The dirt on phytoremediation.
Phytoremediation uses plants to remove or degrade pollutants in the environment. These days, it's not just the plants growing. David J. Glass, a remediation market analyst, projected the U.S. phytoremediation market would expand more than ten-fold between 1998 and 2005, to over $214 million. Researchers like Schwab are matching plants to pollutants, in an effort to increase phytoremediation's effectiveness. Currently, grasses, poplars, cottonwoods, and other plants are cleaning up heavy metals, chemical solvents, explosives, petroleum hydrocarbons, and pesticides.
Two years after the Texas planting, it's easy to see why phytoremediation is attractive. The St. Augustine and milo, along with a winter planting of rye, reduced contaminants by 75 percent. Meanwhile, a control plot's contaminants declined only 45 percent through "intrinsic bioremediation"--a fancy way of saying that the soil's natural microbes did their thing.
In this case, the initial oil spill clean-up had left the farmer with polynuclear aromatic hydrocarbons (PAHs), heavy petroleum compounds that are considered carcinogenic. "The PAHs are rings of carbon that are fused together, and without help such as phytoremediation, many of these compounds can persist for decades," says Schwab. "If you can break those rings, then you really accelerate the degradation of these compounds, and you also get away from the carcinogenic properties."
So how does a scientist break the rings of microscopic molecules? He recruits microscopic workers to do it for him. "Plants exude carbohydrates, enzymes, water, phosphorous, and nitrogen, creating a wonderful environment for microbes," says Schwab. "Roots explore the soil, and where you have roots, you see an increase in the microbial population of usually 100-fold, or as much as 10,000-fold. Plants create an ideal environment for hydrocarbon degradation."
Unlike phytoextraction of heavy metals like lead--where plants take up and retain contaminants--phytostimulation fuels microbial activity around organic pollutants like PAHs. This type of phytoremediation offers the attractive solution of degrading the contaminants so they actually disappear from the environment.
Numerous PAH-contaminated sites around the country give scientists opportunities to refine their techniques. After producing cars for almost eighty years, a coking operation at the Ford Motor Company's Rouge Manufacturing Complex near Dearborn, Michigan had contaminated the surrounding soil with PAHs. A traditional clean-up method would hire trucks to haul out contaminated, excavated material--an expensive process that's highly intrusive on the land. But the company's environmentally conscious chairman, William Clay Ford, Jr., charged the plant with a mission to redesign itself into a model of sustainable manufacturing practices. Enter Clayton Rugh, an assistant professor of phytoremediation at Michigan State University.
Rugh, along with colleagues at Applied Phyto Genetics and the University of Michigan at Dearborn, is researching the most effective phytoremediation strategies for the Ford Rouge Center. They first screened 55 native plant species, evaluating how well the plants degraded contaminants under greenhouse conditions. They then planted the 22 best contenders in test plots offsite containing contaminated coke oven soil imported from the Rouge complex. Preliminary analysis three months after the initial plantings showed 20 to 40 percent reductions of PAHs in the test plot soils. Rugh reports some plants are helping degrade the pollutants 20 to 50 percent faster than the control plots.
Next year, the most effective plants from the test plots may go into the 30-acre area around the coking plant. "We have to sell this as a more environmentally-friendly remediation option than other methods," says Don Russell, Manufacturing Sustainability Manager at Ford's Environmental Quality Office. It's Russell's hope that the demonstration plot will show the Michigan Department of Environmental Quality--with confirming data--that the strategy will work. The challenge then becomes working under prior clean-up agreements. "We signed a Consent Order to clean up the property within the next four years," says Russell, "We can't really set a schedule for the plants." In other words, can the plants clean up the property under the terms of the agreement and to the Michigan Department of Environmental Quality's satisfaction by 2005? If not, then the company may have to supplement clean-up with more traditional methods.
It's a time crunch, and Ford's not the only one to run into the problem. As Julie Bargmann, a landscape architect at the University of Virginia, puts it, "By the time the landscape architects are onsite, we're usually involved in 'cap and cover' or 'hog and haul' operations." In other words, the site is being capped with clay and covered with a green veneer, or the contaminated material is being excavated and disposed of in a landfill. "Folks want the problem to go away quickly," she says.
