PNNL

Abstract

Organophosphorus extractants have been widely investigated for application in the reprocessing of spent nuclear fuel. For example, the previously developed ALSEP (Actinide Lanthanide Separation) process makes use of a neutral diglycolamide ligand in conjunction with the acidic organophosphorus extractant 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester, HEH[EHP]. However, while the role of the diglycolamide ligand is fairly well understood, the role of the HEH[EHP] ligand is still not uncertain. In this study, the extraction of Th(IV), U(VI), and U(IV) from nitric acid media is examined using HEH[EHP] in heptane. Based on UV-Vis spectroscopy, the U(IV) sample series was observed to oxidize to U(VI) in the organic phase, and is annotated as U(IV->VI). The distribution ratio as a function of nitric acid concentration was quantified using UV-Vis spectroscopy, and then the speciation of HEH[EHP]-metal complexes in the organic phase was investigated using FTIR and low-temperature 31P NMR. In addition to perturbation of the vibrational modes proximal to the phosphonic moiety in HEH[EHP] in the FTIR spectra, the appearance of a nitrate signal was found in the organic phase following extraction from the highest acidity conditions for U(IV->VI) and U(VI). The 31P NMR spectra of the organic phase at a low temperature (-70 °C) exhibited a surprising number (n) of resonances (n = 7 for Th(IV), n = 11 for U(IV?VI) and n = 11 for U(VI), with the distribution between these resonances changing with the initial concentration of nitric acid in the aqueous phase. Although unique spectral assignments of the species observed with FTIR and 31P NMR spectroscopy remains an active area of research, these results indicate that the compositions of the inner and outer sphere surrounding extracted actinides in the organic phase are more diverse than initially thought. Ultimately, an improved understanding of the interactions between HEH[EHP] and actinides as well as the role of the nitric acid media in modifying these interactions will help to predict extraction behavior and simplify the ALSEP flowsheet.