A concentrated charge against water germs

Biology teaches us that water is life. Reality teaches otherwise: water contaminated with pathogens kills hundreds of thousands of people every year in places where water treatment is lacking or inadequate. To put an end to this, the availability of clean water for all people has been part of the United Nations' (UN) Global Sustainability Agenda since 2015. In line with this goal, Empa research teams are working with the Eawag Water Research Institute to develop new materials and technologies to remove pathogens from drinking water that have so far been difficult to eliminate using conventional methods, or have required expensive and time-consuming processes.

Tiny pathogens

The researchers are targeting the smallest of germs: Tiny pathogens that - unlike the Sars-Cov-2 coronavirus - spread with drinking water and cause various diseases, such as polio, diarrhea and hepatitis. These pathogens include rotavirus, which is just under 70 nanometers in size.

"Conventional water filters are ineffective against rotaviruses," explains Empa researcher Thomas Graule from the "High Performance Ceramics" laboratory in Dübendorf. But it is precisely these tiny germs that are among the most common pathogens of gastrointestinal infections. According to the World Health Organization (WHO), around 130,000 children died of rotavirus infections worldwide in 2016. Now, researchers have developed strategies for filtration technologies based on new materials that trickily circumvent the problem of tininess. This is because one property of the virus particles can be exploited for a new type of filter: the negative electrical charge of the virus particles in many cases. Based on this idea, the researchers began to develop suitable materials that would allow adsorption of the negatively charged virus surfaces.

This is because it has been difficult to create easily regenerable positively charged surfaces with high adsorption capacity, and systematic experimental studies have been scarce. The researchers therefore used a model virus for their studies that is even smaller than the rotavirus: the bacteriophage MS2, which is only 27 nanometers in size, a virus that infects bacteria but is harmless to humans. Using this model virus, the scientists were able to show that viruses in water adsorb to the filter surface to different degrees depending on the pH. "This needs to be considered when developing new water treatment and filtration technologies," Graule said.

Porous nanocoating

To develop filter technologies that can capture viruses at the nanometer scale, Graule is relying on composite materials that are functionalized to specifically bind viruses. "In water, the surface of the virus particles is negatively charged. We were able to show how the virus particles attach to positively charged surfaces," he explains. For example, the researcher is working in an international team on ceramic granules made of aluminum oxide, whose fine granules are coated with nanometer-thin layers of copper oxide. "The highly porous copper layer forms a composite with the ceramic with a positively charged and immensely large specific surface area," Graule says. The researchers were also able to coat tiny multilayer carbon nanotubes with copper oxide to enable virus elimination.

To develop a cost-effective and sustainable filter technology, the researcher specifically uses materials that can be recovered after use in the sense of a closed material cycle. It is also important that no filter components are washed out into the purified water. To this end, analytical methods for nanosafety still have to be developed so that the optimum composite material can be determined. The ultimate goal is to have a filter technology that is also suitable for water treatment in developing countries, with their particularly high incidence of rotavirus and other waterborne diseases.

Source: Chemie.de – Geballte Ladung gegen Wasserkeime

Image source: Wikimedia Commons: Andri Bryner, Eawag – Eawag Dübendorf, Forum Chriesbach