P. Gary Eller

Speciation of uranyl sorbed at multiple binding sites on montmorillonite

Abstract We have investigated the structures of U (VI) complexes as uranyl moieties sorbed onto a reference montmorillonite, SAz-1, using X-ray absorption fine structure spectroscopy (XAFS). The uranyl-loaded clays were prepared from aqueous solutions of uranyl nitrate in the pH range from 3.0 to 3.5. The U concentrations on the clay ranged from 1.7 to 34.6% of the reported cation exchange capacity (CEC = 1.2 meq/g) of the clay. For all samples, XAFS results indicate that there are two axial oxygen atoms at 1.78–1.80 A, as expected for the uranyl moiety. The average numbers and distances of equatorial oxygen atoms about uranyl sorbed on the clay vary significantly as a function of surface coverage. At high coverage (34.6% CEC), the average number and distance of equatorial oxygen atoms are near those found for the fully hydrated uranyl species in aqueous solution. However, there are fewer equatorial oxygen atoms at a shorter average distance about uranyl sorbed at low coverage (1.7% CEC). At moderate coverage (7.3% CEC), the average number and distance of equatorial oxygen atoms are intermediate between those at higher and lower coverage. These changes suggest that sorbing U is reacting with at least three different sites on the clay as U concentration increases. The existence of multiple surface sites and sorption complexes which are structurally distinct from solution species need to be considered for rigorous modeling of sorption processes.

Mode of incorporation of Sr2+ in calcite: Determination by X-ray absorption spectroscopy

By probing the local atomic environment of strontium coprecipitated with natural and synthetic calcites, X-ray absorption spectroscopy (XAS) reveals that the strontium is incorporated in the calcite by substitution at Ca2+ structural sites, forming a dilute solid solution. Both X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) demonstrated that the local structural environment of Sr2+ in natural and synthetic calcites was similar to that of Ca2+ in calcite and quite different from that of Sr2+ in strontianite. The Sr2+-O2− distance derived from EXAFS, 2.58 ± 0.03A, is consistent with the sum of the radii of these two ions in six- and three-fold coordination, respectively, of 2.57 A. The XANES and EXAFS further eliminate such modes of incorporation as adsorption, occlusion, and the presence of trace amounts of strontianite or other Sr2+-rich phase. However, evidence of a possible relict Sr2+-rich aragonite was encountered in a diagenetic calcite, indicating that XAS may prove a sensitive tool for detecting incomplete mineralogical alteration in carbonates. Because Sr2+ is bound at Ca2+ lattice sites, the record of biomineralization, diagenesis and age encoded in Sr2+Ca2+ ratios, and strontium isotopes in geologic calcites is secure; likewise, 90Sr sequestered in natural calcite should not undergo significant preferential leaching.

Optical spectroscopic studies of the sorption of UO2+2 species on a reference smectite

Abstract The speciation of UO 2+ 2 (uranyl) on a reference smectite (SAz-1 from Cheto, Arizona, USA) has been investigated by electronic emission and Raman vibrational spectroscopies. The spectroscopic studies have been done on uranyl-bearing clays prepared from aqueous solutions of uranyl nitrate in the pH range from ~2.5 to 7 and high initial ionic strength (~0.1–0.3 M). The uranyl loading levels in these samples ranged from ~0.1% to ~53% of the reported cation exchange capacity (~1.2 meq/g). Vibronically resolved emission spectra have been obtained for all samples. These spectra vary significantly in intensity and band-shape as a function of uranyl concentration in the clays and the equilibrium pH of the solutions from which the clays were prepared. For most clay samples the measured emission spectrum is a composite of spectra from multiple uranyl emitters. At the lowest loading levels a uranyl sorption complex with an apparent vibronic spacing of ~750 cm −1 dominates the spectra. At intermediate loading levels an additional uranyl sorption complex also having an apparent vibronic spacing of ~ 750 cm −1 is present at an approximately constant concentration ratio to the species in the most dilute samples. At the highest loading levels, a uranyl sorption complex with a vibronic spacing of ~ 850 cm −1 dominates the spectra. Raman spectra have been obtained for the more concentrated uranyl/clay samples. Two distinct bands (855 cm −1 and 883 cm −1 ) are seen in the spectral region of the totally symmetric uranyl stretch. The 855 cm −1 band correlates with the dominant high-coverage species, while the 883 cm −1 band arises from an additional sorption complex. Comparison of these results with aqueous solution spectral data suggests that monomeric uranyl moieties are responsible for the observed spectral responses in the clay samples, and the multiple spectral components are a result of occupancy by these moieties in several structurally and/or energetically different sites within the clay. It is proposed that the uranyl species responsible for the dominant components in the emission spectra in the low and intermediate coverage clay sample are sorbed to amphoteric edge site(s). The uranyl species responsible for the dominant component in the emission spectrum and the 855 cm −1 Raman band in the high-coverage clays is proposed to be exchanged into the fixed charge site(s). The additional complex identified by the Raman band at 883 cm −1 is also proposed as a sorption complex at fixed-charge sites.

