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Membranes that look like honeycomb make desalination more efficient

Developments in nanotechnology could help create a more efficient form of desalination, potentially changing the game in the global fight for universal access to safe drinking water.

“Four billion people live under conditions of severe water scarcity at least one month of the year [and] half a billion people live under severe water scarcity all year,” said Amir Barati Farimani, Assistant Professor of Mechanical Engineering at Carnegie Mellon University in the US. 

In a peer-reviewed paper published in the American Chemical Society’s journal Nano Letters, Barati Farimani and his research team found that developments in nanomaterials could lead to the creation of nanoporous materials for desalination that have “3 to 6 orders of magnitude higher water permeation compared to that of traditional membranes”.

The engineers say that one of the most effective methods for desalination is membrane desalination, where water is pushed through a thin membrane with tiny holes. Water then flows through the pores, leaving behind salt ions and producing only freshwater on the other side. Using an ultra-thin conductive metal-organic framework (MOF), the researchers found that the technology enhanced efficiency in three ways.  

First, the MOF is only a few atoms thick, which means there’s very little friction as the water molecules pass through the pores.  

The pore design of the MOF also reduces pressure on the walls of the membrane because they are not adjacent, which Barati Farimani said is like the difference between pouring water through a funnel, which places pressure to squeeze water through a tight space, and pouring it through a strainer.

Finally, the pentagonal design of the membrane, which looks like a honeycomb, means that it has more structural integrity than other materials. 

Ordinarily, scientists have to drill tiny holes in order to create the needed pores, which limits the amount that can be created per surface area. 

“If you want to make a lot of pores, graphene or MoS2 can’t do that,” Barati Farimani said.

“Structurally they can’t hold the pressure.” 

Designed for desalination

The design is a critical factor in the energy efficiency of the technology. Barati Farimani said the membranes “consist of both the metal centre and organic compound,” which connect in a pentagonal pattern, leaving a hole in the centre that serves as a pore.

This honeycomb structure means that it is intrinsically porous, allowing for a higher ratio of pores to surface area. It also saves on time and energy, since the pores don’t need to be drilled, or even adjusted in size.

Given there are about 15,000 desalination plants around the world, and a typical desalination plant can have billions of pores, the scale of the efficiencies of the technology could be enormous. 

Barati Farimani said that the research was motivated by the growing global freshwater crisis, which exists while 71% of the world’s surface is covered by seawater. 

“We need to provide fresh water for many underprivileged people, like in Africa or other places,” he said. 

“Basically that’s our mission – to make it so energy-efficient that we have water desalination everywhere.”