Thermodynamics of Technetium Related to Nuclear Waste Disposal: Solubilities of Tc(IV) Oxides and the Electrode Potential of the Tc(VII)/Tc(IV)-Oxide Couple.

摘要

Solubilities of Tc(IV) oxides have been determined in a number of solutions in the pH range 1 to 10. Technetium oxide was prepared either by precipitation upon purified sand with a 30% excess of hydrazine or by electrodeposition. The concentrations 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. In basic solutions, the concentrations of Tc(IV) species in equilibrium with the oxide tended to increase slightly with pH. In acid solutions the concentrations were much higher and were consistent with literature data on the hydrolytic species of Tc(IV). These data can be used to estimate Tc(IV) solubilities for cases where solubility limits transport of technetium in reducing environments of high-level waste repositories.