Speciation-Dependent Kinetics of Uranium(VI) Bioreduction

Abstract

The kinetics of uranium(VI) reduction by Shewanella oneidensis strain MR-1 was studied for varied pH and concentrations of dissolved inorganic carbon (DIC) and calcium. These are key variables affecting U(VI) speciation in aqueous systems. For all conditions studied, a nearly log-linear decrease of [U(VI)] suggested pseudo–first-order kinetics with respect to U(VI). The reduction rate constants (k) decreased with increasing DIC and calcium concentration, and were sensitive to pH. A positive correlation was found between k and the logarithm of the total concentration of U(VI)-hydroxyl and U(VI)-organic complexes. Linear correlations of the rate constant with the redox potential (E_H) of U(VI) reduction and the associated Gibbs free energy of reaction (ΔG_r) were found for both Ca-free and Ca-containing systems. Both E_H and ΔG_r are strong functions of aqueous U(VI) speciation. Because the range in ΔG_r among the experimental conditions was small, the differences in k are more likely to be due to differences in E_H or to differences in individual rate constants of U(VI) species. Calculation of conditional reduction rate constants for the major groups of U(VI) complexes revealed highest constants for the combined groups of U(VI)-hydroxyl and U(VI)-organic species, lower rate constants for the U(VI)-carbonate group, and much lower constants for the Ca-U(VI)-carbonate group. Mechanistic explanations for these findings are discussed.

Keywords:

• uranium(VI)
• aqueous speciation
• bioreduction
• reduction kinetics
• Shewanella oneidensis MR-1

Notes

*Pitzer ionic interaction parameters used by Moore et al. (1999).