Grantee Research Project Results
Final Report: Electrochemical Extraction and Remediation of PFAS in Soils
EPA Contract Number: 68HERC20C0054Title: Electrochemical Extraction and Remediation of PFAS in Soils
Investigators: Lee, Katherine
Small Business: Faraday Technology, Inc.
EPA Contact: Richards, April
Phase: II
Project Period: June 1, 2020 through May 31, 2022 (Extended to May 31, 2023)
Project Amount: $300,000
RFA: Small Business Innovation Research (SBIR) - Phase II (2020) Recipients Lists
Research Category: Small Business Innovation Research (SBIR)
Description:
The technology developed in this program addresses the need for cost-effective methods to extract and destroy per- and polyfluoroalkyl substances (PFAS) from soil, sediment, and groundwater. Soil, sediment, and groundwater have been identified as long-term environmental reservoirs for PFAS and a potential entry point for PFAS into the food chain. Clean-up of PFAS in these environmental reservoirs will be critical for preventing human exposure to PFAS.
A fundamental challenge with managing environmental PFAS is its widespread distribution in the environment and the variability of the PFAS concentration between these sites. Aggregated soil-survey reports on environmental PFAS contamination at sites around the globe found PFAS present at almost every site tested with soil concentrations ranging from mg/kg to mg/kg levels. Groundwater contamination is also observed at these sites, with PFAS concentrations that are typically orders of magnitude lower than soil concentrations. Given the magnitude and variability of PFAS-impacted sites, effective management of environmental PFAS will require flexible and cost-effective treatment technologies. Another challenge is that fully treated sites can be re-contaminated with PFAS from atmospheric deposition. To address this challenge, treatment options must be non-invasive and capable of operating over long timescales.
Current treatment approaches are limited to excavation/off-site disposal for contaminated soil and ex situ pump and treat systems for groundwater plumes. However, excavation is expensive, invasive, ill-suited for long-term management of PFAS, and impractical for treatment of deep contamination/contamination under structures. Meanwhile, pump and treat methods are economically impractical for treatment of large volumes of dilute PFAS contaminated groundwater. As such, there is a pressing need for cost-effective treatment methods to extract and destroy PFAS in soil, sediment, and groundwater without large-scale excavation and/or pumping of dilute groundwater feeds.
To address the need for cost-effective and efficient technologies for the extraction and destruction of PFAS in soil, sediment, and groundwater, Faraday Technology and the Terran Corporation have developed an innovative PFAS treatment system that combines nondestructive electrokinetic (EK) extraction with destructive electrochemical oxidation (EO) technology. The EK extraction process uses an applied electric field to transport PFAS into electrodes that are submerged in the soil, removing the PFAS from the soil and generating a PFAS-concentrated water solution in the electrode compartment. The potential for large-scale implementation of EK technology has already been demonstrated for treatment of other contaminants. For example, Terran has previously installed large scale EK systems similar to the design proposed in this work for the remediation of chlorinated solvents. These instillations were operated on year-long timescales at active sites without interfering with facility operations and were successful in treating contaminants under buildings.
EO is an emerging PFAS destruction technology that has the unique ability to drive the complete mineralization of PFAS. EO uses an electrical current/voltage applied to an electrode to drive electron transfer reactions that break PFAS down into environmentally benign products (e.g., CO2. F‒). Current EO systems are designed to treat PFAS in contaminated water sources and have been validated with groundwater that has been extracted from the environment via traditional pump-and-treat methods.
The novelty of the tandem EK/EO system developed in this program lies in the application of EK for transport of PFAS, the use of EO for in situ treatment of groundwater, and the coupling of EK/EO to create an integrated treatment technology. Three integrated EK/EO system designs were developed and experimentally validated in this program:
- System Design 1: Stand-Alone EK/EO System - EK extraction generates a PFAS-contaminated water solution in the soil-submerged electrode compartments. EO treatment by the soil-submerged electrodes destroys the PFAS in the contaminated water solution.
- System Design 2: In Situ EK/EO Treatment Train - EK drives transport of PFAS towards the soil-submerged electrodes. During EK transport, PFAS is destroyed by EO at a second set of soil-submerged electrodes that are located near the EK anode.
- System 3: Ex Situ EK/EO Treatment Train -The PFAS concentrated water collected at the soil-submerged electrodes is pumped into a separate EO water treatment reactor.
To experimentally validate these three approaches, Faraday Technology and Terran Corp first identified EK process parameters that control the mechanism and rate of EK transport for sulfonate and carboxylate PFAS with varying chain lengths, including PFOS and PFOA. (Note that we use the following nomenclature to represent chain length: CN, where N = the number of carbons in the fluorocarbon chain.) This work was conducted on the bench-scale using Ohio till soil that been spike with mixtures of sulfonate and carboxylate PFAS. Through this work, we were able to identify conditions that extracted PFAS from the soil and subsequently destroyed the PFAS in the electrode compartment, thereby validating System Design 1 (Stand-Alone EK/EO System). Results from these studies provided the foundation for the design and construction of an apparatus and process that integrated the EK/EO technologies as part of an in situ treatment train (System Design 2). System Design 3 was validated using simulated soil-derived electrolytes reflective of those that would be collected in the EK electrode compartment. Using the experimental parameters from this work, Faraday and Terran conducted cost analysis for the three approaches.
