Complexation of Uranium(VI) with Nitrate in Nonaqueous and Aqueous solutions: Speciation and Structural Studies

Understanding the complexation behavior of uranium(VI) with nitrate ions in solution is important for the assessment of environmental impact on the geological disposal of radioactive wastes or for the development of nuclear fuel reprocessing since the present reprocessing plants employ PUREX process, that separates uranium and plutonium from other actinides and fission products in nitric acid solution and, as a result, produces radioactive liquid wastes in nitrate solution. In this study, the chemical speciation and the structure of uranium(VI) nitrate complexes have been investigated in nonaqueous (acetonitrile) and aqueous (nitric acid) solutions by UV-visible absorption spectroscopy, U LIII-edge x-ray absorption spectroscopy, and quantum chemical calculations.

In acetonitrile, the complexation between uranium(VI) and nitrate ions is very strong and the trinitrato complex, [UO2(NO3)3]-, becomes the dominant species when [NO3]/[U] is above 3. The extended x-ray absorption fine structure (EXAFS) spectrum of this trinitrato species indicates that the nitrate ions coordinate to the uranyl ion in a bidentate mode and the average U-Oax, U-Oeq, and U-N distances are 1.77, 2.50, and 2.95 Å, respectively. These values are in good agreement with the results of DFT calculations in acetonitrile (i.e. 1.76, 2.50, and 2.96 Å for U-Oax, U-Oeq, and U-N distances, respectively). By varying the [NO3]/[U] ratio from 0 to 3, we observe a complex speciation diagram composed of four species (UO22+, [UO2NO3(H2O)n]+, [UO2(NO3)2(H2O)m]0, and [UO2(NO3)3]-). However, it is impossible to determine the complex structures of these species directly by EXAFS. In order to estimate their structure, we have isolated their individual EXAFS spectra from the spectra of the mixtures by performing factor analysis [1] on the basis of the speciation diagram obtained from the UV-visible spectral titration. The data analysis of the isolated EXAFS spectra demonstrates that the dinitrato complex has a coordination number of 6 in the equatorial plane of the uranyl ion and forms the complex [UO2(NO3)2(H2O)2]0. On the other hand, the estimated EXAFS spectrum for the mononitrato complex indicates that it has a coordination number of 5 and forms the complex [UO2NO3(H2O)3]+. These results are supported by DFT calculations. The same data treatment (i.e. factor analysis combined with the speciation diagram from spectral titration) has also applied to the EXAFS spectra in aqueous solutions of nitric acid. The results suggest that the nitrate complexation of uranium(VI) is far weaker in aqueous solution than that in acetonitrile and almost no trinitrato complex is formed even when the [NO3]/[U] ratio exceeds 360.