Technetium-99 is a radionuclide of environmental concern due to its long half-life (2.13x105 years), radiotoxicity and high mobility under oxidizing conditions. Poor past radioactive waste management practices led to the release of Tc-99 into the environment, where under oxidizing conditions, it exists as highly soluble pertechnetate (TcO4-), whereas under reducing conditions Tc(IV) forms predominate, which are sparingly soluble. Manganese minerals, such as manganite (MnOOH) can be effective oxidizing agents, due to the fact that the high redox potential of Mn(IV)/Mn(II) makes oxidation of reduced metals in the presence of Mn-minerals almost always thermodynamically favorable. In our experiments, we compare the ability of manganite, a commonly occurred Mn(III) mineral and bixbyite (α-Mn2Ο3) to oxidize TcO2 to soluble pertechnetate under anoxic conditions at different pH values and at different TcO2:Mn-mineral ratios. Oxidative dissolution was found to be higher at pH 6 compared to pH 8 for both minerals and the interaction of bixbyite with TcO2 resulted in much higher pertechnetate release on the aqueous phase, compared to manganite. Furthermore, we studied the effect of Ca2+ and Mn2+ sorption on the mineral surface on the mineral’s redox capacity and ability to oxidize TcO2. Calcium sorption did not affect bixbyite’s redox capacity towards TcO2, whereas Mn2+ sorption was negligible on both minerals at pH 6.

Geochemical controls of bixbyite and manganite on the mobility of Tc-99: TcO2 oxidative dissolution under anoxic conditions

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