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Uranium (U(VI)) sorption in systems containing titanium dioxide (TiO(2)) and various Fe(III)-oxide phases was investigated in the acidic pH range (pH 2.5-6). Studies were conducted with physical mixtures of TiO(2) and ferrihydrite, TiO(2) with coprecipitated ferrihydrite, and with systems where Fe(III) was mostly present as crystalline Fe(III) oxides. The presence of ferrihydrite resulted in decreased U(VI) sorption relative to the pure TiO(2) systems, particularly below pH 4, an unexpected result given that the presence of another sorbent would be expected to increase U(VI) uptake. In mixtures of TiO(2) and crystalline Fe(III) oxide phases, U(VI) sorption was higher than for the analogous mixtures of TiO(2) with ferrihydrite, and was similar to U(VI) sorption on TiO(2) alone. X-ray absorption spectroscopy of the TiO(2) system with freshly precipitated Fe(III) oxides indicated the presence of an Fe(III) surface phase that inhibits U(VI) sorption-a reaction whereby Fe(III) precipitates as lepidocrocite and/or ferrihydrite effectively blocking surface sorption sites on the underlying TiO(2). Competition between dissolved Fe(III) and U(VI) for sorption sites may also contribute to the observed decrease in U(VI) sorption. The present study demonstrates the complexity of sorption in mixed systems, where the oxide phases do not necessarily behave in an additive manner, and has implications for U(VI) mobility in natural and impacted environments where Fe(III) (oxyhydr)oxides are usually assumed to increase the retention of U(VI).

Titanium dioxide (TiO(2)) has often served as a model substrate for experimental sorption studies of environmental contaminants. However, various forms of Ti-oxide have been used, and the different sorption properties of these materials have not been thoroughly studied. We investigated uranium sorption on some thoroughly characterized TiO(2) surfaces with particular attention to the influence of surface area, surface charge, and impurities. The sorption of U(VI) differed significantly between samples. Aggressive pretreatment of one material to remove impurities significantly altered the isoelectric point, determined by an electroacoustic method, but did not significantly impact U sorption. Differences in sorption properties between the various TiO(2) materials were related to the crystallographic form, morphology, surface area, and grain size, rather than to surface impurities or surface charge. In-situ attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopic studies showed that the spectra of the surface species of the TiO(2) samples are not significantly different, suggesting the formation of similar surface complexes. The data provide insights into the effect of different source materials and surface properties on radionuclide sorption.

Davidite (A1-xM21O38) samples from five different geological localities contain approximately 0.2 to 9.5 wt% UO2 (0.02 to 0.65 atoms per formula unit) and <0.1 to 1.3 wt% ThO2 (<0.01 to 0.09 atoms per formula unit). Maximum amounts of other notable cations include 3.7 wt% V2O3, 4.1 wt% Cr2O3, 2.5 wt% Y2O3, 5.6 wt% La2O3, 6.0 wt% Ce2O3, 4.0 wt% MnO, 2.4 wt% ZnO, 2.7 wt% SrO, and 4.9 wt% PbO. As a result of the variation in age and Th-U content, the calculated α decay dose ranges from ~0.2 to 44 x 1016 α/mg (~0.06 to 14.5 dpa). For samples with ages of 275--295 Ma, the critical dose for amorphization based on electron diffraction is ~0.8 x 1016 α/mg. Natural davidite is commonly altered to rutile, ilmenite, titanite, and other minor phases. [ABSTRACT FROM AUTHOR]

Studies the behavior of uranium (U) in zirconolite, CaZrTi2O7, which is an important constituent of synroc. Study of U in a range of atmospheres; Related pyrochlore-structured titanate phase; Processing atmosphere of solid materials to immobilize multivalent waste ions such as actinides.

In situ neutron powder diffraction experiments are applied to physical, kinetic, and microstructural characterization of uranium mononitride as a promising light water reactor fuel material. The temperature-variable coefficient of thermal expansion and isotropic Debye Waller factors are obtained by sequential Rietveld refinement over 499–1873 K. Oxidation of a UN pellet (95.2% density) under flow of 11 mg/min D 2 O is observed to initiate above 623 K and the rate increases by a factor of approximately 10 from 673 to 773 K, with activation energy 50.6 ± 1.3 kJ/mol; uranium oxide is the only solid corrosion product. [ABSTRACT FROM AUTHOR]

