ETH Zurich develops new method for PFAS degradation

Fire-fighting foams, non-stick cookware, water-repellent textiles and pesticides all have one thing in common: they all contain dangerous so-called PFAS (perfluorinated and polyfluorinated alkyl substances). Researchers from the group of Salvador Pané i Vidal, Professor at the Institute of Robotics and Intelligent Systems at ETHZ, have developed a new method to break down a subgroup of PFAS, PFOS (perfluorooctane sulfonates). PFOS are now severely restricted or even banned due to their toxicity. "The main problem is that the molecules consist of long carbon chains surrounded by fluorine atoms. This carbon-fluorine bond is so strong that you need a lot of energy to break it," says Andrea Veciana, PhD student at Pané i Vidal. In order to break up the PFOS molecules and thus degrade them in water, the researchers used piezo catalysis for the first time. Piezo refers to piezoelectricity, an electrical voltage that is generated during mechanical deformation. Catalysis refers to the acceleration of a chemical reaction using suitable substances. "We have developed nanomaterials that are piezoelectric. To the naked eye, this material looks a bit like sand," says Veciana. In an ultrasonic bath, these particles become electrically charged and act as a catalyst. Pané i Vidal adds: "It is this electrical charge that sets the whole chain of reactions in motion and breaks down the PFOS molecules piece by piece. That's why the nanoparticles are called piezoelectric." In order to measure the PFOS concentration in their samples, the researchers worked together with Samy Boulos, an analysis specialist at the Food Biochemistry Laboratory. Using a mass spectrometer, the researchers were able to prove that 90.5 percent of the PFOS molecules were degraded. "It must be added, however, that we worked with a very high concentration of four milligrams per liter," says Veciana. "In nature, for example in lakes and rivers, the PFOS concentration is less than one microgram per liter. The lower the concentration, the longer it takes for the PFOS to break down."

Some technologies that are currently being developed first concentrate the water and only then destroy the PFOS. This would also be an important step for piezocatalysis, which would have to be implemented in a specific application such as wastewater from the chemical industry. The potential of the new method becomes clear when one considers the existing options for degrading PFAS. "One method is thermal decomposition, which is very energy-intensive at over 1000 degrees Celsius," says Veciana. PFAS can also be degraded by photocatalysis. This is a similar process to piezocatalysis, but instead of mechanical energy, light is used to activate the catalyst. The main problem with this method is that in practice it is used to treat waste water. This is cloudy and only little light can penetrate it. Veciana mentions a third method: "You can also use absorption. This involves using a type of sponge that absorbs the pollutants from the water. But this shifts the problem from one place to another. Because now a solution has to be found for the PFAS-permeated sponge." The disadvantages of the existing methods were one of the reasons for the ETH researchers to look for a new way to break down PFAS. Piezocatalysis has the advantage that it works with different mechanical energy sources. "If water has to be purified in wastewater treatment plants and there is already turbulence in the water, this energy could perhaps be used to break down PFAS in the water," says Veciana. Unfortunately, what the researchers have achieved in the laboratory with water samples of 50 milliliters cannot yet be transferred to practice. "The scalability of our method is one of the biggest challenges," says Pané i Vidal. "However, we have succeeded in showing that piezocatalysis works as a method for degrading PFOS and has advantages over previous methods." The method can also be applied not only to PFOS, but to all PFAS and other micropollutants.