Schematic illustration of the photocatalytic system.Nanoparticles stuck onto the surface of quartz beads offer a simple means of breaking down potentially hazardous organic dyes, according to researchers [Zhou and Srinivasan, Materials Today Chemistry 1-2 (2016) 1-6, http://dx.doi.org/10.1016/j.mtchem.2016.09.001].
The textile industry relies on organic dyes but they are damaging to the environment if discharged, toxic, and non-biodegradable. Before wastewater from textile production plants can be released, treatment to remove organic dyes is essential. Current approaches mostly rely on filtration and absorption, but this creates further – secondary – waste that also has to be treated.
As an alternative, scientists are looking for ways to break up organic dyes into smaller, harmless molecules. These so-called degradation reactions require light-activated catalysts to drive the process. Semiconducting materials, particularly TiO2, have attracted interest as potential photocatalysts because of their high stability and activity combined with low cost and toxicity.
A simple photocatalytic system based on TiO2 nanoparticles grown on quartz beads could be the answer, according to Ruitao Zhou of the Hong Kong Polytechnic University in China and M. P. Srinivasan of RMIT University in Australia, because it provides a large surface area for reactions to occur, long-term durability, and is self-cleaning.
“We invented this technique to avoid the disadvantages of common nanoparticles in catalysis, such as aggregation, blocking by surfactants, difficulties with recycling, and scalability,” explains Zhou.
The TiO2 nanoparticles that form on the surface of the quartz beads have a crystalline core/disordered shell structure. A thin film of the functional polymer 4-methoxycalix arene sticks the nanoparticles onto the quartz surface ensuring that they do not bunch together or become blocked by the organic molecules.
The treatment system itself consists of a quartz tube filled with the nanoparticle-covered beads. To demonstrate its capabilities, an organic dye – in this case methyl orange (MO) – was fed into the glass tube. When exposed to visible or UV light, electron-hole pairs generated by the TiO2 nanoparticles produce radicals (OH•), which break up the dye molecules into smaller species.
Zhou and Srinivasan’s photocatalytic setup successfully treated a continuous flow of MO over 140 hours, with activity only showing the first signs of decrease after 120 hours. Moreover, a simple rinse through with de-ionized water for an hour rejuvenates the treatment system.
“The TiO2 nanoparticles could be used for treating water polluted with organic waste,” says Zhou. “Compared with existing approaches, this system can treat polluted water in situ because no secondary waste is generated.”
The researchers believe the approach could be easily scaled up and could be applied to the fabrication of other nanostructures.