The University Network

Northeastern University Researchers Remove Salt From Seawater Using Nanotubes

Current desalination processes are expensive, energy-intensive, and involve large-scale facilities. Now, however, researchers from Northeastern University have discovered a method that could make the process of removing salt from seawater easier, faster and cheaper. The new method will help alleviate concerns about viable water resource as the global population continues to grow.

Meni Wanunu, associate professor of physics at Northeastern, and postdoctoral student Robert Henley collaborated with scientists from the Lawrence Livermore National Laboratory in California on the research.

The study is published in Science.

Wanunu and Henley built their research on what scientists at Lawrence Livermore had discovered last year. Led by Aleksandr Noy, the scientists found carbon nanotubes ideal for transporting protons.

A carbon nanotube is a tube-shaped material, made of carbon, about a nanometer in diameter. A nanometer is one-billionth of a meter. The diameter of a human hair, in comparison, is 50,000 times wider than a carbon nanotube.

Ordinarily, it is difficult to filter water as hydrogen binds the water molecules together, so it takes considerable energy to separate the molecules. Nanotubes, however, took care of the problem easily.

In their study, the researchers used carbon nanotubes that were exactly 0.8 nanometer in size, which was perfect as it allows just one water molecule to pass through at a time, breaking the hydrogen bonds in the process and allowing water to pass through the tubes quickly.

Wanunu used the analogy of a group of people running through a hallway to illustrate his point. It is faster to run through the hallway single-file than to run holding hands with others.

The other element that made desalination possible is that carbon nanotubes have a negative electric charge, which causes them to reject anything with similar charges. Salt contains negative ions, so it’s prevented from entering the tubes.

The tubes, as prepared by the Noy lab, naturally contain negatively charged rims, which act as highly effective ion filters,” Wanunu said. “That is, they allow small cations to pass through, while rejecting anions efficiently.” Cations are ions with a positive charge, and anions are ions with a negative charge.

The team’s study demonstrates that carbon nanotubes are ideal for removing salt from seawater. The technology could have a lasting effect on the world.

“I believe that clean water will be one of the defining future environmental challenges for humanity, and this is why we need new technological solutions that exploit new and interesting physics to help us get clean water better, faster and cheaper,” said Noy.

The team was recently awarded a National Science Foundation/Binational Science Foundation grant that will help them further their study and build water filtration platforms based on their method.

“What we did was to demonstrate physical principles that could lead to very efficient water purification membranes,” said Noy. “Now the challenge is to make those membranes in a form that’s useful for applications.”

Wanunu is excited about the method’s potential and its impact on water security. I am excited about how one, or a few, of these biomimetic porins have such interesting properties,” he said.

Combining many of these small entities into a functional membrane can truly make an impact on water security.

The team has also partnered with a leading water purification company based in Israel. Wanunu would “be very proud to be a part of such an effort, especially coming from Israel, where potable water has always been scarce, precious, and appreciated.”

Likewise, Noy is “very excited about the possibility to work together with him (Wanunu) and colleagues in Israel on this topic.”