Scrubbing Water Clean—Nano Technology-Based Solutions

Industry, pollution and the growing population together increasingly constitute both the demand for and scarcity of clean water. While river-fed sources are becoming less accessible, groundwater is also sinking deeper or getting contaminated.

Conserving and reusing the commodity has become vital and technology to turn bad water to good has become more important than ever.

Marc Andelman, Massachusetts-based inventor, Professor Tony Cass from Imperial College, London, and Professor Sung Jae Kim from Seoul National University presented three new nano technology-based solutions for extracting potable water from inferior sources at the eighth India Nano Meet organised by the S&T Promotion Society, government of Karnataka, at Bengaluru on 8 March. The packed audience was of industrialists, start-ups and students and Professor T Pradeep of IIT, Chennai, who conducted the meeting, pressed for emerging technology to be picked up and used for the benefit of the country and the world.

Andelman’s innovation is an improvement of the method called capacitative de-ionisation, or CDI, where a pair of oppositely charged surfaces, the electrodes, fish out contaminants, mainly salt, from water that is made to flow between the charged plates. The voltage used is low, and there is no current between the surfaces, but dissolved contaminants, which are split in the water medium into oppositely charged halves called ions drift to opposite ends till the ends collect full charge and their drifting stops. While the water that flows through gets purified when the charge is on, the surfaces can now be discharged to release a concentrate of contaminants for disposal. CDI, which extracts dissolved contaminants, is energy efficient compared to other methods like distillation or the now common Reverse Osmosis, which work the other way about, extracting water from a salt solution.

Andelman explained that the material of the electrodes had to be porous so that there was high surface area and greater capacity to collect charge for the same voltage applied. A limitation of the basic design, however, was that when charged ions of the contaminant piled up very near the electrodes, oppositely charged ions were also inserted into the region just beyond, a region called the diffuse layer, and this reduced the efficiency of the extraction of contaminants. A first improvement has hence been to insert an ion exchange membrane that would not impede the movement of the contaminant ions but act as a barrier to the counter-current of opposite charges. This did improve efficiency, but the membrane is expensive and takes space in the water channel.

Andelman’s innovation was to replace the membrane by directly coating the electrodes with a material that contained charged components that were drawn, half towards and half away from the charge on the electrodes. These separated charges create a layer that behaves like the ion exchange charge barrier in keeping down the counter-current of charges being released from the electrodes. This treatment, of creating a “polarised electrode”, however, is a low-cost procedure and the electrodes themselves are nano-porous carbon, which could come from burnt coconut shells, Andelman said.

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Taxonomy

• Purification
• Nanotechnology