Why KOH is an Efficient Mechanochemical Defluorination Additive for PFOA: A Comprehensive Elucidation from Experiments, Characterizations, and Theoretical Calculations

The mechanochemical technique has emerged as a promising destructive method for degrading per- and polyfluoroalkyl substances (PFAS), particularly in solid waste matrices, and potassium hydroxide (KOH) is recognized as a highly effective additive. However, the molecular-scale mechanism that underlies its exceptional defluorination performance remains obscure, which limits the rational optimization of the process. Here we combine experiments with density functional theory (DFT) calculations to resolve the reaction pathway of KOH-assisted mechanochemical defluorination of perfluorooctanoic acid (PFOA), and identify a well-defined three-step sequential mechanism: solid-phase neutralization → homolytic decarboxylation → SN2 nucleophilic substitution defluorination. Mechanical force first drives the rapid neutralization of PFOA with KOH to form PFOA-K. The C–COO - bond of PFOA-K then undergoes homolytic cleavage, a pathway distinct from conventional ionic decarboxylation, generating a highly reactive •C 7 F 15 radical and a •COO - radical without CO 2 release; this step substantially weakens the C–F bonds of the resulting radical. Finally, OH - supplied by KOH acts as a potent nucleophile that preferentially attacks the α-C site (the most electron-deficient and least sterically hindered position) via an SN2 pathway, giving efficient F - elimination. The defluorination kinetic constant of KOH (1.29 h -1 ) far exceeds those of Na 2 CO 3 and Al 2 O 3 . Beyond providing a molecular-level understanding of PFOA degradation, the elucidated "neutralization–radical formation–nucleophilic attack" sequence offers a mechanistic basis for understanding and designing the mechanochemical destruction of other perfluorocarboxylic acids. These results highlight KOH-assisted mechanochemistry as an efficient destructive route for remediating PFAS-contaminated solid wastes such as soils and spent adsorbents.