Complexation of Actinyl Ions with DHOA and TBP in an Ionic Liquid and a Molecular Solvent: How Similar or Different Are They?

Complexation thermodynamics of uranyl ions with well-known reprocessing ligands like tributyl phosphate (TBP) and dihexyl octanamide (DHOA) was studied in an ionic liquid (IL) versus a molecular solvent. Whereas 1-butyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide (Bumim·Tf2N) was used as an IL due to its favorable viscosity, acetonitrile was the choice of molecular solvent due to its poor coordinating nature. Optical spectroscopy studies revealed that UO22+ ions formed species of the types ML1 and ML2 with both TBP and DHOA, in a stepwise manner. The formation constants (log β) of UO22+ with DHOA in the IL were determined as 3.58 ± 0.03 and 6.54 ± 0.10, while those with TBP were obtained as 3.42 ± 0.05 and 6.38 ± 0.08, respectively. In the case of acetonitrile, on the other hand, these β values were significantly lower than those observed in the IL medium, which was supported by the DFT calculations. Calorimetric titrations of the UO22+ ions with DHOA and TBP confirmed that the complex formation reactions were thermodynamically more favored in the ionic liquid medium than the molecular solvent. The nature of binding through FTIR investigations and DFT calculations suggested that the complexes formed in the IL medium were cationic in nature of the types [UO2(H2O)3(TBP)2]2+ and [UO2(H2O)3(DHOA)2]2+, but neutral complexes were formed in the molecular solvent of the types [UO2(NO3)2(TBP)2] and [UO2(NO3)2(DHOA)2].

Utility of Interchangeable Coordination Modes of N,N'-Dialkyl-2,6-pyridinediamide Tridentate Pincer Ligands for Solvent Extraction of Pd(II) and Zr(IV) from High-Level Radioactive Liquid Waste

A new class of ligands, N,N'-dialkyl-2,6-pyridinediamide (DRPDA), has been designed with the specific intention of exhibiting interchangeable diversity in coordination modes, including organometallic interactions, for the purpose of solvent extraction of elements relevant to the proper treatment of high-level radioactive liquid waste (HLLW) generated after nuclear fuel reprocessing. Consequently, DRPDA has been observed to extract Pd(II) and Zr(IV) from HNO3(aq) to 1-octanol in nearly quantitative yields when the selected ligand is sufficiently hydrophobic. However, concomitance of some of other HLLW components were also found. The extraction selectivity toward Pd(II) and Zr(IV) was markedly enhanced by employing n-dodecane instead of 1-octanol as evidenced by good distribution ratios (DM) of Pd(II) (DPd = 72.5) and Zr(IV) (DZr = 12.9), which is several orders of magnitude greater than DM's of other HLLW components (10-3-10-2), where addition of 20 vol % 1-octanol is still required to accelerate the extraction kinetics. Despite direct contact with the highly acidic aqueous phase, deprotonation from one of the amide NH moieties of DRPDA proceeds to form [Pd(DRPDA-)(NO3)] as a good extractables in the current biphasic system. This Pd(II) complex with a rather unique asymmetric N-^N^O tridentate coordination was characterized by SCXRD, elemental analysis and 1H NMR, and theoretically corroborated by DFT calculations and NBO analysis. In contrast, DRPDA also interacts with Zr4+ in different tridentate O^N^O mode without any deprotonation. Based on mechanistic differences in the extraction chemistry we clarified, Pd(II) and Zr(IV) coextracted to the organic phase were recovered stepwise by using appropriate stripping agents such as 1.0 M HCl(aq) and 0.10 M HNO3(aq), respectively.

Complexation of Trivalent Lanthanides with Hexaalkyl Nitrilotriacetamides into Methylimidazolium-Based Ionic Liquids: Spectroscopic, Electrochemical, Calorimetric, and Theoretical Insights

Exothermic, spontaneous inner-sphere complexation of trivalent lanthanides with N,N,N',N',N″,N″-hexaalkyl-substituted nitrilotriacetamides (HRNTAs) into 1-hexyl-3-methylimidazolium-bis(trifluoromethylsulfonyl)imide (C6mim NTf2) is reported with a predominant presence of ML and ML2 complexes having complexation constants, β1Eu 3.10 ± 0.02 ,β1Nd 2.67 ± 0.03 and β2Eu 5.22 ± 0.04, β2Nd 4.95 ± 0.02, respectively, for the n-butyl-substituted HRNTA (HBNTA); while those for the n-hexyl derivative (HHNTA) are β1Eu 4.27 ± 0.03, β1Nd 3.71 ± 0.03 and β2Eu 7.70 ± 0.03, β2Nd 7.18 ± 0.04, respectively. HHNTA shows better complexing ability; whereas the stronger complexation of the Nd3+ over Eu3+ is attributed to the lanthanide contraction. Furthermore, the nature of the ionic liquid also influences the extent of complexation with the trend: C4mim NTf2 > C6mim NTf2 > C8mim NTf2, which follows the order of their dielectric constants. Judd-Ofelt parameters were calculated from photoluminescence data to get an idea about the symmetry of the Eu3+ complexes. Electrochemical investigations give diffusion coefficient values of 1.17 × 10-7 and 8.26 × 10-8 cm2/s for the Eu3+ complexes of HBNTA and HHNTA, respectively. Changes in the spectral characteristics and peak positions are evidenced in the FTIR spectra on the complexation of Eu3+ with the HRNTA ligands in C6mim NTf2. Structure optimization for the complexes was performed by DFT computations.

Effect of branching in the alkyl chain of diglycolamide on the sequestration of tetravalent actinides: solvent extraction and theoretical studies

DGA (diglycolamide) ligands show a different extraction behavior of trivalent metal ions by changing the branching alkyl chain length as well as the branching at the methylene position. There are no studies of these factors on the extraction efficiency of these DGA derivatives for the extraction of tetravalent actinides. We have evaluated four different DGA derivatives for the extraction of Np, Pu, and Th from molecular diluents. The n-butyl derivative shows enhanced extraction efficiency and branching gives rise to a reduction in the extraction efficiency of tetravalent ions. The distribution ratios are higher in pure octanol than in a mixture of 30% octanol and 70% n-dodecane. This behavior is in marked difference to that of the extraction of trivalent ions where the addition of an alcohol generally decreases the distribution ratio of trivalent ions due to the ligand-modifier intercation, poor aggregation or micelle formation tendency of DGAs in polar solvents. This suggests that micelle-mediated extraction may not be the dominating factor for the extraction of tetravalent metal ions. Slope analysis suggests the involvement of only two DGA molecules in the extracted species suggesting no/poor possibility of micelle formation in the present system. The higher extraction in pure octanol may be due to a better solubility of the extracted complexes in this polar medium compared to the mixture of octanol and n-dodecane. The water and acid uptake, the back extraction, and the radiation stability of the solvent systems were also investigated. DFT studies were performed to get a better insight into the extraction and complexation of the different DGA solvent systems.

Remarkably High Separation of Neodymium from Praseodymium by Selective Dissolution from their Oxide Mixture using an Ionic Liquid Containing aβ-Diketone

A simple, efficient, direct and economical method for the mutual separation of Nd and Pr was developed by the selective dissolution of Nd2O3 from their oxide mixtures in an ionic liquid containing 2-thenoyltrifluoroacetone (HTTA) resulting in an unprecedented separation factor (βNd/Pr)>500, which is 277 times more than the thus far reported βNd/Pr values. The proposed mechanism was supported by DFT computations.

