Biotransformation of Perfluorooctane Sulfonamide (FOSA), 8:2 Fluorotelomer Alcohol (8:2 FTOH), and 6:2 Fluorotelomer Sulfonate (6:2 FTS) in Aqueous Film-Forming Foam (AFFF)-Impacted Soils under Oxic and Anoxic Conditions

Abstract

The use of aqueous film-forming foams (AFFFs) for fire suppression over decades has resulted in contamination of numerous sites by per- and polyfluoroalkyl substances (PFAS). However, the environmental fate and transformation of the polyfluoroalkyl substances (referred to as “precursors”) in AFFF-impacted sites remain largely unknown. This study focused on investigating the biotransformation of three AFFF-derived precursors by native microbial communities under conditions representative of AFFF-impacted sites. Aerobic soil biotransformation of perfluorooctane sulfonamide (FOSA) was first examined. Half-lives of 203.0-335.1 days for FOSA, and the production of perfluorooctanoic sulfonate (PFOS) with molar yields of 21.6-29.5 mol% were observed in two studied soils. Then, biotransformation of 8:2 fluorotelomer alcohol (8:2 FTOH) was investigated under various redox conditions. The biotransformation was much slower under sulfate- and iron-reducing conditions with >60 mol% of initial 8:2 FTOH remained after ~400 days, compared to a half-life of 12.5-36.5 days under nitrate-reducing conditions. Perfluorooctanoic acid (PFOA) was only formed under nitrate-reducing conditions. Further, biotransformation of 6:2 fluorotelomer sulfonate (6:2 FTS) was studied in two AFFF-impacted soils. A half-life of 43.3 days was obtained for one soil, while >60 mol% of initial 6:2 FTS remained in the other soil after a 224-day incubation. A novel “fluorotelomer ketone to PFCA” pathway during 6:2 FTS biotransformation is proposed. These microcosm studies demonstrate that biotransformation of FOSA, 8:2 FTOH, and 6:2 FTS could be strongly influenced by some environmental factors (e.g., redox condition, microbial community), and highlight the need to characterize biogeochemical site properties to accurately assess the potential for precursors’ biotransformation at AFFF-impacted sites. To assess the potential effects of dynamic flow conditions in natural environments, biotransformation of 6:2 FTS was further examined in one-dimensional flow-through columns. By increasing hydraulic residence time from 4.1 to 6.3 days, 22-26% lower concentrations of 6:2 FTS were measured in column effluents. Flow interruptions (2-7 days) posed substantial promoting effects on 6:2 FTS biotransformation after flow resumption. Results revealed that flow conditions play important roles in the 6:2 FTS biotransformation in natural environment. The need to incorporate system conditions (e.g., hydraulic parameters) into experimental systems for PFAS biotransformation investigation is highlighted.