Water supply is a growing global issue, especially with climate change bringing on more droughts. Seawater desalination is used worldwide to filter up to 97.4 million cubic metres per day. Two methods – thermal and reverse osmosis – predominate; both have huge energy costs.
In a pioneering study published in Science, researchers have used a fluorine-based nanostructure to successfully filter salt from water faster and more efficiently than other current technologies. But how does it work?
Most people are familiar with the non-stick properties of Teflon-coated frying pans. Teflon’s key component is fluorine, which is a naturally water-repelling (hydrophobic) lightweight element. Teflon has been used to line pipes to help improve the flow of water, something which inspired a team at the University of Tokyo (Japan) to test on a nanoscale.
“We were curious to see how effective a fluorous nanochannel might be at selectively filtering different compounds, in particular, water and salt,” says lead researcher, Associate Professor Yoshimitsu Itoh. “And, after running some complex computer simulations, we decided it was worth the time and effort to create a working sample.”
The team created nanoscopic fluorine rings between 0.9 and two nanometres (nm) in size – for comparison, a human hair is almost 100,000nm wide. These rings have a super hydrophobic surface made up of densely packed carbon-fluorine (C-F) bonds to repel water.
The rings were stacked and embedded in an otherwise impermeable lipid membrane, just like the surface of plant cell walls. The team tested the effectiveness of the membranes by measuring the presence of chlorine irons, one of the major components of salt, on either side of the test membrane.
“It was very exciting to see the results firsthand,” says Itoh. “The smaller of our test channels perfectly rejected incoming salt molecules, and the larger channels too were still an improvement over other desalination techniques and even cutting-edge carbon nanotube filters.”
One of the added benefits of fluorine is that is has a negative electrical charge as it has an additional electron, meaning it repels other negative ions such as the chlorine ions found in salt. It also helps to break down water clusters, which are loosely bound groups of water molecules, which further enhances its water permeability – ability to let water pass through – at an unprecedentedly high rate.
“The real surprise to me was how fast the process occurred,” says Itoh. “Our sample worked around several thousand times faster than typical industrial devices, and around 2,400 times faster than experimental carbon nanotube-based desalination devices.”
“There are two main ways to desalinate water currently: thermally, using heat to evaporate seawater so it condenses as pure water; or by reverse osmosis, which uses pressure to force water through a membrane that blocks salt. Both methods require a lot of energy, but our tests suggest fluorous nanochannels require little energy, and have other benefits too.”
The team hope to scale this technology up to industrial size over the next several years, aiming to create a membrane around one metre across in diameter. This membrane tech could also be used to filter other chemicals, such as carbon dioxide or other industrial waste products.