Advanced Uranium Removal with Pure, Nonfluorinated Ionic Liquids: Fine-Tuning Hydrophilic and Hydrophobic Properties for Enhanced Selectivity

Ionic liquids (ILs) have significant potential for eco-friendly extraction of uranium from aqueous solutions, which is critical for nuclear technology, fuel cycle management, and environmental protection. This study examines the impact of the adjustable hydrophobic/hydrophilic properties of ILs on the removal of uranium(VI) (UO22+) from aqueous solutions utilizing both a novel hydrophilic IL (1-butoxyethyl-1-methylmorpholinium butoxyethylphosphite - Mor1-2O4-BOEP) and 1-heptyl-1-methylmorpholinium heptylphosphite (Mor1-7-HP) as an example of a hydrophobic IL with a similar structure. The transfer mechanism of uranyl ions from water to organic or solid phases closely depends on the physicochemical properties of ILs, especially their hydrophobicity. The hydrophobic Mor1-7-HP extracts uranyl via neutral complex formation as UO2(NO3)2-(Mor1-7-HP)2. Conversely, hydrophilic Mor1-2O4-BOEP induced selective precipitation as UO2(NO3)-(BOEP), transferring uranyl to the solid phase. Optimization of the working parameters, in terms of acidity of the aqueous solution and amount of ILs used, allowed the extraction of over 98% of U(VI). The stoichiometry of the organic complex and the precipitate was determined using physicochemical techniques. These tunable H-phosphonate-based ILs have advantages over traditional solvent extraction and conventional ILs, allowing easier handling, improved selectivity, and lower environmental impact. This work advances uranium separation techniques with applications in hydrometallurgy, particularly in the treatment of wastewater and radioactive waste for sustainable uranium recovery.

In-Situ-Generated Fluoride-Assisted Rapid Dissolution of Uranium Oxides by Ionic Liquids

A quantitative, rapid, endothermic dissolution of U3O8 in C4mim·PF6 (1-alkyl-3-methyl imidazolium hexafluorophosphate) has been achieved within 2 h at 65 °C by in situ generated fluoride ions by pre-equilibrating the ionic liquid with suitable concentrations of nitric acid. The efficiency of the dissolution followed the trend: UO3 > UO2 > U3O8. The fluoride generation was found to increase with the concentration of nitric acid being equilibrated, the water content of the ionic liquid, and also the time of equilibration. The rate of dissolution of U3O8 followed the trend: C4mim·PF6> C6mim·PF6 > C8mim·PF6. The maximum loading observed for the present case was 200 mg mL-1 which is considered to be quite high with an ionic liquid. The effects of different acid pre-equilibration (HClO4, HCl) on F- generation and subsequent dissolution characteristics have also been investigated. The in situ F- generation, as well as U3O8 dissolution, were found to predominantly follow a pseudo-second-order rate kinetics, while the rate constants for U3O8 dissolution were found to be higher than that of F- generation. The dissolved uranium was successfully electrodeposited on a Cu plate, as confirmed by EDXRF, while the formation of UO2 was revealed from the XRD pattern of the deposit.

Efficient actinide sequestration with ionic liquid-based extraction chromatography resins containing Aza-crown ether functionalized diglycolamides

Two new extraction chromatographic resins (ECRs) were prepared by impregnating two exotic diglycolamide (DGA) ligands (having three or four DGA moieties tethered to aza-crown ether scaffolds) dissolved in an ionic liquid onto an inert solid support. A room temperature ionic liquid (RTIL) was used for enhancing the performance of the ECRs. The ECR containing triaza-9-crown-3 functionalized with three DGA moieties (TAM-3-DGA), and tetraaza-12-crown-4 tethered with four DGA arms (TAM-4-DGA) were evaluated for the separation of Am3+ and Pu4+from nitric acid solutions. The resin capacity for Eu3+ was 9.52 mg/g and 7.24 mg/g for TAM-3-DGA and TAM-4-DGA resins, respectively. Similarly, the resin capacity for Pu4+was 7.44 mg/g and 5.72 mg/g for TAM-3-DGA and TAM-4-DGA resins, respectively. These maximum loading values corresponded to the formation of a 1:1 metal/ligand complex for the Eu3+ ion and a 1:2 metal/ligand complex for the Pu4+ ion. The sorption of Eu3+and Pu4+on the resins followed a chemisorption phenomenon on both resins. The sorbed Eu3+and Pu4+ions from the resin phase could be efficiently desorbed with complexing ligands such as guanidine carbonate/HEDTA and oxalic acid, respectively.

