Formation of perfluorocarboxylic acids (PFCAs) during the exposure of earthworms to 6:2 fluorotelomer sulfonic acid (6:2 FTSA)

Abstract

6:2 fluorotelomer sulfonic acid (6:2 FTSA) is a novel perfluorooctane sulfonate (PFOS) alternative used globally in aqueous film forming foams (AFFFs). Although 6:2 FTSA has been recently detected in the environment, its fate in terrestrial invertebrates remains unclear. The uptake, elimination and biotransformation of 6:2 FTSA in earthworms (Eisenia fetida) were investigated after in vivo and in vitro exposure. 6:2 FTSA could be biodegraded by microorganisms in soil to trifluoroacetic acid (TFA), perfluoropropionic acid (PFPrA), perfluorobutanoic acid (PFBA), perfluoropentanoic acid (PFPeA) and perfluorohexanoic acid (PFHxA). The uptake rate constant (ku) and biota-to-soil accumulation factor (BSAF) of 6:2 FTSA in earthworms were 0.185 goc/gww/d and 0.685 goc/gww, respectively, indicating high bioaccumulative ability in earthworms. Five terminal perfluorocarboxylic acids (PFCAs) metabolites, including TFA, PFPrA, PFBA, PFPeA and PFHxA were observed in both in vivo and in vitro exposure tests, with TFA as the predominant metabolite. However, no perfluoroheptanoic acid (PFHpA) was observed in the present study. The elimination rate constants (ke) increased in the order: 6:2 FTSA (0.057/d) < TFA (0.058/d) < PFPrA (0.071/d) < PFBA (0.084/d) < PFHxA (0.182/d) < PFPeA (0.193/d). Biodegradation of 6:2 FTSA in the earthworm homogenates, cytolchrome P450 (CYP450) enzyme solutions and glutathione-s-transferase (GST) enzyme solutions fitted well with the first order kinetics. The biotransformation rate constants (k) were in the following order: homogenates (0.012/h) > CYP450 (0.009/h) > GST (0.007/h), implying that CYP450 and GST were involved in biotransformation of 6:2 FTSA in earthworms. This study provides important theoretical evidence for the fate of 6:2 FTSA in earthworms.