Biological Reduction of a U(V)-Organic Ligand Complex

Metal-reducing microorganisms such as Shewanella oneidensis MR-1 reduce highly soluble species of hexavalent uranyl (U(VI)) to less mobile tetravalent uranium (U(IV)) compounds. The biologically mediated immobilization of U(VI) is being considered for the remediation of U contamination. However, the mechanistic underpinnings of biological U(VI) reduction remain unresolved. It has become clear that a first electron transfer occurs to form pentavalent (U(V)) intermediates, but it has not been definitively established whether a second one-electron transfer can occur or if disproportionation of U(V) is required. Here, we utilize the unusual properties of dpaea2– ((dpaeaH2═bis(pyridyl-6-methyl-2-carboxylate)-ethylamine)), a ligand forming a stable soluble aqueous complex with U(V), and investigate the reduction of U(VI)–dpaea and U(V)–dpaea by S. oneidensis MR-1. We establish U speciation through time by separating U(VI) from U(IV) by ion exchange chromatography and characterize the reaction end-products using U M4-edge high resolution X-ray absorption near-edge structure (HR-XANES) spectroscopy. We document the reduction of solid phase U(VI)–dpaea to aqueous U(V)–dpaea but, most importantly, demonstrate that of U(V)–dpaea to U(IV). This work establishes the potential for biological reduction of U(V) bound to a stabilizing ligand. Thus, further work is warranted to investigate the possible persistence of U(V)–organic complexes followed by their bioreduction in environmental systems

Products of in situ corrosion of depleted uranium ammunition in Bosnia and Herzegovina soils

Hundreds of tons of depleted uranium (DU) ammunition were used in previous armed conflicts in Iraq, Bosnia and Herzegovina, and Serbia/Kosovo. The majority (>90%) of DU penetrators miss their target and, if left in the environment, corrode in these postconflict zones. Thus, the best way to understand the fate of bulk DU material in the environment is to characterize the corrosion products of intact DU penetrators under field conditions for extended periods of time. However, such studies are scarce. To fill this knowledge gap, we characterized corrosion products formed from two intact DU penetrators that remained in soils in Bosnia and Herzegovina for over seven years. We used a combination of X-ray powder diffraction, electron microscopy, and X-ray absorption spectroscopy. The results show that metaschoepite (UO3(H2O)(2)) was a main component of the two DU corrosion products. Moreover, studtite ((UO2)O-2(H2O)(2)center dot 2(H2O)) and becquerelite (Ca(UO2)(6)O-4(OH)(6)center dot 8(H2O)) were also identified in the corrosion products. Their formation through transformation of metaschoepite was a result of the geochemical conditions under which the penetrators corroded. Moreover, we propose that the transformation of metaschoepite to becquerelite or studtite in the DU corrosion products would decrese the potential for mobilization of U from corroded DU penetrators exposed to similar environments in postconflict areas

Characterization of rare-earth doped Lu2O3 nanopowders prepared with polymer complex solution synthesis

We explored a synthesis route based on the polymer complex solution method for the production of the rare-earth doped Lu2O3 crystalline nanopowders. In this type of synthesis polyethylene glycol is used both as fuel for the combustion reaction, and as nucleation agent for the crystallization process. Synthesized materials were characterized with X-ray diffraction technique, scanning and transmission electron microscopy, EDX technique and photoluminescence spectroscopy with steady state and time domain measurements. X-ray diffraction and electron diffraction analysis showed that presented synthesis procedure yields pure-phase, well crystallized Lu2O3 nanopowders with the particles dimensions in the 30-50 nm range, as observed from TEM images. Luminescence properties of Sm3+ and Tb3+ doped Lu2O3 exhibited characteristic red and pseudo-white emissions from these rare-earth ions, with an average emission lifetime of 0.8 and 0.6 ms, respectively. (C) 2010 Elsevier B.V. All rights reserved

Evaluation of Young’s modulus of MgB2 filaments in composite wires for the superconducting links for the high-luminosity LHC upgrade

