Separation Of Am Research Articles

Theoretical investigation the selective separation of Am(III)/Eu(III) by bis-succinamides ligands

Theoretical investigation the selective separation of Am(III)/Eu(III) by bis-succinamides ligands

Assay of zirconium and americium in the irradiated U-Zr metal alloy fuel slug

An alloy-based fuel comprised of U-Pu-Zr is under development for the futuristic fast reactors to improve fissile atom utilization. As the technical data available with the metal fuel reactors is limited, it is imperative to have an exhaustive comprehension of behavior of fission and activation products before deploying the U-Pu-Zr metal fuel for commercial applications. Towards meeting this objective, the present paper highlights the analytical estimation of Zr and Am in the irradiated U-Zr metal that has experienced a low burn-up. The estimation of Zr was carried out by a modified spectrophotometric method developed in our laboratory. The estimation of Am was accomplished through initial liquid-liquid extraction-based removal of U and Pu by tri-n‑butyl phosphate followed by the separation of Am by TODGA from the raffinate. Am was estimated with the help of alpha spectrometry of the Am-separated TODGA phase, and the same has been substantiated by comparing it with the results of gamma spectrometry.

Exploring the bonding and extraction separation effects of N donor ligands with different skeletons on Am(III) and Eu(III)

Designing ligands for effective separation of actinide(III)/lanthanide(III) is one of the key issues in the treatment of spent nuclear fuel. However, due to the chemical similarity of trivalent actinides and lanthanides, their separation faces a great challenge in the reprocessing of spent nuclear fuel. Nitrogen donor extractants are considered as promising reagents for separation of trivalent actinides and lanthanides. To the best of our knowledge, the pre-organized structure of the ligand has a strong influence on the extraction separation capacity. In this work, we designed four N-donor ligands (L1 and L2 with pyridine and bipyridine skeletons, respectively, and L3 and L4 both with phenanthroline skeletons) with different pre-organized structures and investigated their selective extraction for Am(III) and Eu(III) by using scalar relativistic density functional theory (DFT). The absolute minimum ESP values of these ligands and the higher HOMO level suggest that the ligand L2 may have a stronger affinity for Am(III)/Eu(III) ions than other ligands. The three average bond lengths of each complex indicate that the Am(III) and Eu(III) ions have larger bond lengths with N2 atoms than with N1 atoms. Various theoretical approaches have been used to evaluate the properties of the four ligands as well as the coordination structures, bonding properties and thermodynamic properties of the complexes of the four ligands with Am(III) and Eu(III). The WBIs, QTAIM, NBO, DOS and EDA analysis indicate that the metal–ligand bonds are mainly ionic in character, the Am-N bond has more covalency than the Eu-N bond because the Am 5f orbitals are more involved in bonding with ligand than Eu 4f orbitals. According to the extended transition state and natural orbital chemical valence theory (ETS-NOCV) analysis, electrons are obviously transferred from the lone pair of the ligand N atom to the hybrid s, d and f orbitals of the metal ion. The thermodynamic results show that the four ligands can coordinate well with the Am and Eu ions, and also have suitable separation capacity for Am and Eu. Among the four ligands, the L2 and L3 ligands have the strongest coordination ability for metals and the best extraction separation effect for Am and Eu. It can be seen that the bridging skeleton of the ligand and the number of N atoms on the five-membered ring and also different organic solvents have obvious effects on the extraction and separation capacity of the ligand. This study provides a theoretical basis for the efficient separation of Am(III)/Eu(III) by adjusting the preorganization level of ligands, and provides a theoretical basis for the design and screening of ligands by preorganization.

First Report on a 1-Cycle Solid-Phase Extraction Method for Selective Separation of Am3+ and Eu3+: Use of BTP-Grafted Silica Decorated on Activated Carbon

First Report on a 1-Cycle Solid-Phase Extraction Method for Selective Separation of Am³⁺ and Eu³⁺: Use of BTP-Grafted Silica Decorated on Activated Carbon

Insight into selective separation of Am(III)/Eu(III) with novel ligands using (1,10-phenanthroline-2,9-diyl)bis(indolin-1-ylmethanones) as a new skeleton: A scalar relativistic DFT study

