Have you ever thought that is was possible to drink seawater and save energy all in one go?

Inspired by the way water travels inside the bark of a tree, a team of researchers at Indian Institute of Technology Gandhinagar (IITG) have developed an energy-free water desalination technique that does just that. In a paper in Nature Communications journal, the scientists say the process can successfully remove more than 99% of salt ions and other impurities to make seawater potable.

According to the World Health Organisation, nearly a fifth of the world population lives in areas lacking clean drinking water. Increases in population and rising energy demand have put immense pressure on conventional clean water resources.

More and more countries will have to turn to seawater desalination to meet demand, but existing techniques, while widely used, are energy-intensive.

To find an alternative and affordable solution, a research team led by Gopinadhan Kalon, assistant professor of physics and materials engineering at IITG, took inspiration from nature, and the way trees naturally take in water. They use a capillary action to selectively transport molecules and ions around the trunk and branches, an effect that can be seen when a liquid is naturally drawn up between the hairs of a paint-brush.

The researchers observed that mimicking the naturally occurring channels in trees could result in highly efficient filtration systems, which do not require an energy source or any external pressure.

The controllable water transport channels have been created in graphite crystal, with the help of an electric field and potassium chloride ions, which allow only fresh water to pass and blocks the salt ions. The team claim that this is the first time a controllable method to manipulate graphite inside aqueous solutions, without damaging its structural integrity, has been developed.

Co-author of the paper, doctoral student Lalita Saini said, “Natural graphite is not absorptive to water or any ions, including protons. However, by its nature, the graphite crystal also does not allow any water molecules to pass through it, because there is not enough space for the movement of these molecules.

"This issue was solved by using an electric field and inserting potassium chloride ions in it, which created some space inside the graphite crystal and provided a stable structure for easy passage of water molecules, at the same time hindering the movement of any salt ions, giving drinkable water.”

The researchers found that the technique is self-sufficient and can successfully remove more than 99% of salt ions and other impurities from seawater, making it completely safe for drinking. Moreover, carbon materials like graphite are antimicrobial, reducing the number of filters required in the desalination process.

Professor Kalon said, "Our method is not only limited to graphite, but also to a large number of layered materials, like clays, that could be explored for high performance separation applications. With abundant seawater and appropriate plant design optimisation, our method holds a bright future in realising the dream of drinking water for everyone on the planet."

Carbon is abundant in nature, and India is the second largest producer of graphite in the world. The experiment used natural graphite, but the team is also devising a method that can instead synthesise graphene - a one-unit layer of graphite - from other resources, including waste, plastics, wheat, sugar and even chocolate.

The present 2mm x 2mm sized device made by the research team has flow rates comparable to RO technology, that too without using electricity. Since this involves less process filters, it is also expected to have less wastage of water. The water evaporation and water filtration processes used in this technique do not involve any electricity, and therefore it does not produce any gas emissions, making it environment-friendly. The team is now working to develop a direct point-of-use water filter using this technique to make it accessible for the masses.

Moreover, this technique can also prove to be useful in designing filters for gas purification, proton exchange in a fuel cell, chemical separation, recovery of precious metal from waste etc. It can also be suitable for dehumidification applications as expanded graphite has high water evaporation rates.

The research team includes IITG PhD scholars Lalita Saini, Aparna Rathi, Suvigya Kaushik, and post-doctoral fellow Siva Sankar Nemala.