In the U.S., a research group based at the Department of Energy’s Oak Ridge National Laboratory have been working on a low-energy efficient desalination process. The newly devised method deploys a porous membrane made of graphene.
Graphene is a very strong, low weight material. It is 100 times stronger than steel and it conducts heat and electricity with great efficiency. The material is being investigated for many potential applications, including water purification.
The graphene membrane is, according to the researchers, more effective and uses less energy compared with current polymeric membranes, which work on the basis of reverse osmosis. With reverse osmosis an applied pressure is used to overcome osmotic pressure; this allows water to pass through a membrane whilst at the same time particles are retained.
With the new method the most important aspect is making the pores in the graphene. The size here is important: large enough to allow water molecules to pass through but sufficiently small to stop salt molecules from traversing the mesh.
The reason that the graphene process is more energy efficient comes down to the size of the mesh. Graphene is considerably thinner (just one atom in thickness) than the plastic polymers and the result of this is that less energy is required to push the fluid through.
The graphene structure was manufactured by passing methane gas through a tube furnace at 1,000 degrees C over a copper foil. This decomposed the methane into carbon and hydrogen. The carbon then assembled into a hexagonal configuration of one atom thick molecules. The graphene was then mounted onto a silicon nitride support. Small pores in the graphene are created using a plasma (an ionized gas.) Pores were created at the rate of one pore for every 100 square nanometers of graphene.
In experimental runs the graphene filter was used to remove salt from sea water in order to create water of drinking water quality. The test runs were effective with almost 100 percent of the salt removed.
The research has been published in the journal Nature Nanotechnology. The title of the paper is “Water Desalination Using Nanoporous Single-Layer Graphene.”