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 •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.