An investigation of the interactions of Eu³⁺ and Am³⁺ with uranyl minerals: implications for the storage of spent nuclear fuel

Uranium Oxide Hydrate Frameworks with Dy(III) or Lu(III) Ions: Insights Into the Framework Structures With Lanthanide Ions

Two uranium oxide hydrate frameworks (UOHFs) with either Dy3+ or Lu3+ ions, Dy1.36(H2O)6[(UO2)10UO13(OH)4] (UOHF-Dy) or Lu2(H2O)8[(UO2)10UO14(OH)3] (UOHF-Lu), were synthesized hydrothermally and characterized with a range of structural and spectroscopic techniques. Although SEM-EDS analysis confirmed the same atomic ratio of ~5.5 for U : Dy and U : Lu, they displayed different crystal morphologies, needles for UOHF-Dy in the orthorhombic C2221 space group and plates for UOHF-Lu in the triclinic P-1 space group. Both frameworks are composed of β-U3O8 type layers linked by pentagonal bipyramidal uranium polyhedra, with the Dy3+/Lu3+ ions inside the channels. However, the arrangements of Dy3+/Lu3+ ions are different, with disordered Dy3+ ions well aligned at the centers of the channels and single Lu3+ ions well-separated in a zigzag pattern in the channels. While the characteristic vibrational modes were revealed by Raman spectroscopy, the presence of a pentavalent uranium center in UOHF-Lu was confirmed with diffuse reflectance spectroscopy. The formation of two types of UOHFs with lanthanide ions, high or low symmetry, and the structure trend were discussed regards to synthesis conditions and lanthanide ionic radius. This work highlights the complex chemistry driving the formation of UOHFs with lanthanide ions and has implications to the spent nuclear fuel under geological disposal.

Synthetic uranium oxide hydrate materials: Current advances and future perspectives

Uranium oxide hydrate (UOH) materials, a group of minerals and synthetic phases, have attracted recent attention due to their high structural flexibility and diversity as well as their primary relationship with natural weathering of the mineral uraninite and the alteration of spent nuclear fuel (SNF) in geological disposal. Due to the limited structural and chemical understanding of UOH minerals, synthetic UOH phases provide a unique opportunity to fill existing knowledge gaps through the exploration of further structural diversity and distinctive properties, as well as potential applications. Some of the latest developments of synthetic UOH phases include the incorporation of 3d transition metal and lanthanide ions, the evolution of uranyl oxide hydroxide layers driven by interlayer charge, the structural diversity of uranyl oxide hydrate frameworks, and the intrinsic driving force for the formation of diversified structural types. The purpose of this review is to provide a comprehensive summary of the latest advancements of synthetic UOH phases with 3d transition and lanthanide metal ions, including their syntheses, structural diversities, microstructures, uranium valences, vibration modes, and structural and chemical complexities. It also highlights the subsequent implications of these advancements on uranium geochemistry and SNF alterations, amongst other potential applications. A further discussion on technical challenges and knowledge gaps is included to identify areas for future research.

Reaction of Cu(II) Chelates with Uranyl Nitrate to Form a Coordination Complex or H-Bonded Adduct: Experimental Observations and Rationalization by Theoretical Calculations

Four new heterometallic Cu(II)-U(VI) species, [{(CuL¹)(CH₃CN)}UO₂(NO₃)₂] (1), [{(CuL²)(CH₃CN)}UO₂(NO₃)₂] (2), [{(CuL³)(H₂O)}UO₂(NO₃)₂] (3), and [UO₂(NO₃)₂(H₂O)₂]·2[CuL⁴]·H₂O (4), were synthesized using four different metalloligands ([CuL¹], [CuL²], [CuL³], and [CuL⁴], respectively) derived from four unsymmetrically dicondensed N,O-donor Schiff bases. Single-crystal structural analyses revealed that complexes 1, 2, and 3 have a discrete dinuclear [Cu-UO₂] core in which one metalloligand, [CuL], is connected to the uranyl moiety via a double phenoxido bridge. Two chelating nitrate ions complete the octa-coordination around uranium. Species 4 is a cocrystal, where a uranyl nitrate dihydrate is sandwiched between two metalloligands [CuL⁴] by the formation of strong hydrogen bonds between the H atoms of the coordinated water molecules to U(VI) and the O atoms of [CuL⁴]. Spectrophotometric titrations of these four metalloligands with uranyl nitrate dihydrate in acetonitrile showed a well-anchored isosbestic point between 300 and 500 nm in all cases, conforming with the coordination of [CuL¹], [CuL²], [CuL³], and the H-bonding interaction of [CuL⁴] with UO₂(NO₃)₂. This behavior of [CuL⁴] was utilized to selectively bind metal ions (e.g., Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, and La³⁺) in the presence of UO₂(NO₃)₂·2H₂O in acetonitrile. The formation of these Cu(II)-U(VI) species in solution was also evaluated by steady-state fluorescence quenching experiments. The difference in the coordination behavior of these metalloligands toward [UO₂(NO₃)₂(H₂O)₂] was studied by density functional theory calculations. The lower flexibility of the ethylenediamine ring and a large negative binding energy obtained from the evaluation of H bonds and supramolecular interactions between [CuL⁴] and [UO₂(NO₃)₂(H₂O)₂] corroborate the formation of cocrystal 4. A very good linear correlation (r² = 0.9949) was observed between the experimental U═O stretching frequencies and the strength of the equatorial bonds that connect the U atom to the metalloligand.

