Thomas Samuel Neill

Technetium-99 is a long-lived, high-yield fission product whose solubility is controlled by redox conditions. Here, to explore targeted effluent treatment approaches, pertechnetate (Tc(VII)O4–) reductive retention as Tc(IV) during coprecipitation of Fe(II)-doped Ni–Al–CO3 layered double hydroxide (LDH) was investigated. During LDH formation via base addition to an acidic Ni2+, Al3+, Fe2+, and Tc(VII)O4– bearing solution, there was essentially complete TcO4– removal at low (9.6 Bq/mL) and high (2.9 kBq/mL) concentrations. Characterization of experiments using X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, and X-ray absorption spectroscopy (XAS) confirmed that, during base titration, a two-stage process occurred, with Al3+ forming amorphous Al (hydr)oxide between pH 4 and 6 in the first stage, followed by hydrolysis of Fe2+/Ni2+ and formation of LDH particles between pH 6 and 9 in stage two. During the first stage, TcO4– was removed from solution, and XAS confirmed it was reduced to Tc(IV) by Fe(II) associated with the amorphous Al (hydr)oxide, forming hydrous Tc(IV)O2-like chains on the Al (hydr)oxide surface. During the second stage, XAS showed that Tc(IV) was incorporated within the newly formed LDH structure. Overall, these data provide novel insights into the application of LDH formation in Tc sequestration and suggest novel effluent treatment pathways to produce an inorganic solid.

Treatment of 99Tc in contaminated environments remains an important challenge in the remediation of legacy nuclear sites, including Hanford (USA) and Sellafield (UK). The highly mobile pertechnetate anion (Tc(VII)O4–) dominates in oxic shallow subsurface environments. Immobilization driven by biogeochemical reduction of Tc(VII)O4– to poorly soluble Tc(IV) species has been a key focus for remediation approaches. Here, we explore a novel 99Tc remediation strategy employing bisphosphonate ligands (e.g., medronic acid, MDP), which form soluble 99Tc(IV)-phosphate complexes that can subsequently be incorporated into calcium (Ca)-phosphate phases. We explored this MDP ligand-based approach in batch experiments containing 99Tc, hydroxyapatite, sediment, and synthetic groundwater representative of key nuclear sites. 99Tc concentration and speciation were tracked using a multitechnique approach including X-ray absorption spectroscopy. Additionally, we explored MDP-enhanced Tc scavenging in the column experiments. Here, in situ planar and tomographic gamma imaging employing 99mTc as a radiotracer showed enhanced removal of MDP-ligated 99mTc(IV) under dynamic flow conditions and field-relevant 99mTc concentrations. Finally, the feasibility of microbially mediated transformations to form MDP-ligated Tc(IV) was examined to explore the potential for this approach to support in situ bioremediation of technetium contamination. Here, 99Tc(VII) was bioreduced in the presence of MDP using the metal-reducing bacteria Shewanella oneidensis MR-1 to form the Tc(IV)-MDP precursor for enhanced removal by Ca phosphate. Overall, these findings demonstrate that 99Tc(VII) (bio)reduction and complexation with Tc(IV)-MDP enhanced 99Tc scavenging by Ca-phosphate phases.

The role of the equatorial ligands and their influence on the electronic structures and bonding properties of uranyl and other actinyls are not well understood and are thus at the forefront of actinide research. In the study presented here, we found that the good energy match of uranyl(VI) with F^- valence orbitals leads to substantial changes in the uranyl electronic structure, compared to uranyl-Cl^- and uranyl-Br^-. The good energy match between uranyl(VI) and F^- likely enhances the stability of the uranyl-F^- bond, contributing to the higher U-F^- affinity in aqueous solution compared to uranyl-Cl^-/Br^-, which is also demonstrated for plutonyl(VI). These findings are based on studies of equatorial and axial ligand covalency in three uranyl halides: NaRb₈(UO₂)₅F₁₉·2H₂O, Rb₂UO₂Cl₄·2H₂O, and Rb₂UO₂Br₄·2H₂O. We describe covalent uranium-halide interactions, following the trend Br^- ≈ Cl^- > F^-. Ligand K-edge XANES and DFT (including TDDFT and LFDFT) reveal significant electronic structure differences, with the F-based compound having a uranyl-based HOMO, while Cl- and Br-based compounds show predominant ligand p character in the HOMO. A newly introduced theoretical index evaluates bond covalency. U M₄ edge HR-XANES and RIXS exhibit unexpected σ* peak trends not directly correlated with U═O bond lengths but well explained by LFDFT RIXS calculations.

