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Nanotubes squeeze salt from water

Lawrence Livermore researchers hope carbon-nanotube membrane could be used to desalinate water inexpensively.

Researchers at the Lawrence Livermore National Laboratory have developed a carbon-nanotube membrane that could one day be used to desalinate water inexpensively.

Common methods for removing salt from water rely on reverse osmosis, or moving freshwater through a semipermeable membrane by applying pressure; but it can be expensive because of the energy needed to produce large amounts of pressure. According to the national security lab, more-permeable nanotube membranes could offset energy costs of desalination by as much as 75 percent, comparatively.

"This is like having a garden hose that can deliver as much water in the same amount of time as (a) fire hose that is 10 times larger," Olgica Bakajin, Lawrence Livermore's lead scientist on the project, said in a statement.

The developed membrane is a novel compilation of materials, according to the lab, which is based in Livermore, Calif. Molecules made of carbon atoms are arranged to form hollow nanotubes, which are about 50,000 times thinner than a human hair. Billions of those nanotubes are combined to act like pores on a silicon chip the size of a quarter. The slick surface inside the tubes lets liquids and gases flow through, but the tiny openings of the tubes block larger molecules. A ceramic matrix material is used to adhere the tubes.

The pores are so small, only six water molecules can pass through their diameter.

Typical simulations predict that fluids or gases would flow quickly through carbon nanotubes. But the Lawrence Livermore team was able to measure water flow at 100 to 10,000 times faster than previous predictions, according to Bakajin.

Commercial companies are developing similar membranes based on carbon nanotubes. Vermont-based Seldon Laboratories, for example, sells "Nano mesh," a nanotechnology water filter that can remove bacteria and viruses so that they're purportedly at levels better than Environmental Protection Agency standards. The company is building a seawater desalinization device based on the nanotechnology, which it says should be available in 2007.

For its part, Lawrence Livermore believes its findings will support other applications, including demineralization and gas separation. The journal Science featured the research in this week's issue.