EXAFS and UV-VIS investigation of the first coordination sphere of the uranyl ion in UO2(NO3)2(TBP)2

The coordination behaviour of the uranyl ion UO22+ has regained interest due to the problem of nuclear waste management and environmental issues. At the end of the nuclear fuel cycle, reprocessing of the spent nuclear fuel mainly occurs by means of liquid-liquid extraction. During the PUREX process, the spent fuel is dissolved in a nitric acid solution. Uranium is extracted from the aqueous nitric acid fuel solution with an organic phase containing tri-n-butylphosphate (TBP; extracting agent). However, little is known about the structure of the chemical species involved in the liquid-liquid extraction process. The first coordination sphere of the uranyl ion UO22+ in UO2(NO3)2(TBP)2 was studied using UV-Vis absorption spectroscopy, EXAFS spectroscopy and 31P-NMR. The characteristic vibrational fine structure in the UV-Vis absorption spectrum points at a high symmetrical complex. The uranium polyhedron is composed of two axial oxygens located at 1.77 ± 0.01 Å, four nitrate (bidentate) oxygens at 2.52 ± 0.01 Å and two phosphate oxygens (TBP) located at 2.38 ± 0.01 Å.