An N-pivot diglycolamide extractant (DGA-TREN) was synthesized for the first time and its complexation behaviour was studied towards trivalent lanthanide/actinide ions. The solvent extraction studies suggested a unique selectivity reversal in the extraction of trivalent actinides versus trivalent lanthanides which was observed performing extraction studies in an ionic liquid vis-à-vis a molecular diluent for a tripodal TREN-based diglycolamide ligand (DGA-TREN) vs. a tripodal diglycolamide ligand (T-DGA) which may have great significance in radioactive waste remediation. The nature of the bonding to Eu(3+) ion was investigated by EXAFS as well as by DFT calculations.

A multiple diglycolamide (DGA)-containing ligand having four DGA arms tethered to a tetraaza-12-crown-4 ring, viz. 2, 2′,2′′,2′′′-(((1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayl)tetrakis(2-oxoethane-2,1-diyl)) tetrakis (oxy)) tetrakis(N,N-dioctylacetamide) (T12C4ODGA), was synthesized and evaluated for the extraction of different actinide and lanthanide ions, viz. Am 3+ , Eu 3+ , Pu 4+ , Np 4+ , and UO 2 2+ . The extraction efficiency of the present ligand was found to be the highest reported so far, more specifically for the trivalent metal ions Am 3+ and Eu 3+ , when one considers the very low ligand concentration used in the present study, compared to that of the various previously reported multiple DGA-based ligands. The nature of the complexes formed during the extraction of Eu 3+ was investigated using time-resolved fluorescence (TRFS) and extended X-ray absorption fine structure (EXAFS) spectroscopy. Both the solvent extraction and TRFS studies indicated the presence of 1:1 and 1:2 complexes during the extraction of Am 3+ and Eu 3+ having three inner-sphere water molecules in the 1:1 complex. Density functional theoretical (DFT) studies were performed on the Am 3+ and Eu 3+ complexes of both T12C4ODGA and an analogous compound having methyl groups in place of the n-octyl groups, and the DFT results of the T12C4ODGA nicely explain the extraction behavior of Am 3+ and Eu 3+ .

Small-angle neutron scattering (SANS) studies were carried out to compare the aggregation behavior of 1.1 M solutions of tributyl phosphate (TBP) and N,N-dihexyl octanamide (DHOA) dissolved in different deuterated diluents, viz., n-dodecane, chloroform, and benzene, during the extraction of Th(IV) from nitric acid medium. The scattering data was treated using the Baxter sticky spheres model. The third phase formed in the case of DHOA displayed higher aggregation tendency compared to that of TBP. These studies have demonstrated that the nature of the diluents plays an important role in the aggregation behavior of the extracted species (reverse micelles). No third phase was observed in the case of chlorinated and aromatic diluents like chloroform and benzene during the extraction of Th(IV) from nitric acid medium. Theoretical calculations were also performed to gain insights into the binding of thorium nitrate with TBP and DHOA models. These calculations suggest that two to three molecules of both DHOA and TBP strongly coordinate to Th(NO3)4. It is noted that the highly charged Th(IV) cations are screened by nitrates and extractants which enables the interaction of second unit of such complex through noncovalent interactions.

This paper reports the solvent extraction of Am and Eu using N,N,N',N',N'',N''-hexa-n-octylnitrilotriacetamide (HONTA) as the extractant in n-dodecane. The results are in variance with those reported previously with respect to the nature of the extracted species. The solvent extraction data were entirely different from those reported previously as the extracted species conformed to 1 : 2 (M : L) species for both Am and Eu ions. The structure of the extracted complex was determined by EXAFS demonstrating the three amidic 'O' atoms of the HONTA complex with the Eu ion. In the case of the Am ion, the pivotal 'N' atom is suggested to bond to the metal ion, which may explain the significantly more favourable extraction of Amvis-à-vis Eu. The absence of HO molecules in the inner coordination sphere of the Eu-HONTA extract was confirmed by luminescence spectroscopic measurements. Complexation studies in MeOH and EtOH indicated the formation of both 1 : 1 and 1 : 2 complexes with Nd ions. The results are explained on the basis of DFT calculations using HMNTA, the corresponding hexamethyl analogue of HONTA.

