Technetium-99 is a risk driving radionuclide for several U.S. Department of Energy Nuclear sites across the United States because of its long half-life (2.13*105 years) and high environmentally mobility. Tc-99 has no natural abundance and is a by-product from fission of U-235 for nuclear power production, and formerly from the production and testing of nuclear weapons. Past poor waste management practices have led to the release of substantial amounts of Tc-99 into the environment at locations such as the Hanford and Savannah River Sites. The oxidation state of Tc-99 is the controlling factor in its environmental mobility. Under most environmental conditions, Tc-99 exists as Tc(VII)O 4- which is highly soluble and unable to sorb to soil or mineral surfaces, giving it a high environmental mobility. However, TcO4- can be reduced to TcO2, which is insoluble and environmentally immobile. Thus, reductive immobilization has historically been the favored strategy of Tc-99 remediation. The efficacy of Tc-99 immobilization is contingent on the presence of oxidants in the subsurface. The present study investigates for the first time the role of Mn(III)-pyrophosphate (Mn(III)-PP) aqueous complexes, which are strong oxidants, influencing the stability of reduced technetium forms (TcO2) under anoxic conditions. on the Oxidative dissolution of TcO2 by Mn(III)-Pyrophosphate was carried out in batch experiments in an anaerobic glovebox (&lt;0.1 ppm O2). Mn(III)-PP was able to completely solubilize Tc-99 in a matter of hours, in fact faster than oxygen under oxic conditions. The reaction kinetics are sensitive to pH, Mn(III):PP ratio, and Mn(III) concentration. Our results show that Mn(III)-PP can drastically increase the environmental mobility of Tc-99 and can be a liability for the long-term stability even under anoxic condition where long-term stability would be expected.

Rapid oxidation of TcO2 by aqueous Mn(III)-pyrophosphate under anoxic conditions: A liability for the remediation of technetium-99

ACS Spring 2022

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