Three new coordination polymers of uranium(VI) with pyromellitic acid (H 4 btca) have been synthesized and structurally characterized. (ED)[(UO 2 )(btca)]·(DMSO)·3H 2 O ( 1 ) (ED=ethylenediammonium; DMSO=dimethylsulfoxide) has a lamellar structure with intercalation of ED and DMSO. (NH 4 ) 2 [(UO 2 ) 6 O 2 (OH) 6 (btca)]·~6H 2 O ( 2 ) has a 3D framework built from 7-fold coordinated uranyl trinuclear units and btca ligands with 1D diamond-shaped channels (~8.5 Å×~8.6 Å). [(UO 2 ) 2 (H 2 O)(btca)]·4H 2 O ( 3 ) has a 3D network constructed by two types of 7-fold coordinated uranium polyhedron. The unique μ 5 -coordination mode of btca in 3 enables the formation of 1D olive-shaped large channels (~4.5 Å×~19 Å). Vibrational modes, thermal stabilities and fluorescence properties have been investigated. [ABSTRACT FROM AUTHOR]

The reaction of uranium(VI) or thorium(IV) nitrate with p -toluenesulfonic acid (PTSA) in aqueous solutions at room temperature led to the formation of U(VI)/Th(IV) coordination polymers (CPs) with deprotonated μ 2 -bridging PTS anions. Both [(UO 2 )(PTS) 2 (PTSA)]·4H 2 O ( 1 ) and Th(PTS) 4 (H 2 O) 3 ( 2 ) have one-dimensional (1D) undulating polymeric structures built with pentagonal bipyramidal uranyl polyhedrons and 9-fold coordinated thorium polyhedrons, respectively, linked through μ 2 -bridging PTS anions. Both CPs have similar thermal decomposition patterns as they have similar structural type in the presence of the same ligand. Vibrational modes have been investigated and the structural features have been discussed together with relevant structures. [ABSTRACT FROM AUTHOR]

The incorporation of uranium into gadolinium zirconate (Gd2Zr2O7) is investigated by synchrotron X-ray powder diffraction and X-ray absorption near-edge structure (XANES) spectroscopy. The results suggest that the uranium cation is largely located on the pyrochlore B-site instead of the targeted A-site. Cation disorder in Gd2Zr2O7 and U-doped Gd2Zr2O7 is investigated by positron annihilation lifetime spectroscopy (PALS) which demonstrates the absence of cation vacancies in these systems. This work provides direct evidence for cation antisite (A- and B-site mixing) disorder in U-doped and off-stoichiometric Gd2Zr2O7 pyrochlore. [ABSTRACT FROM AUTHOR]

Abstract: A series of uranium-containing gadolinium zirconate samples have been fabricated at 1450°C in oxidizing, inert and reducing atmospheres. X-ray diffraction, Raman spectroscopy and transmission electron microscopy have been utilized to confirm adoption of pyrochlore or defect fluorite structures. X-ray diffraction allowed determination of the bulk averaged structure while Raman spectroscopy and transmission electron microscopy were used to determine ordering at the microdomain scale. Diffuse reflectance, X-ray absorption near edge structure and X-ray photoelectron spectroscopies indicated a predominantly U6+ oxidation state for all the air-sintered samples, even when Ca2+ or A-site vacancies were incorporated to charge balance for U4+, a mixed U5+/U6+ oxidation state was found for samples sintered in argon, while a mixed U4+/U5+ oxidation state occurred for sintering under N2–3.5%H2. This demonstrates a degree of uranium oxidation state control through sintering conditions, and the potential of using gadolinium zirconates as host materials for uranium in nuclear waste applications. [Copyright &y& Elsevier]

Abstract: Calixarene-based picolinamides and malonamides are among the promising extracting ligands for selective separation of minor actinides. However, the understanding of coordination chemistry of actinide ions with simple picolinamide- and malonamide-type ligands is still far from comprehensive. In this work, we report the first two picolinamide coordinated uranium (VI) and thorium (IV) complexes with different 1D polymeric structures as well as a malonamide coordinated uranium (VI) molybdate with a 2D layered structure. [Copyright &y& Elsevier]

Reaction of UO2(NO3)2, NaOH, H2O2, and H3BO3 in aqueous solution leads to the formation of yellow crystals of Na14 [(UO2)(O2)1.5] 28·nH2O (I) at pH 8.5 within a few days, followed by transformation of (I) at room temperature to light yellow-green crystals of Na6 [UO2{B16O24(OH)8}] ·14H2O (II) (<80% yield). [ABSTRACT FROM AUTHOR]