Mechanistic Understanding of Superoxide Radical-Mediated Degradation of Perfluorocarboxylic Acids
2021
Bai, Lu | Jiang, Ying | Xia, Deming | Wei, Zongsu | Spinney, Richard | Dionysiou, Dionysios D. | Minakata, Daisuke | Xiao, Ruiyang | Xie, Hong-Bin | Chai, Liyuan
Perfluorocarboxylic acids (PFCAs) exhibit strong persistence in sunlit surface waters and in radical-based treatment processes, where superoxide radical (O₂•–) is an important and abundant reactive oxygen species. Given that the role of O₂•– during the transformation of PFCAs remains largely unknown, we investigated the kinetics and mechanisms of O₂•–-mediated PFCAs attenuation through complementary experimental and theoretical approaches. The aqueous-phase rate constants between O₂•– and C3–C8 PFCAs were measured using a newly designed in situ spectroscopic system. Mechanistically, bimolecular nucleophilic substitution (SN2) is most likely to be thermodynamically feasible, as indicated by density functional theory calculations at the CBS-QB3 level of theory. This pathway was then investigated by ab initio molecular dynamics simulation with free-energy samplings. As O₂•– approaches PFCA, the C–F bond at the alpha carbon is spontaneously stretched, leading to the bond cleavage. The solvation mechanism for O₂•–-mediated PFCA degradation was also elucidated. Our results indicated that although the less polar solvent enhanced the nucleophilicity of O₂•–, it also decreased the desolvation process of PFCAs, resulting in reduced kinetics. With these quantitative and mechanistic results, we achieved a defined picture of the O₂•–-initiated abatement of PFCAs in natural and engineered waters.
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