The solubility of technetium(IV) at high pH

Technetium-99 is a β-emitting fission product of 238U. It is important when considering a safety case for nuclear waste disposal because of its high yield in radioactive waste and long half-life. The aqueous chemistry of Tc in a cementitious repository is likely to be dominated by Tc(VII), as TcO4 -, in aerobic areas and by Tc(IV), as TcO2 (am), in anaerobic. Some previous studies have shown an increase in Tc(IV) solubiltiy with increasing pH above pH 9.5 whilst others have not observed this increase. This study found that the solubility of Tc(IV) remains independent of pH until around 13.5, when a small increase can be seen which continues to increase linearly with pH. Modelling suggests that this increase occurs as the species TcO(OH)3 - is formed. The formation constant was estimated with data from this study and was found to be log K2 = -21.6 ± 0.3.