By Brooke K. Mayer, PhD, PE

Per- and polyfluoroalkyl substances (PFAS) are a diverse group of chemicals that are widely used in consumer products, such as specialized garments, stain-resistant coatings, non-stick cookware and aqueous film-forming foam used for firefighting.[1] These man-made micropollutants have been dubbed forever chemicals due to PFAS’ extreme resistance to degradation, stemming from their high polarity, solubility and strong C-F bonds.[2] Widespread use and extreme stability led to pervasive PFAS occurrence in the environment, wildlife and humans. Based on evidence that continued exposure to certain PFAS above threshold levels may lead to adverse health effects, US EPA established the current drinking water health advisory level at 70 ng/L for total perfluorooctanoic acid (PFOA) + perfluorooctane sulfonate (PFOS).[3] In 2021, the agency issued a final regulatory determination for these two compounds.

While PFOA and PFOS are by far the most well studied, there are thousands of other PFAS chemicals, encompassing a wide range of distinct physical and chemical properties. Long-chained PFAS (such as PFOA and PFOS) were phased out in the early 2000s. Nonetheless, given their recalcitrance, they are still detected in the environment. This is exacerbated by the breakdown of long-chain PFAS to short chain over time, as well as increasing use of short-chain PFAS such as perfluorobutanoic acid (PFBA), perfluorobutane sulfonic acid (PFBS) and other analogues such as GenX, ADONA and F-53B. The most common short chain PFAS, PFBA and PFBS, have been widely detected in drinking water, sediment, sewage sludge and polar snow/ice.[4] Accordingly, US EPA’s proposed Unregulated Contaminant Monitoring Rule (UCMR 5) includes all 29 PFAS with validated agency drinking water methods (Figure 1).[5]

Figure 1. The 29 PFAS on US EPA’s proposed UCMR 5.5 These compounds include a range of non-polymeric PFAS chemical classifications, structures and chain lengths (short-chain PFCA < 8C, short-chain PFSA < 6C). The classifications were adapted from Buck et al. (2011)6 and Dixit et al. (2021)1) and an example chemical structure (from ChemSpider or ChemSrc) is shown for each.

In general, PFAS molecules consist of i) a hydrophobic, nonionic tail composed of a fluorinated carbon chain (either partially or fully substituted with F) and ii) a charged ionic head (terminal functional group such as -SO3- or -COO-). Most PFAS are negatively charged (anionic)[7], although some are positively charged (cationic) or both positively and negatively charged (zwitterionic), as increas­ingly identified using advanced analytical techniques, e.g., Barzen-Hanson (2017).[8] Wide variation in PFAS structure and net surface charge influences their fate and transport in environmental and engineered systems.[9]

Conventional drinking water technologies such as coagulation, flocculation, sedimentation, filtration, biofiltration and oxidation (either conventional or advanced) are often ineffective for remov­ing PFAS.[2] US EPA’s Drinking Water Treatability Database[10] indicates that granular activated carbon (GAC), ion exchange (IX), some novel adsorptive media and membrane separation can achieve >99 percent removal of several types of PFAS (with the most exten­sive testing for PFOA and PFOS). Adsorption using GAC or IX has emerged as the most widely used approach to control PFAS.[9] For removal of long-chain PFAS, GAC can be effective; however, it offers poorer removal of shorter-chain analogues and its lack of selectivity can inhibit PFAS removal when organics or anionic competitors are present. Compared to GAC, Murray et al. (2021)[11] found that IX provided more cost-effective adsorption of long- and short-chain PFAS.[9] In particular, IX performance exceeds GAC for removal of short-chained PFAS.[1],[9] Accordingly, IX is a promising technology for effective PFAS removal based on its effectiveness, ease of operation, large adsorption capacity, small footprint and potential for regeneration and reuse.[1]

Strong base anion exchange resins are composed of a neutral, hydrophobic copolymer backbone with positively charged ex­change sites loaded with counterions (e.g., chloride).[7] The primary mechanisms of PFAS removal using IX are electrostatic inter­actions and hydrophobic effects, whereas GAC adsorption relies on hydrophobic interactions (such that higher hydrophobicity and molecular weight PFAS are increasingly amenable to GAC adsorption).[1],[9],[11] The hydrophobic perflourinated PFAS tail can adsorb to the hydrophobic IX backbone and/or resin cross linkers, while the hydrophilic, charged PFAS head is electrostatically attracted to the resin’s IX site, causing it to replace the pre-loaded Cl- counterion, as illustrated in Figure 2.[7]

In their comprehensive study of 75 PFAS and 13 IX materials plus GAC, Fang et al. (2021)[9] found that anion exchange resins pro­vided significantly greater PFAS removal compared to cation exchange resins, non-ionic resins and GAC, regardless of the PFAS’ [predicted] charge. The relative order of PFAS removal using anion exchangers was anionic > zwitterionic > cationic. Notably, tailored, PFAS-selective resins exhibit superior PFAS removal compared to conventional IX materials.[1],[9] With increasing PFAS chain length and net negative charge, PFAS removal also increased.[9] Longer-chain perfluoroalkyl acids (PFAAs) are charac­terized by lower solubility and stronger hydrophobic interactions and therefore, more efficient removal. Additionally, long-chain perfluoroalkyl sulfonic acids (PFSAs) tend to exhibit more efficient removal compared to long-chain perfluoroalkyl carboxylic acids (PFCAs) based on IX resins’ substantially higher adsorption capacity of sulfonates relative to carboxylates.[11]

Beyond the character of the PFAS itself, a number of IX resin properties can impact the rate of PFAS removal, including polymer composition (polystyrene versus polyacrylic), functional group (quaternary amine, tertiary amine, etc.) and pore structure due to the extent of cross-linking (macroporous versus gel).[1],[2] Greater PFAS affinity has been observed using polystyrene resins with complex amino functional groups compared to polyacrylic resins with quaternary or tertiary amine functional groups, indicating that hydrophobic composition (e.g., polystyrene) is most effective.[1],[9]

