Nanofiber membrane filters 99.9% of salt in minutes

The World Health Organization estimates that more than 785 million people do not have access to a source of safe drinking water. Even though 71% of the Earth’s surface is covered with seawater, we cannot drink this water. Scientists around the world are working to find effective ways to desalinate seawater at low cost. Now, a team of Korean scientists may have found a way to do it in a matter of minutes.

Fresh water, necessary for most human activities, represents only 2.5% of the total water available on the planet. Changing climatic conditions are causing changes in rainfall and dry rivers, prompting countries to declare water shortages for the first time in their history. It is hardly surprising that desalination of seawater is the easiest way to solve this problem. But these processes have their own limits. One of these limitations is the wetting of the membrane.

When using membranes to filter seawater, the membrane must remain dry for long periods of time. If the membrane becomes wet, the filtration process becomes inefficient and allows large amounts of salt to pass through the membrane. For the operations of long duration, one regularly observes a progressive wetting of the membrane which is resolved by changing the membrane.

Researcher Yunchul Woo and his team at the Korea Institute of Civil Engineering and Building Technology (KICT) have now developed a membrane that is less sensitive to wetting and long-term stable.

The membrane is made of nanofibers that have been fabricated in a three-dimensional hierarchical structure, this was achieved using a type of nanotechnology called electrospinning. Thanks to this technology, the researchers were able to manufacture a highly hydrophobic, that is to say water-repellent, membrane.

The hydrophobic nature of the membrane is useful because it is designed not to let water molecules through. Instead, a temperature difference is applied to both sides of the membrane which causes the water from one end to evaporate into water vapor. The membrane allows water vapor to pass through which then condenses on the colder side. Called membrane distillation, this is a commonly used desalination method using membranes. Since the salt particles are not converted to a gaseous state, they are left out on one side of the membrane, giving highly purified water on the other side.

Korean researchers also used silica airgel in their membrane manufacturing process, which further improved the flow of water vapor through the membrane, providing faster access to desalinated water. The team tested their technology for continuous operation for 30 days and found that the membrane continued to filter 99.9% of the salt without any wetting issues.

The research study was published in the Journal of Membrane Science.

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