Crystal structures of α-UF5 and U2F9 and spectral characterizationof U2F9

Crystalsl of α-UF5 and the mixed-valence compound U2F9 have been obtained from the reaction of sulfur dioxide and uranium hexafluoride at 160 °C. The structures have been refined by single crystal X-ray methods. In accordance with conclusions of previous powder determinations, we find a six-coordinate chain structure for α-UF5 (UF (bridging) = 2.235(1), UF(non-bridging) = 1.995(7) A) and a nine-coordinate, three-dimensionally bridged structure for U2F9 (UF = 2.19(2)-2.40(2) A). Absorption spectral studies of U2F9 indicate no unusual oxidation state for uranium; U(IV) and U(V) are present. The principal structural disparity between earlier structure determination and our study is the finding of a UF (non-bridging) distance of 1.995(7) A for α-UF5, more than 0.2 A shroter than the earlier estimate but in good accord with recent vond length-bond strenth considereations. Cell data: α UF5, I4/m, Z = 2, a = 6.18(4), c = 4.470(1) A, R = 0.022 for 161 reflections with I⩾ 2α(I); U2F9, 143m, Z = 4, a = 8.462(2) A, R = 0.032 for 104 reflections with I ⩾2μ(I).

Stabilization of actinides and lanthanides in high oxidation states

Abstract The stabilization of actinides and lanthanides in unusually high oxidation states can be rationalized on the basis of the interplay of crystal-lattice, acid-base, charge, size and redox factors. In this paper we apply arguments based on such influences to selected high-valent actinide and/or lanthanide compounds. Two new structural studies of the incorporation of f-element cations into the important perovskite structure are briefly discussed. Recently developed superoxidizer-superacid techniques which hold great promise for synthesis of new high-valent f-element fluorides are also mentioned.

Low-temperature conversion of uranium oxides to uranium hexafluoride using dioxygen difluoride

AbstractEfficient high-purity uranium hexafluoride formation has been observed from the reaction of gaseous dioxygen difluoride with U3O8 at room temperature.

Enthalpy of Formation of BaPuO3; Stability of Perovskite as a Nuclear-Waste Matrix for Pu4+

