Date of this Version
Environ. Sci. Technol. 2002, 36, 1452-1459
Pyrolusite (β-MnO2(s)) was used to assess the influence of a competitive electron acceptor on the kinetics of reduction of aqueous uranyl carbonate by a dissimilatory metal-reducing bacterium (DMRB), Shewanella putrefaciens strain CN32. The enzymatic reduction of U(VI) and β-MnO2(s) and the abiotic redox reaction between β-MnO2(s) and biogenic uraninite (UO2(s)) were independently investigated to allow for interpretation of studies of U(VI) bioreduction in the presence of β-MnO2(s). Uranyl bioreduction to UO2(s) by CN32 with H2 as the electron donor followed Monod kinetics, with a maximum specific reduction rate of 110 µM/h/108 cells/mL and a half-saturation constant of 370 µM. The bioreduction rate of β-MnO2(s) by CN32 was described by a pseudo-first-order model with respect to β-MnO2(s) surface sites, with a rate constant of 7.92 x 10-2 h-1/108 cells/mL. Uraninite that precipitated as a result of microbial U(VI) reduction was abiotically reoxidized to U(VI) by β-MnO2(s), with concomitant reduction to Mn(II). The oxidation of biogenic UO2(s) coupled with β-MnO2(s) reduction was well-described by an electrochemical model. However, a simple model that coupled the bacterial reduction of U(VI) and β-MnO2(s) with an abiotic redox reaction between UO2(s) and β-MnO2(s) failed to describe the mass loss of U(VI) in the presence of β-MnO2(s). Transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) revealed that the particle size and spatial distribution of the biogenic UO2(s) changed dynamically in systems with, as compared to without, β-MnO2(s). These observations suggested that the surface properties and localization of UO2(s) in relation to the cell and β-MnO2(s) surfaces was an important factor controlling the abiotic oxidation of UO2(s) and, thus, the overall rate and extent of U(VI) bioreduction. The coupled model that was modified to account for the “effective” contact surface area between UO2(s) and β-MnO2(s) significantly improved the simulation of microbial reduction of U(VI) in the presence of β-MnO2(s).