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and offsets costs if possible." Using plants to clean up sites may take longer, but if the value of the recovered metals pays for the cost and the ecosystem is re stored, this method "is much more effec tive than a million-dollar-per-acre kind of program," he adds. The idea of using plants to strip pol lutants directly from the soil has been around since the 1980s. The technolo gy has not yet been commercialized, but many researchers in addition to A. Maureen Rouhi the steep costs, polluted sites usually are Chaney and Angle are convinced of its C&EN Washington cleaned only enough to eliminate the risk merits and are working to make it eco of contaminating groundwater or of ex nomically viable. For example, research fter a fire leveled part of a forest on posing children to pollutants on surface being done in California seeks to use ι a mountain near Palmerton, Pa., the soil. Such treatments do not guarantee that plants to alleviate the problem of soils Larea remains bare. Plants cannot remediated sites can support wildlife or containing naturally high amounts of the nonmetallic element selenium. And grow because of high levels of zinc in farming. the soil, remnants of pollution from a Plants that gobble u p metals offer an in New Jersey, a venture-capital compa zinc smelter operation in the valley. Cov alternative that may prove better. They ny is developing plant-based technolo ered with contaminated soil, the area is could be grown on contaminated sites, gies for removing toxic metals, such as littered with dead wood and shows little harvested mechanically like hay, and lead, from soil and water. sign of animal life. Restoring it so that dried. The biomass could be burned to People are aiming to plants, because, once again trees can grow, animals can produce ash with a high content of met as Chaney says, techniques currently roam, and humans can enjoy its beauty al that could be recovered. The econom used to clean up soils are expensive yet would take a lot of money using current ic value of the metal and of the heat pro aren't good enough. With t h e soilmethods. duced during ashing—which could b e washing technology, for example, the Hundreds of contaminated sites like used to generate electricity or to retrieve soil is mixed with acid to strip off the that near Palmerton exist in the U.S. Cur the metals—could be enough to pay for metals, he explains. The treatment kills rent technologies to remediate such sites the cost of the cleanup. soil microorganisms and takes away nu include hauling the contaminated soil to a Such is the vision of Rufus L. Chaney, trients. When the soil is washed, the clay landfill and replacing it with clean soil, a research agronomist at the Environ is removed, leaving behind low-organicchemical immobilization of metals, and mental Chemistry Laboratory of the De matter sand with very l o w fertility. leaching metals with acid washes. Accord partment of Agriculture's Agricultural Re "They put this stuff back on the ground ing to industry sources, cleanups in the search Service, Beltsville, Md., and J. and call it soil," Chaney says. "It would U.S. by these methods are estimated to Scott Angle, a soil microbiologist at the need a lot of help to be able to grow cost tens of billions of dollars. Because of University of Maryland, College Park. something again." Test plots in a public garden in Palmer While others in the field remediate con taminated sites for a profit, "we're public ton show what might be done with a servants," Chaney says. "Our strategy is mountainside made barren by high levels to find the least expensive, appropriate of metals. Lawn grass such as Kentucky method that protects the environment bluegrass ordinarily can't survive because of the high amounts of % zinc in the soil. But after £ being planted with a ω small shrub called Alpine g pennycress (Thlaspi cae rulescens), the plots are lush and green. Alpine pennycress is a hyperaccumulator. The term is based on the un usual ability to accumu late at least 10 to 100 times the concentration of a metal tolerated by ordinary crops, explains Chaney. T h e trait ap pears to have evolved Zinc-contaminated sites such as those on a mountain near Palmerton, Pa., (left) might be restored by metal-accumulating over millions of years on metal-rich natural soils. plants such as Thlaspi caerulescens, which is being grown in T. caerulescens detest plots in a public garden In Palmerton (above).
