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Graphene has an important role in the desalination of seawater

26 March 2015

US researchers desalinate salt water using free-standing porous graphene, in a process that outperforms reverse osmosis membranes by an order of magnitude.

Nanopores in graphene (red, and enlarged in the circle to highlight its honeycomb structure) stabilised with silicon atoms (yellow). Orange represents a non-graphene residual polymer (image: Oak Ridge National Laboratory)

A team of researchers, led by the US Department of Energy's Oak Ridge National Laboratory, has demonstrated an energy-efficient desalination technology that uses a porous membrane made of strong, slim graphene - a carbon honeycomb one atom thick. The results of their work are published in the March 23 advance online issue of Nature Nanotechnology.

"Our work is a proof of principle that demonstrates how you can desalinate salt water using free-standing, porous graphene," said Shannon Mark Mahurin of ORNL's Chemical Sciences Division, who co-led the study with Ivan Vlassiouk in ORNL's Energy and Transportation Science Division.

"It's a huge advance," said Vlassiouk. "The flux through the current graphene membranes was at least an order of magnitude higher than [that through] state-of-the-art reverse osmosis polymeric membranes."

Current methods for purifying water include distillation and reverse osmosis. Distillation, or heating a mixture to extract volatile components that condense, requires a significant amount of energy. Reverse osmosis, a more energy-efficient process that nonetheless requires a fair amount of energy, is the basis for the ORNL technology.

Making pores in the graphene is key. Without these holes, water cannot travel from one side of the membrane to the other. The water molecules are simply too big to fit through graphene's fine mesh. But poke holes in the mesh that are just the right size, and water molecules can penetrate.

Salt ions, in contrast, are larger than water molecules and cannot cross the membrane. The porous membrane allows osmosis, or passage of a fluid through a semi-permeable membrane into a solution in which the solvent is more concentrated.

"If you have saltwater on one side of a porous membrane and freshwater on the other, an osmotic pressure tends to bring the water back to the saltwater side. But if you overcome that, and you reverse that, and you push the water from the saltwater side to the freshwater side--that's the reverse osmosis process," Mahurin explains.

Today reverse-osmosis filters are typically polymers. A filter is thin and resides on a support. It takes significant pressure to push water from the saltwater side to the freshwater side. "If you can make the membrane more porous and thinner, you can increase the flux through the membrane and reduce the pressure requirements, within limits," Mahurin says. "That all serves to reduce the amount of energy that it takes to drive the process."

Graphene's mechanical and chemical stabilities make it promising in membranes for separations. A porous graphene membrane could be more permeable than a polymer membrane, so separated water would drive faster through the membrane under the same conditions. "If we can use this single layer of graphene, we could then increase the flux and reduce the membrane area to accomplish that same purification process," Mahurin says.

To make graphene for the membrane, the researchers flowed methane through a tube furnace at 1,000°C over a copper foil that catalyzed its decomposition into carbon and hydrogen. The chemical vapour deposited carbon atoms that self-assembled into adjoining hexagons to form a sheet one atom thick.

The researchers transferred the graphene membrane to a silicon nitride support with a micrometre-sized hole. Then the team exposed the graphene to an oxygen plasma that knocked carbon atoms out of the graphene lattice to create pores. The longer the graphene membrane was exposed to the plasma, the bigger the pores that formed, and the more made.

The prepared membrane separated two water solutions - salty water on one side, fresh on the other. The silicon nitride chip held the graphene membrane in place while water flowed through it from one chamber to the other. The membrane allowed rapid transport of water through the membrane and rejected nearly 100 percent of the salt ions (positively charged sodium atoms and negatively charged chloride atoms).

WIth the help of colleagues at ORNL, the researchers determined the optimum pore size for effective desalination was 0.5 to 1 nanometres. They also found the optimal density of pores for desalination was one pore for every 100 square nanometres. While more pores improve the efficiency of the membrane, there comes a point when its mechanical stability is compromised.

The researchers believe that making the porous graphene membranes used in the experiment is also viable on an industrial scale, and while other methods of production of the pores can be explored, the oxygen plasma approach appears to be the best.

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