Graphene-Oxide Sieve Makes Seawater Drinkable

A graphene-oxide membrane capable of removing common salts from seawater – leaving clean drinkable water – has the potential to be a game-changer when it comes to affordable desalinisation technology, especially in places where large scale plants are unfeasible.

A single layer of carbon atoms arranged in a hexagonal lattice, graphene has many unusual properties – extraordinary tensile strength and electrical conductivity to name a few. This makes it one of the most promising materials for a wide range of potential applications, including providing sanitary drinking water for the millions of people who do not have access to adequate clean water sources.

Graphene-oxide membranes have garnered considerable interest as promising candidates for filtration technologies and researchers from the University of Manchester – where graphene was discovered in 2004 - report they have developed a membrane which can sieve common salts from water.

Developed at the National Graphene Institute at the University, these membranes have already shown their potential for removing small nanoparticles, organic molecules and even large salts from water. But they have been unable to sieve common salts as it requires much smaller pores.

Previous research found that when immersed in water, graphene-oxide membranes became slightly swollen, blocking large ions and molecules while allowing smaller salts to flow through the membrane with water.

Now they have developed a strategy to avoid this swelling when exposed to water. By placing walls made of epoxy resin on either side of the graphene oxide membrane, the team were able to stop the swelling. This has also allowed then to precisely control the pore size in the membrane, allowing them to remove common salts from the briny water, making it safe to drink.

When common salts are dissolved in water, a ‘shell’ of water molecules always forms around the salts molecules. The tiny capillaries of the graphene-oxide membrane are able to block the salt from flowing along with the water, while allowing individual water molecules to pass through the membrane barrier. Water molecules flow anomalously fast, which makes these membranes ideal for use in desalination.

“Realisation 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,” said Professor Rahul Nair, Professor of Materials Physics and Royal Society University Research Fellow at the National Graphene Institute.

“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 graphene-based membranes with required sieve sizes.”

Professor Rahul Nair, National Graphene Institute.

But it is not just desalination where these membranes might have a use:

“The developed membranes are not only useful for desalination, but the atomic scale tunability of the pore size also opens new opportunity to fabricate membranes with on-demand filtration capable of filtering out ions according to their sizes,” said Jijo Abraham, joint lead author of the paper, which has been published in Nature Nanotechnology.

Clean, drinkable water is taken for granted in developed countries, but by 2025 the UN expects that 14% of the world’s population will encounter water scarcity – and that’s in modern, wealthy countries as well as developing nations. This research has the potential to revolutionise the way water filtration is currently performed across the world, especially in places which cannot afford large scale desalination operations.

It has previously been difficult to manufacture graphene-based membranes on an industrial scale, but researchers hope that their graphene-oxide membrane systems can be built on smaller scales making the technology accessible to countries without the financial infrastructure to fund large plants without compromising the yield of the fresh water produced.

Reference

1. University of Manchester

2. Nature

3. Image Credit: Shutterstock.com/NOBUHIROASADA