Extraction of Np⁴⁺ and Pu⁴⁺ from nitric acid feeds using three types of tripodal diglycolamide ligands

Studies on the liquid – liquid extraction of Np⁴⁺ and Pu⁴⁺ were carried out using three tripodal diglycolamide ligands with different centers viz. a carbon atom (termed as C-pivot tripodal DGA or T-DGA), a nitrogen atom (termed as N-pivot tripodal DGA or TREN-DGA) and a benzene centered tripodal diglycolamide (termed as TAETEB) in an aliphatic diluent mixture of n-dodecane (95%) and isodecanol (5%). The extraction efficiency of the ligands followed the trend: TAETEB > T-DGA > TREN-DGA, while Pu⁴⁺ was far more efficiently extracted than Np⁴⁺. A solvation type extraction mechanism was proposed based on the extraction profiles obtained as a function of the feed nitric acid concentration where the extracted species was found out to be M(NO₃)₄·nL (where M = Np or Pu and n was in between 1 and 2). The extraction of other relevant metal ions was also investigated which followed the trend: Am³⁺ > UO₂²⁺ ~ NpO₂²⁺ > Sr²⁺ > Cs⁺. While Am³⁺ extraction was lower with TREN-DGA as compared to that of Np⁴⁺ and Pu⁴⁺, the trend being Am³⁺ < Np⁴⁺ < Pu⁴⁺, the relative extraction trend changed with the extractants to Np⁴⁺ < Am³⁺ < Pu⁴⁺ for T-DGA and to Np⁴⁺ < Pu⁴⁺ < Am³⁺ for TAETEB. The radiolytic stability of the extractants showed contrasting behavior with respect to the metal ion extraction. While the solvent containing 1 × 10⁻³ M solutions of the ligands showed decreased metal ion extraction for T-DGA and TAETEB when exposed to 200 kGy gamma radiation, an opposite trend was seen for TREN-DGA where the metal ion extraction was found to increase by over 2 times. The metal ions were efficiently stripped using a mixture of 0.5 M oxalic acid and 0.5 M nitric acid. Temperature variation studies were carried out to determine the thermodynamic parameters.