Better than water into wine

Expected to produce everything from towers into space to supercool electronics, carbon nanotubes now promise to make water from brine

Alex Noy and team members with a model of a carbon nanotube

Alexsandr Noy, senior scientist at LLNL, and team members Kyunghoon Kim and Jia Geng hold up a model of a carbon nanotube that can be inserted into live cells. Photo: Julie Russell

Given the wonderful stories told about it, all that carbon nanotubes had left to do was to filter water from the sea and so free the world of fears of those dreaded water wars.

Now that too is a possibility, thanks to Aleksandr Noy and other researchers from Lawrence Livermore National Laboratory, the University of California-Merced, and Northeastern University.

Noy and his team had shown last year that protons – those positively charged particles found in atoms – could flow through tiny, one-atom-thick tubes of carbon. That was a channel 0.8 nanometers wide. In a paper about it in Nature Nanotechnology, he said their work suggested that “strong spatial confinement is a key factor in enabling efficient proton transport.”

At the time Noy discussed the potential of wider nanotubes, but then keeping the nanotube width to a slender 0.8 nanometers has the additional advantage of making water molecules shuffle through in single file.

The carbon nanotube filter

The carbon nanotube filter. Image: Y Zhang

To kind of appreciate the width of a carbon nanotube, feel atop your head or anyplace else you can find some foliage, and feel how wide one a hair is. Well, just so you know it, that strand is 50,000 times thicker than Noy’s carbon nanotube.

Noy is not claiming that he is the first to selectively wick out water. For, in the cells that make up living things from bacteria through humans, there are protein tubes – called aquaporins – that do just that. Nor is this the only way to use carbon to desalinate water; for example, salt has been separated from water using filters of single-layered graphene – graphene being the same stuff you might call pencil “lead.”

But trying to strain pure water through a tube comes with its own distinct problems.

Thing is, in atoms, the number of protons at their center equal those of the electrons orbiting them. In ions, they are not, resulting in a net positive charge when they have fewer electrons, and a negative charge when there are more. And one reason water (H2O) is a liquid is because the hydrogen bonding to oxygen can be rather fickle, bonding to other oxygen ions and back again quickly.

Researchers Meni Wanunu and Robert Yousseifian Henley

Meni Wanunu, associate professor of physics and chemistry/chemical biology, with graduate student Robert Yousseifian Henley. Photo: Adam Glanzman/Northeastern University

This makes water molecules “stickier.” Making them go through the tube singly stops this clustering up of water molecules and speeds up their flow rate.

As Wanunu put it, “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.”

Noy’s team at Lawrence Livermore National Laboratories wanted some way to select specific ions – and so called upon Meni Wanunu of Northeastern University who has made a speciality of this.

While size alone can play a role in separating small water molecules from the larger ones of salt (sodium chloride), there is also an issue of effective charge.

The small, compact water molecule is effectively neutral. The sodium and chloride in salt are equal in charge but are farther apart. The carbon nanotube has a negative charge – presumably due to negative groups like carboxylic acid (COOH for the truly curious) attached to the ends, going by Noy’s previous work. This pushes off the chloride ions in the salt and other molecules, clearing the way for the water to enter and slide swiftly over the smooth insides of the nanotube.

Though the technology is not ready for prime time yet, it has promise in a world where fresh water levels are dropping. To the parched, brine to water anytime beats water to wine.

The team’s research was published in the August 25 issue of Science.

A video explaining the researchers’ work: