The overarching goal of this project was to develop an integrated treatment system for the effective treatment of soils impacted with per- and polyfluoroalkyl substances (PFAS) in source zones. Specifically, this project aimed to test the treatment strategy that was based on the “Release-Capture-Destruction” concept. The technical objectives were to determine factors affecting biodegradation of soil-bound precursors, to accelerate biodegradation of precursors in soil via bioaugmentation, to determine the efficiency of PFAS removal by magnetic activated carbon (MAC) in soil, and to assess whether the hydrothermal alkaline treatment (HALT-PFAS) technology can destroy PFAS in the spent MAC.

Four tasks were performed to address the project technical objectives to test associated hypotheses. These four tasks were to (i) determine factors affecting biodegradation of soil-bound precursors; (ii) accelerate biodegradation of precursors via bioaugmentation; (iii) determine the efficiency of PFAS removal by MAC in PFAS-impacted soils, and (iv) assess if HALT-PFAS technology can destroy PFAS in spent MAC.

Capstone 1157 is a mixture of n:2 fluorotelomer sulfonamidoalkyl betaine (FtSaB) (n= 6, 8, 12), n:2 fluorotelomer sulfonamido amine (n= 6, 8), 6:2 fluorotelomer sulfonate (FtS), and 6:2 fluorotelomer alcohols (FTOH). Rhodococcus jostii strain RHA1 (RHA1) degraded 6:2 FtS in Capstone only under sulfur-limited conditions. The 6:2 FtS defluorination ability of RHA1 was inhibited by four different hydrocarbon surfactants, but promoted by three other hydrocarbon surfactants. Soil cultures enriched with alcohols (ethanol,1- propanol, and 1-butanol), alkane (hexane and octane), aromatics (phenol), and cocamidopropyl betaine showed the ability to degrade different precursors: FTOH, 6:2 FtS, and 6:2 FtSaB. Potential suspects were found in 6:2 FtS and 6:2 FtSaB microcosms containing enrichment cultures in S-limited media. MAC effectively removed PFAS from PFAS-spiked soils and aqueous film-forming foam (AFFF)-impacted soils, but the removal efficiencies were affected by high organic contents in soils. Spent MAC was quickly separated from treated soils. Application of HALT (350^oC, 1 M NaOH) led to near-complete destruction of perfluorooctane sulfonate (PFOS) adsorbed onto MAC and stoichiometric generation of fluoride ion without detection of any fluoro-organic intermediates. Removal of PFOS was significantly lower for the AFFF-impacted soil-derived MAC sample (81% as opposed to >97% in the PFOS-loaded MAC) at the same treatment residence time and alkali dose.

Results of this study filled the knowledge gaps on the poorly understood effects of co-occurring hydrocarbon surfactants on the fate and biotransformation of PFAS precursors, and the factors controlling biodegradation of the soil-bound precursors into more mobile perfluoroalkyl acids (PFAA) for subsequent treatments (i.e., capture and destruction technologies). The new knowledge on promoting biodegradation of precursors can lead to the success of the proposed treatment train of PFAS in soils and provides a foundation for the future development of precursor bioremediation. The successful application of MAC to sorb mobile PFAA produced from precursor biodegradation, followed by using hydrothermal liquefaction for the destruction of PFAS-laden MAC, provided strong proof for the proposed “Release-Capture-Destruction” remediation strategy for PFAS-laden soils which are present in numerous Department of Defense sites. (Project Completion - 2023)