ELECTROCHEMICAL REDOX REACTIONS OF URANYL(VI) COMPLEXES ON MAGNETITE

Magnetite (Fe₃O₄) is a common corrosion product of iron under anoxic conditions, and it is likely to be form during the corrosion of metallic canisters used as containers for the geological disposal of nuclear waste. Magnetite can reduce U(VI) in aqueous solution thereby reducing its mobility in the environment. Previous research has shown that the reduction of U(VI) by the iron at the surfaces of magnetite or by the ferrous iron released by the dissolution of magnetite are both likely to occur. However, the mechanism of electron transfer from the magnetite surface to the uranyl ion in aqueous solution remains unclear.

In this work, electrochemistry methods, including linear sweep voltammetry and cyclic voltammetry (CV), have been applied in order to investigate redox reactions of different aqueous uranium species on the surface of synthetic magnetite particles (200nm). The redox potentials of different U(VI)/U(V) complexes vary with their corresponding coordination environment. The peak seperations, ΔE, of all of the CV of these four types of U⁶⁺/U⁵⁺redox couples are greater than ΔE = 56 mV for a reversible redox reaction, indicating that the reduction of uranyl complexes fall within the regime of electron transfer-limited kinetic behavior under the experimental condition (scan rate = 50mv/s).

The reaction rate constant, k⁰, of UO₂(SO₄)_(aq) could be calculated from electrochemical data because of the favorable reversibility of its redox peaks. The reaction rate constant, k⁰, of UO₂(SO₄)_(aq) obtained is less than the reported value for U⁶⁺/U⁵⁺ redox couple, which may due to the semiconducting properties of magnetite by which the kinetics of electron transfer is governed by the electron density in its conduction band in addition to mass transport and electron transfer barriers. The positive shift of the uranium reduction peak from -0.28V to 0.04V (vs. Ag/AgCl) was observed, which resulted from adsorption of reduced U(V) species onto the magnetite surface. This could be the reason for the presence of U(IV) species on magnetite surfaces in batch reaction experiments.