The energetics and kinetics of uranyl reduction on pyrite, hematite, and magnetite surfaces: A powder microelectrode study

摘要

There are many studies describing the influence of parameters such as pH, pCO_2, and complexing ligands on the sorption of the aqueous uranyl species onto mineral surfaces. However, few of these studies describe the reduction reaction mechanisms and the factors that influence the rate of reduction, despite the fact that the oxidation state of uranium is the most important factor controlling the mobility of uranium. In this study, the energetics and kinetics of the U(VI) reduction half-reaction on pyrite, hematite, and magnetite were investigated by electrochemical methods using a powder microelectrode (PME) as the working electrode. Anodic and cathodic peaks corresponding to the 1e- redox couple, U(VI)/U(V), were identified in cyclic voltammograms of pyrite, hematite, and magnetite at pH 4.5. A second oxidation peak, corresponding to the oxidation of U(IV), was identified and provides evidence for the formation of reduced uranium phase(s) on the mineral surfaces. In addition, uranium-containing precipitates were identified on pyrite surfaces after polarization in a PME. This study identifies the disproportionation of U(V) species on the surface as a possible rate-limiting step in the two-step U(VI) reduction mechanism: (1) charge transfer to form U(V) followed by, (2) a disproportionation reaction that forms U(IV) and U(VI). The Tafel slope (i.e., the derivative of the electrode potential with respect to log[current]) was used to evaluate electrochemical mechanisms. High Tafel slopes (>220mV/(logunit of current) on all minerals evaluated) suggest that uranyl reduction is mediated by insulating (hydr)oxide layers that are present on the semiconducting mineral surfaces. The onset potential for uranyl reduction was determined for pyrite (>+0.1V vs. Ag/AgCl), and hematite and magnetite (between-0.02 and-0.1V vs. Ag/AgCl). The onset potential values establish a baseline kinetic parameter that can be used to evaluate how solution conditions (e.g., dissolved reductants, complexing ligands, and polarizing ions) affect the kinetics of uranyl reduction.The results of this study demonstrate the potential for using PMEs to evaluate redox potentials and mechanisms for U(VI) reduction by Fe-oxides and sulfides under more complex solution conditions as well as other environmentally-relevant mineral-analyte systems. However, it should be noted that the determination of redox kinetics using Butler-Volmer theory has limitations when applied to semiconductor mineral electrodes. Charge depletion in semiconductor surface states can affect the kinetic values obtained for redox reactions on the surface. These limitations and a discussion of the flat band potential are considered in the interpretation of U redox kinetics in this study.