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Abstract: Ferrihydrite was exposed to U(VI)-containing cement leachate (pH 10.5) and aged to induce crystallization of hematite. A combination of chemical extractions, TEM, and XAS techniques provided the first evidence that adsorbed U(VI) (?3000 ppm) was incorporated into hematite during ferrihydrite aggregation and the early stages of crystallization, with continued uptake occurring during hematite ripening. Analysis of EXAFS and XANES data indicated that the U(VI) was incorporated into a distorted, octahedrally coordinated site replacing Fe(III). Fitting of the EXAFS showed the uranyl bonds lengthened from 1.81 to 1.87 Å, in contrast to previous studies that have suggested that the uranyl bond is lost altogether upon incorporation into hematite. The results of this study both provide a new mechanistic understanding of uranium incorporation into hematite and define the nature of the bonding environment of uranium within the mineral structure. Immobilization of U(VI) by incorporation into hematite has clear and important implications for limiting uranium migration in natural and engineered environments.

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Abstract: At nuclear contaminated sites, microbially-mediated Fe(III) reduction under alkaline conditions opens up the potential for co-treatment of the groundwater contaminants 99Tc, though reduction to less mobile Tc(IV) phases, and 90Sr, through increased sorption and/or precipitation promoted at higher pH. In the experiments described here, microbial enrichment cultures derived from representative Sellafield sediments were used to probe the effect of microbially-mediated Fe(III) reduction on the mobility of 99Tc and Sr (as stable Sr2+ at elevated concentrations and 90Sr2+ at ultra-trace concentrations) under both neutral and alkaline conditions. The reduction of Fe(III) in enrichment culture experiments at an initial pH of 7 or 9 resulted in the precipitation of an Fe(II) bearing biomineral comprised of siderite and vivianite. Results showed that added at 1.6 × 10−6 M was removed (>80%) from solution concurrent with Fe(III) reduction at both pH 7 and pH 9. Furthermore, X-ray absorption spectroscopy of the reduced biominerals confirmed reduction of Tc(VII) to Tc(IV). To understand Sr behaviour in these systems, Sr2+ was added to enrichment cultures at ultra-trace concentrations (2.2 × 10−10 M (as 90Sr2+)) and at higher concentrations (1.15 × 10−3 M (as stable Sr2+)). In ultra-trace experiments at pH 7, microbially active systems showed enhanced removal of 90Sr compared to the sterile control. This was likely due to sorption of 90Sr2+ to the Fe(II)-bearing biominerals that formed in situ. By contrast, at pH 9, the sterile control showed comparable removal of 90Sr to the microbially active experiment even though the Fe-minerals formed were of very different character in the active (vivianite, siderite) versus sterile (an amorphous Fe(III)-phase) systems. Overall, 90Sr bioreduction experiments showed 60–70% removal of the added 90Sr across the different systems: this suggests that treatment strategies involving bioreduction and the promotion of Fe(III)-reducing conditions to scavenge Tc(IV) are not incompatible with treatment of groundwater 90Sr contamination. In systems with elevated Sr2+ concentrations and an initial pH of 7, microbially active systems showed <20% removal of added Sr2+ following Fe(III) reduction with little or no removal in sterile controls. At pH 9, significant Sr2+ was removed from solution in both sterile and microbially active experiments and was attributed to Sr-sorption to mineral phases both chemically precipitated in sterile controls, and biologically precipitated in the microbially active systems. These results confirm that in systems with an elevated natural or anthropogenic Sr2+ loading, bioreduction at modestly alkaline pH is compatible with co-treatment of both and 90Sr2+. These data are discussed in terms of aqueous geochemistry trends, X-ray diffraction and morphological data, and thermodynamic modelling. The results demonstrate the potential for removal of trace levels of 99Tc and 90Sr2+ from groundwaters during stimulated bioreduction and highlight that in the presence of stable Sr2+, optimal removal for technetium and strontium is likely to occur under mildly alkaline, reducing conditions.

