Graphene used to sieve salts from seawater

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Scientists continue to explore the potential of graphene, aka the one-atom thin sheet of bonded carbon atoms that was created more than a decade ago after researchers first peeled a few layers off a block of graphite with some Scotch Tape — before going on to refine their technique until a sheet just a single atom thick was produced.

The nanomaterial has been posited for all sorts of potentially revolutionary uses, from faster, thinner and even transparent electronics; to biotech implants; to better battery capacity. Now researchers at the University of Manchester say they have come up with a method for controlling the permeation of graphene oxide membranes so they can act as a sieve to desalinate seawater.

By controlling the size of the pores in the membranes the team was able to prevent common salts passing through the material — turning seawater into drinking water.

While graphene has previously been demonstrated for filtering small nanoparticles, organic molecules and even large salts, the challenge with common salts found in seawater is their small size. Graphene membranes also swell in size when immersed in water — meaning smaller sieves were required in order to block these common salts.

The team at Manchester say they used physical confinement to control the interlayer spacing within graphene laminates immersed in water — enabling them to achieve what they describe as “accurate and tunable ion sieving”, and to provide a sieve size smaller than the diameters of hydrated ions.

And while they found that permeation rates for the membranes decrease exponentially with decreasing sieve size, they also report that water transport itself is only “weakly affected” — meaning the filtered water flows through the membrane relatively quickly; an important factor if the aim is to develop affordable desalination technology.

As well as potential uses for desalination, the team envisages wider industrial applications for “on-demand filtration capable of filtering out ions according to their sizes”. Their research is detailed in a paper published in the journal Nature Nanotechnology.

Commenting in a statement, one of the researchers, Professor Rahul Nair, said: “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.

“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.”

Scientists at the university, which houses the £61M National Graphene Institute research center, have also been looking at using graphene membranes for gas separation, such as filtering carbon dioxide molecules from power station flue gases — hinting at a possible route for enabling economic carbon capture and storage at scale.