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

Radioactive technetium-99 ($^{99}$Tc) is present in nuclear and medical wastes. Its immobilization by magnetite (Fe$^{II}$Fe$^{III}$$_2$O$_4$) has been studied in the last decades, showing that magnetite reduces pertechnetate (Tc$^{VII}$O$_4$$^–$) to Tc$^{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 Fe$^{2+}$ through magnetite and subsequent release of Fe$^{II}$ in solution (redox conveyor belt model), “burying” TcIV into the magnetite structure. ... mehrDFT calculations indicate that Tc$^{IV}$ incorporates in magnetite by an exchange of two FeII atoms for one Tc$^{IV}$, keeping the charge balanced by creating a vacancy.