It may soon become possible to convert brackish water into drinking water at about 12 paisa per litre right on the kitchen table by using a potential difference of just 1.8 volts, thanks to researchers at the Indian Institute of Technology, Madras (IIT-M).
The researchers used a stack of tissue papers and carbonised them at high temperature to make graphene. Graphite electrodes were then coated with the graphene produced in the lab. When a small potential is applied to the electrodes, the brackish water gets deionised to become potable water. The results were published in the journal ACS Applied Materials & Interfaces.
“An electrode for such deionisation purposes should have high surface area, high electrical conductivity and high porosity,” says Mr. Soujit Sen Gupta, a coauthor of the paper from the Department of Chemistry, IIT Madras. “The graphene coating gives both high surface area and conductivity.” To render the graphene porous, silica precursors were added to the graphene and removed subsequently. The removal of silica makes the graphene porous while retaining the structural integrity.
Two electrodes are separated from each other by just 500 microns to 1 mm by a nylon membrane. The very short separation distance is to enhance the absorption capacity of the electrodes.
Compared with reverse osmosis, which results in 65-70 per cent water wastage, it is only 25 per cent in the case of capacitive deionisation.When the electrodes are dipped into brackish water and 1.8 volt potential is applied to the electrodes, the sodium ions move towards the cathode and chloride ions move to the anode and get adsorbed. In about five minutes the brackish water turns into potable water with less than 500 ppm of sodium chloride, which is less than the permissible limit for drinking water. Further reduction is possible to bring the concentration below 100 ppm, they say.
“The removal of sodium chloride continues as sodium and chloride ions get adsorbed on the electrodes. The concentration of the ions in water keeps reducing with time and after 30 minutes the electrodes stop adsorbing as they are saturated with ions,” Mr. Sen Gupta says. “The polarity of the electrodes is then reversed [cathode become anode and anode becomes cathode] and the adsorbed ions get desorbed and get released into the solution.” The water with the released ions is rejected as waste. Five minutes after desorption, the electrodes are once again ready to turn brackish water into potable water. Other ions can also be removed similarly.
Compared with reverse osmosis, which is energy intensive and results in 65-70 per cent of water to be rejected as waste, the wastage is only 25 per cent in the case of capacitive deionisation (CDI) technology and it can work independent of the grid by using solar energy.
A prototype has already been developed and tests are under way.
“At the core of the technology are carbon-based electrode materials with high adsorption capacity,” says Prof. T. Pradeep, the corresponding author of the paper from the Department of Chemistry, IIT Madras. “This technology is limited by affordable materials, although there are commercial community installations in the recent past. Our idea was to develop plant-derived active materials to ensure sustained supply of electrodes. Instead of tissue paper, we can use waste wood or rice husk to make the electrodes. The cost of electrodes is nearly 100 times cheaper than commercially available electrodes.”
Prof. Pradeep is optimistic that very soon CDI technology using his electrodes will become a viable alternative for producing potable water at homes especially in coastal regions where salt water intrusion is widely prevalent. “It can be used for providing drinking water even when the TDS is 2500-3000. There’s no need to change the electrodes so the filter will last for 10 years,” Prof. Pradeep says.
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