László Forró, the Aurora and Thomas Marquez Professor of Physics of Complex Quantum Matter in the University of Notre Dame's Department of Physics and Astronomy. Credit: University of Notre Dame
Water contaminated with disease-causing organisms such as parasites, viruses and bacteria is a major health concern in many parts of the world, with an estimated 1.8 billion people regularly consuming contaminated water due to lack of sanitation resources, according to the World Health Organization. Many of those affected live in rural areas without the infrastructure to support water purification technology, including areas that lack electricity and transportation networks. Researchers from École polytechnique fédérale de Lausanne (EPFL) and the University of Notre Dame have recently developed a novel nanomaterial filter that could enable the purification of drinking water using just sunlight and gravity.
The new filter device leverages the photocatalytic properties of titanium dioxide (TiO2), which produces hydroxyl radicals and superoxide radical anions on its surface when exposed to UV light, water and oxygen molecules. These reactive oxygen species can attack and destroy pathogens and other organic contaminants in their immediate vicinity. To create the TiO2-based filters, the team leveraged a process previously developed at EPFL to efficiently produce large quantities of TiO2 nanowires, which involves shear mixing and heating of titanium-containing starting material and base solution in a base resistive vessel at 70°C, followed by pressing or centrifugation to separate the nanowires from other residues. This process produces up to one kilogram of TiO2 nanowires per day, as opposed to just a few grams using conventional methods.
The nanowires were mixed with carbon nanotubes, and the mixture was cut into 30 μm thin films, which were calcinated at 600°C, causing the nanowires to fuse together. The result was a thin filter membrane with a large surface area for purifying water. To complete the device, the researchers placed the filter between two panes of glass, with an opening at the top and bottom for water to flow in and out. The purification device removes contaminants by three processes – mechanical filtration, pasteurization and destruction of microbes and parasites by free radicals. The pores of the filter material block larger particles from passing through the membrane, while sunlight hitting the filter produces both reactive oxygen species and heat that kills harmful organisms. The team tested their prototype on river water containing E. coli and found that the filter completely eradicated the E. coli from the water. The purification device was also tested using water mixed with a “cocktail” of nine different micropollutants, including drug residues, pesticides, hormones and cosmetics. While the filter did not remove all the pollutants, it significantly reduced levels of gabapentin and metformin, suggesting the potential for photocatalytic nanomaterials to aid in future development of solar-powered purification systems. This research was published in npj Clean Water.
“Now one device can sanitize two liters of water a day,” said corresponding author László Forró. “That is enough for one person’s drinking water, but what about other people and other uses like cooking and washing? We think larger filters and larger scale applications will add greater benefits.”