Solvent Extraction Behavior of Neptunium (IV) Ions between Nitric Acid and Diluted 30% Tri‐butyl Phosphate in the Presence of Simple Hydroxamic Acids

Abstract

Formo‐ and aceto‐hydroxamic acids are very effective reagents for stripping tetravalent actinide ions such as Np(IV) and Pu(IV) ions from a tri‐butyl phosphate phase into nitric acid. Distribution data for Np(IV) in the presence of these hydroxamate ions have now been accumulated and trends established. Stability constants for aceto‐hydroxamate complexes of Np(IV) and Np(V) ions have also been determined in a perchlorate medium, and these reaffirm the affinity of hydroxamate ligands for actinide (IV) ions over actinyl (V,VI) ions.

Keywords:

• Formohydroxamic acid
• acetohydroxamic acid
• Advanced Purex
• distribution coefficient
• stability constant
• actinide separations

Acknowledgements

The authors wish to thank G. Brown (Nexia Solutions) and Analytical Services, Sellafield Ltd., for assistance in the course of this work. This work was funded by BNFL and the Nuclear Decommissioning Authority.

Notes

¹Note that the wavelengths of the proposed 1∶1 Np(V)‐AHA peak in Fig. 6 and the 1∶3 Np(IV)‐AHA peak in Fig. 5 are coincident (∼988 nm). In Fig. 5, an assignment of the 988 nm absorption to Np(IV) rather than Np(V) is supported by: (1) the Full Width at Half Maximum (FWHM) is 22 nm, which is two times higher than a FWHM of 11 nm in the 1∶1 Np(V)‐AHA complex in Fig. 6; (2) the molar absorptivity of the 1∶1 complex of Np(V) is 380 M⁻¹cm⁻¹, so if we assume that the 988 nm peak in the Np(IV) titration experiment belongs to the 1∶1 Np(V)‐AHA complex, it gives an expected optical absorbance for this species of 0.00219 (M)×380 (M⁻¹cm⁻¹)×1 (cm) ×1 (yield)=0.83. This disagrees with an experimentally measured value of ∼0.092. Clearly other solution phase structural studies would be more conclusive but these were outside the current scope.