Biomimetic degradation of perfluorinated acids by vitamin B12 with nano-zero-valent iron/nickel bimetal: effects of their self-structure and coexisting substances

Perfluorinated acids (PFAs) are a new class of persistent organic pollutants that are difficult to defluorinate or remove. The reductive degradation of various representative PFAs in a biomimetic system composed of vitamin B 12 (VB 12 ) as a catalyst and nano-zero-valent iron-nickel bimetal (nFe 0 /Ni 0 ) as a reductant was investigated in this study. The effects of the self-structures of PFAs and the coexisting substances in natural water were also discussed. The results indicated that the defluorination and removal rates of PFAs were highly dependent on the length and terminal functional groups of the perfluorocarbon chain. Only Perfluorocarboxylates with C > 11 and Perfluorosulfonates with C > 6 were significantly degraded. Based on the analysis of the degradation products of perfluorobutanesulfonate (PFBS), perfluorohexanesulfonate (PFHxS), prefluorooctanesulfonate (PFOS), and 2-perfluoroctyl ethanol (8:2 FTOH), hydrolysis followed by the scission of C–S or C–C connecting the terminal functional groups was the dominant degradation pathway of long-chain PFAs instead of cleavage of C–C in the perfluorocarbon chain. The perfluorocarbon chain length affects the product type. It is speculated that the high bond dissociation energies of C–F bonds in short-chain PFAs hinder the occurrence of the decarboxylation-hydroxylation-elimination-hydrolysis (DHEH) pathway and make the addition of (–CF 2 –) n dominant. Meanwhile, the inhibition of SO 4 2− removal by PFOS was significant, whereas humic acid, Cl − , and dissolved oxygen had only a slight influence. Overall, this study provides new insights on the degradation of PFAs containing multiple structures and highlights the impact of the self-structure on PFAs removal.