Special Treatment

Tiny technology tackles mega messes

Nearly 4 decades after it sent into orbit its last spacecraft—Apollo 7, which carried a three-man crew around Earth 163 times—NASA’s Launch Complex 34 at Cape Canaveral Air Force Station in Florida resembles an abandoned amusement park. Massive, rusting structures that once fired Saturn rockets into space dominate the desolate concrete launch pad. The surrounding landscape is overgrown with scrappy vegetation.

GOING UNDERGROUND. When injected into groundwater tainted with chemicals such as trichloroethylene from a contaminated source, iron nanoparticles intercept and decontaminate the toxic plume. The iron, in turn, is oxidized into rust. E. Roell
ONE-TWO PUNCH. The 10-nanometer-wide particles in this solution (grouped in micrograph, center) are gold spheres spotted with clusters of palladium atoms (illustration, right). A synergistic effect between the two metals enables the particles to break down environmental contaminants 100 times as fast as pure palladium particles do. Wong

Yet Launch Complex 34 is once again at the forefront of science. Instead of space vehicles, however, the site is launching new technologies for cleaning up the environment.

Throughout the 1960s, it was used for cleaning rocket engines and degreasing equipment, practices that soaked the earth surrounding the site’s facilities with tons of solvents now known to be toxic and carcinogenic. Launch Complex 34 is not alone in this vice. Thousands of government and industrial sites across the country are contaminated with a host of troublesome substances.

Chlorinated solvents such as trichloroethylene (TCE), which is widely used by industry to degrease metals and electronic parts, are particularly problematic. When they are dumped into the environment, these oily chemicals persist in soils and gradually leach into groundwater. According to the Environmental Protection Agency, the number of sites in the country polluted with TCE and other so-called dense nonaqueous-phase liquids ranges from 15,000 to 25,000.

Over the years, government agencies and companies have spent billions of dollars trying to clean up these contaminated sites. Time and again, existing technologies have proved inadequate. Only a fraction of the sites has been completely cleaned up.

One of the most frequently used remediation techniques is pump-and-treat, in which contaminated groundwater is brought to the surface. There, contaminants are removed with special filters before the remediated water is injected back into the ground. Gallon by gallon, the technique works, but it’s so laborious that a major contamination site can take decades to be fully cleaned. Bioremediation—the breakdown of contaminants by naturally occurring microbes bolstered by nutrients injected into the ground—can also take years to produce substantial results.

“A real fresh approach is needed,” says Michael Wong, a chemical engineer at Rice University in Houston. For their part, Wong and his colleagues are experimenting with nanosize metallic particles that could theoretically break down soil and groundwater contaminants faster and more cheaply than the existing technologies can. Other scientists have already taken this tactic beyond the laboratory. Since 2002, several research groups have injected metallic nanoparticles into the ground at several contaminated test sites, including Launch Complex 34.

The results so far are promising. The concentration of toxic chemicals in soil at these sites nosedives once the nanoparticles get to work. “It’s a very powerful technology,” says Barbara Karn of EPA’s National Center for Environmental Research in Washington, D.C. If future large-scale field tests pan out, the technology could alleviate a longstanding environmental headache.

Pure and simple

When it comes to the capacity of metallic nanoparticles to break down pollutants efficiently, size is everything. Because each particle is only about 10 nanometers to 100 nm across, about the width of a virus, it can zigzag its way through soil particles or flow with groundwater to hard-to-reach areas, such as those under buildings and airport runways.

Also, because nanoscale particles have extremely high surface areas relative to their volumes, more of the metal is available to contact and react with contaminants.

“When you make small particles, the reaction rate increases substantially,” says Wei-xian Zhang of Lehigh University in Bethlehem, Pa. Experiments in his lab have shown that 1 kilogram of nanoscale iron particles, which looks like black powder, is as reactive as about 1,000 kg of micrometer-size iron particles, which looks more like black sand. “You can clean up a site much faster and use a smaller amount of material,” says Zhang.

What’s more, the faster reaction rates lead to fewer undesirable by-products. The breakdown of a chemical usually involves several steps—A gets broken down to B, then B to C, and finally C to D. If the reaction is too slow and stops somewhere in the middle, then intermediate compounds B and C build up. “In many cases, these intermediates are more toxic than the original compound,” says Zhang.

