Extraction of Np(IV/VI) from nitric acid medium using three different tripodal diglycolamide ligands in ionic liquid: Batch extraction, spectroscopic and electrochemical investigations

Complexation of Np(IV) and Np(VI) ions was studied in a room temperature ionic liquid (C₄mim.NTf₂) using three tripodal ligands with an ‘N’ atom, a ‘C’ atom and a benzene ring at the center, which are termed as L_I, L_II and L_III, respectively. The studies include solvent extraction, visible-near infrared (vis-NIR) spectroscopy, and cyclic voltammetry. The trend of the extraction of Np⁴⁺ was L_III > L_II > L_I, which resembled that reported before in a mixture of molecular diluents but with about 10 times higher ligand concentration. Np(VI) was extracted to a much lower extent than Np(IV). Slope analysis of Np(IV) extraction suggested formation of Np(NO₃)₄.L species for L_II and L_III, while a cationic species of the type [Np(NO₃)₂.L]²⁺ was seen for L_I. On the other hand, for Np(VI), formation of NpO₂(NO₃)₂.L species was proposed for L_I and L_III and [NpO₂(NO₃).L]⁺ for L_II. All the extracted species conform to a ‘solvation’ type extraction except for Np(IV) with L_I and Np(VI) with L_II which point to a ‘cation-exchange’ mechanism. The complexation of the metal ions with the tripodal ligands was supported by peak shifts in the vis-NIR as well as the cyclic voltammetric studies.