MgB2 wire is a promising superconductor for the superconducting links for the high-luminosity upgrade of the large Hadron collider at CERN. The mechanical properties of MgB2 must be fully quantified for the cable design, and in this study, we evaluate the Young's modulus of MgB2 filaments in wires with a practical level of critical current. The Young's moduli of MgB2 filaments by two different processes, in situ and ex situ, were compared. Two different evaluation methods were applied to an in situ MgB2 wire, a single-fiber tensile test and a tensile test after removing Monel. In addition, the Young's modulus of the few-micron-thick Nb–Ni reaction layer in an ex situ processed wire was evaluated using a nanoindentation testing technique to improve the accuracy of analysis based on the rule of mixtures. The Young's moduli of the in situ and ex situ MgB2 wires were in the range of 76–97 GPa and no distinct difference depending on the fabrication process was found

Fabrication of polycrystalline (Y0.7Gd0.3)(2)O-3:Eu3+ ceramics: The influence of initial pressure and sintering temperature on its morphology and photoluminescence activity

Nanocrystalline (Y0.7Gd0.3)(2)O-3 powder, synthetised via polymer complex solution method, was compacted into 25 pellets applying high pressures (173-867 MPa) for 30 s that were subsequently sintered at different temperatures (800-1400 degrees C) for 18 h. The morphology and optical characteristics of the starting powder and prepared ceramic samples were monitored and discussed in order to identify the changes induced with the variations of initial compacting pressure, which influence is often neglected, and with sintering temperature. The grain size tends to decrease significantly with increasing pressure, even when elevated temperatures are used for annealing, while low compacting pressure resulted in grain coarsening and, in some cases, even in anomalous morphology of ceramic samples. Luminescence emission in ceramic samples decays faster than in nanopowders, that is in complete agreement with the grain formation and gradual transformation to the bulk material. Judd-Ofelt intensity parameters and branching ratios were calculated taking into account the difference in effective refractive index for nanopowder and ceramic samples. (C) 2011 Elsevier Ltd and Techna Group S.r.l. All rights reserved

Preparation, structural and spectroscopic studies of (YxLu1-x)(2)O-3:Eu3+ nanopowders

Lutetium and yttrium oxides are promising scintillating materials suitable for use in medical planar X-ray imaging and mammography. In this paper the procedure for preparation of europium doped mixed lutetium-yttrium oxide nanopowders using polymer complex solution synthesis method is presented. Detailed information on nanopowder phase, morphology and crystallinity are obtained using X-ray powder diffraction. SEM and TEM while optical properties are investigated by photoluminescence and radioluminescence measurements. Constituting nanoparticles are 20-40 nm in size, and have excellent structural ordering in cubic bixbyite-type. Unit cell parameter, ionic coordinates, crystal coherence size and microstrain are determined from Rietveld analysis. All powders show strong Eu3+-characteristic red emission, with an average D-5(0) emission lifetime of 1.5 ms. Radioluminescence efficiency is about 15% of the commercial micron-sized Gd2O2S:Eu3+ powder while negligible level of afterglow is found. (C) 2010 Elsevier B.V. All rights reserved

Effect of Aging on the Stability of Microbially Reduced Uranium in Natural Sediment

Reductive immobilization of uranium has been explored as a remediation strategy for the U-contaminated subsurface. Via the in situ biostimulation of microbial processes, hexavalent U is reduced to less soluble tetravalent species, which are immobilized within the sediment. Although the mineral uraninite (UO2) was initially considered the dominant product of biological reduction, non-crystalline U(IV) species (NCU(IV)) are found to be abundant in the environment despite their greater susceptibility to oxidation and remobilization. However, it has been recently proposed that, through aging, NCU(IV) might transform into UO2, which would potentially enhance the stability of the reduced U pool. In this study, we performed column experiments to produce NCU(IV) species in natural sediment mimicking the environmental conditions during bioremediation. Bioreduced sediment retrieved from the columns and harboring NCU(IV) was incubated in static microcosms under anoxic conditions to allow the systematic monitoring of U coordination by X-ray absorption spectroscopy (XAS) over 12 months. XAS revealed that, under the investigated conditions, the speciation of U(IV) does not change over time. Thus, because NCU(IV) is the dominant species in the sediment, bioreduced U(IV) species remain vulnerable to oxidation and remobilization in the aqueous phase even after a 12-month aging period