The use of aqueous film-forming foam (AFFF) in fire control measures has led to the detection of recalcitrant per- and polyfluoroalkyl substances (PFAS) in groundwater throughout the United States and other countries. Cost effective remediation technologies will be required for environmental restoration since microbial and abiotic degradation of PFAS are too slow to allow for natural attenuation. Remediation strategies are complicated by the numerous types of PFAS and their breakdown products found in the AFFF formulations. In this proposed project, mesoporous organosilica sorbents will be evaluated for the removal of a wide range of PFAS, including anionic, cationic, and non-ionic forms. Adsorption will be designed to be reversible allowing for economical on-site regeneration by solvent rinse. Successfully designed sorbent media could be used for ex situ pump-and-treat remediation systems or point-of-entry water treatment. 

This project is being conducted in two phases. In Phase 1 of this project, rationally-designed organosilica adsorbents were synthesized to study the adsorption of PFAS from water. The goals of this proof-of-concept project were to better understand mechanisms of adsorption and thus design optimal adsorbents to minimize costs of remediation. Swellable organically modified silica (SOMS) was used as a platform as it can be modified by use of different silane monomers and entrapped polymers. A unique feature of SOMS is the ability to volumetrically swell significantly, enlarging the microscale pore structure. Swelling enhances capacity and improves adsorption kinetics. 

The overall goal of Phase II of this project is to develop high capacity, reusable adsorbents to remove PFAS from groundwater. A mechanistic understanding of adsorption will be gained through experimental work to understand the interactions that best remove PFAS from water. Knowledge about adsorption of PFAS will be used to create cost-efficient adsorbents optimized to remove short-chain PFAS from water by current technologies. Adsorbents will be studied for effectiveness in field relevant conditions including treating water co-impacted with hydrocarbons and for use in remediation infrastructure such as horizontal reactive media treatment wells (HRX Wells®). Pilot testing of optimized media will be performed to gather data for comparison with other treatment technologies.

For Phase I of this project, a stepwise approach was used. First, a diverse set of SOMS materials with fluoroalkyl groups and/or cationic groups added to a hydrophobic resin were synthesized. Materials were then screened for PFAS adsorption using kinetics and adsorption isotherms. Adsorbents with optimal performance were further studied using a series of bench-scale column experiments. Measurements were done in comparison to activated carbon and ion exchange resins currently used in water treatment. Finally, the best SOMS adsorbent was evaluated in a pilot test installed on a side stream at the Former Joint Reserve Base Naval Air Station Willow Grove. The reversibility of the resins also were tested.

During Phase II of this project, adsorbents will be based on swellable organically-modified silica, a hydrophobic resin with pores that expand upon absorption of organic compounds.  Silica resins can be modified with various functional groups to evaluate the interactions that best adsorb PFAS from groundwater. The silica adsorbent coatings will be evaluated in combination with sustainable natural substrates. Adsorption of different types of PFAS, total capacity, and regeneration capability will be tested to benchmark performance and overall operational costs.

Phase I Results

The key findings from Phase I are summarized as follows and are available in the Phase I Final Report.

Optimal PFAS adsorbents were found to be highly porous SOMS materials possessing hydrophobicity and cation groups. Cationic groups help to bind anionic PFAS. Data suggest that PFAS self-assemble into aggregates that enhance the adsorption of long-chain PFAS. SOMS is ideally suited to take advantage of PFAS self-aggregation since the resin has an expanded pore structure obtained via swelling. For purposes of evaluation, during the pilot test, poly-SOMS adsorbent developed in this study yielded an adsorption capacity of 2,600 μg/g of PFAS given an influent concentration of 40 μg/g total PFAS and a contact time of one minute.

In Phase II of this project, preliminary experimental work at the bench-scale and pilot tests indicated substantial removal of longer-chain PFAS from groundwater. Organosilica adsorbents were also shown to be regenerable by rinsing with solvents. Finally, the resins were durable and were found to stable during pilot-scale experiments involving >100,000 applied bed volumes.

PFAS are found in many natural waters due to their persistence and widespread application in various products. Purification by adsorption is currently the most implemented method to treat PFAS-impacted water. Lowering the costs of adsorbent resins will make water purification more cost effective. A specific benefit of this project is to develop technology to remove short-chain PFAS which current technologies have limited capacity to adsorb. Results of Phase I of this project demonstrated that SOMS-based PFAS adsorbents have improved capacity over current technologies. Scale-up of SOMS was accomplished during the study making the material an option for full-scale PFAS remediation activities.

SOMS-based resin has two benefits. First, it has a higher capacity than ion exchange resin, especially in the presence of natural organic matter. Second, SOMS can be regenerated using a solvent rinse. Regeneration allows resins to be used and PFAS to be concentrated for residue management. The new adsorbent technology appears useful for economical remedial action to manage groundwater resources. (Anticipated Phase II Completion - 2025)

Kim, Y., K.A. Pike, R. Gray, J.W. Sprankle, J.A. Faust, and P.L. Edmiston. 2023.  Non-Targeted Identification and Semi-Quantitation of Emerging Per- and Polyfluoroalkyl Substances (PFAS) in US Rainwater. Environmental Science: Processes & Impacts, 25:1771-1787. doi.org/10.1039/D2EM00349J.

Pike, K.A., P.L. Edmiston, J.J. Morrison, and J.A. Faust. 2021. Correlation Analysis of Perfluoroalkyl Substances in Regional US Precipitation Events. Water Research, 190:116685. doi.org/10.1016/j.watres.2020.116685.

Stebel, E.K., K.A. Pike, H. Nguyen, H.A. Hartmann, M.J. Klonowski, M.G. Lawrence, M. Rachel,  R.M. Collins, C.E. Hefner, and P.L. Edmiston. 2019. Absorption of Short-Chain to Long-Chain Perfluoroalkyl Substances Using Swellable Organically Modified Silica. Environmental Science: Water Research & Technology, 5:1854-1866. doi.org/10.1039/C9EW00364A.