Researchers at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) are testing two technologies designed to destroy per- and polyfluoroalkyl substances (PFAS), a vast family of industrial chemicals known for resisting natural and conventional breakdown processes.
The first approach uses hydrodynamic cavitation, which creates and collapses tiny vapor bubbles in contaminated water. The second combines cold atmospheric plasma with gas dispersion. Analyses performed by specialists at the Helmholtz Centre for Environmental Research (UFZ) confirmed that PFAS molecules were being degraded and that fluoride was released during treatment.
Once the technologies are sufficiently developed for commercial use, they could help industries treat contaminated wastewater before it reaches rivers, lakes, and oceans.
Why PFAS Are So Difficult to Destroy
Some PFAS are suspected of damaging genetic material and raising cancer risk, although the biological effects of many compounds in this group are still poorly understood. More than 10,000 short-chain and long-chain industrial chemicals are classified as PFAS.
Their extraordinary durability comes from highly stable carbon-fluorine bonds, which are among the strongest bonds found in organic chemistry. This resistance has earned PFAS the nickname “forever chemicals.”
PFAS can enter rivers and oceans through wastewater and have now spread across the globe. Researchers have also recently detected high concentrations in the Elbe River, creating a possible health risk for plants, animals, and people.
As part of Germany’s “National Water Strategy,” which aims to protect water resources and secure the country’s drinking water supply, scientists at HZDR are investigating ways to reduce PFAS contamination and systematically destroy the chemicals rather than simply removing or relocating them.
A preliminary study launched in 2022 was led by postdoctoral researcher Dr. Ysabel Huaccallo-Aguilar. Her team examined whether hydrodynamic cavitation could degrade PFAS in water.
Collapsing Bubbles Create Extreme Conditions
“In hydrodynamic cavitation, we pass PFAS-enriched water through a constriction, generating small vapor bubbles,” explains Dr. Sebastian Reinecke, head of the Department of Water and Environmental Technologies at HZDR.
Long-chain PFAS behave like surface-active substances, causing them to collect on the bubbles. As the water moves beyond the constricted section, pressure rises and the bubbles violently collapse.
“When the bubbles burst under the rising ambient pressure in the water downstream of the constriction, the PFAS that are attached to the bubbles are exposed to local temperature spikes of several thousand degrees Celsius,” Reinecke explains.
Cavitation also creates highly reactive hydroxyl radicals. These molecules react readily with nearby substances and may help destroy compounds produced during the initial stages of PFAS breakdown.
“Our hypothesis is that they attack the intermediate products, significantly boosting PFAS degradation.”
Huaccallo-Aguilar and her colleagues demonstrated that cavitation could degrade PFAS in tap water while also mineralizing organically bound fluorine. As treatment continued, the amount of fluoride measured in the water steadily increased, indicating that fluorine was being separated from the original PFAS molecules.
The experiments focused on perfluorooctane sulfonate (PFOS), a well-studied and exceptionally persistent member of the PFAS family. By the conclusion of the test, the process had degraded about 37 percent of the dissolved PFOS molecules while maintaining a stable rate of breakdown.
“We are now conducting follow-up experiments to increase the degradation rate,” Reinecke explains. “Our goal is to improve the process to a degradation rate of more than 80 percent of the PFAS in the solution and mineralizing more than 50 percent of the fluorine that is bound in the chemicals — that means, breaking down the carbon-fluorine bonds that are typical of PFAS.”
Cold Plasma Rapidly Attacks PFAS
In a separate set of experiments, environmental engineer Dr. Amit Kumar combined cold atmospheric plasma with gas dispersion to destroy PFAS.
The method works under normal surrounding conditions and does not require catalysts or added chemicals. During his PhD research, Kumar had studied how reactive chemical species produced by plasma could break down micropollutants. He adapted those findings for the PFAS experiments.
“We generated plasma at the water surface while simultaneously introducing gas into the PFAS-contaminated water,” says Sebastian Reinecke, explaining the experimental setup. “The PFAS attach to the surface of the gas bubbles. As they rise, the water is constantly circulated. This brings the PFAS to the surface, where they are broken down in the plasma.”
The plasma treatment almost completely degraded both long-chain and short-chain PFAS. It also released about 35 percent of the fluorine atoms originally bound within the chemicals, converting them into fluoride salts.
The results were faster than those produced through cavitation, but the method also had important drawbacks.
“While this method has significantly faster reaction kinetics than cavitation, it also consumes far more energy per volume unit,” Reinecke notes. “In addition, the process generates numerous transformation products that we have not yet been able to investigate in detail — for instance, gaseous compounds that form during the reaction.”
Additional experiments are underway to determine whether any of these transformation products could pose health risks. The researchers also want to identify ways to prevent potentially hazardous substances from forming.
Combining Cavitation and Plasma
The team is now adapting the plasma system to treat larger amounts of contaminated water. By using multiple electrodes and a technical gas injector, the researchers are increasing the reaction volume from approximately 50 milliliters to five liters.
Their longer-term goal is to combine plasma treatment with hydrodynamic cavitation, bringing the strengths of both technologies into a single system.
“I believe we’ll achieve high degradation rates by combining the highly reactive species from the plasma with the effects of cavitation,” says Reinecke.
If the combined approach proves successful, it could lead to a new and more efficient technology for destroying PFAS in contaminated water before the chemicals spread into the environment.
This research was funded by the Helmholtz Association’s Impulse and Networking Fund via the Clean Water Technology Lab (CLEWATEC), a Helmholtz Innovation Lab, under reference number HIL-A02. The projects “HyKaPro SAB-EFRE” and “Plasma4PFAS SAB-EFRE” are co-financed by the European Union and tax revenue as approved by the Saxon Parliament in its state budget.