Stable U(IV) Complexes Form at High-Affinity Mineral Surface Sites

Uranium (U) poses a significant contamination hazard to soils, sediments, and groundwater due to its extensive use for energy production. Despite advances in modeling the risks of this toxic and radioactive element, lack of information about the mechanisms controlling U transport hinders further improvements, particularly in reducing environments where UIV predominates. Here we establish that mineral surfaces can stabilize the majority of U as adsorbed UIV species following reduction of UVI. Using X-ray absorption spectroscopy and electron imaging analysis, we find that at low surface loading, UIV forms inner-sphere complexes with two metal oxides, TiO2 (rutile) and Fe3O4 (magnetite) (at <1.3 U nm–2 and <0.037 U nm–2, respectively). The uraninite (UO2) form of UIV predominates only at higher surface loading. UIV–TiO2 complexes remain stable for at least 12 months, and UIV–Fe3O4 complexes remain stable for at least 4 months, under anoxic conditions. Adsorbed UIV results from UVI reduction by FeII or by the reduced electron shuttle AH2QDS, suggesting that both abiotic and biotic reduction pathways can produce stable UIV–mineral complexes in the subsurface. The observed control of high-affinity mineral surface sites on UIV speciation helps explain the presence of nonuraninite UIV in sediments and has important implications for U transport modeling

Charge distribution and oxygen diffusion in hyperstoichiometric uranium dioxide UO\u3csub\u3e2+x\u3c/sub\u3e (x ≤ 0.25)

Quantum-mechanical techniques were used to determine the charge distribution of U atoms in UO2+x (x ≤ 0.25) and to calculate activation-energy barriers to oxygen diffusion. Upon optimization, the reduction in unit-cell volume relative to UO2, and the shortest (U-O) and (O-O) bond-lengths (0.22 and 0.24 nm, respectively) are in good agreement with experimental data. The addition of interstitial oxygen to the unoccupied cubic sites in the UO2 structure deflects two nearest-neighbor oxygen atoms along the body diagonal of uranium-occupied cubic sites, creating lattice oxygen defects. In (1 × 1 × 2) supercells, the partial oxidation of two U4+ atoms is observed for every interstitial oxygen added to the structure, consistent with previous quantum-mechanical studies. Results favor the stabilization of two U5+ over one U6+ in UO 2+x. Calculated activation energies (2.06-2.73 eV) and diffusion rates for oxygen in UO2+x support the idea that defect clusters likely play an increasingly important role as oxidation proceeds. © 2011 Elsevier B.V. All rights reserved

The corrosion of UO2 versus ThO2: a quantum mechanical investigation

Quantum mechanical surface energy calculations have been performed on both uranium dioxide (UO{sub 2}) and thorium dioxide (ThO{sub 2}) (111), (110), and (100) surfaces to determine their relative reactivities. While UO{sub 2} and ThO{sub 2} both have the fluorite structure Fm3m, they differ in that uranium has two dominant oxidation states, U{sup 4+} and U{sup 6+}, while thorium only has one, Th{sup 4+}. Furthermore, UO{sub 2} is an intrinsically weak p-type semi-conductor with a band gap of 2.14 eV (Killeen, 1980), while ThO{sub 2} is an insulator. Dissolution and spectroscopic studies indicate that UO{sub 2} and ThO{sub 2} have different solubilities (Sunder and Miller, 2000). We use the quantum mechanical program, CASTEP (CAmbridge Scientific Total Energy Package) to perform surface and adsorption energy calculations on the (111) surface of both materials, with specific attention to O, H{sub 2}O, and combined adsorption cases. UO{sub 2} and ThO{sub 2} bulk unit cells were optimized to find the most stable configuration of atoms. Surface slabs were ''cleaved'' from the relaxed bulk for each orientation, placed in a 10 {angstrom} vacuum gap in order to simulate a free surface and were optimized. Relative surface energy trends and atomic relaxation were compared between the surfaces of UO{sub 2} and ThO{sub 2}. The (111) surface is found to have the most energetically stable configuration of atoms in both cases, although ThO{sub 2} has higher surface energy values than UO{sub 2} on all three surfaces. The (111) surface slab is doubled in width in order to increase the number of surface sites, and different starting positions for adsorbates are tested in order to calculate the most energetically favorable adsorption sites. Adsorption energy results indicate that adsorption is more favorable on the UO{sub 2} (111) surface than the ThO{sub 2} (111) surface. Adsorption calculations are accompanied by partial density of state (PDOS) and bandstructure analyses in order to understand the role of electrons during adsorption on semi-conducting versus insulating mineral surfaces