Per- and polyfluoroalkyl substances (PFAS) are prevalent chemicals in the groundwater at many sites throughout the country. These compounds were contained in aqueous film forming foam (AFFF), which was used to suppress fires at hundreds of sites. PFAS pose a serious human health risk, and thus the U.S. Environmental Protection Agency has issued a drinking water health advisory level of 70 ng/L for six PFAS. As a result, there have been extensive investigations of impacted groundwater at Department of Defense (DoD) sites, which generates a large volume of waste containing PFAS, termed investigation derived waste (IDW).

PFAS removal from water is complicated by low volatility, general lack of reactivity to biodegradation and traditional oxidative treatment processes, and poor desorption kinetics from granular activated carbon (GAC) and other sorbents. Therefore, novel and effective remediation technologies are needed to treat PFAS-impacted IDWs, which would allow for on-site disposal options that would lower the overall cost of site management.

This work is being conducted in two phases. The overall objectives of the Phase I work was to utilize a cost-effective reactive electrochemical membrane (REM) for the treatment of PFAS in IDWs. Specific technical objectives associated with this work included:

1. Development of REMs for destructive PFAS removal in IDW water samples;
2. Determination of the optimal operational mode; and
3. Calculation of energy requirements for the REM-based system and comparison to other destructive treatment technologies.

Results from Phase I can be found in the Final Report and are summarized in the Interim Results section below.

In Phase II of this project, the specific technical objectives are as follows:

1. Development of an optimized REM reactor for PFAS removal;
2. Determine the impact of solution conditions and co-occurring chemicals on PFAS removal;
3. Development of an optimized mathematical model with predictive ability;
4. Determine the stability of REM reactors using longevity studies; and
5. Development of life cycle and cost analyses for comparison to other technologies.

The work performed during Phase I of this project consisted of REM synthesis, a series of bench-scale experimental studies, and a preliminary energy cost assessment. The six PFAS on the USEPA UCMR-3 list were investigated, which included perfluorooctanoic acid (PFOA), perfluorooctanesulfonic acid (PFOS), perfluorononanoic acid (PFNA), perfluorohexanesulfonic acid (PFHxS), perfluoroheptanoic acid (PFHpA), and perfluorobutanesulfonic acid (PFBS). A specific focus on treatment of PFOS and PFOA was made, which allowed comparison of results to the existing literature. Experimental parameters that were explored included: 1) adsorption capacity; 2) necessary residence time in the reactor; 3) reactor operational mode; and 4) energy usage (kWh m-3 per log removal of PFAS).

In Phase II of this project we will explore many practical issues of REM design, with the ultimate goal of developing a prototype REM reactor that is ready for pilot-scale testing of IDW solutions at the end of this project. Testing will be expanded to include various PFAS-impacted groundwater solutions and AFFF samples.

Phase I Results

In Phase I, the oxidation of PFOA and PFOS in synthetic solutions indicated that approximately 5-log removal of PFOA and PFOS was achieved in a single pass through the REM (hydraulic residence time ~ 11 s) at applied potentials of 3.3 V/SHE for PFOA and 3.6 V/SHE for PFOS. Permeate concentrations were < 86 ng L- for PFOA and 35 ng L-1 for PFOS, from initial concentrations of 4.14 and 5 mg L-1, respectively. Removal rates were as high as 3415 ± 203 µmol m-2 h-1 for PFOA and 2436 ± 106 µmol m-2 h-1 for PFOS at a permeate flux value of 720 L m-2 h-1 (residence time of ~ 3.8 s). The energy consumption (per log removal) to remove PFOA and PFOS to below the detection limits were 5.1 kWh m-3 and 6.7 kWh m-3, respectively. In addition, non-normalized observed reaction rate constants for this data were calculated as 607 h-1 for PFOA and 210 h-1 for PFOS. These values are more than two orders of magnitude higher than those reported by other destructive technologies (i.e, ultrasonication, photocatalysis, vacuum UV photolysis, microwave-hydrothermal decomposition).

Additional work involving real groundwater samples was also conducted. Results showed that single-pass flow-through mode could achieve only ~50% PFAS destruction, which was attributed to background organic constituents and low solution conductivity. However, operating in recycle-mode with high permeate fluxes achieved > 99% destruction of the total measured PFASs, with individual concentrations < 61 ng L-1. Recycle-mode required a total energy consumption of only 2.9 kWh m-3 per log removal, which was significantly lower than other destructive technologies.

Schematic Diagram of Proposed REM Treatment for PFAS-Impacted IDW Solutions

The high rate constants reported above are the highest reported for electrochemical oxidation of PFAS, and the low energy consumptions are much more favorable then other destructive treatment technologies (i.e., ultrasonication, photocatalysis, vacuum UV photolysis, microwave-hydrothermal decomposition), demonstrating the promise of the REM technology for PFAS treatment. In order to determine the full feasibility of this technology, further research and development is needed, which will be assessed during Phase II of the study. Completion of the Phase II research is needed before the REM technology can be confidently deployed to remediation sites.

However, expected benefits include 1) a better understanding of the use of electrochemical technologies for groundwater remediation; 2) the generation of proof of concept data that can be used to develop a field-scale prototype REM for the remediation of PFAS-impacted groundwater; and 3) an energy cost assessment for using the REM technology at impacted sites, which can be used by practitioners to assess the REM technology as a viable remediation option. (Anticipated Phase II Completion - 2024)

Khalid, Y., S.N. Misal, S. Mehraeen, and B.P. Chaplin. 2021. Reactive Transport Modeling of Electrochemical Oxidation of Perfluoroalkyl Substances in Porous Flow-through Electrodes. Environmental Science and Technology Engineering, 2(4):713-725. doi.org/10.1021/acsestengg.1c00362.

Le, T.X.H., H. Haflich, A.D. Shah, and B.P. Chaplin. 2019. Energy-Efficient Electrochemical Oxidation of Perfluoroalkyl Substances Using a Ti₄O₇ Reactive Electrochemical Membrane Anode. Environmental Science and Technology Letters, 6(8):504-510. doi.org/10.1021/acs.estlett.9b00397.