There are the similar physical and chemical properties between the trivalent actinides (An(III)) and the lanthanides (Ln(III)), which makes the separation process very difficult for nuclear waste reprocessing. Designing efficient ligands with the strong ability to separate An(III)/Ln(III) is one of the ways to solve the problem, and various ligands with 1,10-Phenanthroline as the skeleton have become a hotspot in recent years. Herein we designed a series of novel ligands using (1,10-phenanthroline-2,9-diyl)bis(indolin-1-ylmethanones)(IDL-DAPhen) as a new skeleton. Using scalar relativistic density functional theory (DFT), we systematically unraveled the properties of four IDL-DAPhens modified with electron-donating groups (–CH3, –OCH3) at different substitution sites, and explored the structures of the ligands L1, L2, L3 and L4, bonding properties and thermodynamic behaviors of their complexes coordinated with Am(III)/Eu(III). Molecular orbitals (MOs), IGMH, ETS-NOCV and other analyses showed that the selectivity of the four ligands to Am(III) was higher than that to Eu(III). The bond length, dihedral angle, WBI, QTAIM and thermodynamic results and other ones indicated that L3 and L4 with para-substituents have stronger coordination ability, while L1 and L3 with methyl-substituted exhibit better Am(III)/Eu(III) separation selectivity. The ΔΔG values of L1 for Am(III)/Eu(III) reached −7.71, −7.77 and −11.57 kcal/mol in water, 1-octanol and aniline, respectively. This study provides theoretical guidance for the design of novel ligands for efficient separation of Am (III)/Eu (III), and could expand a new pathway for the design of ligands with better potential selectivity for An (III)/Ln (III) separation.

Theoretical investigation on the ligands constructed from phenanthroline and five-membered N-heterocyclic rings for bonding and separation properties of Am(III) and Eu(III).

The ligands, derived from the combination of phenanthroline and various five-membered N-heterocyclic rings, were subject to a comprehensive investigation for their potential in the extraction and separation of actinides and lanthanides. This study employed DFT methods to thoroughly explore the properties of both phenanthroline (Ph) and the diverse five-membered N-heterocyclic rings (R1-R8). Additionally, tridentate ligands RlPh (l = 1-8) and tetradentate ligands RlPhRr (l, r = 1-8) were analyzed in detail, encompassing their electrostatic potential (ESP), protonation energy, coordination bonding with the metals Am(III) and Eu(III), and the thermodynamics of extraction separation for Am(III) and Eu(III). The findings highlight that the electrostatic potential (ESP) and binding capabilities of the five-membered N-heterocyclic ring units serve as effective predictors for the properties of intricate tridentate and tetradentate ligands, as well as their coordination bonding affinity with metals. The ligands' binding energy is closely associated with their ESP, and notably, the binding energy of tridentate and tetradentate ligands correlates well with the binding energies of their constituent structural units. The computational results reveal that the R2 unit, along with its corresponding tridentate ligand R2Ph and tetradentate ligands R2PhRr, exhibits the highest ESP, superior binding energies, and the strongest coordination bonding affinity with the metals. The theoretical calculations further identify several promising extractants for the effective separation of Am(III) and Eu(III). The tridentate ligands R1Ph, R7Ph, and R4Ph, and the tetradentate ligands R4PhR4, R6PhR6, R2PhR2, R1PhR5 and R3PhR6 were identified as having excellent separation performance for Am(III) and Eu(III). This study would provide insights for the design of extractants for the separation of Am(III) and Eu(III) by use of five-membered N-heterocyclic rings as structural units.

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.

Theoretical insights into selective extraction of Am(III) from Cm(III) and Eu(III) with asymmetric N-heterocyclic ligands.

