Researchers at the University of Birmingham have demonstrated a new method to break down toxic pollutants in wastewater, using sunlight and molecular-thin catalysts created using an innovative ‘mechanical’ approach -  with confidence that the method could be used to produce photocatalysts at an industrial scale.

Non-degradable dyes originating from industries such as textiles, cosmetics, food, pharmaceuticals, and printing, are amongst the most prominent sources of industrial pollution. Left untreated, they disperse in both land and water, leading to contamination that poses serious risks to human health and the environment.

Most industries use a combination of techniques to remove the chemicals from wastewater - however dye-polluted water persists. Textile dyes alone are the second-largest contributor to water pollution worldwide. As well as contaminating drinking water, they can disrupt ecosystems, reduce photosynthesis in freshwater and marine plants, and alter the life cycles of fish and other aquatic animals.

Photocatalysis, which uses a light-activated catalyst to accelerate decomposition, has shown promise as a sustainable method for breaking down these harmful pollutants. However large-scale manufacture of suitable catalysts that can be activated by sunlight has proved challenging, and current methods use toxic solvents, which creates a further waste stream to deal with.

Dr Jason Stafford from the Department of Mechanical Engineering and the Birmingham Centre for Mechanochemistry and Mechanical Processing collaborated with researchers specialising in analytical chemistry and biosciences to demonstrate that catalysts produced by a novel, water-based mechanical method can degrade pollutants, including those containing the tough-to-break carbon-fluorine bonds that feature in many sources of persistent pollution.

The method uses high intensity turbulent shear stresses to exfoliate molecular-thin sheets of material and also assemble them into heterostructures, which are made of two semiconductor materials, with bespoke photoelectronic properties. The most recent work, which is published in npj 2D Materials and Applications, shows it can be used to significantly enhance photocatalytic performance in under 90 minutes without using toxic chemicals.

The research team chose graphitic carbon nitride (g-C3N4) and molybdenum disulfide (MoS2), as the raw material for making catalysts. These compounds are known for their visible-light responsiveness, high stability, and low cost, making them ideal for solar-to-chemical energy conversion.

The manufactured catalysts comprised of ultra-thin layers of the raw material and atomic edge creation, so increasing the surface area on which the chemical decomposition reaction takes place, while simultaneously preserving the electronic structure of the material and maintaining charge generation and transport, which are essential for the catalyst to work.

They tested the effectiveness of the catalysts using Indigo Carmine, Rhodamine B, and Acid Red 266 as ‘model pollutants’. All three are synthetic chemicals used in textile dying, industrial processes or biotechnology research, and Acid Red 266 contains carbon-fluorine bonds.

Their study demonstrated these sustainably produced 2D catalysts increased the degradation performance on the model pollutants by up to 2.5 times compared to the bulk raw material. Notably, the process rapidly achieves these levels of performance after only 10 minutes of mechanical treatment.

“Confident that method could be used to produce photocatalysts at an industrial scale”

Dr Stafford explained:

“While there are several methods for mopping up, coagulating, or filtering to remove pollutants from water, a more desirable option is to degrade them to simpler, non-toxic compounds. In the case of synthetic dyes, the products of degradation are carbon dioxide, water, and inorganic salts. A major roadblock to pursuing this approach has been the lack of sustainable and scalable ways of making catalysts in a format that enables efficiency in the chemical reaction. We have shown this is possible, and are confident that the method could be used to produce photocatalysts at an industrial scale.”

A method for the mechanical synthesis of materials is the subject of a patent application submitted by University of Birmingham Enterprise. The researchers are interested in speaking to commercial companies who are interested in advanced materials and wastewater treatment technologies and want to explore partnership or licensing to bring new products, processes or services to market.

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