PLANTS TO THE RESCUE
Crops with big appetites for certain elements could be key players in soil cleanup
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JANUARY 13, 1997 C&EN 2 1
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vours huge quantities of zinc, cadmium, says. "That a a high-grade ore." And the ing with Baker and others, Li has collect and nickel. Most plants accumulate only ash doesn't contain "junk" elements like ed more than 20 strains of the plant from 400 to 500 mg of zinc per kg of dry iron, aluminum, and silicon found in Europe. He has tested them for metal up leaves before being injured by the zinc. most metal ores, making it easier to re take and yield in nutrient solutions, as In contrast, Alpine pennycress can suck cover the zinc. By Chaney's calculation, well as in the field in Palmerton. up zinc to a whopping 25,000 mg per zinc and cadmium farming by growing As Li reported in 1996 at the annual kg. Similarly, regular plants tolerate cad high-biomass-yielding hyperaccumulators meeting of the American Society of mium concentrations of only 5 to 100 mg on metal-rich but agriculturally poor soils Agronomy in Indianapolis, their efforts per kg. But Alpine pennycress can accu could bring in twice as many dollars as have yielded strains that accumulate cad mulate up to 5,000 mg per kg without growing alfalfa or corn on normal agri mium more selectively than the original injury. And this metal muncher eats up cultural land. Belgian and British strains they have to 16,200 mg of nickel per kg, when or Despite the potential, Chaney says been studying. Selective accumulation of dinary flora tolerate only 50 to 100 mg there's been little money to do the re cadmium is a highly desirable trait, says per kg. search that would bridge the gap be Chaney, and the team will be testing its At the Palmerton site, high levels of tween the basics and commercializa inheritability. Li is attempting to combine hyperac zinc in the soil prevent most plant tion. He hopes further studies can ad growth. But zinc is not the only problem. dress the problems that are keeping cumulating characteristics with rapid The zinc smelters also spit out cadmium, mining and remediation by T. caerule growth by conventional plant breeding. which can have adverse human health scens and similar plants from becoming Meanwhile, Eric P. Brewer, a University of Maryland graduate student, and James effects. Long-term exposure to low levels commercially viable. A. Saunders, a cell culof cadmium can lead to kidney disease, % ture scientist at the lung damage, and fragile bones, and the |- Agricultural Research Environmental Protection Agency has set 8 Service in Beltsville, a limit of 5 ppb for cadmium in drinking ο are using a different water. Because of cadmium, Chaney thinks the Alpine pennycress is the best 1 approach. They are plant for cleaning up the site. Other zinc 2 physically fusing the hyperaccumulators do not have as much S genomes of T. caeappetite for cadmium. I rulescenswiXhBrassi1 ca napus, a tall-growMany hyperaccumulators have been 1 ing oilseed plant that identified, especially through the work < is also a member of of Chaney's collaborator Alan J. M. Baker, the Brassicaceae fam a professor in the department of animal ily. Because the two and plant sciences at the University of plants are not close Sheffield, England. For example, at least ly related, crossing 300 plant species hyperaccumulate nick caerulescens through pollination el. Others take up cobalt, copper, seleni U (from left), Chaney, and Angle Inspect Thlaspi um, or manganese. Some feast on radio plants growing In a greenhouse. doesn't work. nuclides. Many, like T. caerulescens, be Instead, Brewer long to the family Brassicaceae. One such problem is slow growth. and Saunders are using a technique called How plants are able to tolerate such When the soil is poor, says Chaney, "the protoplast fusion, which uses an electric high levels of metals is not yet fully un best bet for a plant is to grow only to the shock to combine the genomes of the derstood. Most metals are chelated and extent that it can make enough seeds to two plants, producing so-called somatic hybrids. In Indianapolis, Brewer reported stored in cell vacuoles. Some are precip propagate." itated. Others are stored in secretory The leaves of mature T. caerulescens that from millions of fused protoplasts he hairlike organs on the leaf surface, called grow only to about 20 to 30 cmfromthe has grown zinc-tolerant hybrids in tissue trichomes. ground in one year, and they are small. culture. The plantlets appear to have Notwithstanding the many questions Even if the plant takes up a lot of zinc long internodes, like B. napus, but they about the physiology and biochemistry and cadmium, the amount it actually are much more tolerant of zinc than B. of hyperaccumulation, Chaney believes removes from the soil would hardly napus itself. Although it is too early to the "proof of concept" that plants like T. be staggering because the plant is puny. tell, "it is extremely likely that these new caerulescens have potential for environ And because T. caerulescens grows plants will show hyperaccumulation," mental remediation has been achieved. close to the ground, it is also difficult says Chaney. Because of the slow growth and low Indeed, he thinks that T. caerulescens to harvest with the usual hay-making biomass of T. caerulescens, other re and similar plants have potential uses be equipment. yond remediation: In some cases, they Chaney's team is trying to combine searchers doubt it will be practical for could be used to farm metals as an alter optimum hyperaccumulating properties environmental cleanup. They are focus native to mining. with the ability to grow taller and faster. ing their efforts on species that may not When the biomass of a hyperaccumu- His other collaborator, Yin-Ming Li, a be as voracious as T. caerulescens but lator is pyrolyzed, the accumulated metal plant geneticist at the Agricultural Re grow faster. The bottom line, says Gary S. Banuebecomes concentrated in the ash. For ex search Service in Beltsville, is studying ample, the ash of T. caerulescens can the genetics of metal hyperaccumulation los, is how much metal the plant re contain as much as 40% zinc, Chaney and tolerance in T. caerulescens. Work moves from the soil. Banuelos is a scien22 JANUARY 13, 1997 C&EN
