New process to help desalinate water faster, cheaper

In a finding that could make the process of removing salt from seawater easier, faster and cheaper than ever before, researchers have discovered that carbon nanotubes of a certain size act as the perfect filter for salt — the most abundant water contaminant.

Currently, desalinisation is too expensive and energy intensive for large-scale feasibility.

The study, published in the journal 'Science', showed that carbon nanotubes are better at desalinisation than any other existing method — natural or man-made.

Filtering water is tricky because water molecules want to stick together.

The "H" in H2O is hydrogen, and hydrogen bonds are strong, requiring a lot of energy to separate. Water tends to bulk up and resist being filtered. But nanotubes do it rapidly, with ease.

A carbon nanotube is like an impossibly small rolled up sheet of paper, about a nanometer in diameter.

For comparison, the diameter of a human hair is 50 to 70 micrometers — 50,000 times wider.

The tube's miniscule size, exactly 0.8 nm, only allows one water molecule to pass through at a time.

This single-file lineup disrupts the hydrogen bonds, so water can be pushed through the tubes at an accelerated pace, with no bulking, the study said.

To conduct the research, Meni Wanunu, Associate Professor of Physics at Northeastern University in Boston and post doctoral student Robert Henley collaborated with scientists at the Lawrence Livermore National Laboratory in California.

In addition to being precisely the right size for passing single water molecules, carbon nanotubes have a negative electric charge.

This causes them to reject anything with the same charge, like the negative ions in salt, as well as other unwanted particles.

"While salt has a hard time passing through because of the charge, water is a neutral molecule and passes through easily," Wanunu said.

The finding offers a novel system that could have major implications for the future of water security.