Polydentate N,O-Ligands Possessing Unsymmetrical Urea Fragments Attached to a p-Cresol Scaffold

In this study, three series of polydentate N,O-ligands possessing unsymmetrical urea fragments attached to a p-cresol scaffold are obtained, namely mono- and bi-substituted open-chain aromatics, synthesised using a common experiment, as well as fused aryloxazinones. Separate protocols for the preparation of each series are developed. It is found that in the case of open-chain compounds, the reaction output is strongly dependent on both bis-amine and carbamoyl chloride substituents, while oxazinones can be effectively obtained via a common protocol. The products are characterized via 1D and 2D NMR spectra in solution and using single-crystal XRD. A preliminary study on the coordination abilities of the products performed via ITC shows that there are no substantial interactions in the pH range of 5.0-8.5 in general.

Selective Complexation and Separation of Uranium(VI) from Thorium(IV) with New Tetradentate N,O-Hybrid Diamide Ligands: Synthesis, Extraction, Spectroscopy, and Crystallographic Studies

An unmet challenge in the thorium-uranium fuel cycle is the efficient separation of uranium from thorium. Herein, two new tetradentate N,O-hybrid ligands, N,N'-diethyl-N,N'-di-p-tolyl-2,2'-bipyridine-6,6'-dicarboxamide (Et-Tol-BPDA) and N,N'-diethyl-N,N'-di-p-tolyl-2,2'-bipyrimidine-4,4'-dicarboxamide (Et-Tol-BPymDA), comprising a bipyridine or bipyrimidine core and amide moieties were designed and synthesized for selectively complexing and separating U(VI) from Th(IV). The high U(VI)/Th(IV) extraction selectivity was achieved by Et-Tol-BPDA (SF_U/Th = 33 at 3 M HNO₃) and Et-Tol-BPymDA (SF_U/Th = 73 at 3 M HNO₃) in nitric acid solutions. The extraction process for U(VI) or Th(IV) with these two ligands primarily proceeded through the solvation mechanism, as evidenced by slope analyses. Thermodynamic studies for the extraction of U(VI) and Th(IV) revealed a spontaneous process. Results from UV-vis spectroscopic titration and slope analyses demonstrated that U(VI) and Th(IV) each form a 1:1 complex with the two ligands both in the monophasic organic solution and the biphasic extraction system. The stability constants of the 1:1 complexes of Et-Tol-BPDA or Et-Tol-BPymDA with U(VI) were found to be larger than those with Th(IV), which coincide well with the high U(VI)/Th(IV) extraction selectivity. The solid-state structures of Et-Tol-BPDA, Et-Tol-BPymDA, and 1:1 complexes of the two ligands with U(VI) or Th(IV) were analyzed by X-ray diffraction technique. The results from this work implicate the potential of bipyridine- and bipyrimidine-derived diamide ligands for uranium/thorium separation.

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⁺.

A comparative study on the uptake of lanthanides from acidic feeds using extraction chromatography resins containing N,N,N',N'-tetra-n-alkyl diglycolamides with varying alkyl chain lengths in an ionic liquid

