New method to permanently break down chemicals uses nanoparticles and ultrasound

What do firefighting foams, nonstick cookware, water-repellent textiles, and pesticides have in common? They all contain perfluoroalkyl and polyfluoroalkyl substances, or PFAS, synthetic chemicals that don’t break down naturally. It’s no wonder, then, that PFAS are now contaminating soil and water and are also being detected in the bodies of humans and animals. The dangers are well known: These timeless chemicals can damage the liver, trigger hormonal disruption, and cause cancer, to name a few of their effects.

Researchers from the group led by Salvador Pané i Vidal, a professor at the Institute of Robotics and Intelligent Systems at ETH Zurich, have developed a new method to break down a subgroup of PFAS called perfluorooctane sulfonates, or PFOS. Due to their toxicity, PFOS are now strictly regulated or even banned. The study is published in the journal Little science.

“The main problem is that the molecules are made up of long carbon chains surrounded by fluorine atoms. This carbon-fluorine bond is so strong that it takes a lot of energy to break it,” explains Andrea Veciana, a doctoral student at Pané i Vidal.

Breaking down molecules with ultrasound and nanoparticles

To break down PFOS molecules and thus degrade them in water, the researchers used piezocatalysis for the first time. The term “piezo” refers to piezoelectricity, an electrical charge generated during mechanical deformation, and the term “catalysis” refers to the acceleration of a chemical reaction with suitable substances.

“We have developed piezoelectric nanomaterials. To the naked eye, this material looks a bit like sand,” explains Veciana. In the ultrasonic bath, these particles become electrically charged and act as a catalyst. Pané i Vidal adds: “It is this electrical charge that triggers the entire reaction chain and breaks down the PFOS molecules piece by piece. That is why the nanoparticles are called piezoelectric.”

To measure the concentration of PFOS in their samples, the researchers worked with Samy Boulos, an analysis specialist at the Food Biochemistry Laboratory. Using a mass spectrometer, the researchers were able to prove that 90.5% of the PFOS molecules were degraded.

“It should be noted, however, that we were working with a very high concentration of 4 milligrams per liter,” explains Veciana.

“In nature, such as in lakes and rivers, the concentration of PFOS is less than 1 microgram per liter. And the lower the concentration, the longer it takes for PFOS to degrade.”

Some technologies currently under development first concentrate the water and then destroy the PFOS. This would also be a key step in piezocatalysis, which should be implemented in a specific application such as a chemical industry effluent.

Better than previous methods

The potential of this new method becomes clear when considering existing options for degrading PFAS. “One method is thermal decomposition, but it requires a temperature above 1,000 degrees Celsius, which makes it very energy intensive,” Veciana says.

PFAS can also be degraded by photocatalysis. This process is similar to piezocatalysis but uses light to activate the catalyst instead of mechanical energy. The main problem with this method is that in practice the goal is to treat wastewater, and since wastewater is turbid, light penetration is low.

Veciana suggests a third method: “There is also absorption, which involves using a kind of sponge to absorb pollutants from the water. But that just moves the problem from one place to another; now we have to find a solution for the PFAS-impregnated sponge.”

Researchers at ETH Zurich have been looking for a new method to break down PFAS due to the disadvantages of existing methods. Piezocatalysis has the advantage of being able to work with different mechanical energy sources.

“If water is to be purified in wastewater treatment plants and there is already turbulence in the water, perhaps that energy could be harnessed to break down the PFAS in it,” Veciana says.

Let’s fight PFAS together

Unfortunately, what the researchers managed to achieve in the laboratory with 50 milliliter water samples has not yet been translated into practice. “The scalability of our method is one of the biggest challenges,” says Pané i Vidal.

“However, we have managed to demonstrate that piezocatalysis works as a method for degrading PFOS and has advantages over previous methods.” Moreover, their method can be used not only on PFOS, but also on any other PFAS and micropollutants.

In general, PFAS degradation methods must be used before the chemicals enter the environment, i.e. in industrial wastewater treatment plants or in agricultural water collected for reuse. “Companies must take all possible measures to ensure that the water they release into the environment is as clean as possible,” says Pané i Vidal.

Veciana adds: “PFAS are a global problem that must be addressed first and foremost through policy change and greater transparency.” There is already a lot of media talk about banning PFAS and stricter regulations to force industry to be more transparent about its use of these chemicals.

Veciana says: “Nevertheless, it is also important to continue to innovate through research to reduce and remediate existing exposure to PFAS as much as possible.”