The objective of this project was to develop a simple, low-cost method to destroy per- and polyfluoroalkyl substances (PFAS). Currently, the dominant method for removing aqueous PFAS is sorption using granular activated carbon (GAC) or ion exchange resins. This method results in impacted GAC or resins which must undergo additional treatment to destroy the PFAS. The goals of this project were to use hydrodynamic cavitation (HC) as a novel and potentially less expensive alternative technology for thermal destruction of PFAS in water, and to conduct research to investigate its feasibility and scalability.

PFAS are exceptionally stable; therefore, destroying them requires extreme reaction conditions. The project team believes that hydrodynamic cavitation (HC) could generate the high temperatures and oxidation conditions needed to destroy PFAS in a process that is scalable and does not produce toxic byproducts. The research builds on prior literature that demonstrated PFAS destruction by ultrasonic cavitation. Researchers have shown how the high temperatures and pressures produced by collapsing cavitation bubbles enable complete breakdown of PFAS into nontoxic byproducts via pyrolytic removal of the ionic head group, pyrolytic breakage of the carbon chain, and oxidative breakage of the C-F bonds. The project team hypothesized that the same mechanisms for PFAS destruction occur in cavitation produced by HC. Prior literature shows that HC is a viable method for destruction of other persistent water-borne chemicals such as chloroform and dyes. In this project, the team conducted an experimental study to investigate the effects of HC reactor parameters on PFAS destruction. Laboratory testing of the byproducts produced and analysis of how the experimental results from the laboratory-scale testing would scale up to use in the field was performed.

The project team has shown that HC can degrade perfluorooctanesulfonic acid (PFOS) across a wide range of operating conditions. This proof-of-concept project successfully met the project objectives and demonstrated that hydrodynamic cavitation is a feasible technology for destruction of PFAS. Single-pass destruction efficiencies are currently up to 0.03%, and it is hoped to improve this efficiency by at least one or two orders of magnitude with further research and development. A majority of the PFOS is fully broken down to yield fluoride ions, though small concentrations of shorter-chain PFAS (sulfonic and carboxylic acids) are also present. Results of these tests indicate that it may be feasible to use HC to destroy PFAS on a scale that is relevant to commercial water treatment facilities. Lessons learned from these exploratory tests can be applied to improve future experiments.

This project has demonstrated that HC is a feasible technology for thermal destruction of PFAS and has deepened the understanding of the important reactor parameters. In the near term, a successful program will foster expanded research and development, allowing further experimental and computational study of the reaction conditions produced by HC and the resulting PFAS destruction pathways and kinetics. Further research and development would also allow exploration into other factors needed for successful scale-up of the technology. Long-term results of a successful program will be the deployment of HC facilities that support cost-effective and efficient DoD and municipal remediation of PFAS‑impacted sites. (Project Completion - 2023)