tist at the Water Management Research Laboratory of the Agriculture Research Service in Fresno, Calif. His research seeks to remove selenium from soils. Although selenium is not a metal, Banuelos' thinking about using plants to remove it applies equally well to metals: Bigger plants with moderate appetites can extract more than hyperaccumulating, but tiny, plants. Banuelos has tested the use of plants good enough for soup—garlic, onion, mustard greens, broccoli, and brussels sprouts—to remove selenium from California soils. About half a million acres of soils in central California are naturally rich in selenium from marine-type sediments, he says. The soils have "hot" spots, containing up to 20 mg of selenium per kg. Extensive irrigation of farm lands in central California mobilizes large amounts of the selenium originally bound in the soil, washing it into the drainage system, which empties the effluent into very large evaporation ponds. "It's like waking up a sleepy monster," he says. "The huge ponds are like magnets for wildlife, and their high selenium content, when transferred through the food chain, disrupts the reproductive cycles of birds and leads to mutations in other wildlife." The plants Banuelos uses—also members of the Brassicaceae family—require high amounts of sulfur. Because of the similarity between sulfur and selenium, the sulfur-loving plants take up selenium as well. The plants also take up large quantities of other elements, such as boron, cadmium, sodium, and chlorine. This ability is useful, he points out, because the central California soils are high not only in selenium but also in various salts. The problem of selenium in evaporation ponds can be avoided by preventing it from leaching into the drainage system. Banuelos thinks the solution is for farmers in central California to alternate regular crops that do not have a high sulfur requirement (and therefore do not take up selenium)—such as tomatoes, alfalfa, and cotton—with plants that can remove selenium. At present, Banuelos is testing B. napus as a selenium scavenger. The plant accumulates 200 to 700 mg of selenium per kg of dry weight. It also volatilizes selenium in the form of dimethyl selenide, which can be blown away. Many areas of the U.S. are deficient in selenium, says Banuelos. Movement of the volatile compound through the atmo-
Bahuelos examines Brassica napus crop on selenium-rich California soil.
sphere serves to redistribute the element naturally. Preliminary tests show that within four years B. napus can reduce by half the selenium in the top meter of the soil. The oil from the seeds can be used for cooking or perhaps as diesel fuel for farm machinery, and the remaining biomass can be used as a supplement for animal feed in selenium-deficient areas. Research toward commercialization also must consider farming practices that will ensure the long-term viability of harvesting metals by plants. As metals are removed with each cropping, whatever remains must be made more available through soil management. Other factors that affect metal uptake by plant roots and metal translocation to the shoots also need to be addressed. The importance of agronomic practices was recently demonstrated by Phytotech Inc., a biotechnology company based in Monmouth Junction, N.J. Last September, the company treated an abandoned, lead-polluted industrial site in Trenton, N.J., where lead batteries had been manufactured for about 30 years. Surface concentrations of lead at the site ranged from 200 to 1,800 mg per kg. Before treatment, about 60% of the area was clean; that is, it contained lead at less than the regulatory limit of 400 mg per kg. Michael J. Blaylock, a Phytotech soil chemist, improved the site by growing Brassica juncea (Indian mustard),
another oilseed crop. Within three croppings, an additional 15% of the area was made clean as defined by the regulatory limit. Analyses of the plant shoots showed that Indian mustard accumulates as much as 3,900 mg of lead per kg dry weight. Most plants do not accumulate more than 100 mg per kg in their shoots. Phytotech's success is noteworthy because lead is insoluble—tightly bound in soil as salts or adsorbed on clay particles—and thus not easily taken up by plants. Blaylock says the key was to coax the lead from the soil to the plant roots and to the shoots by using soil and plant supplements. "Up to now, we haven't found plants that can solubilize lead in soil to the degree that we're looking for solely through root exudates," says Blaylock. Phytotech's proprietary soil supplements help release lead and make it available to plants. Once the lead is in the roots, the plant needs to take it to the aboveground parts so it can be harvested easily. Blaylock says Phytotech's proprietary plant supplements enhance translocation to the shoots. The lead-rich crop of B. juncea is treated as a hazardous material, and Phytotech is examining ways to dispose of it. The best case is to recover the lead, says Blaylock. The worst case would be to take the material to an appropriate disposal facility. Yet even this worst case is not so bad, he explains, "because we've greatly reduced the amount of material going into the disposal facility by concentrating the metal in the plant." One active ingredient in Phytotech's proprietary supplements is ethylenediaminetetraacetic acid (EDTA). By chelating lead, EDTA prevents the metal from being precipitated by phosphates and other anions, keeping the metal solubilized in the soil system and allowing it to move to the roots. But the Agricultural Research Service's Chaney sees a problem: When it rains, the EDTA complex can move through the soil. "You can't let chelated metals leach down the soil just so we can get them taken up by plants," he says. Besides, cleaning up a site to reduce contaminants to regulatory levels is not enough, Chaney says. He believes the goal should be to restore the ecosystem. "I expect to see a delightfully green, well-vegetated, noneroding farm," he says. "It's so important to have the technology for a holistic approach that seeking ways to make it work is driving me crazy. " ^ JANUARY 13, 1997 C&EN 23
