The future of desalination: Using a Teflon-like membrane to purify water
Water scarcity is a growing problem around the globe. In Africa alone, it is estimated that about 230 million people will face water shortages by 2025, with up to 460 million living in water-stressed regions.
Water covers 70% of Earth, so it is easy to assume that it will always be abundant. However freshwater is very scarce. One technology designed to help produce more freshwater is desalination plants. Water desalination is the process of removing salt from seawater to produce fresh water that can be processed further and safely used. A desalination plant converts about half of the water it receives into drinkable water.
Although seawater desalination is a well-established way of producing drinking water, it comes with a high energy cost. Researchers have successfully filtered salt from water for the first time using fluorine-based nanostructures. These fluorous nanochannels are more effective than conventional desalination technologies because they operate quicker, use less pressure, are a more effective filter, and use less energy.
You’ve probably seen how effortlessly wet ingredients slide across a nonstick Teflon-coated frying pan if you’ve ever used one. Fluorine, a lightweight ingredient that is inherently water-repellent, or hydrophobic, is a crucial component of Teflon. Teflon can also be used to enhance the flow of water by lining pipes with it. Associate Professor Yoshimitsu Itoh of the University of Tokyo’s Department of Chemistry and Biotechnology, as well as his colleagues, were intrigued by this behavior. Thus, they were inspired to investigate how fluorine pipelines or channels may work on a different scale, the nanoscale.
“We were curious to see how effective a fluorous nanochannel might be at selectively filtering different compounds, in particular, water and salt. And, after running some complex computer simulations, we decided it was worth the time and effort to create a working sample,” said Itoh. “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 researchers developed test filtration membranes by chemically manufacturing nanoscopic fluorine rings that were stacked and implanted in an otherwise impenetrable lipid layer, similar to the organic molecules found in cell walls. They developed multiple test samples with nanorings ranging in size from 1 to 2 nanometers. A human hair is almost 100,000 nanometers wide for comparison. Itoh and his colleagues evaluated the presence of chlorine ions, one of the major components of salt (the other being sodium), on either side of the test membrane to determine the effectiveness of their membranes.
“It was very exciting to see the results firsthand. 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,” said Itoh. “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.”
As fluorine is electrically negative, it repels negative ions such as the chlorine found in salt. But an added bonus of this negativity is that it also breaks down what is known as water clusters, essentially loosely bound groups of water molecules, so that they pass through the channels quicker. The team’s fluorine-based water desalination membranes are more effective, faster, require less energy to operate, and are made to be very simple to use as well, so what’s the catch?
“At present, the way we synthesize our materials is relatively energy-intensive itself; however, this is something we hope to improve upon in upcoming research. And, given the longevity of the membranes and their low operational costs, the overall energy costs will be much lower than with current methods,” said Itoh. “Other steps we wish to take are of course scaling this up. 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. In parallel with these manufacturing concerns, we’re also exploring whether similar membranes could be used to reduce carbon dioxide or other undesirable waste products released by industry.”
Reference: “Ultrafast water permeation through nanochannels with a densely fluorous interior surface” by Yoshimitsu Itoh, Shuo Chen, Ryota Hirahara, Takeshi Konda, Tsubasa Aoki, Takumi Ueda, Ichio Shimada, James J. Cannon, Cheng Shao, Junichiro Shiomi, Kazuhito V. Tabata, Hiroyuki Noji, Kohei Sato and Takuzo Aida, 12 May 2022, Science.