March/April 1997
Never heard of phytoremediation? It’s a new environmental technology that uses plants to clean up contaminated soil and water. And it could have promising applications in the scrap recycling industry.
By Lynn R. Novelli
Lynn R. Novelli is a writer based in Russel, Ohio.
Will scrap recycling facilities of the 21st century include an acre or two of Indian mustard plants, a grove of poplar trees, or a field of sunflowers?
It’s possible thanks to the growing interest in phytoremediation, a cutting-edge environmental technology that uses plants to clean contaminated soil and water.
Commercial phytoremediation has already been applied successfully to remediate contamination in the chemical, petroleum, and defense industries. Likewise, the scrap recycling industry could find the technology useful and appealing thanks to its relatively low cost, high efficiency, and environmentally friendly profile.
According to Ray Hinchman, a plant physiologist at Argonne National Laboratory (Argonne, Ill.), who has been investigating phytoremediation for three years, “On the right site and under the right conditions, phytoremediation could be an attractive alternative to other methods of contaminant removal.”
A Natural Idea
Phytoremediation is based on the biological principle that plants naturally absorb substances from the soil or groundwater through their roots. These substances include many inorganic elements that are considered environmental contaminants, such as arsenic, lead, cadmium, mercury, and zinc. Certain plants, known as metal hyperaccumulators, are able to absorb unusually high levels of heavy metals in their roots and greenery. Phytoremediation capitalizes on this natural process, sometimes using soil or plant additives to enhance uptake or degradation of the toxins by the plants.
Organic contaminants, such as petroleum products, can similarly be removed from soil or water, using different plant species that are able to metabolize these compounds.
When phytoremediation was first proposed as a remediation technology in 1982, scientists were looking at plants that grew naturally in contaminated soils. While these plants did indeed draw contaminants from soil and water, the plants were so small that remediating a site would have taken 20 years or longer.
Guided by research conducted by Ilya Raskin, a scientist at Rutgers University, interest shifted to introducing crops of hyperaccumulators—plants with well-developed root systems that rapidly grow to a large size—to contaminated sites. Now, some scientists are experimenting with genetic engineering to develop plants that will absorb even higher concentrations of metals.
More Than Just Planting
While phytoremediation may sound simple, commercial application of the process is more complex than it appears, cautions Edward Gatliff, president of Applied Natural Sciences Inc. (Fairfield, Ohio), a commercial phytoremediation firm and a research partner with Argonne National Laboratory. “Phytoremediation isn’t just a matter of planting some grass or trees on a site and watching it grow,” he says. “Anybody can plant something on their property, but whether you achieve the optimal results depends on how the site is managed.”
To be successful, phytoremediation should be based on a site-specific plan executed by experts, Gatliff stresses. The plan is developed based on a comprehensive site survey that determines the site’s physical characteristics and soil or water testing to analyze the contaminants present, as well as their concentrations and depth of penetration into the soil or water. All of these factors are considered in developing a plan that details the plants to be used, their placement at the location, and the planting and harvesting schedule.
After a site-specific plan is developed, appropriate trees or plants are selected and sown on the site. Alternatively, plants may be sown in gravel beds in a specially constructed compartment near the contaminated site, with contaminated leachate or runoff being directed through their root system. In another variation, deep-rooted trees are planted to create a natural pump-and-treat system for contaminated groundwater.
As the plants grow, they absorb contaminants from the soil or water. During the growing season, contamination levels at the site are monitored to evaluate the program’s effectiveness. The more technologically advanced phytoremediation companies use remotely accessed computerized sensors to monitor conditions, activities, and progress at the site, coupled with periodic on-site monitoring.
Soil or plant additives may be applied to increase the plants’ uptake or degradation abilities. In some cases, the entire site may be managed agronomically to induce longer root growth to reach deep groundwater.
Other strategies include amending the soil with metal chelating agents to maintain the metal’s availability for plant uptake and applying ammonium-containing fertilizers or soil acidifiers that maintain soil pH at a moderately acidic level, which also increases plant uptake. Through a combination of these techniques, roots and plant materials can attain concentrations of up to 10,000 ppm of the toxic material, or 1 percent of the plant. Under greenhouse conditions, phytoremediation has yielded even more impressive results, with hybrid poplars absorbing up to 38,000 ppm of certain contaminants, or about 4 percent of the plant, Hinchman reports.
When maximum concentrations are achieved, the plants are harvested, sometimes roots and all. At this point, the plants are considered toxic waste, and the best solution to their disposal has yet to be worked out.
