About 18 months ago, I wrote about an MIT project in which computer models demonstrated that graphene could act as a filter in the desalination of water through the reverse osmosis (RO) method. RO is slightly less energy intensive than the predominantly used multi-stage-flash process. The hope was that the nanopores of the graphene material would make the RO method even less energy intensive than current versions by making it easier to push the water through the filter membrane.
The models were promising, but other researchers in the field said at the time it was going to be a long road to translate a computer model to a real product.
“Manufacturing the very precise pore structures that are found in this paper will be difficult to do on a large scale with existing methods,” said Joshua Schrier, an assistant professor of chemistry at Haverford College. However, Schrier also believed that “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.”
It would seem that the MIT researchers agreed it was worth the effort and accepted the challenge to go from computer model to a real device as they announced this week that they had developed a method for creating selective pores in graphene that make it suitable for water desalination.
The MIT group collaborated with a team from Oak Ridge National Laboratory and researchers from Saudi Arabia, a country that has been quite active in trying to use nanotechnology to finding cheaper water desalination processes. They published their results in the journal Nano Letters ("Selective Ionic Transport through Tunable Subnanometer Pores in Single-Layer Graphene Membranes").
The process the researchers developed for creating these nanopores in the graphene involves a two-step process. First the graphene is bombarded with gallium ions. This process breaks the carbon bonds in the hexagonal array of carbon atoms that makes up graphene. The next step involves etching the resulting material with an oxidizing solution that reacts strongly with the disrupted carbon bonds. This creates holes in the material in every spot where the gallium ions disrupted the carbon. The researchers were able to control the pore size by timing how long the graphene is kept in the oxidizing solution.
“We’ve developed the first membrane that consists of a high density of subnanometer-scale pores in an atomically thin, single sheet of graphene,” said Sean O’Hern, a graduate student who led the research along with MIT professor of mechanical engineering Rohit Karnik, in a press release.
The pores per square centimeter are well suited for filtration applications, with measurements indicating that there are 5 trillion pores per square centimeter. “To better understand how small and dense these graphene pores are, if our graphene membrane were to be magnified about a million times, the pores would be less than 1 millimeter in size, spaced about 4 millimeters apart, and span over 38 square miles, an area roughly half the size of Boston,” O’Hern explained in the release.
Of course, this is just an initial step in developing graphene as a filter medium for these purposes. To scale the graphene filter up to the size that would be needed for water filtration remains a challenge. However, O’Hern believes that initial applications in biofiltration of molecules, such as the removal of unreacted reagents from DNA, could be a more attainable application in the short term.
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.