Carbon nanotube-based membranes will dramatically cut the cost of desalination.
A water desalination system using carbon nanotube-based membranes could significantly reduce the cost of purifying water from the ocean. The technology could potentially provide a solution to water shortages both in the United States, where populations are expected to soar in areas with few freshwater sources, and worldwide, where a lack of clean water is a major cause of disease.
The new membranes, developed by researchers at Lawrence Livermore National Laboratory (LLNL), could reduce the cost of desalination by 75 percent, compared to reverse osmosis methods used today, the researchers say. The membranes, which sort molecules by size and with electrostatic forces, could also separate various gases, perhaps leading to economical ways to capture carbon dioxide emitted from power plants, to prevent it from entering the atmosphere.
The carbon nanotubes used by the researchers are sheets of carbon atoms rolled so tightly that only seven water molecules can fit across their diameter. Their small size makes them good candidates for separating molecules. And, despite their diminutive dimensions, these nanopores allow water to flow at the same rate as pores considerably larger, reducing the amount of pressure needed to force water through, and potentially saving energy and costs compared to reverse osmosis using conventional membranes.
Indeed, the LLNL team measures water flow rates up to 10,000 times faster than would be predicted by classical equations, which suggest that flow rates through a pore will slow to a crawl as the diameter drops. "It's something that is quite counter-intuitive," says LLNL chemical engineer Jason Holt, whose findings appeared in the 19 May issue of Science. "As you shrink the pore size, there is a huge enhancement in flow rate."
The surprising results might be due to the smooth interior of the nanotubes, or to physics at this small scale -- more research is needed to understand the mechanisms involved. "In some physical systems the underlying assumptions are not valid at these smaller length scales," says Rod Ruoff, a physical chemist and professor of mechanical engineering at Northwestern University (who was not involved with the work).
To make the membranes, the researchers started with a silicon wafer about the size of a quarter, coated with a metal nanoparticle catalyst for growing carbon nanotubes. Holt says the small particles allow the nanotubes to grow "like blades of grass -- vertically aligned and closely packed." Once grown, the gaps between the nanotubes are filled with a ceramic material, silicon nitride, which provides stability and helps the membrane adhere to the underlying silicon wafer. The field of nanotubes functions as an array of pores, allowing water and certain gases through, while keeping larger molecules and clusters of molecules at bay.
This illustration demonstrates one form a carbon nanotube can take.
Experiments during the past several years examining membranes formed from carbon nanotubes have demonstrated potential for a variety of applications, perhaps most notably economical filtration of water for desalination.  But something that has consistently confused researchers is that the rate of flow for water passing through these special membranes is considerably higher than would be predicted by classical physics. In some experiments the flow rate was up to 10,000 times the speed expected.  In addition, recent measurements by a team at Lawrence Livermore National Laboratory conducted via nuclear magnetic resonance showed that the water passing through nanotubes exhibits very different properties in comparison to bulk water.

In a press release Tuesday the News Bureau of the University of Illinois at Urbana-Champaign highlighted a recent finding that offers one possible explanation for the phenomenon.  The research, published in ''Physical Review Letters'' last month (), used molecular modeling to analyze the predicted behavior and interactions of individual water molecules passing through a nanotube.

UIUC researchers Narayana R. Aluru, a professor of mechanical science and engineering, and doctoral student Sony Joseph found that the chemical polarity of H2O and what they describe as "confinement effects" within the bounds of smaller nanotubes cause the molecular movements to couple.  Due to the water molecules interacting and orienting themselves in response to one another they flow in a more orderly helical or corkscrew pattern through the nanotube.  This flow pattern may explain some of the experimental results obtained by other scientists.

Hopefully this discovery is another step towards an affordable technology to produce plentiful potable water for drinking, irrigation of crops, and other purposes, a vital resource that is in short supply in many parts of the world.