Electrochemical oxidation (EO) is among the few remediation technologies that have been proven effective for destructive per- and polyfluoroalkyl substances (PFAS) treatment and that can be conceivably implemented in situ. The adaptability, small footprint, relatively low power requirements, and modular nature of EO water treatment enables highly flexible reactor design and operation. Under ESTCP project ER21-5045, the Horizontal Treatment Well (HRX Well^®) technology has been adapted to incorporate in situ reactors (in addition to or as a substitution for sorptive treatment media) to destroy PFAS and other chemicals. The goal of this project is to build on this prior work and demonstrate in situ destructive treatment via EO utilizing an existing HRX Well that is currently treating PFAS (using granular activated carbon). Specific technical objectives of this project include the following. 

• Develop and test an EO reactor system that can be installed in an HRX Well.
• Measure the effectiveness of PFAS treatment by an EO module placed within a full-scale HRX Well.
• Assess the energy demand of the EO-HRX Well and benefits over conventional approaches.
• Assess the ease of reactor installation, and operation and maintenance (O&M).
• Compare the lifecycle costs of the EO-HRX Well to other viable in situ and ex situ approaches for PFAS.

EO is an emerging technology for the treatment of persistent impacted groundwater such as PFAS, chlorinated volatile organic compounds, and 1,4-dioxane. A direct current is applied to dimensionally stable electrodes, driving anodic oxidation reactions either through reactive oxygen species, such as hydroxyl radicals, or via direct electron transfer. The efficiency of EO largely depends on site-specific water quality (e.g., electrical conductivity, precipitable species), electrode material, and reactor design. EO has been recently demonstrated with Magnéli phase Ti₄O₇ anodes as an efficient and reliable treatment technology for PFAS destruction in two ongoing SERDP projects (ER-2717 and ER18-1320), and other researchers have reported corroborative results. EO with Ti₄O₇ anodes has been shown to degrade PFAS into benign products via direct electron transfer at the anode surface in concert with indirect oxidation by hydroxyl radicals and other reactive species that are also produced on the anode. The electric energy per order of degradation for perfluorooctane sulfonate in water by Ti₄O₇-based EO was reported in the low single digit (kWh/m3), orders of magnitude lower than those reported for the other few PFAS destruction processes. The superior performance of Ti₄O₇ anode in degrading PFAS is partly owing to its porous structure, high oxygen evolution potential, and catalytic activity toward fluorinated compounds. In addition, Ti₄O₇ materials are durable, robust, and can be readily fabricated at large scale with low production cost.

EO offers several advantages over other treatment technologies, including, straightforward adjustment to variations in the water composition and flow rate, no need for chemical addition, modular and flexible reactor design, and compatibility with other treatment technologies. Based on recent advances, it is anticipated that small modular EO reactors can be designed and constructed that are appropriate for deployment in situ within an HRX Well. The HRX Well provides precise control of the influent flow rate to the EO reactor and the length of the HRX Well makes the installation of multiple reactors (in series) possible, providing further control on residence time and treatment efficiency.

Results from this work will facilitate an improved capability to cost-effectively characterize, remediate, and manage soil and waters impacted by chemicals of concern. EO completely destroys PFAS; there is no lasting liability of eventual desorption from an in situ adsorbent strategy. Compared to groundwater extraction and ex situ treatment, HRX Well and EO represents cost savings in terms of adsorbent changeout, transport and disposal of spent adsorbents, and O&M of the groundwater extraction system. The approach also allows access under active surface infrastructure (e.g., flightlines) and features, and system above-ground footprint requirements are minimal. Successful completion of this effort will ultimately improve the cost effectiveness of PFAS treatment technologies, directly benefiting the warfighter and installation communities. (Anticipated Project Completion - 2027)