The primary goal of this project is to explore the use of air bubbles to extract and remove per- and polyfluoroalkyl substances (PFAS) from impacted water and soil. PFAS exhibits both oleophobic/lipophobic (oil/lipid-repellent) and hydrophobic (water-repellent) properties, and thus tend to accumulate at the air-water interface. The interaction of PFAS with air bubbles during treatment is unrecognized and has not been studied in-depth thus far. This project is geared to address this important knowledge gap to provide mechanistic understanding of the air-water interface accumulation of PFAS and to develop a simple, but effective and cheap, bubbling-assisted treatment process to efficiently extract and remove PFAS from impacted waters and soils. This research addresses the following objectives: developing a cost-effective treatment approach for PFAS-impacted matrices and evaluating the impact of common environmental treatment complications (e.g., water quality parameters, co-occurring chemicals, etc.). The specific research objectives are to: (i) identify conditions to reduce surface tension (ST) and increase air-water partitioning of PFAS (with cationic modifiers) to improve its separation from impacted matrices using bubbles; and (ii) test the application of air-bubbling at optimal conditions in combination with conventional coagulants (e.g., ferric chloride and alum) for effective removal of PFAS from water and soil.

The governing hypotheses of this study are as follows: (A) introduction of nano- to micro-sized air bubbles in a water or soil column can effectively capture PFAS molecules and concentrate them at the air-water interface; and (B) extraction efficiency of PFAS from the water/soil column is dependent on the individual PFAS’s ST and the presence of cationic modifiers (e.g. cationic polymers, metal cross-linkers). There will be four tasks to achieve the objectives.

• Task 1 will investigate the accumulation of PFAS at the air-water interface with modification of solution chemistry (pH, ionic strength, cationic modifiers, and coagulants). Batch experiments will be performed to measure ST of PFAS to determine the enrichment of PFAS at the air-water interface as a function of solution chemistry.
• Task 2 will evaluate the effect of bubbling on the extraction of PFAS from lab-scale water and soil columns using ideal conditions identified in Task 1.
• Task 3 will determine the effect of air-bubbling post-coagulation/flocculation of impacted water.
• Task 4 will apply the optimized air-bubbling treatment approach to AFFF-impacted water and soil samples collected from impacted Department of Defense and Department of Energy sites.

Tasks 1 through 3 will be conducted for selected individual and mixtures of PFAS to study the impact of chain length and functional groups on the performance of the treatment approach. For Task 4, the project team will perform both targeted and nontargeted PFAS analysis and total precursor assay to assess the removal of novel PFAS and precursors using this approach.

This research will identify a simple solution to enhance the physical separation and removal of PFAS, including short-chain compounds, using bubbling-assisted treatment processes by taking advantage of the surfactant nature of PFAS. The approach involves the use of only air-bubbles and avoids the use of reactive chemicals, thus providing a cost-effective process to be implemented for remediation of PFAS-impacted matrices. An important advantage is that this approach can serve as a pre-concentration step to be coupled with other destructive treatment technologies (e.g. electrochemical oxidation and electron beam) to make the overall process of PFAS destruction more efficient. Results from this work will provide great cost-saving benefits for existing treatment facilities. For example, water utilities employing conventional coagulation-flocculation treatment are widespread in the U.S. and installation of a simple bubble diffuser in the coagulation tank is a cheaper alternative than installing a granular activated carbon column for PFAS treatment. Dissolved air flotation systems are currently unrecognized as a treatment approach for PFAS and the research will additionally inform on the feasibility of utilizing existing DAF systems for efficient removal of PFAS. Furthermore, this approach can easily be modified and engineered for remediation of AFFF-impacted soil, as suggested in this study, providing great flexibility in its application for treating other impacted matrices (e.g., sludge, landfill leachate etc.). (Anticipated Project Completion - 2024)