Aqueous film-forming foam (AFFF) formulations have been used for decades to suppress hydrocarbon fuel-based fires. Per- and polyfluoroalkyl substances (PFAS) are key constituents of AFFF and their occurrence in groundwater has been widely reported. Developing cost-effective remediation strategies to remove PFAS from groundwater remains an important challenge.

This work is being conducted in two phases. The goal of Phase I was to design, characterize, and evaluate the implementation of novel polymer adsorbents for the in situ or ex situ remediation of PFAS-impacted groundwater. Some of the major deficiencies of contemporary adsorption-based processes were addressed, including their insufficient affinity for shorter-chain and more hydrophilic PFAS, the propensity for fouling by natural organic matter and other matrix constituents, and costs associated with the regeneration of spent adsorbents. The technical objectives of Phase I were to:

1. discover cyclodextrin (CD)-based polymer adsorbents with high affinity, high capacity, and rapid kinetics for priority and environmentally relevant PFAS;
2. characterize the performance of each polymer under scenarios relevant to groundwater remediation; and
3. optimize synthesis pathways and evaluate adsorption process alternatives to transition towards implementation of the new polymer adsorbents into remediation processes.

The goal of Phase II is to optimize the design and performance of modular, next-generation β-CD-based polymer adsorbents for implementation in ex situ packed-bed filtration processes. Phase I research will be built upon to optimize the design and synthesis of CD-based polymer adsorbents that exhibit rapid adsorption kinetics and high adsorption affinity for short- and long-chain perfluoroalkyl acids (PFAAs) and to limit adsorption inhibition caused by natural organic matter and other matrix constituents. Adsorbents will be synthesized in morphologies that are compatible with existing packed-bed adsorption processes, and offer a variety of options for handling of spent adsorbent media. The technical objectives of Phase II are to:

1. identify the ideal steric and electronic chemical environment around the β-CD monomer to enable rapid and selective adsorption of target PFAS;
2. optimize adsorbent particle size, morphology, and durability by means of emulsion-based polymerization to generate materials amenable to packed-bed filtration processes; and
3. evaluate destruction technologies to simultaneously mineralize bound PFAS and spent adsorbent media to eliminate regeneration waste or off-site disposal.

The project team discovered a promising class of novel polymer adsorbents that can be rationally designed to target specific chemicals. These include the first mesoporous polymers of β-cyclodextrin, which exhibit rapid pollutant uptake, high pollutant capacity, and facile regeneration potential. In Phase I of this project, the project team designed and characterized new cyclodextrin-based polymer adsorbents that target PFAS and controlled polymer structure and morphology to enable their implementation into groundwater remediation processes. The project was structured around six scientific tasks:

1. Develop new polymer adsorbents that maximize the adsorption kinetics, affinity, and capacity for priority PFAS.
2. Characterize the uptake of diverse PFAS in laboratory experiments and from AFFF-impacted groundwater.
3. Develop structure-activity relationships to understand and strengthen the non-covalent interactions by which promising polymer derivatives sequester PFAS.
4. Optimize the synthesis of the most promising polymers to obtain useful morphologies and high yields to facilitate commercialization.
5. Evaluate alternatives for ex situ adsorption-based remediation strategies with consideration for adsorbent regeneration and the handling of ancillary waste streams.
6. Develop covalent organic frameworks (COFs) with novel functional groups and translate the most effective functionalities into future generations of CDPs.

Recently, the project team has discovered a new way to synthesize modular cyclodextrin-based adsorbents with higher yields and reliable morphology that significantly outperform the CDPs developed under Phase I for the removal of PFAS from water. As part of the Phase II project, the project team will optimize the synthesis, morphology, and performance of these so-called next-generation CDPs for implementation in ex situ packed-bed filtration processes. The project is structured around four scientific tasks:

1. Explore comonomers with varying properties to identify the best-performing derivatives.
2. Characterize the performance of next-generation CDPs under a variety of scenarios relevant to groundwater remediation.
3. Develop emulsion polymerization methods for particle size and morphology control.
4. Evaluate the extent of mineralization of spent media through physical and chemical destruction techniques.

Phase I Results

Phase I resulted in the synthesis of several notable CDP derivatives that exhibit enhanced uptake of PFAS and that were evaluated to explore adsorption kinetics, adsorption affinity, and adsorption inhibition under a variety of groundwater scenarios. Modeling studies identified physicochemical properties of CDPs that enhance their affinity for perfluoroalkyl acids. The chemistry of the crosslinker plays a large and unexpected role in determining the selectivity and affinity for PFAS. For example, only CDPs synthesized with crosslinkers that provide a positive surface charge exhibit rapid adsorption kinetics and high adsorption affinity for PFAS containing fewer than six carbon atoms. These observations suggest that the hydrophobic interior cavity of the cyclodextrins is accessible and a suitable binding site for more hydrophobic PFAAs, but that complementary electrostatic interactions are essential for enhancing the adsorption kinetics and adsorption affinity for less hydrophobic PFAAs. We have also discovered techniques to control particle size and morphology, demonstrated long-term mechanical and chemical durability of CDPs, and established facile regeneration and reuse procedures. The results of the Phase I study are summarized in the Phase I Final Report.

