Characterization of PFOA isomers from PFAS precursors and their reductive defluorination
Jun Sun, Wanutcha Lorpaiboon, Nicholas Fox, Adele M. Jones, Junming Ho, Michael Manefield, Naresh Kumar, Denis M. O’Carroll, Matthew Lee
Abstract
• Identified environmentally relevant pathways for branched PFOA formation from PFAS precursors. • AFFF studies reveal underestimation of branched PFOA in the environment. • Branched PFOAs show varying reactivities with VB 12 -ZVZ treatment. • Modelling found that structural differences in PFOA significantly affect its degradability. Perfluorooctanoic acid (PFOA) including linear and branched isomers is one of only three PFAS included in the Stockholm convention on Persistent Organic Pollutants. Unfortunately, PFOA branched isomers have received less attention than the linear due to analytical difficulties and perceived lower environmental concentrations. In this study, we revealed a environmentally relevant pathway for the formation of branched PFOA from PFAS precursors. AFFF samples showed a doubling of branched PFOA concentrations (138 mg/L) after TOP assay oxidation (307 mg/L). These findings indicate that branched PFOA may be more pervasive in the environment than previously thought. Additionally, we investigated the reductive degradability of PFOA using vitamin B12 (VB 12 ) (a naturally occurring electron shuttle) in combination with either zero-valent zinc (ZVZ) or zero-valent iron (ZVI). Linear PFOA, as well as two branched isomers (3-methyl PFOA and 5,5-dimethyl PFOA), resisted reductive defluorination under the experimental conditions. However, all other branched isomers degraded within 10 days in the ZVZ-VB 12 system. The experimental rate constants for specific PFOA isomers generally correlate with their calculated reduction potentials, except for 6-methyl PFOA. A potential defluorination pathway was proposed based on high-resolution mass spectrometry (LC-Orbitrap) and density functional theory (DFT) studies.