The selection of ceramic nuclear waste forms requires the identification of oxides with cation sites that immobilize radionuclides effectively. It is appropriate to study the bonding of model classes of oxides to tailor the composition and stability of ceramic matrices. A very important model oxide system, which incorporates +3, +4, +5, and +6 ions under appropriate conditions, is perovskite (CaTi03). Perovskite is one constituent of the important ceramic waste form known as SYNROC [1]. The Heavy Elements Chemistry research program at Argonne has included a major component that has determined the stability of +3, +4, and +6 ions of f-elements in oxides [2 — 5]. Since the lanthanides and actinides are of primary interest for nuclear waste immobilization, we have determined enthalpies of formation of the perovskites BaCe03, BaPr03, BaTb03, and BaU03 which contain tetravalent f-element cations in the six-coordinate Β site. The research reported in this paper represents the culmination of this effort: determination of the enthalpy of formation of BaPuO 3 , a small sample of which was made available following its synthesis and structure determination at Los Alamos National Laboratory [6]. BaPu03 was first reported by RUSSELL et al. [7] as a black simple-cubic (ideal perovskite, α0 = 4.39 Â) powder from heating BaC03 with an equimolar amount of Pu02 in air at 1500-1650°C. KELLER [8] prepared BaPu03 from BaO and Pu02 in an H2 or Ar stream at 1200° 1300°C. In both of these studies, the reaction of 1 :1 starting mixtures left some unreacted Pu02 . To obtain pure BaPu03, KELLER found it necessary to use excess BaO and to extract the excess BaO with methanol. He reported a cubic perovskite with a0 = 4.357 ±0.007 Â. CHACKRABURTTY et al. [9] also prepared and reported a simple cubic BaPu03 product with a0 = 4.373 ±0.003 Â. In a study of BaU03, WILLIAMS et al. [3] attempted a parallel preparation of BaPuO3 by using a 1 :1 ratio of BaO and Pu02 starting materials, the Pu02 having been ignited at a relatively low temperature (800 °C). Their product appeared cubic (a0 = 4.3839 ± 0.0004 Â) and showed no Pu02 X-ray diffraction lines by the DEBYESCHERRER method. However, the product was not pure since it did not dissolve completely in dilute acid. It is known from neutron powder diffraction and high resolution X-ray powder diffraction [10] that the f-element perovskites BaM03 are rhombohedrally or orthorhombically distorted. ΡΕΝΝΕΜΑΝ and ELLER [11] predicted that Pu has a sufficiently large ionic radius that BaPu03 should also exhibit a distorted perovskite structure. For the purpose of neutron diffraction studies, a large sample of high-purity BaPuO 3 was prepared by CHRISTOPH et al. by extensive mixing and heating (to 1200°C in Ar) of a 1 :1 ratio of BaC03 and 2 4 Pu02 [6] As predicted [11], the product was found to be orthorhombic (GdFe03) with no other phases observed. A small amount was made available for this study.

Thermochemistry of uranium compounds XII. Standard enthalpies of formation of the α and β modifications of uranium pentafluoride The enthalpy of the β-to-α transition at 298.15 K

A solution-calorimetric determination of the enthalpies of reaction of the α and β modifications of UF5 with Ce(SO4)2 + H2SO4 is described. Auxiliary measurements were made of the enthalpies of reaction of UO2, γ-UO3, and HF(aq). From these results are calculated the standard enthalpies of formation ΔHfo(298.15 K) of β-UF5, −(2083.0 ± 6.3) kJ·mol−1 and of α-UF5, −(2075.5 ± 6.7) kJ·mol−1. The enthalpy of the (β-to-α) transition is (7.5 ± 2.1) kJ·mol−1 at 298.15 K. Suggested ΔHfo values are also given for U4F17 and U2F9.

Reactions of dioxygen difluoride with neptunium oxides and fluorides

Abstract Neptunium dioxide and tetrafluoride are converted in essentially quantitative yield to volatile neptunium hexafluoride by dioxygen difluoride (O2F2), both in gas–solid reactions at ambient temperatures and in liquid anhydrous hydrogen fluoride at −78°C. Neptunium dioxydifluoride was identified by Raman spectroscopy as a dominant reaction intermediate in the neptunium dioxide reaction. Direct reaction of NpF4 with liquid O2F2 resulted in violent decomposition of O2F2 with little or no conversion to NpF6. These reaction temperatures contrast markedly with the elevated temperatures (above 200°C) required for NpF6 generation using elemental fluorine or halogen fluorides.

Synthesis and structural characterization of CaTiO3 doped with 0.05–7.5 mole% gadolinium(III)

Abstract Titanates containing 0.05–7.5 mole% gadolinium(III) in the CaTiO3 perovskite lattice have been prepared and characterized by X-ray diffraction and absorption, electron paramagnetic resonance, and Raman methods. The structural studies confirm the predicted site substitution of Gd3+ ions into the Ca2+ sites and suggest charge compensation by a cation deficiency mechanism (Ca2+ and/or Ti4+ vacancies). Evidence is presented for a significant difference between local Gd3+-substituted sites and the average Ca(Gd) sites, resulting from the difference in charge and size of the two ions. A least-squares refinement of single-crystal X-ray data was carried out on (Ca0.925Gd0.075)TiO3, with resulting cell parameters and major structural features within three standard deviations of those previously found for stoichiometric CaTiO3: space group Pbnm, a = 5.38(1), b = 5.449(1), c = 7.647(1) A, Z = 4, R = 4.1% for 236 reflections with I ≥ 2σ(I).