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Nanopores provide efficient route to desalination

12 November 2015

University of Illinois engineers have demonstrated an energy-efficient desalination process using a nanometre-thick sheet of nanoporous molybdenum disulphide.

Computer model of a nanopore in a single-layer sheet of MoS2 shows that high volumes of water can pass through the pore using less pressure than standard plastic membranes. Salt water on left, fresh water on right (graphic: Mohammad Heiranian)

In a study published in the journal, Nature Communications, the Illinois team modelled various thin-film membranes and found that molybdenum disulphide (MoS2) showed the greatest efficiency, filtering through up to 70 percent more water than graphene membranes.

"Finding materials for efficient desalination has been a big issue, and I think this work lays the foundation for next-generation materials. These materials are efficient in terms of energy usage and fouling, which are issues that have plagued desalination technology for a long time," says Professor Narayana Aluru of the University of Illinois.

Most available desalination technologies rely on a process called reverse osmosis to push seawater through a thin plastic membrane to make fresh water. The membrane has holes in it small enough to prevent salt or dirt passing through, but large enough to let water through. They are very good at filtering out salt, but yield only a trickle of fresh water. Although thin to the eye, these membranes are still relatively thick for filtering on the molecular level, so a lot of pressure has to be applied to push the water through.

"Reverse osmosis is a very expensive process," says Aluru. "It's very energy intensive. A lot of power is required to do this process, and it's not very efficient. In addition, the membranes fail because of clogging. So we'd like to make it cheaper and make the membranes more efficient so they don't fail as often. We also don't want to have to use a lot of pressure to get a high flow rate of water."

One way to dramatically increase the water flow is to make the membrane thinner, since the required force is proportional to the membrane thickness. Researchers have been looking at nanometre-thin membranes such as graphene but graphene presents its own challenges in the way it interacts with water.

Aluru's group has previously studied MoS2 nanopores as a platform for DNA sequencing and decided to explore its properties for water desalination. Using a supercomputer simulation, they found that a single-layer sheet of MoS2 outperformed its competitors thanks to a combination of thinness, pore geometry and chemical properties.

A MoS2 molecule has one molybdenum atom sandwiched between two sulphur atoms. A sheet of MoS2 has sulphur coating either side with the molybdenum in the centre. The researchers found that creating a pore in the sheet that left an exposed ring of molybdenum around the centre of the pore created a nozzle-like shape that drew water through the pore.

"MoS2 has inherent advantages in that the molybdenum in the centre attracts water, then the sulphur on the other side pushes it away, so we have much higher rate of water going through the pore," says graduate student Mohammad Heiranian, the first author of the study. "It's inherent in the chemistry of MoS2 and the geometry of the pore, so we don't have to functionalise the pore, which is a very complex process with graphene."

In addition to the chemical properties, the single-layer sheets of MoS2 have the advantages of thinness, requiring much less energy, which in turn dramatically reduces operating costs. MoS2 is also a robust material, so even such a thin sheet is able to withstand the necessary pressures and water volumes.

The Illinois researchers are establishing collaborations to experimentally test MoS2 for water desalination and to test its rate of fouling - a major problem for plastic membranes. MoS2 is a relatively new material, but the researchers believe that manufacturing techniques will improve as its high performance becomes more sought-after for various applications.

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