Bacteria Found That Can Clean Up PCBs Without Dredging
ATLANTA, Georgia, May 1, 2007 (ENS) - Researchers have identified a group of bacteria that can detoxify a common type of polychlorinated biphenyls, PCBs. These carcinogenic compounds, once used as coolants and lubricants, have contaminated more than 250 U.S. sites, including lake and river sediments.
The discovery is a first step toward a bioremediation strategy that would naturally detoxify the PCBs without risky removal of the sediments in which they persist.
Development of bioremediation technologies for PCB cleanup would offer an alternative to sediment dredging and disposal in landfills, which is the most commonly used method for removing PCBs used today. Dredging is controversial because of the invasive nature of this technology and the risk of spreading contaminants.
Researchers have known for more than 20 years that naturally occurring microorganisms could slowly dechlorinate PCBs, which were once commonly used by industry. The compounds were banned from production in the United States in 1977 because of their toxicity to humans and animals.
In research funded by the National Science Foundation and General Electric, PCB expert Donna Bedard, a biology professor at Renssalaer Polytechnic Institute, analyzed sediments from the Housatonic River in Massachusetts.
Bedard collaborated with microbiologists at the Georgia Institute of Technology to study microbial degradation in Aroclor 1260, a common, highly chlorinated PCB mixture.
Working with sediment samples from the Housatonic, the team was able to determine that bacteria in the Dehalococcoides, Dhc, group were responsible for the dechlorination of Aroclor 1260.
These microbes replace the chlorine atoms in Aroclor 1260 with hydrogen, which fuels their growth and initiates the PCB degradation process, explained Frank Loeffler, an associate professor in the Georgia Tech School of Civil and Environmental Engineering and the School of Biology.
Once Dhc bacteria dechlorinate Aroclor 1260 to a certain level, other microbes will degrade it further and completely detoxify PCBs, Loeffler said.
"Identifying the bacteria responsible for Aroclor degradation represents a crucial step. Now we can start to design tools to look for these microbes in sediments and then develop engineering approaches to stimulate their growth and activity in river or lake sediments," Loeffler said.
"Then the decontamination will occur more rapidly. Instead of taking decades, the microbes might be able to degrade the PCBs in a few years," he said.
Loeffler is optimistic about a bioremediation strategy for PCBs because of his lab’s earlier success in identifying microbes that degrade the common solvents tetrachloroethene, PCE, and trichloroethene, TCE. These toxic compounds, which contaminated subsurface environments and groundwater decades ago when their use was unregulated, are used in dry cleaning operations and degreasing of metal components.
After Loeffler’s discovery, it took less than five years for scientists and engineers to develop and implement bioremediation strategies that use these microbes to detoxify PCE and TCE.
"The situation with PCBs is a little more complicated because they are in river and lake sediments instead of groundwater and subsurface environments, but in principle, the same sequence of events could occur," Loeffler said. "We need industry, engineers and scientists to work together to develop a bioremediation approach for PCBs."
Loeffler predicts that bioremediation technologies for addressing PCB detoxification will be developed first for lakes, such as Lake Hartwell in South Carolina. Then it will be refined to clean up river sediments, where the flow rate is greater and bioremediation may be more difficult to implement.
Polychlorinated biphenyls are mixtures of up to 209 individual chlorinated compounds.
PCBs have been used as coolants and lubricants in transformers, capacitors, and other electrical equipment because they do not burn easily and are good insulators.
The manufacture of PCBs was stopped in the United States in 1977 because of evidence they build up in the environment and can cause harmful health effects. Old fluorescent lighting fixtures and electrical devices and old microscope and hydraulic oils may still contain PCBs.
PCBs entered the air, water, and soil during their manufacture, use, and disposal; from accidental spills and leaks during their transport; and from leaks or fires in products containing PCBs.
These compounds do not readily break down in the environment. They can travel long distances in the air and be deposited in areas far away from where they were released. On land, PCBs bind strongly to soil.
In water, a small amount of PCBs may remain dissolved, but most stick to organic particles and bottom sediments.
PCBs are taken up by small organisms and fish in water. They are also taken up by other animals that eat these aquatic animals as food. PCBs accumulate in fish and marine mammals, reaching levels that may be many thousands of times higher than in water.
The main dietary sources of PCBs are fish, especially sportfish caught in contaminated lakes or rivers, meat, and dairy products.
The Department of Health and Human Services has concluded that PCBs may reasonably be anticipated to be carcinogens. The U.S. Environmental Protection Agency and the International Agency for Research on Cancer have determined that PCBs are probably carcinogenic to humans.