For some parts of the world desalination of seawater is an important option for accessing fresh drinking water. In 2007 the estimates were that worldwide desalination reached 30 billion liters a day. But the cost of that desalination was at the exorbitant levels of $0.50 to $0.85 per cubic meter.
Because of this huge expense, most desalination production remains in the oil-producing countries of the Persian Gulf, where they can afford the huge energy costs of running the multi-stage flash (MSF) processes. But outside of the Middle East, the predominant method for desalination is Reverse Osmosis (RO), which is only slightly less energy consuming and expensive than MSF processes.
Researchers at MIT are looking to replace the membrane materials used now in RO with nanoporous graphene.
Currently, RO depends on comparatively thick membranes that effectively block the salt ions when water molecules are hydraulically pushed through them. In the process envisioned by the MIT researchers, which was published in the journal Nano Letters, one-atom-thick grapheme with nanometer-sized pores would replace those membranes. Because the graphene is a thousand times thinner than the traditional membrane materials it requires far less force—and therefore energy—to push the water molecules through it. A video describing the benefits can be seen below.
The key to making nanoporous graphene work in this desalination process is getting the size of the pores just right. If the pores are too big, the salt can pass right through; and, conversely, if the they are too small, the water will be blocked. According to Jeffrey Grossman, the Carl Richard Soderberg Associate Professor of Power Engineering in MIT’s Department of Materials Science and Engineering, the ideal size range is extremely limited and looks to be 1 nanometer. If the pores are slightly smaller, 0.7 nanometers, the water won’t pass through the membrane at all.
At this point, the research seems largely centered around computer modeling of an RO process using the nanoporous graphene. And in the MIT press release Joshua Schrier, an assistant professor of chemistry at Haverford College, points out that translating this research from computer models to the real world will not be an easy step.
“Manufacturing the very precise pore structures that are found in this paper will be difficult to do on a large scale with existing methods,” he says. However, he also believes 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.”
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.