A Remarkable Strategy to Hijack U(VI) from Mixed Metal Ion Solutions Using 1-Hydroxy-2-pyridone.

The present work envisages a chelation driven, facile, selective, and rapid method for uranium(VI) separation from a (U, Th) mixture using 1-hydroxy-2-pyridone (1,2-HOPO). Herein, U(VI) was selectively precipitated as the neutral [UO2(HOPO)2(H2O)]·nH2O (orange colored) complex while Th(IV) and other metal ions remained in the solution. The pH of the medium played a key role in facilitating the separation process.

Pyridine Diphosphonate Ligand for Stabilization of Tetravalent Uranium and Neptunium in Aqueous Medium under Aerobic Conditions.

Though uranium is usually present in its +6 oxidation state (as uranyl ion) in aqueous solutions, its conversion to oxidation states such as +4 or +5 is a challenging task. Electrochemical reduction and axial oxo activation are the preferred methods to get stable unusual oxidation states of uranium in an aqueous medium. In previous studies, dicarboxylic acid has been used to stabilize UO2+ in aqueous alkaline solutions. In the present work, a diphosphonate ligand was chosen due to its higher complexing ability compared to that of the carboxylate ligands. Neptunium complexation studies with 2,6-pyridinediphosphonic acid (PyPOH) indicated the formation of different species at different pH values and the complexation facilitates disproportionation of NpO2+ to Np4+ and NpO22+ at pH 2. Hexavalent actinides form insoluble complexes in aqueous media at pH = 2, as confirmed by UO22+ complexation studies. The in situ complexation-driven precipitation resulted in conversion to pure Np4+ in aqueous media as the Np4+-PyPOH complex. A strong complexing ability of the PyPOH ligand toward the Np4+ ion is also seen for the stabilization of the electrochemically generated U4+ in aqueous medium under aerobic conditions. The U4+-PyPOH complex was found to be stable for 3 months. Raman, UV-vis, fluorescence, and cyclic voltametric studies along with density functional theory (DFT) calculations were done to get structural insights into the PyPOH complexes of actinides in different oxidation states.

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+.

Aggregation Behavior of Nitrilotriacetamide (NTAmide) Ligands in Thorium(IV) Extraction from Acidic Medium: Small-Angle Neutron Scattering, Fourier Transform Infrared, and Theoretical Studies.

Two tripodal amides obtained from nitrilotriacetic acid with n-butyl and n-octyl alkyl chains (HBNTA(LI) and HONTA(LII), respectively) were studied for the extraction of Th(IV) ions from nitric acid medium. The effect of the diluent medium, i.e., n-dodecane alone and a mixture of n-dodecane and 1-decanol, onto aggregate formation were investigated using small angle neutron scattering (SANS) studies. In addition, the influence of the ligand structure, nitric acid, and Th(IV) loading onto ligand aggregation and third-phase formation tendency was discussed.The LI/LII exist as monomers (aggregarte radius for LI: 6.0 Å; LII:7.4 Å) in the presence of 1-decanol, whereas LII forms dimers (aggregarte radius for LII:9.3 Å; LI does not dissolve in n-dodecane) in the absence of 1-decanol. The aggregation number increases for both the ligands after HNO3 and Th(IV) loading. The maximum organic concentration (0.050 ± 0.004 M) of Th(IV) was reached without third-phase formation for 0.1 M LI/LII dissolved in 20% isodecanol +80% n-dodecane. The interaction of 1-decanol with LII and HNO3/Th(IV) with amidic oxygens of LI/LII results in shift of carbonyl stretching frequency, as shown by attenuated total reflectance-Fourier transform infrared (ATR-FTIR) studies. The structural and bonding information of the Th-LI/LII complex were derived from the density functional theoretical (DFT) studies. The molecular dynamics (MD) simulations suggested that the aggregation behavior of the ligand in the present system is governed by the population of hydrogen bonds by phase modifier around the ligand molecules. Although the theoretical studies suggested higher Gibbs free energy of complexation for Th4+ ions with LI than LII, the extraction was found to be higher with the latter, possibly due to the higher lipophilicity and solubility of the Th-LII aggregate in the nonpolar media.

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.

Separation of neptunium (IV) from actinides by solid phase extraction using a resin containing Aliquat 336.

An extraction chromatographic resin material containing Aliquat 336 as the liquid anion exchanger extractant and Chromosorb W as the solid support was prepared and tested for the uptake of UO22+, Np4+, Pu4+, and Pu3+ from nitric acid feed solutions. The resin beads were characterized by thermogravimetry/differential thermogravimetry (TG/DTG) and scanning electron microscopy (SEM) surface morphology analysis. The uptake trend for the metal ions from 3 M HNO3 was found to be Pu4+ >> Np4+ >> UO22+ > Pu3+ which clearly followed the trend of their ionic potentials. In view of the significant difference in the uptake of Np4+ with respect to those of UO22+ and Pu3+, a separation scheme was developed for the selective separation of Np from feeds containing U, Np and Pu in nitric acid. The purity of the product was verified by alpha spectrometry.

Liquid-liquid extraction and facilitated transport of f-elements using an N-pivot tripodal ligand.

Diglycolamide (DGA)-functionalized tripodal ligands offer the required nine-coordinated complex for effective binding to a trivalent lanthanide/actinide ion. A N-pivot tripodal ligand (TREN-DGA) containing three DGA pendant arms was evaluated for the extraction and supported liquid membrane transport studies using PTFE flat sheets. Solvent extraction studies indicated preferential extraction of 1:1 (M:L) species, while the metal ion extraction increased with increasing HNO3 concentration conforming to a solvated species extraction. Flat sheet-supported liquid membrane studies, carried out using 4.0 × 10-3 M TREN-DGA in 95% n-dodecane + 5% iso-decanol indicated faster mass transport for Eu3+ ion as compared to Am3+ ion. The determined transport parameters indicated slow diffusion of the M-TREN-DGA (M = Am or Eu) complex being the rate-determining step. The transport of lanthanides and actinides followed the trend: Eu3+ > Am3+∼ Pu4+ >> UO22+ and Am can be selectively separated from a mixture of U and Pu by oxidizing the latter to its +6 oxidation state. The liquid membrane stability was not encouraging and was deteriorating the transport efficiency with time, which was attributed to carrier loss into the aqueous phases.