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FEATURE
Phytoremediation on the Brink of Commercialization MYRNA E. WATANABE
Plants ranging from pennycress to poplar trees are proving their worth as cleanup tools.
fter more than a decade and a half of research scrutiny, phytoremediation is moving into the realm of the usable. Academic, government, and corporate researchers have a body of data on the ability of certain plants to either remove pollutants from the environment or render them harmless (1), and they are looking for ways to improve these traits through plant breeding and molecular techniques. In the past three years, at least three new companies have formed to use plants to clean sites contaminated with heavy metals or organics. One of these companies—Phytotech, Inc,, of Monmouth Junction, N.J.—expects to begin commercialization of a lead extraction technique this year. In phytoremediation, "the greatest progress is in removal of heavy metals," said Ilya Raskin, a professor at Rutgers University in New Brunswick, N.J., and a founder of Phytotech. Phytoremediation is a natural process carried out by plants, especially those that have been able to survive in contaminated soil and water. Hyperaccumulators are plants that can absorb high levels of contaminants with their roots and concentrate them either there or in shoots and leaves. Phytoremediation researchers Alan J. M. Baker of the University of Sheffield in England and R. R. Brooks of Massey University in New Zealand define metal hyperaccumulators as plants that contain more than 1000 milligrams per gram (mg/g) of cobalt, copper, chromium, lead, or nickel or 10,000 mg/g (1%) of manganese or zinc in the dry matter (2). Researchers have found hyperaccumulator species by collecting plants in areas where soil contains greater than usual amounts of metals or other potentially toxic compounds because of geological factors or pollution (2,3). Among the plants that have been collected and used in field trials are species from the genus Thlaspi, or Alpine pennycress, which accumulate zinc, cadmium, or lead (4), and Alyssum species, which accumulate nickel. Both genera belong to the mustard family Brassicaceae. Plants from many other families also have been shown to remove cobalt, copper, chromium, manganese, or selenium from contaminated soils (5). According to a Department of Energy report on phytoremediation research (6), the best hyperaccumulators should have the following traits: high accumulation rate, even at low environmental concentration of the contaminant; ability to accumulate very high levels of contaminants; ability to accumulate several metals; fast growth; high biomass production; and resistance to diseases and pests.
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Scott Angle, a University of Maryland soil microbiologist works with U.S. Department of Agriculture researchers studying metal-scavenging Thlaspi plants like the one shown here. (Photo courtesy USDA Agricultural Research Service)
Getting the metals out Of all the metals contaminating the environment, lead is the most common, according to EPA (7). But plants cannot just be stuck into the soil in the hope that they will take up the contaminant. "We don't have any plant that honestly accumulates lead under real soil conditions," said Rufus L. Chaney, senior research agronomist at the U.S. Department of Agriculture (USDA) in Beltsville, Md., and one of the founders of the phytoremediation field. He points out that in the case of lead contamination, chelators such as EDTA (ethylenediaminetetraacetic acid) have to be added as soil amendments to make the lead more readily available to the plant (8). "You can still get some uptake [of lead] without adding soil amendments, but it's small amounts," said Bert Ensley, Phytotech's president and CEO. Lead uptake can also be boosted sometimes when soil pH is reduced. After several years of field trials, Ensley believes his company has plants that are ready to be commercialized for lead phytoextraction using amend-
ments. Phytotech has been using Indian mustard, Brassica juncea, in its lead phytoextraction experiments. "We have plants that could take up 1.5% of their dry weight in the shoots, the above-ground part of the plant," said Ensley. He reported that last summer Phytotech did a field trial on a brownfield area in Trenton, N.J. The former industrial site had been occupied by manufacturers of Magic Marker pens and batteries. Ensley says the treated site is almost clean after one summer. Phytotech also treated an industrial site in Bayonne, N.J. "When we started, most [of the site] was above New Jersey industrial standards [1000 ppm of lead]. After one summer, most of the treated area was below industrial standards." Phytotech expects to complete its current lead remediation projects in two summers, deflating a major premise in phytoremediation. "One of the dogmas in the field is that it takes 10 to 20 years [to clean up lead]," said Ensley. As lead phytoextractors move toward commercial development, Chaney and colleagues are searchVOL.31, NO. 4, 1997/ENVIRONMENTAL SCIENCE S TECHNOLOGY / NEWS " 1 8 3 A
ating the plant's ability to remove zinc and cadmium for the Zinc Corporation of America in a park in Palmerton, Pa., the site of a former zinc smelter. Chaney and colleagues from USDA and the University of Maryland, College Park, also have predicted that there may be financial gains from harvesting plants used to phytoremediate zinc and cadmium contamination. Assuming 20 tons of biomass/hectare/harvest, the researchers have hypothesized that the metals in the plants would be worth $1069/ha (9). There is not yet a market for harvested hyperaccumulators, however.
