Solubility study of Tc(Ⅳ) in a granitic water

The deep geological disposal of the high level radioactive wastes is expected to be a safe disposal method in most countries. The long-lived fission product ⁹⁹Tc is present in large quantities in nuclear wastes and its chemical behavior in aqueous solution is of considerable interest. Under oxidizing conditions technetium exists as the anionic species TcO₄^- whereas under the reducing conditions, expected to exist in a deep geological repository, it is generally predicted that technetium will be present as TcO₂-nH₂O. Hence, the mobility of Tc(Ⅳ) in reducing groundwater may be limited by the solubility of TcO₂·nH₂O under these conditions. Due to this fact it is important to investigate the solubility of TcO₂·nH₂O. The solubility determines the release of radionuclides from waste form and is used as a source term in radionuclide migration analysis in performance assessment of radioactive waste repository. Technetium (Ⅳ) was prepared by reduction of a technetate solution with Sn²⁺. The solubility of Tc(Ⅳ) has been determined in simulated groundwater and redistilled water under aerobic and anaerobic conditions. The effects of pH and CO₃^2-concentration of solution on solubility of Tc(Ⅳ) were studied. The concentration of total technetium and Tc(Ⅳ) species in the solutions were periodically determined by separating the oxidized and reduced technetium species using a solvent extraction procedure and counting the beta activity of the ⁹⁹Tc with a liquid scintillation counter. The experimental results show that the rate of oxidation of Tc(Ⅳ) in simulated groundwater and redistilled water is about (1.49-1.86)x10^-9 mol·L^-1 d^-1 under aerobic conditions, while no Tc(Ⅳ) oxidation was detected in simulated groundwater and redistilled water under anaerobic conditions. Under aerobic or anaerobic conditions the solubility of Tc(Ⅳ) in simulated groundwater and redistilled water is equal on the whole after centrifugation or ultrafiltration. The solubility of Tc(Ⅳ) increases with the decrease of pH at pH <2, increases with the increase of pH at pH > II and is pH independent in the range 2 < pH < 11. The concentrations of Tc(Ⅳ) species were in the range of 10^-8 to 10^-9 mol·L^-1 at 2 < pH < 11. The solubility of Tc(Ⅳ) slightly increases with increasing the increase of CO₃^2- concentration. Geochemical modelling showed a good agreement between our experimental results and thermodynamic constants from the NEA TDB review. These data could be used to estimate the Tc(Ⅳ) solubility for cases where solubility limits transport of technetium in reducing environments of high-level waste repositories. (authors)