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)