Role of pH in the Transformation of Perfluoroalkyl Carboxylic Acids by Activated Persulfate: Implications from the Determination of Absolute Electron-Transfer Rates and Chemical Computations

Abstract

Perfluoroalkyl carboxylic acids (PFCAs) are ubiquitous contaminants known for their bioaccumulation, toxicological harm, and resistance to degradation. Remediating PFCAs in water is an ongoing challenge with existing technologies being insufficient or requiring additional disposal. An emergent approach is using activated persulfate, which degrades PFCAs through sequential scission of CF 2 equivalents yielding shorter-chain homologues, CO 2 and F − . This transformation is thought to be initiated by single electron transfer (SET) from the PFCA to the activate oxidant, SO 4•− . A pronounced pH effect has been observed for thermally activated persulfate PFCA transformation. To evaluate the role of pH during SET, we directly determined absolute rate constants for perfluorobutanoic acid and trifluoroacetic acid oxidation by SO 4•− in the pH range of 0.5−4.0 using laser flash photolysis. The average of the rate constants for both substrates across all pH values was 9 ± 2 × 10 3 M −1 s −1 (±2σ), implying that acid catalysis of thermal persulfate activation may be the primary culprit of the observed pH effect, instead of pH influencing the SET step. In addition, density functional theory was used to investigate if SO 4•− protonation might enhance PFCA transformation kinetics. We found that when calculations include explicit water molecules, direct SO 4•− protonation does not occur.

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

“…Regarding the stepwise investigation, we calculated with eqn (7), 26,32 which accounts for both the formation of the intermediate radical species and the resulting bond cleavage as an example of an RX bond. 26,32 where BDE is the bond dissociation energy of the cleaved bond, T is the absolute temperature in Kelvin, Δ S is the gaseous-phase entropy of the cleaved bond, ΔΔ G solv is the difference in solvation energy between the parent compound and the two radical products in eqn (8), and is the reduction potential of the cleaved aqueous atom.ΔΔ G solv = Δ G solv (R˙) + Δ G solv (X˙) − Δ G solv (RX)The BDE of the cleaved RX bond was calculated using the enthalpies ( H ) of the parent compound and the two radical products produced upon cleavage (eqn (9)). BDE = −[ H (RX) − H (R˙) − H (X˙)]…”

“…Several DFT-based methods were used to investigate the thermodynamics and kinetics of electron-induced reactions with halogenated compounds such as polychlorinated ethylenes, 21,22 polybrominated electrophiles, 23 and PFAS. 24–26 The dissociation and reductive cleavage of a given molecule were investigated based on the optimized electronic structures, bond dissociation energies and reduction potentials of the corresponding bond. The k exp values represent the overall reactivities, and thus, the elementary reaction mechanisms of the overall reaction cannot be known.…”

Mechanistic Understanding of Superoxide Radical-Mediated Degradation of Perfluorocarboxylic Acids

“…However, conventional water treatment technologies to remove these notorious contaminants are fraught with challenges. , For example, PFCAs cannot be biodegraded by activated sludge treatment processes to any appreciable extent, while advanced oxidation technologies (AOTs), an effective method for treating pharmaceutical and personal care products, also exhibit limited removal efficiency for PFCAs. In particular, a hydroxyl radical ( • OH) with a redox potential of E ° = 2.72 V versus a normal hydrogen electrode (NHE) at 20 °C is only slightly reactive toward certain PFCAs ( e.g., trifluoroacetic acid, CF 3 CO 2 – and perfluorooctanoic acid (PFOA), C 7 F 15 CO 2 – ) with low rate constants ( k , in the unit of M –1 s –1 ). , The reactivity of a sulfate radical (SO 4 •– , E ° = 2.5–3.1 V vs NHE at 20 °C) with PFCAs was reported to be in the range of 0.9 × 10 4 to 4.4 × 10 4 M –1 s –1 . , Thi et al revealed that only using UV 254 irradiation at 400 W, PFOA was degraded to 52.1% after 12 h of contact time, while simultaneously with UV 254 and carbonate radical (CO 3 •– ) treatment, PFOA was completely removed within the same time frame . The contact time of 12 h is not pragmatic for any realistic operation in wastewater treatment plants.…”

Role of Explicit Hydration in Predicting the Aqueous Standard Reduction Potential of Sulfate Radical Anion by DFT and Insight into the Influence of pH on the Reduction Potential

“…• in the presence of three and six water molecules. 28 In that study, H 3 O + was coordinated by two H 2 O molecules, whereas in the present study the three explicit H 2 O molecules were spatially separated from each other. Beyond the structures already described, there were no other isomers involving HSO 4…”