We are still far from achieving universal access: 26% of people do not have access to safe drinking water; nearly half are without safe sanitation; and nearly one-third without basic soap and water. Due to the high specific surface area of nano-sized photocatalytic particles, their efficient photo-generation of ROS, as well as large contact surface and direct interactions, the photocatalytic systems based on nanoparticle forms of TiO2 provide a powerful disinfecting strategy against waterborne microorganisms and pathogens. However, despite the significant potential and many advantages of such an approach, no operational photocatalytic sunlight-driven titania-based devices have been elaborated up to date. The current situation can be explained in part by the technical complications, which are directly related to the nanoparticle form of titania. More specifically, in slurry-type reactors, after completed photocatalytic cycles, it is difficult to collect and recycle TiO2NPs (20–100 nm in size), which are dispersed in suspension containing the pollutants. In a new research paper, published in the Nature partner journal Clean Water, Swiss scientists led by Professor László Forró at École polytechnique fédérale de Lausanne (EPFL) have accomplished, with a new water purification filter that combines titanium dioxide (TiO2) nanowires and carbon nanotubes powered by nothing but sunlight.
The authors designed and characterized the highly efficient and durable photocatalytic water filter, which is based on a nanocomposite material consisting of titanium dioxide nanowires (TiO2NWs) and carbon nanotubes (CNTs). The device has a very simple (planar) configuration, in which the contaminated water passes through a multi-layered filter material, the pathogens are trapped on the filter surface, whereas the UV component of the sunlight, being strongly absorbed by TiO2NWs, generates ROS. Therefore, in addition to the mechanical retention of pathogens, the photo-triggered ROS attack and kill diverse range of pathogenic species, including bacteria and germs, thus leading to even more efficient water decontamination. Furthermore, implementation of CNTs in the herein developed composite contributes to enhancing the photocatalytic action under exposure to sunlight
The research team first show that the TiO2 nanowires by themselves can efficiently purify water in the presence of sunlight. But interweaving the nanowires with carbon nanotubes forms a composite material that adds an extra layer of decontamination by pasteurizing the water. The idea is that when UV light from the visible spectrum of sunlight hits the filter, it causes it to produce a group of molecules called Reactive Oxygen Species (ROS). These include hydrogen peroxide (H2O2), hydroxide (OH), and oxygen (O2-), and are known to be effective pathogen killers.
The researchers tested their device with a variety of pathogens including: E. Coli, bacteria, Campylobacter Jejuni, Giardia Lamblia, Salmonella, Cryptosporidium, the Hepatitis A virus, and Legionella Pneumophila. The device is exceptionally adept at removing all the pathogens from water, and shows promising results even for eliminating micropollutants, such as pesticides, drug residues, cosmetics etc.
It was a multidisciplinary collaboration between chemists, physicists, and biologists, where the researchers developed a very efficient water purification device, which does not need any energy source but sunlight. The prototype can supply clean drinking water even at remote places to small populations and could be easily scaled-up. According to the authors it is a great achievement and an important “side-product” of this project is that it has attracted a large number of talented and motivated students who care for environmental issues, for sustainability.
In a nutshell, EPFL reports an innovative tailored synthetic method to prepare a solar water purifier, its basic active components, as well as its thorough characterization in terms of photo-generation of ROS towards the removal of a wide spectrum of toxic chemicals and pathogens from drinking water.
E. Horváth, J. Gabathuler, G. Bourdiec, E. Vidal-Revel, M. Benthem Muñiz, M. Gaal, D. Grandjean, F. Breider, L. Rossi, A. Sienkiewicz, L. Forró. Solar water purification with photocatalytic nanocomposite filter based on TiO2 nanowires and carbon nanotubes. npj Clean Water, 2022; 5 (1) DOI: 10.1038/s41545-022-00157-2