Structure-Dependent Reactivity and Transformation Products of Polyfluoroalkyl Substances by Hydroxyl Radicals

Polyfluoroalkyl substances (PoFAS) can be oxidized by hydroxyl radicals (^•OH) in both natural and engineered treatment systems, yet a comprehensive understanding of their structure-dependent reactivity and transformation products remains unexplored. Herein, we investigated the ^•OH-mediated transformation of 28 PoFAS among four distinct classes using UV/H₂O₂ and unraveled the mechanisms governing their reactivity and product formation. The second-order rate constants revealed a reactivity order of alkenyl PoFAS (C═C, ∼10⁹ M^-1·s^-1) > PoFAS with H-substitution at the headgroup (-SO₂N-) or α-C (-CH₂-, ∼10⁸ M^-1·s^{- 1}) ≫ H-substitution at ω-C (H-CF₂-, ∼10⁵ M^-1·s^-1). Transition state theory effectively identified initial ^•OH attacking sites and unraveled how H-substitution patterns, headgroup chemistry, and steric hindrance collectively dictate PFAS reactivity. The experimentally derived rate constants were well described (R² = 0.790, p < 0.01) by the calculated free energy of activation (3.79-14.8 kcal/mol). Analysis of transformation pathways based on products identified revealed the capability of ^•OH to convert diverse nonperfluorinated moieties in PoFAS and their hydroxylated and ketone intermediates into terminal perfluoro(di)carboxylates of varying chain lengths. This study provides the first mechanistic framework linking the PoFAS structure to ^•OH-mediated transformation kinetics and pathways, enabling predictions of environmental persistence, vulnerable oxidation sites, intermediates, and terminal products for novel PoFAS.