Evaluation of two aza-crown ether-based multiple diglycolamide-containing ligands for complexation with the tetravalent actinide ions Np4+ and Pu4+: extraction and DFT studies

Two multiple diglycolamide (DGA)-containing extractants where the DGA arms are tethered to the nitrogen atoms of two aza-crown ether scaffolds, a 9-membered aza-crown ether containing three 'N' atoms (L-I) and a 12-membered aza-crown ether containing four 'N' atoms (L-II), were evaluated for the extraction of the tetravalent actinide ions Np4+ and Pu4+. The tripodal ligand with three DGA arms (L-I) was relatively inferior in its metal ion extraction properties as compared to the tetrapodal ligand with four DGA arms (L-II) and Pu4+ ion was better extracted than Np4+ ion with both the ligands. A solvation extraction mechanism, where species of the type ML(NO3)(4) are extracted, was found to be operative for both the ligands involving both the tetravalent actinide ions. While the extraction of the metal ions increased with the feed nitric acid concentration up to 4 M, a sharp decline in the extraction was seen after that. Quantitative extraction (>99%) of the actinide ions was observed with L-II from 4 M HNO3, suggesting the possible application of the ligands for actinide partitioning of high-level waste. The structure and the composition of the complexes were optimized by DFT computations.