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 present in seawater on a commercial scale. When the spacing between two successive graphene oxide sheets was nearly 10 Å (1 angstrom is 0.1 nanometre), more than 97% of table salt (NaCl) was removed from seawater. The removal of salts will be even better when the spacing between two successive sheets is reduced to below 8 Å.

The use of grapene oxide as a molecular sieve to filter common salts from seawater while allowing water to pass through it is already known. But grapene oxide 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 without being filtered.

A team led by Prof. Rahul Raveendran Nair from the National Graphene Institute, University of Manchester has addressed this problem by developing graphene oxide membranes that do not swell when immersed in water and are able to sieve common salts. In a paper published on April 3 in the journal Nature Nanotechnology, the researchers were able to achieve a desirable interlayer spacing by storing the membranes in high humidity and then physically restraining the membranes from swelling by embedding them in epoxy. “The grapene oxide sheets adjust their interlayer spacing according to the humidity of the air (hygro-responsive property),” Prof. Nair says in an email. Embedding the membranes in epoxy did not alter the rate at which water permeated through the membranes.

Alternative technique

The researchers also tried an alternative technique of adding graphene flakes to graphene oxide 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 graphene oxide instead,” he says.

The higher the ion charge, the stronger it attracts water molecules. So ions with higher free energies have water molecules that are strongly bound. The salt ions with water molecules strongly bound have larger diameter and experience larger barriers to enter the tiny space between the sheets. On the other hand, water molecules have weak hydrogen bonding and need very little energy to strip the surrounding water from the water molecules before entering the tiny space between the graphene oxide layers. So water is able to pass through the membrane more easily.

Making use of energy barriers

“Both KCl and NaCl have similar energy barriers and the [rejection] increases with decreasing spacing. Similar to the case of KCl, even the channel with a spacing of 10 Å has a finite energy barrier, but that barrier increases nearly three fold when the spacing is reduced to below 8 Å,” Prof. Nair says.

In the case of graphene oxide membranes that have graphene flakes (Gr) added to it, the ion permeation rate was suppressed by two orders of magnitude compared with graphane oxide membranes embedded in epoxy. As a result, salt rejection was as high as 97% in the case of Go-Gr membranes when the spacing was nearly 10 Å. There was about 20% reduction in water permeation rate across the membranes.

Treating wastewater

The graphene oxide-graphene flakes membranes are not mechanically strong and are ill-suited for conventional reverse osmosis process. Instead, they can be used for treating wastewater using forward osmosis (and where hydraulic force is not applied).

“Instead of hydraulic pressure, naturally occurring osmotic pressure due to the concentration gradient of salt/molecular solution present on both sides of the membrane is used to drive water molecules across a membrane,” he says. “If we separate wastewater (something needs to be filtered) and sugar solution by using a membrane, then, water from the wastewater solution will flow towards the sugar solution if the concentration of sugar solution is higher than the wastewater solution. This process enables the dilution of sugar solution by pulling the pure water from the wastewater solution to the sugar solution. This technology can be used to separate clean water from the wastewater including sea water. Currently, forward osmosis is mainly using as a pre-treatment to RO process to reduce the energy consumption.”

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.

Published in The Hindu on April 3, 2017