Nanopores could take the salt out of seawater, scientists claim

By @iamkarlatecson on
Desalination process
A desalination pump constructed by the San Francisco Exploratorium that displays the process of turning salt water to fresh water at a public parklet - a space the size of two parking spots in the Mission neighborhood in San Francisco, California May 18, 2015. Reuters/Robert Galbraith

A team of U.S. engineers has found a low-cost, energy-efficient material that could remove salt from seawater, which may lead to resolving the ongoing water crisis.

The University of Illinois study has found that a nanometer-thick sheet of molybdenum disulfide (MoS2) riddled with tiny holes called nanopores could filter through up to 70 percent more water than other materials.

“Even though we have a lot of water on this planet, there is very little that is drinkable. Finding materials for efficient desalination has been a big issue, and I think this work lays the foundation for next-generation materials,” said study leader Narayana Aluru, a professor of mechanical science and engineering in the University of Illinois. 

These materials are efficient in terms of energy usage and fouling, which are issues that have plagued desalination technology for a long time, he added.

Desalination is a process that let high volumes of water through but keep salt and other contaminates out. Most available desalination technologies rely on a process called reverse osmosis to push seawater through a thin plastic membrane to make fresh water. While the membrane’s holes are small enough to not let salt or dirt through, they are also large enough to let water pass. This means that the process is good in filtering out salt but yields only a trickle of fresh water. 

Reverse osmosis is also a very expensive process, Aluru noted. Since the process requires a lot of power, it’s not considered  efficient. In addition, the membranes fail because of clogging. The research team hopes to address these issues with their new method, making the process cheaper and more efficient so they won't fail as often. 

According to the team, 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 nanometer-thin membranes such as graphene. However, graphene presents its own challenges in the way it interacts with water.

Aluru’s group previously studied MoS2 nanopores as a platform for DNA sequencing and decided to explore its properties for water desalination. Using the Blue Waters supercomputer at the National Center for Supercomputing Applications, they found that a single-layer sheet of MoS2 outperformed its competitors, thanks to a combination of thinness, pore geometry and chemical properties.

The researchers also 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.

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

For their next steps, the researchers aim to establish collaborations to experimentally test MoS2 for water desalination and to test its rate of fouling, or clogging of the pores, which is a major problem for plastic membranes. While MoS2 is a relatively new material, the team believes that manufacturing techniques will improve as its high performance becomes more sought-after for various applications.

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