Straight chain amide N,N-dihexyloctanamide (DHOA) has been found to be a promising alternative extractant to tri-n-butyl phosphate (TBP) for the reprocessing of irradiated uranium- and thorium-based fuels. Unlike TBP, DHOA displays preferential extraction of Pu(IV) over U(VI) at higher acidities (≥3 M HNO3) and poor extraction at lower acidities. Density functional theory (DFT) based calculations have been carried out on the structures and relative binding energies of U(VI) and Pu(IV) with the extractant molecules. These calculations suggest that the differential hardness of the two extractants is responsible for the preferential binding/complexation of TBP to uranyl, whereas the softer DHOA and the bulky nature of the extractant lead to stronger binding/complexation of DHOA to Pu(IV). In conjunction with quantum chemical calculations, small angle neutron scattering (SANS) measurements have also been performed for understanding the stoichiometry of the complex formed that leads to relatively lower extraction of Th(IV) (a model for Pu(IV)) as compared to U(VI) using DHOA and TBP as the extractants. The combined experimental and theoretical studies helped us to understand the superior complexation/extraction behavior of Pu(IV) over U(VI) with DHOA.

HTTA forms M(TTA)2+, M(TTA)2+, M(TTA)3° and M(TTA)4− complexes with lanthanides. Complexation is strongly favoured when the salt of TTA− is used.

There are contradicting reports on the thermodynamics of cation-cation interactions (CCIs; inner/outer sphere) involving NpO2(+) and UO2(2+). This paper revisits CCIs of NpO2(+) (2 × 10(-4) M) under varying conditions such as reaction time, nitric acid (2 × 10(-3)-4 M HNO3)/uranium (up to 1.2 M) concentrations, and temperature (283-343 K) by spectrophotometric measurements. This study reports for the first time the appearance of a signature peak of Np(IV) (∼964 nm) in addition to NpO2(+) (980 nm) and the NpO2(+)-UO2(2+) complex (992 nm). For a pure NpO2(+) solution at 4 M HNO3, there is a gradual increase in Np(IV) peak intensity with increasing temperature and correspondingly the Np(V) peak diminishes. The CCIs are more favored at higher uranium concentrations. However, the intensity of the 992 nm peak decreases steadily with increasing temperature suggesting the exothermic nature of the complexation process. The thermodynamic data and reported structural studies indicate the formation of an inner-sphere complex under the conditions of this study. In addition, the spectral changes also suggest the formation of Np(IV) even in the presence of uranium at elevated temperatures. Solvent extraction studies using 1.1 M TBP and 1.1 M DHOA solutions in n-dodecane show that NpO2(+)-UO2(2+) complexes are extractable leaving NpO2(+) in the aqueous phase.

A novel tripodal diglycolamide ligand containing a triazamacrocycle center (2,2′,2′′-(((1,4,7-triazonane-1,4,7-triyl)­tris­(2-oxoethane-2,1-diyl)) tris­(oxy)) tris­(N,N-dioctylacetamide), abbreviated as T9C3ODGA) was synthesized and characterized by conventional techniques. The ligand resulted in efficient extraction of actinide/lanthanide ions yielding the trend: Eu3+ > Pu4+ > Am3+ > NpO2 2+ > UO2 2+ > Sr2+ > Cs+. Similar to most of the other diglycolamide (DGA) ligands, Eu3+ was preferentially extracted as compared to Am3+; the separation factor (D Eu/D Am) value at 3 M HNO3 was ca. 4.2. In contrast, separation from UO2 2+ ion was less effective as compared to that of other tripodal DGA ligands studied earlier. Solvent extraction studies indicated extraction of species of the ML2 (where L is T9C3ODGA) stoichiometry. The formation of an inclusion complex with no inner-sphere water molecule was confirmed from luminescence spectral studies. DFT computations predicted the presence of an inner-sphere nitrate ion in the most preferred complex, which was also supplemented by EXAFS and luminescence studies. The selectivity of T9C3ODGA could be explained on the basis of its more favorable interactions with Eu3+ as compared to those with Am3+ both in the gas and the solution phases.