Revolutionizing PFAS Treatment: Plasma Spinning Disk Reactor for Complete Degradation and Defluorination of Short-chain Poly- and Perfluoroalkyl Substances

Poly- and perfluoroalkyl substances (PFAS) are a group of human-made chemicals notoriously difficult to degrade due to their strong carbon-fluorine bonds. While plasma is very effective in degrading long-chain PFAS, treating short-chain homologues and achieving complete mineralization remains a challenge.

This work introduces the Plasma Spinning Disk Reactor (PSDR) as one of the most effective designs for treating short-chain PFAS. Operating by treating thin liquid layers on the surface of a spinning disc, this system is able to completely degrade perfluorobutane sulfonate (PFBS), a PFAS compound consisting of four carbon atoms. The results of operational parameter investigations revealed that the removal rate of this compound is primarily governed by its diffusion to the gas-liquid interface. Fluoride ions measurements revealed that upon completion of the treatment, only 50% of the compound is defluorinated. Plasma experiments conducted in the presence of ferrous ions (i.e., plasma-assisted Fenton reaction) suggested that OH radicals, predominantly present at the plasma-liquid interface, play a major role in PFAS defluorination. We discovered that initially, the plasma transforms PFAS into hydrofluorocarbon intermediates, which are immediately oxidized and defluorinated at the plasma-liquid interface. Intermediates that escape this interfacial oxidation can only be degraded in the bulk liquid through auxiliary oxidation processes that generate OH radicals. By combining the Fenton reaction with the plasma process, close to 98% defluorination of PFBS was achieved. These mechanistic insights can be used broadly by the PFAS community and extended to other fluorinated compounds to find a comprehensive solution to this global challenge.