A comparative study on the uptake of several rare earth element (REE) ions viz. La(III), Ce(III), Pr(III), Nd(III), Sm(III), Gd(III) and Dy(III) was carried out from nitric acid feeds using four extraction chromatography resins which contained the diglycolamide (DGA) ligands, N,N,N',N'-tetra-n-alkyldiglycolamide with n-pentyl (TPDGA), n-hexyl (THDGA), n-octyl (TODGA) and n-decyl (TDDGA) groups taken in a room temperature ionic liquid (C₄mim·NTf₂). The uptake of the lanthanides followed the trend: La(III) < Ce(III) < Pr(III) < Nd(III) < Sm(III) < Gd(III) < Dy(III), which is similar to their ionic potential values and the uptake trend of the resins was TPDGA > THDGA > TODGA > TDDGA. The uptake of the metal ions was very high (>10⁴ g/mL) for all the lanthanide ions and was found to increase with increasing nitric acid concentrations. Based on the encouraging batch data, column studies were carried out with all the four extraction chromatography resins with the lanthanide ions used in this work. The column studies were carried out with both individual lanthanide ions and their mixtures. While the loading studies were carried out with 80 mg/L solutions of the metal ions (with respect to each of those) in the mixture of REEs, the elution studies were carried out using a solution of 0.05 M EDTA in 1 M guanidine carbonate. For the column studies involving individual REEs, 550 mg/L solutions were used. The elution profiles appeared to be sharp as >95% elution of the metal ions was accomplished in only 3 mL of the eluent which amounted to only 1.6 bed volumes which is highly impressive. When the studies were carried out with the mixture of the lanthanide ions, the breakthrough of Dy(III) was last while that of La(III) was seen at much lower volumes which was dependent on the nature of the extractant in the resins.

Fate of Neptunium in nuclear fuel cycle streams: state-of-the art on separation strategies

Neptunium, with a half life of 2.14 million years is one of the most notorious activation products in the nuclear fuel cycle. It has been more than 5 decades in the reprocessing of nuclear fuels by the well documented PUREX process, but the fate of Np in the PUREX cycle is still not well controlled. Although Np being stable in its pentavalent state in low acid media, its starts to undergo disproportionation at higher acidities. This disproportionation along with the oxidizing conditions of the HNO3 medium makes Np to exits as Np(IV), Np(V) and Np(VI) in the dissolver solution. The overall extractability of Np in the co-decontamination step of the PUREX cycle is dependent on its oxidation state in the medium as Np(VI) and Np(IV) being extractable while Np(V) being least extractable. The present review article discusses about the speciation of Np in HNO3 and its disproportionation. The variety of redox reagents are discussed for their effectiveness towards controlling Np redox behavior in the HNO3 media. The extraction of Np with the different class of extractant has also been discussed and the results are compared for better understanding. Solid phase extraction of Np using both commercially available resin and lab based synthesized resins were discussed. The anion exchange resins with the different cationic centers were shown to behave differently towards the uptake of Np form the acidic medium. The present review also highlight the chemical conditions required for controlling or minimizing the fate of Np in different process streams of the nuclear fuel cycle.

Liquid-Liquid Extraction of the Eu(III) Cation by BTP Ligands into Ionic Liquids: Interfacial Features and Extraction Mechanisms Investigated by MD Simulations

What happens at the ionic-liquid (IL)/water interface when the Eu³⁺ cation is complexed and extracted by bis(dimethyltriazinyl) pyridine "BTP" ligands has been investigated by molecular dynamics and potential of mean force simulations on the interface crossing by key species: neutral BTP, its protonated BTPH⁺ form, Eu³⁺, and the Eu(BTP)₃³⁺ complex. At both the [BMI][Tf₂N]/water and [OMI][Tf₂N]/water interfaces, neither BTP nor Eu(BTP)₃³⁺ are found to adsorb. The distribution of Eu(BTP)₂³⁺ and Eu(BTP)³⁺ precursors of Eu(BTP)₃³⁺, and of their nitrate adducts, implies the occurrence of a stepwise complexation process in the interfacial domain, however. The analysis of the ionic content of the bulk phases and of their interface before and after extraction highlights the role of charge buffering by interfacial IL cations and anions, by different amounts depending on the IL. Comparison of ILs with octanol as the oil phase reveals striking differences regarding the extraction efficiency, the affinity of Eu(BTP)₃³⁺ for the interface, the effects of added nitric acid and of counterions (NO₃^- vs Tf₂N^-), charge neutralization mechanisms, and the extent of "oil" heterogeneity. Extraction into octanol is suggested to proceed via adsorption at the surface of water pools, nanoemulsions, or droplets, with marked counterion effects.

