Cheap Drinking Water from the Ocean

[theski]

This sounds great...

Carbon nanotube-based membranes will dramatically cut the cost of desalination.
By Aditi Risbud

A carbon-nanotube membrane (quarter shown for scale). The nanotubes are located at the center of each square. Despite their tiny size, they can filter water more efficiently than current larger membranes. (Credit: Science)
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.




Holt estimates that these membranes could be brought to market within the next five to ten years. "The challenge is to scale up so we can produce usable amounts of these membrane materials for desalination, or gas separation, the other high-impact application for these membranes," he says, adding that the fabrication process is "inherently scalable."


Eventually, the membranes could be adapted for a variety of applications, ranging from pharmaceuticals to the food industry, where they could be used to separate sugars, for example, says co-author Olgica Bakajin, a physicist at LLNL. "Practically, the next step is figuring out how to take a general concept and modify it to a specific application," Bakajin says.


"There are many studies that one can imagine to build upon this study," says Northwestern's Ruoff. "Our understanding of molecular processes will be helped by experiments of this type. There are interesting possibilities for nanofluidic applications, such as in nanoelectromechanical systems and in 'smart' switching [on and off] of the flow through such small channels."

tech review

[Zac Naloen]

This Drought suffering Isle (The UK) could do with some cheap desalinisation soon. Or else im gonna get pissed off and hose pipe my street out of spite.

[Acidburns]

This Drought suffering Isle (England) could do with some cheap desalinisation soon. Or else im gonna get pissed off and hose pipe my street out of spite.

^Fixed the above. Scotland is fine thanks.

[Vehrec]

Based on some of the stuff that this month's National Geographic showed nanotech doing(converting raw crude into diesel) I am not suprized by this development, just that it is here now. The main obsticle would be the possible health risks of nanotubes in your water. Does anyone know if this is potentially hazardous?
From the way he says the process of making these is inherently scalable, I expect it to be a bottom up assembly method. Which is good, because it will be cheep, effective and probably will be doable in any lab in the world. And there are a lot of countries that could benefit from this. Even in the US, this would be a godsend. No more need to depend on one river to water the southwest.

[drachefly]

1) the jury is still out on whether nanotubes are dangerous. On the one side, you have groups that have been successfully using nanotubes as a framework for cell growth... which suggests that it isn't all that bad in those circumstances. On the other side, you have groups which did experiments where it appeared toxic. Yet another group showed that in those cases it was the combination of the nanotube with its surfactant which was toxic, not the tube itself.

So, it may depend on circumstances and what the tube is with.

2) It's really easy to filter out uncoated nanotubes: nanotubes stick very hard to silicon oxides, e.g. sand. So just run the water through a tank of sand after the desalinization step.