Influence of Diluent on Extraction Parameters of Systems for Separation Am(III) and Ln(III) Based on 1,10-Phenanthroline-2,9-Diamide

A systematic study of extraction systems for the separation of f-elements using the tetradentate N,O-donor diamide of 1,10-phenanthroline-2,9-dicarboxylic acid (L) in various molecular and ionic solvents was performed. It was demonstrated that the nature of a diluent has a significant impact on solvent extraction of Am(III) and Ln(III) and the stoichiometry of formed complexes with f-elements. The mechanism of complexation and forms of complexes in different diluents were investigated by radiometric methods, UV-vis titration, and XRD.

Ionic Liquid as a N,O-Donor Ligand-Based Extraction System Modifier: Establishing the Mechanism of Am(III)-Selectivity Increasing

In this work, we studied the extraction systems for the separation f-elements based on the tetradentate N,O-donor ligand di(N-ethyl-4-ethylanilide) 2,2'-dipyridyl-6,6'-dicarboxylic acid (L). The organic phase of these systems was perspective fluorine-containing organic solvents-metanitrobenzotrifluoride (F-3), ionic liquid C4mimNTf2 (IL), and their mixture. The increase of Am(III) selectivity in the presence of Ln(III) in cases of the diluent mixture was shown. The mechanism of the f-element complexation leading to the improved properties of the extraction systems was studied by UV-visible, Raman-spectroscopy, XRD-study, and density functional theory calculations.

Theoretical investigations into the bonding and separation properties of non-rigid, partially rigid, and rigid ligands derived from Et-Tol-PTA with trivalent lanthanides and actinides

The separation of trivalent actinide elements from lanthanide elements represents one of the most formidable challenges within the context of nuclear waste partitioning and transmutation (P&T) processes. Consequently, we embarked on a systematic investigation aimed at elucidating the bonding properties and thermodynamic behavior of a N-ethyl-N-tolyl-2-amide-1,10-phenanthroline (Et-Tol-PTA) ligand in conjunction with trivalent actinide and lanthanide elements. This investigation involved the utilization of various density functional theory (DFT) methods and a comparative analysis between small-core pseudopotential basis sets and all-electron basis sets. It was found that well-performing results were achieved using the PBE0 functional in both bond length and thermodynamic energy calculations, with minimal impact being exerted by the basis set on the results. Furthermore, an exploration was carried out into the bonding and thermodynamic properties of trivalent actinides and lanthanides with ligands derived from Et-Tol-PTA, encompassing non-rigid (La), partially rigid (Lb, Lc), and rigid (Ld) ligands. Thermodynamically, advantages in the separation of Am(III)/Eu(III) were exhibited by Lb and Lc ligands, while excellent performance in the separation of Am(III)/Cm(III) was demonstrated by the La ligand. Analyses conducted using quantum theory of atoms in molecules (QTAIM), reduced density gradient (RDG), and natural bond orbital (NBO) methodologies revealed the presence of partial covalent character in the bonds between oxygen (O) and metal (M), as well as between nitrogen (N) and metal (M), with a higher degree of covalent character being observed in O-Am and N-Am bonds compared to O-Cm/Eu and N-Cm/Eu interactions.

Simple, Fast, and Selective Dissolution of Eu2O3 in an Ionic Liquid as a Sustainable Paradigm for Lanthanide-Actinide Separations in Radioactive Waste Remediation

