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 (O-2(center dot-)) is an important and abundant reactive oxygen species. Given that the role of O-2(center dot-) during the transformation of PFCAs remains largely unknown, we investigated the kinetics and mechanisms of O-2(center dot-)-mediated PFCAs attenuation through complementary experimental and theoretical approaches. The aqueous-phase rate constants between O-2(center dot-) and C3-C8 PFCAs were measured using a newly designed in situ spectroscopic system. Mechanistically, bimolecular nucleophilic substitution (S(N)2) 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 O-2(center dot-) approaches PFCA, the C-F bond at the alpha carbon is spontaneously stretched, leading to the bond cleavage. The solvation mechanism for O-2(center dot-)-mediated PFCA degradation was also elucidated. Our results indicated that although the less polar solvent enhanced the nucleophilicity of O-2(center dot-), 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 O-2(center dot-)-initiated abatement of PFCAs in natural and engineered waters.