INEEL And WSU Researchers Coax Bacteria To Clean Up Toxic Chromium

October 08, 1998

Idaho Falls, ID -- A team of researchers has tricked bacteria from contaminated soil into cleaning up the toxic heavy metal chromium. The Department of Energy has awarded the collaborators funds to study the basic science behind this bacterial conversion. Eventually, the researchers hope to optimize the microbial action at contaminated sites simply by adding the appropriate nutrients.

James Petersen, chemical engineering professor and director of the Center for Multiphase Environmental Research at Washington State University, and scientific fellow William Apel, at the Idaho National Engineering and Environmental Laboratory, received a three-year grant of $767,000 from the Natural and Accelerated Bioremediation Research (NABIR) program of the DOE.

Established last year, NABIR recognizes that existing bioremediation technology is insufficient to clean up many of DOE's contaminated sites. The program supports basic research into how biological systems can be used to remediate hazardous and radioactive contaminants in soils and groundwater.

Chromium is a heavy metal naturally found in several different forms. Hexavalent chromium, often in a form called chromate, is toxic even at low concentrations. At high concentrations, it can cause cancer and damage DNA. Chromate found in the environment is generated almost exclusively by human activities-it is left over from mining operations, agricultural procedures, and oil refining. It is a contaminant in almost one third of Superfund sites. Trivalent chromium, on the other hand, is a benign form of the metal that is actually an essential element for good nutrition.

In previous research funded by the INEEL University Research Consortium, Petersen and biochemistry colleagues at WSU, in collaboration with microbiologists led by Apel at INEEL, demonstrated that bacteria normally found in contaminated soils are capable of converting the hexavalent chromium to trivalent chromium by a chemical process called reduction. However, a major block to this process is the microbe's preference for reducing nitrate rather than chromate.

Nitrate is in great supply in the microbe's natural environment. The ultimate goal of this bioremediation approach is to induce resident bacteria to clean up toxic chromium within the contaminated settings. Therefore, the researchers needed to find conditions under which chromate is reduced instead.

Bringing bacteria from several contaminated sites-including DOE's Superfund-listed Hanford site in southeastern Washington state-to the lab, the researchers found feeding conditions under which bacteria would reduce chromate to nearly undetectable levels. "We've converted a carcinogenic, hazardous compound to a non-hazardous one," Petersen said. The researchers suspect that, under certain growth conditions, the bacteria commandeer an enzyme with a different function.

"Petersen has been playing a trick on the bacteria, in a way," said Apel. "The bacterial enzymes are promiscuous." "No enzyme has evolved to use chromium," said Petersen. He thinks the bacteria might be using the same enzyme they use to reduce nitrate to reduce chromium. "We are trying to determine what are the best sugars and nutrients to get chromium reduced, to determine what effect other contaminating species (such as uranium and technetium) have on chromium reduction and what the relative concentrations of chromate and nitrate do to the amount of reduction," said Petersen.

The team will also study the biochemical pathway that the bacteria use to convert hexavalent chromium to trivalent chromium. And they plan to demonstrate the ability of resident microbes to decontaminate heavy metals using bacteria-containing soil obtained from contaminated DOE sites, including the Hanford site. The three-year project started in September of this year.

DOE/Idaho National Laboratory

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