Researchers have just turned a corner when it comes to efficient desalination by introducing a graphene oxide membrane that sieves salt right out of seawater. Today, the technique is still limited to the lab, but it has the exciting potential to turn one of our most abundant resources—seawater—into one of our most necessary, drinking water.
Rahul Nair from the University of Manchester in the UK and his team of researchers have shown that the sieve can efficiently filter out salts. The next step is to determine whether the technology works against existing desalination membranes.
“Realization of scalable membranes with uniform pore size down to atomic scale is a significant step forward and will open new possibilities for improving the efficiency of desalination technology,” Nair wrote. “This is the first clear-cut experiment in this regime. We also demonstrate that there are realistic possibilities to scale up the described approach and mass produce grapheme-based membranes with required sieve sizes.”
What Makes Grapheme-oxide Membranes Special
Graphene oxide membranes have been considered a promising candidate for filtration and desalination for some time. But scientists have had difficulty getting beyond removing large particles to eliminating salt. That process requires even smaller sieves that scientists have struggled to create. One major challenge is that when grapheme-oxide membranes are immersed in water, they swell—allowing salt particles to flow through engorged pores. The Manchester team was able to overcome this challenge by building walls of epoxy resin on either side of the graphene oxide membranes. The key to the process is what when salts are dissolved in water, they form a ‘shell’ of water molecules around themselves.
“Water molecules can go through individually, but sodium chloride cannot. It always needs the help of the water molecules,” Nair told Paul Rincon from BBC. “The size of the shell of water around the salt is larger than the channel size, so it cannot go through.”
The result was seawater that was fresh enough to drink, and water molecules that could flow more efficiently through the membrane barrier. This is important progress, given that major desalination plants today are using polymer-based membranes to filter out salt—which is both expensive and inefficient.
Clean drinking water is still incredibly difficult to come by in many parts of the world. In the next eight years, 14 percent of the world’s population is expected to encounter water scarcity—and many of those countries won’t be able to afford desalination plants. The researchers are optimistic that the graphene-based sieve will be as effective as large plants on the small scale, so it’s easier and cheaper for these countries to roll out.
The research was originally published in Nature Nanotechnology.