P.G. Allen

The structure of uranium (VI) sorption complexes on silica, alumina, and montmorillonite

Abstract We have investigated the adsorption of the uranyl ion (UO22+) in contact with amorphous silica (SiO2), γ-alumina (Al2O3), and montmorillonite surfaces in the pH range of 3.1–6.5, using X-ray absorption fine structure (XAFS) spectroscopy to observe the local structure around the uranium atom. Analysis shows that in all samples the uranyl ion structure is preserved, with two axial oxygen atoms detected at ca. 1.8 A. For the montmorillonite samples at low pH a single equatorial oxygen shell is observed at ca. 2.4 A, with a coordination number of 6 ± 1. At near-neutral pH (6.41) and high ion concentration (0.1 M NaCl), two separate equatorial shells are observed with bond lengths of ca. 2.30 and 2.48 A. The samples of uranyl on silica and γ-alumina are all observed to have two separate equatorial shells with bond lengths of ca. 2.30 A and 2.49 A. A uranium shell at ca. 4.0 A is observed in the near-neutral pH (∼6.5) samples of uranyl on silica and on γ-alumina. A silicon shell at ca. 3.10 A is observed in the sample of uranyl on silica at pH 6.5. These results suggest that adsorption of the uranyl ion onto montmorillonite at low pH occurs via ion exchange, leaving the inner-sphere uranyl aquo-ion structure intact. At near-neutral pH and in the presence of a competing cation, inner-sphere complexation with the surface predominates. Adsorption of the uranyl onto the silica and γ-alumina surfaces appears to occur via an inner-sphere, bidentate complexation with the surface, with the formation of polynuclear surface complexes occurring at near-neutral pH.

EXAFS and principal component analysis: a new shell game

The use of principal component (factor) analysis for the interpretation of EXAFS spectra is described. The components derived from EXAFS spectra share mathematical properties with the original spectra. As a result, the abstract components can be analyzed using standard EXAFS methodology to yield bond distances and other coordination parameters. The number of components that must be analyzed is usually less than the number of original spectra. The method is demonstrated using a series of spectra from aqueous solutions of uranyl ions.

The structure of U (super 6+) sorption complexes on vermiculite and hydrobiotite

The sorption of the uranyl oxo-cation (UO22+)at different types of binding sites on layer silicate mineral surfaces was investigated. Well-characterized samples of vermiculite and hydrobiotite were exposed to aqueous uranyl under conditions designed to promote surface sorption either at fixed charge ionexchange sites or at amphoteric surface hydroxyl sites. The local structure of uranium in the sorption samples was directly measured using uranium L3-edge extended X-ray absorption fine structure (EXAFS). Polarized L1- and L3-edge X-ray absorption near-edge structure (XANES) measurements were used to characterize the orientation of uranyl groups in layered samples. X-ray diffraction (XRD) measurements of interlayer spacings were used to assess the effects of ion-exchange and dehydration upon the mineral structure. The most significant findings are: (1) Under conditions which greatly favor ion-exchange sorption mechanisms, uranyl retains a symmetric local structure suggestive of an outer-sphere complex, with a preferred orientation of the uranyl axis parallel to the mineral layers; (2) Upon dehydration, the ionexchange complexes adopt a less symmetric structure, consistent with an inner-sphere complex, with less pronounced orientation of the uranyl axis; and (3) For conditions which favor sorption at surface hydroxyl sites, uranyl has a highly distorted equatorial shell, indicative of stronger equatorial ligation, and the detection of a neighboring U atom suggests the formation of surface precipitates and/or oligomeric complexes.

Multiconfigurational nature of 5f orbitals in uranium and plutonium intermetallics

Uranium and plutonium’s 5f electrons are tenuously poised between strongly bonding with ligand spd-states and residing close to the nucleus. The unusual properties of these elements and their compounds (e.g., the six different allotropes of elemental plutonium) are widely believed to depend on the related attributes of f-orbital occupancy and delocalization for which a quantitative measure is lacking. By employing resonant X-ray emission spectroscopy (RXES) and X-ray absorption near-edge structure (XANES) spectroscopy and making comparisons to specific heat measurements, we demonstrate the presence of multiconfigurational f-orbital states in the actinide elements U and Pu and in a wide range of uranium and plutonium intermetallic compounds. These results provide a robust experimental basis for a new framework toward understanding the strongly-correlated behavior of actinide materials.

