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UC and UMeC2 (Me = Fe, Zr, Mo) carbides were studied by the high‐energy‐resolution fluorescence‐detected X‐ray absorption (HERFD‐XAS) technique at the U M4 and L3 edges. Both U M4 and L3 HERFD‐XAS reveal some differences between UMeC2 and UC; there are differences also between the M4 and L3 edge results for both types of carbide in terms of the spectral width and energy position. The observed differences are attributed to the consequences of the U 5f, 6d–4d(3d) hybridization in UMeC2. Calculations of the U M4 HERFD‐XAS spectra were also performed using the Anderson impurity model (AIM). Based on the analysis of the data, the 5f occupancy in the ground state of UC was estimated to be 3.05 electrons. This finding is also supported by the analysis of U N4,5 XAS of UC and by the results of the AIM calculations of the U 4f X‐ray photoelectron spectrum of UC. [ABSTRACT FROM AUTHOR]

The structural characterisation of actinide nanoparticles (NPs) is of primary importance and hard to achieve, especially for non‐homogeneous samples with NPs less than 3 nm. By combining high‐energy X‐ray scattering (HEXS) and high‐energy‐resolution fluorescence‐detected X‐ray absorption near‐edge structure (HERFD XANES) analysis, we have characterised for the first time both the short‐ and medium‐range order of ThO2 NPs obtained by chemical precipitation. By using this methodology, a novel insight into the structures of NPs at different stages of their formation has been achieved. The pair distribution function revealed a high concentration of ThO2 small units similar to thorium hexamer clusters mixed with 1 nm ThO2 NPs in the initial steps of formation. Drying the precipitates at around 150 °C promoted the recrystallisation of the smallest units into more thermodynamically stable ThO2 NPs. HERFD XANES analysis at the thorium M4 edge, a direct probe for f states, showed variations that we have correlated with the breakdown of the local symmetry around the thorium atoms, which most likely concerns surface atoms. Together, HEXS and HERFD XANES are a powerful methodology for investigating actinide NPs and their formation mechanism. [ABSTRACT FROM AUTHOR]

A general strategy for the determination of Tc oxidation state by new approach involving X-ray absorption near edge spectroscopy (XANES) at the Tc L3 edge is shown. A comprehensive series of 99Tc compounds, ranging from oxidation states I to VII, was measured and subsequently simulated within the framework of crystal-field multiplet theory. The observable trends in the absorption edge energy shift in combination with the spectral shape allow for a deeper understanding of complicated Tc coordination chemistry. This approach can be extended to numerous studies of Tc systems as this method is one of the most sensitive methods for accurate Tc oxidation state and ligand characterization. [ABSTRACT FROM AUTHOR]

The dilution of aqueous neptunium carbonate complexes induces the intrinsic formation of nanocrystalline neptunium dioxide (NpO2) particles, which are characterised by UV/Vis and X-ray absorption spectroscopies and transmission electron microscopy. This new route of nanocrystalline NpO2 formation could be a potential scenario for the environmental transport of radionuclides from the waste repository (i.e. under near-field alkaline conditions) to the geological environment (i.e. under far-field neutral conditions). [ABSTRACT FROM AUTHOR]

Keywords: actinides; nanoparticles; thorium; X-ray absorption spectroscopy; X-ray scattering EN actinides nanoparticles thorium X-ray absorption spectroscopy X-ray scattering 1 1 1 01/06/21 20210104 NES 210104 B Oh, now I see b ! The precipitates formed during Th SP IV sp hydrolysis are ThO SB 2 sb nanoparticles, which in their initial stages of formation are a mixture of ultra-small nanoparticles and Th hexamer clusters. Actinides, nanoparticles, thorium, X-ray absorption spectroscopy, X-ray scattering. [Extracted from the article]

The structural characterisation of actinide nanoparticles (NPs) is of primary importance and hard to achieve, especially for non‐homogeneous samples with NPs less than 3 nm. By combining high‐energy X‐ray scattering (HEXS) and high‐energy‐resolution fluorescence‐detected X‐ray absorption near‐edge structure (HERFD XANES) analysis, we have characterised for the first time both the short‐ and medium‐range order of ThO2 NPs obtained by chemical precipitation. By using this methodology, a novel insight into the structures of NPs at different stages of their formation has been achieved. The pair distribution function revealed a high concentration of ThO2 small units similar to thorium hexamer clusters mixed with 1 nm ThO2 NPs in the initial steps of formation. Drying the precipitates at around 150 °C promoted the recrystallisation of the smallest units into more thermodynamically stable ThO2 NPs. HERFD XANES analysis at the thorium M4 edge, a direct probe for f states, showed variations that we have correlated with the breakdown of the local symmetry around the thorium atoms, which most likely concerns surface atoms. Together, HEXS and HERFD XANES are a powerful methodology for investigating actinide NPs and their formation mechanism. [ABSTRACT FROM AUTHOR]

The mechanism of selenium(IV) uptake by maghemite was investigated on both the macroscopic and the molecular level. Maghemite nanoparticles exhibited fast adsorption kinetics toward selenium(IV). Batch experiments showed a decreased sorption with increasing pH (3.5-11). Ionic strength variations (0.01 to 0.1 M NaCl) had no significant influence on selenium(IV) uptake. Electrophoretic mobility measurements revealed a significant shift toward lower values of the isoelectric point of maghemite upon selenium(IV) uptake, suggesting the formation of inner-sphere surface complexes. At the molecular level, using X-ray Absorption Fine-Structure Spectroscopy (EXAFS), the formation of both bidentate binuclear corner-sharing (2C) and bidentate mononuclear edge-sharing (1E) inner-sphere surface complexes was observed, with a trend toward solely 1E complexes at high pH. The absence of a tridentate surface complex as observed for arsenic(III) and antimonite(III) might be due to the relatively small size of the SeIVO3 unit. These new spectroscopic results can be implemented in reactive transport models to improve the prediction of selenium migration behavior in the environment as well as its monitoring through its interaction with maghemite or maghemite layers at the surface of magnetite. Due to its chemical stability even at low pH and its magnetization properties allowing magnetic separation, maghemite is a promising sorbing phase for the treatment of Se polluted waters. [ABSTRACT FROM AUTHOR]