Alpine pennycress, a small herb, accumulates such large amounts of zinc and cadmium that researchers are evaluating its phytoremediation potential. (Photo courtesy USDA Agricultural Research Service)
ing worldwide for different plants that will hyperaccumulate other metals. For zinc and cadmium remediation, they are concentrating on Thlaspi caerulescens. This small herb accumulates up to 25,000 mg of zinc per kilogram of dry matter of shoots and 1000 mg of cadmium per kilogram. A plant that could take up both cadmium and zinc would be of great practical use: "Where pollutants are almost always co-occurring, we think we should have a technology that would remove both of them," added Chaney. Zinc is the limiting factor in cadmium uptake. When zinc builds to a certain level, the plant is prevented from removing additional cadmium. But Chaney and his colleagues have found some plant germ plasm sources that are better able to remove cadmium in the presence of zinc. "We can increase the annual removal of cadmium 10-fold before zinc inhibits removal of cadmium," Chaney said. Chaney and colleagues do plant-breeding studies to identify genes responsible for the metal hyperaccumulation. They are using genetic engineering techniques to implant genes from more efficient accumulators into other plants' cells. Results of these new techniques are "some extremely zinc-tolerant [Thlaspi] plants that are taller than Thlaspi usually is," stated Chaney. Being taller can be advantageous for a bioaccumulator plant, because the increased biomass means it can pick up larger amounts of contaminants. Chaney's research group is evalu1 8 4 A • VOL. 31, NO. 4, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
Volatilizing contaminants Hyperaccumulation is not the only option in phytoremediation. Phytovolatilization, the process of turning absorbed metal contaminants into gases using plants, is also under study. For instance, Richard Meagher, head of the Genetics Department at the University of Georgia at Athens and a principal in Phytoworks, Inc., of Gladwyne, Pa., has developed a phytovolatilizing method for mercury. The method involves introducing the bacterial gene called mercury reductase into Arabidopsis plants, which are members of the mustard family. The gene expresses in the transgenic Arabidopsis, where it reduces mercury to Hg (0), and results in the metal volatilizing as a gas. Even nonmetals like selenium can be volatilized. Gary Banuelos of USDA's Agricultural Research Service (ARS) Water Management Research Laboratory in Fresno, Calif., and colleagues have found that some plants grown in high-selenium media produce volatile selenium in the form of dimethylselenide and dimethyldiselenide {10). In greenhouse experiments, Banuelos and colleagues have shown that rice, broccoli, cabbage, and other plants have especially high selenium volatilization rates. Volatilization, however, is not without risk. Both lead and selenium are toxic at high concentrations, and Raskin said that no one is quite sure that volatilization of mercury into the air is something that should be done, because mercury is a highly toxic heavy metal. Organics pose their own challenges Although phytoremediation of organics could be widely used because more industrial sites are contaminated with volatile organic compounds (8) than with heavy metals, research on it is not as advanced as work on phytoextraction of heavy metals; according to academic researchers, sufficient data do not yet exist to support its commercialization. "It's much more difficult to work with organics," stated Raskin. "It's very difficult to analyze them, very difficult to look at transformation. Metals are much easier to measure, and they do not form metabolites." Despite this dearth of data on endpoints and on assessment of phytoremediation of organics such as volatiles, there is a commercial demand. Sedimentologist Paul Thomas of Thomas Consulting, Inc., in Cincinnati, Ohio, has worked for "industrial clients who have sites that aren't under tremendous regulatory pressure " Using ph.ytoremedi3.tion now, rather than waiting the 5 to 10 years that may be needed to srenerate the delta, on fate and transport may be
