Scientists at the Central Salt & Marine Chemicals Research Institute (CSMCRI), Bhavanagar, Gujarat have been able to completely degrade three industrial dyes — methyl orange, methylene blue and reactive black-5 — in the presence of sunlight.
The researchers developed a photocatalyst using titanium dioxide doped with red seaweed polymer carrageenan to degrade the dyes. The results were published recently in the journal RSC Advances.
Despite stringent environmental regulations, a comprehensive method of treating industrial dye is not available. The methods available are expensive and do not completely break down the dye molecules to non-toxic constituents but merely concentrate the contaminants.
“Annually, more than 500 tonnes of non-degradable textile colour wastes are being disposed of in natural streams without adequate treatments,” the paper says.
The photocatalyst can be reused up to six times with the degradation efficiency remaining at over 97 per cent.Titanium dioxide has conventionally been used for photocatalytic degradation of industrial dyes, but it takes a long time to degrade dyes. So the researchers doped titanium dioxide nanoparticles with sulphur and carbon by treating it with carrageenan. Though carrageenan with one, two and three sulphate groups can be utilised for treating industrial dyes, carrageenan with three sulphates was most efficient in degrading the dyes.
The nanocomposite was found to behave as an excellent photocatalyst that helped degrade industrial dyes quickly in a single-step process.
“The energy required to activate the catalyst is less when it is doped and this makes the dye degradation faster,” says Dr. Ramavatar Meena, the senior author of the paper from CSMCRI.
Solar concentrator used
Unlike a commercial titanium-dioxide-based catalyst that did not clear the dye solutions, the photocatalyst prepared in the lab was found to degrade the dyes when exposed to direct sunlight between noon and 2 pm during May-July.
“The titanium-dioxide-doped photocatalyst degraded reactive black-5 and methylene blue in about one-and-half hours and 60 per cent of methyl orange in two hours,” says Dr. Meena. “Visible light is mainly responsible for degradation; ultraviolet radiation intensity was just 3 per cent.” Also, the degradation was poor when only ultraviolet radiation was used to breakdown the dyes.
When a solar concentrator was used, the degradation process was hastened. “Reactive black-5 and methylene blue degraded within five minutes and methyl orange degraded completely in 20 minutes,” says Dr. Meena. There was no significant colour change in the case of control titanium dioxide sample that was not doped.
“When a solar concentrator is used the intensity of visible light is more and this plays an important role in the degradation process,” says Jai Prakash Chaudhary, the first author of the paper from CSMCRI.
A 15 degree C increase in temperature was seen when solar concentrator was used. But the temperature increase had no role to play in hastening the degradation process as the researchers did not get reproducible results when the dyes kept in an oil bath at 60 degree C were exposed to sunlight.
The researchers are now planning to conduct studies during winter to assess the photocatalyst’s ability to break down the dyes when bright sunlight is not available.
The nanocomposites are thermally stable and can be reused up to six times with the degradation efficiency remaining at over 97 per cent. This allows the nanocomposite photocatalyst to safely and completely treat harmful dyes in an eco-friendly and cost-effective manner.