Nanoscale mechanism of UO2 formation through uranium reduction by magnetite

Uranium (U) is a ubiquitous element in the Earth’s crust at ~2 ppm. In anoxic environments, soluble hexavalent uranium (U(VI)) is reduced and immobilized. The underlying reduction mechanism is unknown but likely of critical importance to explain the geochemical behavior of U. Here, we tackle the mechanism of reduction of U(VI) by the mixed-valence iron oxide, magnetite. Through high-end spectroscopic and microscopic tools, we demonstrate that the reduction proceeds first through surface-associated U(VI) to form pentavalent U, U(V). U(V) persists on the surface of magnetite and is further reduced to tetravalent UO2 as nanocrystals (~1–2 nm) with random orientations inside nanowires. Through nanoparticle re-orientation and coalescence, the nanowires collapse into ordered UO2 nanoclusters. This work provides evidence for a transient U nanowire structure that may have implications for uranium isotope fractionation as well as for the molecular-scale understanding of nuclear waste temporal evolution and the reductive remediation of uranium contamination

Aqueous U(VI) interaction with magnetic nanoparticles in a mixed flow reactor system: HR-XANES study

The redox variations and changes in local atomic environment of uranium (U) interacted with the magnetite nanoparticles were studied in a proof of principle experiment by the U L3 and M4 edges high energy resolution X-ray absorption near edge structure (HR-XANES) technique. We designed and applied a mixed flow reactor (MFR) set-up to maintain dynamic flow conditions during U-magnetite interactions. Formation of hydrolyzed, bi- and poly-nuclear U species were excluded by slow continuous injection of U(VI) (10-6 M) and pH control integrated in the MFR set-up. The applied U HR-XANES technique is more sensitive to minor changes in the U redox states and bonding compared to the conventional XANES method. Major U(VI) contribution in uranyl type of bonding is found in the magnetite nanoparticles after three days operation time of the MFR. Indications for shortening of the U-Oaxial bond length for the magnetite compared to the maghemite system are present too

Studies of Actinides Reduction on Iron Surfaces by Means ofResonant Inelastic X-ray Scattering

The interaction of actinides with corroded iron surfaces was studied using resonant inelastic x-ray scattering (RIXS) spectroscopy at actinide 5d edges. RIXS profiles, corresponding to the f-f excitations are found to be very sensitive to the chemical states of actinides in different systems. Our results clearly indicate that U(VI) (as soluble uranyl ion) was reduced to U(IV) in the form of relatively insoluble uranium species, indicating that the iron presence significantly affects the mobility of actinides, creating reducing conditions. Also Np(V) and Pu (VI) in the ground water solution were getting reduced by the iron surface to Np(IV) and Pu (IV) respectively. Studying the reduction of actinides compounds will have an important process controlling the environmental behavior. Using RIXS we have shown that actinides, formed by radiolysis of water in the disposal canister, are likely to be reduced on the inset corrosion products and prevent release from the canister

Comparative U, Np and Pu M edge high energy resolution X-ray absorption spectroscopy (HR-XANES) investigations of model and genuine active waste glass

Genuine radioactive glass sampled from the vitrification plant Karlsruhe and actinide doped model 2 glasses  are  investigated  by  U/Pu/Np  M4/M5  high  energy  resolution  X‐ray  absorption near edge structure (HR‐XANES), U L3 EXAFS and XPS spectroscopy techniques to characterize and compare the U, Pu and Np oxidation states and their local atomic environments. The importance of the results will be discussed in terms of the strategy of using simplified simulated waste glasses to understand more complex industrial glass samples. The final goal of these studies is to predict the long term behavior of vitrified nuclear waste stored in a nuclear waste repository. Highly active waste concentrate (HAWC) from nuclear fuel reprocessing is immobilized in borosilicate glass matrices to generate a disposable waste form [1]. Between 2009 and 2010, the vitrification plant Karlsruhe (VEK) was operated for vitrification of liquid process residues left over from operation of the former reprocessing plant Karlsruhe (WAK). About 56 m3 HAWC were processed, resulting in 50 t of waste  glass  [2].  The  long  term  radiotoxicity  of  U,  Np,  Pu  and  other  actinide  elements  (An),  minor constitute of the reprocessed waste, is of great concern in safety assessment studies of nuclear waste repositories. For example, in case of water intrusion and interaction with the glass matrix, corrosion processes will take place which might facilitate the release of radionuclides into the geosphere. The An redox state and bonding characteristics in the glass matrix determine their release mechanisms and retention processes taking place in near and far field of the repository [3]. Understanding the long term behavior of vitrified nuclear waste requires full and detailed characterization of the materials including their characteristics as synthesized and after exposure to  groundwater. Genuine radioactive waste glass has a complex chemical composition. Therefore we take a simplified approach by investigating and comparing the oxidation states of U, Pu and Np in high level waste (HLW) glass sampled from the VEK vitrification process (VEK glass) and in model glasses. The model glasses doped with U and Pu have the same borosilicate glass frit composition as the VEK glass, whereas  the  model  glass  doped  with  Np  has  a  base  glass  composition  (R7T7)  typically  used  for  vitrification of HLW in France. U/Pu/Np  M4/M5  edge  high  energy  resolution  X‐ray  absorption  near  edge  structure  (HR‐XANES)  spectroscopy technique [4] is applied to characterize the An oxidation states

