Uranium (VI) complexation with aqueous silicates under elevated temperatures


Uranium (VI) complexation with aqueous silicates under elevated temperatures

Lösch, H.; Huittinen, N.; Stumpf, T.

For the safety assessment of high-level nuclear waste repositories in deep geological formations, a sound understanding on the migration behavior of mobilized radionuclides from the spent nuclear fuel (SNF) is required. In recent decades, the solution chemistry of actinides, including hydrolysis and complexation with inorganic ligands such as silicates and carbonates, as well as the interactions of actinides at interfaces present in the geo- and biosphere, have been intensely investigated. These studies are, however, often conducted at ambient temperatures (20-30°C), which hampers the prediction of actinide migration in the environment of heat generating high-level nuclear waste repositories. It is estimated that maximum temperatures in the near field of a SNF repository could reach 100°C, depending on the waste form and host rock under consideration. Thus, for a reliable safety assessment of future SNF repositories, already existing thermodynamic data at 25°C (logK(T), ΔrH, ΔrS, ΔrG) should be expanded to higher temperatures [1, 2].
Silicon is ubiquitous in natural waters as ionic or colloidal species, as a result of weathering of amorphous silica or silicate minerals. U(VI) complexation with silicates in solution has been already subject in previous studies, but only in the temperature range between 20-25°C. For the U(VI)-silicate complexation reaction presented below, a log10*K° between (-1.44) - (-2.65) has been reported [3-4].

UO22+ + Si(OH)4(aq) ↔ UO2SiO(OH)3+ + H+

In the present study, we investigate the complexation of U(VI) with aqueous silicates at temperatures between 0-60°C, thus, allowing the extraction of complexation constants at elevated temperatures as well as thermodynamic parameters for the identified species.
The complexation of U(VI) with dissolved silicates will be investigated as a function of temperature, silicate concentration and pH. The U(VI) concentration will be fixed at 5*10-6 M and the silicate concentration will be varied between 3*10-4 - 2*10-3 M. The complexation will be analyzed with a combination of time-resolved laser-induced fluorescence (TRLFS) and UV-vis absorption spectroscopy to extract the complexation constants at elevated temperatures and subsequent determination of thermodynamic constants (ΔH, ΔS, ΔG). Slope analysis is used to calculate the complex formation constants, which are recalculated to standard conditions with the van´t Hoff equation and the SIT model.
The complexation constants, thermodynamic data and potential solubility products will be included in a thermodynamic database. Thermodynamic speciation modelling of the U(VI) silicate complexation behavior will be done based on the experimental data. Here, comparisons using existing data for ambient conditions can be directly made to estimate the influence of elevated temperatures on the complexation behavior and phase equilibria of U(VI) in the presence of silicates.
Results from the U(VI)-silicate complexation study at elevated temperatures will be presented at the conference.

[1] Warwick P., Hall A., Zhu J., Dimmock P.W., Robbins R., Carlsen L., Lassen P., (1997), Chemosphere, 35, 2471-2477
[2] Rao L., Jiang J., Zanonato P., Bernardo P., Bismondo A., Garnov A.Y., (2002), Radiochim. Acta, 90, 581-588
[3] Guillaumont, R., Fanghänel, T., Neck, V., Fuger, J., Palmer, D.A., Grenthe, I., Rand, M.H., (2003), “Update on the Chemical Thermodynamics of Uranium, Neptunium, Plutonium, Americium and Technetium, Chemical Thermodynamics 5”, Nuclear Energy Agency, Elsevier Science Publisher
[4] Moll H., Geipel G., Brendler V., Bernhard G. and Nitsche H., (1998), J. Alloys Compounds 271–273, 765–768.

Keywords: Uranium (VI); silicates; complexation; aqueous system; elevated temperatures

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    Migration 2017, 10.-15.09.2017, Barcelona, Spanien

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Publ.-Id: 25274