Exploring the Reduction Mechanism of Tc-99(VII) in NaClO4: A Spectro-Electrochemical Approach

Technetium (Tc) is an environmentally relevant radioactive contaminant whose migration is limited when Tc(VII) is reduced to Tc(IV). However, its reaction mechanisms are not well understood yet. We have combined electrochemistry, spectroscopy, and microscopy (cyclic voltammetry, rotating disk electrode, X-ray photoelectron spectroscopy, and Raman and scanning electron microscopy) to study Tc(VII) reduction in non-complexing media: 0.5 mM KTcO4 in 2 M NaClO4 in the pH from 2.0 to 10.0. At pH 2.0, Tc(VII) first gains 2.3 +/- 0.3 electrons, following Tc(V) rapidly receives 1.3 +/- 0.3 electrons yielding Tc(IV). At pH 4.0-10.0, Tc(IV) is directly obtained by transfer of 3.2 +/- 0.3 electrons. The reduction of Tc(VII) produced always a black solid identified as Tc(IV) by Raman and XPS. Our results narrow a significant gap in the fundamental knowledge of Tc aqueous chemistry and are important to understand Tc speciation. They provide basic steps on the way from non-complexing to complex media.