Elucidating degradation mechanisms for a range of per- and polyfluoroalkyl substances (PFAS) via controlled irradiation studies

Per- and polyfluoroalkyl substances (PFAS) are a challenging class of environmental pollutants due to a lack of avail-able destructive remediation technologies. Understanding the fundamental mechanisms for degradation of PFAS is key for the development of field scalable and in-situ destructive based remediation technologies. This study aimed to elucidate and refine the current understanding of PFAS degradation mechanisms in water through a series of controlled gamma irradiation studies. Gamma irradiation of PFAS was performed using a cobalt-60 source ina batch irradiation up to 80 kGy at the Australian Nuclear Science and Technology Organisation. Perfluorooctanoic acid (PFOA), perfluorooctanesulfonic acid (PFOS), 6:2 fluorotelomer sulfonate (6:2 FTS), anda suite of thirteen different PFAS (in-cluding C4-C12 PFCAs, C4, C6, C8 PFSAs, and FOSA) were irradiated to investigate degradation, influence of pH, chain length, and transformation. High resolution mass spectrometry was used to identify more than 80 fluorinated transformation products throughout the degradation experiments. These included the -F/+H, -F/+OH, -F/CH2OH ex-changed PFAS and n - 1 PFCA, amongst others. Given the reactive species present (hydroxyl radicals (center dot OH), hydrogen radicals (center dot H) and aqueous electrons (e(aq)(-))), and the degradation products formed it was shown that aqueous electrons were the key reactive species responsible for initial PFAS degradation. Most importantly, based on degradation product formation, we found that the initial -F/+H does not have to occur at the alpha-fluoride (nearest the func-tional head group), rather occurring throughout the chain length leading to more complex degradation pathways than previously postulated. While our results support some of the reaction steps postulated in the literature, we have developed a unified 16 step and 3 pathway schematic of degradation supported by experimental observations.

Automated summary

This study aimed to elucidate and refine the current understanding of PFAS degradation mechanisms in water through controlled gamma irradiation experiments. It was found that aqueous electrons were the key reactive species responsible for initial PFAS degradation, and the initial -F/+H can occur throughout the chain length, leading to more complex degradation pathways.