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Abstract: Lanthanide(III) complexes with N-donor extractants, which exhibit the potential for the separation of minor actinides from lanthanides in the management of spent nuclear fuel, have been directly synthesized and characterized in both solution and solid states. Crystal structures of the Pr3+, Eu3+, Tb3+, and Yb3+ complexes of 2,9-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-1,10-phenanthroline (CyMe4-BTPhen) and the Pr3+, Eu3+, and Tb3+ complexes of 6,6?-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-2,2?-bypyridine (CyMe4-BTBP) were obtained. The majority of these structures displayed coordination of two of the tetra-N-donor ligands to each Ln3+ ion, even when in some cases the complexations were performed with equimolar amounts of lanthanide and N-donor ligand. The structures showed that generally the lighter lanthanides had their coordination spheres completed by a bidentate nitrate ion, giving a 2+ charged complex cation, whereas the structures of the heavier lanthanides displayed tricationic complex species with a single water molecule completing their coordination environments. Electronic absorption spectroscopic titrations showed formation of the 1:2 Ln3+/LN4-donor species (Ln = Pr3+, Eu3+, Tb3+) in methanol when the N-donor ligand was in excess. When the Ln3+ ion was in excess, evidence for formation of a 1:1 Ln3+/LN4-donor complex species was observed. Luminescent lifetime studies of mixtures of Eu3+ with excess CyMe4-BTBP and CyMe4-BTPhen in methanol indicated that the nitrate-coordinated species is dominant in solution. X-ray absorption spectra of Eu3+ and Tb3+ species, formed by extraction from an acidic aqueous phase into an organic solution consisting of excess N-donor extractant in pure cyclohexanone or 30% tri-n-butyl phosphate (TBP) in cyclohexanone, were obtained. The presence of TBP in the organic phase did not alter lanthanide speciation. Extended X-ray absorption fine structure data from these spectra were fitted using chemical models established by crystallography and solution spectroscopy and showed the dominant lanthanide species in the bulk organic phase was a 1:2 Ln3+/LN-donor species.

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Abstract: High-resolution synchrotron radiation x-ray powder diffraction (HR-XRPD) combined with Hf L3-edge extended x-ray absorption fine structure allowed us to determine the structure of a Hf-UiO-66 metal-organic framework (MOF) showing that it is isoreticular to Zr-UiO-66 MOF [ Cavka et al. J. Am. Chem. Soc. 130 13850 (2008)]. Thermal gravimetric measurements (coupled with mass spectroscopy) and temperature-dependent synchrotron radiation XRPD proved the high thermal stability of the Hf-UiO-66 MOF. The Langmuir surface area (849 m2/g) combined with the high stability of the UiO-66 framework and with the high neutron absorption cross section of Hf suggest that among all microporous crystalline materials the Hf-UiO-66 MOF possesses the physical and chemical requirements for the interim storage of radioactive waste in a much safer way than is currently available. The first results proving the synthesis of a MOF material with UiO-66 topology realized by a B-containing linker are also reported, allowing a further improvement of the neutron shielding power of this class of materials.

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Abstract: The average number of 5f electrons making up the valence state in plutonium metal together with the electronic fluctuations on each metal site has been a recent subject of debate. For the delta phase of Pu, where compared to the alpha phase increased localization (more atomiclike character) leads to decreased overlap and volume increase, an f count close to either 5 or 6 has been proposed depending on the type of electronic structure calculation. In order to resolve the controversy, we analyze the Pu 4f photoemission spectrum, which displays well screened and poorly screened peaks that can be used as a measure for the degree of localization. A simple analytical two-level model already shows on general grounds that the f count for Pu must be between 5 and 5.5. Furthermore, we present detailed Anderson impurity model calculations including the full multiplet structure for Pu 4f photoemission, which are compared to previous experimental results obtained from 1 to 9 monolayers thin films of Pu on Mg and from Pu metal in the alpha and delta phases. The trend in the satellite to main peak intensity ratio as a function of the Pu layer thickness gives a clear indication that Pu metal has an 5f(5) like ground state. For the Pu allotropes and thicker films an f count of 5.22 is obtained with a Coulomb interaction U=4 eV. The 5f fluctuations in Pu metal are very prominent and strongly material dependent. The calculations give a ground state with 9.6% f(4), 58.8% f(5), and 31.6% f(6) for the alpha phase and 5.7% f(4), 66.4% f(5), and 27.8% f(6) for the delta phase while for the thin films the amount of f(5) and the localization strongly increase with reduced thickness. The obtained findings are in agreement with recent electronic structure calculations for delta Pu using local-density approximation with dynamical mean-field theory and with the branching-ratio analysis of the Pu N-4,N-5 edge in electron-energy-loss spectroscopy.