1 research outputs found
Electrochemical and Spectroscopic Evidence on the One-Electron Reduction of U(VI) to U(V) on Magnetite
Reduction of UÂ(VI) to UÂ(IV) on mineral
surfaces is often considered
a one-step two-electron process. However, stabilized UÂ(V), with no
evidence of UÂ(IV), found in recent studies indicates UÂ(VI) can undergo
a one-electron reduction to UÂ(V) without further progression to UÂ(IV).
We investigated reduction pathways of uranium by reducing UÂ(VI) electrochemically
on a magnetite electrode at pH 3.4. Cyclic voltammetry confirms the
one-electron reduction of UÂ(VI) to UÂ(V). Formation of nanosize uranium
precipitates on the magnetite surface at reducing potentials and dissolution
of the solids at oxidizing potentials are observed by in situ electrochemical
atomic force microscopy. XPS analysis of the magnetite electrodes
polarized in uranium solutions at voltages from −0.1 to −0.9
V (E<sup>0</sup><sub>U(VI)/U(V)</sub>= −0.135 V vs Ag/AgCl)
show the presence of only UÂ(V) and UÂ(VI). The sample with the highest
UÂ(V)/UÂ(VI) ratio was prepared at −0.7 V, where the longest
average U–O<sub>axial</sub> distance of 2.05 ± 0.01 Å
was evident in the same sample revealed by extended X-ray absorption
fine structure analysis. The results demonstrate that the electrochemical
reduction of UÂ(VI) on magnetite only yields UÂ(V), even at a potential
of −0.9 V, which favors the one-electron reduction mechanism.
UÂ(V) does not disproportionate but stabilizes on magnetite through
precipitation of mixed-valence state UÂ(V)/UÂ(VI) solids