There are very few examples in nature for U(VI) compounds with carbonate ligands other than the well known tricarbonates. Especially examples of U(VI) dicarbonato compounds are nearly completely missing. Even in aqueous solutions, the dicarbonato complex was found as a species of minorimportance only. On the basis of structural data on the ligands H2O and carbonate as well as the available data on U(VI) coordination compounds, steric requirements of equatorial coordination are studied for aqueous solution species. A pentagonally coordinated monocarbonato species [UO2CO3(H2O)3] is found as the most likely coordination. For the dicarbonato species, hexagonally coordinated [UO2(CO3)2(H2O)2] with D2h symmetry is found as most probable structure. Possible causes of the instability of U(VI) dicarbonato species are discussed.
The solubility of NaNpO2CO3(s) in 0.1M perchlorate solution at 25°C in equilibrium with 1.0% CO2/N2 atmosphere has been investigated as a function of pH/lg [CO32-]. The solid phase was found hexagonal with a=1008.1±0.3 pm and c=991.1±0.2 pm. A solubility product of lg Ksp(NaNpO2CO3)=–10.22±0.02 and a formation constant of the first Np(V) carbonato species of lg ß01 = 4.52±0.02 was evaluated. For the dicarbonato species an upper limit of lg lg ß02 < 6.6 was derived. Comparison of the present data with solubility values of Np(V) in equilibrium with 0.03% CO2 partial pressure gave evidence that carbonato species are prevailing in solutions at both 1% and 0.03% CO2 partial pressures.
UV-Vis spectra of solutions in solid-aqueous phase equilibrium with UO3·2H2O(s) and 0.03 kPa CO2 partial pressure are quantitatively analyzed by single component spectra of hydrolysis species UO
. From the deconvoluted spectra, single species concentrations are obtained and interpreted by various statistical methods.
Relationship of UV-Vis data to fluorescence spectroscopic analysis of the same system is discussed. Calculation of thermodynamic
quantities gave consistent results, both within experimental data and with results from solubility studies and spectroscopic
analysis from literature. A reinterpretation of some literature data is proposed.
The application of UV-Vis and time-resolved laser-induced fluorescence (TRLF) spectroscopies to direct speciation of uranium(VI) in environmental samples offers various prospects that have, however, serious limitations. While UV-Vis spectroscopy is probably not sensitive enough to detect uranium(VI) species in the majority of environmental samples, TRLFS is principially able to speciate uranium(VI) at very low concentration levels in the nanomol range. Speciation by TRLFS can be based on three parameters: excitation spectrum, emission spectrum and lifetime of the fluorescence emission process. Due to quenching effects, the lifetime may not be expected to be as characteristic as, e.g., the emission spectrum. Quenching of U(VI) fluorescence by reaction with organic substances, inorganic ions and formation of carbonate radicals is one important limiting factor in the application of U(VI) fluorescence spectroscopy. Fundamental photophysical criteria are illustrated using UV-Vis and fluorescence spectra of U(VI) hydrolysis and carbonato species as examples.
Authors:B. Delakowitz, G. Meinrath, and W. Spiegel
A comparative review of mineral-specific radionuclide sorption databases created for performance assessment and reported in both the open literature and grey literature (e.g. technical reports) has in part shown poor quality of documentation describing the database selection procedures. Inadeguate information is available on the chemical species of the radionuclide under consideration and the laboratory conditions for determining Kd-values. Sorption data derived from literature are neither comparable nor generally applicable due to the wide range in the composition of the aqueous and the solid phases applied in migration experiments. Subsequently, standardized characterization and determination procedures are needed. To improve the reliability of a mathematical model for the prediction of radionuclide sorption on cementitious and ash-type binder materials, determination of mineral-specific sorption coefficients is indispensable.
The behavior of (UO2)2(OH)
has been investigated in solid-liquid equilibria under 100%, 8%, 1%, 0.3% and 0.03% CO2 partial pressure as well as in undersaturated systems in equilibrium with air at 24±2°C in 0.1M NaClO4 solutions. From spectroscopic investigations by UV-Vis-and time-resolved laser-induced fluorescence (TRLF) spectroscopies, single component absorption and emission spectra are suggested for the (UO2)2 (OH)
species. The lifetime
22 of the fluorescence emitting electronically excited state of (UO2)2(OH)
was determined as
22 = 2.9 ± 0.9 s. The formation constant of (UO2)2(OH)
was found to be log K22=–5.97 ± 0.06. Interpretation of the experimental data was also made assuming the species (UO2)2(OH)
, but unsatisfactory results have been obtained.