UK researchers create graphene-sieve to make salt-water drinkable

A team of researchers from UK just created a graphene-based sieve designed to remove salt from seawater possibly aiding millions of people who have no ready access to clean drinking water.

This new graphene-oxide sieve could possibly be highly competent at filtering salts and now is to be tested against existing desalination membranes. The results of the research were published in the widely-acclaimed Science journal, ‘Nature Nanotechnology’, led by Dr Rahul Nair; scientists from the University of Manchester depicted how they solved some of the hurdles by employing the use of a chemical derivative called Graphene Oxide.  First created by a team led by University of Manchester in 2004, Graphene constitutes of a single layer of carbon atoms placed in a hexagonal lattice. It’s odd properties, such as extraordinary tensile strength and electrical conductivity has deemed it as one of the most promising materials for future applications.

However, it sure has been rather troublesome to produce large quantities of single-layered graphene using current methods, such as chemical vapor deposition (CVD), which also are quite costly. Nonetheless Dr Nair whilst talking to foreign media said, "graphene oxide can be produced by simple oxidation in the lab. 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."

On the single-layer graphene he commented, "To make it permeable, you need to drill small holes in the membrane. But if the hole size is larger than one nanometre, the salts go through that hole. You have to make a membrane with a very uniform less-than-one-nanometre hole size to make it useful for desalination. It is a really challenging job."

Graphene oxide membranes have already proven their worth in sieving out small nanoparticles, organic molecules and even large salts. But until now, they couldn't be used to filter out common salts, which require even smaller sieves.

Previous experiments had revealed that Graphene Oxide membranes became slightly swollen when submerged in water consequently granting smaller salts a safe passage to flow through the pores along with the water molecules. However, now Dr Nair and his colleagues lay showed that placing walls made of epoxy resin on either side of the Graphene Oxide membrane was suffice to stop the said expansion. Limiting the swelling in this way also permitted the scientists to adjust the properties of the membrane, letting through less or more common salt for example.

When common salts are dissolved in water, they always form a "shell" of water molecules around the salt molecules. This allows the tiny capillaries of the graphene-oxide membranes to block the salt from flowing through along with the water.

"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," said Dr Nair.

By contrast, water molecules flow exceptionally fast through the membrane barrier, which makes it ideal for use in desalination.

"When the capillary size is around one nanometre, which is very close to the size of the water molecule, those molecules form a nice interconnected arrangement like a train," Dr Nair explained.

"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."

"This is our first demonstration that we can control the spacing [of pores in the membrane] and that we can do desalination, which was not possible before. The next step is to compare this with the state-of-the-art material available on the market," said Dr Nair.