Characterization of Fluorinated Organic Chemicals in Pyrolysis Byproducts of Biosolids Using Complementary Analytical Approaches

Pyrolysis is a promising thermal treatment technique for removing per- and polyfluoroalkyl substances (PFASs) from biosolids. In this study, biosolids and pyrolysis-derived byproducts from a full-scale plant were characterized using liquid chromatography–tandem mass spectrometry and high-resolution mass spectrometry (LC-MS/MS, LC-HRMS), combustion ion chromatography (CIC), and fluorine-19 nuclear magnetic resonance spectroscopy (¹⁹F-NMR) to detect and identify fluorinated organic compounds, including PFASs. CIC-based extractable organic fluorine (EOF) analysis showed only a 12% reduction in EOF in biochar after pyrolysis. In contrast, ¹⁹F-NMR revealed an 85% reduction in PFAS-specific −CF₃ groups and an 81% reduction in total organic fluorine (TOF) in biochar relative to biosolids. Targeted LC-MS/MS identified nine PFASs in biosolids, with perfluorobutanesulfonic acid (PFBS, 0.056 nmol/g), 6:2 fluorotelomersulfonic acid (6:2 FTS, 0.018 nmol/g), and 8:2 fluorotelomersulfonic acid (8:2 FTS, 0.012 nmol/g) as the three most abundant. Only five PFASs were detected in biochar, with the shorter chain, perfluorobutanoic acid (PFBA) and PFBS being the most abundant. These elevated concentrations in biochar (PFBA: 0.121 nmol/g; PFBS: 0.162 nmol/g) compared to biosolids (PFBA: 0.036 nmol/g; PFBS: 0.056 nmol/g) suggest transformation of long-chain PFASs into shorter chain analogues during pyrolysis. Trifluoroacetic acid (TFA) was enriched in biochar, further supporting the accumulation of short-chain products formed through chain-shortening reactions.