Slipping away from salt

A new membrane based on carbon nanopores can desalinate water at unprecedented rates, pointing the way towards faster, cheaper desalination. 

The membrane, which was designed and tested in a collaboration between the King Abdullah University of Science and Technology and several Chinese universities, is a composite consisting of a nanoporous carbon layer grown on top of a porous ceramic base. In desalination tests, it removed all of the salt from saline solutions while achieving a throughput several times faster than existing membranes. 

“We also tested the membrane with Red Sea water and achieved the same performance as in the lab. Desalination performance in a real application won’t be a problem,” says KAUST’s Zhiping Lai, who led the project. “The big challenge in real applications will be membrane fouling, which is the Achilles' heel of any membrane process.” 

“The almost complete salt rejection was completely unexpected,” says Lai. The team discovered that the membrane desalinates water because the carbon nanotubes slightly repel water, leading to the formation of a miniscule air gap between the carbon and the ceramic. 

Water vapour evaporates from the saltwater side of the gap, leaving behind the salt and condensing as freshwater on the ceramic side of the membrane.

The high flow rate through the carbon and the narrowness of the air gap enable the membrane to filter water significantly faster than existing polymer membranes. The carbon composite also recovers most of the heat that would otherwise be lost. Together with the high flow rate, this creates the possibility of engineering significantly more efficient desalination tools.

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