In research published in the journal Environmental Microbiology, Laboratory chemist Mary Neu and her colleagues describe their study of different naturally occurring bacteria used to treat actinide contamination. Actinides are the elements above atomic number 89 and are usually radioactive. The study's results indicate that actinide toxicity is primarily chemical, rather than radiological, and so a bacteria's resistance to radiation does not necessarily ensure a tolerance for radionuclides. In fact, the bacteria's worst enemy in a nuclear waste site may not be the radioactive elements, but rather, the other toxic metals that might also be found at the site.
The study also shows that contrary to the conventional wisdom, when it comes to these environmental bacteria, plutonium is less toxic than uranium and, in general, actinides are less toxic than other types of metals. This suggests that actinide toxicity will not impede bioremediation using naturally occurring bacteria.
"This study" said Neu, "is exciting because even though we've known for years that bacterial bioremediation can be a preferred method for cleaning up actinide contamination, we've never really known whether or not radioactivity or chemical toxicity will stifle the process. Our study found that actinides are chemically toxic to bacteria only at high levels far, far above concentrations at contaminated sites, and that common toxic metals, such as cadmium, nickel, and chromium, are mo re likely to cause problems for the bacteria."
Generally, bacteria used for bioremediation are selected to target a specific form and oxidation state of toxic pollutants, such as soluble hexavalent uranium carbonate for uranium contamination. However, a single chemical rarely contaminates soils and groundwater and combinations of actinides, radionuclides, organic chemicals and metal regularly exist at many nuclear sites. Based on the results of this study, if bioremediation is to be effective at these types of sites, the operative microorganisms must be able to grow, function and do better than other bacteria in the presence of all kinds of contaminants.
The study examined the effects of toxicity of actinides, metals and chelators on different bacteria being evaluated for radionuclide bioremediation, Deinococcus radiodurans and Pseudomonas putida, along with the toxicity of plutonium on the bacteria Shewanella putrefaciens.
In addition to Neu, the bacteria bioremediation study team includes Christy Ruggiero and Hakim Boukhalfa of the Chemistry Division, and Joseph Lack and Larry Hersman from the Laboratory's Bioscience Division.