Artist's rendition of carbon nanotube filter

Carbon nanotubes could turn brine into drinking water

Image credit: Lawrence Livermore National Laboratory

US researchers have found that carbon nanotube pores are capable of filtering seawater to exclude salt, leaving pure water behind. Such pores could eventually be incorporated into water purification technology.

Today, four billion people struggle with water scarcity, and as the global population continues to explode, the need for fresh water and water purification technologies becomes ever more pressing.

In the future, carbon nanotubes – hollow structures essentially made from atom-thick graphene rolled into a tube – could play a role in providing safe drinking water to billions. Carbon nanotubes are remarkable for their strength, electrical and thermal conductivity, as well as being extremely stretchy and flexible.

They also possess very high water permeability, due to their smooth interiors which do not interact with passing water molecules.

Studying this specific property of carbon nanotubes, researchers at Lawrence Livermore National Laboratory and Northwestern University discovered that nanotube structures with diameters smaller than a 0.8nm have remarkably high water permeability.

“The narrow hydrophobic channel forces water to translocate in a single-file arrangement, a phenomenon similar to that found in the most efficient biological water transporters,” said Ramya Tunuguntla, a PhD student at Lawrence Livermore National Laboratory, and co-author of the Science study.

Larger salt ions are caught by the carbon nanotube’s pores, meaning that the nanotubes function as miniaturised water filters.

Through computer simulations and experimentation, the researchers found that wider nanotubes did not separate salt nearly as efficiently as the narrow ones. Water permeability for the 0.8nm nanotubes was approximately 10 times higher than for 1.5nm nanotubes.

“Carbon nanotubes are a unique platform for studying molecular transport and nanofluidics,” said Dr Alex Noy, a senior scientist at Lawrence  Livermore National Laboratory and principal investigator. “Their sub-nanometre size, atomically smooth surfaces and similarity to cellular water transport channels make them exceptionally suited for this purpose, and it is very exciting to make a synthetic water channel that performs better than nature’s own.”

The carbon nanotube pores that the US researchers developed, if scaled up sufficiently, could be incorporated into current water purification technologies.

In April, it was reported that researchers at the University of Manchester had succeeded in creating a “graphene sieve”; a membrane made of the atom-thick material capable of purifying sea water.

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