A new approach to water desalination

Graphene sheets with precisely controlled pores have potential to purify water more efficiently than existing methods.


The availability of fresh water is dwindling in many parts of the world, a problem that is expected to grow with populations. One promising source of potable water is the world’s virtually limitless supply of seawater, but so far desalination technology has been too expensive for widespread use.

Now, MIT researchers have come up with a new approach using a different kind of filtration material: sheets of graphene, a one-atom-thick form of the element carbon, which they say can be far more efficient and possibly less expensive than existing desalination systems.


When water molecules (red and white) and sodium and chlorine ions (green and purple) in saltwater, on the right, encounter a sheet of graphene (pale blue, center) perforated by holes of the right size, the water passes through (left side), but the sodium and chlorine of the salt are blocked.
Graphic: David Cohen-Tanugi

“There are not that many people working on desalination from a materials point of view,” says Jeffrey Grossman, the Carl Richard Soderberg Associate Professor of Power Engineering in MIT’s Department of Materials Science and Engineering, who is the senior author of a paper describing the new process in the journal Nano Letters.

Grossman and graduate student David Cohen-Tanugi, who is the lead author of the paper, aimed to “control the properties of the material down to the atomic level,” producing a graphene sheet perforated with precisely sized holes. They also added other elements to the material, causing the edges of these minuscule openings to interact chemically with water molecules — either repelling or attracting them.

“We were very pleasantly surprised” by how well graphene performed compared to existing systems in computer simulations, Grossman says.

One common method of desalination, called reverse osmosis, uses membranes to filter the salt from the water. But these systems require extremely high pressure — and hence, energy use — to force water through the thick membranes, which are about a thousand times thicker than graphene. The new graphene system operates at much lower pressure, and thus could purify water at far lower cost, the researchers say.

While reverse osmosis has been used for decades, “really basic mechanisms of separating salt from water are not well understood, and they are very complex,” Cohen-Tanugi says, adding that it’s very difficult to do experiments at the scale of individual molecules and ions. But the new graphene-based system, he says, works “hundreds of times faster than current techniques, with the same pressure” — or, alternatively, the system could run at similar rates to present systems, but with lower pressure.

The key to the new process is very precise control over the size of the holes in the graphene sheet. “There’s a sweet spot, but it’s very small,” Grossman says — between pores so large that salt could pass through and ones so small that water molecules would be blocked. The ideal size is just about one nanometer, or one billionth of a meter, he says. If the holes are just a bit smaller — 0.7 nanometers — the water won’t flow through at all.

Other research groups have worked to create pores in graphene, Cohen-Tanugi says, but at very different sizes and for very different purposes — for example, making much bigger holes to filter large molecules such as DNA, or to separate different kinds of gases. The methods used for those processes were not precise enough to make the tiny holes needed for desalination, he says, but more advanced techniques — such as helium-ion bombardment to make precise holes in graphene, chemical etching and self-assembling systems — might be suitable.

For now, Grossman and Cohen-Tanugi have been doing computer simulations of the process to determine its optimal characteristics. “We will begin working on prototypes this summer,” Grossman says. 

Because graphene is the subject of research into many different applications, there has been a great deal of work on finding ways of making it inexpensively and in large quantities. And for desalination, because graphene is such a strong material — pound for pound, it’s the strongest material known — the membranes should be more durable than those presently used for reverse osmosis, Grossman says.

In addition, the material needed for desalination does not need to be nearly as pure as for electronic or optical uses, he says: “A few defects don’t matter, as long as they don’t open it up” so that salt could pass through.

Joshua Schrier, an assistant professor of chemistry at Haverford College, says, “Previous simulations had studied the flow of water through very small holes in graphene, and the design of pores that selectively allow ion passage, but — despite the social and engineering relevance to desalination — nobody had thought to examine the intersection of these two fields.” The work by the MIT team could open a whole new approach to desalination, he says.

Schrier adds, “Manufacturing the very precise pore structures that are found in this paper will be difficult to do on a large scale with existing methods.” However, he says, “the predictions are exciting enough that they should motivate chemical engineers to perform more detailed economic analyses of … water desalination with these types of materials.”

The work was funded by the MIT Energy Initiative and a John S. Hennessy Fellowship, and used computer resources from the National Energy Research Scientific Computing Center.


