Water-soluble Schiff base-actinyl complexes and their effect on the solvent extraction of f-elements

Conventional solvent extraction of selected f-element cations by bis(2-ethylhexyl)phosphoric acid (HDEHP) yields increased extraction from aqueous to organic solution along the series Np(v) < Cm(iii) < Eu(iii) < U(vi), with distribution ratios all within two orders of magnitude. However, in the presence of the water-soluble tetradentate Schiff base (N,N′-bis(5-sulfonatosalicylidene)-ethylenediamine or H₂salenSO₃), selective complexation of the two actinyl cations (Np(v) and U(vi)) resulted in an extraction order of Np(v) < U(vi) ≪ Eu(iii) < Cm(iii). The extraction of neither Cm(iii) or Eu(iii) by HDEHP are significantly impacted by the presence of the aqueous phase Schiff base. Despite observed hydrolytic decomposition of H₂salenSO₃ in aqueous solutions, the calculated high conditional stability constant (β₁₁ = 26) for the complex [UO₂(salenSO₃)]^2- demonstrates its capacity for aqueous hold-back of U(vi). UV-visible-NIR spectroscopy of solutions prepared with a Np(vi) stock and H₂salenSO₃ suggest that reduction of Np(vi) to Np(v) by the ligand was rapid, resulting in a pentavalent Np complex that was substantially retained in the aqueous phase. Results from ¹H NMR of aqueous solutions of H₂salenSO₃ with U(vi) and La(iii), Eu(iii), and Lu(iii) provides additional evidence that the ligand readily chelates U(vi), but has only weak interactions with trivalent lanthanide ions.