Extraction of Lanthanide and Actinide Nitrate and Thiocyanate Salts by 2:6-Bis[(bis(2-n-Octyl)phosphino)methyl]pyridine N:P:P′-trioxide in Toluene

Abstract

Organophosphorus reagents for solvent extraction have played a particularly important role in actinide science and technology. A wide variety of compounds of varying basicity, denticity, and geometric arrangement of donor groups have been reported. Of relevance to this report, some features of the extraction of actinides by 2,6-bis[(bis(2-ethylhexyl)phosphino)methyl]pyridine N,P,P′-trioxide, TEH(NOPOPO), have been previously reported. The presence of a chiral center at the ethylhexyl groups makes purification difficult, though its solubility is improved by the presence of four chiral centers. In contrast, the structurally-related extractant 2,6-bis[(bis(2-n-octyl)phosphino)methyl] pyridine N,P,P′-trioxide, TO(NOPOPO), can be purified by crystallization (at the price of lower solubility limits relative to TEH(NOPOPO). This work reports new results describing the extraction of Am(III), Th(IV), Pu(IV), Np(V), and U(VI) from acidic nitrate and thiocyanate media by the trifunctional organophosphorus ligand TO(NOPOPO)) in toluene. Trans-lanthanide trends for extraction are also explored. TO(NOPOPO) behaves as a classic solvating extractant reagent, wherein extraction preference increases as the effective charge of actinide metal cation increases. The corresponding lanthanide extraction patterns are either flat (nitrate) or tending to discriminate between light and heavy lanthanides (thiocyanate). The extractant stoichiometry of the phase transfer reaction of the actinides into TO(NOPOPO) in toluene varies between 1 and 2 for the suite of metal nitrates and thiocyanates investigated. The back extraction of trivalent ions and Np(V) can be accomplished with a shift to less acidic media, but stripping of hexavalent and tetravalent ions requires a complexant. Results indicate that TO(NOPOPO) in toluene is a strong extractant for actinide nitrates in all oxidation states and potentially a suitable platform to explore for trivalent actinide-lanthanide separations from thiocyanate media.