According to project lead Dr. Rahul Nair, they can make graphene oxide sieves simply by pouring the solution onto a substrate or porous material. To make typical graphene permeable for desalination, you'd need to drill holes absolutely no larger than one nanometer into the membrane. "It is a really challenging job," Nair said.
The team still had to find a way for graphene oxide membranes to filter out salts, though. In previous studies, scientists found that the membranes swell when immersed in water, enlarging their pores and allowing salts to pass through. To solve that problem, Nair and his team coated both sides of the membranes with epoxy resin.
The epoxy-coated membranes' pores are too small for salt-and-water molecules to pass through. However, they allow pure water molecules to flow through really quickly since their one-nanometer pores are very close to the size of water molecules. Nair explained:
"When the capillary size is around one nanometer, which is very close to the size of the water molecule, those molecules form a nice interconnected arrangement like a train. That makes the movement of water faster: if you push harder on one side, the molecules all move on the other side because of the hydrogen bonds between them. You can only get that situation if the channel size is very small."
Someday, these graphene-based sieves could change lives around the world. But before that happens, the team has to make sure they can withstand prolonged contact with seawater. They also need to test the material against current membranes desalination processes use. "The ultimate goal," Nair said, "is to create a filtration device that will produce potable water from seawater or wastewater with minimal energy input."