Imagine a cross-section of a strand of hair. That tiny surface — roughly one-millionth of a meter in diameter — is huge compared to the pores in a new type of filter developed by engineers at the University of Tokyo in Japan.
In a paper published Thursday in the peer-reviewed journal Science, the researchers unveiled their new method for desalinating water using rings of fluorine just one to two nanometers in diameter. The chemical’s hydrophobic properties contributed to its remarkable ability to filter salt molecules with impressive speed and efficiency.
Laid out end-to-end, it would take nearly 100,000 of the rings to stretch all the way across the cut surface of a human hair.
“It was very exciting to see the results firsthand,” says materials engineer Yoshimitsu Itoh, one of the paper’s co-authors. “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.”
Fluorine is the perfect element for the impossibly small pores
The key innovation in this new desalination technology is fluorine, a hydrophobic element that that’s long been prized for its desire to be left alone. It’s no accident that fluorine is a key ingredient in Teflon, which is used on non-stick pans to keep fried eggs from sticking and inside pipes to make fluids flow more efficiently. At the nanoscopic level, fluorine repels negatively charged ions, including the chlorine in salt (NaCl). Its electric properties also break down clumps of water molecules that can keep the liquid from flowing as freely as possible.
The researchers created membranes by stacking several fluorous rings on top of each other to form tubes. They embedded the tubes, side by side, in a water-tight layer of lipid molecules, creating something that resembles a cell membrane. Water molecules are welcome to pass through, and salt molecules are not.
“The real surprise to me was how fast the process occurred,”. “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.”
A long way to commercialization
Transforming sea water into something humans can drink is a vital technological capacity that’s becoming more and more important.
“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,” Itoh says.
While those technologies are proven to work at large scales, they require a lot of energy. These early results suggest that fluorine nanostructures could be the key to desalination techniques that are far more efficient. “[O]ur tests suggest fluorous nanochannels require little energy,” Itoh says.
Right now, the process of manufacturing the new material requires a lot of energy, but the researchers think they can bring those costs down. “And, given the longevity of the membranes and their low operational costs, the overall energy costs will be much lower than with current methods,” Itoh says.
The current study is impressive, but it’s far from a functional prototype that a community can rely on. “Our test samples were single nanochannels, but with the help of other specialists, we hope to create a membrane around 1 meter across in several years,” Itoh says. The researchers also have plans to look beyond water desalination.
“[W]e’re also exploring whether similar membranes could be used to reduce carbon dioxide or other undesirable waste products released by industry,” Itoh says.