Drinking water can now be made completely free of E. coli in about 30 minutes by exposing it to sunlight, thanks to a catalyst developed by researchers at the Indian Institute of Science (IISc), Bengaluru. The bacterium E. coli is responsible for most of the water-borne bacterial infections. The results were published on September 2, 2016 in the journal RSC Advances.

Conventional methods that rely on UV light to kill pathogenic bacteria are often expensive and need relatively more sophisticated process. Now, the IISc researchers have made it possible to easily rid the water of E. coli bacteria by synthesising a zinc oxide photocatalyst that absorbs both UV and visible light to kill the bacteria. “We studied E. coli, but the catalyst can potentially kill all harmful bacteria,” says Prof. Giridhar Madras from the Department of Chemical Engineering at IISc and the corresponding author of the paper.

“Our catalyst is unique as we have doped it with a metal and a non-metal (copper and nitrogen) so that it absorbs both visible and UV light,” says Prof. Madras. “Our catalyst is far efficient than conventional catalysts as it absorbs both.”

Visible light comprises more than 40 per cent of the electromagnetic spectrum and UV light 4 per cent. The catalyst absorbs both components and generates free radicals that kill the bacteria. Such is the efficiency of the catalyst in the presence of sunlight that it is able to reduce the E. coli load in water from 10 million to zero in about 60 minutes. “The rate of killing the bacteria increases with an increase in the intensity of sunlight. We did our experiments between 11 am and 3 pm,” Prof. Madras says.

But to be effective, the catalyst (in a powder form) must be kept in suspension to increase the chances of interacting with the bacteria and killing them. “We kept stirring the water to keep the catalyst in suspension, else it will settle at the bottom and its efficacy will be reduced. We are now trying to coat the catalyst on a glass plate and suspend the glass plate in water to kill the bacteria,” says Prof. Madras.

“Conventional catalysts like TiO2 are active only in the UV region as it has a wide band gap. In the case of ZnO, we have reduced the band gap by co-doping it with copper and nitrogen,” says Rimzhim Gupta from IISc and the first author of the paper. “The co-doped ZnO catalyst will be able to absorb even the longer wavelength of 400-700 nm which is the visible range of the spectrum.”

The band gap of a semiconductor determines the wavelength of light required to activate a photocatalyst and kill the bacteria by producing free radicals. In this case, copper and nitrogen have unique roles in reducing the band gap. While the nitrogen shifts the valence band, the copper shifts the conduction band.

“When you shine light of appropriate wavelength on a photocatalyst, the electrons and holes get separated. The electrons and holes themselves can produce free radicals that kill the bacteria. Superoxide radicals (free radicals) can be generated when electrons from the conduction band react with dissolved oxygen, and holes in the valence band react with hydroxyl group and produce hydroxyl radicals,” says Neerugatti KrishnaRao Eswar from IISc and a coauthor of the paper. “We found superoxide and hydroxyl radicals were more effective in rupturing the cell wall and killing the bacteria.”