4 Apr 2017

Sieve turns seawater into drinking water

4:05 pm on 4 April 2017

A UK-based team of researchers has created a graphene-based sieve capable of removing salt from seawater.

Pacific Ocean

Researchers have invented a membrane that could turn the ocean into drinking water. Photo: Public Domain

The sought-after development could help millions of people without ready access to clean drinking water.

The promising graphene oxide sieve could be highly efficient at filtering salts. It will be tested against existing desalination membranes.

It was previously difficult to manufacture graphene-based barriers on an industrial scale.

Reporting their results in the Nature Nanotechnology journal, University of Manchester scientists, led by Dr Rahul Nair, show how they used a chemical derivative called graphene oxide to solve some of the challenges.

Isolated and characterised by a University of Manchester-led team in 2004, graphene comprises a single layer of carbon atoms arranged in a hexagonal lattice.

Its unusual properties, such as extraordinary tensile strength and electrical conductivity, have earmarked it as one of the most promising materials of the future.

"To make it permeable, you need to drill small holes in the membrane, but if the hole size is larger than one nanometre, the salts go through that hole... It is a really challenging job."

Graphene oxide membranes have already proven their worth in sieving out small nanoparticles, organic molecules and large salts. Until now, they could not be used to filter out common salts, which require smaller sieves.

Previous work had shown that graphene oxide membranes became slightly swollen when immersed in water, allowing smaller salts to flow through the pores along with water molecules.

Dr Nair and colleagues demonstrated that placing walls made of epoxy resin (a substance used in coatings and glues) on either side of the graphene oxide membrane stopped the expansion.

By contrast, water molecules flowed fast through the membrane barrier, which made it ideal for use in desalination.

By 2025, the United Nations expects 14 percent of the world's population to encounter water scarcity.

As the effects of climate change continue to reduce urban water supplies, wealthy modern countries are investing in desalination technologies.

Current desalination plants around the world use polymer-based membranes.

"The next step is to compare this with the state-of-the-art material available on the market," Dr Nair said.

Ram Devanathan, from the Pacific Northwest National Laboratory in Richland, US, said more work was needed to produce graphene oxide membranes inexpensively at industrial scales.

Scientists needed to demonstrate the durability of the membranes under prolonged contact with seawater and ensure the membrane was resistant to "fouling" by salts and biological material, which required the barriers to be periodically cleaned or replaced.

"The ultimate goal is to create a filtration device that will produce potable water from seawater or wastewater, with minimal energy input."