Application of Ionic Liquids for the Recycling and Recovery of Technologically Critical and Valuable Metals

Population growth has led to an increased demand for raw minerals and energy resources; however, their supply cannot easily be provided in the same proportions. Modern technologies contain materials that are becoming more finely intermixed because of the broadening palette of elements used, and this outcome creates certain limitations for recycling. The recovery and separation of individual elements, critical materials and valuable metals from complex systems requires complex energy-consuming solutions with many hazardous chemicals used. Significant pressure is brought to bear on the improvement of separation and recycling approaches by the need to balance sustainability, efficiency, and environmental impacts. Due to the increase in environmental consciousness in chemical research and industry, the challenge for a sustainable environment calls for clean procedures that avoid the use of harmful organic solvents. Ionic liquids, also known as molten salts and future solvents, are endowed with unique features that have already had a promising impact on cutting-edge science and technologies. This review aims to address the current challenges associated with the energy-efficient design, recovery, recycling, and separation of valuable metals employing ionic liquids.

Highly efficient Plutonium(IV) uptake from acidic feeds using four extraction chromatography resins containing diglycolamides and ionic liquid

Quantitative recovery of plutonium from lean effluents is one of the most challenging tasks for separation scientists. Four extraction chromatography (XC) resins containing substituted diglycolamide ligands viz. N,N,N',N'-tetra-n-pentyl diglycolamide (TPDGA), N,N,N',N'-tetra-n-hexyl diglycolamide (THDGA), N,N,N',N'-tetra-n-octyl diglycolamide (TODGA) and N,N,N',N'-tetra-n-decyl diglycolamide (TDDGA) and a room temperature ionic liquid (RTIL) were tested for the extraction of plutonium (IV) from nitric acid feed solutions. The relative efficiency of uptake of the metal ion in the entire range of HNO₃ studied was: TPDGA > THDGA > TODGA > TDDGA, which was opposite to the chain length of the attached alkyl groups. Also, for all the four XC resins the uptake of Pu(IV) was found to decrease with increasing nitric acid concentration in the lower acidity range followed by an increase thereafter. The uptake of Pu(IV) with all the four XC resins was fitted into different kinetic and isotherm models. It was found that all the four resins followed the pseudo-second order kinetic model and Langmuir monolayer adsorption model. Column studies with these XC resins using a loading solution containing 1.2 g/L Pu(IV) in 3 M HNO₃ showed early breakthrough for the higher homolog DGA ligands as compared to the lower homologs. Effective elution of the loaded Pu(IV) from the column was done in about 5.5 column volumes using a solution containing 0.5 M oxalic acid in 0.5 M HNO₃.

Unraveling the Coordination Approach of Eu(III) in Cyphos Nitrate Ionic Liquid – A Comprehensive Luminescence Spectroscopy Study

In consideration of the mounting attention of the ionic liquid: Cyphos 101 (trihexyl(tetradecyl)phoshonium chloride: [P66614][Cl]) in the recovery of rare earth and other valuables from their waste matrices, an effort...

Isolation of single crystals of a homoleptic UO22+-diglycolamide complex from a room temperature ionic liquid: X-ray crystallography and complexation studies

Complexation and structural investigations of the solid complex of UO22+ ion and TMDGA isolated from an ionic liquid for the first time revealed that the nature and structural features of the complex are identical with those of the complex isolated from the aqueous medium.

Metal chloride anion based ionic liquids: synthesis, characterization and evaluation of performance in hydrogen sulfide oxidative absorption

Three metal chloride anion based ionic liquids (MCABILs) were synthesized and characterized for high conversion of hydrogen sulfide (H2S).

