First Report on the Complexation of Actinides and Lanthanides Using 2,2′,2′′-(((1,4,7-Triazonane-1,4,7-triyl)tris(2-oxoethane-2,1-diyl)) tris(oxy)) tris(N, N-dioctylacetamide):Synthesis, Extraction, Luminescence, EXAFS, and DFT Studies

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+ &gt; Pu4+ &gt; Am3+ &gt; NpO2 2+ &gt; UO2 2+ &gt; Sr2+ &gt; Cs+. Similar to most of the other diglycolamide (DGA) ligands, Eu3+ was preferentially extracted as compared to Am3+ the separation factor (DEu/DAm) 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.</p

A diglycolamide-functionalized TREN-based dendrimer with a ‘crab-like' grip for the complexation of actinides and lanthanides

A generation 1 dendrimer, based on tris(2-aminoethyl)amine (TREN), containing six diglycolamide (DGA) pendent arms (termed TREN-G1-DGA) was synthesized and evaluated for the extraction of actinides and fission product ions. Solvent extraction studies indicated preferential extraction of Eu3+ over Am3+ with a separation factor value of ca. 4.5 in line with the extraction behaviour of multiple DGA ligands in previous reports. The distribution values of Am3+ and Eu3+ were about 12 and 9 times higher, respectively, than those obtained in the case of TREN-DGA using the 1 × 10−3 M ligand in 5% iso-decanol/95% n-dodecane at 3 M HNO3. The 1 : 1 (M : L) extracted species suggested ‘inclusion' complex formation where more than one DGA moiety participates in the complex formation. The extracted species were devoid of any inner-sphere coordinated water molecules as confirmed by luminescence spectroscopy. The structure of the complex was also studied by DFT computations and EXAFS which suggested binding of three DGA arms around the central metal ion in the absence of any inner-sphere nitrate ions.</p

Role of ‘O’ Substitution in Expanded Porphyrins on Uranyl Complexation: Orbital and density based analyses

The need for efficient macrocyclic ligands that can sequester U(VI) has gained immense importance due to the increased applications of U(VI) in various sectors, including but not limited to nuclear energy. Structural attributes such as number and type of donor centers ("hard" and "soft") of ligands are essentially the key components for providing the adequate bonding scenario for uranyl. Beside hard or soft-donor-based binding cavity, the mixed-donor ligands are also finding popularity for achieving optimized performances. However, many aspects are still unknown about how and at what extent the ratio of hard-to-soft donor centers tune the bonding attributes with uranyl. Moreover, a consensus is yet to be reached on the nature and role of underlying covalent interaction between U and donors upon complexation, particularly in the mixed-donor ligand environment. In this work, using relativistic density functional theory (DFT), we attempted to address these important issues by systematically investigating the impact on the bonding characteristics of uranyl ion and an expanded porphyrin, viz. sapphyrin with increasing number of \u27O\u27 substitution at the cavity. Our results suggest that in the O-substituted sapphyrin variants, UO$_2^{2+}$ prefers to bind N over O donor sites, and decrease in N donor sites at the cavity prompts UO$_2^{2+}$ to have better interaction with the rest of N donor centers. Extended transition state (ETS) with natural orbital for chemical valences (NOCV) analysis shows that at equatorial plane N acts as better $\sigma$ donor to uranyl than O donor. Molecular orbital (MO) and density of states (DOS) analysis shows favorable bonding-interaction between U(d) and donor\u27s p orbitals, the participation of U(f)-orbitals in bonding are of low-extent but non-negligible. Energy decomposition analysis (EDA), natural population analysis (NPA) along with thermodynamic analyses confirms the dominance of electrostatic interaction on the thermodynamic stability of the complexes. However, the U-N/O bonds at the equatorial plane do carry appreciable amount of covalent character. Analysis of quantum theory of atoms in molecules (QTAIM) descriptors in conjugation with MO analysis and overlap integral calculations confirms its nature as near-degeneracy driven type. Statistics of mixed-orbitals and overlap integral further suggest that the O donor does not act as adequate replacement of N for uranyl binding despite having more number of mixed MOs due to the variation in the amplitude of overlap

Electrodriven Selective Transport of Cs⁺ Using Chlorinated Cobalt Dicarbollide in Polymer Inclusion Membrane: A Novel Approach for Cesium Removal from Simulated Nuclear Waste Solution

The work describes a novel and cleaner approach of electrodriven selective transport of Cs from simulated nuclear waste solutions through cellulose tri acetate (CTA)/poly vinyl chloride (PVC) based polymer inclusion membrane. The electrodriven cation transport together with the use of highly Cs⁺ selective hexachlorinated derivative of cobalt bis dicarbollide, allows to achieve selective separation of Cs⁺ from high concentration of Na⁺ and other fission products in nuclear waste solutions. The transport selectivity has been studied using radiotracer technique as well as atomic emission spectroscopic technique. Transport studies using CTA based membrane have been carried out from neutral solution as well as 0.4 M HNO₃, while that with PVC based membrane has been carried out from 3 M HNO₃. High decontamination factor for Cs⁺ over Na⁺ has been obtained in all the cases. Experiment with simulated high level waste solution shows selective transport of Cs⁺ from most of other fission products also. Significantly fast Cs⁺ transport rate along with high selectivity is an interesting feature observed in this membrane. The current efficiency for Cs⁺ transport has been found to be ∼100%. The promising results show the possibility of using this kind of electrodriven membrane transport methods for nuclear waste treatment

