Study demonstrates desalination with nanoporous graphene membrane

March 25, 2015
Researchers created nanopores in graphene (red, and enlarged in the circle to highlight its honeycomb structure) that are stabilized with silicon atoms (yellow) and showed their porous membrane could desalinate seawater. Orange represents a non-graphene residual polymer. Credit: Oak Ridge National Laboratory, US Dept. of Energy

Less than 1 percent of Earth's water is drinkable. Removing salt and other minerals from our biggest available source of water—seawater—may help satisfy a growing global population thirsty for fresh water for drinking, farming, transportation, heating, cooling and industry. But desalination is an energy-intensive process, which concerns those wanting to expand its application.

Now, a team of experimentalists led by the Department of Energy's Oak Ridge National Laboratory has demonstrated an energy-efficient desalination technology that uses a porous made of strong, slim graphene—a carbon honeycomb one atom thick. The results 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 saltwater 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, pointing out a wealth of travels through the porous graphene membrane. "The flux through the current graphene membranes was at least an order of magnitude higher than [that through] state-of-the-art 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 and cannot cross the membrane. The allows osmosis, or passage of a fluid through a semipermeable 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 explained.

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 said. "That all serves to reduce the amount of energy that it takes to drive the process."

Graphene to the rescue Graphene is only one-atom thick, yet flexible and strong. Its mechanical and chemical stabilities make it promising in membranes for separations. A porous graphene membrane could be more permeable than a , so separated water would drive faster through the membrane under the same conditions, the scientists reasoned. "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 said.

To make graphene for the membrane, the researchers flowed methane through a tube furnace at 1,000 degrees C over a copper foil that catalyzed its decomposition into carbon and hydrogen. The chemical vapor 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 micrometer-sized hole. Then the team exposed the graphene to an oxygen plasma that knocked carbon atoms out of the graphene's nanoscale chicken wire 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, e.g., positively charged sodium atoms and negatively charged chloride atoms.

To figure out the best pore size for desalination, the researchers relied on the Center for Nanophase Materials Sciences (CNMS), a DOE Office of Science User Facility at ORNL. There, aberration-corrected scanning transmission electron microscopy (STEM) imaging, led by Raymond Unocic, allowed for atom-resolution imaging of graphene, which the scientists used to correlate the porosity of the graphene membrane with transport properties. They determined the optimum pore size for effective desalination was 0.5 to 1 nanometers, Mahurin said.

They also found the optimal density of pores for desalination was one pore for every 100 square nanometers. "The more pores you get, the better, up to a point until you start to degrade any mechanical stability," Mahurin said.

Vlassiouk said making the porous membranes used in the experiment is viable on an industrial scale, and other methods of production of the pores can be explored. "Various approaches have been tried, including irradiation with electrons and ions, but none of them worked. So far, the oxygen plasma approach worked the best," he added. He worries more about gremlins that plague today's reverse osmosis membranes—growths on membrane surfaces that clog them (called "biofouling") and ensuring the mechanical stability of a membrane under pressure.

Explore further: Imperfect graphene opens door to better fuel cells

More information: Water Desalination Using Nanoporous Single-Layer Graphene, Nature Nanotechnology, 2015. DOI: 10.1038/nnano.2015.37

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MR166
5 / 5 (5) Mar 25, 2015
An inexpensive process to remove salt from sea water would be a boon to many areas of the world.
hopper
5 / 5 (5) Mar 25, 2015
An inexpensive process to remove salt from sea water would -- enable desert farming and essentially double the size of the habitable earth.
betterexists
1 / 5 (3) Mar 25, 2015
Is it HB Pencil OR 2H Pencil?
I have a dozen of them lying around!
betterexists
1 / 5 (4) Mar 25, 2015
I SAY WHERE IS THE NEED TO WORRY! Instead of Osmosis, try Gravity...Newton's Gravity.
"The porous membrane allows osmosis, or passage of a fluid through a semipermeable 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."
Just Put Salt Water ATOP Graphene with PORES.
Water TRICKLES/FLOWS/GUSHES DOWN.
" ONE-WAY is The Right Way! "
verkle
Mar 25, 2015
This comment has been removed by a moderator.
betterexists
1 / 5 (2) Mar 25, 2015
Just Spray into a container with Graphene (with pores) at its bottom. The bigger the container, the greater the amount of fresh water yield below that! Just keep going.
betterexists
1 / 5 (2) Mar 25, 2015
'He worries more about gremlins that plague today's reverse osmosis membranes—growths on membrane surfaces that clog them (called "biofouling").'
IS IT REALLY A THING TO WORRY ABOUT?
Just Alternate with a periodical wash with Bleach and Divert the Bleached Water into a different channel .......AND REUSE IT from time to time!
betterexists
1 / 5 (2) Mar 26, 2015
While washing off the Salt ions, Turn the Container upside down. As long as they do not mix up again, that is fine!
Mike_Massen
3 / 5 (2) Mar 26, 2015
The process of desalination, if at least by evaporation, has been well known for a very long time - perhaps millennia. The problem is organisation, commitment and correct use of "net present costing".

