Solubility of Tc(IV) oxides

Full text of publication follows: The deep geological disposal of the high level radioactive wastes is expected to be a safer 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 the reducing conditions, expected to exist in a deep geological repository, it is generally predicted that technetium will be present as TcO2.nH2O. The solubility of Tc(IV) is used as a source term in performance assessment of radioactive waste repository. Technetium oxide was prepared by reduction of a technetate solution with Sn2+. The solubility of Tc(IV) oxide has been determined in simulated groundwater and re-distilled water under aerobic and anaerobic conditions. The effects of pH and CO32- concentration of solution on solubility of Tc(IV) oxide 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 re-distilled water is about (1.49∼1.86) x 10-9 mol/(L.d) under aerobic conditions, but Tc(IV) in simulated groundwater and re-distilled water is not oxidized under anaerobic conditions. Under aerobic or anaerobic conditions the solubility of Tc(IV) oxide in simulated groundwater and re-distilled water is equal on the whole after centrifugation or ultrafiltration. The solubility of Tc(IV) oxide decreases with the increase of pH at pH < 2, increases with the increase of pH at pH > 10 and is pH independent in the range 2 < pH < 10. The concentrations of Tc(IV) species were in the range of 10-8 to 10-9 mol/L at 2 < pH < 10. The solubility of Tc(IV) oxide increases with the increase of CO32- concentration. 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)