Ultra-short chain fluorocarboxylates exhibit wide ranging reactivity with hydrated electrons

Abstract

Recent reports demonstrate that technologies generating hydrated electrons (eaq−; e.g., UV-sulfite) are a promising strategy for destruction of per- and polyfluoroalkyl substances, but fundamental rate constants are lacking. This work examines the kinetics and mechanisms of eaq− reactions with ultra-short chain (C2–C4) fluorocarboxylates using experimental and theoretical approaches. Laser flash photolysis (LFP) was used to measure bimolecular rate constants (k2; M−1 s−1) for eaq− reactions with thirteen per-, and for the first time, polyfluorinated carboxylate structures. The measured k2 values varied widely from 5.26 × 106 to 1.30 × 108 M−1s−1, a large range considering the minor structural changes among the target compounds. Molecular descriptors calculated using density functional theory did not reveal correlation between k2 values and individual descriptors when considering the whole dataset, however, semiquantitative correlation manifests when grouping by similar possible initial reduction event such as electron attachment at the α-carbon versus β- or γ-carbons along the backbone. From this, it is postulated that fluorocarboxylate reduction by eaq− occurs via divergent mechanisms with the possibility of non-degradative pathways being prominent. These mechanistic insights provide rationale for contradictory trends between LFP-derived k2 values and apparent degradation rates recently reported in UV-sulfite constant irradiation treatment experiments.