Reactivity of Per – and Poly- fluorinated species (PFAS) under subcritical water conditions

Abstract

Per- and polyfluoroalkyl substances (PFAS) are a class of synthetic organic contaminants that are ubiquitous in the environment. Many PFAS are environmentally persistent due to the strength of the carbon–fluorine bond and the stability of fluorinated carbon chains. Conventional treatment methods, such as granular activated carbon adsorption, focus on removing PFAS from contaminated water but do not destroy them, which results in concentrated waste materials requiring further treatment. Subcritical water treatment is a potential destructive treatment method that leverages pressurized liquid water at elevated temperatures to promote chemical transformations. Although previous research has shown that PFAS can be mineralized under hydrothermal alkaline treatment conditions, less is known about how structurally diverse PFAS behave under subcritical water treatment without alkaline amendment. This research aims to determine how PFAS structure affects stability and degradation under subcritical water conditions. To evaluate PFAS behavior, 27 compounds, including 8 perfluoroalkyl carboxylic acids (PFCAs), 4 perfluorosulfonic acids (PFSAs), 3 fluorotelomer sulfonates (FTSs), 8 perfluoroether carboxylic acids (PFECAs), 3 perfluoroether sulfonic acids (PFESAs), and Nafion by-product 4 (Nafion BP4) were treated in stainless steel batch reactors under varied SWT conditions. Treated samples were analyzed using liquid chromatography tandem mass spectrometry (LC-MS/MS), and degradation behavior was evaluated using normalized response ratios relative to control samples. PFCAs showed substantial degradation at elevated temperatures, indicating susceptibility to SWT induced degradation. In contrast, PFSAs, FTSs and PFESAs were generally more stable under the tested conditions. Susceptible side chain containing PFAS showed increased degradation susceptibility compared with non-side-chain species. Chain length was found to have little to no measurable effect on the degradation efficiency across subgroups accessed. These results suggest that terminal functional group identity is a strong predictor of PFAS degradation efficiency under subcritical water conditions. This research expands the understanding of PFAS behavior under non-alkaline subcritical water conditions and provides a foundation for future studies focused on product identification, and reaction pathways identification.