title: Uranium Immobilization and Nanofilm Formation on Magnesium-Rich Minerals.
abstract: Polarization-dependent grazing incidence X-ray absorption spectroscopy (XAS) measurements were completed on oriented single crystals of magnesite [MgCO3] and brucite [Mg(OH)2] reacted with aqueous uranyl chloride above and below the solubility boundaries of schoepite (500, 50, and 5 ppm) at pH 8.3 and at ambient (PCO2 = 10(-3.5)) or reduced partial pressures of carbon dioxide (PCO2 = 10(-4.5)). X-ray absorption near edge structure (XANES) spectra show a striking polarization dependence (χ = 0° and 90° relative to the polarization plane of the incident beam) and consistently demonstrated that the uranyl molecule was preferentially oriented with its Oaxial═U(VI)═Oaxial linkage at high angles (60-80°) to both magnesite (101̅4) and brucite (0001). Extended X-ray absorption fine structure (EXAFS) analysis shows that the "effective" number of U(VI) axial oxygens is the most strongly affected fitting parameter as a function of polarization. Furthermore, axial tilt in the surface thin films (thickness ∼ 21 Å) is correlated with surface roughness [σ]. Our results show that hydrated uranyl(-carbonate) complexes polymerize on all of our experimental surfaces and that this process is controlled by surface hydroxylation. These results provide new insights into the bonding configuration expected for uranyl complexes on the environmentally significant carbonate and hydroxide mineral surfaces.
authors.forename: Arjen
authors.initials: A
authors.lastname: van Veelen
authors.affiliation: Department of Photon Science and Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory , 2575 Sand Hill Road, Menlo Park, California 94025, United States.
authors.forename: John R
authors.initials: JR
authors.lastname: Bargar
authors.forename: Gareth T W
authors.initials: GT
authors.lastname: Law
authors.affiliation: Department of Photon Science and Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory , 2575 Sand Hill Road, Menlo Park, California 94025, United States.|Surface and Aqueous Geochemistry Group, Department of Geological Sciences, School of Earth, Energy, and Environmental Sciences, Stanford University , Stanford, California 94305-2115, United States.
authors.forename: Gordon E
authors.initials: GE
authors.lastname: Brown
authors.forename: Roy A
authors.initials: RA
authors.lastname: Wogelius
mesh_terms: D002712:Chlorides / Q000737:chemistry*; D003460:Crystallization; D006863:Hydrogen-Ion Concentration; D008274:Magnesium / Q000737:chemistry*; D008276:Magnesium Hydroxide / Q000737:chemistry; D049329:Nanostructures / Q000737:chemistry; D012995:Solubility; D014501:Uranium / Q000737:chemistry*; D056928:X-Ray Absorption Spectroscopy; D014961:X-Ray Diffraction
chemical_list: D002712:Chlorides; C005479:magnesium carbonate; D014501:Uranium; D008274:Magnesium; D008276:Magnesium Hydroxide; C408075:uranyl chloride