BioCycle August 2005, Vol. 46, No. 8, p. 30
Purdue University researchers develop new approaches to use phytoremediation to degrade hydrocarbons and decontaminate dredged sediments.
Jennifer Cutraro

IMAGINE driving past a field that glows with a fluorescent green light as pollutants from a nearby factory leak into the ground. Or planting a crop that’s harvested not as food, but for its ability to extract contaminants from the soil and store them in its shoots and leaves. Sound far-fetched? Perhaps, but a multidisciplinary team of Purdue University researchers is getting closer to making these and other innovative methods of cleaning up the environment a reality.
Plants that glow and crops that clean are just two of the many applications of research in phytoremediation – the use of plants to clean up hazardous compounds. At Purdue, a team of plant physiologists, molecular biologists, soil scientists and engineers is identifying the genes responsible for key steps in phytoremediation and developing plants to use in remediating different types of contaminants around the world. This research holds the promise of not only cleaning up polluted soils but also developing new technologies to detect soil and water contamination.
HARVESTING POLLUTANTS
Phytoremediation is a far less destructive method of environmental remediation than many traditional technologies, says David Salt, a Purdue plant molecular physiologist. “Up until now, the two basic methods of remediation were either to dig up the contaminated soil and haul it away to a hazardous waste landfill or to leach that contamination out by applying chemicals to the soil,” he says. “Both methods are very expensive. With the first option, you’re left with no soil, while the other method leaves you with something that barely resembles soil.” By harnessing the natural ability that some plants have to take up metals, contamination can be removed while keeping the soil in place – without the use of harsh chemicals that render the soil unsuitable for future use.
Salt studies plants collectively known as metal hyperaccumulators, which take up and store amounts of metals in their shoots and leaves that would kill most other plants. Only a handful of plants found in nature have this unique ability. Salt not only studies how these plants hyperaccumulate metals but also how to engineer other plants so that they may one day remove metal contaminants from the soil as well.
“We want to understand how these natural accumulators work at the molecular level, and then use that information to genetically engineer plants that would be ideal for phytoremediation,” he says. Ultimately, they could be planted on polluted sites, grown for a season, then harvested like any other crop – the difference being that these plants would be full of metals that could later be extracted from the leaves and stems and possibly even put into commercial use. “It’s like farming metal out of the soil,” he says.
Addressing the kinds of basic research questions behind phytoremediation has led to applications beyond cleaning up polluted fields. Salt also is developing indicator plants, which change their appearance when they detect metals in the environment. He envisions a day when factories could establish sensor plants around their perimeter to set off a visual alarm should pollutants leach into the soil or water.
A KNOCKOUT PUNCH
Metals are only part of the problem – organic compounds make up another class of pollutants. Some of the most ubiquitous organic contaminants, known as polycyclic aromatic hydrocarbons, or PAHs, are by-products of oil refining or gasoline combustion and are a problem nearly everywhere in the world.
“It’s hard to find a place without some degree of PAH contamination,” says Purdue agronomist Paul Schwab. “PAHs are a combustion product – when we burn gasoline, they come right out of the tailpipe. They get into the air, they fall to the ground and they find their way into the soil and watersheds.”
Surprisingly, it turns out that PAHs make ideal candidates for phytoremediation. “If you put appropriate plants in the soil, and they establish a good root system, they’ll remove the petroleum contamination by degrading it. Even the PAHs can be degraded in this way,” he says.
Schwab and Kathy Banks, professor of civil engineering, remediate field sites contaminated with organic pollutants. Their combined expertise in the fields of soil chemistry and microbial degradation of organic compounds has been a central component of their success in cleaning up contaminated sites all over the country, from an oil pipeline spill in Texas to PAH-contaminated groundwater in southern Indiana.
Their latest research project uses plants to decontaminate dredged sediments that have been removed from Wisconsin’s Milwaukee Harbor and stored in a large containment facility. “The idea is, rather than leave the sediments in storage indefinitely, it’s better to remove the water, decontaminate the sediments and then turn them into something useful, like fill material for construction or landscaping,” Schwab says.
Schwab and Banks are currently studying methods for establishing plants in these sediments. Ultimately, they hope to create a wetland that will first remove the water from the sediments and then will begin the process of breaking down the organic compounds.
Unlike metals, some organic contaminants are not taken up and stored by plants. In fact, it’s not the plant that degrades organic compounds in the soil, but the microbes associated with the plant’s roots. “Plants are the means by which we can deliver the microorganisms to the contaminants,” says Schwab. “Roots penetrate into parts of the soil that the microbes can’t access by themselves. Once the roots move downward, the microbes move in right along with them. That’s how phytoremediation works for the organic materials we’ve studied.”
Microbes degrade the organic contaminants, using the carbon as a source of energy and breaking the organics into smaller, less toxic compounds. “Plants and their associated microbes change the whole structure of the contaminated soil,” Banks explains. “We can take sludge from an industrial site, put a plant in it and, after 12 months, the sludge will resemble normal soil in almost every way.”
ONE PLANT DOESN’T FIT ALL
The number of variables at work in a contaminated site means that one type of plant won’t be sufficient for every remediation project. “The ideal plant varies, depending on the contaminant and what type of resource is contaminated,” Banks says. For example, a groundwater contaminant is best cleaned up using plants with a deep taproot system, whereas plants with a branching root system are better at cleaning up contaminants that attach to the soil.
“Polluted soils exist everywhere in the world, so we’re going to need to develop a whole battery of crops,” Salt explains. “Depending on which part of the world you’re in, you’ll need different plants to suit different growing conditions.”
Jennifer Cutraro is a science writer at Purdue University in West Lafayette, Indiana. She may be contacted via e-mail at jenny@nasw.org.
VENTURES IN PHYTOREMEDIATION
THE FIELD of phytoremediation is being advanced by companies putting the concepts and research into action. Ecolotree, based in North Liberty, Iowa, has developed engineered tree systems to remove and contain pollutants. Its engineered forests have been installed over the past 15 years at landfills, industrial facilities, gas stations and fertilizer storage areas. The ECap is a crop system that reduces water percolation through the subsurface, e.g., of landfills or contaminated sites, thus containing contaminants from moving into groundwater. It consists of specially prepared soils planted with fast-growing, deep-rooting trees and understory grasses. Soil pores hold precipitation until plant roots can access the water. Ecolotree’s EBuffer uses poplar trees and understory grasses to filter sediments and pollutants from groundwater, surface water and irrigation water. The buffers can be installed in the form of a plantation, or placed as the final filter at streamside or around a site perimeter. The trees also can be managed for biomass yield and harvested for their energy value. According to the Ecolotree website (www.ecolotree.com), the company has installed over 60 caps and buffers around the country.
Edenspace Systems Corporation, based in Dulles, Virginia, was founded in 1998. Its phytoremediation products and services use plants to detect, concentrate and remove lead, arsenic, radionuclides, chlorides, hydrocarbons and other minerals in water and soil. It also focuses on plants with traits that improve yields of renewable energy sources such as ethanol. The company’s patents relate to plant biosensing, phytoextraction, hyperaccumulation and rhizofiltration. Edenspace recently announced an expansion of a project it is doing with the Army Corps of Engineers to remove soil arsenic from residential properties in a neighborhood in Washington, D.C. In 2004, about 2,800 of the company’s patented fern plants were installed in 14 test plots at three different sites. According to information on Edenspace’s website (www.edenspace.com), the ferns removed an average of 9 parts/million of soil arsenic across all sites from starting concentrations that ranged from 16 to 127 ppm. The new project scope will use about 10,000 ferns to be planted on up to 35 plots at 14 residential sites. -N. G.