Uranium(VI) complexation with aqueous silicates in the acidic to alkaline pH-range

INTRODUCTION
An important parameter for safety assessments of radioactive waste respositories is the prediction and modelling of aqueous complex formation reactions between actinides and common dissolved inorganic and organic ligands. For this assessment, the knowledge of dissolved ligands in the groundwater produced by dissolution of waste canisters, backfill material, and host rock is required. Due to the ubiquitous existence of silicon in these materials, aqueous silicate species are important ligands to consider in the metal-ligand speciation, especially in contact zones of cement pore water with clay or granite where high silicate concentrations are expected as a result of alteration processes [1]. Depending on the used host rock and backfill material, the pH of the groundwater will vary between the neutral to alkaline range. However, in this pH-range, reported An-Si species are scarce or non-existent, and there is a lack of reliable thermodynamic data. In the acidic pH-range, only the 1:1 An(VI)-Si complex, i.e. An(VI)O ₂OSiOH₃+, is known for U(VI), Np(VI), and Pu(VI), and the complex formation constants differ by one order of magnitude.
In the alkaline pH-range (pH ~8), Yusof et al. [3] postulated the formation of either a ternary Pu-OH-Si PuO₂(H₂O)₃(OH)OSi(OH)₃ complex with the H₃SiO₄- ligand or a binary Pu-Si PuO₂(H₂O)₃O₂Si(OH)₂ complex with H₂SiO₄ ²-. For other hexavalent actinides, no complexes in the alkaline pH-range have been reported, however, following the analogy of the hexavalent cations comparable complexes should also exist for U(VI) and Np(VI).

Experimental
In this study the in-situ speciation of U(VI) in solution in the presence of silicates was monitored with laser-induced luminescence spectroscopy (TRLFS) and , the Schubert method. For the TRLFS measurements, a U(VI) concentration of 5×10-⁶ M was used, while the silicon concentration was varied between 3×10-⁴ and 2×10-³ M depending on the pH. Temperature-dependent measurements were performed in the T-range from 1°C to 40°C to improve the signal to noise ratio and to enable the extraction of thermodynamic parameters, such as the enthalpy ΔRH° and entropy ΔRS° of reaction. The TRLFS measurements were performed at two excitation wavelengths of 266 nm and 394 nm.
In the Schubert method, the desorption of U(VI) from an inert solid phase as a result of silicate complex formation in solution, is monitored. Here, monoclinic ZrO₂ was used as a solid phase and investigations were performed for a U(VI) concentration of 1×10-⁷ M and silicate concentrations between 5×10-⁵ and 5×10-³ M, at pH values ranging from 7.0 to 11.5. LSC measurements of the 233U activity were used to determine the U(VI) concentration in solution.

RESULTS
Based on the TRLFS investigations in the acidic pH-range, the formation of the 1:1 U-Si complex UO₂OSi(OH)₃+ could be confirmed in addition to a hitherto unidentified silicate species. The normalized emission spectra clearly show a change in the peak shape with increasing silicate concentration. Next to the change in the spectral shape, a significant increase in the luminescence intensity could be observed. Such an increase of the luminescence intensity speaks for the formation of a polynuclear U(VI)-silicate complex. However, investigations to confirm this hypothesis are still ongoing.

In the alkaline pH-range it was possible to identify a ternary U-OH-Si complex, most likely either a monodentate UO₂(OH)₂OSi(OH)₃- or a bidentate UO ₂(OH)OSi(OH)₂- complex with a complex formation constant of logβ0 = -15.6. Preliminary speciation calculations in clay and cement pore water show, that this ternary U-OH-Si will dominate the U(VI) speciation in the pH-range between 9.0 and 11.5. To resolve the stoichiometry of this complex, TRLFS investigations are planned together with complementary DFT calculations.

REFERENCES

1. D. SAVAGE, Mineralogical Magazine, 75, 2401-2418 (2011).
2. R. GUILLAUMONT, Update on the chemical thermodynamics of Uranium, Neptunium, Plutonium, Americium and Technetium 5, p. 252, Nuclear Energy Agency, Elsevier Science Publisher (2003).
3. A.B. YUSOF, A.M. FEDOSEEV, Russ. J. Coord. Chem., 29, 582-590, (2003).