Water Desalination: A New Approach from MIT

via MIT News Office
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.

“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.

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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.

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