There is an imminent need to develop degradation pathways for Per- or Polyfluoroalkyl Substances (PFAS) to relieve current global contamination. PFAS are toxic, ubiquitous and consist of the C-F bond, which is recalcitrant to common degradation processes. Photolysis with UV-C irradiation is a promising alternative for PFAS degradation but is limited by low conversion and production of toxic and persistent by-products. This work considers various conditions to improve the C–F bond breaking efficiency in the photolysis (275 nm) of perfluorooctanoic acid (PFOA). Adjusting the aqueous PFOA solution to pH 2 with HCl or H2SO4 has been found to significantly improve the photolytic degradation of PFOA compared to direct photolysis. This process is hypothesised to rely on radical species from the photolysis of water in the presence of acid, such as OH and H radicals, which are thought to have sufficient redox potential to break C–F bonds under these conditions.
PFAS conversion is monitored via 19F Nuclear Magnetic Resonance Spectroscopy and Liquid Chromatography-High Resolution Mass Spectrometry (LC-HRMS). LC-HRMS is further used to quantify production of short-chain PFAS, which are undesirable products of degradation as they are toxic and persistent. Fluoride production is measured using a F- Ion Selective Electrode, to monitor the desired, C–F bond breaking ability.
Degradation reactions are carried out in custom-made flow photoreactors which have been developed for both photolytic and photocatalytic pollutant remediation. A single-pass flow system is used to incorporate in-line and real-time analyses, and a circulating flow system is used to improve pollutant conversion/mineralisation at increased flow rates. The use of continuous flow photoreactors is a promising and scalable approach for environmental remediation, with potential for integration into wastewater treatment infrastructure.