Untargeted analysis of per- and polyfluoroalkyl substances: an analysis of contaminated waters and advanced treatment processes

Per- and polyfluoroalkyl substances (PFAS) are persistent and widespread environmental contaminants. As a result, destructive treatment techniques have been developed to degrade these pollutants. However, such processes may generate transformation products (TPs), whose identities and potential risks remain unknown. This thesis investigates the formation of TPs using high-resolution mass spectrometry (HRMS). The processes investigated include electrochemical oxidation, ultraviolet (UV) degradation, and plasma degradation. An untargeted HRMS workflow was developed and validated against an in-house targeted PFAS method.
The findings elucidated the formation of TPs resulting from the electrochemical oxidation treatment in groundwater and landfill leachate. During treatment, ammonia present in the sample was oxidised to reactive nitro-substances that subsequently reacted with the PFAS present in the matrix leading to the formation of TPs such as perfluoroalkane sulfonhyponitrites (FASHN). The plasma degradation study further highlighted the influence of sample matrices on TP formation. A new class of PFAS, not previously reported, was detected, termed twoto-one fluorotelomer ethers (2:1-FtE), likely originating from larger precursor compounds. This class of PFAS had a consistent backbone but featured diverse functional groups.
The UV degradation study was performed in ultrapure water spiked with PFAS and induced the formation of hydrogen-substituted and unsaturated PFAS species. The treatment also revealed extensive isomerisation and the previously unreported di-hydrogen-substituted unsaturated PFOS (H2-UPFOS).
Toxicity evaluation using in silico predictions showed that some PFAS TPs were predicted to be more toxic than their precursors. Notably, H₂-UPFOS and the novel 2:1-FtE exhibited the highest predicted toxicity. This thesis demonstrates that untargeted HRMS can identify novel and potentially toxic PFAS TPs, improving the evaluation of PFAS remediation technologies.