Evolution of Acid-Dependent Am³⁺ and Eu³⁺ Organic Coordination Environment: Effects on the Extraction Efficiency

Abstract

Coordination of trivalent lanthanide and actinide metal ions by lipophilic diglycolamides and phosphonic acids has been proposed for their separation through extraction from aqueous nitric acid solutions. However, the nature of M3+ coordination complexes in these combined solvent systems is not well understood, resulting in low predictability of their behavior. This work demonstrates that a combination of N,N,N′,N′-tetrakis­(2-ethylhexyl)­diglycolamide (T2EHDGA) and weakly acidic 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEH­[EHP]) in n-dodecane exhibits a complicated extraction mechanism for Eu3+ and Am3+, which continuously evolves as a function of the aqueous phase acidity. At low aqueous phase nitric acid concentrations, M3+ ions are primarily extracted via exchange of the phosphonic acid proton and coordination with HEH­[EHP]. At high aqueous phase nitric acid concentrations, HEH­[EHP] remains protonated, and M3+ ions are transported to the organic phase by the coextraction of nitrate anions from the aqueous phase, thus forming complex species with T2EHDGA. At moderate acid regimes, both ligands participate in the coordination of M3+ ions and show a synergistic relationship resulting in considerable enhancement of M3+ transport into the combined solvent system over the simple sum of the individual extractants. The observed synergism is caused by differences in organic phase M3+ speciation and has a significant impact on the performance of the organic solvent. Distribution studies with Eu3+ indicate that nominally two or three T2EHDGA ligands participate in metal extraction in the presence of phosphonic acid, while nominally three diglycolamide ligands participate in the presence or absence of phosphonic acid. While synergistic behavior has been observed in many solvent-extraction processes, this system demonstrates a clear correlation between the continuously changing organic speciation of M3+ and its transport into the organic solvent. This paper reports the spectroscopic characterization of the organic phase M3+ species by IR, X-ray absorption, and visible spectroscopies. Spectroscopic evidence indicates a mixed-ligand complex, i.e., a ternary complex at the moderate acid regime, where the greatest degree of synergism is observed. Differences in synergistic extraction of Am3+ and Eu3+ at the low acid regime were observed, indicating their dissimilar extraction behavior.

Engineering lanmodulin's selectivity for actinides over lanthanides by controlling solvent coordination and second-sphere interactions

“…It also demonstrates the crucial importance of second-sphere interactions with at least one of those waters impacting selectivity and affinity. Whereas other studies 20,21,24,26 have focused on rst-coordination-sphere effects to alter An 3+ / Ln 3+ selectivity of ligands, here we show that LanM's selectivity can also be substantially enhanced by modulation of a single, second-sphere hydrogen bond. These results have important implications for both the physiological function and technological applications of LanM.…”

“…18,19 For instance, separation of lanthanide ssion products from heavy actinides (i.e., Am 3+ and Cm 3+ ) present in nuclear spent fuel is a critical challenge to overcome for closing the nuclear fuel cycle, but processes able to separate the 4f and 5f series are notoriously arduous to develop and implement 11 equivalent to the separation of individual lanthanides but with additional radiation-related constraints. Typical approaches to increase the selectivity of small-molecule 20,21 and peptide 22 ligands for actinides focus on alteration of the rst coordination sphere, exploiting differential aqueous ligand complexation kinetics, 23 or differences in the extractant speciation in the organic phase 24 (in the case of liquid-liquid extraction). Here, we show how dissection of the supramolecular architecture of the metal-binding sites in lanmodulina natural protein highly selective for f-elementsallows engineering of higher An 3+ /Ln 3+ selectivity by a new approach, tuning solvent and second-sphere interactions.…”

Effect of metal complexation on diglycolamide radiolysis: a comparison between ex situ gamma and in situ alpha irradiation

“…The ethyl hexyl derivative, T2EHDGA (tetra 2-ethyl hexyl diglycolamide), and methylated derivatives of TODGA have also been under investigation for nuclear fuel reprocessing. [22][23][24][25][26][27][28][29][30][31][32] Radiolytic and hydrolytic degradation of DGA has been under recent investigation. Many studies have utilized external gamma irradiation sources to irradiate TODGA under different solvent conditions to induce degradation.…”

Coordination Sphere of Lanthanide Aqua Ions Resolved with Ab Initio Molecular Dynamics and X-ray Absorption Spectroscopy

“…Resolving lanthanide (Ln) solution structures, at atomic resolution, is relevant for rare earth separation and purification, − medical contrast agents, − single-molecule magnets, − and luminescent materials, − to mention only a few applications. The most common oxidation state of Ln ions is +III and they are characterized by large coordination spheres that can accommodate seven to nine coordination sites.…”