Spectroscopic characterization of trivalent f-element (Eu, Am) solid carbonates

Abstract Solubility studies show that trivalent f-element carbonate compounds are the predominant solids that primarily limit the soluble metal ion concentration under environmental conditions. We systematically investigated the spectroscopic characteristics of a series of solid f-element (Eu and 243 Am) carbonates. Varying pH, ionic strength, and carbonate concentration results in the formation of M(OH) 3 , MOHCO 3 , M 2 (CO 3 ) 3 ·nH 2 O, and NaM(CO 3 ) 2 ·nH 2 O, where M=Eu(III) or Am(III). Solids were characterized by FTIR, fluorescence, and EXAFS spectroscopies that determined and confirmed the coordination environment, and by their individual X-ray diffraction powder patterns. The number of coordinated crystal waters was determined to be 2–3 for Eu 2 (CO 3 ) 3 ·nH 2 O and 5 for NaEu(CO 3 ) 2 ·nH 2 O using thermogravimetric/differential thermal analysis and fluorescence lifetime. We report studies of the fluorescence of Am(III) and the effect of carbonate coordination on the 5 D 1 → 7 F 1 transition.

237Np: Oxidation State in Vivo and Chelation by Multidentate Catecholate and Hydroxypyridinonate Ligands

Chemically, 237Np(V) is as toxic as U(VI), and radiologically, about as toxic as 239Pu. Depending on redox conditions in vivo, 237Np exists as weakly complexing Np(V) (NpO2+) or as Np(IV), which forms complexes as stable as those of Pu(IV). Ten multidentate catecholate (CAM) and hydroxypyridinonate (HOPO) ligands with great affinity for Pu(IV) were compared with CaNa3-DTPA for in vivo chelation of 237Np. Mice were injected intravenously with 237NpO2Cl: those in a kinetic study were killed 1 to 2880 min; in ligand studies, fed mice were injected intraperitoneally with a ligand 5, 60, or 1440 min after 237Np(V) (molar ratio 5.6 to 73), mice fasted for 16 h were gastrically intubated with a ligand 3 min after 237Np(V) (molar ratio 5.6 to 274), and all were killed 24 h after ligand administration; tissues and excreta were radioanalyzed. Rapid plasma clearance and urinary excretion of 237Np(V) resemble U(VI); deposition and early retention in skeleton and liver resemble Pu(IV). The x-ray absorption near edge structure spectroscopy (XANES) spectra of femora of 237Np(V)-injected mice, compared with spectra of Np(V) and Np(IV) from reference solids, showed predominantly Np(IV). Significant in vivo 237Np chelation was obtained with all of the HOPO and CAM ligands injected at molar ratio 22; the HOPO ligands reduced 237Np in skeleton, liver, and other soft tissue, on average, to 72, 25, and 25% of control, respectively, while CaNa3-DTPA was ineffective. Two HOPO ligands injected 60 min after 237Np (molar ratio 5.6) significantly reduced body and liver 237Np, and three HOPO ligands given orally (molar ratio > or = 73) significantly reduced body and liver 237Np, compared with controls. Combined with earlier work, these results indicate that: the dominant neptunium species circulating and excreted in urine is Np(V), while that in bone and liver deposits is Np(IV); Np(V) must be reduced to Np(IV) before it can be stably chelated; efficient decorporation of neptunium requires multidentate ligands that form exceptionally stable actinide(IV) chelates and facilitate Np(V) reduction.

Oxygen and Phosphorus Coordination Around Iron in Crystalline Ferric Ferrous Pyrophosphate and Iron-Phosphate Glasses with UO2 or Na2O

Fe {ital K}-edge x-ray absorption fine-structure (XAFS) measurements were performed on glass samples of (Fe{sub 3}O{sub 4}){sub 0.3}(P{sub 2}O{sub 5}){sub 0.7} with various amounts of Na{sub 2}O or UO{sub 2}. Near-edge and extended XAFS regions are studied and comparisons are made to several reference compounds. We find that iron in the base glass is {approximately}25{percent} divalent, and that the Fe{sup 2+} coordination is predominantly octahedral, while Fe{sup 3+} sites are roughly split between tetrahedral and octahedral coordinations. Also, we measure roughly one Fe{endash}O{endash}P link per iron. Substitution of Na{sub 2}O or UO{sub 2} up to 15 molh{percent} primarily affects the first Fe{endash}O shell. The results are compared to data from the related material Fe{sub 3}(P{sub 2}O{sub 7}){sub 2}. {copyright} {ital 1999 Materials Research Society.}

