Among the energy sources currently available that could address our insatiable appetite for energy and minimize our CO2 emission, solar, wind, and nuclear energy currently occupy an increasing portion of our energy portfolio. The energy associated with these sources can however only be harnessed after mineral resources containing valuable constituents such as actinides (Ac) and rare earth elements (REEs) are extracted, purified and transformed into components necessary for the conversion of energy into electricity. Unfortunately, the environmental impacts resulting from their manufacture including the generation of undesirable and, sometimes, radioactive wastes and the non-renewable nature of the mineral resources, to name a few, have emerged as challenges that should be addressed by the scientific community. In this perspective, the recent development of functionalized solid materials dedicated to selective elemental separation/pre-concentration could provide answers to several of the above-mentioned challenges. This review focuses on recent advances in the field of mesoporous solid-phase (SP) sorbents designed for REEs and Ac liquid-solid extraction. Particular attention will be devoted to silica and carbon sorbents functionalized with commonly known ligands, such as phosphorus or amide-containing functionalities. The extraction performances of these new systems are discussed in terms of sorption capacity and selectivity. In order to support potential industrial applications of the silica and carbon-based sorbents, their main drawbacks and advantages are highlighted and discussed.

Functionalization of mesoporous materials for lanthanide and actinide extraction

Recently, phenanthroline-based ligands have received increasing attention due to their excellent separation capabilities for trivalent actinides over lanthanide. In this work, we designed a soft-hard donor combined tetradentate phenanthroline-based extractant, tetraethyl (1,10-phenanthrolin-2,9-diyl)phosphonate (C2-POPhen), for the selective separation of trivalent Am(III) over Ln(III) from HNO3 media. The solvent extraction and complexation behaviors of Am(III) and Ln(III) by C2-POPhen were investigated both experimentally and theoretically. C2-POPhen could selectively extract Am(III) over Eu(III) with an extremely fast extraction kinetics. NMR titration studies suggest that only 1:1 complexes of Ln(III) with C2-POPhen formed in CH3OH in the presence of a significant amount of nitrate, while both 1:1 and 2:1 complexes species could form between C2-POPhen and Ln(III) perchlorate in CH3OH without nitrate ions. The stability constants for the complexation of Am(III) and Ln(III) with C2-POPhen in CH3OH were determined by spectrophotometric titrations and the Am(III) complexes are approximately 1 order of magnitude stronger than those of Ln(III), which is consistent with the extraction results. Theoretical calculations indicate that the Am-N bonds in Am/C2-POPhen complexes possess more covalent characters than the Eu-N bonds in Eu/C2-POPhen complexes, shedding light on the underlying chemical force responsible for the Am/Eu selectivity by C2-POPhen. This work represents the first report utilizing phenanthroline-based phosphonate ligands for selective separation of actinides from highly acidic solutions.

Selective Separation and Complexation of Trivalent Actinide and Lanthanide by a Tetradentate Soft-Hard Donor Ligand: Solvent Extraction, Spectroscopy, and DFT Calculations.

With the fast development of nuclear energy, the issue related to spent nuclear fuel reprocessing has been regarded as an imperative task, especially for the separation of minor actinides. In fact, it still remains a worldwide challenge to separate trivalent An(iii) from Ln(iii) because of their similar chemical properties. Therefore, understanding the origin of extractant selectivity for the separation of An(iii)/Ln(iii) by using theoretical methods is quite necessary. In this work, three ligands with similar structures but different bridging frameworks, Et-Tol-DAPhen (La), Et-Tol-BPyDA (Lb) and Et-Tol-PyDA (Lc), have been investigated and compared using relativistic density functional theory. The electrostatic potential and molecular orbitals of the ligands indicate that ligand La is a better electron donor compared to ligands Lb and Lc. The results of QTAIM, NOCV and NBO suggest that the Am-N bonds in the studied complexes have more covalent character compared to the Eu-N bonds. Based on the thermodynamic analysis, [M(NO3)(H2O)8]2+ + L + 2NO3- = [ML(NO3)3] + 8H2O should be the most probable reaction in the solvent extraction system. Our results clearly verify that the relatively harder oxygen atoms offer these ligands higher coordination affinities toward both of the An(iii) and Ln(iii) ions compared to the relatively softer nitrogen atoms. However, the latter possess stronger affinities toward An(iii) over Ln(iii), which partly results in the selectivity of these ligands. This work can afford useful information on achieving efficient An(iii)/Ln(iii) separation through tuning the structural rigidity and hardness or softness of the functional moieties of the ligands.

