Per- and polyfluoroalkyl substances (PFAS) are relatively recalcitrant against chemical degradation; the combination of a destruction process (relatively slow and energy-intensive) with rapid separation may provide a cost-effective treatment train to remove, concentrate, and destroy PFAS in groundwater. This project aimed to develop an ex situ treatment system with PFAS adsorption by organoclay and defluorination by strongly reducing (e.g., hydrated electron) and oxidizing species (e.g., hydroxyl radical). Specific project objectives included: 

• assessing the performance of advanced oxidation processes as pre- and post-treatment; 
• developing a class of organoclay materials for rapid and efficient PFAS adsorption; 
• investigating the mechanisms of PFAS defluorination and improving the system performance by ultraviolet (UV) irradiation and common chemicals; and 
• coupling adsorbent regeneration with PFAS destruction for groundwater remediation.

A General Summary of the Mechanistic Insights into PFAS Degradation under UV/Sulfite Treatment: Confirmation of Two Pathways Using Three Legacy PFAS Structures (Perfluorocarboxylic Acids, Perfluoroalkane Sulfonic Acids, and Fluorotelomeric Carboxylic Acids) with Various Fluoroalkyl Chain Lengths

PFAS destruction experiments were conducted in photochemical reactors using a 254 nanometer UV lamp to irradiate the aqueous solution of PFAS and sulfite. PFAS degradation was evaluated by the rate and extent of fluoride ion release, parent compound degradation, and transformation product formation by the combined use of fluoride selective electrode, high-resolution mass spectrometry, and ion chromatography. Theoretical calculations were conducted to correlate the experimental observation with C−F bond dissociation energy and spontaneous bond dissociation. The abundance and lifetime of hydrated electrons under various conditions were measured by laser flash photolysis. The organoclay was synthesized from a series of clay materials and organic modifiers and tested with batch PFAS adsorption in various water matrices. The sorbent materials were further evaluated by regeneration, reuse, and multiple loading tests. 

For the PFAS destruction module, the degradation and defluorination of more than 65 individual PFAS under UV/sulfite and heat/persulfate treatment were investigated. Multiple transformation pathways and reactive species were identified. Hydroxyl radicals were effective in converting H-containing structures. Near complete defluorination was achieved by the integration of reductive and oxidative processes. The UV/sulfite system performance was substantially enhanced by pH adjustment and iodide addition. The final UV/sulfite+iodide at pH 12 enabled rapid, efficient, and deep defluorination of most PFAS. The optimized system realized deep defluorination of concentrated PFAS mixture in still bottom brines from ion exchange resin regeneration. For the separation module, a novel type of organoclays for fast, robust, and regenerable PFAS adsorption in various water chemistry was developed.

Findings from this project substantially advance the knowledge of PFAS adsorption and degradation as well as the general environmental chemistry of halogenated organics. The structure-reactivity relationships provide mechanistic insights for designing PFAS treatment strategies. The bench-scale results serve as proof-of-concept for near-complete adsorption and defluorination of various PFAS pollutants. The optimized photochemical system provides a unique homogeneous method to degrade concentrated PFAS pollutants in salt- and organic-rich water matrices. The system is being further evaluated under ESTCP project ER21-5152. Successful completion of this research effort will ultimately lead to more cost-effective PFAS treatment technologies, directly benefiting the warfighter and installation communities. (Project Completion - 2024)

Bentel, M.J., Y. Yu, L. Xu, Z. Li, B.M. Wong, Y. Men, and J. Liu. 2019. Defluorination of Per- and Polyfluoroalkyl Substances (PFAS) with Hydrated Electrons: Structural Dependence and Implications to PFAS Remediation and Management. Environmental Science and Technology, 53(7):3718-3728. doi.org/10.1021/acs.est.8b06648.

Bentel, M., Y. Yu, L. Xu, B. Wong, Y. Men, and J. Liu. 2020. Degradation of Perfluoroalkyl Ether Carboxylic Acids with Hydrated electrons: Structure-Reactivity Relationships and Environmental Implications. Environmental Science and Technology, 54(4):2489-2499. doi.org/10.1021/acs.est.9b05869.

Bentel, M., Z. Liu, Y. Yu, J. Gao, Y. Men, and J. Liu. 2020. Enhanced degradation of Perfluorocarboxylic Acids (PFCAs) by UV/Sulfite Treatment: Reaction Mechanisms and System Efficiencies at pH 12. Environmental Science and Technology Letters, 7(5):351-357. doi.org/10.1021/acs.estlett.0c00236.

Dong, Q., X. Min, J. Huo, and Y. Wang. 2021. Efficient Sorption of Perfluoroalkyl Acids by Ionic Liquid-Modified Natural Clay. Chemical Engineering Journal Advances, 7:100135. doi.org/10.1016/j.ceja.2021.100135.

Gao, J., Z. Liu, M. Bentel, Y. Yu, Y. Men, and J. Liu. 2021. Defluorination of Omega-Hydroperfluorocarboxylates (ω-HPFCAs): Distinct Reactivities from Perfluoro and Fluorotelomeric Carboxylates. Environmental Science and Technology, 55(20):14146-14155. doi.org/10.1021/acs.est.1c04429.

Liu, Z. M. Bentel, Y. Yu, C. Ren, J. Gao, V. Pulikkal, M. Sun, Y. Men, and J. Liu. 2021. Near-Quantitative Defluorination of Perfluorinated and Fluorotelomer Carboxylates and Sulfonates with Integrated Oxidation and Reduction. Environmental Science and Technology, 55(10):7052-7062. doi.org/10.1021/acs.est.1c00353.

Liu, Z., Z. Chen, J. Gao, Y. Yu, Y. Men, C. Gu, and J. Liu. 2022. Accelerated Degradation of Perfluorosulfonates (PFSAs) and Perfluorocarboxylates (PFCAs) by UV/Sulfite + Iodide: Reaction Mechanisms and System Efficiencies. Environmental Science and Technology, 56(6):3699-3709. doi.org/10.1021/acs.est.1c07608.