Separation of lanthanide (Ln) and minor actinide (MA) elements and mutual separation between minor actinide elements (e.g. Am(III) and Cm(III)) represent a crucial undertaking. However, separating these elements poses a significant challenge owing to their highly similar physicochemical properties. Asymmetric N-heterocyclic ligands such as N-ethyl-6-(1H-pyrazol-3-yl)-N-(p-tolyl)picolinamide (Et-p-Tol-A-PzPy) and N-ethyl-N-(p-tolyl)-1,10-phenanthroline-2-carboxamide (ETPhenAm) have recently received considerable attention in the separation of MAs over Ln from acid solutions. By changing the central skeleton structures of these ligands and introducing substituents with different properties on the side chains, their complexation behavior with Am(III), Cm(III), and Eu(III) may be affected. In this work, we explore four different asymmetric N-containing heterocyclic ligands, namely Et-p-Tol-A-PzPy (L1), N-ethyl-6'-(1H-pyrazol-3-yl)-N-(p-tolyl)-[2,2'-bipyridine]-6-carboxamide (L2), N-ethyl-9-(1H-pyrazol-3-yl)-N-(p-tolyl)-1,10-phenanthroline-2-carboxamide (L3), and ETPhenAm (L4) using density functional theory (DFT). The calculated results demonstrate the potential of ligands L1-L4 for the extraction and separation of Am(III), Cm(III), and Eu(III). Ligand analysis shows that ligand L3 binds more easily to the central metal atom, in line with the stronger extraction capacity of L3. In spite of the higher covalence between the side chain and the central metal atom for complexes with L1-L3, the main chain seems to control the stability of the extraction complexes. The preorganized 1,10-phenanthroline backbone also further enhances the extraction performance of L3 and L4. The difference in coordination ability between the side chain donors of these ligands and metal ions may affect their separation efficiency. This work presents theoretical insights into synthesizing novel ligands for separating trivalent actinides by adjusting N-heterocyclic ligands.

Theoretical design of polyazole based ligands for the separation of Am(III)/Eu(III)

The extraction of radioactive minor actinides (An(III)) from lanthanides (Ln(III)) is an extremely important step in nuclear waste reprocessing. Designing ligands with high-performance actinide-selectivity remains an essential task. Recent works have reported that some polyazole based ligands exhibit good An(III)/Ln(III) separation performance. Herein, we first evaluated the effects of different polyazole side chains on the Am(III)/Eu(III) selectivity by exploring three pyridine-derived polyazole ligands L1, L2 and L3 with 1,2,4-triazole, 1,2,3-triazole, and pyrazole side chains, respectively, using scalar relativistic theoretical methods. The coordination structures, bonding properties and thermodynamic behaviors of AmL(NO3)3 and EuL(NO3)3 complexes were investigated, which clarifies that the side chains do affect the electronic structure of ligand and its selectivity for Am(III)/Eu(III) ions. Moreover, L1 with 1,2,4-triazole side chains exhibited the highest selectivity for Am(III) over Eu(III) while the lowest complexation ability for metal ions among the three pyridine-derived polyazole ligands. Subsequently, we designed a new ligand L4 containing 1,2,4-triazole side chains and a preorganized phenanthroline backbone. Theoretically, such a new ligand was verified to show stronger complexation ability and higher selectivity for Am(III)/Eu(III) ions than L1. This work clarifies the complexation nature of polyazole based ligands with Am(III)/Eu(III) ions and provides design strategies for highly efficient polyazole based ligands for An(III)/Ln(III) separation.

Mutual separation of Am and Cm using ADAAM (Alkyl DiAmide AMine) and reduction of volumes for liquid waste generated via batch-wise multistage extractions

ABSTRACT The mutual separation of Am and Cm is conducted using an alkyl-diamide amine (ADAAM(EH)) extractant. ADAAM(EH) exhibits an extremely high separation factor (SF) with respect to Am and Cm separation (SF = 5.9) in an HNO3-n-dodecane system. The batch-wise multistage extractions are performed using a system containing 0.2 M ADAAM(EH) and 1.5 M HNO3, with their distribution ratios (D) of the system for Am and Cm equal to 2.55 and 0.434, respectively. In this multistage extraction, 3 stages of feed, 9 stages involving scrubbing, and 11 stages involving the use of an organic solvent give 96.5% and 1.06% yields of Am and Cm, respectively. After the mutual separation, the aqueous and the organic phases including actinides increase four times in volume compared to the feed solution. Additional extraction steps are to reduce the volume of the solutions including actinides. We use tetraethyl diglycolamide (TEDGA)/water for Am concentration, tetraoctyl diglycolamide/n-dodecane for the intermediate extraction, and TEDGA/water for stripping in the Cm concentration step. Taking the additional steps, Am and Cm can be recovered in just two or three stages in the aqueous phases.

