The complexation and solvent extraction properties of a phenanthroline diamide extractant for trivalent actinide and lanthanide ions

Abstract

Characterization of trivalent actinide (An(III)) chemistry with nitrogen and oxygen donor (N,O-donor) extractants is relevant to closing the nuclear fuel cycle. In general the extraction of transplutonium actinides has not been extensively explored, but is essential to fundamental chemical understanding and separation schemes. A popular N,O-donor extractant, phenanthroline diamide (DAPhen), has incurred interest for f-element separations due to numerous positive qualities as an extractant. DAPhen ligands are stable in molar concentrated acids and have a pre-organized binding mode that encourages faster complexation. Additionally, the amide moieties can be functionalized to tune interactions between metals and ligands. In this work, solvent extraction experiments were completed using a derivative of the DAPhen extractant in nitrobenzene that revealed selectivity for multiple An(III) (An = Am, Cm, Bk, Cf) metals in comparison to Eu(III). The extraction of all surveyed An(III) cations was effective. Surprisingly, the degree to which each An(III) cation was extracted was not periodic. Extraction was most effective for Am(III), intermediate for Cf(III) and Bk(III), and lowest for Cm(III). To provide additional insight into the extraction process, we characterized the corrdiantion chemistry between Am(III) and numerous Ln(III) cations (Pr(III), Nd(III), Tb(III), Lu(III)) with TEtDAPhen. Lanthanide speciation and stability constants were measured using UV-Visible spectroscopy for multiple Ln(III) metals (Nd(III), Eu(III), Gd(III)). Those experiments showed increasing stability constants from Nd(III) – Gd(III) and that the metal-to-ligand stoichiometry was one-to-one. Consistent with that interpretation were single crystal X-ray crystallography studies on M(TEtDAPhen)(NO₃)₃ complexes.