Extraction of Neptunium (IV) Ions into 30% Tri-Butyl Phosphate from Nitric Acid

The distribution of Np(IV) between 0.08-4.5M HNO3(aq,eqm) and 30% tributyl phosphate has been modelled, accounting for the formation of 1:1 and 1:2 nitrate complexes and Np(IV) hydrolysis in the aqueous phase and the extraction of Np(NO3)4(TBP)2 into TBP. The potential formation and extraction of NpOH(NO3)3(TBP)2 and Np(NO3)4(TBP)2.HNO3 species, including spectroscopic evidence, and oxidations of Np(IV) to Np(V) and Np(VI) in the solvent phase have also been considered. The model highlights some key gaps in the available thermodynamic data.