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

Abstract

Per- and polyfluoroalkyl substances (PFASs) are synthetic contaminants found in drinking groundwater sources and a wide variety of consumer products. Because of their adverse environmental and human health effects, remediation of these persistent compounds has attracted significant recent attention. To gain mechanistic insight into their remediation, we present the first ab initio study of PFAS degradation via hydrated electronsa configuration that has not been correctly considered in previous computational studies up to this point. To capture these complex dynamical effects, we harness ab initio molecular dynamics (AIMD) simulations to probe the reactivities of perfluorooctanoic (PFOA) and perfluorooctane sulfonic acid (PFOS) with hydrated electrons in explicit water. We complement our AIMD calculations with advanced metadynamics sampling techniques to compute free energy profiles and detailed statistical analyses of PFOA/PFOS dynamics. Although our calculations show that the activation barrier for C–F bond dissociation in PFOS is three times larger than that in PFOA, all the computed free energy barriers are still relatively low, resulting in a diffusion-limited process. We discuss our results in the context of recent studies on PFAS degradation with hydrated electrons to give insight into the most efficient remediation strategies for these contaminants. Most importantly, we show that the degradation of PFASs with hydrated electrons is markedly different from that with excess electrons/charges, a common (but largely incomplete) approach used in several earlier computational studies.

Transient Kinetics of Short-Chain Perfluoroalkyl Sulfonate with Radiolytic Reducing Species

“…The negligible chain length dependence on e aq – kinetics (10 8 –10 9 M –1 s –1 ) found by Maza et al has raised divergent defluorination mechanistic understanding. − Therefore, it is also unclear whether the difference in calculated C–F bond dissociation energy (BDE) necessarily determines the initial rate constant . However, Vyas et al have noted that the aqueous vertical attachment energy (VAE, energy released from instantaneous electron addition) of PFxS varied with chain length.…”

“…Moreover, the measured value is an order of magnitude higher than that determined by previous pulse radiolysis of PFOS (7.3 × 10 7 M –1 s –1 ) . Such a discrepancy has posed significant challenges in completing the degradation model and validating the theoretical predictions. − Priority of research with a distinct e aq – ̵ generation method should therefore be given to clarify the long-standing controversy.…”

Impact of Salinity and Temperature on Removal of PFAS Species from Water by Aeration in the Absence of Additional Surfactants: A Novel Application of Green Chemistry Using Adsorptive Bubble Fractionation

“…Recent advances in computational materials science using density functional theory and molecular dynamic may in time provide significant insights into these mechanisms and guide further process improvements. 21,22 In recent field trials in Australia at a military base contaminated from historical use of PFAS, removal using aeration has been demonstrated to achieve removal efficiency for PFOS, PFOA, and PFHxS of ≥99.8, ≥99.8, and 98.4%, respectively. 23 When multi-stage aeration was used in combination with an anionic exchange resin on the product water, all trace detectable PFAS species were removed.…”

“…It is also apparent that the mechanisms for surface adsorption and the effects of changes to the surrounding media, such as the presence of cations, anions, and temperature, are not well understood or are contested. Recent advances in computational materials science using density functional theory and molecular dynamic may in time provide significant insights into these mechanisms and guide further process improvements. , …”

Atomistic insights into the hydrodefluorination of PFAS using silylium catalysts

“…For reduction via hydrated electrons and photo-induced degradation, AIMD is important to account for intermolecular interactions that influence the resulting chemical degradation. 88,89 Therefore, selecting an appropriate computational method is crucial when investing PFAS remediation techniques.…”