This project provides remediation project managers and the scientific community with alternative adsorbents to implement in adsorption-based remediation processes. These novel polymer adsorbents have tailored affinity for various classes of PFAS, limited interactions with natural organic matter and other matrix constituents, and the potential for facile regeneration and reuse. These features address the most important deficiencies of conventional adsorbents and may lead to the development of more cost-effective adsorption-based remediation processes for AFFF-impacted groundwater. (Anticipated Phase II Completion - 2026)

Ateia, M., D.E. Helbling, and W.R. Dichtel. 2020. Best Practices for Evaluating New Materials as Adsorbents for Water Treatment. ACS Materials Letters, 2(11):1532–1544. doi.org/10.1021/acsmaterialslett.0c00414.

Ching, C., M.J. Klemes, B. Trang, W.R. Dichtel, and D.E. Helbling. 2020. β-Cyclodextrin Polymers with Different Cross-Linkers and Ion-Exchange Resins Exhibit Variable Adsorption of Anionic, Zwitterionic, and Nonionic PFASs. Environmental Science and Technology, 54(19):12693–12702. doi.org/10.1021/acs.est.0c04028.

Ching, C., Y. Ling, B. Trang, M. Klemes, L. Xiao, A. Yang, G. Barin, W.R. Dichtel, and D.E. Helbling. 2022. Identifying the Physicochemical Properties of β-Cyclodextrin Polymers that Determine the Adsorption of Perfluoroalkyl Acids. Water Research, 209:117938. doi.org/10.1016/j.watres.2021.117938. 

Ching, C., Z. Lin, W.R. Dichtel, and D.E. Helbling. 2023. Evaluating the Performance of Novel Cyclodextrin Polymer Granules to Remove Perfluoroalkyl Acids (PFAAs) from Water. ACS ES&T Engineering. doi.org/10.1021/acsestengg.2c00379. 

Ji, W., L. Xiao, Y. Ling, C. Ching, M. Matsumoto, R.P. Bisbey, D.E. Helbling, and W.R. Dichtel. 2018. Removal of GenX and Perfluorinated Alkyl Substances from Water by Amine-Functionalized Covalent Organic Frameworks. Journal of the American Chemical Society, 140(40):12677−12681. doi.org/10.1021/jacs.8b06958.

Klemes, M.J., L.P. Skala, M. Ateia, B. Trang, D.E. Helbling, and W.R. Dichtel. 2020. Polymerized Molecular Receptors as Adsorbents to Remove Micropollutants from Water. Accounts of Chemical Research, 53(10):2314–2324. doi.org/10.1021/acs.accounts.0c00426.

Klemes, M.J., Y. Ling, C. Ching, C. Wu, L. Xiao, D.E. Helbling, and W.R. Dichtel. 2019. Reduction of a Tetrafluoroterephthalonitrile-β-Cyclodextrin Polymer to Remove Anionic Micropollutants and Perfluorinated Alkyl Substances from Water. Angewandte Chemie International Edition, 58(35):12049-12053. doi.org/10.1002/anie.201905142.

Wang, R., C. Ching, W.R. Dichtel, and D.E. Helbling. 2020. Evaluating the Removal of Per- and Polyfluoroalkyl Substances from Contaminated Groundwater with Different Adsorbents Using a Suspect Screening Approach. Environmental Science and Technology Letters, 7(12):954–960. doi.org/10.1021/acs.estlett.0c00736.

Wang, R., Z.W. Lin, M.J. Klemes, M. Ateia, B. Trang, J. Wang, C. Ching, D.E. Helbling, and W.R. Dichtel. 2022. A Tunable Porous β-Cyclodextrin Polymer Platform to Understand and Improve Anionic PFAS Removal. ACS Central Science, 8(5):663-669. doi.org/10.1021/acscentsci.2c00478.

Wu, C., M.J. Klemes, B. Trang, W.R. Dichtel, and D.E. Helbling. 2020. Exploring the Factors that Influence the Adsorption of Anionic PFAS on Conventional and Emerging Adsorbents in Aquatic Matrices. Water Research, 182:115950. doi.org/10.1016/j.watres.2020.115950.

Xiao, L., C. Ching, Y. Ling, M. Nasiri, M.J. Klemes, T.M. Reineke, D.E. Helbling, and W.R. Dichtel. 2019. Cross-linker Chemistry Determines the Uptake Potential of Perfluorinated Alkyl Substances by β-Cyclodextrin Polymers. Macromolecules, 52(10):3747-3752. doi.org/10.1021/acs.macromol.9b00417.

Yang, A., C. Ching, M. Easler, D.E. Helbling, and W.R. Dichtel. 2020. Cyclodextrin Polymers with Nitrogen-Containing Tripodal Crosslinkers for Efficient PFAS Adsorption. ACS Materials Letters, 2(9):1240−1245. doi.org/10.1021/acsmaterialslett.0c00240.

Patents

Dichtel, W.R., A. Alsbaiee, B.J. Smith, J. Hinestroza, D. Alzate-Sanchez, L. Xiao, Y. Ling, and D. Helbling. 2021. Porous Cyclodextrin Polymeric Materials and Methods of Making and Using Same. US Patent 11,491,460 B2. 

Dichtel, W.R., D. Helbling, C. Li, D. Alzate-Sanchez, J.M. Spruell, and Y. Ling. 2018. Grafted Porous Cyclodextrin Polymeric Material and Methods of Making and Using Same. International Patent International Patent WO2023107968A2.

Dichtel, W.R., D. Helbling, R. Wang, and M.J. Klemes. 2023. Functionalized Cyclodextrin Monomer and Polymer for Water Remediation. International Patent WO2023107968A2.

Helbling, D., Y. Ling, C. Ching, W.R. Dichtel, and M.J. Klemes. 2020. Post-Synthetically Modified Cyclodextrin Polymeric Materials and Methods of Making and Using Same. International Patent WO2020167375A1.