One choice is incineration, a process that produces a residual ash that has at least a 40-percent concentration of metal. The metal contained in this ash “can be recovered or the ash can be disposed of in a hazardous waste landfill,” says Burt Ensley, president and CEO of Phytotech Inc. (Monmouth Junction, N.J.), a commercial phytoremediation company. “Alternatively, the plants can be compressed and landfilled. Either way, phytoremediation reduces the mass of landfilled material 85 to 98 percent.”
Proof Positive
With the right plants and under the right conditions of contaminant level and depth of penetration, phytoremediation can theoretically be applied to any heavy metal or organic compound that a plant will accumulate. A dozen or so heavy metals are being targeted for phytoremediation, and the related technology is at different stages of development. While many of the field tests conducted thus far have focused on lead, other elements such as nickel, copper, selenium, cadmium, chromium, and uranium are also being examined for plant-based removal, as are volatile organics found in fuel and oil.
Field trial data have been encouraging so far. “On a properly selected site, phytoremediation can reduce the level of a contaminant to below U.S. EPA and state regulatory levels,” says Ensley.
In a 1996 field test, for example, Phytotech planted Indian mustard at a lead-contaminated abandoned battery recycling site in Trenton, N.J. Before treatment, contamination ranged from 100 to 1,200 ppm, with 40 percent of the area exceeding the regulatory limit of 400 ppm and 7 percent exceeding 1,000 ppm. In one growing season consisting of three crops, approximately 70 percent of the treated area was cleaned to the New Jersey EPA standard of less than 400 ppm, says Ensley, noting that the remaining portion of the site is scheduled to be remediated this summer.
Other successful field trials have included a uranium-contaminated former nuclear weapons site in Ashtabula, Ohio; Department of Energy sites in Montana, Alabama, and Tennessee; a nitrogen-contaminated aquifer in New Jersey; and—perhaps the most dramatic—fields and ponds near Chernobyl, Russia, that were contaminated with radioactive cesium and strontium.
Such success stories have enabled phytoremediation to catch the attention of corporate heavyweights such as Procter & Gamble Co., Kaiser Aluminum & Chemical Corp., and DuPont Chemical, all of which have phytoremediation programs. In addition, the U.S. EPA has begun evaluating phytoremediation on select Superfund sites. “On many existing Superfund sites, nothing has been done because of the expense involved,” Ensley says. “With phytoremediation, there’s a higher probability that sites will get cleaned.”
A Host of Benefits
According to experts in the process, phytoremediation offers a handful of advantages over traditional cleanup methods such as incineration, excavation, soil washing, and thermal desorption.
Since it is plant-based and solar-driven, for instance, the process is intrinsically environmentally friendly, low-tech, and aesthetically pleasing.
The process can also be used in situ—that is, in place—thus preserving, and in most cases enhancing, the biological component of the soil and minimizing environmental disruption.
Another of phytoremediation’s primary selling points is its low cost compared with conventional cleanup methods, which are often expensive. In many instances, companies have relied on digging, packaging, and hauling contaminated soil away from a site for disposal in a hazardous waste landfill. “There hasn’t been any method for removing metal from soil, so business owners in this situation have had little choice but to dig up the site,” Ensley points out.
The cost of such conventional cleanups can be $500,000 to $1 million an acre, and indirect costs include the long-term environmental impact of removing topsoil and disturbing the ecosystem.
Typical costs for phytoremediation, in contrast, run between $50,000 and $100,000 an acre. Fixed costs may include $5,000 to $20,000 a year for monitoring and site management. “On cost considerations alone, phytoremediation is an attractive alternative,” Gatliff says, adding, “Selling clients on the idea is not too difficult once they see the numbers.”
It should be noted, however, that the cost of phytoremediation is relative to the level of contamination, with the general rule for heavy metal removal being that “the less contamination, the cheaper it is to clean the site,” Ensley notes.
Phytoremediation also holds out the promise of potentially freeing companies from downstream liability related to the disposal of contaminated material from their sites. Under the U.S. EPA’s current cradle-to-grave liability rules, companies own the liability related to a hazardous material, even after they dispose of it. This applies whether a company digs, seals, and landfills contaminated soil, or whether it disposes of heavy-metal containing plants used in phytoremediation.
Phytoremediation’s liability promise, however, lies in the potential to recover the metals contained within the plants or, if incinerated, the ash and, hence, eliminate the related liability of discarding hazardous waste. “When metals can be extracted from the plants or the ash and sold, phytoremediation can break the chain of liability,” Ensley says. “If the owner can recover and sell the metal, he sells the liability as well.”
Not a Panacea
While phytoremediation offers some encouraging success stories and promising benefits, it also comes with a number of drawbacks.
