Diglycolamic acid for the mutual separation of lanthanides and actinides from dilute nitric acid solution: solvent extraction, dynamic light scattering, and spectroscopic investigations

Abstract

N,N-Dialkyl diglycolamic acid (HDRDGA) derivatives with alkyl groups varying from n-hexyl to n-dodecyl were synthesized, and the promising ligands such as N,N-dioctyl diglycolamic acid (HDODGA) and N,N-didecyl diglycolamic acid (HD²DGA) were investigated for the mutual separation of trivalent lanthanides and actinides from dilute nitric acid solution. Since diglycolamic acid is strongly polar, it was dissolved in a traditional diluent, n-dodecane, with the help of an isodecyl alcohol (IDA) phase modifier. Nevertheless, the hexyl and didodecyl derivatives were sparingly soluble even in the presence of 30% (v/v) IDA in n-dodecane, and therefore, the extraction behavior of the lanthanide and actinide representatives, Eu(III) and Am(III), respectively, was studied only in a solution of HDODGA or HD²DGA present in 30% (v/v) IDA/n-dodecane. The cation-exchange mechanism of extraction and the stoichiometry of trivalent metal to HDRDGA were elucidated from the slope analysis of the extraction data. Since the stoichiometry of the metal to ligand complex in the organic phase was unusually varied from 1 : 3 to 1 : 6 depending upon the Nd(III) loading into the organic phase, the reverse micellar aggregation behavior and the coordination chemistry of Nd(III) in the organic phase were probed by dynamic light scattering and visible spectroscopy, respectively. The results revealed that three molecules of anionic DRDGA were coordinated to Nd(III) in the inner-sphere at lower metal concentrations, and also three more molecules of HDRDGA in the outer-sphere at higher metal concentrations. The good separation factor achieved with the use of these ligands showed their potential ability to separate lanthanides from actinides in nitric acid medium.