Highly efficient uptake of tetravalent actinide ions from nitric acid feeds using an extraction chromatography material containing tetra-n-butyl diglycolamide and a room temperature ionic liquid

An extraction chromatography (XC) material containing N,N,N',N'-tetra-n-butyl diglycolamide (TBDGA) and 1-butyl-3-methylimidazolium bis(trifluoromethanesulphonyl) imide (C₄mim∙NTf₂), a room temperature ionic liquid, was used for the uptake of the tetravalent actinide ions Th(IV), Np(IV), and Pu(IV) from nitric acid feed solutions. The uptake of the metal ions followed the trend: Th(IV) < Np(IV) < Pu(IV), which is the same as that of their ionic potential values. While a decrease in the Np(IV) and Pu(IV) uptake was seen with increasing HNO₃ concentration at lower acidities, an opposite trend was observed at higher acidities. Th(IV) uptake was not affected with the acid concentration. In view of the very high uptake and its importance in the nuclear fuel cycle, the major part of the studies was carried out with Pu(IV) ion. The loaded Pu(IV) was back extracted from the XC material using a mixture of 0.5 M oxalic acid and 0.5 M nitric acid. The Pu(IV) uptake by the XC material was fitted into different kinetic and isotherm models. The results conformed to the pseudo-second order kinetic model and the Langmuir monolayer sorption model. Column studies were carried out using a feed having 1.6 mg/L Pu solution in 3 M HNO₃. While the breakthrough was obtained after passing ca. nine bed volumes, a sharp elution peak was obtained with >99% recovery in about seven bed volumes.

In Situ Preconcentration during the Di-(2-ethylhexyl) Phosphoric Acid-Assisted Dissolution of Uranium Trioxide in an Ionic Liquid: Spectroscopic, Electrochemical, and Theoretical Studies

Dissolution of uranium oxide was carried out using a solution of HD2EHP in C₈mim·NTf₂, which was apparently facilitated by the in situ generation of water during the complex formation reaction. The dissolved complex in the ionic liquid phase led to splitting of the latter into a light phase and a heavy phase where the former contained predominantly the UO₂(HL₂)₂ complex (HL = HD2EHP), while the latter contained the ionic liquid as supported by FTIR and UV-Visible spectral analyses. The complexation of the uranyl ion was suggested to take place in the equatorial plane where two dimeric units of the H-bonded HD2EHP molecules took part in complexation. An increase in temperature facilitated the dissolution rate with an activation energy of 31.0 ± 2.8 kJ/mol. The cyclic voltammetry studies indicated potential chances of recovery of the dissolved uranium by electrodeposition at the cathode. The proposed dimeric structure of HD2EHP in the complexation with U(VI) was supported by DFT studies also.

Bis-(1,2,4-triazin-3-yl) ligand structure driven selectivity reversal between Am3+ and Cm3+: solvent extraction and DFT studies

Selectivity between Am3+ and Cm3+ was investigated after their aqueous complexation with three structurally tailored hydrophilic bis-(1,2,4-triazin-3-yl) ligands followed by their extraction with N,N,N'N'-tetraoctyl diglycolamide (TODGA) dissolved in an ionic liquid (C4mim·Tf2N). The three hydrophilic ligands used were SO3PhBTP, SO3PhBTBP, and SO3PhBTPhen. It was evident from the solvent extraction studies that SO3PhBTP formed a stronger complex with Cm3+ than with Am3+, but SO3PhBTPhen showed better complexation ability for Am3+ than for Cm3+, and SO3PhBTBP showed no selectivity for the two actinide ions. DFT calculations indicated that the coordinating 'N' atoms in BTP were more co-planar in the complex and this co-planarity was higher in the Cm3+ complex as compared to that in Am3+. In the case of BTBP and BTPhen ligands, on the other hand, the co-planarity was more pronounced in the Am3+ complexes. Mayer's bond order calculations of M-N bonds in the complexes also indicated a reversal of the complexation ability of the BTP and BTPhen ligands for Am3+ and Cm3+. Calculations of the complexation energies further supported the higher selectivity of the BTP ligand for Am3+ by -52.0 kJ mol-1, and better selectivity of the BTPhen ligand for Cm3+ by -24.7 kJ mol-1.

