The effects of uranium speciation on the rate of U(VI) reduction by Shewanella oneidensis MR-1

Abstract

We measured the kinetics of U(VI) reduction by Shewanella oneidensis MR-1 under anaerobic conditions in the presence of variable concentrations of either EDTA or dissolved Ca. We measured both total dissolved U and U(VI) concentrations in solution as a function of time. In separate experiments, we also measured the extent of U(VI) adsorption onto S. oneidensis in order to quantify the thermodynamic stabilities of the important U(VI)–bacterial surface complexes. In the EDTA experiments, the rate of U(IV) production increased with increasing EDTA concentration. However, the total dissolved U concentrations remained constant and identical to the initial U concentrations during the course of the experiments for all EDTA-bearing systems. Additionally, the U(VI) reduction rate in the EDTA experiments exhibited a strong correlation to the concentration of the aqueous U 4+–EDTA complex. We conclude that the U(VI) reduction rate increases with increasing EDTA concentration, likely due to U 4+–EDTA aqueous complexation which removes U(IV) from the cell surface and prevents UO 2 precipitation. In the Ca experiments, the U(VI) reduction rate decreased as Ca concentration increased. Our thermodynamic modeling results based on the U(VI) adsorption data demonstrate that U(VI) was adsorbed onto the bacterial surface in the form of a Ca–uranyl-carbonate complex in addition to a number of other Ca–free uranyl complexes. The observed U(VI) reduction rates in the presence of Ca exhibit a strong negative correlation to the concentration of the Ca–uranyl-carbonate bacterial surface complex, but a strong positive correlation to the total concentration of all the other Ca–free uranyl surface complexes. Thus, the concentration of these Ca–free uranyl surface complexes appears to control the rate of U(VI) reduction by S. oneidensis in the presence of dissolved Ca. Our results demonstrate that U speciation, both of U(VI) before reduction and of U(IV) after reduction, affects the reduction kinetics, and that thermodynamic modeling of the U speciation may be useful in the prediction of reduction kinetics in realistic geologic settings.