A Study on Identification of Uranyl Complexes in Aqueous Solutions Containing Carbonate Ion and Hydrogen Peroxide

Complex formations in aqueous solutions (pH = 9.7∼11.8) containing uranyl ions (5 or 20 mM, M = mol dm-3) carbonate ions (0.04, 0.12, or 1.05 M), and hydrogen peroxide have been studied using UV-vis and NMR spectroscopic methods. In 17O NMR spectra, a peak due to uranyl “yl” oxygen of UO2(CO3)34− was observed at 1099 ppm, and with an increase in [H2O2], the intensity of this peak decreases and two peaks were newly appeared at 1094 and 1103 ppm, respectively. In 13C NMR spectra, a carbonate peak of UO2(CO3)34− and two peaks due to other carbonate complexes were observed at 170.4, 169.8, and 169.9 ppm, respectively. In addition, it was found that the peak intensities of 17O signals observed at 1094 and 1103 ppm change in proportion to those of 13C signals observed at 169.9 and 169.8 ppm. From the NMR peak areas, two new uranyl complexes were proposed as species with the molar ratio of UO22+: CO2- =1: 2. Furthermore, the UV-vis absorption spectral changes with the addition of H2O2 to the aqueous solutions containing uranyl ions and carbonate ions were analyzed by the least-square fitting on the basis of the formation of UO2(O2)(CO3)24− and (UO2)2(O2)(CO3)46- complexes except for UO2(CO3)34-. As a result, the formation constants (logK) for UO2(O2)(CO3)24− and (UO2)2(O2)(CO3)46− were estimated as 7.9 ± 0.6 and 31.9 ± 0.6, respectively. Using the obtained formation constants, the molar absorptivity of UO2(O2)(CO3)2 and (UO2)2(O2)(CO3)46- were evaluated. It was found that the UO2(O2)(CO3)24- complex has an absorption peak at 350.6 nm and a shoulder around 420.0 nm, and their ɛ values are 1260 ± 21 and 893 ± 18 M-1 cm-1, and that the absorption spectrum of (UO2)2(O2)(CO3)46- have a shoulder around 346.0 nm (ɛ = 1720 ± 24).