Electrodriven Transport of Cs⁺ through Polymer Inclusion Membrane as “Solvent Separated Ions”

In our earlier work, we reported [<i>Env. Sci. Technol.</i> <b>2014</b>, <i>48</i>, 12994] a novel electrodialysis based selective separation of Cs⁺ from nuclear waste solution using chlorinated cobalt dicarbollide (HCCD) loaded polymer inclusion membrane (PIM). In continuation with that work, the mechanism of electrodriven transport of Cs⁺ through HCCD loaded polymer inclusion membranes has been explored. PIMs containing fixed amount of cellulose triacetate and nitrophenyl octyl ether (NPOE) but different concentrations of the carrier have been prepared. The experimental flux of Cs⁺ across the PIMs, for two different concentrations of the metal ion in the initial feed solution, has been measured using the radiotracer technique. On the basis of the Nernst–Planck equation, an attempt has been made to calculate the time dependence of concentration changes of the metal ion in the feed compartment. The experimental parameters of the membrane., viz., length, self-diffusion coefficient, distribution ratio, electrical resistance, and current, have been used in the calculation. The experimental results indicate that the transport of Cs⁺ by mobile carrier diffusion or fixed site jumping is not possible. It has been proposed that, under applied electric field, Cs⁺ is mostly transported as “solvent separated ions” through the polar lipophilic solvent NPOE. The proposed mechanism has been substantiated by comparing the experimental and the calculated results

Insight into the Complexation of Actinides and Lanthanides with Diglycolamide Derivatives: Experimental and Density Functional Theoretical Studies

Extraction of actinide (Pu4+, UO2 2+, Am3+) and lanthanide (Eu3+) ions was carried out using different diglycolamide (DGA) ligands with systematic increase in the alkyl chain length from n-pentyl to n-dodecyl. The results show a monotonous reduction in the metal ion extraction efficiency with increasing alkyl chain length and this reduction becomes even more prominent in case of the branched alkyl (2-ethylhexyl) substituted DGA (T2EHDGA) for all the metal ions studied. Steric hindrance provided by the alkyl groups has a strong influence in controlling the extraction behavior of the DGAs. The distribution ratio reduction factor, defined as the ratio of the distribution ratio values of different DGAs to that of T2EHDGA, in n-dodecane follows the order UO2 2+ > Pu4+ > Eu3+ > Am3+. Complexation of Nd3+ was carried out with the DGAs in methanol by carrying out UV-vis spectrophotometric titrations. The results indicate a significant enhancement in the complexation constants upon going from methyl to n-pentyl substituted DGAs. They decreased significantly for DGAs containing alkyl substituents beyond the n-pentyl group, which corresponds to the observed trend from the solvent extraction studies. DFT-based calculations were performed on the free and the Nd3+ complexes of the DGAs both in the gas and the solvent (methanol) phase and the results were compared the experimental observations. Luminescence spectroscopic investigations were carried out to understand the complexation of Eu3+ with the DGA ligands and to correlate the nature of the alkyl substituents on the photophysical properties of the Eu(III)-DGA complexes. The monoexponential nature of the decay profiles of the complex revealed the predominant presence of single species, while no water molecules were present in the inner coordination sphere of the Eu3+ ion

Unusual Reversal in Pu and U Extraction in an Ionic Liquid Using Two Tripodal Diglycolamide Ligands: Experimental and DFT Studies

The extraction of UO2 2+ and Pu4+ ions was studied from nitric acid medium using three diglycolamide (DGA) extractants, viz. TODGA (N,N,N′,N′-tetra-n-octyldiglycolamide), T-DGA (tripodal diglycolamide), and TREN-DGA (N-pivot tripodal diglycolamide) in a molecular diluent mixture (9:1 mixture of n-dodecane and iso-decanol) and an ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C4mim][Tf2N]). Using 1.0 × 10–3 M solutions of the ligands in [C4mim][Tf2N], T-DGA yielded an exceptionally high extraction of Pu4+ ion (DPu > 3.2 × 103) from 3 M HNO3 as compared to DPu values of 4 and 17 obtained with TREN-DGA and TODGA, respectively. Similarly, while the extraction of UO2 2+ ion was significantly lower than that of Am3+ ion for all the three DGA ligands in the molecular diluent and with TODGA and TREN-DGA in [C4mim][Tf2N], a reversal in the extraction trend was found in the case of T-DGA. Density Functional Theory (DFT) computational studies were carried out to understand the structures of the extracted complexes. PuL2(NO3)n (4–n)+ species with n = 2, 3, or 4 were considered for the geometry optimization. DFT data indicated longer M–O bonds with the etheric ‘O’ atom as compared to the carbonyl ‘O’ atom. The metal-ligand bond length and bond order analysis indicated the extraction of neutral complexes of the type PuL2(NO3)4 as compared to cationic species of the type PuL2(NO3)2 2+ and PuL2(NO3)3 + for all the three types of ligands (TODGA, TREN-DGA, and T-DGA)

Understanding the complexation of Eu3+ with three diglycolamide-functionalized calix[4]arenes: Spectroscopic and DFT studies

Complexation of Eu3+ with three diglycolamide-functionalized calix[4]arene (C4DGA) ligands was investigated by UV-Vis and luminescence spectroscopy measurements in acetonitrile medium. The complexation thermodynamics was studied by micro-calorimetry while structural information was obtained from DFT calculations