In the 1967 film The Graduate, Dustin Hoffman’s character is a recent college graduate contemplating his future. A family friend offers him one word of career advice: “plastics”. A 2012 version of the movie might contain a different suggestion: “graphene”. Like plastics, graphene promises to be a game-changing material that could have far-reaching impacts.
Graphene is a two-dimensional material that consists of a single layer of carbon atoms arranged in a honeycomb structure that resembles chicken wire. Not only is it the thinnest known material, it is the strongest ever measured. Since it was first isolated in 2004, researchers have begun to look at countless ways that graphene might be put to practical use.
Earlier this year, University of Manchester researchers studying graphene’s ion permeation properties found that water molecules from a container diffused through a graphene membrane at the same evaporation rate whether the container was closed or open. Professor Andre Geim, a recipient of the 2010 Nobel Prize in Physics for his research with graphene, told WDR, “Its properties are so unusual that it is hard to imagine that they cannot find some use in the design of filtration, separation or barrier membranes and for selective removal of water.”
Now, results published by MIT Professor Jeffrey Grossman and PhD candidate David Cohen-Tanugi in the July issue of the journal Nano Letters show that a single-layer, nanoporous graphene membrane can act as a molecular sieve to effectively separate salt from water. Using classical molecular dynamics simulations, they were able to demonstrate how a graphene membrane removes dissolved salts at a permeability rate that is two to three orders of magnitude higher than diffusive membranes used in conventional RO systems.
Cohen-Tanugi told WDR that the membrane performance varies as a function of pore size, pore chemistry and applied pressure. “Not only is it necessary that the membrane has an extremely narrow pore size distribution, it is also important that the unsaturated carbon atoms at the pore edges be passivated with the right chemical functional groups. We found that hydroxylated pores exhibit higher water permeability, while hydrogenated pores are more effective at rejecting salts,” he said.
Of the various techniques that could be used to introduce nanopores into graphene, he said that helium ion beam drilling or spontaneous self-assembly currently show the most promise.
Although the MIT study didn’t consider the membrane’s mechanical stability, he also stressed that the membrane must be able to perform under high applied pressures. “It may be necessary to add a highly porous layer to support the active membrane layer, similar to the way polysulfone is used to support a conventional RO membrane,” he suggested.
Cohen-Tanugi and Grossman acknowledge that research on graphene membranes for desal applications has just started, and that its commercial use is some years away. However, they note that the work done on nanocomposite membranes was only announced in 2006, and they are now in use in more than 50 commercial seawater RO installations.
“Our approach suggests that a bottom-up, systematic redesign of desal membrane materials can yield significant improvements over existing technological methods. We expect that this work will add to the understanding of next-generation membranes,” said Grossman.
WDR’s current CDR rating for this technology is 4.7.