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 (Fe^IIFe^III₂O₄) has been studied in the last decades, showing that magnetite reduces pertechnetate (Tc^VIIO₄^-) to Tc^IV, which is either incorporated into the magnetite structure or forms Tc^IV-Tc^IV-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 Tc^VII 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 Tc^IV incorporation. We observed that the sorption of Tc^IV-Tc^IV-dimers initially occurs at pH 5 and pH 7, while Tc^IV incorporation in magnetite prevails at longer times and at pH 10. We suggest that Tc^IV-Tc^IV-dimer sorption on magnetite is due to (surficial) maghemitization of the magnetite nanoparticles, whereas Tc^IV incorporation is due to the electron transfer from sorbed Fe²⁺ through magnetite and subsequent release of Fe^II in solution (redox conveyor belt model), "burying" Tc^IV into the magnetite structure. DFT calculations indicate that Tc^IV incorporates in magnetite by an exchange of two Fe^II atoms for one Tc^IV, keeping the charge balanced by creating a vacancy.