Layer-structured uranyl-oxide hydroxy-hydrates with Pr(iii) and Tb(iii) ions: hydroxyl to oxo transition driven by interlayer cations

We report the hydrothermal synthesis and characterization of two uranyl-oxide hydroxy-hydrate compounds with Pr(iii) (U-Pr) and Tb(iii) (U-Tb) ions prepared via direct hydrothermal reactions of lanthanide (Ln = Pr or Tb) ions with a uranyl-oxide hydroxy-hydrate phase, schoepite. Both compounds U-Pr and U-Tb show thin plate morphologies with atomic ratios of 2 (U : Pr) and 6 (U : Tb) and have been characterized by multiple techniques. The layered structures with interlayer hydrated Pr(iii) or Tb(iii) ions formed via uranyl-Pr/Tb interactions have been confirmed by synchrotron single crystal X-ray diffraction studies. In addition, the evolution of the uranyl oxide hydroxide layers and anion topologies upon increasing the concentration of interlayer cations by using different U : Ln (Ln = Pr or Tb) ratios has been discussed. The success in the preparation and characterization of compounds U-Pr and U-Tb with different U : Ln (Ln = Pr or Tb) ratios highlights the flexibility of the uranyl oxide hydroxide layers with respect to the incorporation of interlayer cations via a gradual hydroxyl to oxo transition. The study has direct implications in regard to the natural weathering of uraninite mineral and the alteration of spent nuclear fuels during the long-term geological disposal.

Uranyl oxide hydrate frameworks with lanthanide ions

The first two uranyl oxide hydrate frameworks incorporating lanthanide ions (Ln = Eu³⁺/Gd³⁺) have been synthesized hydrothermally and characterized.

Americium incorporation into studtite: a theoretical and experimental study

Studtite, [UO₂(η²-O₂)(H₂O)₂]·2H₂O, and metastudtite, [UO₂(η²-O₂)(H₂O)₂], are important phase alterations of UO₂ in a spent nuclear fuel repository and have previously been shown to react with Np(v). In this work we extend the study to Am(v) on a tracer scale and show spectroscopic evidence that the Am is incorporated into the structure of studtite as Am(iii). A computational study on the possible mechanisms for the incorporation of Np and Am shows that protonation of the -yl oxygen is the favoured route and the calculated incorporation energies are large and positive. The results suggest that Am is less favoured compared to Np but energetically more favoured to incorporate both actinide ions into metastudtite rather than studtite. Finally, we have shown that once incorporated, Am readily leaches into water but spectroscopic measurements suggest subtle changes in the structure of studtite.

Detection and identification of solids, surfaces, and solutions of uranium using vibrational spectroscopy

The purpose of this review is to provide an overview of uranium speciation using vibrational spectroscopy methods including Raman and IR. Uranium is a naturally occurring, radioactive element that is utilized in the nuclear energy and national security sectors. Fundamental uranium chemistry is also an active area of investigation due to ongoing questions regarding the participation of 5f orbitals in bonding, variation in oxidation states and coordination environments, and unique chemical and physical properties. Importantly, uranium speciation affects fate and transportation in the environment, influences bioavailability and toxicity to human health, controls separation processes for nuclear waste, and impacts isotopic partitioning and geochronological dating. This review article provides a thorough discussion of the vibrational modes for U(IV), U(V), and U(VI) and applications of infrared absorption and Raman scattering spectroscopies in the identification and detection of both naturally occurring and synthetic uranium species in solid and solution states. The vibrational frequencies of the uranyl moiety, including both symmetric and asymmetric stretches are sensitive to the coordinating ligands and used to identify individual species in water, organic solvents, and ionic liquids or on the surface of materials. Additionally, vibrational spectroscopy allows for the in situ detection and real-time monitoring of chemical reactions involving uranium. Finally, techniques to enhance uranium species signals with vibrational modes are discussed to expand the application of vibrational spectroscopy to biological, environmental, inorganic, and materials scientists and engineers.

Sensing Uranyl(VI) Ions by Coordination and Energy Transfer to a Luminescent Europium(III) Complex

The release of uranyl(VI) is a hazardous environmental issue, with limited ways to monitor accumulation in situ. Here, we present a method for the detection of uranyl(VI) ions through the utilization of a unique fluorescence energy transfer process to europium(III). Our system displays the first example of a "turn-on" europium(III) emission process with a small, water-soluble lanthanide complex triggered by uranyl(VI) ions.