Oxidation and Mobilization of Tc-99 Reduced Phases by a Mn(III)–Pyrophosphate Aqueous Complex under Anoxic Conditions: Implications for Remediation of a Risk-Driving Radionuclide

Abstract

The environmental fate of technetium-99 is tied to its oxidation state. Under oxidizing conditions, Tc-99 predominates as the Tc(VII)O₄^– anion, which exhibits high solubility and is precluded from sorption to mineral surfaces, making it highly mobile in the environment. Under reducing conditions, Tc-99 predominates as Tc(IV) [Tc(IV)O₂, Tc(IV)₂S₇, or Tc(IV)-bearing mineral phases, e.g., Tc(IV) incorporation into iron oxides], which shows low solubility. There has been significant interest in developing reductive immobilization strategies for Tc-99, particularly in anoxic environments where Tc(IV) is conventionally assumed to be stable. However, O₂ is not the only common environmental oxidant. Many high-valent manganese species are prolific in anoxic environments, and they can create localized oxidizing conditions in otherwise reducing environments. Our work aims to bridge the knowledge gap on the remobilization of Tc(IV) species under the conditions mentioned, by studying the oxidation of Tc(IV) by a Mn(III)–pyrophosphate complex. Mn(III)–ligand complexes in particular have been overlooked due to the assumption that Mn(III) will be disproportionate in aqueous systems. In this work, the Mn(III)–pyrophosphate complex rapidly oxidized Tc(IV) to Tc(VII) in the absence of oxygen, resulting in dissolution and release of Tc-99 to the aqueous phase. This work presents novel information about the redox interface chemistry of Tc-99, which is crucial to developing effective remediation methods.