Summary/Accomplishments (Outputs/Outcomes):
Key Finding 1: The rate and efficacy of EK transport/extraction depends on PFAS properties. EK transport and extraction was found to be most effective for shorter-chain (C4‒C6) PFAS and PFAS with carboxylate head groups, which could be effectively extracted from the soil using lower applied voltages that are commonly employed during field-scale instillations (i.e., direct current waveforms, 0.5 V/cm soil length). Once extracted from the soil, EO destruction in the electrode compartment was most effective for sulfonate species, with 50‒70% destruction observed for C4-C6 sulfonate species.
Key Finding 2: The rate and efficacy of EK transport/extraction can be enhanced through the use of elevated voltages. EK trials that used elevated direct current voltages (2.5 V/cm soil length) were able to extract 30-80% of C4/C6/C8/C10 sulfonate PFAS and C6/C8/C10/C12 carboxylate PFAS. Once extracted, all species were largely destroyed.
Key Finding 3: Despite the improved EK/EO performance of these elevated voltages, direct current waveforms using these high voltages cannot be reasonably applied at the field-scale. However, Faraday and Terran demonstrated that pulsed-voltage waveforms using elevated peak voltages (2.5 V/cm soil length), but field relevant average voltages (0.5 V/cm soil length), had the potential to enable EK extraction of longer chain PFAS. These results are important because they provide a pathway to further tune and enhance EK concentration/extraction of PFAS through pulsed-voltage waveform engineering.
Key Finding 4: System Design 2 (In Situ EK/EO Treatment Train) was effective at destroying mobilized PFAS during EK transport. For the proof-of-concept studies conducted in this work, destruction of up to 60% of mobilized PFAS at the secondary EO electrode was demonstrated.
Key Finding 5: For ex situ treatment of soil-derived electrolytes, the use of pulsed-voltage waveforms provided significant improvement in EO performance relative to direct current waveforms. Over short timescales (30 minutes), pulsed-voltage EO waveforms were identified that achieved 2.5x faster PFAS destruction rates while requiring 10x less energy than direct current waveforms. Over longer timescales (24 hours), the use of pulsed-voltage EO waveforms prevented significant (30‒60x) and economically unreasonable increases in energy requirements. We anticipate that use of these pulsed-voltage EO waveforms will be critical for the economic success of ex situ treatment strategies.
Key Finding 6: Cost analysis of the stand-alone EK/EO system (System Design 1) shows that this treatment approach is cost-competitive with excavation and disposal. The cost-estimates for System Design 1 were ~$150/ton of soil treated. In comparison, excavation and disposal costs are estimated at ~$130/ton excluding soil transportation fees. As these transportation fees can double to triple the cost of excavation/disposal, the ability to operate the stand-alone EK/EO system on-site (there-by avoiding hefty soil transportation fees) is expected to provide considerable additional cost-savings relative to excavation/disposal. The cost-effectiveness of the in situ EK/EO treatment train (System Design 2) is contingent on identifying inexpensive electrode materials. System Design 3 (ex situ EK/EO treatment train) will be cost-competitive when coupled with a pre-concentration technology (such as foam fractionation) that can generate a low-volume, highly concentrated PFAS solution for EO treatment.
Conclusions:
Results from the Phase II activate validate the technical feasibility of coupling non-destructive EK extraction with EO PFAS destruction as a stand-alone technology and as part of an in situ or ex situ treatment train. Technoeconomic analysis validates that stand-alone EK/EO system design is cost-competitive with current treatment methods (i.e., soil excavation and disposal). Pathways to achieve commercial viability for the in situ (less expensive electrode materials) and ex situ treatment train (coupling with a pre-concentration technology) have also been identified and will be considered during future work.
Key commercial benefits of this tandem EK/EO approach include (1) that it is flexible enough to be used for a variety of site sizes and PFAS concentration levels, (2) it can be operated over the course of multiple years without interfering with site operation, and (3) it can be operated on-site, thereby circumventing costly transportation of soil to specialized facilities. During this program, Faraday presented this work at a number of conferences and held one-on-one meetings with commercial, academic, and government entities interested in PFAS destruction technology. Insight from these discussions was used to develop our technology transition plan. Through these discussions, two critical market entry points were identified - the Department of Defense and agricultural sector - which are currently trying to manage widespread contamination stemming from legacy use of AFFF firefighting foam and residuals such as biosolids, respectively.
Work conducted as part of this program largely focused on soil from a single region of the country (Dayton, OH till soil) and exclusively used artificially spiked soil samples. As part of our commercialization/transition plan, Faraday and Terran are conducting ongoing outreach to military instillations and agricultural sites in order to obtain naturally contaminated soil samples for bench-scale validation studies. These bench-scale validation studies will be used to identify soil types that are most compatible with the EK/EO approaches discussed above. Results from these studies will be used to identify sites for large-scale field demonstrations.
Journal Articles:
No journal articles submitted with this report: View all 5 publications for this projectSBIR Phase I:
Electrochemical Extraction and Remediation of PFAS in Soils | Final ReportThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.