Coordination chemistry of actinides
Fundamental knowledge on the coordination chemistry of actinides is of high importance in several fields of actinide research, such as safety research for nuclear waste disposal, the development and optimization of extraction processes in the framework of actinide recycling, and in the field of toxicology. Therefore, spectroscopic investigations on the coordination of actinides and lanthanides with inorganic, organic, and biological ligands are performed to provide fundamental information on the bonding and the coordination structure. Our research focuses on the following topics:
- Spectroscopic speciation studies on the complexation of lanthanides and actinides with inorganic and organic ligands in the temperature range ≤ 200°C.
- Fundamental spectroscopic investigations on lanthanide and actinide complexes with N-donor ligands. Aim of these studies is a deeper understanding of the complexation behavior of trivalent lanthanides and actinides and the bonding (share of covalence) in f-element complexes with soft-donor ligands.
- Interaction of actinides with blood serum proteins (transferrin, albumin etc.) to obtain detailed information on the bioavailability and toxicity of actinides in the human body.
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Solvent extraction chemistry
Solvent extraction is a widely used technique for separating and purifying ionic species. Solvent extraction is applied on an industrial level e. g. for the reprocessing of spent nuclear fuels by the PUREX process and for the large-scale copper production by the SX/EW (solvent extraction, electrowinning) technique.
Principle: A ionic solute in an aqueous phase forms a complex with a complexing agent present in an immiscible organic phase. This complex is preferentially soluble in the organic phase; the solute is being extracted. This way, the solute is separated from other solutes not forming complexes with the complexing agent.
We develop and study solvent extraction systems both in the nuclear and the non-nuclear context. Within the nuclear context, we have been focusing on solvent extraction systems for actinide separations, most notably for separating trivalent actinides, An(III), from the chemically similar lanthanides, Ln(III). Since the early 1990’s, these studies have continuously been performed in the framework of EURATOM research programmes. A common denominator throughout these projects is the use of heterocyclic nitrogen donor complexing agents. Such compounds exhibit a high selectivity for An(III) over Ln(III), with molecules based on the BT(B)P (bis-triazinyl-(bi)-pyridine) core being among the most efficient.
The work is carried out in close cooperation with the Coordination Chemistry of Actinides and NMR Spectroscopy working groups. Fundamental studies on the coordination chemistry of trivalent actinide and lanthanide ions with nitrogen donor ligands are carried out within projects funded by the BMBF. The objective is to deepen our understanding of such ligands' pronounced selectivity on a molecular level.
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Nuclear Magnetic Resonance Spectroscopy (NMR)
Nuclear Magnetic Resonance Spectroscopy (NMR) is the most important and versatile spectroscopical method for organic as well as inorganic chemical compounds. It has been a state-of-the-art method in research for decades, and represents a method of routine analysis as well as an active area of scientific research.
So far, NMR has found only limited use in radiochemistry. KIT-INE is one of the very few research institutes across the world that has a NMR spectrometer installed within a radioactive controlled area, thus permitting the analysis of radioactive materials. The NMR working group uses a Bruker Avance III 400 spectrometer with a proton resonance frequency of 400 MHz (corresponding to a magnetic flux density of 9.4 T). Several different probes are available, allowing direct and indirect measurement of all common NMR-active nuclei (1H, 13C, 15N, 19F, 31P, …) as well as research on more “exotic” nuclei. With more specialized probes, nuclei with a very low magnetogyric ratio can be measured. Another probe is dedicated to the radioactive hydrogen isotope tritium (3H) in liquid samples. Spectroscopy on solid state samples (Magic Angle Spinning) is possible as well, but limited to non-radioactive materials.
The focus of the working group is on the elucidation of structure and bonding in radioactive coordination compounds of the transuranium elements in liquid samples. While actinde nuclei cannot be observed directly, 1H, 13C and especially 15N spectra of the ligand nuclei give insight into the bonding in the complexes. The paramagnetism of the actinide ions has several effects on NMR spectra. The origin of the effects is the transfer of electron through covalent bods and the coupling of electron spins, located on the metal ion, to nuclear spins in the ligands. By comparison of the actinide complexes’ spectra to those of isostructural lanthanide complexes and the application of sophisticated mathematical methods, the effect of covalently transferred electron spin density can be evaluated. This is also an indication of the degree of covalency in the metal-ligand bond. Another working field of the group is the synthesis of novel N-donor ligands, as well as isotope labelling of these new ligands, predominantly with 15N.
For routine measurements in liquid state, the NMR spectrometer is available for other working groups at INE in walk-up mode. More complex, off-routine experiments for internal and external scientists are possible upon request.