Recent reports have suggested the late actinides participate in more covalent interactions than the earlier actinides, yet the origin of this shift in chemistry is not understood. This report considers the chemistry of actinide dipicolinate complexes to identify why covalent interactions become more prominent for heavy actinides. A modest increase in measured actinide:dipicolinate stability constants is coincident with a significant increase in An 5f energy degeneracy with the dipicolinate molecular orbitals for Bk and Cf relative to Am and Cm. While the interactions in the actinide-dipicolinate complex are largely ionic, the decrease in 5f orbital energy across the series manifests in orbital-mixing and, hence, covalency driven by energy degeneracy. This observation suggests the origin of covalency in heavy actinide interactions stems from the degeneracy of 5f orbitals with ligand molecular orbitals rather than spatial orbital overlap. These findings suggest that the limiting radial extension of the 5f orbitals later in the actinide series could make the heavy actinides ideal elements to probe and tune effects of energy degeneracy driven covalency.

Fundamental understanding of the selective recognition and separation of f-block metal ions by chelating agents is of crucial importance for advancing sustainable energy systems. Current investigations in this area are mostly focused on the study of inner-sphere interactions between metal ions and donor groups of ligands, while the effects on the selectivity resulting from molecular interactions in the outer-sphere region have been largely overlooked. Herein, we explore the fundamental origins of the selectivity of the solvating extractant N,N,N′,N′-tetraoctyl diglycolamide (TODGA) for adjacent lanthanides in a liquid–liquid extraction system, which is of relevance to nuclear fuel reprocessing and rare-earth refining technologies. Complementary investigations integrating distribution studies, quantum mechanical calculations, and classical molecular dynamics simulations establish a relationship between coextracted water and lanthanide extraction by TODGA across the series, pointing to the importance of the hydrogen-bonding interactions between outer-sphere nitrate ions and water clusters in a nonpolar environment. Our findings have significant implications for the design of novel efficient separation systems and processes, emphasizing the importance of tuning both inner- and outer-sphere interactions to obtain total control over selectivity in the biphasic extraction of lanthanides.

Processing of hazardous materials is a crucial example where online monitoring can significantly reduce operation risk, cost, and time. This is particularly true in the case of the Hanford site, where nuclear materials from the Cold War era are being processed for environmental cleanup efforts. In exceedingly complex streams such as those at Hanford, online and real-time monitoring can be challenging due to the complexity of instrument signals. Further obstacles are imposed by the caustic nature of processing streams, as well as the radiation damage inflicted on instruments and probes. Online monitoring based on Raman spectroscopy enables the detection of many Hanford tank species of interest. Nine chemical species that comprise the majority of tank waste by volume, including Al­(OH)4 –, C2O4 2–, CO3 2–, CrO4 2–, NO3 –, NO3 –, OH–, PO4 3–, and SO4 2–, were detected and quantified. Real-time analysis of Raman signal allows for immediate quantification of target analytes and was successfully accomplished through the use of chemometric models. Furthermore, irradiation tests revealed that Raman monitoring systems can effectively continue to operate even after receiving 1 × 107 rad of γ dose. The online, real-time monitoring system developed here was successfully used to simultaneously quantify nine target analytes in a real sample collected from Hanford tank AP-105.

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Actinide-lanthanide separation (ALSEP) has been a topic of interest in recent years as it has been shown to selectively extract problematic metals from spent nuclear fuel. However, the process suffers from slow kinetics, prohibiting it from being applied to nuclear facilities. In an effort to improve the process, many fundamental studies have been performed, but the majority have only focused on the thermodynamics of separation. Therefore, to understand the mechanism behind the ALSEP process, molecular dynamics (MD) simulations were utilized to obtain the dynamics and solvation characteristics for an organic extractant, 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEHEHP). Simulations were conducted with both pure and biphasic solvent systems to evaluate the complex solvent interactions within the ALSEP extraction method. The MD simulations revealed solvation and dynamical behaviors that are consistent with the experimentally observed chemical properties of HEHEHP for the pure solvent systems (e.g., hydrophobic/hydrophilic behaviors of the polar head group and alkyl chains and dimer formation between the ligands within an organic solvent). When present in a biphasic solvent system, interfacial behaviors of the ligand revealed that, at low concentrations, the alkyl side chains of HEHEHP were parallel to the interfacial plane. Upon increasing the concentration to 0.75 M, tendency for the parallel orientation decreased and a more perpendicular-like orientation was observed. Analysis of ligand solvation energies in different solvents through the thermodynamic integration method demonstrated favorability toward n-dodecane and biphasic solvents, which is in agreement with the previous experimental findings.

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Uranyl nitrate/extractant complexes form long-range isotropic pairs and short-range ordered dimeric assemblies with a unique square configuration comprised of noncovalent ligand and acid anion mediated interactions, further stabilized by organic phase solvation.