NANOPARTICLES of iron could dramatically reduce the levels of toxic chemicals at tens of thousands of contaminated sites. Laboratory and field tests have shown that the particles can break down common pollutants such as trichloroethene, carbon tetrachloride and dioxins in a matter of hours.
A decade ago, chemists discovered that iron could play an important role in destroying polluting chemicals. Under the right conditions, the metal breaks down water to produce hydrogen, as well as reacting with oxygen to form iron oxide, better known as rust. But when other chemicals are present, the chemistry can be much more complex.
In 1994, Robert Gilham and Stephanie O鈥橦annesin from the University of Waterloo in Ontario, Canada, showed that iron particles could break down a number of common pollutants such as pesticides, organic dyes and chlorinated benzenes, albeit relatively slowly.
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The reaction takes place on the surface of the iron. So when the coarse powder is replaced with particles only a few nanometres across, which vastly increases the surface area, the reaction proceeds up to a 1000 times as fast. Now Wei-xian Zhang, an environmental engineer specialising in hazardous waste treatment and environmental clean-up at Lehigh University in Pennsylvania, has tested the ability of nanoparticles to clean up chlorinated organic solvents such as trichloroethene, which are a common class of pollutant. In lab tests, he found that the particles could cut the level of all chlorinated contaminants in groundwater by 99 per cent in only 24 hours.
The particles also performed well in field tests, in which they were mixed with water and injected into the ground. They are small enough to soak several metres into soil, but because they are insoluble they do not spread into the surrounding groundwater. A kilogram of the nanoparticles, enough to clean up almost 10 square metres of contaminated land, costs about $45.
Zhang says that flushing nanoparticles into soil can profoundly reduce contamination around a site within 48 hours (Journal of Nanoparticle Research, vol 5, p 323).