The partitioning of uranium and neptunium onto hydrothermally altered concrete

Partition coefficients (Kd) of U(VI) and Np(V) on untreated and hydrothermally altered concrete were measured in 0.01 M NaCl and 0.01 M NaHCO3 solutions as functions of concentration of the radionuclides, pH, and time. The partition coefficients for both U(VI) and Np(V) on hydrothermally altered concrete are significantly lower than those on untreated concrete. The partition of both U(VI) and Np(V) are pH dependent, although the pH dependence does not appear to reflect precipitation of U and Np-bearing phases. Both sorption and precipitation are likely processes controlling partitioning of U to concrete; sorption is the most likely process controlling the partitioning of Np to concrete. The presence of 0.01 M carbonate species in solution decreases Kd of U(VI) for both hydrothermally altered and untreated concrete from ≥ 104 mL/g to ~400 to 1000 mL/g indicating a significant impact on U(VI) sorption. In contrast, the presence of carbonate only reduced the Kdof Np(V) by one order of magnitude or less. X-ray absorption spectroscopy analysis of U/concrete mixtures at different pHs and times indicate that uranyl ions are partitioned as monomeric species on untreated concrete, but oligomeric species on hydrothermally altered concrete. Similar analysis of Np/concrete mixtures shows that about half of the partitioned Np(V) is reduced to Np(IV) over a period of 6 months.

Precipitation of crystalline neptunium dioxide from near-neutral aqueous solution

Abstract We report experimental evidence that crystalline neptunium dioxide, NpO2(cr), will precipitate from near-neutral aqueous solutions that initially contain NpO2+(aq), even under mildly oxidizing conditions. Observed decreases in aqueous Np concentrations with time are balanced by corresponding decreases in pH and accompanied by formation of high-purity crystals of NpO2. The crystalline NpO2 was characterized by X-ray powder diffraction (XRD), X-ray absorption fine structure (XAFS) spectroscopy, and scanning electron microscopy (SEM). Thermodynamic data in the literature suggest that NpO2(cr) is possibly the stable solid phase in aqueous systems containing dissolved NpO2+(aq); however, the precipitation of NpO2(cr) from solutions containing initially NpO2+(aq) had not been observed experimentally until now, possibly because of slow reaction kinetics. Our experiments were conducted at 200°C in order to overcome slow reduction reactions and/or precipitation kinetics. Because long time scales may render slow reaction kinetics irrelevant, thermodynamically stable NpO2(cr) may keep dissolved Np concentrations well below the relatively high Np concentrations (10-3 M) associated with meta-stable Np(V) solids observed in previous laboratory experiments. Projects investigating the suitability of sites for an underground repository for high-level nuclear waste disposal may not need be so concerned with high dose rates due to migration of Np over long times, because these may not apply to repository-relevant time scales over which NpO2(cr) is likely to be stable.

Vibrational properties of Ga-stabilized δ-Pu by extended x-ray absorption fine structure

Temperature dependent extended x-ray absorption fine structure (EXAFS) spectra were measured for a 3.3 at% Ga stabilized Pu alloy over the range T= 20 - 300 K at both the Ga K-edge and the Pu L_III-edge. The temperature dependence of the pair-distance distribution widths, \sigma(T) was accurately modeled using a correlated-Debye model for the lattice vibrational properties, suggesting Debye-like behavior in this material. We obtain pair- specific correlated-Debye temperatures, \Theta_cD, of 110.7 +/- 1.7 K and 202.6 +/- 3.7 K, for the Pu-Pu and Ga-Pu pairs, respectively. These results represent the first unambiguous determination of Ga-specific vibrational properties in PuGa alloys, and indicate the Ga-Pu bonds are significantly stronger than the Pu-Pu bonds. This effect has important implications for lattice stabilization mechanisms in these alloys.