One of these is that the process takes time, even when using the most efficient, bioengineered plants. In general, completely cleaning a site to within federal EPA specifications usually requires a minimum of two to three growing seasons. As Gatliff comments, “We try to get positive, measurable results in 18 months, but this is not a solution for a firm that wants to clean a site quickly for resale or development purposes.”
The depth of contamination in the soil is also something of a constraint to phytoremediation’s effectiveness. For example, Indian mustard, a lead hyperaccumulator, can extract metal no more than 2 feet below the surface. Deeper penetration can be achieved using poplar or willow trees, however, with some bioengineered strains able to extend their roots as deep as 20 feet to filter contaminated groundwater.
Another limitation is that high levels of contamination may rule out phytoremediation, though Ensley asserts that property owners are often too quick to reject the technology in such instances. “Granted, remediating a heavily contaminated site by phytoremediation alone might take 100 years,” he says, “but that doesn’t mean that sites with small pockets of higher concentration should be ruled out as totally unsuitable for phytoremediation.” A processing plant, for instance, might have widespread low-level contamination with small pockets of high contamination, such as near a loading dock or where a spill occurred. A remediation plan that includes phytoremediation in combination with other technologies could make cleanup financially realistic, Ensley asserts. “Maybe we could treat 90 percent of the site with plants, leaving 10 percent to be dug,” he says, noting, “That’s still a lot cheaper than having to dig the entire site.”
As Hinchman sums up, “Phytoremediation is not a magic bullet that solves every problem,” recommending that “it should be considered another tool to apply under certain conditions.”
Gaining Acceptance
Another impediment to phytoremediation’s progress is the slow acceptance of the technology by environmental engineering firms, as well as state and federal government agencies.
Part of the problem is that many engineers and regulators simply don’t have a firm understanding of the technology, according to Gatliff.
Also, despite compelling arguments and evidence in phytoremediation’s favor, many major environmental engineering firms are waiting to see results based on longer experience and at more locations before they sell clients on the process. “It’s too new a technology,” says Don Basch, supervising engineer for Montgomery Watson (Madison, Wis.), an environmental engineering firm. “Other than a few specialty companies, there’s not a lot of involvement from engineering companies yet.”
Even so, Montgomery Watson has tried phytoremediation on a limited basis and has tested it successfully in several cases. According to Basch, clients have included an oil company that cleaned up widespread low-level contamination at an idle oil terminal and a Superfund site in Illinois where vegetation is treating radioactive leachate from a landfill.
While some state agencies have quickly embraced phytoremediation, other state and federal agencies aren’t completely sold on the process, though overall acceptance at both levels is improving, says Gatliff. “Their skepticism has been one of the barriers to phytoremediation technology developing as rapidly as it could and should, but the situation is changing,” he offers. “A year ago, if you talked with an EPA official about phytoremediating a site, they would have pooh-poohed the idea. Now, at least they’re willing to hear you out and possibly consider phytoremediation as a reasonable option for at least a portion of the site.” If the U.S. EPA’s interest in phytoremediation for Superfund and other sites continues to increase, he adds, that could hasten its acceptance by the private sector.
In the meantime, as phytoremediation supporters wait for the technology to gain broader acceptance, they are being careful to comply with all state and federal environmental regulations during their projects and field studies, even though property owners technically don’t need state or federal permission to phytoremediate their property. (After harvest, however, plants used to phytoremediate contamination may, in some cases, be considered toxic waste and must be disposed of in accordance with state and federal regulations.) As Ensley explains, “We’re very careful to comply with all EPA regulations during site preparation, management, and harvesting to ensure protection of the immediate and surrounding environment.”
A Green Future
Advances in the identification and bioengineering of hyperaccumulator plants and agronomics in the past six years have made phytoremediation a reasonable, cost-effective option for remediating contaminated sites. Ultimately, phytoremediation could be used as a preventive strategy rather than simply as a post-contamination remediation process. “Contamination could be managed proactively by creating a barrier of trees that would head off site problems,” explains Gatliff, noting that “trees could be introduced into areas at risk or around the perimeter of a site to prevent migration of contaminated groundwater.”
Research also continues into the most effective plants and trees for phytoremediation, as well as new techniques for enhancing contaminant uptake and degradation. Phytoremediation processes to remove cadmium and uranium, for example, will reportedly be available commercially within two years.
And scientists are eager to expand the technology to other metals and organics, not to mention other industries—including the scrap recycling industry. •
Never heard of phytoremediation? It’s a new environmental technology that uses plants to clean up contaminated soil and water. And it could have promising applications in the scrap recycling industry.