Extraction of tetra- And hexavalent actinide ions from nitric acid solutions using some diglycolamide functionalized calix[4]arenes

Neptunium is one of the most important minor actinide elements with some of its isotopes having very long half-lives, therefore necessitating its separation from acidic radioactive wastes. Solvent extraction of Np⁴⁺ and NpO₂²⁺ was studied using three multiple diglycolamide (DGA) extractants with n-propyl, n-octyl and 3-pentyl substituents termed as L_I, L_II and L_III, respectively, in a mixed diluent of 5% isodecanol and 95% n-dodecane. For comparison purpose, the extraction of Pu⁴⁺ and UO₂²+ was carried out under identical conditions. The extraction efficiency of the ligands for the tetravalent ions followed the trend: L_II > L_I > L_III, which changed to L_III > L_II > L_I for the hexavalent ions. While the extraction of the tetravalent ions was reasonably good (ca. 90-98%) with an extremely low (5.0 × 10⁻⁵ M) ligand concentration, poor extraction (ca. 5-16%) of the hexavalent ions was seen even with a 20 times higher concentration of the ligand. In general, Pu⁴⁺ was better extracted than Np⁴⁺, while NpO₂²⁺ was marginally better extracted then UO₂²⁺. A 'solvation' type extraction mechanism was proposed based on the extraction profiles obtained as a function of the concentrations of the feed nitric acid, extractant as well as nitrate ion. The extracted species were found out to be M(NO₃)₄·mL and MO₂(NO₃)₂·nL (M = Np or Pu, 1 < m < 2, n ≃ 1).