There are more than 4,000 per- and polyfluoroalkyl substances (PFAS), many of which have been used in aqueous film-forming foam (AFFF) for fire-fighting activities across the United States. The growing presence of PFAS in surface and groundwater is raising concern due to their potential toxicity to humans. As a result, the U.S. Environmental Protection Agency and several states have set health advisory goals or regulatory limits on many PFAS. Since PFAS are stable and persistent in the environment, impacted sites face numerous remediation challenges. Among ex situ treatment options, granular activated carbon (GAC) adsorption is a widely used approach, wherein groundwater is pumped through beds of GAC before being reintroduced into the aquifer. Many long-chain PFAS adsorb well to GAC because of their moderate to high hydrophobicity. Some PFAS, especially short-chain species, do not adsorb well to GAC, resulting in limited removal if treatment criteria are based on long-chain PFAS removal or high GAC use rates if short-chain PFAS removal is targeted. The objective of this project was to determine if PFAS adsorption to activated carbon (AC) could be enhanced by applying low levels of electricity.
To fulfill the overall objective, the project team completed four primary tasks. First, after validating and benchmarking a continuous-flow, capacitive electrochemical cell using sodium chloride, the project team compared the adsorption of perfluorooctanoic acid (PFOA) onto AC cloth electrodes with and without an applied voltage (1.0 V). Second, the project team used the standard operating procedure developed for PFOA to test the adsorption of five other representative PFAS (perfluorobutane sulfonic acid [PFBS], perfluorooctane sulfonic acid [PFOS], perfluorononanoic acid [PFNA], perfluorohexane sulfonic acid, perfluoroheptanoic acid [PFHpA]) in the presence and absence of an applied voltage. Third, the project team examined the effect of the inorganic ion chlorine (Cl-) (an abundant anion in groundwater) on the electrosorption of PFBS, a model short-chain PFAS. Last, the project team qualitatively assessed electrode short-circuiting as a method to desorb PFAS from AC.
The adsorption of all PFAS increased when the project team applied a voltage across two AC electrodes. The magnitude of the enhancement was dependent on PFAS properties. Short-chain species (e.g., PFBS, PFHpA) yielded the largest enhancement with voltage (2.6–3.7 times), while long-chain species (e.g., PFOS, PFNA) had the smallest enhancement (1.5–1.7 times). The carboxylic acids generally had greater enhanced adsorption than the sulfonic acids (e.g., PFOA > PFOS). When Cl- was present at a low concentration (1 mM), the electrosorption of PFBS was not negatively impacted. At a higher Cl- concentration (10 mM), electrosorption was simlar to nonelectrically assisted adsorption. A qualitative desorption test conducted with PFOA showed that effluent PFOA concentrations decreased relative to the influent when the electrodes were short-circuited. Application of a voltage after short-circuiting the electrodes resulted in a large increase in PFOA concentration in the effluent, suggesting that some of the PFOA that physically adsorbed during short-circuit operation was released back until solution. Results of this study are available in the Final Report.
The findings of this project indicate that PFAS adsorption onto AC can be increased with the application of a low voltage. The enhancement depended on PFAS properties and ranged from 1.5 – 3.7 times, with greater enhancement for short-chain PFAS. In a real-world situation, this work implies that integrating electrodes into existing GAC systems might provide a simple approach to increase the lifetime of GAC beds. Future research needs include identifying optimal GAC properties for enhanced PFAS electrosorption and assessing the benefits of electrosorption in electrically-controlled GAC columns. Completing these additional tasks will lay the foundation for large-scale testing and adoption of electrosorption for enhanced PFAS removal. (Project Completion - 2020)