Degradation of Perfluoroalkyl Ether Carboxylic Acids with Hydrated Electrons: Structure–Reactivity Relationships and Environmental Implications

Abstract

This study explores structure–reactivity relationships for the degradation of emerging perfluoroalkyl ether carboxylic acid (PFECA) pollutants with ultraviolet-generated hydrated electrons (eaq –). The rate and extent of PFECA degradation depend on both the branching extent and the chain length of oxygen-segregated fluoroalkyl moieties. Kinetic measurements, theoretical calculations, and transformation product analyses provide a comprehensive understanding of the PFECA degradation mechanisms and pathways. In comparison to traditional full-carbon-chain perfluorocarboxylic acids, the distinct degradation behavior of PFECAs is attributed to their ether structures. The ether oxygen atoms increase the bond dissociation energy of the C–F bonds on the adjacent −CF2– moieties. This impact reduces the formation of H/F-exchanged polyfluorinated products that are recalcitrant to reductive defluorination. Instead, the cleavage of ether C–O bonds generates unstable perfluoroalcohols and thus promotes deep defluorination of short fluoroalkyl moieties. In comparison to linear PFECAs, branched PFECAs have a higher tendency of H/F exchange on the tertiary carbon and thus lower percentages of defluorination. These findings provide mechanistic insights for an improved design and efficient degradation of fluorochemicals.

Molecular Tuning of Redox‐Copolymers for Selective Electrochemical Remediation

“…While the molecular control of the redox‐copolymer allows for the selective separation of organic contaminants, the fate of the enriched pollutant waste after separation process must be addressed. [ 18 ] Here, we propose a unique approach to achieve tandem separation and degradation of the model compound PFOA within the same electrochemical unit, by leveraging an asymmetric electrochemical design that combines the P(TMA x ‐ co ‐TMPMA 1− x )‐functionalized CNT working electrode with a BDD counter electrode. While the redox‐copolymer acts as a selective adsorbent, the BDD counter‐electrode acts as a synergistic electrocatalyst during the regeneration step.…”

Understanding and Managing the Potential By‐Products of PFAS Destruction

“…This explains the long residence times to achieve mineralization, especially in complex solutions, because as the fluorinated alkyl chain shortens, the associated bond dissociation energy necessary to achieve defluorination increases while the potential for rapid scavenging reactions remains constant. The perfluoroalkyl ether compounds (e.g., “Gen X”) that have become more prevalent in recent years have higher bond dissociation energies adjacent to the ether linkages, making these molecules less likely to exchange fluorine for hydrogen in ARP‐mediated defluorination and form recalcitrant by‐products (Bentel et al ). By‐products that have been detected in the ARP treatment of perfluoroalkyl ethers are predominantly polyfluorinated ether compounds (Bentel et al ).…”

“…The perfluoroalkyl ether compounds (e.g., “Gen X”) that have become more prevalent in recent years have higher bond dissociation energies adjacent to the ether linkages, making these molecules less likely to exchange fluorine for hydrogen in ARP‐mediated defluorination and form recalcitrant by‐products (Bentel et al ). By‐products that have been detected in the ARP treatment of perfluoroalkyl ethers are predominantly polyfluorinated ether compounds (Bentel et al ).…”

Reductive Defluorination and Mechanochemical Decomposition of Per- and Polyfluoroalkyl Substances (PFASs): From Present Knowledge to Future Remediation Concepts

“…In contrast to oxidative processes, reduction involves the direct transformation of electrons from a reducing agent with a lower reduction potential compared to the substrate. Advanced reduction processes (ARPs), reductive metals (e.g., zero-valent iron (ZVI)), hydride radicals (H·) and most notably hydrated electrons (E 0 = −2.9 V) have been used to achieve the effective cleavage of C−F bonds in PFASs [ 66 , 67 , 68 , 69 , 70 , 71 , 72 , 73 ]. The investigation of PFAS reduction reactions have shown that they are sensitive towards the choice of reagents, their concentrations and reaction times as well as pH value of the solution [ 74 , 75 , 76 ], thus can show variable efficiencies.…”

“…This finding was assigned to an elevated OH − concentration in solution since the degradation of PFASs in the presence of pure NaOH was reported previously (see also Section 2.1 ). In a different report, various perfluoroalkyl ether carboxylic acids (PFECAs) were degraded at a pH of 9.5 [ 73 ]. All compounds were degraded but the defluorination rates were only between 30 and 98% after 48 h of treatment.…”