Bioreduction of Uranium:  Environmental Implications of a Pentavalent Intermediate

Abstract

The release of uranium and other transuranics into the environment, and their subsequent mobility, are subjects of intense public concern. Uranium dominates the inventory of most medium- and low-level radioactive waste sites and under oxic conditions is highly mobile as U(VI), the soluble uranyl dioxocation (UO2)2+. Specialist anaerobic bacteria are, however, able to reduce U(VI)to insoluble U(IV), offering a strategy for the bioremediation of uranium-contaminated groundwater and a potential mechanism for the biodeposition of uranium ores. Despite the environmental importance of U(VI) bioreduction, there is little information on the mechanism of this transformation. In the course of this study we used X-ray absorption spectroscopy (XAS) to show that the subsurface metal-reducing bacterium Geobacter sulfurreducens reduces U(VI) by a one-electron reduction, forming an unstable (UO2)+ species. The final, insoluble U(IV) product could be formed either through further reduction of U(V) or through its disproportionation. When G. sulfurreducens was challenged with the chemically analogous (NpO2)+, which is stable with respect to disproportionation, it was not reduced, suggesting that it is disproportionation of U(V) which leads to the U(IV) product. This surprising discrimination between U and Np illustrates the need for mechanistic understanding and care in devising in situ bioremediation strategies for complex wastes containing other redox-active actinides, including plutonium.

Mechanism of uranium removal from the aqueous solution by elemental iron

“…[64,65] With this study, the potential of bulk reactions with selected additives for providing mechanistic information on aqueous contaminant removal has been demonstrated. This applicable technique can be very useful to obtain qualitative information on Fe 0 removal mechanisms for several contaminants; particularly for other radionuclides, metals and some organics which are known to interact with iron oxide.…”

“…At Bodo Canyon Site (Durango, Colorado) uranium was present nearly exclusively as U(VI). [64,65] Another important fact in favor of U(VI) co-precipitation with aging corrosion products is given by Morrison et al [22] , who conducted laboratory column experiments under anoxic conditions and could not accurately model their results according to reductive precipitation or U(VI) adsorption onto Fe(III) oxides. They stated that "while a reductive precipitation mechanism is not inconsistent with the results of the laboratory column experiment, the decrease in Eh in the bulk is suspect.…”

Biogenic Uraninite Nanoparticles and Their Importance for Uranium Remediation

“…Subsequent studies have also reported biogenic uraninite particle sizes in the 2 to 10 nm range and slight but significant contraction of the first U-U shell (Renshaw et al 2005;Singer et al 2006;Senko et al 2007;Burgos et al 2008;Komlos et al 2008). Similar results have been reported for uraninite nanoparticles produced by abiotic reduction of U(VI) by green rust (O'Loughlin et al 2003).…”

“…U(V) is unstable as an aqueous complex (Nagaishi et al 1996), and it is therefore possible that enzymatic reduction proceeds from U(VI) to U(V), followed by disproportionation to U(IV) and U(VI). Tentative evidence in support of this mechanism has been provided by Renshaw et al (2005) and Grossmann et al (2007), but further work is required to unambiguously demonstrate the relevance of this reduction pathway.…”

Uranium(VI) reduction and removal by high performing purified anaerobic cultures from mine soil

“…Natural attenuation processes such as bacterial reductive/precipitation and immobilization of soluble uranium is gaining much interest (Dodge and Francis, 2008). For example, dissimilatory metal-reducing microorganisms have been investigated for their capability to selectively remove uranium from aqueous solutions (Lovley et al, 1992;Renshaw et al, 2005). These bacteria can use U(VI) as an electron acceptor thereby reducing soluble U(VI) to the precipitable tetravalent state [U(IV)] (Lovley et al, 1992;Lloyd and Renshaw, 2005;.…”