XAFS and DFT Insights into the Kinetics and Mechanisms of Technetium Reduction by Nanoparticulate Magnetite

Radioactive technetium-99 (⁹⁹Tc) is present in nuclear and medical wastes. Its immobilization by magnetite (FeIIFeIII_2O_4$) has been studied in the last decades, showing that magnetite reduces pertechnetate (Tc$VIIO_4^–) to TcIV, which is either incorporated into the magnetite structure or forms TcIV–TcIV-dimers attached to the magnetite surface. The distribution between both phases and the incorporation mechanism remain, however, unclear. Therefore, we investigated the molecular environment of Tc after contacting TcVII with synthesized nanoparticulate magnetite as a function of pH (2–13) and time (up to 7 weeks). X-ray absorption spectroscopy was combined with density functional theory (DFT) simulations to decipher the mechanism of TcIV incorporation. We observed that the sorption of TcIV–TcIV-dimers initially occurs at pH 5 and pH 7, while TcIV incorporation in magnetite prevails at longer times and at pH 10. We suggest that TcIV–TcIV-dimer sorption on magnetite is due to (surficial) maghemitization of the magnetite nanoparticles, whereas TcIV incorporation is due to the electron transfer from sorbed Fe²⁺ through magnetite and subsequent release of FeII in solution (redox conveyor belt model), “burying” TcIV into the magnetite structure. DFT calculations indicate that TcIV incorporates in magnetite by an exchange of two FeII atoms for one TcIV, keeping the charge balanced by creating a vacancy.