Redox transformations and cation–cation interactions of neptunium in organic solutions

Abstract

Neptunium is a minor actinide with diverse redox chemistry and high tendency to form unique structures with cation–cation interactions. As a result, the fate of neptunium during nuclear fuel reprocessing remains underexplored. In this work, we aim to understand neptunium chemistry in organic solutions with a tetradentate N,O-donor ligand that is pre-organized to stabilize 5f-elements in contact with concentrated nitric acid solutions. Crystal structures of neptunium(V) and (VI) complexes with ligands revealed the formation of three types of species: [LNp^VIO₂NO₃][Np^VIO₂(NO₃)₃], [LNp^VIO₂NO₃]₂[Np^VIO₂(NO₃)₄] ion pairs, and [LNp^VO₂NO₃]₂·Np^VIO₂(NO₃)₂. The latter one is a mixed-valence Np^V–Np^VI complex with ligand-coordinated neptunium(V), which stabilizes poorly extractable neptunium(V) in organic solution. Neptunium speciation in organic phase after extraction from neptunium(V) and (VI) aqueous solutions was investigated using a combination of XAS and ¹H-NMR spectroscopies, supported by thermodynamic calculations. The results demonstrate that neptunium(V) is extracted as LNp^VO₂NO₃ and then oxidized to form [LNp^VIO₂NO₃]NO₃. Extraction of neptunium(VI) leads to the formation of ion pairs. The formation of mixed-valence Np^V–Np^VI complexes was not observed in solvent extraction systems, but they were found to be stable in acetonitrile. This study provides valuable insights into the chemical processes behind the extraction of neptunium(V) and (VI) under conditions relevant to spent nuclear fuel reprocessing.