Mechanistic Understanding of Superoxide Radical-Mediated Degradation of Perfluorocarboxylic Acids

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.