College |
Campbell A.J.,University of Maryland University College |
Shofner G.A.,University of Maryland University College |
Lord O.T.,University of Bristol |
And 2 more authors.
Earth and Planetary Science Letters | Year: 2011
Wüstite, Fe1-xO, is an important component in the mineralogy of Earth's lower mantle and may also be a component in the core. Therefore the high pressure, high temperature behavior of FeO, including its phase diagram and equation of state, is essential knowledge for understanding the properties and evolution of Earth's deep interior. We performed X-ray diffraction measurements using a laser-heated diamond anvil cell to achieve simultaneous high pressures and temperatures. Wüstite was mixed with iron metal, which served as our pressure standard, under the assumption that negligible oxygen dissolved into the iron. Our data show a positive slope for the subsolidus phase boundary between the B1 and B8 structures, indicating that the B1 phase is stable at the P-T conditions of the lower mantle and core. We have determined the thermal equation of state of B1 FeO to 156GPa and 3100K, finding an isothermal bulk modulus K0=149.4±1.0GPa and its pressure derivative K0'=3.60±0.4. This implies that 7.7±1.1wt.% oxygen is required in the outer core to match the seismologically-determined density, under the simplifying assumption of a purely Fe-O outer core. © 2011 Elsevier B.V.