The role of the equatorial ligands and their influence on the electronic structure and bonding properties of uranyl and other actinyls are not well understood and thus at the forefront of actinide research. In the study presented here, we found that the good energy match of uranyl(VI) with F- valence orbitals leads to substantial changes in the uranyl electronic structure, compared to uranyl-Cl- and uranyl-Br-. The good energy match between uranyl(VI) and F- likely enhances the stability of the uranyl-F- bond, contributing to the higher U-F- affinity in aqueous solution compared to uranyl-Cl-/Br-, which is also demonstrated for plutonyl(VI). These findings are based on studies of equatorial and axial ligand covalency in three uranyl halides—NaRb8(UO2)5F19·2H2O, Rb2UO2Cl4·2H2O, and Rb2UO2Br4·2H2O. We describe covalent uranium-halide interactions, following the trend Br- ≈ Cl- > F-. Ligand K-edge XANES and DFT (including TDDFT and LFDFT) reveal significant electronic structure differences, with the F-based compound having a uranyl-based HOMO, while Cl- and Br-based compounds show predominant ligand p character in the HOMO. A newly introduced theoretical index evaluates bond covalency. U M4 edge HR-XANES and RIXS exhibit unexpected σ* peak trends, not directly correlated with U=O bond lengths but well explained by LFDFT RIXS calculations.

The actinides possess a complex electronic structure, making their chemical and physical properties among the least understood in the periodic table. Advanced spectroscopic tools, able to obtain deep insights into the electronic structure and binding properties of the actinides, are highly desirable. Here, we introduce two sensitive spectroscopic tools: one determines the number of localized 5 f electrons on an actinide atom, and another assesses the covalent character of actinide-ligand bonding. Both tools are based on the multiplet structure present in actinide M ₄ edge core-to-core resonant inelastic X-ray scattering (CC-RIXS) maps. The spectral intensity of different many-body final-state multiplets directly depends on the local many-electron ground-state symmetry including the local 5 f spin configuration. By comparing U M ₄ edge CC-RIXS data for 21 U, Np, Pu and Am compounds, we demonstrate the ability to compare the number of localized 5 f electrons and bond covalency across the actinide series.

Lanthanides are widely assumed not to form covalent bonds due to the localized nature of their 4f valence electrons. This work demonstrates that the ionic bond of Sm(II) with cyclononatetraenyl (eta(9)-C9H9-) in [Sm(eta(9)-C9H9)(2)] can be modulated and becomes more covalent by photon-induced transfer of Sm 4f electrons to Sm 5d orbitals. This photon-induced change in bonding properties facilitates a subsequent reconfiguration of [Sm(eta(9)-C9H9)(2)]. As a result, Sm-C bond length contraction is detected and the local Sm coordination environment exhibits more extensive disorder. Both Sm 4f and 5d electrons have increased participation in covalent Sm-ligand interactions. The Sm L-3-edge valence band resonant inelastic X-ray scattering (VB-RIXS), high-resolution X-ray absorption near-edge structure (HR-XANES), and quantum chemical computations showcase a spectroscopic methodology for in-depth studies of bond covalency of lanthanide atoms.

Spent nuclear fuel contains both uranium (U) and high yield fission products, including strontium-90 (⁹⁰Sr), a key radioactive contaminant at nuclear facilities. Both U and ⁹⁰Sr will be present where spent nuclear fuel has been processed, including in storage ponds and tanks. However, the interactions between Sr and U phases under ambient conditions are not well understood. Over a pH range of 4–14, we investigate Sr sorption behavior in contact with two nuclear fuel cycle relevant U(IV) phases: nano-uraninite (UO₂) and U(IV)–silicate nanoparticles. Nano-UO₂ is a product of the anaerobic corrosion of metallic uranium fuel, and UO₂ is also the predominant form of U in ceramic fuels. U(IV)–silicates form stable colloids under the neutral to alkaline pH conditions highly relevant to nuclear fuel storage ponds and geodisposal scenarios. In sorption experiments, Sr had the highest affinity for UO₂, although significant Sr sorption also occurred to U(IV)–silicate phases at pH ≥ 6. Extended X-ray absorption fine structure (EXAFS) spectroscopy, transmission electron microscopy, and desorption data for the UO₂ system suggested that Sr interacted with UO₂ via a near surface, highly coordinated complex at pH ≥ 10. EXAFS measurements for the U(IV)–silicate samples showed outer-sphere Sr sorption dominated at acidic and near-neutral pH with intrinsic Sr-silicates forming at pH ≥ 12. These complex interactions of Sr with important U(IV) phases highlight a largely unrecognized control on ⁹⁰Sr mobility in environments of relevance to spent nuclear fuel management and storage.