Theme: 07. Per- and Polyfluorinated Alkyl Substances (PFAS)

Topic Area: 7f. In Situ PFAS Treatment Approaches

Formatted Title:

Adsorption of Per- and Polyfluoroalkyl Substances onto Ion Exchange Resins for In Situ Sequestration

Background/Objectives:

Ion exchange resins (IXRs) are commonly used as an adsorbent for dissolved minerals and natural organic matter to soften and purify water. Recently, investigations of IXRs for per- and polyfluoroalkyl substances (PFAS) adsorption have demonstrated their potential as a superior alternative to other adsorbents, such as activated carbon, for the removal of PFAS from drinking water and groundwater. IXRs are typically split into two broad categories: strong base and weak base, and further categorized based on intrinsic properties, such as functional group, cross-linking, and polymeric matrix, with these properties playing a role in adsorption capacity and selectivity for particular contaminants. The objective of this study was to determine properties of IXRs that influence adsorption of both long- and short-chain PFAS, and evaluate their use as injectable amendments for in situ treatment of PFAS-impacted groundwater.

Approach/Activities:

Batch adsorption experiments were conducted with four IXRs, each with different combinations of functional group, cross-linking, and polymeric matrix. For each IXR, adsorption isotherms were generated for single-solute perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA), and for a mixture of six compounds (PFOA, PFOS, perfluorohexanesulfonic acid [PFHxS], perfluoroheptanoic acid [PFHpA], perfluorononanoic acid [PFNA], and perfluorobutanesulfonic acid [PFBS]) to determine competitive adsorption effects. The best performing IXR was then selected for column experiments that were designed to simulate in situ delivery of polymer-stabilized IXR into aquifer materials and assess the viability of the resulting permeable adsorptive barrier for the sequestration of long- and short-chain PFAS. Synthetic groundwater containing either a mixture of PFOA and PFOS or the six PFAS compounds was injected, and effluent samples were collected continuously to monitor for PFAS breakthrough. After terminating the column experiments, the solid phase was destructively sampled to measure the retained PFAS distribution along the IXR-treated aquifer material. All PFAS analyses were performed using a Waters Acquity H-Class ultra-performance liquid chromatograph (UPLC) equipped with a Waters BEH C-18 column with an eluent gradient of ammonium acetate in water and methanol. Quantification of PFAS was achieved using a Waters Xevo tandem quadrupole mass spectrometer (TQ-S Micro) equipped with a Unispray ionization source.

Results/Lessons Learned:

Adsorption isotherm results for PFOS, PFOA, PFHxS, PFHpA, PFNA, and PFBS on the four different IXRs were fit to a Freundlich adsorption isotherm using non-linear regression procedures. The resulting data were correlated to determine major IXR properties that lead to the greatest adsorption of both long- and short-chain PFAS. The resulting data were also compared to activated carbons (i.e., Darco 100) to compare adsorption effectiveness. In situ application of the top performing IXR was demonstrated in column adsorption experiments to represent the potential for field application of an IXR permeable adsorptive barrier for more effective removal of PFAS from groundwater in situ.

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Alexis Meservey