The Earth's surface is largely made up of water. The irony is, this highly abundant resource is also generally scarce, particularly in its potable form. While the Earth's surface may be 71 percent water, 96.5 percent of this come from the planet's oceans. Salt water, as everyone knows, isn't potable. The quest to find ways to tap into this abundant source and turn it into drinkable water is well underway, facilitated mainly by desalination plants. But these plants are expensive to maintain and don't necessarily serve the populations that need them most.
The question, then, is how to make these plants cheaper and more efficient? That's just what a study published in the journal Nature Nanotechnology recently explored. The research was conducted by a UK-based team led by Rahul Nair from the University of Manchester. Their solution to the world's drinking problem? A graphene oxide sieve that can remove salt from seawater.
Ideal for Desalination
Since it was first discovered in 2004, graphene has become a wonder material capable of remarkable feats. Graphene's remarkable nature is at least in part thanks to its unique properties, such as electrical conductivity and tensile strength. Graphene oxide "can be produced by simple oxidation in the lab," Nair told the BBC. "As an ink or solution, we can compose it on a substrate or porous material. Then we can use it as a membrane. In terms of scalability and the cost of the material, graphene oxide has a potential advantage over single-layered graphene."
Graphene oxide has proven effective for sieving out nanoparticles, organic molecules, and large salts. To filter out common salt, Nair and his colleagues used walls made of epoxy resin on both sides of a graphene oxide membrane. This prevented the graphene oxide from expanding when immersed in water. It also gave the researcher the ability to control these pores.
When dissolved in water, common salts form a "shell" of water molecules around the molecules of salt. Tiny capillaries from the graphene oxide membranes can then block these salts from flowing through. This also allows the water molecules to flow through the membrane much faster. "Water molecules can go through individually, but sodium chloride cannot. It always needs the help of the water molecules. The size of the shell of water around the salt is larger than the channel size, so it cannot go through," Nair explained.
Once these graphene oxide membranes can be industrially (and cheaply) produced, they can become a more efficient option than the polymer-based membranes currently used in desalination plants. "The selective separation of water molecules from ions by physical restriction of interlayer spacing opens the door to the synthesis of inexpensive membranes for desalination," Ram Devanathan, from the Pacific Northwest National Laboratory wrote as a review to Nair's research. "The ultimate goal is to create a filtration device that will produce potable water from seawater or wastewater with minimal energy input."