a viable option to a property owner. At the very least, phytoremediation will not harm the situation and is likely to help it. Ari Ferro, a founder of Phytokinetics, Inc., in North Logan, Utah, said that his company is evaluating field-scale sites, and he thinks this coming growing season will provide essential data. The three-yearold firm is conducting a field evaluation with EPA's Superfund Innovative Technology Evaluation (SITE) program. Phytokinetics is using grasses to clean up surface soils and poplars to remediate plumes of groundwater at a former Chevron fuel transfer terminal contaminated with total petroleum hydrocarbons (TPHs). The poplars are arranged in barrier strips both to block the flow of the contaminated plumes and to remove the TPHs Poplars also are being used by EPA at another SITE demonstration at the U S Army's Aberdeen Proving Ground in Aberdeen Md There the poplars control a groundwater plume contaminated with 1 1 2 2-PCA (tetrachloroethane) The contamination at the Aberdeen Proving Ground came from open-burning/open-detonation pits. The Army used open pits to burn or detonate munitions slated for destruction. Although there are nofielddata for either site as yet, Ferro said, "Based on modeling, theoretical considerations, this should be an effective way of blocking the flow of plumes of contaminated groundwater." But Ferro adds that there is no guarantee that phytoremediation can sufficiently reduce the contamination to meet EPA cleanup targets. There is also some concern that "a plant could take up an organic compound intact and it could contaminate the food chain," according to Ferro. He thinks this is unlikely, because compounds are broken down either by the microorganisms in the plant's root zone or within the plant itself. But, if there is any doubt, scientists can "put bird-netting over the site to try to prevent food chain contamination," Ferro added. Milton Gordon and co-workers Lee Newman and Stuart Strand at the University of Washington, Seattie, believe it is possible that poplars may protect the food chain from contamination. Working with funding from the National Institute of Environmental Health Sciences, Gordon and colleagues are finding that poplar trees appear to break down trichloroethylene "into insoluble, nonextractable material" that is "biologically inert." The team, according to Gordon, believes that these end products are stored in the lignin of the tree. To test the hypothesis that these end p r o d u c t s 3.TG nontoxic Gordon said that they plan to carry out experiments in which they feed the material from the poplar trees to animals have mice nest in the material and test its toxicity on so uatic or23.nis.ms "So fox we have no in dication that it's hazardous at all" Gordon said Phytoworks takes a different approach to finding plants that might remove organics. To rapidly locate plants that can do a particular kind of organic remediation, Laura Carreira, the company's principal scientist, studies the soil around the plant's roots first. Plants, she explained, release proteins, including enzymes, into the soil, and these enzymes provide hints about what the plants can break down. Af-
An Indian mustard plant {Brassies juncea) being commercialized by Phytotech. Inc., is being used in lead-contaminated cleanups like this site in New Jersey. (Photo courtesy Phytotech, Inc.)
