The core objective of this proof-of-concept project is assessment and initial development of in situ approaches for enhancing cometabolic transformation of polyfluoroalkyl substances and subsequent removal of the resulting perfluoroalkyl acids (PFAA) in PFAS-impacted soils and aquifers. As much as 97% of the PFAS mass in historical fire training areas are cationic and zwitterionic precursor compounds. The physicochemical characteristics of these compounds often result in strong sorption and low recovery during groundwater or soil extraction. Developing cost-effective techniques to convert these compounds to more extractable, treatable PFAAs, will result in improved treatment strategies for aqueous film-forming foam (AFFF) source areas.
This one-year project consists of two main tasks to achieve the core objective of improving the understanding of potential cometabolic transformation reactions of polyfluoroalkyl precursor compounds, and the extent to which these reactions lead to the formation of extractable PFAAs.
During Task 1, the project team will identify candidate cometabolic processes that enhance precursor transformation using impacted soils and/or aquifer sediments and assess enhanced extractability of PFAS after biotransformation. Two AFFF field sites known to have a high concentration of polyfluorinated precursors will be selected for study. The site solids will be amended with oxygen and substrates known to enhance aerobic cometabolic activity, such as methane and propane, among others. The conversion of precursors to PFAAs will be monitored over time versus anoxic controls, killed controls, and samples receiving oxygen only. This task will provide an indication of:
- precursors that are subject to cometabolic oxidation to PFAAs in site samples;
- collective transformation products of the precursors;
- cometabolic substrates that promote this activity; and
- the extent to which extractability of total PFAS is enhanced by the process (by comparing the soil [adsorbed] and aqueous total PFAS over time).
Based on the data, the project team will enrich cometabolic organisms from the site samples (e.g., methanotrophs if methane stimulates significant conversion of different precursors) and conduct studies with these cultures (enrichments and/or pure cultures) to assess their abilities to transform precursors to PFAAs under controlled conditions, including relevant kinetics and degradation products. Transformation of select precursor compounds by known cometabolic organisms also will be evaluated.
The project will provide an initial evaluation of the role of microbial cometabolic processes in the conversion of polyfluoroalkyl precursor compounds to PFAAs in AFFF-impacted environments. This will ultimately lead to a better understanding of these biological conversions, as well as potential field approaches to convert strongly sorbed precursors to more extractable, treatable PFAAs in AFFF source areas.