Nanosecond Transient Absorption of Hydrated Electrons and Reduction of Linear Perfluoroalkyl Acids and Sulfonates

Abstract

We report the rate constants (k q ) and activation energies (E a ) associated with the initial reduction of linear perfluoroalkyl carboxylates (PFxA, x = the number of carbons present in the surfactant) and perfluoroalkyl sulfonates (PFxS) by the hydrated electron (e aq − ) as measured using temperature-dependent transient absorption spectroscopy. The reduction of PFxA and PFxS by e aq − displays a negligible dependence on the number of carbons present in the surfactant and occurs with an apparent k q on the order of 10 8 to 10 9 M −1 s −1 with the k q corresponding to PFxA reduction being on average two times smaller than those for PFxS. There are no significant differences in the E a associated with PFxA or PFxS reduction; the magnitude of E a (∼11 kJ mol −1 ) is consistent with the reduction of PFxA and PFxS being a diffusion controlled process. These results are discussed within the framework of recent theoretical and experimental work on the degradation of PFxA and PFxS.

Reactivities of hydrated electrons with organic compounds in aqueous-phase advanced reduction processes

“…The predicted k chem values of perfluorobutanoic acid (3 carbon chains, 6.87 × 10 7 M −1 s −1 ), perfluorohexanoic acid (5 carbon chains, 6.66 × 10 8 M −1 s −1 ), perfluorooctanoic acid (7 carbon chains, 5.78 × 10 8 M −1 s −1 ) and perfluorononanoic acid (8 carbon chains, 7.96 × 10 8 M −1 s −1 ) for the associative mechanism with CO were in excellent agreement with the recently reported k exp values of (5.4 ± 1.2) × 10 8 M −1 s −1 for perfluorobutanoic acid, (5.4 ± 0.1) × 10 8 M −1 s −1 for perfluorohexanoic acid, (7.1 ± 0.6) × 10 8 M −1 s −1 for perfluorooctanoic acid, and (6.4 ± 0.4) × 10 8 M −1 s −1 for perfluorononanoic acid. 162 Although this experimental study did not determine the mechanism for those measured rate constants, we believe they measured the rates of the associative mechanism. In contrast, the k exp values for C–F cleavage (10 6 –10 7 M −1 s −1 ) were previously reported 134 and used for the determination of our LFERs (compound no.…”

Charge carrier dynamics and reaction intermediates in heterogeneous photocatalysis by time-resolved spectroscopies

“…157 Also, recently Maza et al applied temperature-dependent TAS to deduce rate constants and activation energies associated with the initial reduction of linear perfluoroalkyl carboxylates and perfluoroalkyl sulfonates by hydrated electrons. 158 Therefore, we note that this method may similarly be applied to deduce rate constants and activation energies associated with heterogeneous photocatalysis. On the other hand, electrochemical techniques are coupled with TRIR to validate the occurrence of ET in the photocatalytic processes.…”

Degradation of Per- and Polyfluoroalkyl Substances with Hydrated Electrons: A New Mechanism from First-Principles Calculations

“…Finally, to probe the energetics of these various degradation mechanisms, we carried out well-tempered metadynamics simulations to calculate free energy activation barriers for C- bond dissociation is 3 times larger for solvated PFOS than PFOA, resulting in a slower defluorination of PFOS. However, the free energy activation barrier for both PFOS and PFOA falls below the diffusion-controlled limit, 12,29 and, therefore, the degradation process essentially becomes independent of the overall chemical rate constant. 64,65 The relatively low free energy activation barriers obtained from our AIMD calculations provide atomistic details in this process to rationalize these previous experimental observations.…”

“…Regardless of the specific experimental approach, the use of hydrated electrons has been shown to effectively reduce both PFOA and PFOS when used in sufficiently large quantities and in a conducive environment (i.e., correct pH, solute concentration, and temperature). 16,18,19,[22][23][24][25][26][27][28][29][30] Despite their effectiveness, a detailed mechanistic and atomistic understanding of how these hydrated electrons accelerate PFAS degradation remains unclear. Of the numerous experimental studies on PFAS degradation with hydrated electrons, most have either lacked the spectroscopic resolution to resolve these ultrafast dynamics or used fairly crude theoretical approximations.…”

“…However, there have been a few studies in the scientific literature that have explored time-resolved PFAS degradation kinetics with either hydroxyl radicals 33 or hydrated electrons. 12,29 In particular, pulse radiolysis experiments by Szajdzinska-Pietek have shown that PFOA is largely unreactive with hydroxyl radicals, 33 resulting in biomolecular rate constants far lower than ∼ 10 7 M −1 s −1 . Huang and co-workers explored photochemical reductive degradation (which produces hydrated electrons) of short-and long-chain PFASs 12 and showed that the rate constant increases with chain length.…”