Cell Density Dependent Reduction Kinetics of Hexavalent Uranium by Shewanella oneidensis Lisa Mullen † , Vanja Klepac-Ceraj * , Chanathip Pharino * , Ken Czerwinski † , Martin Polz* † Department of Nuclear Engineering, * Department of Civil and Environmental Engineering, Massachusetts Institute of Technology Abstract Shewanella oneidensis is a widely distributed species of bacteria and is known to utilize several elements such as iron, manganese and sulfur as electron acceptors. In an anoxic environment lacking more electrochemically favourable electron acceptors S. oneidensis is shown to reduce uranium, changing its oxidation state from hexavalent to tetravalent, by the following reaction: H 2 + UO 2 2+ → 2H + + UO 2 . The uranyl solution concentration (U(VI)) was measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES), and the reduction data were fit to first order. Several cell concentrations were examined and both the rate of uranyl reduction and the total amount of uranyl reduced are found to be dependent upon cell density. The largest rate constant was 0.7 hr -1 corresponding to a cell density of 2.4*10 9 cells/mL and an initial reduction rate of 1414 µM/hr. A cell concentration of 6.6*10 8 cells/mL gave rise to an initial reduction rate of 400µM U(VI) per hour, and had, within a period of 72 hours approximately 98% of the original 2 mM uranyl acetate reduced, as opposed to only 87% for 2.4*10 9 cells/mL. Introduction Mining of uranium ores involves solublizing any uranium present in crushed rock removed from a mine and leaching it out under acidic conditions. Only one to five pounds of uranium can be extracted from every ton of ore, and up until the late 1970’s excess tailings could be discarded or stored without governmental regulation. This resulted in the contamination of 24 sites in the United States [1] and most likely many more throughout the world. Cleanup efforts at these sites have consisted mainly of removal of contaminated soil from the site or securing the soil onsite to prevent any further spread of uranium; both of which are often costly and large- scale processes [1]. Bioremediation of soils contaminated with other heavy metals such as chromium, and mercury [2] have been shown in many cases to be a useful and more efficient process than traditional chemical or electric methods of treatment. Bioremediation often involves a microbially facilitated change in oxidation state of the target metal to reduce either the mobility or toxicity of the element. In the case of chromium, for example, the soluble and carcinogenic Cr(VI) can be reduced by bacteria to Cr(III), which is not only less mobile in the soil and ground water, but also much less toxic to animals [3]. Exploitation of a naturally occurring process for cleanup of metal contaminated soils generally causes less stress on the environment than other methods and requires less human management. The metal reducing soil bacterium S. oneidensis is known to metabolize several different metals including uranium [4]. In an anoxic environment S. oneidensis will reduce soluble uranium(VI) to uranium(IV), which is insoluble at a wide range of pH. Uranium in its insoluble form is much less mobile and reactive in soil and groundwater and microbially mediated reduction of uranium would help to sequester uranium at the site of contamination. It has been II3.12.1 II3.12.1 Mat. Res. Soc. Symp. Proc. Vol. 757 © 2003 Materials Research Society