ter determining what the enzymes do, Carreira can track them down within plants. She and other researchers do this by raising antibodies to the enzymes found in the soil. She then treats plants with the antibodies to see if they latch onto the same enzyme in the plant. Using these techniques, Carreira has traced several enzymes, including a nitroreductase that breaks down 2,4,6-trinitrotoluene (TNT). The rate of TNT breakdown is directly proportional to the amount of nitroreductase in the plant. Approximately 20% of the plant species tested using the antibodies show nitroreductase activity, including poplars, some Researchers grasses, and many aquatic plants. Car- have found reira also has isolated an aryldehalogenase hyperaccumulator that will degrade ethylene-containing com- species by collecting pounds. plants from areas Carreira believes that searching for the organ- where soil contains ics-degrading enzymes first is the most eco- high levels of metals nomical way of pursuing phytoremediation of because of geological organics. Researchers can go out and find factors or pollution. plants that can do what they want them to do; but if they don't do the enzymology cind instead ttike f& trial-and-error course it will cost & lot more she ssid
Inching toward acceptance Although phytoremediation has been tested on sites contaminated with petroleum products, heavy metals, munitions, and radionuclides and at abandoned mines, wood treatment sites, and sewage treatment sites, it remains unclear how large the phytoremediation market will be. Phytoremediation experts say the growth of interest in the field is driven by its relative costefficiency compared to standard remediation methVOL.31, NO. 4, 1997/ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 1 8 5 A
Gary Banuelos, a U.S. Department of Agriculture scientist, has found that Brassies plants can volatilize excess selenium. (Photo courtesy USDA, Agricultural Research Service)
ods for government-mandated site cleanup. Scott Cunningham of DuPont, in Newark, Del, and colleagues have estimated that in situ remediation of a contaminated site may cost between $10 and $100 per cubic meter, whereas ex situ remediation, in which the contaminated material is removed—sometimes permanendy, sometimes for cleanup offsite and eventual return—may cost as much as $30-$300/m3. "The acceptance of In comparison, "land farming" techniques [phytoremediation] is such as phytoremediation, in which plants are not going to be cultivated die same way immediate. People a farmer plants a field or orchard may are going to have to $0 05/m3 (11) EPAs Steve Rock said be shown it works." that phytoremediation is "best suited for cleanBert Ensley, ups over a wide area, Phytotech, Inc. with contaminants in low to medium concentrations." But before much more than lead phytoremediation is commercialized, more research must be done to monitor die results of field trials, to find more efficient bioaccumulators, and to find better ways to dispose of plant refuse—especially for heavy metal accumulators once the plants have been harvested.
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"I think the acceptance of this is not going to be enthusiastic or immediate," said Phytotech's Ensley. "People are going to have to be shown it works before it's commonly accepted."
References (1) Raskin, I.; Smith, R. D.; Salt, D. E. Current Opinions in Biotechnology, in press. (2) Baker, A.J.M.; Brooks, R. R. Biorecovery 1989, 1, 81. (3) Banuelos, G. S. et al. /. Soil Water Conserv. 1993, 48(6), 530. (4) Baker, A.J.M. et al. Resources, Conservation and Recycling 1994, 11, 41. (5) Reeves, R. D.; Baker, A.J.M.; Brooks, R. R. Mining Environmental Management 1995, 3(3), 4. (6) U.S. Department of Energy. "Summary Report of a Workshop on Phytoremediation Research Needs, December 1994"; Dec. 1994; DOE/EM- 0224. (7) U S. Environmental Protection Agency. Cleaning Up the Nation's Waste Sites: Market and Technology Trends; April, 1993; EPA/542/R-92/012. (8) Huang, J. W. et al. Environ. Sci. Technol. 1997, 31, 80005. (9) Chaney, R. L. et al. Mining Environmental Management 1995, 3(3), 9. (10) Banuelos, G. S. et al. /. Soil Water Conserv., in press. (11) Cunningham, S. D. et al. In Phytoremediation of Soil and Water Contaminants; Krueger, E. L.; Anderson, T. A.; Coats, J. R., Eds.; American Chemical Society: Washington, DC, 1997.
Myrna Watanabe is a biotechnology consultant and freelance writer. She ii a frequent contributor ro The Scientist.
