The process requires a lot less energy than other approaches

Feb 27, 2014 07:58 GMT  ·  By

Massachusetts Institute of Technology (MIT) graduate student Sean O’Hern and professor of mechanical engineering Rohit Karni announce the development of a graphene membrane that holds great promise for enabling the clean and energy-efficient filtering of water. 

The advancement comes about one and a half years after the same MIT group first proposed a computer model that revealed that potential graphene has in filtering liquids. The model indicated that sheets of carbon outfitted with nanopores could remove salts from water fairly easily.

The process used here is a part of the reverse osmosis (RO) family, as opposed to the multi-stage-flash process that is commonly used. RO has the great advantage of being less energy-intensive, but it is more technologically-complex. By using graphene, it may become easier, and require even less energy.

This, the model revealed, was made possible by the fact that the nanopores in graphene would contribute to pushing the water through the hexagonal, 2D membrane. Graphene is made up entirely out of carbon atoms, arranged in a hexagonal, single-atom-thick lattice.

Many scientists commented 18 months ago that the road from this computer model to a real-life device would be long and very difficult, so the MIT group took it upon itself to demonstrate that their critics were wrong. Now, they have developed the first nanopore graphene sheet for water desalination.

The work was carried out in conjunction with scientists from the US Department of Energy's (DOE) Oak Ridge National Laboratory (ORNL) and researchers from Saudi Arabia. Details of the investigation appear in a paper entitled “Selective Ionic Transport through Tunable Subnanometer Pores in Single-Layer Graphene Membranes,” published in the latest issue of the journal Nano Letters.

“We’ve developed the first membrane that consists of a high density of subnanometer-scale pores in an atomically thin, single sheet of graphene,” says O’Hern, who led the new investigation. He explains that the team's approach to creating nanopores in graphene featured two stages.

First, graphene sheets are bombarded with gallium ions, a process that destroys some of the carbon bonds between the atoms in the graphene lattice. The second step is to oxidize the material with a solution featuring chemicals that react strongly with the broken carbon bonds.

As such, wherever the gallium bombardment has created holes in the graphene sheet, new nanopores appear. The size of these holes can be adjusted by controlling the amount of time the 2D material spends in the oxidizing solution, the team reveals, quoted by IEEE Spectrum.

“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 concludes.