Engineering news
The research by a team at Manchester University, led by Rahul Nair, could help many developing countries cope with water shortages. Worldwide, about 700 million people don’t have access to enough clean water, while the UN expects that by 2025, 14% of the world's population will face water scarcity.
“Any new technology that could create a cheap source of water is of great relief since water scarcity is becoming a real global problem,” said Amir Peleg, the founder and CEO of TaKaDu, an Israeli water network monitoring firm. “We need to conserve it but also to generate new sources. In low cost.”
The new source identified by the UK team in Manchester is rather plentiful: our blue planet’s oceans and seas. The researchers developed a so-called ‘sieve’ out of graphene that is, they say, highly efficient at filtering salts. Graphene was first discovered in 2004 at the University of Manchester, by a different group of scientists, who later shared a Nobel Prize in Physics for their work. It is composed of a single layer of carbon atoms arranged in a hexagonal lattice, and has extraordinary tensile strength and electrical conductivity.
Scientists have tried to make graphene-based membranes in the past, but it’s been a challenge to produce them on a mass scale – both financially and because the existing methods, such as chemical vapour deposition (CVD), were not very efficient.
But Nair’s team took a different approach. It decided to make the membranes using graphene oxide, a chemical derivative that can be produced by simple oxidation in the lab. The team has been working with graphene-oxide membranes for a while, and found during previous research that if immersed in water, the membranes become slightly swollen; this means that while smaller salts flow through them along with the water, larger ions or molecules are blocked.
The latest study, published in the journal Nature Nanotechnology, describes how the scientists have developed these membranes even further, and found a way to prevent the membrane from swelling when exposed to water. They have developed a sieve where the pore size can be precisely controlled, which makes it possible to stop common salts in seawater from going through, making the water drinkable.
The researchers "showed that the slits between graphene-oxide laminates could be narrowed so that even the smallest of the ions, sodium-chloride salts, could be rejected, all while retaining a significant flow of water in the slits," said Slaven Garaj, a physicist at National University of Singapore who was not involved in Nair's research. "This neatly demonstrates that pursuit of graphene-oxide membranes for desalination is not a pipe-dream, but an exciting technology on the cusp of a breakthrough. The Manchester group opened the doors wide, and the field is ripe for innovations."
Nair says that it is possible in the future to scale up the development and mass produce graphene-based membranes with required sieve sizes. “Our work shows that the efficiency of water filtration could be improved by using graphene-based membranes and this can potentially make the drinking water cheap,” he told PE.
Going commercial
Nair added that his team is currently collaborating with a number of companies “to evaluate the membrane in realistic environments and further pilot scale testing. This may take another few years’ time.”
Around the world there have been many efforts to improve traditional desalination processes to tap into one of the Earth’s most plentiful resources: seawater. A team at Alexandria University in Egypt, for example, developed in 2015 a cost-effective desalination technology capable of filtering salty water in minutes, using membranes containing cellulose acetate powder. The powder, together with other components, stops the salt particles on their way through and acts like a sieve.
There are desalination plants in many countries, and one in Israel, called Israel Desalination Enterprises, or IDE Technologies, is one of the biggest in the world. It uses a conventional desalination technology called reverse osmosis (RO) to turn seawater into drinking water cheaply and at a mass scale, producing 627,000 cubic meters of water daily and providing about half of the country’s total water supply.
While the graphene-oxide membranes produced by Nair and his team "are still not on par with the conventional membranes in terms of pure performance," says Garaj, "these results show that [these] membranes could surpass the existing technologies in the near future."
"They would be able to reduce the energy or price expenditure of desalination only by a smaller fraction, since the reverse osmosis technology is already fairly close to its thermodynamic limit. However, their strength lays in the fact that they could offer much smaller footprint and require smaller capital investment – a huge factor for implementing desalination technologies in the developing world."