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These microscopic tubes may one day help turn seawater to drinking water

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Six times faster than natural proteins

A carbon nanotube channeling water molecules.
Video: Y. Zhang, A. Noy, 2017

Scientists have created tiny carbon nanotubes that can filter water very efficiently, and could one day help turn seawater into drinkable water.

The nanotubes work similarly to certain biological proteins used to filter water in living cells, but they’re six times more efficient, according to a study published today in Science. Though the nanotubes weren’t used to desalinate actual seawater, the researchers showed that they can successfully filter water that’s just as salty. To be able to desalinate seawater, the nanotubes will have to be able to withstand much higher pressures.

The idea for the nanotubes came from natural proteins called aquaporins, says study co-author Aleksandr Noy, an associate adjunct professor at the University of California Merced. Aquaporins are found all over the human body: they’re used to transport water through membranes and filter out ions so that cells can remain healthy. Aquaporins work by transporting water molecules through channels just 0.3 nanometers wide, essentially creating a super fast train of water molecules and filtering out all ions. “It’s really really tiny,” Noy tells The Verge. “That’s what makes it selective.”

A computer representation of a carbon nanotube.
Photo: Y. Zhang, A. Noy, 2017

Aquaporins, however, are hard to use in artificial filtration systems because the proteins are pretty fragile and hard to reproduce, says Zuzanna Siwy, a professor of physics and astronomy at UC Irvine, who was not involved in the study. So Noy and other researchers decided to use carbon nanotubes, which are incredibly strong. These nanotubes are 0.8 nanometers wide, and just like aquaporins, they channel water molecules in a single line, filtering water six times faster than aquaporins, the study shows. But there’s a caveat, Noy says. “It looks like we were able to beat nature at its own game,” he says. “But in my experience, any time you want to beat nature you have to think twice.”

The aquaporins are slower because they filter out all ions, including protons. To do this, the water molecules are flipped 180 degrees in the aquaporin channels — and that chemical reaction slows the molecules down. The carbon nanotubes don’t filter out protons, Noy says, and that allows for a faster filtration system. “That’s why we beat the proteins in speed,” he says.

That still makes the nanotubes good candidates for filtering out seawater, however. The nanotubes could one day be included in artificial membranes that take ions out of seawater and turn it into freshwater. Because they’re super robust and very efficient, the carbon nanotubes have the potential to bring the cost of desalination down. But we’re not there yet.

Simulation of water molecules passing through a carbon nanotube.
Video: Y. Zhang, A. Noy, 2017

To make that happen, the nanotubes need to be scaled up and made even more robust, Noy says. They also need to be tested to make sure they work properly and can actually desalinate water. “I don’t want to sound like we have a membrane that can be used in a plant in Dubai tomorrow,” Noy says. “In fact, we’re still quite far from that membrane, but the physics of the process is there. We showed that this ... can work.”

For Siwy, the research also shows there’s always room for improvement. Other scientists had created carbon nanotubes to filter water, but those were larger and couldn’t transport water as fast as aquaporins, Siwy tells The Verge. But the nanotubes described in today’s study rival nature itself. “It looks like scientific imagination should not have any bounds,” she says, “we should dream as high as we can because we can always do better.”