Desalination is by far the best water production technique the world has at its disposal, but despite technological improvements, the process is still energy intensive. However, some low energy or no energy desalination techniques are on the horizon. They are still a long way from commercial adoption but, nonetheless, signal the nature of things to come.
the Quantum The June 21 issue featured a brief mention of a process for producing hydrogen from seawater. Now scientists experimenting with this process are working to take a step forward, to produce water.
The process is straightforward. Add metallic aluminum, preferably nicely shredded, to the seawater and nature will do the rest. Metals naturally have a strong affinity for oxygen. Aluminum will pick up oxygen from seawater and become alumina (which is actually the substance of aluminum ore, bauxite). When oxygen leaves the water and joins the metal, hydrogen in the water is released.
This absurdly simple process, which isn’t a big scientific revelation, would have long since become ubiquitous in commerce without the fact that it is expensive – it would take a lot of aluminum to make it work.
However, Professor Satya Chakravarthy of the Aerospace Engineering Department at IIT-Madras, who is also the president of the National Center for Combustion Research and Development, tried to cut costs by converting alumina back to aluminum, by electrolysis, using non-consumable (or renewable) electrodes.
Once you produce hydrogen, it can be used for many different uses. The original idea of the researchers was to react hydrogen with carbon dioxide, which is available in abundance, to produce methanol.
From salty to drinkable
Now, Chakravarthy and his team are looking to react hydrogen with atmospheric oxygen to produce water. So, incoming seawater, outgoing drinking water. Since the process involves recycling the alumina, the cost is expected to be very low.
Of course, it takes energy to recycle alumina. Some of the energy is produced in the process. Two constituents of the process produce heat (exothermic). First, the reaction of (sea) water with aluminum 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 in the process itself is harnessed, then little or no external energy is needed for the conversion of alumina to aluminum. For any recharge, solar could be involved, which would be much less than the solar energy required for the electrolysis of seawater to produce hydrogen.
Chakravarthy and the start-up he supervises, X2Energy Fuels, also play with the idea of using aluminum nanoparticles for better efficiency. IIT-Madras has more than 20 years of experience in the generation of nanoaluminum “by explosion of electric wire subjected to a high voltage”, he says.
Once perfected, seawater reacting with aluminum, with the resulting recycled alumina, could prove 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 calculated.
Meanwhile, another low cost desalination technique is baking in another lab.
Professor Sarith P Sathian from the Department of Applied Mechanics at IIT-Madras performed computer simulation work to design special membranes that can replace conventional membranes used in reverse osmosis desalination plants.
Be like the human cell
It has been known for some time that graphitic carbonaceous materials make wonderful membranes in reverse osmosis plants. However, water does not easily pass through the membranes because of a phenomenon called ‘hydrodynamic resistance’ at the entry of the carbon tubes. If you could make a membrane that would allow water to pass through it more easily, while leaving behind the salts, you have a winning product.
Sathian and his team were inspired by something that is happening in our body, at the cellular level. The water channels in cell membranes – aquaporins – are not tubes but hourglass shaped. These water channels let water through but say “no entry” to salts, for reasons that are not well understood.
Sathian decided to check if the same hourglass structure could be used to make membranes based on carbon nanomaterials. His study reveals the mechanisms responsible for the increased permeation of water through hourglass-shaped nanopores. Basically, the density distribution of water induced by the curvature inside the nanopores influences the transport of water through the nanopores.
“The results are useful for designing novel nanopore-based separation membranes that mimic the shape of biological nanochannels,” notes a recent article by Sathian, published in the international journal Desalination.
Calculations show that if you used these membranes you would need 60-70% less energy for desalination, compared to conventional methods, Sathian says, but insists that this is work based. on computer simulations and that they must be validated by physical experiments.
For this, Sathian and his team collaborate with scientists from the Institute of Light and Matter, France. Together, the researchers are working on different tilt angles of the nanopores to find out which one works best.
Basically, it’s possible to make membranes with hourglass-shaped nanopores, Sathian explains Quantum. It is possible to “design a geometry of nanopores with a very high permeation capacity without compromising the rejection of the ions,” he says. It is the scientific language for “letting the water pass but stopping the salts”.