The goal of this project was to develop a combined photo/electrochemical reduction process capable of degrading recalcitrant per- and polyfluoroalkyl substances (PFAS), as well as other co-occurring chemicals found in investigation-derived waste generated during the study of groundwater. This process relies on electron transfer (ET) between negatively charged hydrophobic cathodes and sorbed hydrophobic chemicals, 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 chemical lowers the activation energy of the carbon-fluorine bond, dramatically increasing the reaction rate. This process is an innovative modification of typical PFAS reductive defluorination methods, dramatically increasing PFAS degradation rates up to 30 times in a non-optimized system when compared to the control. This, in turn, will lead to lower overall energy consumption and shorter hydraulic retention times.

The researchers examined how potential-induced ET processes can dramatically increase the defluorination rate of perfluorooctane sulfonate (PFOS) molecules by ultra violet (UV)-generated hydrated electron in an additive-free system. Importantly, this ET reaction is performed at potentials below water electrolysis, which dramatically reduces energy consumption in the system. The defluorination reactions were explored under different electrode surface chemistries, aqueous constituents, and applied potentials. The degradation products and defluorination rates were characterized by liquid chromatography – mass spectroscopy, ion selective electrodes, and ion chromatography. In addition, density functional theory was used to explore the impact of electron addition to the PFOS molecule in terms of bond lengths and the C-F bond activation energy.

The results demonstrated how the specific adsorption of PFOS onto electrodes with tailored chemical properties can facilitate ET between the electrode and the adsorbed PFOS, which enables UV-generated hydrated electrons to rapidly defluorinate PFOS. To the best of the project team's knowledge, this is the first study to demonstrate the coupling of electrochemically induced ET to photo-assisted degradation reactions, which dramatically increases the degradation rate of aqueous recalcitrant organic chemicals. The project team also extended this study to investigate the effect of the head group (carboxyl versus sulfonate) and alkyl chain length on the extent of PFAS degradation, and on other co-occurring chemicals including chlorinated solvents such as trichloroethene and cis-dichloroethene, which are found in groundwater along with PFAS.

The specific benefits of this research include:

1. the development of a fundamental understanding of the ET activation process;
2. the determination that ET activation and subsequent reduction via hydrated electrons is an effective degradation pathway for PFAS and other co-occurring chemicals typically found in groundwater; and
3. the prioritization of operational parameters including 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.

(Project Completion - 2020)

Rao, U., Y. Su, C.M. Khor, B. Jung, S. Ma, D.M. Cwiertny, B.M. Wong, and D. Jassby. 2020. Structural Dependence of Reductive Defluorination of Linear PFAS Compounds in a UV/Electrochemical System. Environmental Science & Technology, 54(17):10668-10677. doi.org/10.1021/acs.est.0c02773.

Su, Y., U. Rao, C.M. Khor, M.G. Jensen, L.M. Teesch, B.M. Wong, D.M. Cwiertny, and D. Jassby. 2019. Potential-Driven Electron Transfer Lowers the Dissociation Energy of the C–F Bond and Facilitates Reductive Defluorination of Perfluorooctane Sulfonate (PFOS). ACS Applied Materials & Interfaces, 11(37):33913-33922. doi.org/10.1021/acsami.9b10449.