The magic sieve that could solve a water crisis

Photograph: The Hindu

Producing potable water through desalination may become more efficient and less energy-intensive if researchers at the University of Manchester are able to successfully use graphene oxide (GO) membranes to filter common salts in seawater on a commercial scale.

The use of GO as a molecular sieve to filter common salts from seawater while allowing water to pass through it is already known. But GO membranes have a tendency to slightly swell when immersed in water and this results in increased spacing between successive sheets (akin to increasing the pore size of a sieve). The increased spacing allows smaller salts to flow through the membrane along with water without being filtered.

A team led by Professor Rahul Raveendran Nair from the National Graphene Institute, University of Manchester, has addressed this problem by developing GO membranes that do not swell when immersed in water and are able to sieve common salts. In a paper titled “Tunable sieveing of ions using graphene oxide membranes” published on April 3 in Nature Nanotechnology, the researchers write that they were able to achieve a certain interlayer spacing by storing the membranes in high humidity and then physically restraining them from swelling by embedding them in epoxy. This altered the rate at which water permeated through the membranes.

The researchers also tried an alternative technique of adding graphene flakes to GO to prevent the membranes from swelling. Though the epoxy coating gives better control over swelling, large area membrane fabrication may be difficult and time-consuming. Producing scalable membranes for desalination application will be possible by adding graphene flakes to GO instead.

The water molecules that get strongly bound to common salts increase the diameter of salt ions and are hence unable to pass through the tiny space between the sheets; water molecules with weak hydrogen bonding are easily able to pass through the membrane. The membranes developed by the team can be used for waste water treatment even when no energy is supplied.

Preliminary experiments by the team found clogging of the membranes with salt was negligible and the membrane can be recovered to the original state by a simple washing process. They do not anticipate any significant fouling due to the inertness of graphene surface. More studies are needed before the membranes can be commercialised.

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