Topics: Graphene, Materials science, MIT Energy Initiative (MITEI), Nanoscience and nanotechnology, Water, Desalination, Research, Jeffrey Grossman, Research Laboratory of Electronics

Comments

The time has come to quit talking about great ideas and inventions and start doing something with them. I think this discovery and concept is profound and sophisticated. When will great institutions like MIT start organizing and forming new startups that the average man can participate by purchasing common stock ? Please, don’t listen to the typical corporate shmuck’s ,blah, blah, blah. Let these startups start returning profits in the form of dividends as soon as earnings go positive ! Let the system work as it should and not as it has been for the last 30 or so years. Let’s start seeing one idea after another come to market and the common man. As an addendum I note that I see no reason why this idea could not be used to mine gold and silver from seawater, or deuterium, or any precious metal or material for that matter. I see no reason why you could not purify gasoline or other fuels for cleaner and more efficient burns using this method ! I see no reason why you could not filte
How about getting the following teams together to create precise arrays of holes in graphene needed for desalination? http://web.mit.edu/newsoffice/2012/graphene-water-desalination-0702.html A new approach to water desalination Graphene sheets with precisely controlled pores have potential to purify water more efficiently than existing methods. The key to the new process is very precise control over the size of the holes in the graphene sheet. “There’s a sweet spot, but it’s very small,” Grossman says — between pores so large that salt could pass through and ones so small that water molecules would be blocked. http://web.mit.edu/newsoffice/2012/self-assembling-rectangular-chips-research-update-0719.html Research update: Chips with self-assembling rectangles This system can also produce features, such as arrays of holes in the material, whose spacing is much closer than what can be achieved using conventional chip-making methods.
@Wm. James: While there has obviously been much intervention from politics and greedy corporations preventing such ideas from ever becoming a reality, things aren't as simple as many would seem. If such things went from working R&D prototypes directly to commercialized production, we'd have a major increase in lawsuits against manufacturers in a short time. The point of posting such news is for more R&D, collaboration, and to raise the hopes of those who would benefit the most from such findings. However, to simply suggest such a process could be used to filter seawater for gold, silver, or other metals....well, that's what material science 101 is for. We're talking atomic level here, you know H2O, pretty small molecule. Comparably smaller than that of an i.e: sodium-water molecule. It would be impossible to filter out everything but precious metals through such a membrane. Nice idea, but not that simple. Purify fuels? It's not the fuel that's the main problem.
This new graphene sheet stuff is great, but I was just wondering what ever became of the ion concentration polarization research by Sung Jae Kim and Jongyoon Han, both in MIT’s Department of Electrical Engineering and Computer Science, and colleagues in Korea? It was reported on in 2010. http://web.mit.edu/press/2010/desalination
Dear Folks, Looking to solve the water problem is a huge task but if we simplify the systems on a natural way drinking water can be obtained out of seawater at 0.02 €/m³. I’m not an academic but after 20 years of experimenting in the backyard I’m able to de-bound seawater molecules leaving the salt etc. behind and re-hydrogenbound the H and O molecules to H²O. I’m not able to change the world way of thinking that only academics can find answers and solutions so I’ll put it for sale on the market. The system can produce 113.000 m³/day, day and night. The results of the module are great at 40 ppm. I’m waiting on the official lab results. Who knows maybe someone will start thinking and change their way of looking at things around them. Tony
Seems like this will be a much more efficient process, but there's still a basic thermodynamic limit on separating salt from water. There must be over 300 psi driving pressure between seawater and fresh water in any membrane process, or the water will go backwards. That's why it's called reverse osmosis, the pressure is used to reverse the pressure gradient caused by osmosis of water from low concentrations of ions to higher concentrations to achieve equilibrium.
I have been researching desalinization for future benefits. I already understand the mining benefits plausible; however, in my model, I am trying to incorporate solar and sodium combustion as a means of continual energy via byproduct on a miniature mobile plant. My question revolves around the reclamation of pure sodium to be used with part of the intended product for combustion. My system would use considerable solar energy and time for startup, and I have no idea what the economical outcome could be, or side affects to the environment if successful in mass operation? Any ideas?
Can you tell me more about your system and the status of your prototypes and testing. Thank you, Dudley Burton dudleyjburton@gmail.com
There has been recent concern rising about whether or not graphene can enter into the lungs much like asbestos and cause inflammation. Due to its nano properties, is this a concern that should be taken into account when desalinating water?
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