Highly efficient separation of Am(III) and Pu(IV) from lean feeds and soil using an extraction chromatographic resin containing a diglycolamide in an ionic liquid

A TODGA based extraction chromatographic resin containing an ionic liquid was used for the separation of actinide ions such as Am3+ and Pu4+ from samples such as lean effluents emanating from laboratory waste, environmental water as well as soil samples adjacent to a nuclear plant site. The methodology involved feed adjustment to 3 M HNO3 followed by conditioning of the column, loading, washing (3 M HNO3), and elution of the actinide ions. The elution of Am3+ was done using EDTA in a buffered medium (1 M guanidine carbonate) while that of Pu4+ was carried out using a mixture of 0.5 M oxalic acid and 0.5 M HNO3. The elution peaks were sharp with almost no tailing suggesting the efficiency of the separation method. The results obtained were compared with the literature results which suggested the high efficiency of the present method.

Selective separation of Am(III)/Eu(III) by effectively adjusting the aliphatic ring in bis‐1,2,4‐triazine ligands: A theoretical study

AbstractIn recent years, bis‐1,2,4‐triazine ligands have been widely used in actinide separation processes for their good separation ability and stability in high acidity. However, the hard stripping of BTP, kinetics sluggish of BTBP, and inexhaustive separation of BTPhen are still issues to be improved. Here, we evaluated the effects of bis‐1,2,4‐triazine ligands containing 6‐membered aliphatic rings L1‐3 and bis‐1,2,4‐triazine ligands containing 5‐membered aliphatic rings L4‐6 on selective separation of Am(III)/Eu(III) to understand the structure‐effect relationship of ligands (skeleton of pyridine L1,4, bipyridine L2,5, and phenanthroline L3,6). Six bis‐1,2,4‐triazine ligand structures are optimized based on density function theory(DFT). Herein, complexations of L2, L3 L5 L6 with Am/Eu ions were investigated. The results show that all BTPs ligands preferentially bind to Am(III). The electronic structures of the [M(L)2(NO3)]2+ (M = Am, Eu) complex and the bonding properties between metal and ligand were investigated. The results suggest that phenanthroline containing six‐membered aliphatic rings L3 shows the best affinity for Am(III) and phenanthroline containing five‐membered aliphatic rings L6 show faster dynamics. The calculated thermodynamic and separation factors indicate that L6 has the best selective separation of Am(III)/Eu(III). In conclusion, the skeleton of phenanthroline has the best extraction effect on Am(III), while the kinetics and separation effect can be improved by adjusting aliphatic rings.

Tuning the Alkyl Chain of Nitrilotriacetamide for Selectively Extracting Trivalent Am over Eu Ions.

The successful management and safe disposal of high-level nuclear waste necessitate the efficient separation of actinides (An) from lanthanides (Ln), which has emerged as a crucial prerequisite. Mixed donor ligands incorporating both soft and hard donor atoms have garnered interest in the field of An/Ln separation and purification. One such example is nitrilotriacetamide (NTAamide) derivatives, which have demonstrated selectivity in extracting minor actinide Am(III) ions over Eu(III) ions. Nevertheless, the Am/Eu complexation behavior and selectivity remain underexplored. In the work, a comprehensive and systematic investigation has been conducted for [M(RL)(NO3)3] complexes (M = Am and Eu) utilizing relativistic density functional theory. The NTAamide ligand (RL) is substituted with various alkyl groups, namely, methyl, ethyl, propyl, n-butyl, n-pentyl, n-hexyl, n-heptyl, and n-octyl. Thermodynamic calculations show that the alkyl chain length in NTAamide is capable of tuning the separation selectivity of Am and Eu. Moreover, the differences in calculated free energies between Am and Eu complexes are more negative for R = Bu-Oct than Me-Pr. This indicates that elongation of the alkyl chain can increase the efficiency of selective separation of Am(III) from Eu(III). Based on the quantum theory of atoms in molecules and charge decomposition analyses, it has been observed that the strength of Am-RL bonds is higher than that of Eu-RL bonds. This disparity is attributed to a greater degree of covalency in Am-RL bonds and a higher level of charge transfer from ligands to Am within complexes containing these bonds. Energies of occupied orbitals with the central N character are recognized overall lower for [Am(OctL)(NO3)3] than for [Eu(OctL)(NO3)3], indicative of stronger complexation stability of the former. These results offer valuable insights into the separation mechanism of NTAamide ligands, which can help guide the development of more powerful agents for An/Ln separation in future applications.

