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.

Extraction of Am 3+ from acidic feed solutions was investigated using novel solvent systems containing a tripodal diglycolamide (T-DGA) in three room temperature ionic liquids (RTIL), viz. [C 4 mim][NTf 2 ], [C 6 mim][NTf 2 ] and [C 8 mim][NTf 2 ]. Compared to the results obtained with N,N,N9,N9-tetra-n-octyl diglycolamide (TODGA), T-DGA gave significantly higher distribution coefficients in these RTILs. The D Am values decreased with increasing carbon chain length in the RTILs, which was related to the solubility of the RTIL in the aqueous phase. The distribution studies included the effect of equilibration time, aqueous phase acid concentration variation and T-DGA concentration variation. In general, significantly higher equilibration times were observed for the extraction systems, which was partly due to the viscous RTIL phase and partly due to the slow conformational changes of the T-DGA ligand during complexation. Apart from Am 3+ , extraction of Pu 4+ , UO 2 2+ , Eu 3+ , Sr 2+ and Cs + was also investigated, since they have significant implications in radioactive waste processing. Stripping studies indicated .99% stripping in three stages using 0.5 M EDTA or DTPA in 1 M guanidine carbonate. Slope analysis indicated the extraction of 1 : 1 complexed species of Am(III) with T-DGA. Time resolved laser fluorescence spectroscopy (TRLFS) studies showed a strong complexation with no inner-sphere water molecules in the Eu(III)-T-DGA complexes for [C 4 mim][NTf 2 ] as the diluent. Radiolytic degradation studies of the solvent systems containing T-DGA in the three RTILs were also carried out and while the D Am values decreased marginally when the solvents were exposed to 500 kGy absorbed dose, a relatively sharp decrease (60%) was seen when the solvents were exposed further to 1000 kGy absorbed dose, suggesting the possibility of recycling. Extraction studies were also carried out at varying temperatures and the thermodynamic parameters were calculated.

Three room temperature ionic liquids (RTILs), viz. C(4)mim(+)·PF(6)(-), C(6)mim(+)·PF(6)(-) and C(8)mim(+)·PF(6)(-), were evaluated as diluents for the extraction of Am(III) by N,N,N',N'-tetraoctyl diglycolamide (TODGA). At 3 M HNO(3), the D(Am)-values by 0.01 M TODGA were found to be 102, 34 and 74 for C(4)mim(+)·PF(6)(-), C(6)mim(+)·PF(6)(-) and C(8)mim(+)·PF(6)(-), respectively. The extraction of Am(III) decreased with increasing feed acidity for all three diluents, indicating an ion exchange mechanism for the extraction. The stoichiometry of the extracted species suggested that two TODGA molecules were associated with Am(III) during the extraction for all three RTILs and the conditional extraction constants have been determined. The D(M)-values for different metal ions followed the order: 75 (Am(III)) > 30.7 (Pu(IV)) > 3.9 (Np(IV)) > 1.19 (Pu(VI)) > 0.52 (U(VI)) > 0.12 (Cs(I)) > 0.024 (Sr(II)). The distribution behaviour of Am(III) was also studied with a recently synthesized calix[4]arene-4DGA (C4DGA) extractant dissolved in C(8)mim(+)·PF(6)(-). Using this extractant diluent combination, the D(Am)-value was 194 at 3 M HNO(3) using 5 × 10(-5) M C4DGA, suggesting a very high distribution coefficient at very low extractant concentrations. The stoichiometry of the extracted species containing Am was found to be 1:2 (M:L) in C(8)mim(+)·PF(6)(-). The thermodynamics of the extraction was also studied for both extractants in C(8)mim(+)·PF(6)(-). The use of RTILs gives rise to significantly improved extraction properties than the commonly used n-dodecane and an unusual increase in separation factor values was seen for the first time which can lead to selective separation of Am from wastes containing a mixture of U, Pu and Am.

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Diglycolamide-functionalized calix[4]arenes (C4DGAs) with varying structural modifications were evaluated for actinide complexation from their extraction behavior toward actinide ions such as UO2(2+), Pu(4+), PuO2(2+), and Am(3+) in the room temperature ionic liquid (RTIL) 1-n-octyl-3-methylimidazolium bis(trifluoromethane)sulfonamide (C8mimNTf2). The formation constants were calculated for Am(3+) which showed a significant role of ligand structure, nature of substituents, and spacer length. Although the alkyl substituents on the amidic nitrogen increase the extraction efficiency of americium at lower acidity because of the inductive effect of the alkyl groups, at higher acidity the steric crowding around the ligating site determines the extraction efficiency. All C4DGAs formed 1:1 complexes with Am(3+) while for the analogous Eu(3+) complexes no inner sphere water molecules were detected and the asymmetry of the metal ligand complex differed from one another as proved by time-resolved laser induced fluorescence spectroscopy (TRLIFS). Thermodynamic studies indicated that the extraction process, predominant by the Am(3+)-C4DGA complexation reaction, is exothermic. The unique role of the medium on Am(3+) complexation with the C4DGA molecules with varying spacer length, L-IV and L-V, was noticed for the first time with a reversal in the trend observed in the RTIL compared to that seen in a nonpolar molecular diluent like n-dodecane. Various factors leading to a more preorganized structure were responsible for favorable metal ion complexation. The solvent systems show promise to be employed for nuclear waste remediation, and sustainability options were evaluated from radiolytic stability as well as stripping studies.

Two new diglycolamide-based task-specific ionic liquids (DGA-TSILs) were evaluated for the extraction of actinides and lanthanides from acidic feed solutions. These DGA-TSILs were capable of exceptionally high extraction of trivalent actinide ions, such as Am(3+), and even higher extraction of the lanthanide ion, Eu(3+) (about 5-10 fold). Dilution of the DGA-TSILs in an ionic liquid, C(4)mim(+)·NTf(2)(-), afforded reasonably high extraction ability, faster mass transfer, and more efficient stripping of the metal ion. The nature of the extracted species was studied by slope analysis, which showed that the extracted species contained one NO(3)(-) anion, along with the participation of two DGA-TSIL molecules. Time-resolved laser fluorescence spectroscopy (TRLFS) analysis showed a strong complexation with no inner-sphere water molecule in the Eu(III)-DGA-TSIL complexes in the presence and absence of C(4)mim(+)·NTf(2)(-) as the diluent. The very high radiolytic stability of DGA-TSIL 6 makes it one of the most-efficient solvent systems for the extraction of actinides under acidic feed conditions.

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Three benzene-centered tripodal diglycolamides (Bz-T-DGAs) were synthesized and evaluated for actinide, lanthanide, and fission product ion extraction. 1,3,5-Triethylbenzene-based tripodal DGA (LI) showed high distribution ratio (D) values for Am and Eu in a mixture of 95% n-dodecane and 5% iso-decanol at 3 M HNO. Eu/Am separation factors, in the range of 8-10, were obtained at 1 M HNO which decreased at higher acidities with the exception of LII which did not show much change. Benzene-1,3,5-triamide-based tripodal DGA (LII) exhibited a high D-value for Pu compared to the other ligands. Slope analysis showed the formation of 1 : 1 or 1 : 2 complexes is dependent on the ligand. The nature of the complexes was further studied with luminescence spectroscopy (Eu complexes) and DFT calculations (Am complexes).