Hydroxyl radical is the major oxidant in most advanced oxidation processes and can degrade various contaminants efficiently, but its reactivity for perfluoroalkyl acids (PFAAs) is very limited. Previous studies have shown that PFAA precursors that contain nonfluorinated groups can be transformed by hydroxyl radical into different chain length of PFAAs, but the reaction mechanism and pathways are not clear. In this study, we used density functional theory (DFT) calculations to investigate the hydroxyl radical-initiated transformation of one representative precursor, 6:2 fluorotelomer sulfonate (6:2 FTS) and illustrate the detailed reaction path. Different reaction pathways were proposed, and the free energy of the reactions was calculated at the M06-2X/6-311++G(2d,2p) level of theory using SMD as the solvation model. The kinetic feasibility of the reactions was compared by calculating the free energy of activation at the same level of theory and the optimal reaction path was then proposed. The results showed that the reaction started with a H-abstraction from the ethyl carbons of 6:2 FTS followed by a hydroxyl radical addition and a second H-abstraction. The addition of O<sub>2</sub> to the alkyl radical is critical as it generated the ketone radicals, which tend to capture any available electrons from adjacent functional groups and cause C−C cleavage. The C−C cleavage resulted in the formation of perfluoroalkyl or polyfluoroalkyl attached to carboxyl groups or aldehyde groups, and perfluorohexyl radical, which were further transformed to different chain length of perfluoroalkyl carboxylic acids (PFCAs). The rate constants of each step were calculated based on transition state theory. The product profile from DFT calculations were comparable with our experimental data where 6:2 FTS was efficiently transformed by hydroxyl radical into different PFCAs simultaneously, with perfluorohexanoic acid being the predominant product. The involvement of O<sub>2</sub> in the reaction path was verified experimentally and there was no reaction of 6:2 FTS under anaerobic conditions. The intermediates proposed in the optimal reaction path such as 6:2 fluorotelomer carboxylic acid and OH-substituted sulfonate and carboxylate were detected in the reaction solution by high resolution mass spectrometer. This study provides valuable insight into the degradation and transformation of precursors in hydroxyl radical-based oxidation processes.

Density functional theory calculations on the reaction pathways for the transformation of 6:2 fluorotelomer sulfonate to perfluoroalkyl carboxylic acids initiated by hydroxyl radical in aqueous phase

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.