The research on this project contributes to the fundamental understanding of the factors controlling reductive defluorination of perfluoroalkyl acids (PFAAs) using a membrane catalyst-film reactor (MCfR) with elemental palladium (Pd⁰) and other precious metal catalysts. The cooperation of catalytic reductive defluorination and biodegradation achieved in the synergistic platform reveals a novel strategy for the treatment of persistent PFAAs. It also laid the foundation for developing a reliable and cost-effective synergistic platform for treating PFAAs. Four tasks were designed to demonstrate proof-of-concept of the novel synergistic platform and to explore strategies to optimize the catalytic-biological synergy. The specific tasks were: (1) reductive defluorination of perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) in the hydrogen based MCfR (H₂-MCfR); (2) oxidative defluorination and mineralization of partially fluorinated octanoic acid (OA)/octanesulfonic acid (OS) in the oxygen based membrane biofilm reactor (O₂-MBfR); (3) synergistic defluorination of PFOA and PFOS; and (4) cost analysis.

This project had three tasks of experimental work. In Task 1, the research team determined the optimal catalyst synthesis method and catalytic conditions that yielded fast PFOA/PFOS removal with least-fluorinated products. The intermediates and products of PFOA and PFOS reductive defluorination were determined to investigate the reaction mechanisms. In Task 2, the project team conducted continuous operation of the O₂-MBfR for partially fluorinated OA/OS for oxidative defluorination and mineralization using non-fluorinated counterpart as the primary substrate. The functional microbial community and genes were identified by analyzing the shallow-metagenomic sequencing results of biofilms. In Task 3, the project team operated a complete synergistic system with the two membrane-film reactors in series and successfully achieved PFOA and PFOS continuous removal. In Task 4, the project team estimated capital and annual operating costs of a 100-gpm system treating low or high concentrations of PFOA and PFOS.

Fast adsorption of PFOA and PFOS and the release of the fluoride ion and partially and fully defluorinated compounds verified that the H₂-MCfR catalytically removed and destroyed PFAS. Defluorination, preceded by PFOA adsorption in an orientation parallel to the Pd⁰ surface, enabled a fast reaction between fluorine substituents on PFOA and PFOS and activated hydrogen on the Pd⁰ surface. The addition of a promoter metal enabled palladium-based bimetallic catalysts to defluorinate PFOA and PFOS at neutral potential hydrogen. The MCfR was capable of sustained removal of PFOA at environmentally relevant concentrations, averaging 97% removal, to well below 70 nanograms/liter, for continuous flow for more than two months. The continuous oxidative biomineralization of partially defluorinated PFOA/PFOS in the O₂-MBfR proved the capability of MBfR biofilms for further biodegradation and mineralization of PFOA and PFOS-hydrodefluorination products from the H₂-MCfR. Continuous experiments with the synergistic platform proved that the H₂-MCfR and O₂-MBfR worked as expected when linked together in the synergistic platform; partially defluorinated products from the MCfR were further defluorinated in the membrane biofilm reactor (MBfR). The defluorinated ratio in H₂-MCfR affected the biodegradation in O₂-MBfR, with more hydrodefluorination in the MCfR allowing more oxidative biodefluorination in the MBfR.

This research contributes to a fundamental understanding of the factors controlling reductive defluorination of PFAAs using MCfR with Pd⁰ and other precious metal catalysts. The cooperation of catalytic reductive defluorination and biodegradation achieved in the synergistic platform reveals a novel strategy for the treatment of persistent PFAAs. It also lays the foundation for developing a reliable and cost-effective synergistic platform for treating PFAAs which is of intense interest to Department of Defense.

Long, M., J. Donoso, M. Bhati, W.C. Elias, K.N. Heck, Y.H. Luo, Y.S. Lai, H. Gu, T.P. Senftle, C, Zhou, M.S. Wong, and B.E. Rittmann. 2021. Adsorption and Reductive Defluorination of Perfluorooctanoic Acid over Palladium Nanoparticles. Environ. Sci. Technol, 55(21):14836-14843.

Long, M., W.C. Elias, K.N. Heck, Y.H. Luo, Y.S. Lai, Y. Jin, H. Gu, J. Donoso, T.P. Senftle, C. Zhou, M.S. Wong, and B.E. Rittmann. 2021. Hydrodefluorination of Perfluorooctanoic Acid in the H2-Based Membrane Catalyst-Film Reactor with Platinum Group Metal Nanoparticles: Pathways and Optimal Conditions. Environ. Sci. Technol, 55(24):16699-16707.

Zhou, C., Y. Luo, C. Zheng, M. Long, X. Long, Y. Bi, X. Zheng, D. Zhou, and B.E. Rittmann. 2021. H2-Based Membrane Catalyst-Film Reactor (H2-MCfR) Loaded with Palladium for Removing Oxidized Contaminants in Water. Environ. Sci. Technol, 55(10):7082-7093.