Structure-Dependent Transformation of Per- and Polyfluoroalkyl Substance Precursors during Drinking Water Disinfection.

Transformation of per- and polyfluoroalkyl substance (PFAS) precursors during drinking-water disinfection may generate persistent perfluoroalkyl acids (PFAAs), yet the roles of precursor structure and disinfection configuration remain unclear. Here, four representative precursors-N-methyl perfluorooctane sulfonamidoacetic acid (N-MeFOSAA), perfluorooctane sulfonamide (PFOSA), 8:2 fluorotelomer sulfonate (8:2 FTS), and 8:2 fluorotelomer phosphate diester (8:2 diPAP)-were compared across individual, simultaneous, and sequential ultraviolet (UV)/chlor(am)ination processes. Individual treatments showed limited precursor transformation, whereas simultaneous UV/disinfectant treatment increased conversion to 16.7-40.8%, with sulfonamide-based precursors exhibiting higher reactivity than fluorotelomer precursors. Fluorine mass balance revealed structure-dependent fluorine fates: sulfonamide precursors were transformed more extensively toward quantified PFAAs, whereas fluorotelomer precursors retained larger estimated unmeasured fluorine fractions. The same transformation behavior was also observed in actual drinking water. Mechanistic analyses integrating non-target high-resolution mass spectrometry (HRMS), reactive-species probe experiments, and density functional theory (DFT) calculations suggested that precursor structure governed reactive-species susceptibility and transformation pathways, with hydroxyl radicals playing a major role in precursor conversion toward PFAAs and chlorine radicals contributing to the initiation of one-electron oxidation steps that enabled subsequent chain shortening. Simultaneous UV/disinfectant treatments also increased PFAA-associated ecological risks and JEG-3 cytotoxicity, and the two endpoints were strongly positively correlated, suggesting that greater PFAA formation during precursor transformation may be accompanied by increased health-relevant toxicity. These findings highlight the importance of precursor structure in determining PFAS transformation and its potential health implications during drinking water disinfection.