Eg. I live in Western Australia, where climate change appears to be the main factor in reducing rain-fall, ie more cooler water off coast due to Antarctic ice melts reduces temps current temperatures which reduces evaporation which reduces precipitation etc...

Powered reverse osmosis has taken upper hand but, its not efficient, used because it entails low risk for politicians who seek re-election, the expense however is immense, not just capital but ongoing re power needs !

However, the potential to use sea fed canals with coverage to collect evaporation during day/night thermal changes has primarily only a capital cost with negligible comparative maintenance costs. Its easy & only needs to address scale but, governments, with negligible science see it as a risk !
vic1248
not rated yet Mar 26, 2015
Vlassiouk said making the porous graphene membranes used in the experiment is viable on an industrial scale,..


That's the key, I hope it works, that would be a great service to humanity!

I always wondered about "desalination" to solve the water shortage problem around the globe but apparently that's not economically viable by the energy-intensive evaporation process.

Meanwhile, I wonder how underground water comes about and if that natural process could be replicated for seawater.
Mike_Massen
3 / 5 (2) Mar 26, 2015
vic1248 claimed
I always wondered about "desalination" to solve the water shortage problem around the globe but apparently that's not economically viable by the energy-intensive evaporation process
If u were throwing a barb at my comment, perhaps I should have confirmed Solar - there is a tremendous amount of Insolation, its a matter of simply arranging canals, coverings & collection ducts & especially take advantage of dew-point in association with engineering re Psychrometry.
https://en.wikipe...ew_point
https://en.wikipe...ometrics

vic1248 asked
Meanwhile, I wonder how underground water comes about and if that natural process could be replicated for seawater
Primarily precipitation in conjunction with diffusion & capillary action from lakes, rivers, ponds etc. Re seawater, there isnt any sort of natural process by which seawater can diffuse into groundwater and drop off its salt content on the way.

Best use of Sol is ideal !
vic1248
5 / 5 (2) Mar 26, 2015
@Mike_Massen
If u were throwing a barb at my comment,..


Not at all! As a matter of fact, I shared the same sentiment months ago on another forum discussing Bill Gates' humanitarian project for recycling waste water.

Thanks for the rest of information and insights.
betterexists
1 / 5 (1) Mar 27, 2015
I SAY WHERE IS THE NEED TO WORRY! Instead of Osmosis, try Gravity...Newton's Gravity.
Well, How to use Newton's Gravity? Just Dig Deep Wells (Containers) with a layer of Graphene in the Center at Sea Shores. As the sea water starts flowing into them , Collect all the filtered Fresh Water at the bottom of wells into Waiting Trucks/Trains! Do this at places wherever nearby towns are in dire need of lots & lots of fresh water or wherever there are Pepsi and Coca Cola Plants.
Mike_Massen
3 / 5 (2) Mar 28, 2015
vic1248 responded well
@Mike_Massen
If u were throwing a barb at my comment,..
Not at all! As a matter of fact, I shared the same sentiment months ago on another forum discussing Bill Gates' humanitarian project for recycling waste water
Ah great :-) its just you mentioned the nature of it being energy intensive as IF we need to supply that without taking advantage of Insolation.

FWIW Nature of forum comments being devoid of feedback at the time Eg as re face to face conversation, one tends to second guess as a matter of course, unfortunately the global warming denier camp is the worst source & I recognise, especially here at phys.org, it is often full of uneducated barbs and implications by way of exclusions.

vic1248 added
Thanks for the rest of information and insights
Np. I'm interested in the field as I hired my Son, recent chem eng graduate, to investigate viability of alternate water extraction from air exploiting alt approaches re UN program.
antialias_physorg
5 / 5 (2) Mar 28, 2015
I always wondered about "desalination" to solve the water shortage problem around the globe

I hope there's a better way (and I think most people will agree who have ever had the opportunity to try and use desalinated water for drinking or bathing. Those who have not might want to try taking a holiday in a place where that is an option. It's a pretty terrible experience.)

Conservation and optimization of usage of current resources should go hand in hand with any desalination efforts. Desalination (as importnat as it undoubtedly is and will become) should not be seen as a panacea to keep wasting our water resources the way we are.
Urgelt
1 / 5 (1) Mar 30, 2015
Vlassiouk is worrying about the right things. Those tiny pores will be clogged rapidly by seawater; and constructing membranes an atom thick makes for a very vulnerable membrane.

Anti, water is water; if desalinization can be made much more economical, then seawater is a water resource like all of the rest (only vaster). I can't see much of a downside in shifting reliance to seawater; it will ease the strain on aquifers and rivers.
Accata
not rated yet Mar 30, 2015
Before some time it was presented, that the graphene oxide membrane itself can be used for desalination - i.e. without puncturing of artificial holes in it. Apparently, no progress has been done with it.

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