The liquid-liquid extraction (LLE) process for lanthanide-actinide separation from the nuclear fuel cycle has several drawbacks such as, the requirement of cooling for decay heat control, the handling of large volumes of toxic volatile organic compounds (VOCs), and secondary waste generation. Alternatively reprocessing without spent fuel cooling is done by pyroprocessing, which uses high-temperature corrosive molten salts and requires elevated temperature, and is an energy-intensive process. In recent years, some of the shortcomings of both LLE and pyroprocessing are overcome by the use of room temperature ionic liquids (RTILs) as the solvents. In the present work, an attempt was made to exploit the potential of the neoteric, less-corrosive, low-VOC RTILs toward direct dissolution-based separations at ambient conditions. The present paper involves the selective dissolution of Eu₂O₃ in an RTIL, i.e., C₄mim·NTf₂ containing 2-thenoyltrifluoroacetone (HTTA) within ca. 30 min at ambient conditions; while the dissolution of AmO₂ and UO₂ were found to be very poor, making this an attractive method for lanthanide-actinide separation, a key step in radioactive waste management, i.e., an actinide partitioning and transmutation strategy. The quantitative dissolution of Eu₂O₃ from simulated spent nuclear fuel with different Eu₂O₃ loading was also shown. Water plays a crucial role in deciding the kinetics of dissolution and amount of the dissolved oxide. The combination of X-ray absorption, fluorescence, and UV-vis spectroscopic studies suggested the formation of the dehydrated anionic complex Ln(TTA)₄^- to play pivotal role in the oxide dissolution process. The structure of the complex was analyzed by density functional theory and extended X-ray absorption fine structure. The mechanism of oxide dissolution was proposed and electrochemical studies were performed to understand the possible recovery option using electrodeposition of the dissolved Eu³⁺.

Complexation Behaviors of a Tridentate Phenanthroline Carboxamide Ligand with Trivalent f-Block Elements in Different Anion Systems: A Thermodynamic and Crystallographic Perspective

The counteranion has a strong influence on the complexation behavior of tridentate phenanthroline carboxamide ligands with actinides and lanthanides, but the thermodynamic and underlying interaction mechanism at the molecular level is still not clear. In this work, a tridentate ligand, N-ethyl-N-tolyl-2-amide-1,10-phenanthroline (Et-Tol-PTA), was synthesized, and the effects of different anions (Cl^-, NO₃^-, and ClO₄^-) on the complexation behavior of Et-Tol-PTA with typical lanthanides were thoroughly studied by using ¹H nuclear magnetic resonance (NMR) spectroscopy, ultraviolet-visible (UV-vis) spectrophotometry, and single-crystal X-ray diffraction. The NMR spectroscopic titration of Lu(III) showed that there were three species (1:1, 2:1, and 3:1 ligand-metal complexes) formed in Cl^- solution systems while two species (2:1 and 1:1) were formed in NO₃^- and ClO₄^- solution systems. When Et-Tol-PTA was titrated with La(III), two species (2:1 and 1:1) were formed in NO₃^- systems and only one species (1:1) was formed in Cl^- and ClO₄^- systems. In addition, the stability constant was determined via UV-vis spectroscopic titration, which showed that the complexation strength between Et-Tol-PTA and Eu(III) decreased in the following order: ClO₄^- > NO₃^- > Cl^-. This indicated that Et-Tol-PTA had the strongest complexation ability with Eu(III) in the ClO₄^- system. The structures of Et-Tol-PTA complexed with EuCl₃, Eu(NO₃)₃, and Eu(ClO₄)₃ were further elucidated by single-crystal X-ray diffraction and agreed well with the results of UV-vis titration experiments. The results of this work revealed that the mechanisms of complexation of lanthanides with the asymmetric ligand Et-Tol-PTA were strongly affected by different anionic environments in solution and in the solid state. These findings may lead to the improvement of the separation of trivalent actinides and lanthanides in nuclear waste.

Molecular Dynamics Simulations of Complexation of Am(III) with a Preorganized Dicationic Ligand in an Ionic Liquid

Preorganized ligands with imidazolium arms have been found to be highly selective in extracting Am(III) into ionic liquids (ILs), but the detailed structure and mechanism of the complexation process in the ionic solvation environment are unclear. Here, we carry out molecular dynamics simulation of the complexation of Am(III) with a preorganized 1,10-phenanthroline-2,9-dicarboxamide complexant (L) functionalized with alkyl chains and imidazolium cations in the butylmethylimidazolium bistriflimide ([BMIM][NTf₂]) IL. Both Am:L (1:1) and Am:L₂ (1:2) complexes are examined. In the absence of the ligand, Am(III) is found to be coordinated by six NTf₂ anions via nine O donors in the first solvation shell. In the Am:L complex, Am(III) is coordinated to the ligand via two O donors and four NTf₂ anions via seven O donors in the first coordination shell. In the Am:L₂ complex, Am(III) is coordinated to the two ligands via four O donors and four NTf₂ anions via five O donors. The imidazolium arms of the ligands play an important role in the secondary solvation environment by attracting NTf₂ anions closer to the metal center. As a result, we find that the binding free energy for the second L²⁺ ligand is twice that for the first L²⁺ ligand, making the Am:L₂ complex significantly more stable than the Am:L complex. This work highlights the multiple factors and tunability in using preorganized ligands with charged functional groups in an ionic solvation environment, which could hold the key to achieving desired selectivity in ion extraction efficiency.

