Graphene oxide to ‘sieve’ salt from water

RESEARCHERS at the University of Manchester, UK, have developed graphene oxide membranes which can effectively “sieve” out common salts from seawater to make drinking water.

By 2025, the UN estimates that 14% of the world’s population will suffer from water scarcity. Desalination of seawater is one option, but large-scale plants are expensive and many countries in the developing world cannot afford them. The team at Manchester, led by materials physics professor Rahul Nair, believe that their graphene oxide membranes could offer a realistic alternative, and could revolutionise drinking water filtration around the world.

Researchers at Manchester’s National Graphene Institute have previously made graphene oxide laminate membranes for gas separation and water filtration which can remove small nanoparticles, organic molecules, and large salts. However, thus far, attempts to use graphene oxide membranes to desalinate water have failed as they tend to swell in water. Molecules travel through graphene channels between the graphene oxide layers in the membrane. When the membrane swells, the interlayer spacing increases, allowing the larger salt ions, which are surrounded by a shell of water molecules, to pass through as well as the much smaller water molecules. Nair and the team have now found a way to prevent the membrane swelling, by physical confinement.

The team prepared graphene oxide membranes by dispersing graphene oxide flakes in distilled water and sonicating the mixture to make a suspension. Multilayer flakes were removed by centrifugation, before the membranes were made using a vacuum filtration process. The membranes are then stacked and fixed in place using an epoxy, which prevents the membranes from swelling. When tested, the membranes removed 97% of sodium chloride ions.

“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 Nair. “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.”

Lead author Jijo Abraham believes that the new technique could open the way to fabricate membranes with precisely-tuned pore sizes to filter other ions according to their size.

Nature Nanotechnology doi.org/b46q