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As environmental concerns surrounding water pollution grows, there is a need for one or more solutions to combat worldwide problems. Over 300,000 tonnes of synthetically made dyes are released into the world’s aquatic ecosystems each year and are known to be toxic to both animals and humans.
team of Researchers from the UK have now developed a photocatalytic composite material, composed of tantalum and tungsten compounds, which photodegrades man-made dyes found in marine ecosystems around the world.
With so much man-made dye being released into our water systems each year, it is easy to see how Scientists are concerned. Aside from being toxic, some of these dyes also inhibit photosynthetic activity in these systems due to their strong light absorbance, which is a major concern regarding global oxygen production in the long run.
The pollution on such a scale, contributes further to a worldwide problem- a lack of clean drinking water in many areas. Safe to say, that this is a major issue in itself without the extra contributions through human means. Unfortunately, these synthetic dyes are inherently stable and hard to break down through natural and biological means.
One such way to remove the dyes is through photocatalysis. Both Ta3N5 and W18O49 are known to be good degraders of organic dyes and absorbers of light, respectively. As such, the Researchers have developed a new composite material for the breakdown of these aquatic-occupying dyes. The Ta3N5/ W18O49 composite was formed through the heterogenous growth of W18O49, using solvothermal methods and mixed/reacted with a fine Ta3N5 powder through various wet chemical techniques.
The Researchers characterized the composite through various means, including field emission scanning electron microscopy (FE-SEM, Hitachi S4800), transmission electron spectroscopy (TEM, Jeol 2100), X-ray diffraction (XRD, Bruker D8 Advance), UV-Vis diffuse reflectance spectroscopy (Agilent Cary 100), X-ray photoelectron spectroscopy (XPS, Kratos Axis Supra) and ultraviolet photoelectron spectroscopy (UPS).
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The Researchers tested the breakdown efficiency of the composite by testing it on one of the most common dyes found in water systems- Rhodamine B. The process was not only found to be highly effective, but also scalable, making it a great contender for large-scale commercial applications.
Whilst Ta3N5 alone is known to be a good photodegrader of organic dyes, the composite completely surpassed it and achieved over double the rate of Rhodamine B oxidation using white light. The composite also removed over 90% of the dissolved dye through molecular adsorption- a similar process that is observed with commercially used activated carbon. The photocatalyst was also found to possess a high cyclability, with the rate of Rhodamine decomposition being twice that of when just Ta3N5 was used.
The combination of the two photcatalysts produces a two-photon photocatalytic mechanism, where the quantum efficiency of the catalyst can be enhanced due to an improved separation of electrons and holes within the system. In addition, the high density of oxygen vacancies within the tungsten component acts as a shallow donor state to provide adsorption sites for surface species and improve the catalytic ability of the material.
The mechanism of decomposition has been factored to the removal of hydroxyl and superoxide radicals, by the Ta3N5 component, which enhanced the cleavage of the Rhodamine B chromophore through the loss of conduction band electrons. The Researchers used scavenger molecules to test the mechanism and found that as the superoxide radicals are removed from the sample, it generates a lower concentration of conduction band electrons. This then induces a decrease in the electron-hole recombination process, yielding an increased density of valence holes for the oxidation of Rhodamine B molecules.
The enhanced catalytic activity and scalability of this composite photocatalyst holds significant promise as photocatalysts for dye degradation. If commercialized and optimized, these photocatalysts could be used in the treatment of dye-polluted wastewater on a global scale.
Sources and Further Reading
“Active removal of waste dye pollutants using Ta3N5/W18O49 nanocomposite fibres”- Jones D. R., et al, Scientific Reports, 2017, DOI:10.1038/s41598-017-04240-4