- The Washington Times - Thursday, May 1, 2003

Rufus Chaney fights weeds in his garden, but he embraces them in his research. “Weeds are just plants that are out of place,” says Mr. Chaney, an agronomist with the U.S. Department of Agriculture. “There is nothing inherently wrong with them except they use water and nutrients you supplied for the crop plant.”Mr. Chaney discovered more than a decade ago that certain weeds, known as hyperaccumulators, could be used to extract toxic metals from contaminated soil. Since then, he and a team of researchers around the globe have been tweaking the extraction technology, called phytoextraction.They have learned how to use plants to extract cadmium, nickel and zinc from the soil. They’re working on cobalt and arsenic phytoextraction. Why is this research important?It’s a cheaper way of getting rid of contamination than simply digging up and moving contaminated soil to a landfill and replacing it with clean soil, says Scott Angle, a professor of agronomy at the University of Maryland and a member of the team. “It costs as much as $2 million per acre to move soil from polluted areas to landfills,” Mr. Angle says. “With phytoextraction, it costs about $10,000, or less, per acre.” In certain East and Southeast Asian rice-growing nations with heightened levels of cadmium in the soil, this method of remediation might be the only affordable cleanup method, Mr. Angle says. Too much cadmium in the human body can lead to kidney problems, brittle bones and, in severe cases, neurological disorders, he says. For cadmium phytoextraction, the researchers are using a plant called Alpine pennycress (thlaspi caerulescens).”People discovered thlaspi in the 1800s, but nobody thought about using it this way until now,” Mr. Chaney says. “It’s really a very simple idea.”These bushy little weeds with white flowers can grow and thrive where nothing else can survive. They have an ability to absorb the soil’s toxic minerals in their leaves. The stored minerals actually give the plant protection against chewing insects while not poisoning the weed itself. So, the absorption of toxic minerals is to these weeds what thorns are to roses and oils are to poison ivy.”If a deer eats the thlaspi, it’ll get a stomachache and go somewhere else,” Mr. Chaney says. Pumping, storing contaminantsThe weeds used for phytoremediation seem to store the toxic minerals in areas of the leaves that are not metabolically active, which means the toxicity is compartmentalized and doesn’t spread to other parts of the plant, Mr. Chaney says. “That way it doesn’t interfere with the metabolism of the rest of the plant,” he says. It appears that these weeds have three or four different pump systems that grab hold of metal ions in the soil, pumping them up from the root system to the stem. Another plant pump moves them to a storage area in the leaves. These plants can store up to 2.5 percent of their dry weight in toxic metals, about 100 to 1,000 times more than a nonspecialized plant could, Mr. Angle says.Part of the challenge in using these hyperaccumulators is creating the right soil conditions for the weeds. Thlaspi occurs naturally in Europe in areas where the soil is naturally full of certain minerals. To use the plant elsewhere, the scientists have to know what soil conditions have to be created for the plant to thrive in its new habitat. “We’re learning how to create the best conditions for them how to fertilize them, for example,” Mr. Angle says. They have learned, for example, that certain weeds, such as goldentuft madwort (alyssum), can take up nickel more effectively if the soil has low acidity overall. With high levels of acidity, the same plant can extract cobalt from the soil. This system of absorption only works if the metal is soluble, however. It can’t be in its solid state when it enters the plant because the plant’s xylem, the weed’s vascular system, is unable to pump solid minerals. That means that lead, the No. 1 contaminant of soil, is not treatable through phytoremediation.Making money, energyWhile some metals, such as cadmium, have little value on the metals market, others, such as nickel, are worth recovering from the weeds, Mr. Angle says. The extraction of metals from a plant is called phytomining. To extract nickel, the weeds have to be burned in an incinerator, creating nickel-rich ash. To recover the metal, the ash would be taken to a smelter.”The ash is about 25 percent to 40 percent nickel,” Mr. Chaney says. That means that farmers who own serpentine, or mineral-rich, land finally may be able to make some money, Mr. Angle says. “Serpentine soils used to present severely infertile conditions, but now they might be able to make money,” Mr. Angle says. “These farmers might be able to ‘grow’ nickel. It’s a whole new way of thinking.”While growing hay would bring in about $50 per acre, growing nickel would bring in about $1,000 per acre, Mr. Angle says.Places such as Oregon, Washington state and certain areas of California where the soil is naturally serpentine could benefit from phytomining. In fact, the University of Maryland and the Department of Agriculture licensed the phytomining technology invented by Mr. Angle and Mr. Chaney to a Houston-based company called Veridian Environmental LLC to further develop the business. So far, about 700 acres in Oregon have been planted with alyssum, Mr. Chaney says. “We estimate that you could farm the land for 20 years before the nickel is so low that it’s not economically feasible anymore,” Mr. Angle says. Another way to make money on the hyperaccumulators is to generate energy while burning the plants. This type of energy production is called biomass energy. An added benefit is that growing these weeds helps prevent erosion in areas where mining has stripped the land of vegetation. “They will help stabilize the soil,” Mr. Angle says. Even if the plants are weeds, he adds, chances are they look better than no vegetation. The main limitation of this plant technology is that it’s time-consuming: It may take five to 20 years for a site to be cleaned up using this method. “If the people have to live on the site right now, then this is not the method,” Mr. Angle says. “You’re basically trading time for money.”Nevertheless, the scientists still think there is a bright future for the hyperaccumulators, of which 500 are known. The number keeps growing as scientists find new weeds that work for phytoremediation and phytomining. “I think we will see more of it,” Mr. Chaney says. “The inexpense of using an agricultural method to clean contaminated soil is reason enough.”


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