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.

In recent years, scientists have progressively recognized the role of electronic structures in the characterization of chemical properties for actinide containing materials. High-energy resolution X-ray spectroscopy at the actinide M_4,5 edges emerged as a promising direction because this method can probe actinide properties at the atomic level through the possibility of reducing the experimental spectral width below the natural core-hole lifetime broadening. Parallel to the technical developments of the X-ray method and experimental discoveries, theoretical models, describing the observed electronic structure phenomena, have also advanced. In this feature article, we describe the latest progress in the field of high-energy resolution X-ray spectroscopy at the actinide M_4,5 and ligand K edges and we show that the methods are able to (a) provide fingerprint information on the actinide oxidation state and ground state characters (b) probe 5f occupancy, non-stoichiometry, defects, and ligand/metal ratio and (c) investigate the local symmetry and effects of the crystal field. We discuss the chemical aspects of the electronic structure in terms familiar to chemists and materials scientists and conclude with a brief description of new opportunities and approaches to improve the experimental methodology and theoretical analysis for f-electron systems.

X-ray absorption experiments at Ce L₃ and M₅ edges and theoretical calculations demonstrate that in addition to the nanoceria charge stability, the formation of hydroxyl groups at the surface affects the chemical performance of nanomaterials.

Significance: This work is to our knowledge the first illustration of the ability of the high-energy resolution fluorescence-detection X-ray absorption spectroscopy (HERFD-XAS) technique to directly probe the crystal-field splitting in the 5 f shell of actinides. This is a breakthrough for actinide science, allowing for the easy extraction of desired information from the spectroscopic method, which is easy to interpret and to calculate. Furthermore, the HERFD-XAS technique allows us to resolve the charge-transfer satellites in actinide spectra that were hidden before, thus enhancing the sensitivity to the covalent character of the chemical bonding. Using ThO ₂ as an example, we show that other common techniques underestimate the crystal-field strength and that ThO ₂ is not an anionic compound as previously believed.

Here we provide evidence that the formation of PuO₂ nanoparticles from oxidized Pu^VI under alkaline conditions proceeds through the formation of an intermediate Pu^V solid phase, similar to NH₄PuO₂CO₃, which is stable over a period of several months. For the first time, state‐of‐the‐art experiments at Pu M₄ and at L₃ absorption edges combined with theoretical calculations unambiguously allow to determine the oxidation state and the local structure of this intermediate phase.

A systematic x-ray absorption study at the U $3d$, $4d$ and $4f$ edges of UO$_2$ was performed and the data were analyzed within framework of the Anderson impurity model. By applying the high-energy-resolution fluorescence-detection (HERFD) mode of x-ray absorption spectroscopy (XAS) at the U $3d_{3/2}$ edge and conducting the XAS measurements at the shallower U $4f$ levels, fine details of the XAS spectra were resolved resulting from reduced core-hole lifetime broadening. This multi-edge study enabled a far more effective analysis of the electronic structure at the U sites and characterization of the chemical bonding and degree of the $5f$ localization in UO$_2$. The results support the covalent character of UO$_2$ and do not agree with the suggestions of rather ionic bonding in this compound as expressed in some publications.