Theoretical Insights into the Selectivity of Hydrophilic Sulfonated and Phosphorylated Ligands to Am(III) and Eu(III) Ions.

The separation of lanthanides and actinides has attracted great attention in spent nuclear fuel reprocessing up to date. In addition, liquid-liquid extraction is a feasible and useful way to separate An(III) from Ln(III) based on their relative solubilities in two different immiscible liquids. The hydrophilic bipyridine- and phenanthroline-based nitrogen-chelating ligands show excellent performance in separation of Am(III) and Eu(III) as reported previously. To profoundly explore the separation mechanism, herein, we first of all designed four hydrophilic sulfonated and phosphorylated ligands L1, L2, L3, and L4 based on the bipyridine and phenanthroline backbones. In addition, we studied the structures of these ligands and their neutral complexes [ML(NO3)3] (M = Am, Eu) as well as the thermodynamic properties of complexing reactions through the scalar relativistic density functional theory. According to the changes of the Gibbs free energy for the back-extraction reactions, the phenanthroline-based ligands L2 and L4 have stronger complexing capacity for both Am(III) and Eu(III) ions while the phosphorylated ligand L3 with the bipyridine framework has the highest Am(III)/Eu(III) selectivity. In addition, the charge decomposition analysis revealed a higher degree of charge transfer from the ligand to Am(III), suggesting stronger donor-acceptor interactions in the Am(III) complexes. This study can provide theoretical insights into the separation of actinide(III)/lanthanide(III) using hydrophilic sulfonated and phosphorylated N-donor ligands.

Extraction characteristics and radiation stability of the CHALMEX process for separation of Am(III) and Eu(III) at different temperatures

The behavior of the CHALMEX system comprising CyMe4-BTBP extractant in 30% tri-n-butyl phosphate and 70% phenyl trifluoromethyl sulfone diluents was characterized. The kinetic model based on mass transfer as the rate-controlling process was found to best describe the kinetics of Am(III) and Eu(III) extraction; the overall mass–transfer coefficients were determined. The dependence of distribution coefficients on temperature allowed change in free enthalpy and entropy of the system to be calculated. An investigation into the extraction properties of various CHALMEX systems combinations after irradiation with accelerated electrons up to an absorbed dose of 500 kGy at different temperatures up to 45 °C was carried out together with the characterisation of radiolytic products and their quantification using LC-MS and 1H NMR. The results obtained suggest that some of the radiolytic degradation products/adducts are efficient Am(III)/Eu(III) extractants, extracting Am and Eu by the ion-exchange mechanism. Adducts formed by hydroxylation or transfer of the CF3 moiety to the CyMe4-BTBP molecule are formed under irradiation together with products formed by elimination of a triazine ring or a cleavage of triazine ring(s). The results obtained now allow for a scheme of the main pathways of the CyMe4-BTBP molecule transformation under irradiation to be proposed.

Separation of americium from highly active raffinates by an innovative variant of the AmSel process based on the ionic liquid Aliquat-336 nitrate

A new variant of the AmSel (Americium Selective Separation) system for the separation of Am(iii) from a PUREX raffinate was tested in which the aliphatic diluent was replaced by the ionic liquid Aliquat-336 nitrate.

Theoretical Insights into Phenanthroline-Based Ligands toward the Separation of Am(III)/Eu(III).

The bistriazinyl-phenanthroline representative ligand, BTPhen, shows excellent extraction and separation ability for trivalent actinides and lanthanides. Herein, we first designed three phenanthroline-based nitrogen-donor ligands (L1, L2, and L3), and then studied the structural and bonding properties as well as thermodynamic properties of the probable complexes, ML(NO3)3 (M = Am or Eu and L = L1, L2, or L3), using scalar relativistic density functional theory. Our charge decomposition analysis revealed an obviously higher charge transfer from the ligand to Am(III) compared with the Eu(III) case for the studied complexes. Spin density analysis further showed a more significant degree of Am-to-ligand spin delocalization and the corresponding spin polarization on the ligands. According to the thermodynamic analysis, ligand L3 has the strongest complexation capacity for both Am(III) and Eu(III) ions, while ligand L1 has the highest Am(III)/Eu(III) selectivity in binary octanol/water solutions. We expected that this work can provide valuable theoretical support for the design of effective ligands for actinide(III)/lanthanide(III) separation in high level liquid waste.