For purposes of environmental remediation, few metals have been more thoroughly investigated than iron. In moist settings, including the ground, iron naturally corrodes to iron oxide—rust—by giving up electrons to water molecules. Environmental engineers have long sought to commandeer this trait by designing iron particles that donate electrons to toxic chemicals instead. As iron transforms into rust, many bad chemicals transform into benign products as well. For instance, the extra electrons strip all the chloride groups off TCE, converting the toxic compound into ethane.

Several years ago, Zhang developed a chemical technique for making nanoscale particles of iron. Since then, his team has tested, with approval from EPA, iron nanoparticles at several sites polluted with TCE and its toxic relatives perchloroethylene and dichloroethylene.

One of the first field tests took place in 2002 at an industrial site in Research Triangle Park, N.C. To enhance the particles’ reactivity, the researchers coated the iron nanoparticles with a layer of palladium, a favorite metal for catalyzing the breakdown of chemicals. Then, Zhang’s team mixed a total of 11.2 kg of the nanoparticles—enough to fill a coffee can—into about 6,000 liters of water and slowly injected the resulting slurry into contaminated groundwater running under the site. Within 6 weeks, the concentration of the target chemicals dropped by 99.9 percent in groundwater within 12 meters of the injection site.

Engineers with the consulting firm PARS Environmental in Robbinsville, N.J., have also seen promising outcomes from injecting iron nanoparticles at more than a dozen sites around the United States. “We haven’t seen results [from other remediation strategies] as effective as those we’ve experienced with nanoiron,” says Harch Gill, an engineer with the company.

In addition to being simple, the technology is relatively inexpensive, says Gill. He contrasts the price of injecting a slurry of nanoparticles with alternative strategies. The company recently calculated that the cost of using the pump-and-treat approach to clean up a small, polluted site owned by a New Jersey manufacturing firm would be about $4 million. An alternative, to intercept a plume of polluted groundwater with a permeable iron barrier, would cost about $2 million. The firm chose to experiment with iron nanoparticles, the cheapest option at $450,000.

Targeting the source

Although iron nanoparticles have already proved successful at cleaning up toxic chemicals that spread through groundwater, they don’t go after the source, the polluted, saturated soil under the original dumping sites, says Chris Clausen, a chemist at the University of Central Florida in Orlando. Even after a plume is cleaned up, material from the source can continue leaching out of the soil, forming a new plume.

“Take a dry cleaning operation that dumped chlorinated solvents into the environment,” he says. “If you had nothing more than 25 kg of solvents in that soil and your groundwater flow was relatively slow, you could have a contaminated plume that could last for hundreds of years.”

Iron nanoparticles don’t work very well for treating sources of chemicals because the particles are hydrophilic, or water attracting, says Clausen’s colleague Cherie Geiger, also of the University of Central Florida. In contrast, the organic contaminants in a typical underground source are highly hydrophobic, or water repelling. Instead of penetrating the saturated soil, the iron particles float on top of the contaminated zone.

Clausen and Geiger have adapted iron nanoparticles to circumvent this problem. The researchers encapsulated clusters of particles in hydrophobic membranes of vegetable oil. “In order to get the particles to move to where the contamination is, we wanted to create something that would travel through the [contaminated] soil just like chlorinated solvents do,” says Clausen.

To demonstrate the technology in the field, Clausen and Geiger teamed with researchers at NASA, EPA, and Geosyntec, an engineering firm based in Guelph, Ontario. Their maiden site was Launch Complex 34.

As reported in the March 1 Environmental Science & Technology, the group injected a half-ton of nanoparticles into a small area under one of the complex’s engineering buildings. Within 90 days, soil tests showed that 85 percent of the contaminants—mainly TCE and dichloroethylene—had disappeared from the test site. Within that area, some sections of soil showed 100 percent removal while others showed very little, a disparity that Geiger blames on uneven distribution of the particles.

