Electrodes made of copper plates wound with cellulose thread coated with carbon nanotubes for capacitive deionisation and placed close to each other achieved 84% efficiency in ion removal. With power requirement of just 0.15 mW, the water filter made by IIT Bombay researchers can be operated with a pen-torch battery.
Unlike the conventionally used reverse osmosis which is energy intensive, desalination of tap water to make it potable (less than 200 ppm of ions, which is the WHO standard) is now possible by using very low power. The water filter can desalinate water as it trickles down the electrodes by gravity thus eliminating the need for any water-pumping system.
The water filter developed by a two-member team led by Prof. Chandramouli Subramaniam from the Department of Chemistry at the Indian Institute of Technology (IIT) Bombay has a high desalination rate and capacity; one gram of electrode can remove 139 mg of salt (calcium, magnesium, sodium and potassium ions) at a rate of 3 mg of salt in one minute. The efficiency of ion removal is high at about 84%. The results were published in the journal ACS Applied Materials and Interfaces.
“Our electrodes have more than three times higher salt-removal capacity and two times higher salt-removal rate, which makes them the best among all know materials, says Prof. Subramaniam.
The researchers used a novel way to produce the electrodes which are used for capacitive deionisation to remove salt from water. Carbon nanotubes were synthesised and cellulose thread was dip coated with these nanotubes. The dried carbon nanotube thread was then closely wound across flat copper plates to produce the electrodes.
Making carbon nanotubes hydrophilic
While carbon nanotubes are hydrophobic in nature, cellulose thread is highly water-loving (hydrophilic). By combining the two, the carbon nanotube thread becomes hydrophilic — water shows initial contact angle of 63 degrees which then reduces to 15 degrees in 30 seconds contact time. The highly hydrophilic and mesoporous nature of the thread combined with nano-sized pores and high surface area of nanotubes allow the ions to be effectively and quickly removed from water. “Achieving the right combination of surface area and porosity is critical to achieve this high performance,” says Prof. Subramaniam.
Two electrodes are placed one above the other and separated from each other by 2 mm. The water flows between the plates in a narrow channel of 60 mm length, 6 mm breadth and 2 mm height. “We have currently tested with flow rates of 6 ml per minute” he says. “The volume of water filtered can be scaled up by having a larger channel using longer and wider electrodes,” says Maku Moronshing from the Department of Chemistry at IIT Bombay and the first author of the paper.
The cost of materials to make the device is less than Rs.3 per sq. cm. The device (60 mm x 6 mm x 2 mm) tested in the lab requires only 0.15 mW power and can be operated with a pen-torch battery.
Why only low power is required
The carbon nanotubes coated on the cellulose thread increases the surface area of the nanotube thread to a very large extend. When a material has sharp points (like the lightening conductors on building rooftops) or high surface area, the electric field around the material will also be very high. “Because the nanotube thread has high electric conductivity and high surface area, even a low voltage applied can generate high electric field and this helps in pulling the ions from the water,” says Prof. Subramaniam.
Recycling the electrodes
The electrode has to be deionised after 15-20 ml of water is collected. The researchers tested the electrodes for five cycles with very little change in performance. The electrodes can be recycled using deionised water or by changing the polarity of electrodes. It will take about one minute if deionised water is used and 15 seconds if polarity is reversed. “The water that comes out for 15 seconds when the polarity is reversed should be rejected as waste water,” Prof. Subramaniam says.
“We would really like to take this forward to a commerical scale. We are currently exploring options with both DST and industries to help us set up a pilot scale plant and improve the technology readiness level,” Prof. Subramaniam says. “An Indian patent application has been filed and we are open to direct licensing of technology to any company.”