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

Abstract

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.