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

An important parameter for the safety assessment of radioactive waste repositories is the prediction and modelling of aqueous complex formation reactions between actinides (An) and common dissolved inorganic or organic ligands. Alteration processes at the contact zone between the backfill material, bentonite, or the clay host rock and the cementitious materials of the geotechnical barrier will lead to high silicate concentrations in the groundwater, which may strongly influence the aqueous speciation of actinides such as uranium, which is stable in the hexavalent oxidation state under oxidizing conditions. [1]. A detailed knowledge of the U(VI) –silicate complex formation is therefore very important.
Depending on the used host rock and backfill material, the pH of the groundwater will be in the neutral to alkaline range. However, in this pH-range, reliable thermodynamic data for aqueous An(VI) - silicate complexes are scarce. In the acidic pH-range, only the 1:1 An(VI)-Si complex, i.e. An(VI)O2OSiOH3+, has been determined for U(VI), Np(VI), and Pu(VI), and the complex formation constants differ by almost two orders of magnitude (Table 1) [2].
Table 1: Complex formation constants for An(VI)-Si complexes [2].
An logK0
U(VI) -1,86
Np(VI) -2,61
Pu(VI) -3,65

In the alkaline pH-range (pH ~8), Yusov et al. [3] postulated the formation of either a ternary Pu-OH-Si complex: (PuO2(H2O)3(OH)OSi(OH)3) with the H3SiO4- ligand or a binary Pu-Si complex (PuO2(H2O)3O2Si(OH)2) with H2SiO42-. For other hexavalent actinides, no complexes in the alkaline pH-range have been reported, however, in analogy with Pu(VI), comparable complexes should also exist for U(VI) and Np(VI).
This contribution reports on a study of the U(VI) complexation with silicate in the pH range between 3.4 and 11.5. Two approaches were used: 1) Time-resolved laser-induced luminescence spectroscopy (TRLS) was applied to determine the in situ U(VI) speciation in U(VI) solutions with various silicate concentrations and various pH. 2) U(VI)-silicate complexation constants and complex stoichiometries were determined using Schubert’s method. For the TRLFS measurements, a U(VI) concentration of 5×10-6 M (pH = 3.5) or 1×10-7 M (pH = 9) was used, while the silicon concentration was varied between 3×10-4 and 1.5×10-3 M. To determine the thermodynamic parameters ΔrH0 and ΔrS0, temperature dependent measurements were performed in the range from 1°C to 25°C. The ionic strength was fixed with NaClO4 at 0.2 M. The Schubert method allows determination of complex stoichiometry and complexation constant by measuring the solid/liquid distribution ratio (Rd value) for the U(VI) sorption on a solid phase in absence and in the presence of increasing concentrations of silicate. Here, monoclinic ZrO2 was used as a solid phase. The U(VI) concentration in the experiments was 1×10-7 M and silicate concentrations were varied between 5×10-5 and 5×10-3 M, at pH values ranging from 6.0 to 11.5 at an ionic strength of 0.1 M NaCl. LSC measurements of the 233U activity were used to determine the U(VI) concentration in solution.
In the absence of aqueous silicates, the 1:1 uranium hydrolysis species UO2OH+ plays a significant role in the speciation starting from a pH of 3.5. Therefore, this species has to be taken into account in the speciation. Figure 1 shows the luminescence spectra with increasing Si-concentration at different temperatures. The obtained spectra show a bathochromic shift and an increase in the luminescence intensity with increasing silicate concentration. Based on peak deconvolution, the pure component spectra of the UO2OSi(OH)3+ and UO2OH+ complex were extracted. The species distributions were calculated by a least-square fit method. The following slope analysis resulted in a slope close to 1 for all temperatures, confirming the formation of a UO2OSi(OH)3+ complex at pH 3.5. The obtained complexation constants were corrected to standard conditions using the Davies equation. The obtained stability constant at 25°C is significantly higher than the literature values due to the consideration of the hydroxo complex and the solubility limit of the aqueous silicates [2]. A van’t Hoff plot was used to extract the reaction enthalpy and entropy, which were found to be ΔrH0 = 46.3 kJ∙mol-1 and ΔrS0 = 154.1 J∙K-1∙mol-1.

Figure 1: Emission spectra of the U-Si complexation at pH 3.5 with varying [Si] between 3×10-4 and 1.5×10-3 M, [U] = 5×10-6 M, fixed [NaClO4] = 0.2 M, in the temperature range between 1°C to 25°C.
For the Schubert method, the U(VI) sorption distribution coefficient Rd on ZrO2 was determined by LSC-measurements as a function of the ligand concentration and the pH in the alkaline pH range. By plotting the Rd-values as a function of the ligand concentration, information about the number of involved ligands in the U(VI)-silicate complex could be obtained. When further plotting the fitting constant (obtained from the Rd-plot) as a function of corrected pH, the number of protons involved in the complexation reaction and the conditional complexation constant could be determined. With the obtained stoichiometry, two possible complexes could be proposed in the alkaline pH-range, (i) UO2(OH)O2Si(OH)2- or (ii) UO2(OH)2OSi(OH)3-. DFT-calculations support the formation of the second complex with a corrected stability constant of logK0 = -16.30.

[1] D. Savage, Mineral. Mag.,2011, 75, 2401-2418.
[2] R. Guillaumont et al., Update on the Chemical Thermodynamics of Uranium, Neptunium, Plutonium, Americium and Technetium, 2003, NEA-TBD.
[3] A. B. Yusov, A. M. Fedoseev, Russ. J. Coord. Chem., 2003, 29, 625-634.