Non-hazardous photocatalytic material removes dye from water

Earlier this month, Mumbai was startled by the sight of blue dogs wandering the streets. It turned out that the dogs had been swimming in the Kasadi river, which was polluted with industrial waste and dyes, turning the dogs’ white fur blue. Spilling dye pollutants, whether by accident or on purpose, can have hazardous effects on nature and our own health. In order to combat this problem, researchers at Swansea University have developed a photocatalytic material that can absorb more than 90 per cent of dyes and enhance the rate of their breakdown using visible light.

Environmentally harmful synthetic dye pollutants are released at a rate of nearly 300,000 tonnes (330,693 US tons) a year into the world’s water, endangering the health of humans and animals alike.

The new material is a composite of two compounds: tungsten oxide and tantalum nitride. By heating it at high pressures inside a sealed container, the composite is synthesized by growing ultra-thin “nanowires” of tungsten oxide on the surface of tiny particles of tantalum nitride. The composite provides a huge surface area for dye capture.

The photocatalytic material is breaks down the dye into smaller, harmless molecules using the energy of the sun. After the harmful dyes have been removed from the water, the composite material can simply be filtered and reused.

Tungsten oxide has high electrical conductivity, chemical stability and surface activity, as well as strong light absorbance. In turn, tantalum nitride is a low band-gap semiconductor and has the ability to absorb almost the entire spectrum of visible light. Therefore, it is able to use sunlight to power the degradation processes.

Combined, these two materials strengthen each other. A dye test used in the study showed that the composite broke down the dye at around double the rate of tantalum nitride on its own, while tungsten oxide was incapable of breaking down the dye at all. Both materials are non-hazardous.

Photos: Daniel Jones / Swansea University / Arati Chauhan