Several Department of Energy nuclear sites have widespread Tc-99 contamination. This radionuclide poses an environmental risk due to its long half life (T1/2 = 2,13*105 years) and the high environmental mobility. Under oxic conditions, Tc-99 persists as Tc(VII)O4- which is highly soluble and does not sorb onto most soil or mineral surfaces lending to its high mobility. However, under anoxic conditions, TcO4- can be attenuated by reduction to TcO2, which is sparingly soluble and environmentally immobile. While the potential for in-situ reduction of Tc-99 has been demonstrated experimentally, the presence of naturally occurring oxidants in suboxic zones poses a potential path for reoxidation and remobilization of Tc-99. Mn(III)-ligand complexes are commonly found in suboxic and anoxic waters and act strong oxidizing agents; the standard reduction potential of the Mn(III)/Mn(II) redox pair is similar to that of O2. The ubiquity of Mn(III)-ligand complexes, coupled with their strong oxidizing potential, makes them a strong liability for the remediation of Tc-99 by reductive immobilization. In the present study, the kinetics of the oxidative dissolution of TcO2 by Mn(III)-ligand complexes were examined. Three Mn(III)-ligand complexes were studied: Mn(III)-pyrophosphate, Mn(III)-citrate and Mn(III)-tartrate. All experiments were carried out in an anaerobic glovebox (nitrogen atmosphere) and oxygen was kept below 0.1 ppm as indicated by the glovebox’s oxygen monitoring system. Our results indicate that all three Mn(III)-ligand complexes rapidly oxidize TcO2 releasing TcO4- into the aqueous phase. However, Mn(III)-pyrophosphate is notable in its ability to induce TcO2 oxidation under a variety of pH conditions, whereas Mn(III)-citrate and Mn(III)-tartrate are limited to pH ≥ 8. These results indicate that Mn(III)-ligand complexes are a strong liability for the long term remediation and stewardship of Tc-99 at contaminated field sites.

Oxidative dissolution of Tc-99 by aqueous Mn(III) under anoxic conditions: Redox mediated mobilization of a risk driving radionuclide

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