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

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journal contribution
posted on 29.01.2020, 21:43 by Michael J. Bentel, Yaochun Yu, Lihua Xu, Hyuna Kwon, Zhong Li, Bryan M. Wong, Yujie Men, Jinyong Liu
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.