Thermal Decomposition of Anionic, Zwitterionic, and Cationic Polyfluoroalkyl Substances in Aqueous Film-Forming Foams

• Published in: Environmental science & technology Vol. 55; no. 14; pp. 9885 - 9894
• Main Authors: Xiao, Feng, Sasi, Pavankumar Challa, Alinezhad, Ali, Golovko, Svetlana A, Golovko, Mikhail Y, Spoto, Anthony
• Format: Journal Article
• Language: English
• Published: Easton American Chemical Society 20.07.2021
• Subjects: Amides; Carboxylic acids; Cations; Chain scission; Chains; Contaminants in Aquatic and Terrestrial Environments; Decomposition; Foams; Heat treatment; Low temperature; Perfluoro compounds; Perfluoroalkyl & polyfluoroalkyl substances; Perfluorooctane sulfonic acid; Perfluorooctanoic acid; Recombination; Recombination reactions; Sulfonamides; Sulfonic acid; Thermal decomposition; Zwitterions; decomposition mechanisms; precursor compounds; high-resolution PIS; thermal transformation; thermal generation of PFAS; PFAS; thermal decomposition of PFAS
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/acs.est.1c02125

In this study, we investigated thermal decomposition mechanisms of cationic, zwitterionic, and anionic polyfluoroalkyl substances, including those present in aqueous film-forming foam (AFFF) samples. We present novel evidence that polyfluoroalkyl substances gave quantitative yields of perfluoroalkyl substances of different chain lengths during thermal treatment. The results support a radical-mediated transformation mechanism involving random-chain scission and end-chain scission, leading to the formation of perfluoroalkyl carboxylic acids such as perfluorooctanoic acid (PFOA) from certain polyfluoroalkyl amides and sulfonamides. Our results also support a direct thermal decomposition mechanism (chain stripping) on the nonfluorinated moiety of polyfluoroalkyl sulfonamides, resulting in the formation of perfluorooctanesulfonic acid (PFOS) and other structurally related polyfluoroalkyl compounds. Thermal decomposition of 8:2 fluorotelomer sulfonate occurred through end-chain scission and recombination reactions, successively yielding PFOS. All of the studied polyfluoroalkyl substances began to degrade at 200–300 °C, exhibiting near-complete decomposition at ≥400 °C. Using a high-resolution parent ion search method, we demonstrated for the first time that low-temperature thermal treatments of AFFF samples led to the generation of anionic fluoroalkyl substances, including perfluoroheptanesulfonamide, 8:2 fluorotelomer sulfonic acid, N-methyl perfluorooctane sulfonamide, and a previously unreported compound N-2-propenyl-perfluorohexylsulfonamide. This study provides key insights into the fate of polyfluoroalkyl substances in thermal processes.