Electrochemical destruction of PFAS

The substance group of PFAS (per- and polyfluorinated alkyl substances) comprises around 8000 to 10,000 individual compounds, which are chemically very stable. In many applications, this is a highly desired property and hence they are sometimes considered indispensable. They are used as anti-adhesives and impregnating agents, surfactants, high-performance fire extinguishing agents or as synthesis building blocks. We encounter them in all areas of life, e.g. in functional clothing and medical products. However, their persistence means that they are carried over and accumulate in sewage sludge, soil, water and organisms, where they develop their harmful effects and interfere with cell metabolism. As PFAS cannot really be removed using the currently known cleaning methods for soil and water, the EU has initiated a procedure to ban this class of substances. Unfortunately, we still do not know exactly which of these substances are harmful and so the entire group of substances is under general suspicion. However, the short-chain species of this substance class are considered particularly harmful.

Degradation of short-chain PFAS by total oxidation

This is where the approach we are pursuing in the ZIM project PFCex comes in. Fraunhofer IKTS has now been able to show that the shortest chain of all PFAS, trifluoroacetic acid (CF₃COOH – ‘TFA’), can be quantitatively destroyed electrochemically under certain conditions – leaving only CO₂ and fluoride ions as relatively harmless species. The process is called electrochemical mineralization or total oxidation. We have already been able to demonstrate the destruction of a multitude of organic compounds on this basis. Fig. 1 shows the reduction of the TFA concentration in an industrial waste product solution in a continuous electrolysis test. The corresponding test stand is shown in Fig. 2. The initial concentration was 5960 mg/l. It was demonstrated that more than 90 % of the TFA was consistently destroyed by electrolysis, while the concentration of fluoride ions released during degradation increased in the effluent. In batch tests, the process achieved over 99.9 % degradation. In the project, the range of tests was gradually extended to other short-chain PFAS. It was shown that perfluorooctanoic acid (PFOA) and perfluorobutane sulfonic acid (PFBSA), for example, can also be destroyed electrochemically (Fig. 3). The current density at the anode is a decisive factor here.

Outlook

In the future, the process will be tested under practical conditions for the treatment of landfill leachate and extinguishing agent residues. In addition to the challenges in process development, a further challenge lies in establishing appropriate routinely applicable analytics for the ultra-trace level. The legal limits for these substances are in the ng/l range. Two approaches are pursued here: determining the electrochemically released fluoride after pre-enrichment and finding markers (typical individual compounds) using a technique called LC-MS (liquid chromatography – mass spectrometry) after appropriate sample pre-treatment. Fraunhofer IKTS offers support in solving PFAS problems, including their destruction, for many areas, such as the electronics industry.