Employing Plasma to Combat Hazardous PFAS Chemicals

PFAS harmful chemicals can now be detected in many soils and water bodies. Removing them using conventional filtering techniques is costly and virtually useless. Researchers at the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB are now successfully implementing a plasma-based technology in the joint research project AtWaPlas.

The contaminated water is fed into a combined glass and stainless steel cylinder where it is then treated with ionized gas, i.e. plasma. This reduces the PFAS molecular chains, allowing the toxin to be removed at low cost.

Volcanic and polyfluoroalkyl substances (PFAS) have many special properties. Because it is thermally and chemically stable and resistant to water, grease, and dirt, it can be found in a slew of everyday products: pizza boxes and baking sheets are coated with it, for example, and shampoos and creams also contain PFAS. In industry they act as extinguishing and wetting agents, and in agriculture they are used in plant protection products.

However, traces of PFAS are now being detected where they should not be found: in soil, rivers and groundwater, in food and in drinking water. This is how harmful substances end up in the human body. Due to their chemical stability, eradicating these “chemicals forever” has so far been almost impossible without significant effort and expense.

The AtWaPlas joint research project aims to change that. Abbreviation stands for Air Water Plasma Treatment. The innovative project is currently being run at the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB in Stuttgart in collaboration with industrial partner HYDR.O. Geologen und Ingenieure GbR from Aachen. The goal is to treat and recover water contaminated with perfluorinated sulfonates (PFAS) using plasma treatment.

The research team led by Dr. Georg Ohloff, an expert in functional surfaces and materials, uses the ability of plasmas to attack molecular chains of materials. An electrically conductive gas consisting of electrons and ions is generated when a high voltage is applied. “Our experiments with plasma have successfully shortened the chains of PFAS molecules in water. This is an important step toward efficiently removing these stubborn contaminants,” Umlauf is pleased to report.

The water cycle is in a stainless steel cylinder

Fraunhofer researchers use a cylindrical structure for this plasma process. Inside is a stainless steel tube, which serves as the ground electrode for the electrical circuit. Then the outer copper grid acts as a high voltage electrode and is protected on the inside by a glass insulator. A very small gap is left between the two, which is filled with the air mixture. This air mixture is converted into plasma when a voltage of several kilovolts is applied. It is visible to the human eye through its characteristic glow and discharge in the form of flashes of light.

During the purification process, the water contaminated with PFAS is introduced to the bottom of the stainless steel tank and pumped up. It then travels across the gap between the electrodes, passing through the electrically active plasma atmosphere. The plasma breaks down and shortens the PFAS molecule chains as it discharges.

Water is repeatedly pumped through both the steel reactor and the plasma discharge area in a closed circuit, reducing the PFAS molecule chains each time until completely mineralized. “Ideally, harmful PFAS substances are eliminated to the point where they can no longer be detected in mass spectrometry measurements. This also complies with the strict regulations of the German Drinking Water Ordinance (TrinkwV) regarding PFAS concentrations,” says Umlauf.

The technology developed at the Fraunhofer Institute has a major advantage over traditional methods such as active carbon filtering: “Active carbon filters can bind harmful substances, but they are not able to remove them. This means that filters must be replaced and disposed regularly. On the other hand, the technology AtWaPlas is able to completely eliminate harmful substances without any residue and is extremely efficient and low maintenance,” explains Fraunhofer expert Umlauf.

Real water samples rather than synthetic lab samples

In order to ensure true viability, Fraunhofer researchers are testing plasma purification under more challenging conditions. Conventional test methods involve the use of completely clean water and PFAS solutions that have been mixed synthetically in the laboratory. However, the research team in Stuttgart is using “real” water samples that come from areas contaminated with fluorosulfonates.

Samples are collected by project partner HYDR.O. Geologen und Ingenieure GbR from Aachen. The company specializes in cleaning up contaminated sites and also performs hydrodynamic simulations.

So the real water samples Umlauf and his team work with contain PFAS as well as other particles, suspended solids and organic turbidity. “This is how we verify the purification efficiency of AtWaPlas, not only using synthetic lab samples, but also under real conditions as water quality changes. Process parameters can be adapted and developed at the same time,” explains Umlauf.

This plasma method can also be used to break down other harmful substances, including pharmaceutical residues in wastewater, pesticides and herbicides, as well as industrial chemicals such as cyanide. AtWaPlas can also be used to treat drinking water in mobile applications in an environmentally friendly and cost-effective way.

The AtWaPlas joint research project launched at JuIy 2021. After a successful series of pilot tests using a 5-liter reactor, the Fraunhofer team is now working with the joint research partner to further improve the process.

Georg Umlauf stated, “Our current goal is to completely eliminate toxic PFAS by extending processing times and increasing the number of cycles in the tank. We also want to make AtWaPlas technology available for practical application on a larger scale.” The future could see similar plants being built as stand-alone filtration stages in wastewater treatment plants or used in portable containers on polluted outdoor sites.