Defluorination of Per- and Polyfluoroalkyl Substances (PFASs) with Hydrated Electrons: Structural Dependence and Implications to PFAS Remediation and Management

TLDR

The study examines how structural features of 34 PFASs influence their defluorination by UV‑generated hydrated electrons. The authors used theoretical calculations of C–F bond dissociation energies and identified multiple reaction pathways—H/F exchange, terminal group dissociation, decarboxylation‑triggered HF elimination, and hydrolysis—to explain how structural factors govern PFAS defluorination. The study found that carboxylate PFASs decompose and defluorinate at comparable rates regardless of chain length, whereas telomeric and sulfonate PFASs show chain‑length dependence; most PFASs undergo incomplete defluorination except CF₃COO⁻, which is fully defluorinated, highlighting structural determinants that can guide remediation and fluorochemical design.

Abstract

This study investigates critical structure–reactivity relationships within 34 representative per- and polyfluoroalkyl substances (PFASs) undergoing defluorination with UV-generated hydrated electrons. While CnF2n+1–COO– with variable fluoroalkyl chain lengths (n = 2 to 10) exhibited a similar rate and extent of parent compound decay and defluorination, the reactions of telomeric CnF2n+1–CH2CH2–COO– and CnF2n+1–SO3– showed an apparent dependence on the length of the fluoroalkyl chain. Cross comparison of experimental results, including different rates of decay and defluorination of specific PFAS categories, the incomplete defluorination from most PFAS structures, and the surprising 100% defluorination from CF3COO–, leads to the elucidation of new mechanistic insights into PFAS degradation. Theoretical calculations on the C–F bond dissociation energies (BDEs) of all PFAS structures reveal strong relationships among (i) the rate and extent of decay and defluorination, (ii) head functional groups, (iii) fluoroalkyl chain length, and (iv) the position and number of C–F bonds with low BDEs. These relationships are further supported by the spontaneous cleavage of specific bonds during calculated geometry optimization of PFAS structures bearing one extra electron, and by the product analyses with high-resolution mass spectrometry. Multiple reaction pathways, including H/F exchange, dissociation of terminal functional groups, and decarboxylation-triggered HF elimination and hydrolysis, result in the formation of variable defluorination products. The selectivity and ease of C–F bond cleavage highly depends on molecular structures. These findings provide critical information for developing PFAS treatment processes and technologies to destruct a wide scope of PFAS pollutants and for designing fluorochemical formulations to avoid releasing recalcitrant PFASs into the environment.