Savvy Sieve: Carbon nanotubes filter petroleum, polluted water

Bridging the gap between the nanoworld and the macroworld, researchers have created a membrane out of carbon nanotubes and demonstrated its potential for filtering petroleum and treating contaminated drinking water.

Scientists have long valued carbon nanotubes for their high strength and thermal properties (SN: 6/5/04, p. 363: Available to subscribers at http://sciencenews.org/articles/20040605/bob10.asp), yet it’s been a challenge to assemble nanotubes into useful materials large enough for people to hold in their hands.

Researchers at Rensselaer Polytechnic Institute in Troy, N.Y., and Banaras Hindu University in Varanasi, India, have now devised a method for making such large-scale structures and found an application for them.

The researchers injected a solution of benzene and ferrocene—the materials needed to assemble the carbon nanotubes—into a stream of argon gas and then sprayed the mixture into a quartz tube. The tube was located inside a furnace heated to 900°C.

A dense forest of carbon nanotubes formed on the inner walls of the quartz tube, yielding a hollow black cylinder. The researchers carefully removed the cylinder, which measured several centimeters long and up to a centimeter in diameter. It was composed of trillions of nanotubes. Each nanotube was only a few hundred microns long, essentially the thickness of the carbon cylinder’s wall.

“It’s a pretty amazing structure if you think about it,” says lead investigator Pulickel Ajayan of Rensselaer.

To test their cylinder as a filter, the researchers capped one end and let petroleum flow into it. As the oil passed through the cylinder’s wall, the membrane caught the large and complex hydrocarbons—a necessary step in making gasoline.

In a second experiment, Ajayan and his colleagues tested their filter on contaminated water. The researchers had added Escherichia coli, the bacterium responsible for a common intestinal disease, to a sample of water and passed the sample through the filter. Analysis of the filtered water showed that it was devoid of E. coli. More surprising, when the researchers tried water contaminated with the poliovirus, which is much smaller, not one virus made it through the sieve.

The researchers describe their results in the September Nature Materials.

“It’s very encouraging to see the development of new applications like these for carbon nanotubes,” says Alan Windle, a materials scientist at the University of Cambridge in England. “This is a nice piece of work.”

However, because the researchers didn’t compare their material’s performance with that of conventional ceramic or polymer filters, it’s difficult to gauge how competitive a carbon-nanotube filter would be, Windle adds.

Ajayan considers the new study just a first demonstration of nanotube filtration. However, he says, because the pore sizes in his team’s membrane are more uniform than those in conventional membranes, a carbon-nanotube filter could be especially effective at filtering out selected chemicals or microorganisms. What’s more, because carbon nanotubes can tolerate much higher temperatures than polymers can, periodic doses of heat could unclog the membrane without destroying it.