Graphene-based sieve turns seawater into drinking water

British researchers, lead by Rahul Nair, at the University of Manchester, have discovered a way of efficient desalination by creating a graphene-based sieve that is capable of removing salt from sea water. The graphene sieve has left seawater fresh to drink.

Climate change is wreaking havoc on the world’s urban water supplies. With already rising temperatures and a decreased amount of rainfall – the UN predicts that by 2025, 14% of the world’s population will encounter water scarcity. At the same time, sea water will be something we have in abundance, as Greenland’s coastal ice caps are melting at a rapid rate. If the entire Greenland Ice Sheet melts, our future generations are likely to see oceans that are 7.3 metres higher than what they are today. There are so many countries where access to safe, clean, drinkable water is already limited which is why this discovery is so crucial.

As the effects of climate change continue to reduce modern society’s water supplies, wealthier countries have been investing in desalination technologies. There are already several major desalination plants around the world that are using techniques such as polymer-based membranes that filter out salt as well as distillation through thermal energy. However, these processes have been criticised by environmentalists because they are still largely ineffective and expensive. They also involve using large amounts of energy which produces greenhouse gases which are harming the environment.

Graphene is an ultra-thin sheet of carbon atoms that are organised into hexagonal lattices. This ‘wonder material’ was discovered in 2002 at University of Manchester and had researchers racing to develop graphene-based barriers for desalination on an industrial scale. Initial attempts were futile because when the graphene-oxide membranes were immersed in water, they would swell up, thus letting the salt particles through the engorged pores. The Manchester team overcame this issue by covering each layer of the graphene oxide membrane with epoxy resin. This allowed the researchers to control the pore size in the membrane, creating holes tiny enough to filter out all common salts but at the same time allowing the clean water to pass through. To make the graphene-oxide compound successful the holes needed to be less-than-one-nanometre in size. This is because when salt is immersed in water, water molecules form a shell around the salt – making it bigger than a nanometre, and thus unable to pass through. “Water molecules can go through individually, but sodium chloride cannot. It always needs the help of the water molecules” said Nair. He continued to say that the size of the shell water around the salt is larger than the channel size, so it cannot go through.

“The ultimate goal is to create a filtration device that will produce potable water from seawater or wastewater with minimal energy input,” said Nair. Currently, they are ironing out the kinks and are working on testing it against existing desalination membranes as well as how best to mass produce it, and how long it can stay in salt water without being replaced. So, while this device is still only operational in the laboratory, the researchers hope that this can be mainstreamed within five years. This sought-after development could help the millions of people that already do not have access to clean drinking water.