Perfluoroalkyl compounds (PFCs), such as perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA), are used as surfactants in aqueous film-forming foams (AFFFs) extensively employed in U.S. military fire fighting. Historically, effluents from AFFF fire-fighting activities were neither impounded nor pretreated prior to discharge to water treatment systems or to the environment. Approximately 600 sites under the Defense Environmental Restoration Program (DERP) are categorized as Fire/Crash/Training areas and thus have the potential for an impact to groundwater due to historical use of AFFFs. PFCs comprise an emerging class of water/sediment contaminants that are hazardous to aquatic species and ecological and human health and have thus drawn close regulatory scrutiny. Remediation of PFC-impacted groundwater, however, remains a significant challenge to current technologies.
The objective of this project was to assess the feasibility of using a permeable reactive barrier (PRB) system to induce effective enzyme-catalyzed humification reactions for in situ remediation of PFC-impacted groundwater.
Four research tasks were proposed to achieve the project objective. Task 1 was comprised of batch reactor studies aimed at optimizing enzyme-catalyzed oxidative humification reaction (ECOHR) conditions for enhanced PFC degradation. This was conducted in two types of systems: aqueous phase and soil slurry. Task 2 focused on enzyme immobilization on sand and other media for use in PRB setup. Task 3 was intended to study the use of calcium peroxide as a source of oxidants for ECHOR in anaerobic conditions. Task 4 was a laboratory column study to evaluate the performance of the PRB system to remove PFCs in water flow.
The experimental results from the aqueous phase reactions show that all three tested enzymes (HRP, LiP, and Laccase) are capable of mediating ECOHR to cause PFOA degradation. Laccase appears to be the most promising candidate for remediation use because of its stability and availability. Results indicated that the level of oxygen may be a critical factor that controls PFOA degradation during ECOHR in addition to laccase dosage and mediator concentrations; however, further studies are required to confirm this conclusion. The soil slurry batch reactor studies showed that both PFOA and PFOS degraded significantly in soil through laccase-mediated ECOHR. While soil organic matter was found to serve the role of mediator for ECOHR, the reaction was enhanced when chemical mediators were added. Studies focusing on enzyme immobilization on sand and other media showed that both soil and clay were good support media. Sand was not a good support medium for immobilizing humification enzymes, which is thought to be a result of the limited specific surface area of sand. A double-layer permeable reactive barrier (DL-PRB) system was originally proposed for the study for inducing in situ ECHOR reactions. The DL-PRB system was comprised of an oxidant releasing material layer followed by a layer of quartz sands immobilized with humification enzymes. As noted previously, sand was found to be a poor candidate for enzyme immobilization. In addition, results from the earlier tests showed that oxygen releasing material, such as calcium peroxide, did not need to be in a layer separate from the reactive layer. As a result, the conceptual model evolved from a double-layer reactive barrier and Task 3 was not completed. Instead, remaining efforts focused on the column study experiments. Results from the column studies showed that granular activated carbon (GAC) is a promising candidate material to use in PRBs to induce ECHOR for PFC remediation. ECHOR may be enhanced on the GAC surface through micro-reactor effects, but future studies are needed to verify this.
Results from this study provide a better understanding of how PFCs may be transformed during natural humification processes. These interactions can be enhanced through system engineering to help address groundwater PFC-impacted groundwater; however, additional research is necessary to develop these techniques. (Project Completion - 2013)
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Luo, Q., X. Yan, J. Lu, and Q. Huang. 2018. Perfluorooctanesulfonate Degrades in a Laccase-Mediator System. Environmental Science & Technology, 52(18):10617-10626.
Zhang, D., Q. Luo, B. Gao, S.Y. Chiang, D. Woodward, and Q. Huang. 2016. Sorption of Perfluorooctanoic Acid, Perfluorooctane Sulfonate and Perfluoroheptanoic Acid on Granular Activated Carbon. Chemosphere, 144:2336-2342.