Lanthanide Speciation in Potential SANEX and GANEX Actinide/Lanthanide Separations Using Tetra-N-Donor Extractants

Lanthanide­(III)
complexes with N-donor extractants, which exhibit the potential for
the separation of minor actinides from lanthanides in the management
of spent nuclear fuel, have been directly synthesized and characterized
in both solution and solid states. Crystal structures of the Pr3+, Eu3+, Tb3+, and Yb3+ complexes
of 2,9-bis­(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-1,10-phenanthroline
(CyMe4-BTPhen) and the Pr3+, Eu3+, and Tb3+ complexes of 6,6′-bis­(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-2,2′-bypyridine
(CyMe4-BTBP) were obtained. The majority of these structures
displayed coordination of two of the tetra-N-donor ligands to each
Ln3+ ion, even when in some cases the complexations were
performed with equimolar amounts of lanthanide and N-donor ligand.
The structures showed that generally the lighter lanthanides had their
coordination spheres completed by a bidentate nitrate ion, giving
a 2+ charged complex cation, whereas the structures of the heavier
lanthanides displayed tricationic complex species with a single water
molecule completing their coordination environments. Electronic absorption
spectroscopic titrations showed formation of the 1:2 Ln3+/LN4‑donor species (Ln = Pr3+, Eu3+, Tb3+) in methanol when the N-donor
ligand was in excess. When the Ln3+ ion was in excess,
evidence for formation of a 1:1 Ln3+/LN4‑donor complex species was observed. Luminescent lifetime
studies of mixtures of Eu3+ with excess CyMe4-BTBP and CyMe4-BTPhen in methanol indicated that the
nitrate-coordinated species is dominant in solution. X-ray absorption
spectra of Eu3+ and Tb3+ species, formed by
extraction from an acidic aqueous phase into an organic solution consisting
of excess N-donor extractant in pure cyclohexanone or 30% tri-n-butyl phosphate (TBP) in cyclohexanone, were obtained.
The presence of TBP in the organic phase did not alter lanthanide
speciation. Extended X-ray absorption fine structure data from these
spectra were fitted using chemical models established by crystallography
and solution spectroscopy and showed the dominant lanthanide species
in the bulk organic phase was a 1:2 Ln3+/LN‑donor species.