In carbonate-rich aqueous environments relevant to spent nuclear fuel storage, radiolytic H2O2 and carbonate species promote the formation of uranyl peroxo-carbonate complexes under mild conditions. These complexes are poorly characterized spectroscopically, with many assignments remaining controversial. This work presents the first solution-phase identification of the peroxo-dicarbonato species MxUO2(O2)(CO3)2(4-x)- (M = Na+ or K+) achieved through Raman and 18O-labeled H2O2 Fourier-transform infrared (FTIR) spectroscopy, combined with density functional theory (DFT) calculations. Key vibrational signatures distinguish the peroxo-dicarbonato complex from those of the tricarbonato complex UO2(CO3)34-, including the νs(U-Oyl) stretch at 765-769 cm-1, the ν1(U-Operoxo) mode at 418-440 cm-1, and the ν(O-O) stretch at 845-850 cm-1, which shifts upon 18O substitution. FTIR data further confirm these assignments, providing the first infrared evidence of a uranyl peroxo-carbonato complex in aqueous solution. DFT calculations validate the observed frequencies and isotope shifts, clarifying the role of solvation and counter-cation effects. These results clarify uranyl peroxo-carbonato speciation, resolving long-standing ambiguities and establishing a robust spectroscopic framework for applications in uranium fuel dissolution, environmental mobility, and long-term waste stability.
Margate et al. (Wed,) studied this question.