In the Lab
Scientist: Nanoscale Iron Could Treat Contaminated Soil, Water In Situ
An ultrafine, "nanoscale" powder made from iron could effectively solve some of the most difficult remediation challenges, according to Lehigh University environmental engineer Wei-xian Zhang.
In the Sept. 3 issue of the Journal of Nanoparticle Research, Zhang reviews his eight years of work with the material, which could be used to treat contaminated soil and groundwater in situ.
Iron's cleansing power stems from the fact that it rusts, or oxidizes, Zhang explains. When metallic iron oxidizes in the presence of contaminants such as trichloroethene, carbon tetrachloride, dioxins or PCBs, these organic molecules get caught up in the reactions and are broken down into simple carbon compounds that are far less toxic.
Similarly, oxidizing iron reduces dangerous heavy metals such as lead, nickel, mercury and uranium to an insoluble form that tends to stay locked in the soil, rather than spreading through the food chain.
While many companies now use a relatively course form of metallic iron powder to purify industrial wastes, Zhang notes, these industrial reactors are not very effective in treating pollutants that have already seeped into soil and water. But nanoscale iron particles are some 10 to 1,000 times more reactive than conventional powders because their smaller size collectively gives them a much larger surface area.
In addition, they can be suspended in a slurry and pumped straight into the heart of a contaminated site like an industrial-scale hypodermic injection. This in situ treatment is cheaper than conventional methods that involve removing and then treating contaminated soil and water.
Zhang also points out that unlike in situ biological treatments that use specialized bacteria to metabolize toxins, the iron particles aren't affected by soil acidity, temperature or nutrient levels. Moreover, because they are about 10, 1,000 times smaller than most bacteria, the tiny iron crystals can slip between soil particles and avoid being trapped.
Laboratory and field tests indicate that treatment with nanoscale iron particles can significantly lower contaminant levels around an injection well within a day or two. In a few weeks, contamination is lowered to the point that a formerly polluted site will meet federal groundwater quality standards.
Of course, cost is always an important consideration, but Zhang says nanoscale iron treatment is much less expensive than it was in 1995, when he and his colleagues first developed a chemical route for making the particles. Today, the cost has dropped from about $500 a kilogram to $40 or $50 a kilogram. (Decontaminating an area of about 100 square meters using a single injection well requires 11.2 kilograms.)
Zhang is currently forming a company to mass produce the nanoscale iron particles. In the meantime, he and his colleagues are consulting with clients including pharmaceutical firms, semiconductor manufacturers and many other companies concerned with cleaning up sites.
After nearly a decade of research, he says, "We're entering a phase of exponential growth. There are thousands and thousands of contaminated sites out there. And hopefully, this will be a cost-effective way to deal with many of them."
UGA Researchers Use Transgenic Trees to Help Clean Up Toxic Waste Site
In the first such field test ever done with trees, a team of researchers from the University of Georgia (UGA) hopes to clean up a site contaminated with toxic mercury using genetically altered cottonwood trees.
UGA scientists and city officials in Danbury, Conn., planted some 60 cottonwoods with a special gene at the site of a 19th-century hat factory in that northeastern city on July 16. The results could make clearer the future of phytoremediation -- a technique of using trees, grasses and other plants to remove hazardous materials from the soil.
"We hope to see a significant difference in the levels of mercury in the soil within 18 months, perhaps as much as a twofold reduction," said Richard Meagher, professor of genetics at UGA.
The field test is a collaboration between UGA, Western Connecticut State University, Applied PhytoGenetics Inc. of Athens and the City of Danbury.
A long-time pioneer in phytoremediation, Meagher was the first to demonstrate that a gene called merA can be inserted into plants and used to detoxify mercury in the environment. Meagher's lab has been working to find ways to let transgenic plants or trees grow on polluted sites, draw such heavy metals as mercury into the plants themselves and then either transpire a much less toxic from of the metal into the air or trap the metal aboveground for later harvest.
Meagher's team did the first-ever trial of a genetically engineered plant to sequester mercury when it grew transgenic tobacco in a New Jersey field trial in 2001, but the Danbury trial is the first to use trees, whose larger root systems and year-round life cycle make them better candidates for the long-term cleaning of polluted soil.
The publisher of an early study in 1977 that showed how native plants could transfer mercury from contaminated soils into the atmosphere, Danbury's environmental coordinator, Jack Kozuchowski, convinced officials in Danbury that the so-called Barnum Court (hat factory) site would be a perfect site for a field trial of the genetically engineered trees that Meagher and his collaborator Scott Merkle developed.
The U.S. Environmental Protection Agency (EPA) awarded the city a grant of more than $55,000 to explore use of the technology, and the trial was set up.
Postdoctoral associate Andrew Heaton of Meagher's lab and one other of Meagher's students traveled to Danbury in July to supervise planting the genetically engineered trees on the site.
Because the mercury on the site ranges, depending on location, from five to more than 300 parts per million, trials were set up to measure the effects of the cottonwood trees on progressively more polluted samples of soil. Forty-five plots, most planted with four trees each, are located on the site, which is in a mixed-use urban area and whose total area is less than one acre.
The form of mercury at the Danbury site is ionic mercury, a species that can be sequestered and transformed into less toxic metallic mercury in the transgenic trees and then transpired into the atmosphere.
"This is a field test, not a cleanup," said Meagher. "And we will be measuring mercury in both the soil and the trees to see just how much success we have in reducing the mercury levels in the soil. We are very optimistic that this technology will work."
While the trees at the site will have to be watered, the costs of that pale in comparison to traditional clean-up methods.
A team of researchers from Western Connecticut University will be studying the role of soil microorganisms in the potential clean-up of mercury on the site. According to the City of Danbury, the field test will run through the 2004 growing season, and if results are positive, genetically engineered cottonwood trees will be used to clean the whole site.
"It is our hope that the research will lead to a cleansing of the Barnum Court property so the city can transfer the property for development," said Mark Boughton, mayor of Danbury.
Meagher's mercury phytoremediation technology is exclusively licensed to Applied PhytoGenetics, or APGEN, and that company has been instrumental in helping set up the field trial.
This news item originally appeared in the October 2003 issue of Environmental Protection, Vol. 14, No. 9.
This article originally appeared in the 11/01/2003 issue of Environmental Protection.