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.

       The paper reports that annually, more than 500 tonnes of non-degradable textile colour waste effluents are being disposed off in natural streams without adequate treatments. 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. The nanocomposite was found to behave as an excellent photocatalyst that helped to degrade industrial dyes quickly in a single-step process.

       They have demonstrated that the direct use of sulphate rich seaweed polysaccharides, carrageenans, namely kappa (κ), iota (ι) and lambda (λ) as sources of sulphur and carbon in doping TiO2 synthesis resulted in a highly active photocatalyst. Evaluation of the dye degradation pattern shows rapid degradation of reactive black-5, methylene blue and methyl orange using modified TiO2 nanocomposites in different light sources. Robust dye degradation was achieved between 1 and 4 h under daylight whereas, the use of a solar concentrator reduced the degradation time of Methylene Blue and Reactive Black-5 to <5 min and Methyl Orange solution was turned colourless within 20 min. The present study elaborates the effect of seaweed carrageenans in inducing heteroatoms like sulphur and residual carbon for the photodegradation of industrially important dyes.

       “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.

       “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.

       The researchers are 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. The nanocomposite photocatalyst can safely and completely treat harmful dyes in an eco-friendly and cost-effective manner, the study said.

A view of Common Effluent Treatment Plant.

Source: www.thehindu.com