Technetium (Tc) contamination remains a major environmental problem at nuclear reprocessing sites, e.g., the Hanford Site, WA, USA. The high environmental risk associated with Tc is due to its long half-life (213,000 years) and the mobility of the oxidized anionic species Tc(VII)O4-. Under reducing conditions, TcO4- is readily reduced to Tc(IV), which commonly exists as a hydrous Tc oxide (TcO2.nH2O). Ensuring the stability of Tc is dependent on the solubility and re-oxidation of immobilized Tc(IV)-bearing phases in both the environment and in nuclear waste streams. However, the solubility of crystalline TcO2 or amorphous TcO2.H2O is still higher than the maximum contaminant level (MCL) established by the EPA (900 pCi/L). Sulfide is also an important complexing ligand that can have a significant effect on the adsorption, reduction and solubility of Tc species. In the environment, TcO4- removal by sulfide minerals depends on particle size, contacting solution, pH, ionic strength and Eh, with the principal mechanism of TcO4- attenuation being the reduction to low solubility sulfide phases. Little is known about the mechanisms of formation, stabilization and re-oxidation of Tc sulfides, however, it has been shown that TcO2 is preferentially oxidized to aqueous Tc(VII) relative to sulfide-stabilized Tc. Longer periods of oxidation result in transformation of Tc speciation from Tc(IV) sulfides to a Tc(IV)O2-like environment, but no dissolution of Tc occurs, suggesting that oxidation is not accompanied by oxidation of Tc(IV) to Tc(VII). Thus, the transformation in speciation from Tc(IV) sulfides to Tc(IV) oxides provides an extra energy barrier to overcome prior to Tc re-oxidation to Tc(VII)O4-. The Tc observed diffusivity for a cementitious waste form will be presented to show the effect of Tc stabilization as Tc(IV)2S7 relative to TcO2.nH2O.

Technetium stabilization in low solubility sulfide phases

253rd National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.