Science and Technology

Desalinate using less energy

M Ramesh | Updated on July 18, 2021

Reverse-osmosis water treatment   -  ISTOCK.COM

Emerging technologies promise to remove salts from seawater cheaper

Desalination is, by far, the best water production technique the world has, but, despite technological improvements, the process remains an energy guzzler. However, some low- or no-energy desalination techniques are emerging on the horizon. They are still some distance from commercial adoption but, nevertheless, cue the nature of things to come.

The Quantum issue of June 21 featured a brief mention of a process for producing hydrogen from seawater. Now the scientists who are experimenting with that process are working on taking it a step forward, to produce water.

The process is simple. Add aluminium metal, preferably nicely shredded, to seawater and Nature will do the rest. Metals naturally have a high affinity for oxygen. The aluminium will pick up oxygen from seawater and become alumina (which is really the stuff of the aluminium ore, bauxite). As oxygen leaves water and joins the metal, hydrogen in the water is liberated.

This absurdly simple process, which is no great scientific revelation, would have long become commercially ubiquitous but for the fact that it is expensive — you’d need a lot of aluminium to get it going.

However, Prof Satya Chakravarthy of the Department of Aerospace Engineering at IIT-Madras, who is also the Convenor of the National Centre for Combustion Research and Development, has tried to chip off costs by converting the alumina back into aluminium, through an electrolysis route, using non-consumable (or renewable) electrodes.

Once you produce hydrogen, it can be put to many different uses. The original idea of the researchers was to get the hydrogen to react with carbon dioxide, which is available aplenty, to make methanol.


From salty to potable

Now, Chakravarthy and his team are looking to react the hydrogen with atmospheric oxygen to produce water. So, seawater in, potable water out. Since the process involves recycling of alumina, the cost should be very low.

Of course, energy is required for recycling alumina. Some energy is produced within the process. Two constituents of the process are heat-producing (exothermic). First, the reaction of (sea) water with aluminium to produce hydrogen (and alumina) itself is exothermic. Second, when hydrogen reacts with atmospheric oxygen (to form water), again heat is produced. If the energy that is developed within the process itself is tapped, then little or no external energy is required for the conversion of alumina into aluminium. For any top-up, solar could be brought into play — which would be much less than the solar energy needed for electrolysis of seawater to produce hydrogen.

Chakravarthy and the start-up he mentors, X2Energy Fuels, are also toying with the idea of using aluminium nano particles for better efficiency. IIT-Madras has over 20 years of experience in generating nano aluminium “by explosion of electrical wire subjected to high voltage,” he says.

When perfected, seawater reacting with aluminium, with the resultant alumina recycled, might turn out to be the cheapest route for desalination.

Chakravarthy tells Quantum that, intuitively, the process is very energy-efficient, but the actual costs will have to be worked out.

Meanwhile, another low-cost desalination technique is cooking in another lab.

Prof Sarith P Sathian of the Department of Applied Mechanics at IIT-Madras has been doing some computer simulation work to design special membranes that can replace the conventional ones used in reverse-osmosis desalination plants.

Do it like the human cell

It has been known for some time that graphitic carbon materials make wonderful membranes in RO plants. Still, water does not pass easily through the membranes because of a phenomenon called ‘hydrodynamic resistance’ at the entrance of the carbon tubes. If you could make a membrane that would allow water to pass through it more easily, while, at the same time, leaving behind the salts, you have a winning product.

Sathian and his team took inspiration from something that happens within our bodies, at the cellular level. The water channels in the cell membranes — aquaporins — are not tubes but shaped like an hour-glass. These water channels allow water to pass through but say ‘no entry’ to salts, for reasons that are not well understood.

Sathian decided to check if the same hour-glass structure could be used to make carbon nanomaterial-based membranes. His study reveals the mechanisms responsible for the enhanced water permeation through the hour-glass shaped nanopores. Basically, curvature-induced density distribution of water inside the nanopores influences the transport of water through the nanopores.

“The results are insightful for devising novel separation membranes based on nanopores that mimic the shape of biological nanochannels,” notes a recent paper by Sathian, published in the international journal Desalination.

Calculations show that if you use these membranes, you would need 60-70 per cent less energy for desalination, compared with the conventional methods, says Sathian, but insists that these are computer simulations-based workings and need to be validated by physical experiments.

For this, Sathian and his team are collaborating with scientists of the Institute of Light and Matter, France. Together, the researchers are working on different angles of inclination of the nanopores to find out which works best.

Basically, it is possible to fabricate membranes with hour-glass shaped nanopores, Sathian tells Quantum. It is possible to “conceive a nanopore geometry with very high permeation capacity without compromising on ion rejection,” he says. That is science-speak for ‘allowing water through but stopping salts’.

Published on July 18, 2021

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