Per- and polyfluoroalkyl substances (PFAS) such as perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) are present in the subsurface in part due to the use of aqueous film-forming foam (AFFF) in firefighting at airports and training facilities. In 2022, the U.S. Environmental Protection Agency (EPA) updated its drinking water health advisory levels (HAL) for PFOA (0.004 ng/L), PFOS (0.02 ng/L), perfluorobutane sulfonate (PFBS, 2000 ng/L), and hexafluoropropylene oxide dimer acid (100 ng/L). These values are many orders of magnitude lower than PFOA and PFOS concentrations observed in groundwater at numerous impacted sites. Therefore, there is an urgent need for effective strategies for the remediation of impacted water with environmentally relevant concentrations of PFAS.

The objective of this study was to determine the feasibility of utilizing a sorptive remediation approach that exploits multiple, complementary bonding modes (e.g., electrostatic and hydrophobic interactions) for the remediation of PFAS-impacted groundwater. The innovative sorbents are cationic polyaniline and polypyrrole polymers containing hydrophobic moieties. The hypothesis of this study was that the unique structure of these polymeric materials would allow both strong electrostatic interaction with the anionic head group present in many PFAS and hydrophobic interactions with the fluorinated tail, allowing them to be more selective than granular activated carbon (GAC) and to adsorb a wider range of compounds than anion exchange resins. By using suitable polymer precursors, the charge density and hydrophobicity of these polymers can be tailored to enhance PFAS removal.

 Yaniv Olshansky, Postdoctoral Scientist, and Anton Gomeniuc, Environmental Engineering MS Student, Preparing PFAS Removal Adsorbents at the University of Arizona

To meet the project goals, the research plan included six research tasks that address the technical objectives: 

1. Develop, characterize, test and optimize cationic hydrophobic polymers and activated carbon-cationic polymer composites as ultra-high affinity sorbents to sequester PFAS. 
2. Develop cationic hydrophobic polymers grafted on GAC and activated carbon fibers and evaluate the capacity of these composites to adsorb PFAS. 
3. Evaluate the impact of common co-occurring chemicals and aqueous chemistry on the sorptive removal of PFAS. 
4. Assess the feasibility of regenerating the engineered sorbents to enable sorbent reuse. 
5. Elucidate the molecular-scale adsorption mechanisms of PFAS on the most effective engineered sorbents under various geochemical conditions to gain insights on how to improve the sorbents. 
6. For the most promising sorbents, demonstrate the continuous removal of PFAS in laboratory columns. 

This study examined the sorptive remediation of the six anionic PFAS listed in the EPA’s Unregulated Contaminant Monitoring Rule 3 list (i.e., perfluoroheptanoic acid, PFOA, perfluorononaoic acid, PFBS, perfluorohexane sulfonate, PFOS), and two additional compounds often detected in impacted water (i.e., perfluorobutanoic acid and 6:2 fluorotelomer sulfonate). Screening studies and kinetic experiments were conducted with PFOA. The project team also evaluated the performance of the polyaniline-based polymers with two AFFF-impacted groundwater samples.

The project team characterized the engineered polymers using a multi-faceted approach, including surface areas, charge densities, and molecular structures using N₂-Brunauer, Emmett and Teller adsorption, electrophoretic mobility, and infrared- and X-ray spectroscopy techniques. The affinity and selectivity of these polymers for PFOA were then tested across a wide range of aqueous solution pH and interpreted based on the characterization data. It was found that a balance between the addition of hydrophobic substitutions to the polymer backbone and the presence of cationic amine groups provides the highest affinity of PFOA to the new adsorbents. The data also indicate that the affinity of these polymers can be tailored by altering substituents on the aniline ring.

Among the tested polymers, poly-o-toluidine and polyaniline were the most promising adsorbents. The adsorption capacity of these polymers was comparable to that of activated carbon at low equilibrium concentrations. The tailored polymers performed similar or better than activated carbon, and better than an anion exchange resin, in water containing elevated natural organic matter and ion concentrations. In addition, the novel polymers displayed fast PFAS adsorption kinetics, low desorption of adsorbed PFAS in aqueous media, and facile regeneration using methanol with 1% NaCl at ambient temperature. Additional testing confirmed the chemical stability of the polymers when stored under ambient conditions and their resistance to microbial attack. It was also demonstrated that the polymers can be effectively grafted onto GAC, generating a granular composite suitable for application in continuous-flow packed-bed adsorbers. However, the low polyaniline (PANI) content required to maintain high PFAS adsorption capacity in the GAC polymer composite limits the applicability of these hybrid adsorbents. Furthermore, rapid small-scale column studies conducted with synthetic groundwater and actual groundwater confirmed the outstanding performance of the polymeric adsorbents as PFAS adsorbents.

This project has led to the development of engineered materials with outstanding PFAS adsorptive capacity at environmentally relevant concentrations. These novel cationic hydrophobic polymers display fast PFAS uptake kinetics, excellent performance in the presence of elevated natural organic matter and ion concentrations, low desorption of adsorbed PFAS to aqueous media, and facile regeneration at ambient temperature. Moreover, the polymers can be customized to enhance adsorption of PFAS under the wide range of solution chemistry conditions encountered in environmental systems. Collectively, these findings indicate that the tailored polymeric adsorbents offer great promise for remediation of PFAS-impacted groundwater.

The project team is already working on one field application under SERDP project ER22-3155. It is anticipated that this work will lead to an in situ pilot study and the technology will be applied as a permeable adsorptive barrier to control migration of low PFAS concentrations in groundwater in conjunction with source treatment and management. The research team is also planning to seek funding to demonstrate the feasibility of the new adsorbents at the pilot-scale in an on-site flow-through system. For a field-scale on-site reactor-based application, further development is required to create a granular or pelletized polymer material with high PANI content. (Project Completion - 2023)

He, J., A. Gomeniuc, Y. Olshansky, J. Hatton, L. Abrell, J.A. Field, J. Chorover, and R. Sierra-Alvarez. 2022. Enhanced Removal of Per- And Polyfluoroalkyl Substances by Crosslinked Polyaniline Polymers. Chemical Engineering Journal, 446(5):137246. doi.org/10.1016/j.cej.2022.137246.

Olshansky, Y., A. Gomeniuc, J. Chorover, L. Abrell, J.A. Field, J. Hatton, J. He, and R. Sierra-Alvarez. 2022. Tailored Polyanilines Are High-Affinity Adsorbents for Per- and Polyfluoroalkyl Substances. ACS ES&T Water, 2(8):1402-1410. doi.org/10.1021/acsestwater.2c00166.

Olshansky, Y., A. Gomeniuc, J. Chorover, L. Abrell, J.A. Field, J. Hatton, and R. Sierra-Alvarez. 2021. Synthesis and Characterization of Customizable Polyaniline-Derived Polymers and Their Application for Perfluorooctanoic Acid Removal from Aqueous Solution. ACS ES&T Water, 1(6):1438-1446. doi.org/10.1021/acsestwater.1c00019.

Olshansky, Y., J. Chorover, L. Abrell, J.A. Field, A. Gomeniuc, J. Hatton, and R. Sierra-Alvarez. 2022. Sorption of PFAS by Cationic Hydrophobic Polymers. Abstracts of Papers of the American Chemical Society, 257. 

Theses and Dissertations

Gomeniuc, A. 2021. Novel Sorptive Approach for the Remediation of Per- and Polyfluoroalkyl Substances (PFAS) (Master’s Thesis). University of Arizona.