Modeling the Inhibition of the Bacterial Reduction of U(VI) by β-MnO2(s)

Authors

Chongxuan Liu, Pacific Northwest National LaboratoryFollow
John M. Zachara, Pacific Northwest National LaboratoryFollow
James K. Fredrickson, Pacific Northwest National LaboratoryFollow
David W. Kennedy, Pacific Northwest National LaboratoryFollow
Alice Dohnalkova, Pacific Northwest National Laboratory

Date of this Version

2002

Citation

Environ. Sci. Technol. 2002, 36, 1452-1459

Abstract

Pyrolusite (β-MnO_2(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-MnO_2(s) and the abiotic redox reaction between β-MnO_2(s) and biogenic uraninite (UO_2(s)) were independently investigated to allow for interpretation of studies of U(VI) bioreduction in the presence of β-MnO_2(s). Uranyl bioreduction to UO_2(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-MnO_2(s) by CN32 was described by a pseudo-first-order model with respect to β-MnO_2(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-MnO_2(s), with concomitant reduction to Mn(II). The oxidation of biogenic UO_2(s) coupled with β-MnO_{2(s) reduction} was well-described by an electrochemical model. However, a simple model that coupled the bacterial reduction of U(VI) and β-MnO_2(s) with an abiotic redox reaction between UO_2(s) and β-MnO_2(s) failed to describe the mass loss of U(VI) in the presence of β-MnO_2(s). Transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) revealed that the particle size and spatial distribution of the biogenic UO_2(s) changed dynamically in systems with, as compared to without, β-MnO_2(s). These observations suggested that the surface properties and localization of UO_2(s) in relation to the cell and β-MnO_{2(s) surfaces} was an important factor controlling the abiotic oxidation of UO_2(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 UO_2(s) and β-MnO_{2(s) significantly} improved the simulation of microbial reduction of U(VI) in the presence of β-MnO_2(s).