6:2氟调磺酸在土壤中的有氧微生物转化

Aerobic biotransformation of 6:2 fluorotelomer sulfonic acid in soil

全(多)氟烷基化合物作为主要的活性成分被广泛地应用于水成膜泡沫灭火剂中.在消防演习中,大量的6:2氟调磺酸(6:2 FTSA)及其前体物被释放到土壤及地下水等环境中,然而关于6:2 FTSA在土壤中的归趋和转化的研究仍存在较大的空白.本研究利用室内微宇宙实验探究了6:2 FTSA在好氧土壤中的降解行为.实验进行到90 d时,6:2 FTSA在灭菌对照组和降解实验组中均具有较好的质量平衡结果,分别达到97.4%和76.8%. 6:2 FTSA在好氧土壤中的降解率较低, 90 d时仍有33%没有发生降解. 6:2 FTSA经一系列好氧降解转化,主要降解转化为短链的全氟戊烷羧酸(PFPeA)和全氟己烷羧酸(PFHxA),在另一次要的降解路径中则观察到5:3氟代饱和酸(5:3 acid)的生成.90 d时, PFPeA的生成产率占初始6:2 FTSA的摩尔百分比为20.4%, PFHxA为6.5%, 5:3 acid为9.5%,全氟丁烷羧酸(PFBA)为1.0%. 6:2 FTSA及其降解产物在土壤上的吸附能够降低其降解量.脱磺酸基步骤是6:2 FTSA在土壤中好氧降解的限速步骤.这些研究结果表明,进入到土壤环境中的6:2 FTSA可能是短链羧酸的一个潜在来源.

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)6 CH2 CH2 SO3 H]) 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)6 CH2 CH2 OH]) 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 d. 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 d, 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)5 CH(OH)CH3]) and 5:2 fluorotelomer ketone(5:2 FT ketone, [F(CF2)5 COCH3]) were the most abundant intermediate transformation products at the beginning, and reached highest at day 7 and 14, respectively.Perfluoropentanoic acid(PFPeA, [F(CF2)4 COOH]) and perfluorohexanoic acid(PFHxA, [F(CF2)5 COOH]) 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)5 CH2 CH2 COOH]) and perfluorobutanoic acid(PFBA, [F(CF2)3 COOH]) 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)6 COOH]) 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.