Per- and polyfluoroalkyl substances (PFAS) chemicals are widely used in industry thanks to their thermal and chemical stability, and resistance to water and grease. They are found in everything from pizza boxes and baking paper to shampoo and extinguishers.
They are also toxic and harmful to human health, and their long-lasting stability means they can contaminate soil, rivers, and even food, leading to their ‘forever chemical’ nickname.
Removing them using conventional filter techniques is “costly and almost infeasible”, however, according to the Fraunhofer Institute for Interfacial Engineering and Biotechnology (IGB) in Stuttgart, Germany. The institute and industrial partner Hydr.O in Aachen, Germany, which specialises in cleaning up contaminated sites, set out to tackle the issue with the Atmospheric Water Plasma Treatment (AtWaPlas) project.
The research team, led by functional surface and materials expert Dr Georg Umlauf, used plasma’s ability to attack the molecular chains of substances. Contaminated water is fed into a glass and stainless steel cylinder, where it is then treated with the ionised gas. This reduces the PFAS molecular chains, allowing the toxic substance to be removed at a reportedly low cost.
“Our experiments with plasma have been successful in shortening the PFAS molecule chains in water. This is a significant step towards efficiently removing these stubborn pollutants,” said Dr Umlauf.
The stainless steel tube serves as the ground electrode of the electrical circuit. An outer copper mesh then acts as a high-voltage electrode, protected on the inside by a glass dielectric. A very small gap is left between the two, which is filled with an air mixture, which is converted into plasma when a voltage of several kilovolts is applied. It is visible to the human eye by its characteristic glow and discharge as flashes of light.
During the purification process, the PFAS-contaminated water is introduced at the bottom of the stainless steel tank and pumped upwards. It then travels down through the gap between the electrodes, passing through the electrically active plasma atmosphere.
The plasma breaks up and shortens the PFAS molecule chains as it discharges. The water is repeatedly pumped through both the steel reactor and the plasma discharge zone in a closed circuit, reducing the PFAS molecule chains further each time until they are completely mineralised.
“Ideally, the harmful PFAS substances are eliminated to the point that they can no longer be detected in mass spectrometric measurements. This also complies with the strict German Drinking Water Ordinance regulations regarding PFAS concentrations,” said Dr Umlauf.
The process has a key advantage over conventional methods such as active carbon filtering, he added.
“Active carbon filters can bind the harmful substances, but they are unable to eliminate them. This means that the filters must be replaced and disposed of regularly.
“The AtWaPlas technology, on the other hand, is capable of completely eliminating the harmful substances without any residue and is very efficient and low maintenance.”
To ensure the system’s feasibility, the researchers are testing the plasma purification under more challenging conditions. Conventional test methods involve using perfectly clean water and PFAS solutions that have been synthetically mixed in the laboratory. The team in Stuttgart is instead using real water samples that come from PFAS-contaminated areas. The real water samples therefore contain PFAS as well as other particles and suspended solids.
This plasma method can also be used to break down other harmful substances, including pharmaceutical residues, pesticides and herbicides, but also industrial chemicals such as cyanides. AtWaPlas could also treat drinking water in mobile applications, the Fraunhofer announcement said.
After a successful series of pilot-scale tests with a five litre reactor, the team is now working to optimise the process. “Our current objective is to completely eliminate toxic PFAS by extending process times and increasing the number of circulations in the tank. We also want to make the AtWaPlas technology available for practical application on a larger scale,” said Dr Umlauf.
The future could see installations set up in sewage treatment plants, or used in portable containers on contaminated open air sites, the announcement said.
Want the best engineering stories delivered straight to your inbox? The Professional Engineering newsletter gives you vital updates on the most cutting-edge engineering and exciting new job opportunities. To sign up, click here.
Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.