Formatted Title
Enhanced Anaerobic Biodegradation of Per- and Polyfluoroalkyl Substances (PFAS) in Semi-Arid Region Soils in Areas of Aqueous Film-Forming Foam Release
Background/Objectives
Variable formulations of aqueous film-forming foam (AFFF) have contained large amounts of perfluoroalkyl acids (PFAAs), such as perfluorooctane sulfonate (PFOS), and polyfluoroalkyl substances, or precursors, such as 6:2 fluorotelomer sulfonate that can transform to more persistent PFAAs. Understanding both PFAA and precursor (PFAS) fate is critical for site characterization and remediation, including development of in situ remediation technologies. Remediation approaches are lacking for semi-arid or arid regions with thick vadose zones, where PFAS contamination can extend to depths greater than practical for excavation and act as a continuous source to groundwater plumes. Along with field characterization at PFAS source areas, we conducted anaerobic laboratory experiments with soil from a fire-training area at Cannon Air Force Base (CAFB), New Mexico, to evaluate biodegradation of PFAS with biostimulation or bioaugmentation with an anaerobic dehalogenating culture, WBC-2. This study builds on a previous investigation with soil from another site that showed biodegradation of PFOS with WBC-2 culture addition.
Approach/Activities
Soil cores were collected at five potential PFAS source areas to maximum depths of about 6.5 meters and sub-sectioned for analysis of 28 targeted PFAS, total oxidizable precursors (TOP), microbial communities, and other biogeochemical constituents. The soil used for biodegradation experiments was from a fire-training area that showed some of the highest concentrations of targeted PFAS and precursors identified by the TOP assay at CAFB (total over 5,000 nanograms per gram). In an anaerobic chamber, homogenized soil was mixed with simulated water, adjusted to pH 8.0 (typical for study area), and pre-incubated with lactate for 4 days prior to day 0 sampling for the experiment. WBC-2 was added to bioaugmented treatments at the start of pre-incubation and compared to non-bioaugmented treatments that contained added lactate only. Killed controls and deionized water controls were also prepared. All treatments were prepared in triplicate for sacrificial sampling at each time point. Microcosm soil samples were analyzed for targeted PFAS, TOP, and microbial communities; microcosm water samples were analyzed for targeted PFAS, TOP assay, dissolved organic carbon, anions, and redox constituents.
Results/Lessons Learned
Native anaerobic microorganisms were present in the site soil as evidenced by the rapid onset of anaerobic conditions in all live treatments. In live treatments with only the native microbes, sulfide production occurred immediately, while methane production occurred after 14 days. High methane and sulfide concentrations occurred immediately in microcosm treatments with WBC-2 culture. Total mass (water plus soil) of PFOS and other PFAS initially increased during incubation, and the mass increase was greatest in treatments with the added culture. Precursor transformation was the apparent cause of this initial increase in PFOS and other target PFAS. After 40 days of incubation, the mass of PFOS and a wide range of other PFAS decreased substantially in the bioaugmented treatments compared to the killed controls or the live treatments with only the native microbes. The delay in PFOS degradation observed in the experiments with the CAFB soil compared to our prior study, where PFOS degradation began within the first 20 days, is likely due to the high concentrations of unknown precursors in the CAFB soil. Reduction of sulfur moieties in the precursors could partly account for the initial sulfide production observed. The results of this study show that anaerobic biotransformation of precursors can be an important fate process in semi-arid region soils and indicate the potential for enhancement of anaerobic biodegradation as part of remediation actions. Shifts in the microbial community in the non-bioaugmented and bioaugmented treatments will be presented to help elucidate PFAS biodegradation processes.