Substituent Effect on the Selective Separation and Complexation of Trivalent Americium and Lanthanides by N,O-Hybrid 2,9-Diamide-1,10-phenanthroline Ligands in Ionic Liquid

The extraction and complexation of trivalent americium (Am) and lanthanides (Ln) by four 2,9-diamide-1,10-phenanthroline (DAPhen) ligands with different alkyl substituent groups on the diamide moiety in an ionic liquid (IL), C₄mimNTf₂, were studied through a combination of batch extraction, spectroscopic, and calorimetric approaches. All four DAPhen ligands can achieve selective separation of Am(III) from Eu(III), but the detailed extractability and the extraction kinetics are affected significantly by the length of the alkyl substituent groups. UV-vis absorption spectrophotometric titrations indicate that Ln(III) coordinates with all four ligands in a 1:2 mode in the ionic liquid and the binding strength decreases with the increase of the alkyl chain length. The complexation of the DAPhen ligands with Ln(III) in the ionic liquid is driven by highly positive entropies and opposed by endothermic enthalpies. A luminescence spectroscopy study suggests that each DAPhen ligand coordinates in a tetradentate form with Eu(III). This work further unravels the unique extraction and coordination behavior in an ionic liquid system and offers additional guidelines to design more efficient DAPhen ligands for Ln(III)/An(III) separation.

Exploring the Subtle Effect of Aliphatic Ring Size on Minor Actinide-Extraction Properties and Metal Ion Speciation in Bis-1,2,4-Triazine Ligands

The synthesis and evaluation of three novel bis-1,2,4-triazine ligands containing five-membered aliphatic rings are reported. Compared to the more hydrophobic ligands 1-3 containing six-membered aliphatic rings, the distribution ratios for relevant f-block metal ions were approximately one order of magnitude lower in each case. Ligand 10 showed an efficient, selective and rapid separation of Am^III and Cm^III from nitric acid. The speciation of the ligands with trivalent f-block metal ions was probed using NMR titrations and competition experiments, time-resolved laser fluorescence spectroscopy and X-ray crystallography. While the tetradentate ligands 8 and 10 formed Ln^III complexes of the same stoichiometry as their more hydrophobic analogues 2 and 3, significant differences in speciation were observed between the two classes of ligand, with a lower percentage of the extracted 1:2 complexes being formed for ligands 8 and 10. The structures of the solid state 1:1 and 1:2 complexes formed by 8 and 10 with Y^III , Lu^III and Pr^III are very similar to those formed by 2 and 3 with Ln^III . Ligand 10 forms Cm^III and Eu^III 1:2 complexes that are thermodynamically less stable than those formed by ligand 3, suggesting that less hydrophobic ligands form less stable An^III complexes. Thus, it has been shown for the first time how tuning the cyclic aliphatic part of these ligands leads to subtle changes in their metal ion speciation, complex stability and metal extraction affinity.

Coordination of Nd(iii) and Eu(iii) with monodentate organophosphorus ligands in ionic liquids: spectroscopy and thermodynamics

The thermodynamics and coordination nature of the complexes of Ln(iii) with three monodentate organophosphorus ligands in ionic liquids have been elucidated and illustrated by spectroscopic and calorimetric techniques.

Hydrophilic 2,9-bis-triazolyl-1,10-phenanthroline ligands enable selective Am(iii) separation: a step further towards sustainable nuclear energy

An exciting new family of hydrophilic ligands for the selective extraction of Am(iii) from spent nuclear fuel are reported herein.

Th(iv) and Ce(iv) napthylsalophen sandwich complexes: characterization of unusual thorium fluorescence in solution and solid-state

The synthesis, characterization, and surprising electronic spectroscopy of two ML₂ sandwich complexes, where M = Ce(iv) or Th(iv) and L = napthylsalophen²⁻ are described.