Darn, it seems these days that just about as fast as you can think of something — there’s a researcher around working on it. I’ve mentioned that it would be nice to shape a carbon nanotube filter into a pipe so that you could just stick it out in the ocean and fresh water would flow in. Here’s a researcher who has developed a carbon nanotube pipe for a purpose that might be adapted to water desalination.

Savvy Sieve: Carbon nanotubes filter petroleum, polluted water


Alexandra Goho

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.


CLEAR PASSAGE. The wall of this tube-shaped filter is made of a single layer of densely packed carbon nanotubes.

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.

A later generation carbon nanotube filter might be shaped into the form of a house sized mushroom that sits on the ocean floor not too far from the coast. The carbon nanotube filter would be on the dome of the mushroom. A ram pump would be on the stem. The ram pump would use the weight and momentum of falling water to push the water ashore.

Ram pumps have only two moving parts, making them virtually maintenance-free. The basic idea behind a ram pump is simple. The pump uses the momentum of a relatively large amount of moving water to pump a relatively small amount of water uphill. To use a ram pump, you must have a source of water situated above the pump. For example, you must have a pond on a hillside so that you can locate the pump below the pond. You run a pipe from the pond to the pump. The pump has a valve that allows water to flow through this pipe and build up speed.

Ram pump in action

Once the water reaches its maximum speed this valve slams shut. As it does so the flowing water develops a great deal of pressure in the pump because of its inertia. The pressure forces open a second valve. High-pressure water flows through the second valve to the delivery pipe and the pressure in the pump falls. The first valve can then reopen to allow water to flow and build up momentum again, and so the cycle repeats.

Kind of a neat idea I think. But a couple years from now the cost of photovoltaics might come down sufficiently to make a solar pump an attractive low maintenance option or  carbon nanotube membranes may be able to cheaply extract hydrogen from water so as to make it dirt cheap to pump the water using hydrogen as an energy source. That way you don’t have a big contraption out in the ocean. All you have is a pipe. But a Ram Pump might serve as an intermediary step.

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