Comparative U, Np and Pu M edge high energy resolution X-ray absorption spectroscopy (HR-XANES) investigations of model and genuine active waste glass

Understanding the long term behavior of vitrified nuclear waste requires a full and detailed characterization of the materials including their characteristics as synthesized and after exposure to groundwater. Genuine radioactive waste glass has a complex chemical composition. Therefore we take a simplified approach by investigating and comparing the oxidation states of U, Pu and Np in high level waste (HLW) glass sampled from the VEK vitrification process (VEK glass) and in model glasses. The model glasses doped with U and Pu have the same borosilicate glass frit composition as the VEK glass, whereas the model glass doped with Np has a base glass composition (R7T7) typically used for vitrification of HLW in France. U/Pu/Np M4/M5 edge high energy resolution X-ray absorption near edge structure (HR-XANES) spectroscopy technique [1] is applied to characterize the An oxidation states in model and genuine VEK HLW glass. The HR-XANES analyses suggest predominant existence of U(VI) and Pu(IV) in the HLW and the model glasses as expected from the oxidative vitrification conditions. Weak changes in U oxidation state as a function of the U loading (1.2 – 5 wt% UO2) are discussed on the basis of U M4 edge HR-XANES and X-ray photoelectron spectroscopy (XPS) results. One significant difference found between the model and the genuine HLW glasses is the strong radiation damage induced in the HLW glass by the soft X-ray beam (position of the U M4 edge: 3.73 keV) which was not observed for the U doped model glasses and the previous L3 edge investigations of the HLW glass sample. The dominant U(VI) oxidation state is reduced almost by 50% to U(IV) within 5 h of measurement. The complex chemical composition of the HLW glass leads to different local U atomic environments compared to the model glass as found by EXAFS investigations. EXAFS results confirm that U in the HLW glass is coordinated by Al/Si neighbors in the second coordination sphere, whereas no neighboring atoms are observed at this distance for the model glass. Differences in results obtained for the Np oxidation state for Np doped asprepared and leached R7T7 borosilicate model glasses and the HLW glass will be presented and discussed

Spectator and participator processes in the resonant photon-in and photon-out spectra at the Ce L

We study both theoretically and experimentally the photon-in and photon-out spectra of CeO2, which are caused by the Ce 2p to Ce 5d excitation followed by the three different de-excitation channels: (i) Ce 3d to Ce 2p (denoted by 3d-RXES), (ii) O 2p to Ce 2p (v-RXES), and (iii) Ce 5d to Ce 2p (RIXS). In 3d- and v-RXES, the 5d electron plays a role of a spectator, but in RIXS it is a participator. By extending our single impurity Anderson model (SIAM), which was used recently for our calculations of v- and 3d-RXES spectra of CeO2, we study the polarization dependence in the spectator and participator spectra, and we perform more detailed calculations for 3d- and v-RXES spectral features, as well as new calculations for the RIXS spectrum with charge transfer excitations. The polarization dependence is different for the spectator and participator spectra; we have no polarization correlation between the incident and emitted photons for the spectator spectra but a strong polarization correlation for the participator spectrum. The theoretical calculations predict that the charge transfer excitations in RIXS occur in the transfer-energy range overlapped with v-RXES, but the RIXS and v-RXES spectra can be discriminated by taking advantage of the different polarization dependence. The overlapped RIXS and v-RXES spectra are observed successfully by our experiments and well reproduced by our SIAM calculations

Tuning of the size and the lattice parameter of ion-beam synthesized Pb nanoparticles embedded in Si

The size and lattice constant evolution of Pb nanoparticles (NPs) synthesized by high fluence implantation in crystalline Si have been studied with a variety of experimental techniques. Results obtained from small-angle x-ray scattering showed that the Pb NPs grow with increasing implantation fluence and annealing duration. The theory of NP growth kinetics can be applied to qualitatively explain the size evolution of the Pb NPs during the implantation and annealing processes. Moreover, the lattice constant of the Pb NPs was evaluated by conventional x-ray diffraction. The lattice dilatation was observed to decrease with increasing size of the Pb NPs. Such lattice constant tuning can be attributed to the pseudomorphism caused by the lattice mismatch between the Pb NPs and the Si matrix

Electronic Structure of Two Dimensional Lead II Iodide Perovskites An Experimental and Theoretical Study

Layered two-dimensional (2D) hybrid organic–inorganic perovskites (HOP) are promising materials for light-harvesting applications because of their chemical stability, wide flexibility in composition and dimensionality, and increases in photovoltaic power conversion efficiencies. Three 2D lead iodide perovskites were studied through various X-ray spectroscopic techniques to derive detailed electronic structures and band energetics profiles at a titania interface. Core-level and valence band photoelectron spectra of HOP were analyzed to resolve the electronic structure changes due to the reduced dimensionality of inorganic layers. The results show orbital narrowing when comparing the HOP, the layered precursor PbI₂, and the conventional 3D (CH₃NH₃)­PbI₃ such that different localizations of band edge states and narrow band states are unambiguously due to the decrease in dimensionality of the layered HOPs. Support from density functional theory calculations provide further details on the interaction and band gap variations of the electronic structure. We observed an interlayer distance dependent dispersion in the near band edge electronic states. The results show how tuning the interlayer distance between the inorganic layers affects the electronic properties and provides important design principles for control of the interlayer charge transport properties, such as the change in effective charge masses as a function of the organic cation length. The results of these findings can be used to tune layered materials for optimal functionality and new applications