Interpretation of Reductive PFAS Defluorination with Quantum Chemical Parameters

Advanced reduction using hydrated electrons (e(aq)(-)) is a promising technology for the degradation of per- and polyfluoroalkyl substances (PFAS). To better characterize and understand intrinsic factors that influence reductive PFAS defluorination, we probed the relationships between quantum chemical parameters and the reported overall defluorination ratio (deF%) of multiple PFAS structural categories with quantitative structure-activity relationship (QSAR) models. The Fukui index with respect to nucleophilic attack [f(+)], bond order (BOx), molecular size effect (E-B3LYP), partial charge on carbon atoms [q(C-)(n)], and energy of the lowest unoccupied molecular orbital (ELUMO) are identified as the influencing factors. The optimal QSAR model with both mechanistic and statistic meanings is log(10)(deF%) = -4.333 - 68.710E(LUMO) - 5.873f(+)(x), with the following evaluation indices: R-2 = 0.815, q(2) = 0.728, and Q(ext)(2) = 0.707. The f(+) distribution in PFAS and their degradation intermediates and the optimized structures of excited states support the reported decarboxylation and H/F exchange pathways. This study provides a rapid approach for estimating the overall deF% and gaining new insights into the mechanism of reductive defluorination.