Chinese scientists have developed a cheap, eco-friendly method to extract dyes from wastewater. The waste dyes can then be used to colour plastics, they say.

The dye-calcium carbonate hybrid precipitates as a sludge that can be reused to colour plastics

Azo dyes are commonly used to colour fabrics in the textile industry. However, many of them can cause cancer and so pose a health hazard when released into the environment in wastewater.

Hong-Wen Gao and colleagues from Tongji University, Shanghai, mixed two wastewater samples - one containing a cationic azo dye and the other an anionic one. They added sodium carbonate followed by calcium chloride and the dyes precipitated out of solution as a dye-calcium carbonate hybrid. Spectrophotometry measurements revealed that the process removed over 98 per cent of the dyes.

Currently, dyes are removed from industrial wastewater by either adsorption, for example onto activated carbon, or chemical processes, such as electrolysis or ozonation, which destroy the dye molecules. However, these methods produce a lot of waste and are expensive.

Gao's hybrid can absorb about seven times more dye than conventional adsorbents, prompting Ryan Richards, an expert in dye extraction at the Colorado School of Mines, Golden, US, to comment, 'One of the most impressive aspects of this work seems to be the high adsorption capacity reached using this technology.'

In addition, the hybrid can be used as a colourant for plastics, rubber and paint. This removes the need for waste disposal and replaces the metal oxides commonly used for colouring these materials. The team tested the water resistance of plastics coloured with the hybrid and found that no colour was released, even after 24 hours' immersion in water.

Gao sees no obstacles to industrial implementation: 'Without doubt, the adsorbent can be prepared simply and easily with available inorganic raw materials and used directly in the treatment of wastewater.'

Christina Hodkinson

Enjoy this story? Spread the word using the 'tools' menu on the left or add a comment to the Chemistry World blog.