Sectrophotometric Investigation of U(VI) Chloride Complexation in the Naci/NaClO₄ system

Abstract

Post closure radioactive release scenarios from geologic salt formation, such as the WIPP (Waste Isolation Pilot Plant) (USA) include hydrologic transport of radionuclides through a chloride saturated aquifer. Consequently, the understanding of actinide solution chemistry in brines is essential for modeling requiring accurate knowledge of the interaction between AnO22+ and chloride ions. Complexation constants of two U(VI) chloride species UO2Cl+ and UO2Cl20 have been intensively studied for about 40 years using different methods. However, large uncertainties reflect the general difficulty in determining accurate stability constants of weak complexes. In order to model the behavior of U(VI) in brines, we studied the formation of its chloride complexes by UV-Vis spectroscopy as a function of the NaCI concentration at 25 °C. The experiments were performed at constant ionic strength by varying the concentration ratio of NaCI and NaClO4. Deconvolution resulted in single component absorption sepectrum for UO2Cl+ and UO2Cl20. The apparent stability constants of UO2Cl+ and UO2Cl20 are determined at different ionic strengths. The experimental data are used to parameterize using the SIT approach.

Species Distribution and Coordination of Uranyl Chloro Complexes in Acetonitrile

“…Such chloro complexes in ionic liquids may have some importance for the reprocessing by partitioning and further transmutation of highly radioactive waste . Several experimental and theoretical attempts have been made to reveal U(VI) chloro complexes with higher coordination numbers than those provided in the thermodynamic database. – In aqueous solution up to three U(VI) chloro complexes, [UO 2 Cl] + , [UO 2 Cl 2 ] 0 , and [UO 2 Cl 3 ] − , have been observed by EXAFS (extended X-ray absorption fine structure) and UV–vis spectroscopy. – Nguyen-Trung et al indentified the species [UO 2 Cl] + , [UO 2 Cl 2 ] 0 , [UO 2 Cl] 3− , [UO 2 Cl 4 ] 2− , and [UO 2 Cl 5 ] 3– with Raman spectroscopy. – However, according to the authors knowledge, the pentachloro species has not been confirmed yet by other methods. The actual known crystal structure and spectroscopic data of solids and solution species suggest the UO 2 Cl 4 2− unit as the limiting species.…”

“… ,, In contrast, the pentacoordinated complex UO 2 F 5 3− has been observed under different conditions. , [UO 2 Cl 4 ] 2− has the centrosymmetric symmetry D 4 h which is related with a characteristic vibronic UV–vis spectrum. ,,, The UV–vis absorption spectrum of the limiting species [UO 2 Cl 4 ] 2− is well understood, but structural details of the intermediate species are still under discussion. Paviet-Hartmann and Lin extracted the quantitative species distribution of [UO 2 ] 2+ , [UO 2 Cl] + , and [UO 2 Cl 2 ] 0 in aqueous solution by principal component analysis of UV–vis spectra . However, at present, the analysis of the full U(VI) chloride species distribution including the limiting species UO 2 Cl 4 2− is missing in the literature.…”

“…Because of differences in R i , several backscattering atoms can be separated from each other by factor analysis. It has been shown that factor analysis is able to reveal the species distribution from a series of UV–vis absorption spectra , and is also able to extract the main interatomic distances from EXAFS. ,– UV–vis absorption spectroscopy and EXAFS are both especially sensitive in the same concentration range of U(VI) in solution.…”

“…[4][5][6][7] In aqueous solution up to three U(VI) chloro complexes, [UO 2 Cl] + , [UO 2 Cl 2 ] 0 , and [UO 2 Cl 3 ] -, have been observed by EXAFS (extended X-ray absorption fine structure) and UV-vis spectroscopy. [7][8][9] 3with Raman spectroscopy. [3][4][5][6][7][8][9][10] However, according to the authors knowledge, the pentachloro species has not been confirmed yet by other methods.…”

Comparative EXAFS Investigation of Uranium(VI) and -(IV) Aquo Chloro Complexes in Solution Using a Newly Developed Spectroelectrochemical Cell

“…From this correlation the species UO 2 Cl + , UO 2 Cl 2 0 , UO 2 Cl 3 - , UO 2 Cl 4 2- , and UO 2 Cl 5 3- were deduced in the concentration range 3−22 M [Cl - ]. UV−Vis spectroscopy shows significant spectral features which allowed to discriminate the species UO 2 2+ , UO 2 Cl + , and UO 2 Cl 2 0 quantitatively. , Allen et al recently investigated the structure of U(VI) complexes in aqueous chloride solutions by EXAFS spectroscopy. For concentrations up to 14 M [Cl - ], they observed the species UO 2 (H 2 O) 5 2+ , UO 2 (H 2 O) x Cl + , UO 2 (H 2 O) x Cl 2 , and UO 2 (H 2 O) x Cl 3 - with U−Cl distances varying from 2.71 to 2.73 Å.…”