Stabilization of Plutonium in Subsurface Environments via Microbial Reduction and Biofilm Formation

Plutonium has a long half-life (2.4 x 104 years) and is of concern because of its chemical and radiological toxicity, high-energy alpha radioactive decay. A full understanding of its speciation and interactions with environmental processes is required in order to predict, contain, or remediate contaminated sites. Under aerobic conditions Pu is sparingly soluble, existing primarily in its tetravalent oxidation state. To the extent that pentavalent and hexavalent complexes and small colloidal species form they will increase the solubility and resultant mobility from contamination sources. There is evidence that in both marine environments and brines substantial fractions of the plutonium in solution is present as hexavalent plutonyl, PuO2 2+

Plutonium(IV) Reduction by the Metal-Reducing Bacteria Geobacter metallireducens GS15 and Shewanella oneidensis MR1▿

The bacterial reduction of actinides has been suggested as a possible remedial strategy for actinide-contaminated environments, and the bacterial reduction of Pu(VI/V) has the potential to produce highly insoluble Pu(IV) solid phases. However, the behavior of plutonium with regard to bacterial reduction is more complex than for other actinides because it is possible for Pu(IV) to be further reduced to Pu(III), which is relatively more soluble than Pu(IV). This work investigates the ability of the metal-reducing bacteria Geobacter metallireducens GS15 and Shewanella oneidensis MR1 to enzymatically reduce freshly precipitated amorphous Pu(IV) (OH)4 [Pu(IV)(OH)4(am)] and soluble Pu(IV)(EDTA). In cell suspensions without added complexing ligands, minor Pu(III) production was observed in cultures containing S. oneidensis, but little or no Pu(III) production was observed in cultures containing G. metallireducens. In the presence of EDTA, most of the Pu(IV)(OH)4(am) present was reduced to Pu(III) and remained soluble in cell suspensions of both S. oneidensis and G. metallireducens. When soluble Pu(IV)(EDTA) was provided as the terminal electron acceptor, cell suspensions of both S. oneidensis and G. metallireducens rapidly reduced Pu(IV)(EDTA) to Pu(III)(EDTA) with nearly complete reduction within 20 to 40 min, depending on the initial concentration. Neither bacterium was able to use Pu(IV) (in any of the forms used) as a terminal electron acceptor to support growth. These results have significant implications for the potential remediation of plutonium and suggest that strongly reducing environments where complexing ligands are present may produce soluble forms of reduced Pu species

Plutonium(V/VI) Reduction by the Metal-Reducing Bacteria Geobacter metallireducens GS-15 and Shewanella oneidensis MR-1▿

We examined the ability of the metal-reducing bacteria Geobacter metallireducens GS-15 and Shewanella oneidensis MR-1 to reduce Pu(VI) and Pu(V). Cell suspensions of both bacteria reduced oxidized Pu [a mixture of Pu(VI) and Pu(V)] to Pu(IV). The rate of plutonium reduction was similar to the rate of U(VI) reduction obtained under similar conditions for each bacteria. The rates of Pu(VI) and U(VI) reduction by cell suspensions of S. oneidensis were slightly higher than the rates observed with G. metallireducens. The reduced form of Pu was characterized as aggregates of nanoparticulates of Pu(IV). Transmission electron microscopy images of the solids obtained from the cultures after the reduction of Pu(VI) and Pu(V) by S. oneidensis show that the Pu precipitates have a crystalline structure. The nanoparticulates of Pu(IV) were precipitated on the surface of or within the cell walls of the bacteria. The production of Pu(III) was not observed, which indicates that Pu(IV) was the stable form of reduced Pu under these experimental conditions. Experiments examining the ability of these bacteria to use Pu(VI) as a terminal electron acceptor for growth were inconclusive. A slight increase in cell density was observed for both G. metallireducens and S. oneidensis when Pu(VI) was provided as the sole electron acceptor; however, Pu(VI) concentrations decreased similarly in both the experimental and control cultures