Aerobic biotransformation of 6:2 fluorotelomer sulfonic acid in soil

Abstract

Per- and polyfluoroalkyl substances (PFASs) have been widely used in numerous industrial and household products due to its hydrophobic and lipophobic nature. As the main active ingredient, PFASs have been widely used in aqueous film forming foam (AFFF) used to fight liquid-fueled fires. As the electrochemical fluorination (ECF)-based PFASs were gradually phased out by major PFAS manufacturers worldwide, fluorotelomer-based PFASs, such as 6:2 fluorotelomer sulfonic acid (6:2 FTSA, [F(CF2)6CH2CH2SO3H]) and its precursors, have been widely detected in soil and groundwater effected by firefighting training activities. A few precursors (e.g., 6:2 fluorotelomer alcohol, 6:2 FTOH, [F(CF2)6CH2CH2OH]) have been well studied, but there are still leaving major gaps in understanding of 6:2 FTSA fate and aerobic biotransformation in soil. Soil collected from forest area was dosed with 6:2 FTSA to investigate its biotransformation rate and to identify major transformation products in a flow-through soil incubation system over 90\u2005d. The overall mass balance at day 90 in sterile and live treatments averaged 97.4% and 76.8%, respectively, suggesting that there may be soil-bound residues and conjugates between fluorinated transformation products and dissolved soil components only in live test system not in the sterile soil. The 6:2 FTSA biotransformation was relatively low with a half-life more than 56\u2005d, and 33% of initial 6:2 FTSA remained at day 90. According to the transformation products and the results in 6:2 FTSA biodegradation, the pathways of 6:2 FTSA aerobic biodegradation in the soil were proposed. The volatile 5:2 secondary alcohol (5:2 sFTOH, [F(CF2)5CH(OH)CH3]) and 5:2 fluorotelomer ketone (5:2 FT ketone, [F(CF2)5COCH3]) were the most abundant intermediate transformation products at the beginning, and reached highest at day 7 and 14, respectively. Perfluoropentanoic acid (PFPeA, [F(CF2)4COOH]) and perfluorohexanoic acid (PFHxA, [F(CF2)5COOH]) were produced as end products by 6:2 FTSA through desulfonation, oxidation and other reactions. In another minor pathway, 5:3 polyfluorinated acid (5:3 acid, [F(CF2)5CH2CH2COOH]) and perfluorobutanoic acid (PFBA, [F(CF2)3COOH]) were also formed. The concentrations of these end products, including PFBA, PFPeA, PFHxA, and 5:3 acid, increased in all experiment period. At day 90, the generations of PFPeA, PFHxA, 5:3 acid, and PFBA at a yield of 20.4 mol%, 6.5 mol%, 9.5 mol%, and 1.0 mol% were observed. Perfluoroheptanoic acid (PFHpA, [F(CF2)6COOH]) was not detected in the soil, suggesting that there was no α-oxidation in the process of 6:2 FTSA biotransformation in live soil. NaOH-acetonitrile extraction plus ENVI-Carb™ clean up could breakdown the binding between 5:3 acid and organic matter of the soil, and an enhancement factor up to 18 could be achieved for 5:3 acid by this cleanup method. The sorption of 6:2 FTSA to soil could low the biotransformation. The enzymatic desulfonation step may limit 6:2 FTSA biotransformation in aerobic soil. This study indicates that 6:2 FTSA may be persistent in aerobic soil, and a potential source of short chain perfluorocarboxylic acids (PFCAs) in soil. More studies need to be done to understanding the fate of AFFF-related polyfluoroalkyl precursors in soil environment in the future work.