Future of desalination: Use a Teflon-like membrane for water purification

Water scarcity is an increasing problem all over the world. By 2025, approximately 230 million Africans will be without water, and up to 460 million more people may live in water-stressed areas.

70% of the Earth is water, so it’s easy to assume that fresh water will always be plentiful. Freshwater is extremely scarce. Desalination plants are one technology that can help increase freshwater production. Desalination, which is the process of removing salts from seawater, produces fresh water that can then be used safely. About half the water it receives is converted into drinkable water by a desalination plant.

Seawater desalination, although a proven method of producing drinking water from seawater, comes at a high cost. Fluorine-based nanostructures have been used to filter salt out of water for the first time. Fluorous nanochannels, which are fluorine-based, work faster and require less pressure and energy than traditional desalination techniques.

If you have ever used a Teflon-coated skillet, you will probably have seen the ease with which wet ingredients glide across it. Teflon’s most important component is fluorine. Fluorine is a light ingredient that is naturally water-repellent or hydrophobic. Teflon can be used to increase water flow by lining pipes. This behaviour fascinated Associate Professor Yoshimitsu Itoh of the University of Tokyo’s Department of Chemistry and Biotechnology. They were therefore inspired to study how fluorine channels or pipelines might work at a smaller scale, the nanoscale.

Communities around the globe with limited access to safe drinking water could benefit from reducing the cost of energy and financial costs and simplifying water desalination. Credit: 2022 Itoh et al.

“We wanted to determine how efficient a fluorous microchannel could be in selectively filtering different compounds. In particular, salt and water. After running complex computer simulations, it became clear that creating a working sample was worthwhile.” Itoh said. There are two main methods to desalinate water at the moment. One is thermal. This involves using heat to evaporate seawater and condense it as pure water. The other is reverse osmosis which uses pressure to force water through membranes that block salt. While both methods are energy-intensive, our tests show that fluorous nanochannels use less energy and offer other benefits.

Researchers developed test filtration membranes using chemically manufactured nanoscopic fluorine rings, which were then stacked and embedded in an impenetrable lipid layer. This was similar to organic molecules found within cell walls. Multiple test samples were created with nanorings of sizes ranging from 1 to 2 micrometres. For comparison, human hair measures almost 100,000 nanometers in width. To determine the effectiveness and efficiency of their membranes, Itoh and his associates evaluated the presence of chlorine ions (one of the main components of salt) on either side.

“It was thrilling to see the results in person. Itoh stated that the smaller test channels completely rejected salt molecules. The larger channels improved significantly over other desalination techniques, including advanced carbon nanotube filters. The real surprise was the speed of the process. Our sample performed several thousand times faster than typical industrial devices and approximately 2,400 times faster than experimental carbon nanotube-based desalination systems.

Fluorine repels electrically negative ions like chlorine in salt. This negativity has an additional benefit: it breaks down water clusters, loosely bound groups or water molecules. They pass through the channels faster. Fluorine-based water desalination membranes developed by the team are more efficient, less energy-intensive, and easier to use.

“Currently, the method we use to synthesize our materials requires a lot of energy. However, we are working on ways to reduce this. Itoh stated that the membranes’ longevity and low operational costs would lower the overall energy cost than current methods. We also plan to scale this up. Although our test samples were only single nanochannels, we hope to make a membrane of approximately 1 meter in length with the assistance of other specialists over the next few years. Parallel to these manufacturing concerns, our team is also investigating whether similar membranes can be used to reduce carbon dioxide and other unwanted waste products from the industry.