Automated separation of Am from Sm by two-stage polymer-based HDEHP extraction chromatography

Separation of Am from Sm is very important for gamma spectrometric analysis of 242gAm in some specific samples, and also represents a very difficult challenge due to their similar chemical properties. Here we report a novel automated separation procedure based on two-stage HDEHP extraction chromatography to achieve the fast and highly efficient separation of Am from Sm. The two-stage HDEHP extraction chromatography was established by using HDEHP resin as the stationary phase, and DTPA-lactic acid and HNO3 as the mobile phases respectively. The automated separation method can be realized through process automation design. As a result, the recovery of Am is above 90%, and the decontamination factor (DF) for Sm reaches 2 × 107. The separation speed is improved and the radiation damage of radionuclides to the analysts is greatly reduced. So, this automated separation procedure developed in this work is certified to be convenient and efficient for the analysis of low activity 242gAm in the complex samples.

Lipophilic phenanthroline diamide ligands in 1-octanol for separation of Am(III) from Eu(III)

Although phenanthroline diamide (DAPhen) ligands have recently shown vast opportunity in group separation of actinides, the commonly used diluents in research, such as cyclohexanone with highly aqueous intersolubility, and 1-(trifluoromethyl)− 3-nitrobenzene with strong toxicity and corrosiveness, are becoming clear demerits. Herein, several lipophilic N,N′-dialkyl-N,N′-diphenyl-2,9-diamide-1,10-phenanthroline (R-Ph-DAPhen) ligands were synthesized and employed for selective separation of Am(III) from Eu(III) in 1-octanol, a practical low-toxicity and low-corrosive solvent. The solvent extraction results confirm their separation effect toward Am(III) over Eu(III) in both 1-octanol and TBP/1-octanol (v/v, 3/7) systems. Spectroscopic analysis illustrates that the ligands simultaneously generate M:L= 1:1 and 1:2 complexes with Ln(III), depending on the molar ratio of metal ion to ligand. The steric hindrance caused by the length of the alkyl side chains greatly affects the coordination chemistry of these ligands. Density functional theory (DFT) calculations shed light that these R-Ph-DAPhen ligands have desired An(III) affinity over Ln(III), and compared with F-3 solvent, 1-octanol weakens the extraction ability of the ligands for both lanthanides and actinides. This work contributes to further understanding of the extraction behavior of phenanthroline diamide-based ligands in different solvents and pushing them forward into practical applications in real An(III) extraction.

Theoretical Insights into the Selective Separation of Am(III)/Eu(III) Using Hydrophilic Triazolyl-Based Ligands

Designing ligands with efficient actinide (An(III))/lanthanide (Ln(III)) separation performance is still one of the key issues for the disposal of accumulated radioactive waste and the recovery of minor actinides. Recently, the hydrophilic ligands as promising extractants in the innovative Selective ActiNide Extraction (i-SANEX) process show excellent selectivity for Am(III) over Eu(III), such as hydroxylated-based ligands. In this work, we investigated the selective back-extraction toward Am(III) over Eu(III) with three hydrophilic hydroxylated triazolyl-based ligands (the skeleton of pyridine La, bipyridine Lb, and phenanthroline Lc) using scalar-relativistic density functional theory. The properties of three hydrophilic hydroxylated ligands and the coordination structures, bonding nature, and thermodynamic properties of the Am(III) and Eu(III) complexes with three ligands have been evaluated using multiple theoretical methods. The results of molecular orbitals (MOs), quantum theory of atoms in molecules (QTAIMs), and natural bond orbital (NBO) reveal that Am-N bonds possess more covalent character compared to Eu-N bonds. The thermodynamic results indicate that the complexing ability of Lb and Lc with metal ions is almost the same, which is stronger than that of La. However, La has the best Am(III)/Eu(III) selectivity among three ligands, which is attributed to the largest difference in covalency between Am-Ntrzl and Eu-Ntrzl bonds in MLa(NO3)3. This work provides an in-depth understanding of the preferential selectivity of the hydrophilic hydroxylated ligands with An(III) over Ln(III) and also provides theoretical support for designing potential hydrophilic ligands with excellent separation performance of Am(III)/Eu(III).