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.

Comparative uptake studies on trivalent f-cations from acidic feeds using two extraction chromatography resins containing a diglycolamide in molecular diluent and ionic liquid

Low molecular weight diglycolamide (DGA) extractants were tested for the extraction of europium(III) and americium(III) from nitric acid solutions in n-dodecane, a molecular diluent and 1-butyl-3-methylimidazolium bis(trifluoromethanesulphonyl) imide (C₄mim⋅NTf₂), a room temperature ionic liquid, as the diluents. N,N,N',N'-tetra-n-butyl diglycolamide (TBDGA) was selected for extraction chromatography (XC) studies involving Eu(III) and Am(III). While the TBDGA resin containing n-dodecane gave reasonably high K_d values, that containing the ionic liquid showed higher Eu(III) uptake values. Compared to Eu(III), Am(III) was extracted by the resins to a lower extent. The loaded Eu(III) was back extracted from the resin using 0.05 M EDTA solutions in a buffered medium containing 1 M guanidine carbonate. Reusability studies indicated that, while the ionic liquid-based resin can be conveniently recycled five times with very marginal decrease in the percentage extraction values, there was a sharp decrease in the percent extraction after three cycles with the n-dodecane-based resin. The uptake data was fitted into different isotherm models and the results conformed to the Langmuir model. Based on the batch uptake studies, columns were prepared and the breakthrough as well as elution profiles were obtained. The elution profiles were found to be sharp without any significant tailing.

Selective extraction of trivalent actinides using CyMe4BTPhen in the ionic liquid Aliquat-336 nitrate

The extraction of Am(iii), Cm(iii) and Eu(iii) by 2,9-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-1,10-phenanthroline (CyMe₄BTPhen) from nitric acid solution was studied using the ionic liquid Aliquat-336 nitrate ([A336][NO₃]) as diluent. Results show a high selectivity of the solvent for Am(iii) and Cm(iii) over Eu(iii), but rather slow extraction kinetics. The kinetics of CyMe₄BTPhen were largely improved by the addition of 0.005 mol L^-1 N,N,N',N'-tetra-n-octyl-diglycolamide (TODGA) as a phase transfer reagent and by the use of 1-octanol as co-diluent. The addition of the phase transfer catalyst and co-diluent did not compromise the selectivity towards the actinide/lanthanide separation and thus this four-component system can be successfully applied to separate Am(iii) and Cm(iii) from the lanthanides.

Recent Progress in Ionic Liquid Extraction for the Separation of Rare Earth Elements

This review summarizes recent progress in solvent extraction of rare earth elements (REEs) using an ionic liquid (IL) as the extraction solvent. These IL extraction systems are advantageous owing to the affinity of ILs for both charged and neutral hydrophobic species, in contrast to conventional organic solvent extraction systems. Herein, REE extraction studies using ILs are detailed and classified based on the type of extraction system, namely extraction using anionic ligands, extraction using neutral ligands, synergistic extraction, extraction without extractants, and a specific system using task-specific ionic liquids (TSILs). In IL extraction systems, the extracted complexes are often different from those in organic solvent systems, and the REE extraction and separation efficiencies are often significantly enhanced. Synergistic IL extraction is an effective approach to improving the extractability and separability of REEs. The development of novel TSILs suitable for IL extraction systems is also effective for REE separation.

Sequestration of tetravalent neptunium from acidic feeds using diglycolamide-functionalized dendrimers in a room temperature ionic liquid: extraction, spectroscopic and electrochemical studies

Extraction of Np(iv) was studied using RTIL containing two poly(propyleneimine) based dendrimer ligands from HNO3 solutions. Studies of the extracted complexes were also carried out by Vis-NIR and cyclic voltammetry.