Tc(VII) reductive immobilization by S(-II) pre-sorbed on alumina

Tc-99 is a fission product of U-235 and Pu-239 with a long half-live (2.14∙10⁵ years). Under oxidizing conditions, Tc main species (Tc(VII)O₄⁻) exhibits a high solubility and hardly interacts with minerals. In contrast, under reducing conditions, Tc(IV) presents a more limited mobility, either because Tc(IV) interacts with minerals or Tc(IV)O₂ is formed [1]. However, the formation of Tc(IV)O₂ is not sufficient to ensure the immobilization of Tc, since when it is in contact with O₂, the reoxidation of Tc(IV) to Tc(VII) would be thermodynamically favorable. In contrast, the formation of Tc(IV) polysulfide species (such as TcSx or Tc₂S7) could inhibit Tc oxidation under oxidizing conditions [2]. Therefore, S(-II) seems a promising candidate to immobilize Tc. Sulfide would be present in the nuclear waste repository due to the addition of fly ash in the concrete, as well as the presence of minerals such as pyrite (FeS₂). It has been proven for Fe(II) that Tc(VII) reduction is more favorable when Fe(II) takes part in the mineral structure or it is sorbed on a surface than when Tc(VII) reduction is carried by dissolved Fe(II) homoreduction) [3]. We have recently showed that Tc(VII) heteroreduction (reduction occurring at the mineral-water interface) by Fe(II) pre-sorbed on alumina nanoparticles is highly efficient [4].
Thus, in this work, we have studied kinetically as a function of pH: i) S(-II) sorption on alumina, and ii) subsequent Tc uptake promoted by S(-II) pre-sorbed on alumina. We have also focused on the effect of different sulfide sources on Tc(VII) reduction. All the experiments were performed in a N₂ glove box free of CO₂ and O₂ (< 2 ppm). The alumina
nanoparticles used in the experiments has been previously characterized with 127 m² /g N₂ BET and pH 9 as isoelectric point pH [5]. For the batch sorption experiments, suspensions of alumina (0.5 g/L) containing 50 μM of NaHS at pH 5.3, 6.7 and 7.7 were prepared and shaken for two days. Then, KTcO₄ was added to the suspensions to obtain 5 μM of KTcO₄. Subsequently, the suspensions were placed in a horizontal shaker. The suspension pH was monitored frequently and readjusted when needed. Samples were taken periodically and centrifuged at 14,000 rpm for 45 min. The Tc concentration in the supernatant solution was measured by liquid scintillation counter to determine the percentage of Tc removed.
Figure 1 shows the uptake of Tc in % as a function of time and pH. Tc removal increases with decreasing pH. This is in agreement with the highest anion sorption on alumina nanoparticles at lower pH, when alumina surface is positive charged [5]. The maximum Tc retention is 70% at pH 5.3, being complete after one day of contact. Whereas at higher pH values, Tc removal is significantly lower, i.e., 10% at pH 6.7 and 5% at pH 7.7. It is noteworthy to mention that the NaHS reactant used for the experiments in Figure 1. was partially oxidized. Despite of its oxidation, reduction of Tc(VII) yield at pH 5.3 was above 70% after one day of contact.

Further contact experiments have been performed to isolate the contribution of S(-II) in Tc(VII) heteroreduction, and the effect of the sulfide source on Tc removal. Raman microscopy and X-ray absorption spectroscopy have been used to determine the changes occurring at a molecular level when Tc(VII) is heteroreduced by S(-II).

Acknowledgements: The authors acknowledge the Spanish Ministry of Research and Universities for the abroad internship fellowship (PRE2018-085618) and the project (ENE2017-83048-R). Part of this work was financially supported by the German Federal Ministry of Education and Research (BMBF) NukSiFutur TecRad young investigator group (02NUK072).