Desalination: A useful application may have been found for graphene: improving access to fresh water in the developing world

Desalination: A useful application may have been found for graphene: improving access to fresh water in the developing world

Jun 1st 2013 |From the print edition

ALLOTROPES of carbon—varying forms of the element in which the atoms are stuck together in different patterns—have a mixed record of practical use. Diamonds, famously, are a girl’s best friend. Graphite makes good pencil lead. But buckminsterfullerene, in which the atoms are arranged like the geodesic domes beloved of the eponymous American architect, though hailed as a wonder material, proved largely useless.

Graphene, which looks like atomic-scale chicken-wire, may be in the useful camp. At room temperature, it is the best conductor of heat yet found. It is being developed as a photoreceptor, to convert light into electricity. And now two groups of engineers, one at Lockheed Martin, an American aerospace company, the other at the Massachusetts Institute of Technology (MIT), are trying to use it to desalinate water. That could change the world.Desalination plants usually employ a process called reverse osmosis, in which seawater is put under pressure on one side of a polymer membrane. Polymer molecules are long, snakelike affairs, and if you pick the right ones, they will tangle together in a way that leaves gaps big enough for water molecules to pass, but not big enough for the sodium and chloride ions of which salt is composed. These ions, being electrically charged, attract shells of water molecules that increase their effective diameter. The gaps in commercially available membranes for reverse osmosis are about half a nanometre across (a nanometre being a billionth of a metre). These let water through, but not the ions.

A graphene-based membrane would work similarly, except that the holes through which the water would pass would have to be punched through it deliberately, rather than happening spontaneously. But that would bring several advantages. First, they could be engineered to be of the optimum size. This is 1.2 nanometres across, a diameter that permits the passage of water more easily than a polymer membrane does, but is still small enough to hold back hydrated chloride ions. They, in turn, hold back the sodium ions since, being negatively charged, they attract the positively charged sodium. Second, the holes would all be of the same size, so there would be no gaps large enough to let sodium and chloride ions through. Third, pores punched in this way are straight, rather than being convoluted channels as is the case in a polymer membrane. That also speeds the water molecules’ passage.

The upshot, according to John Stetson, the engineer in charge of Lockheed’s end of the project, is that far less pressure would be needed to desalinate water than a polymer-based system requires. The seawater in a graphene desalination system would need a pressure of about 28 bar (400lb per square inch). That is between a half and a third of that in a traditional plant. Mr Stetson reckons graphene desalination plants should be 15-20% cheaper to run than traditional ones, because their lower pressure means they would need less energy, and they would also have much greater throughput.

The engineering problems remain formidable, though, according to Jeffrey Grossman, who runs the MIT end of things. Punching the holes precisely but cheaply on an industrial scale will be hard. Some chemicals might do it. So might a beam of electrons. David Cohen-Tanugi, also of MIT, says the exact benefits remain to be seen. It may not be diamond. But graphene could yet turn out to be the world’s best friend.