Which is the problem a unique community-based phytoremediation project in Hartford, Connecticut ran into. In 1997, a coalition including the Environmental Protection Agency, the Connecticut Department of Environmental Protection, a middle school, a soup kitchen, and the Hartford-based Knox Parks Foundation, gathered to plan a community garden at an abandoned plot near the school and soup kitchen. An agriculture scientist analyzed the soil and found lead--lots of it. It turns out the land had been the former site of a paint store.
Help came from a local college. "A professor at Trinity College heard about the lead problem, and offered to come in and address it as a summer project for students," says Jack Hale, executive director of the Knox Parks Foundation, which sponsors the community garden. The students measured lead levels over 1,000 parts per million (ppm)--twice the EPA'S limit. So they planted Indian mustard grass in the summer of 1999, allowing the grass to take up lead into its foliage in the process called phytoextraction. That fall, they harvested the grass and found high lead levels in the foliage. The students' lead measurements of the remediated area showed lead levels below 500 ppm in eleven of fourteen hot spots. Subsequent measurements by Hartford's brownfields program confirmed the remaining hot spots required further cleanup.
Since the student project was over, Hale asked the company that provided the mustard plants to come in and finish the job. Unfortunately, the company "had bigger fish to fry," says Hale, and wasn't overly interested in their community garden project. So ultimately, the city sent in the bulldozers. A year after the students obtained promising results from Indian mustard plants taking up lead, workers excavated the soil and replaced it.
"Phytoremediation is not a magic solution," cautions Rugh. "It's highly technical. It's not like going down to the local nursery and picking up a bunch of phytoremediation plants. Someday we hope to assemble a comprehensive database of appropriate plant species and cultivars for particular pollutants and regions, but we're not there yet."
So for now, phytoremediation isn't an appropriate match for many polluted sites. "It's not a solution for sites with high levels of pollutants, mixed pollutants, or conditions intolerant of plant growth," says Rugh. "It's also not fast, and if you can afford excavation and treatment or burial, and you need treatment rapidly, then phytoremediation is not for you."
What the Future Holds
Ultimately, Rugh thinks that plants may provide cheaper, long term solutions in areas beyond the most toxic impact of spills or industrial sites. "Very few remediation scenarios are just one action, anyway," he says. "Most of the time phytoremediation will be used with other clean-up actions to reduce the cost and impact of the other methods." For example, extraction might be used to clean up high levels of contaminants, while plants might be used in fringe areas with low to medium levels of contamination.
Several research labs around the country plan to make phytoremediation more effective in the future by engineering plants that better degrade organic pollutants or take up heavy metals, Researchers at the University of Georgia, for example, have introduced two bacterial genes into eastern cottonwood trees, in an effort to clean toxic mercury from the environment. Mercury enters the environment when people burn fossil fuels and discharge mercury-laden industrial waste and municipal sewage. Bacteria then convert it to methylmercury. And methylmercury passes ominously up the food chain, leading to health advisories against eating large Great Lakes fish, for example.
One of the Georgia lab's genes enables trees to convert methylmercury to mercury ions, while a second gene enables the trees to convert the newly produced ionic mercury to mercury vapor, a gas that is naturally present in the atmosphere. Rich Meagher, a professor in the university's genetics department, plans to plant cottonwoods in the field in six months.
While practitioners wait on advances, phytoremediation continues to bloom. "Commercially, phytoremediation is gaining appeal because it's cheaper" than traditional clean-up methods, says Rugh. "And it naturally controls water, so it can stabilize the site, keep the hazardous material from blowing and washing away."
That's just one more reason to consider phytoremediation in clean-up plans. Of course, if it were easy, everyone would already be doing it. Project designers must carefully consider issues like how to choose the proper species, establish plants in highly contaminated environments, and manage the entire system to maximize clean-up. As Schwab suggests, "A little consulting with someone who has experience in the field can go a long way in increasing the chances for success."
Loriee D. Evans is a freelance writer based out of St. Louis, Missouri.
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|Author:||Evans, Loriee D.|
|Publication:||Journal of Soil and Water Conservation|
|Date:||Jan 1, 2002|
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