Macroporous IX materials generally have higher PFAS adsorption capacity compared to gel IX materials. For microporous IX resins, high molecular weight organics, e.g., humics, can impede PFAS removal by physically blocking pores and thus excluding internal exchange sites. Moreover, aggregation of the PFAS itself, e.g., as micelles (or hemimicelles, as pictured in Figure 2) can impact IX efficiency. Such structures may not be able to access pores on microporous gel material, thereby decreasing PFAS removal. Additional parameters such as the ratio of resin to PFAS, presence of other ionic competitors, material porosity, water temperature and distribution of resin bead diameter can also impact PFAS removal kinetics. Sulfate, phosphate and nitrite are the most competitive inorganic ions for PFAS; fulvic acid and other smaller molecular weight organics can also compete for IX sites.[1]

Reusability of the adsorbent is another consideration for PFAS treatment. Single-use IX is more frequently used versus multi-use regenerable resins given the simplicity of single use operation in small systems.[1] Specifically, regenerable resins often require longer contact times for PFAS adsorption and must then be regen­erated using various salts and/or cosolvent solutions (salts facilitate desorption of the charged head, whereas organic cosolvents facilitate desorption of the hydrophobic tail).[1],[7],[9] Regeneration also necessitates subsequent handling of the PFAS-concentrated brine solution, e.g., using electrochemical treatment,[12],[13] whereas single use systems use incineration, thermal destruction or sulfate-based advanced oxidation to dispose of the spent resin.1 Resin manufacturing often carries the highest life cycle environmental burden, highlighting the benefits of reusable IX materials for efficient PFAS removal by, literally, sticking it to forever chemicals.[1]

References

  1. Dixit F, Dutta R, Barbeau B, Berube P, Mohseni M. “PFAS re­moval by ion exchange resins: A review.” Chemosphere. 2021;272:129777. doi:10.1016/j.chemosphere.2021.129777
  2. Rahman MF, Peldszus S anderson WB. “Behaviour and fate of perfluoroalkyl and polyfluoroalkyl substances (PFASs) in drinking water treatment: A review.” Water Res. 2014;50:318-340. doi:10.1016/j.watres.2013.10.045
  3. US EPA. Research on Per- and Polyfluoroalkyl Substances (PFAS). Published 2021. Accessed December 11, 2021. https://www.epa.gov/chemical-research/research-and-poly­fluoroalkyl-substances-pfas
  4. Li F, Duan J, Tian S, et al. “Short-chain per- and polyfluo­roalkyl substances in aquatic systems: Occurrence, impacts and treatment.” Chem Eng J. 2020;380(June 2019). doi:10.1016/j.cej.2019.122506
  5. US EPA. Fifth Unregulated Contaminant Monitoring Rule. Monitoring Unregulated Drinking Water Contaminants. Published 2021. Accessed December 11, 2021. https://www.epa.gov/dwucmr/fifth-unregulated-contami­nant-monitoring-rule
  6. Buck RC, Franklin J, Berger U, et al. “Perfluoroalkyl and polyfluoroalkyl substances in the environment: Terminol­ogy, classification and origins.” Integr Environ Assess Manag. 2011;7(4):513-541. doi:10.1002/ieam.258
  7. Woodard S, Berry J, Newman B. “Ion exchange resin for PFAS removal and pilot test comparison to GAC.” Remedia­tion. 2017;27(3):19-27. doi:10.1002/rem.21515
  8. Barzen-Hanson KA, Roberts SC, Choyke S, et al. “Discovery of 40 classes of per- and polyfluoroalkyl substances in his­torical aqueous Film-forming foams (AFFFs) and AFFF-im­pacted groundwater.” Environ Sci Technol. 2017;51(4):2047-2057. doi:10.1021/acs.est.6b058439.
  9. Fang Y, Ellis A, Choi YJ, et al. “Removal of per- and polyfluo­roalkyl substances (PFASs) in aqueous film-forming foam (AFFF) using ion-exchange and nonionic resins.” Environ Sci Technol. 2021;55(8):5001-5011. doi:10.1021/acs.est.1c00769
  10. US EPA. Welcome to the Drinking Water Treatability Data­base. Accessed December 11, 2021. https://tdb.epa.gov/tdb/home/
  11. Murray CC, Marshall RE, Liu CJ, Vatankhah H, Bellona CL. “PFAS treatment with granular activated carbon and ion exchange resin: Comparing chain length, empty bed con­tact time and cost.” J Water Process Eng. 2021;44(July):102342. doi:10.1016/j.jwpe.2021.102342
  12. Ryan DR, Mayer BK, Baldus C, Wang Y, Mcnamara PJ. “Elec­trochemical technologies for per‐ and polyfluoroalkyl sub­stances mitigation in drinking water and water treatment residuals.” Water Sci. 2021;3(5):1-23.
  13. McBeath ST, Serrano Mora A, Asadi Zeidabadi F, et al. “Pro­gress and prospect of anodic oxidation for the remediation of perfluoroalkyl and polyfluoroalkyl substances in water and wastewater using diamond electrodes.” Curr Opin Electrochem. 2021;30(October):100865. doi:10.1016/j.coelec.2021.100865

About the author
Dr. Brooke K. Mayer is an Associate Professor in the Department of Civil, Construction and Environmental Engineering as part of the Opus College of Engineering at Marquette University. She holds Bachelors, Masters and doctorate degrees in civil engineering with an emphasis in environmental engineering from Arizona State University. She is a registered Professional Engineer in the state of Arizona.

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