Coupling Raman spectroscopy and DFT study for enhanced description of nitrosyl nitrato nitrite ruthenium(III) complexes in nitric acid

Abstract

Dissolution of spent nuclear fuel in nitric acid forms diverse octahedral complexes of the ruthenium fission product with one nitrosyl and a variable number of nitrate, nitrite, hydroxide and water ligands. Some of these ruthenium complexes can trouble uranium and plutonium extractions by TBP and their purifications. In this study, we analyze the effect of gamma radiation on the structure of ruthenium(III) nitrosyl complexes in 5 M nitric acid solution and highlight the necessity to take into account the nitrato–nitrite complexes [RuNO(NO3)x(NO2)y(H2O)5−x−y]3−x−y. Accordingly, we performed a parametric study to analysis the influence of varying nitrous acid concentrations on ruthenium nitrosyl complexes in 1 M and 5 M nitric acid solutions. Raman spectra show that nitrites from nitrous acid replace nitrate ligands in ruthenium complexes. Resulting nitrite complexes were characterized by comparing the experimental Raman spectra to DFT calculations. This comparison supports a successive formation of mono-, cis- and potentially mer-ruthenium nitrite complexes with increasing nitrous acid concentrations. The degradation of such nitrite complexes has been followed by Raman measurements and the reverting to nitrate complexes occurs within several weeks. The radiolytic formation and the slow degradation shows a relevance of nitrite complexes to understand and hence control ruthenium in solvent extraction processes.