Posted on 03/25/2015 6:59:31 PM PDT by ckilmer
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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. Image credit: Oak Ridge National Laboratory, US Dept. of Energy (hi-res image)OAK RIDGE, Tenn., March 25, 2015—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 membrane 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 water 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 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 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 polymer membrane, 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 graphene 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.
Mahurin, Vlassiouk and Sheng Dai, of both ORNL and the University of Tennessee, Knoxville, conceived the idea and designed the experiments. Vlassiouk prepared membranes and measured ion transport. Sumedh Surwade of ORNL performed water transport experiments and made pores in graphene. Unocic performed aberration-corrected STEM to reveal atomic structure. Gabriel Veith of ORNL revealed the detailed chemical composition with x-ray photoelectron spectroscopy measurements and analyzed the results. Mahurin, Vlassiouk, Surwade, Dai and Sergei Smirnov of New Mexico State University analyzed the data and interpreted the results.
The title of the paper is “Water Desalination Using Nanoporous Single-Layer Graphene.”
Research was sponsored by ORNL’s Laboratory Directed Research and Development Program. A portion of the work was conducted at the CNMS, a DOE Office of Science User Facility at ORNL.
UT-Battelle manages ORNL for DOE’s Office of Science. The single largest supporter of basic research in the physical sciences in the United States, the Office of Science is working to address some of the most pressing challenges of our time.—by Dawn Levy
That would threaten the habitat of the tilt-winged snail darter. We can’t have that!
Paging Moonbeam Brown.
Well that will spoil the Zero Population Growth Folk’s supper.
There you go throwing a monkey wrench into the justification for killing off 5.5 billion excessive unnecessary people. I hope you realize Dr. John Holdren is going to throw the book of Settled Science at you for daring to insinuate that more that 500 million beings can inhabit Mother Earth.
Eco-nuts will hate it and want it banned
Nanoporous graphene RO membranes allow desal at 1% of the energy previously required. The obstacle is the production of the membranes, and the Koreans are working on 3D printing them, so we are close to cost-effective desal.
Would have been a lot better article if they’d provided some estimates on cost or energy savings for the two methods.
Will this method reduce cost of desal by 10% or 90%? Big difference.
What makes this interesting is that its just one of probably a dozen announcements about the production of graphene for various purposes including desalination that I’ve seen in the last year. Here’s a google search of graphene desalination.
http://bit.ly/1NauYgn
You can see that’s there’s a pretty large community including big names with deep pockets working on the problem. And several companies have said they can produce graphene membranes to spec on an industrial scale.
But I have yet to see them in actual products.
There’s still kinks to be worked out.
But probably five years from now the world will look very different.
Its a shame that someone as sharp as Cruz would just promise the moon. That’s not really gob smacking anymore. But promise to turn the world’s desert’s green and double the size of the habitable earth. ...woa... Katy bar the door.
That’s what limited government is all about.
I remember seeing that the Gates foundation had come to the conclusion that the greatest good that could be done for the greatest number, was to find ways to provide plentiful and cheap fresh water and energy.
Pretty much the rest could work itself out, but these were the long poles in the tent of improving the human condition.
I guess that such filters could have a big role in waste water treatment as well.
Perhaps, my grapheme stock will now go up.
well yes.
Its a pretty sound theory. The way you grow a food chain is to feed the bottom of the food chain. In the case of a civilization that means that you lower the cost of water and energy.
The cheap energy side is coming in byo 4th generation msr reactors.
Ya gotta love the dweeb foundation knowing the greatest good.
Julian Simon’s “The Ultimate Resource” has come true.
http://www.juliansimon.com/writings/Ultimate_Resource/
I didn’t know there was a graphene stock.
Generally speaking the water business is not sexy. No way any water company could ever command high PE’s.
Gates has got a company working on 4th generation nuclear reactors. imho he knows what he’s talking about.
agree.
But for americans the age of limited resources only dates from 1970.
What’s more in about 50 years the wider solar system resources will open up.
I'm sure he does. I'm sure he's sure about everything about our lives and deaths. He can keep his New World Order self to himself.
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