Environmental engineer Greg Lowry and chemist Krzysztof Matyjaszewski of Carnegie Mellon University in Pittsburgh are using another material to cover iron nanoparticles. The polymer coatings they’re developing not only facilitate the nanoparticles’ transport through contaminated soil but also enable the particles to selectively seek out chlorinated compounds. “If we can’t get [the particles] to where they need to be, then they’re no good to us,” says Lowry.

The coatings consist of three polymer layers. The outside shell is hydrophilic, so that the particle can move easily through the groundwater. The next layer is hydrophobic, to have an affinity for chemicals such as TCE. The third and innermost layer anchors the entire polymer complex to the iron nanoparticle.

Because it’s often difficult to map precisely where an underground pool of chemicals is and how far it has spread through soil, choosing where to inject the particles can involve a certain amount of guesswork. “You can put the particles in the ground, but they might float right by the source,” says Lowry. His idea is to inject coated particles into groundwater upstream of the chemicals and to let them flow into the contaminated site.

Although field tests might be several years off, preliminary experiments with the three-coat particles look promising, Lowry says.

Particle power

One of the drawbacks of iron nanoparticles is that because the particles dissolve after they’re injected into the ground, “you can’t reuse the iron,” says Rice’s Wong. So, he and his colleagues are pursuing a different tack: synthesizing nanoparticles of palladium on gold that are catalytic and therefore don’t break down. The particles can then be incorporated into an adapted pump-and-treat system.

Using such catalytic nanoparticles could speed up a long and tedious process and make it much cleaner, the researchers reason. Today’s pump-and-treat systems flush contaminated water through a disposable chemical sieve much as water goes through a faucet-mounted filter. The sieve strips the contaminants from the water but it doesn’t actually destroy them. “You don’t solve the problem because the TCE is now in a landfill,” says Wong.

But a filter containing palladium on gold nanoparticles could break down TCE and other compounds, eliminating the contaminants. Palladium is a highly effective catalyst, but it’s also expensive. So, Wong’s group decided to coat nanoparticles of silica, the main ingredient of glass, with a thin layer of palladium to minimize the amount of the metal required for the reaction.

The researchers subsequently found that they could speed up the chemicals’ breakdown by using 10-nanometer-wide gold particles spotted with nanoscale clusters of palladium atoms. “The gold somehow makes the palladium more reactive,” says Wong, who describes the particles in the March 1 Environmental Science & Technology. Laboratory measurements show that the two-metal particles break down TCE 100 times as fast as pure palladium particles do.

In collaboration with Martin Reinhard, an environmental engineer at Stanford University, Wong and his colleagues plan to attach the particles to a porous alumina membrane. The researchers expect to test the filters on contaminated water this summer.

Although nanoscale materials are proving adept at cleaning up the environment, many questions remain regarding their possible harmful effects on natural ecosystems and human health.

So far, Zhang and his coworkers have found that iron nanoparticles injected into the ground not only oxidize to rust but also become virtually indistinguishable from the iron oxide that is naturally abundant in the environment. Still, the material’s potential toxic effects on human health—say, on workers exposed to pure-iron nanoparticles in a plant that makes them—are unknown, he says.

What’s more, researchers are still trying to assess the long-term fate of these particles once they’re injected into the ground. “If we’re really going to gear up and make these nanomaterials and disperse them widely in the environment, are we going to look back 20 years from now and say, ‘Wow, we really shouldn’t have done that’?” asks Lowry.

Recently, several groups around the world have begun investigating the harmful effects of engineered nanoscale materials. For instance, initial studies of carbon molecules known as buckyballs and of carbon nanotubes have shown that at certain doses the materials are toxic to animals (SN: 3/19/05, p. 179: Nano Hazards: Exposure to minute particles harms lungs, circulatory system).

Unfortunately, risk assessments lag far behind the pace of new developments in nanotechnology. EPA’s Karn is aware of such concerns and says that she hopes to fund more studies in the coming year on the safety of nanoparticles designed for environmental remediation.

However, unlike the nanoscale materials that are being developed for use in a wide range of consumer products, these nanoparticles are being injected into waste sites that aren’t environmentally healthy to begin with. Still, Karn says, “We need to make sure these particles don’t move beyond the places where they’re injected and cause any unforeseen consequences.”

That would be unfortunate, she says, since the technology’s ultimate benefit to the environment could be significant.

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