Per- and polyfluoroalkyl substances (PFAS) are the major target chemicals of concern of this study, with a focus on the most persistent perfluoroalkyl acids (PFAA). The occurrence of PFAS in the environment is widespread and has caused serious concerns. PFAS, in particular PFAAs, are toxic and extremely persistent, posing challenges to treatment technologies for their degradation. Recent studies indicated that PFAAs may be effectively degraded by electrooxidation using Magneli phase titanium sub-oxides (TSO) as anodes. Magneli phase TSOs are a series of substoichiometric titanium oxides.
This project is being conducted in two phases, with a proof-of-concept as Phase I. Phase I of this project proved the effectiveness of a novel reactive electrochemical membrane (REM) system in degrading and mineralizing PFAS (Phase I Report). The REM system involves an electric-conductive porous ceramic membrane made of TSO that is operated as a filtration device as well as a three-dimensional (3-D) anode. Built on the result of the Phase I study, a systematic study is planned in Phase II to provide a basis for design and optimization of the TSO-based REM systems for applications on DoD sites to remediate PFAS-impacted groundwater, including the following objectives:
- to further examine the performance of the REM systems on a wide range PFASs under various operation conditions;
- to modify the TSO materials used in the REM systems for improved performances and minimized reactivity towards chloride;
- to examine the feasibility of the REM systems using real water samples from DoD sites.
In Phase I, a proof-of-concept study was performed to verify the electrooxidation treatment efficiency of TSO anodes operated in both batch and REM reactors using a few PFAAs as model chemicals of concern. In Phase II, the study involves tasks that are designed to probe the specific hypothesis that a REM system involving TSO membrane can be optimized by improving its efficiency to degrade PFAS and minimizing its reactivity towards chloride via modification of the REM membrane materials and adopting effective operation scheme and conditions.
The first task involves a systematic assessment of REM performances using a wide variety of TSO materials on a wide spectrum of target PFAS. The REM performance will be tested with the major operation conditions and water conditions systematically varied. Formation of chlorate and perchlorate will also be evaluated in selected systems with chloride present in the samples.
The second task focuses on modification of the TSO materials by manipulating their porous structures and compositions. The goal is to further improve the REM efficiency towards PFAS degradation and minimize its reactivity towards chloride. Molecular simulations based on density functional theory (DFT) will be performed to guide the design of the materials and explore the interactions of PFAS and chloride on the anodes.
In the final task, real water samples from DoD sites impacted with PFAS will be collected and tested in bench scale experiments to assess the feasibility of REM applications on DoD sites. The experiments will be performed simulating REM treatments in a pump-and-treat scenario.
The Phase I study, for a few perfluroalkyl acids, has demonstrated the effectiveness of electrooxidation by TSO anode (e.g. Ti4O7) on degrading and mineralizing PFAAs. Degradation of all studied PFAAs was observed, with near stoichiometric formation of F-, and increased efficiency with increasing applied current density. Electrooxidation (EO) efficiency was greater in the REM system than in a batch reactor system. This was because the substrate interphase mass transfer rate was increased via convection facilitated dispersion, and more anode surface was made available for reaction with the solution filtered through the REM. The results suggested that the EO performance in degrading PFAAs was dependent on the composition of the anode material and their pore structure. Our study also indicated that chlorate and perchlorate were formed during EO when chloride was present in solution, but their formation was much slower on the Ti4O7 anode than on the boron-doped diamond (BDD) anode. This is because the oxidation of Cl− due to direct electron transfer (DET) on Ti4O7 anode did not occur as opposed to BDD.
The comprehensive data to be collected in this study will provide a basis for design and optimization of TSO-based REM applications for on-site implementation. New TSO materials will be developed with improved performance for PFAS degradation and reduced activity towards chloride, which will enable a wider range of REM operation conditions for effective PFAS removal without excessive formation of chlorinated byproducts. The experiments with real water samples from DoD sites will allow for feasibility assessment and help with further refinement of the process design. Overall, the study will help to prepare the TSO-based REM technology ready for on-site applications.
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