Theoretically unraveling the separation of Am(III)/Eu(III): insights from mixed N,O-donor ligands with variations of central heterocyclic moieties

Quantum chemical modelling of extraction separation of minor actinides and lanthanides: the state of the art

Separation of trivalent actinides An(III) from lanthanides Ln(III) is a worldwide challenge owing to their very similar chemical behaviors. It is highly desirable to understand the nature of selectivity for the An(III)/Ln(III) separation with various ligands through theoretical calculations because of their radiotoxicity and experimental difficulties. In this work, we have investigated three dithioamide-based ligands and their extraction behaviors with Am(III) and Eu(III) ions using the scalar-relativistic density functional theory. The results show that the dithioamide-based ligands have stronger electron donating ability than do the corresponding diamide-based ones. All analyses including geometry, Mulliken population, QTAIM (quantum theory of atoms in molecules), and NBO (natural bond orbital) suggest that the Am-S/N bonds possess more covalency compared to the Eu-S/N bonds, and the M-S bonds have more covalent character than the M-N bonds. Thermodynamic results reveal that N2,N9-diethyl-N2,N9-di-p-tolyl-1,10-phenanthroline-2,9-bis(carbothioamide) (L1) has a stronger complexing ability with metal ions owing to its rigid structure and that N6,N6'-diethyl-N6,N6'-di-p-tolyl-[2,2'-bipyridine]-6,6'-bis(carbothioamide) (L2) shows a higher selectivity for the Am(III)/Eu(III) separation. In addition, these dithioamide-based ligands possess Am(III)/Eu(III) selectivity higher than those of the corresponding diamide-based ones, although the former have weaker complexing ability with metal ions, probably due to the greater covalency of the M-S bonds. This theoretical evaluation provides valuable insights into the nature of the selectivity for the Am(III)/Eu(III) separation and information on designing of efficient An(III)/Ln(III) separation with dithioamide-based ligands.

Theoretical Insights into the Selective Extraction of Americium(III) over Europium(III) with Dithioamide-Based Ligands.

Although many heterocyclic N-donor ligands have shown excellent competence for separating actinides from lanthanides, an explanation for why some ligands work whereas others fail is very fundamental but greatly needs to be addressed for designing novel and efficient extractants. In this work, we systematically investigated four phenanthroline-derived ligands, DHDIPhen, BQPhen, Ph2-BTPhen and CyMe4-BTPhen, and their coordination geometrical properties and formation reactions with Am(III) and Eu(III) ions by quasi-relativistic density functional theory. The calculated hardness of ligands, which may help to determine their selectivity toward actinides and lanthanides, yielded an order, from the softest to the hardest, as follows: Ph2-BTPhen < CyMe4-BTPhen < BQPhen < DHDIPhen. It shows that the intramolecular hydrogen bonds and size of a ligand cavity are two dominant factors for metal-ion complexation. Natural population analysis (NPA) reveals that the 5d/6d orbitals of Eu/Am accept significantly more electrons than other orbitals, but partial density of states and molecular orbital analysis prove that the d orbitals with more accepted electrons have little contribution to the metal–ligand bonds. The thermodynamic results suggest that ligand protonation does have a great influence on the complexation of ligands with metal ions but does not change the selectivity of ligands toward metal ions. This work can help in-depth understanding the differences of selectivity of various structurally similar ligands and provide more theoretical insights for designing more innovative ligands for Ln/An separation.

New insights into the selectivity of four 1,10-phenanthroline-derived ligands toward the separation of trivalent actinides and lanthanides: a DFT based comparison study.

Due to the similar chemical properties of actinides An(III) and lanthanides Ln(III), their separation in spent nuclear fuel reprocessing is extremely challenging. A 1,10-phenanthroline dipicolinamide-based ligand (PhenBHPPA) has been identified to possess a selectivity for Am(III) over Eu(III) and could potentially be used for group actinide extraction. In this study, quasi-relativistic density functional theoretical calculations have been used to disclose the interaction mechanisms of Am(III) and Eu(III) complexes with PhenBHPPA. The electronic structures, bonding nature, QTAIM (Quantum Theory of Atoms in Molecules) analyses and thermodynamic behaviors of the Am(III) and Eu(III) complexes with PhenBHPPA have been explored in detail. According to the Wiberg bond indices (WBIs) and QTAIM analyses, interactions between the ligand and metal cations (Am(III) and Eu(III)) exhibit a weakly covalent character. Thermodynamic analyses show that the charged complexes [ML(NO3)2]+ appear to be the most stable species in the complexation processes. Moreover, it is more energetically favorable for PhenBHPPA to bind to Am(III) compared to Eu(III). Our study could render new insights into understanding the selectivity of the ligand towards minor actinides and the separation of An(III) from Ln(III) via liquid–liquid extraction.

Theoretical insights into the separation of Am(III) over Eu(III) with PhenBHPPA.

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New insights into the selectivity of four 1,10-phenanthroline-derived ligands toward the separation of trivalent actinides and lanthanides: a DFT based comparison study.

Theoretical insights into the separation of Am(III) over Eu(III) with PhenBHPPA.