Formatted Title

Background/Objectives

The environmental fate of electrochemical fluorination (ECF)-based per- and polyfluoroalkyl substances (PFAS) emanating from aqueous film-forming foams (AFFFs) remains largely unknown. The lack of prior studies investigating the biotransformation of AFFF-derived ECF-based perfluoroalkyl acids (PFAAs) precursors limits our ability to effectively manage impacted sites and design remediation strategies. In addition, given the competitive or synergistic transformation of AFFF components (both PFAS and non-PFAS), previously reported transformation kinetics of single component AFFF-derived PFAA precursors may differ from what would be anticipated within a mixture at an AFFF-impacted site. To address these knowledge gaps and improve our understanding of precursor fate at AFFF-impacted sites, this study investigated the biotransformation potential of a variety of ECF-based precursors from a historically used AFFF formulation in microcosms prepared with aerobic AFFF-impacted soil.

Approach/Activities

Microcosms were prepared with soil collected from an AFFF-impacted site, the former Loring Air Force Base (Maine, USA), in serum bottles using a 1:10 ratio (wt./vol.) with synthetic groundwater. Three treatments included: (1) live treatment with 30 μL of AFFF solution (0.1% v/v) spiked; (2) abiotic control with 30 μL of AFFF dilute solution spiked along with 1 g/L sodium azide to inhibit microbial activity; and (3) live control with only 1.2 mM diethylene glycol butyl ether (DGBE) spiked (same amount of DGBE added in live treatment and abiotic control but without AFFF). DGBE is an organic solvent in AFFF formulations that has been demonstrated to serve as an electron donor and carbon source for microbial growth. Each bottle was sealed and maintained under aerobic conditions with a syringe needle pierced through the septa and connected with a 0.22 μm sterile polyethersulfone (PES) syringe filter. All microcosms were incubated at room temperature on a horizontal shaker at 150 rpm for 308 days. The molar yields of known biotransformation products were determined using liquid chromatography tandem mass spectrometry (LC-MS/MS). High-resolution mass spectrometry (HRMS) was employed to identify potential unknown transformation products. Amplification and sequencing of microcosm DNA samples evaluated microbial community changes following AFFF exposure. 

Results/Lessons Learned

Fifteen classes of PFAS, including AFFF components and transformation products, were identified or tentatively identified by suspect screening/non-targeted analysis (SSA/NTA) throughout the 308-day incubation. Nine classes were categorized as AFFF components, eight as transformation products, and two as both AFFF components and products. Transformation (abiotic and/or biotic) pathways involving these PFAS classes were delineated. The results demonstrate that AFFF-derived ECF-based precursors may serve as sources of perfluoroalkane sulfonamide (FASA) and PFAAs, which are commonly detected at AFFF-impacted sites. Further, potentially overlooked precursors and/or persistent transformation intermediates were identified. Overall, the study reveals that the environmental stability of ECF-based precursors is likely influenced by PFAS structural characteristics, i.e., perfluoroalkyl chain length, presence of sulfonamide or carboxamide groups, and structures on nitrogen atoms (e.g., a branched carboxyalkyl group). Although the microbial community composition shifted over time, the difference between the live treatment (AFFF amended) and the live control (no AFFF added) was not significant. Generally, the dominate taxa at phylum level in the live treatment and live control were similar, with notable variations observed at the genus level. For example, genus Phaeodactylibacter, belonging to phylum Bacteroidota, was significantly (p<0.05) more abundant in the live treatment than in the live control. These findings provide insights into the environmental fate and transformation pathways of AFFF-derived PFAS and other structurally similar ECF-based PFAS, which may guide the selection of scientifically sound strategies for long-term management and remediation of sites historically-impacted by AFFF.