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 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
phyto-remediation, 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
cleanup 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 cleanup treatment.
Banks' graduate
student Tom Spriggs, from Bargersville, IN, 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 phyto-remediation.
"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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