PCBs, a significant source of environmental pollution, consist of 209 extremely stable, difficult-to-treat compounds prepared by replacing hydrogen atoms in biphenyl, a hydrocarbon compound, with chlorine. Banned from production in 1976, PCBs were once produced as mixtures and used in various industrial applications, printing inks, anti-dusting agents and pesticides. The mixtures were introduced into the environment primarily via industrial discharge. Once in the natural water system, they began to enter the food chain, accumulating in the tissues of plants and animals. This build up has led to accumulation of PCBs in humans, which has been linked to cancer, birth defects and other illnesses. Millions of pounds of PCBs still remain in the environment, particularly in soil, sediments and groundwater.
An ORNL group headed by Dr. K. Thomas Klasson in the Chemical Technology Division has developed an inexpensive remediation method, using natural organisms, to detoxify PCB compounds that contaminate the environment.
"This is an important breakthrough," Klasson says of the results of his research. Previously, researchers have had to mix PCB-contaminated soil with river sediment to cause PCB degradation because sediment is the only known environment in which PCB-degrading bacteria have been found. Klasson's Remediation Technology Group, consisting of both physical scientists and engineers, was able to successfully transfer the bacteria to PCB-contaminated soil without adding any sediment. In addition, Klasson and his colleagues have developed and successfully used a two-stage treatment method called anaerobic-aerobic biodegradation to enhance the removal of these compounds from previously pristine soil.
"The toxicity and persistence of PCB compounds in the environment depend upon the number and location of their chlorine atoms," says Klasson. In the first stage of the treatment method called anaerobic dechlorination, PCB compounds in contaminated soil are converted into less hazardous products by the bacteria which do not require oxygen.
The second stage of the process, aerobic degradation, requires air and converts the less hazardous products obtained during the first stage into carbon dioxide and water. These can be released harmlessly into the environment. Each stage currently requires several months to reach completion, and Klasson's group has achieved a 70-percent total decrease in PCB levels in soil.
"We are proud of our work and are aiming for even better results," says Klasson. The group is now focusing on complete destruction of PCBs.
The work is funded by DOE's Mixed Waste Integrated and Buried Waste Integrated programs, which are part of DOE's Office of Technology Development.
ORNL, one of DOE's multiprogram national research and development facilities, is managed by Lockheed Martin Energy Research Corp.