Don't be salty -- tiny tubes desalinate water one molecule at a time

Earth is 70 per cent water, but only a tiny portion--0.007 per cent--is available to drink.

As potable water sources dwindle, global population increases every year. One potential solution to quenching the planet's thirst is through desalinisation -- the process of removing salt from seawater. While tantalising, this approach has always been too expensive and energy intensive for large-scale feasibility.

Now, researchers from Northeastern have made a discovery that could change that, making desalinisation easier, faster and cheaper than ever before. In a paper published Thursday in Science, the group describes how carbon nanotubes of a certain size act as the perfect filter for salt - the smallest and most abundant water contaminant.

Filtering water is tricky because water molecules want to stick together. The "H" in H2O is hydrogen, and hydrogen bonds are strong, requiring a lot of energy to separate. Water tends to bulk up and resist being filtered. But nanotubes do it rapidly, with ease.

A carbon nanotube is like an impossibly small rolled up sheet of paper, about a nanometer in diameter. For comparison, the diameter of a human hair is 50 to 70 micrometers - 50,000 times wider.

The tube's miniscule size, exactly 0.8 nm, only allows one water molecule to pass through at a time. This single-file lineup disrupts the hydrogen bonds, so water can be pushed through the tubes at an accelerated pace, with no bulking.

"You can imagine if you're a group of people trying to run through the hallway holding hands, it's going to be a lot slower than running through the hallway single-file," said co-author Meni Wanunu, associate professor of physics at Northeastern. Wanunu and post doctoral student Robert Henley collaborated with scientists at the Lawrence Livermore National Laboratory in California to conduct the research.

Scientists led by Aleksandr Noy at Lawrence Livermore discovered last year that carbon nanotubes were an ideal channel for proton transport. For this new study, Henley brought expertise and technology from Wanunu's Nanoscale Biophysics Lab to Noy's lab, and together they took the research one step further.

In addition to being precisely the right size for passing single water molecules, carbon nanotubes have a negative electric charge. This causes them to reject anything with the same charge, like the negative ions in salt, as well as other unwanted particles.

"While salt has a hard time passing through because of the charge, water is a neutral molecule and passes through easily," Wanunu said. Scientists in Noy's lab had theorised that carbon nanotubes could be designed for specific ion selectivity, but they didn't have a reliable system of measurement. Luckily, "That's the bread and butter of what we do in Meni's lab," Henley said. "It created a nice symbiotic relationship."

"Robert brought the cutting-edge measurement and design capabilities of Wanunu's group to my lab, and he was indispensable in developing a new platform that we used to measure the ion selectivity of the nanotubes," Noy said.

The result is a novel system that could have major implications for the future of water security. The study showed that carbon nanotubes are better at desalinisation than any other existing method-- natural or man-made.

To keep their momentum going, the two labs have partnered with a leading water purification organisation based in Israel. And the group was recently awarded a National Science Foundation / Binational Science Foundation grant to conduct further studies and develop water filtration platforms based on their new method.

As they continue the research, the researchers hope to start programs where students can learn the latest on water filtration technology - with the goal of increasing that 0.007 percent.