Unravelling the structure-dependent defluorination mechanisms of per- and polyfluoroalkyl substances by hydrated electrons in UV/sulfite

Abstract

Hydrated electron ( $${\mathrm{e}}_{{\rm{aq}}}^{-}$$ )-based technologies are promising for resolving the global pollution of per- and polyfluoroalkyl substances (PFAS) by efficiently breaking C−F bonds. However, achieving complete defluorination has been challenging, and the fundamental mechanisms behind the stalled efficacy and varied reactivity remain obscure. Here we propose an electron transfer (ET)-limiting mechanism for PFAS degradation by $${\mathrm{e}}_{{\rm{aq}}}^{-}$$ through experimental and theoretical evaluations of 41 structures in UV/sulfite. The degradation rate constants spanned four orders of magnitude, with 34 PFAS achieving ~100% defluorination. We found that the defluorination occurs in a stepwise manner, with ET from $${\mathrm{e}}_{{\rm{aq}}}^{-}$$ to PFAS being rate limiting rather than subsequent C−F bond cleavage. This ET-limiting mechanism was verified by the effectiveness of the free energy of activation (2.33–27.4 kcal mol−1) calculated on the basis of the Marcus theory to predict the distinct reactivity of PFAS. Combined with spin-density analysis, this mechanism reveals that C=C, C−Cl, CF2COO− and (CF2)n≥6 promote the complete defluorination by favouring ET, whereas ET-disfavouring moieties, C−H, −O−, (CH2)n, SO3− and (CF2)n≤3, hinder the defluorination to varying extents. In particular, most PFAS were found to undergo initial attack of $${\mathrm{e}}_{{\rm{aq}}}^{-}$$ either at α-CF2 of CF2COO− or in the middle of (CF2)n≥6, resulting in two defluorination patterns with extensive or absent intermediates, respectively. The ET-limiting mechanism captures these different pathways that align with experimental results by governing the reactivity of potential intermediates. Our theoretical framework explains the $${\mathrm{e}}_{{\rm{aq}}}^{-}$$ -induced defluorination process and provides insights into designing rapid degradable substitutes to tackle the PFAS crisis. Complete defluorination of diverse per- and polyfluoroalkyl substances can be achieved in the UV/sulfite system. Further experimental and theoretical evaluation demonstrates that the rate-limiting step is electron transfer from hydrated electrons, rather than the subsequent C−F bond cleavage.