July 5, 2000 WEST LAFAYETTE, Ind. – A husband and wife research team at Purdue University has pioneered the use of plants to help clean up soil contaminated with petroleum products. The Environmental Protection Agency and industry researchers already use methods developed by the Purdue team at several petroleum spill sites across the nation.
"We're using natural methods to clean up soil pollutants," says Katherine Banks, a Purdue professor of civil engineering. She and her agronomy professor husband, Paul Schwab, were one of the first research teams to develop methods for field-testing phytoremediation, the use of plants to clean up contaminated soil.
Banks says her expertise solving hazardous waste problems combined with Schwab's strong plant and soils background is a marriage that makes phytoremediation work. Their son complains that they always talk about work at home, "which makes us better researchers, although it makes us a little narrow in some ways," Banks says.
Banks and Schwab have used plants to help clean up a Texas oil pipeline spill, contamination at an Indiana manufactured-gas plant, an industrial sludge site in California and diesel spills on Naval bases in Virginia and California. They describe their work in "Bioremediation of Contaminated Soils," a book published last winter by the American Society of Agronomy.
"Soil microbes are actually the ones that break down the petroleum contaminants," Schwab says, "but the plants accelerate the microbes’ action in the soil. They stimulate microbes to degrade contaminants by getting more oxygen into the soil and by supplying nutrients through their roots."
Other biological clean-up methods can do the job faster, but phytoremediation costs much less and leaves the soil structure intact, Schwab says.
"With standard methods you have to dig the soil out and then incinerate, compost or landfill the contaminated material," Schwab says. "Using phytoremediation, we can treat the soil at the spot where a spill occurred."
Near Bedford, Ind., Banks and Schwab are working with the EPA and Indiana Gas Co. to compare the efficiency of several bioremediation methods at a contaminated site at a coal-to-natural gas refinery. Their joint efforts have become a demonstration project for natural gas manufacturers nationwide.
At Bedford, the Purdue researchers have planted grasses and poplar trees on one part of the site to hasten the degradation of the soil pollutants. The EPA is treating other parts of the site by composting soil, land farming (adding nutrients to soil with tillage), or letting natural processes work to degrade the contaminants. During the next few years both EPA and Purdue will compare the cost and speed of each clean-up treatment.
Banks' graduate student Tom Spriggs, from Bargersville, Ind., oversees the Bedford phytoremediation project. The work will become the basis for his doctoral thesis.
The biggest challenge at any site comes in finding the right plant for the job, Schwab says. Part of the challenge comes in matching plants to climate. For example, plants that thrive in southern Indiana may not make it through hurricanes or the heat of an east Texas summer.
Another part of the problem is finding plants that survive in contaminated areas and at the same time encourage microbial growth. Banks and Schwab have worked with crop scientists to find plants that work best with microbes to break down petroleum. No one yet has a complete list of the best plants for the job, Banks says, although researchers have identified some characteristics that make plants good at phytoremediation.
"For this method to work, we've got to get the roots in contact with the contamination," Banks says. "Sod-forming grasses work well in certain situations, because they have a large root surface in contact with the soil."
In field tests, the researchers found that fescue and Bermuda grass work well. Clovers and alfalfa also look promising in certain situations, because their root systems stimulate microbe growth.
Schwab and Banks met on the job when they both worked as professors at Kansas State University. Banks was working on an EPA project and needed help looking at soil characteristics. A friend told her to call Schwab, and their collaboration began.
They'll continue to focus on clean up of petroleum products, in part because petroleum is one of the major soil contaminants around the world and in part because plants are well-suited to petroleum remediation.
"Petroleum isn't very mobile, it adheres tightly to components of the soil," Schwab says. "Phytoremediation works well with compounds like that because the contaminants stay in the top six feet of the soil and are in direct contact with plant roots."
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