A Combined Photo/Electrochemical Reductive Pathway Towards Enhanced PFAS Degradation
David Jassby | University of California, Los Angeles
The overall goal of this project is the development of a combined photo/electrochemical reduction process capable of degrading recalcitrant per- and poly-fluoroalkyl substances (PFAS), as well as other co-contaminants found in investigation-derived waste generated during the study of contaminated groundwater. The reduction process relies on electron transfer (ET) between negatively charged hydrophobic cathodes and sorbed hydrophobic contaminants, where the applied electrical potential is lower than the water splitting potential, followed by a reaction with a hydrated electron. ET between the electrode and the sorbed contaminant lowers the activation energy of the carbon-fluorine bond, dramatically increasing the reaction rate. This approach represents an innovative modification of typical PFAS reductive defluorination methods, dramatically increasing (by 30X in a non-optimized system) PFAS degradation rates compared to the control. This, in turn, can potentially lead to lower overall energy consumption and shorter hydraulic retention times (HRT).
The research objective of this study is the development of a combined photo/electrochemical method that can reduce the activation energy needed to degrade recalcitrant organic contaminants, with a focus on the strong Carbon-Fluorine (C-F) bond. The research team will identify key operational conditions needed to efficiently facilitate ET and C-F bond activation, explore how activation of the C-F bond facilitates its reduction by hydrated electrons, and test this process in the presence of common aqueous constituents (e.g., ionic species and organic co-contaminants). This project is based on the hypothesis that contaminant molecules sorbed onto electrodes can be “activated” through ET, making the molecule more susceptible to reductive treatment. The rationale behind this project is that lowering the activation energy of recalcitrant chemical bonds in an energy efficient manner will lower the overall treatment cost through reductions in chemical use, treatment time, and energy consumption. Researchers intend to test the hypotheses by completing the following specific Tasks:
- Fabricate and test electrodes for their ability to sorb PFAS and other contaminants, facilitate ET with these molecules, and reduce the activation energy of the C-F bond.
- Determine the transformation rates and products of “activated” PFAS and other halogenated contaminants that are undergoing ET on the electrode surface while interacting with hydrated electrons.
- Investigate the impact of aqueous species, pH, and temperature on degradation rates.
- Model impact of ET on C-F bond strength and activation energy using density functional theory simulations.
The specific benefits of this research will include: 1) development of a fundamental understanding of the ET activation process; 2) determining whether ET activation and subsequent reduction via hydrated electrons is an effective degradation pathway for PFAS and other co-contaminants typically found in contaminated groundwater; and 3) prioritizing operational parameters, such as electrode surface properties, applied potentials, hydraulic residence times, temperatures, pH, and the impact of other aqueous constituents, that impact the performance of the photo/electrochemical PFAS degradation process. This information will guide this research approach for a complete proposal the team intends to submit to SERDP that will include the development and demonstration of a larger scale system for the ex-situ treatment of PFAS contaminated groundwater. (Anticipated Completion - March 2019)