Making Nanotech Safer

As thin as one nanometer in diameter and stronger than steel, carbon nanotubes have been called a “poster child for the ‘nanotechnology revolution.’ ” Environmental chemist Desirée Plata, PhD ‘09, wants to make sure that they don’t become a poster child for environmental destruction. In 2007, Plata and colleagues at MIT, the Woods Hole Oceanographic Institution (WHOI), and the University of Michigan found that a common process used in manufacturing nanotubes releases more than a dozen harmful by-products, including chemicals that cause cancer and contribute to global warming. Three years later, the researchers showed how to tweak the process so that it’s more efficient and less toxic.

She calls her approach “preventive science”: instead of waiting for carbon nanotubes to become ubiquitous and then scrambling to clean up the damage, she says, “we can use science to predict environmental problems and engineering to prevent them.” Plata aims to demonstrate, someday, that preventive science makes sense for other industries, too. If she’s right, her work may fundamentally change the way new materials are brought to market.

A native of Portland, Maine, Plata was eight years old when she made her first environmental health hypothesis. Her grandmother, in nearby Gray, had multiple sclerosis. Two doors down from her grandmother, a neighbor had cancer, and so did a neighbor two doors down from that. “Every other house had someone who was sick,” Plata recalls. “I said to my mom, ‘I bet there’s something in the water.’ ” Years later, she learned that there was: trichloroethylene (TCE), a carcinogenic and neurotoxic solvent that had leached from a nearby industrial-waste plant.

When Plata went to Union College in Schenectady, New York, she planned to be a doctor. But as she learned more about how environmental contaminants could affect human health, she says, “I realized how much easier it would be to prevent cancer than to cure it.” When she spent a summer at WHOI doing research on oil spills, she met MIT professor Philip Gschwend, who had revolutionized environmental chemistry by using mathematics and physics to predict how chemicals would behave in the environment. With Gschwend and Christopher Reddy of WHOI as her advisors, she began the MIT-WHOI joint doctoral program in 2003.

For abstracts of Plata’s two key papers on nanotech manufacturing click here and here.

Now Plata is a professor at Mount Holyoke College and a visiting assistant professor in MIT’s Department of Civil and Environmental Engineering, and she and Gschwend are continuing a collaboration begun when she was in grad school in hopes of making nanotube manufacturing safer before the industry ramps up. Carbon nanotubes can be woven into fabric to create smart clothing, incorporated into special inks used to print lightweight batteries, and crafted into artificial muscles that can withstand extremes of cold and heat, among a multitude of potential commercial uses. Analysts forecast that demand for nanotubes will rise from $215 million in 2009 to $1.4 billion in 2014.

All too often, when a new material like this one is introduced, any environmental harm doesn’t become clear until decades later. Then governments may ban the product, but the damage is already done. DuPont, for example, used perfluorooctanoic acid (PFOA) in the process of manufacturing Teflon for years before PFOA was found to be a carcinogen that persists in the environment. In 2005, the company, which the EPA determined had failed to disclose the potential harm, was fined $10.25 million. DuPont also agreed to spend another $6.25 million on related scientific projects and to stop using PFOA by 2015.

Part of the problem is that companies optimize new materials to cut costs and boost performance, Plata says. But they usually don’t think about optimizing them to minimize environmental harm.

Although media reports often portray nanotube fabrication as a marvel of high-tech manufacturing, in some ways it seems more like a modern version of alchemy. Here’s how one of the more common methods, catalytic chemical vapor deposition (CVD), works: pump a carbon-containing gas into a reactor, add a metal catalyst, turn some knobs to heat it to 500 to 1,200 ˚C, and wait about 10 minutes for nanotube forests to grow. Methane and ethylene are popular choices for the gas, iron or nickel for the catalyst.

No one knows the exact mechanism that transforms these materials into nanotube “gold.” But we do know that CVD is woefully inefficient: for every 100 carbon atoms that enter the reactor, only three end up in nanotubes. What happens to the other 97? From a crude analysis, manufacturers concluded that they leave the reactor in the form of a gas that’s pretty much unchanged. Plata surmised, however, that heating a carbon gas to such high temperatures probably does change the gas’s structure, and that only some of those changes are likely to be critical to nanotube formation. The rest might be unnecessary at best, and at worst they could be harmful to the product or the planet.

Plata, Gschwend, Reddy, and John Hart, SM ‘02, PhD ‘06, an assistant professor at the University of Michigan, set out to measure those changes more precisely, using a novel test reactor that Hart developed in the MIT lab of Alex Slocum ‘82, SM ‘83, PhD ‘85. The reactor allows them to separate the heating phase of CVD from the interaction of the gas with the catalyst, so researchers can identify the by-products that result from heating the gas. They cooked up a small batch of nanotubes and found that heating the gas released over 45 carbon by-products—more than a dozen of them harmful, including smog-forming volatile organic compounds (VOCs) such as benzene and butadiene, and cancer-causing polycyclic aromatic hydrocarbons.

Rough calculations suggest that inhaling a half-second puff of these chemicals would do as much damage as smoking one cigarette, Plata says; running the reactor for one 10-minute cycle, to make just half a milligram of nanotubes, would release as many toxins as 1,000 cigarettes. “These compounds are produced in trace quantities, but if you’re producing thousands of tons of carbon nanotubes, it becomes a big problem,” she says.

This past fall, just three years after first documenting the problem, Plata and colleagues offered a solution. They tested the carbon by-products generated by the gas-heating step of CVD to determine which ones were critical to nanotube growth. They also found that adding vinyl acetylene and methyl acetylene to ethylene gas, without any heat, made nanotube production much cleaner than heating ethylene gas alone. The revised technique reduced VOC formation by an order of magnitude, and the researchers found that benzene—a carcinogenic by-­product that does not speed nanotube formation at all—could be eliminated entirely. The proc­ess was also more efficient, using 40 percent less hydrogen, 20 percent less ethylene, and 55 percent less energy. And it yielded the same growth rate as the ethylene-only method that requires heating.

“People are investing tens of millions of dollars to build giant reactors,” Plata says. “We hope to get [these changes] incorporated into big facilities as they are being built so that they don’t have pollution built into them.”

To persuade companies to take this approach, Plata plans to do additional cost-effectiveness studies. The idea is to demonstrate that what she calls “coöptimization”—working to minimize the environmental impact as new materials are developed—can make manufacturing more efficient and save companies the cost of environmental remediation down the road.

Plata believes that a host of industries, including pharmaceuticals and munitions, could use preventive science to reëngineer their products. She wants to show that an ounce of prevention is worth more than the proverbial pound of cure, because often there is no remedy for the unintended damage that chemicals create.

“That’s what my life mission is,” Plata says. “We [usually] don’t know about these problems until after it’s too late. We really need to start thinking about them early on. Otherwise we’re not going to be able to undo the harm.”