The primary goal of this proof-of-concept project is to improve the adsorption of hydrophilic short-chain per- and polyfluoroalkyl substances (PFAS) that are poorly removed by granular activated carbon (GAC) filtration. This project plans to utilize “hydrophobic ion-pairing (HIP)” as a practical pretreatment step to enhance the removal of short-chain PFAS without the need for any modification of GAC material. HIP is a concept by which an ion pair complex is made through the interaction of a hydrophilic ion of interest with a hydrophobic counter-ion, making the overall molecule hydrophobic. In the case of PFAS, this project plans to add a cationic HIP counterion to the influent of GAC systems, resulting in the formation of PFAS-HIP ion pairs with higher hydrophobicity that can be effectively removed by GACs. The specific objectives of this research are to evaluate the following:

• Selectivity and performance of biodegradable HIP additives with varying characteristics
• Impact of HIP performance on different GAC properties with respect to raw material, mesh number, pore size distribution, and surface charge
• Effect of water matrix on HIP performance including pH, ionic strength, and dissolved organic matter
• HIP performance to treat complex PFAS mixtures

The governing hypothesis of this study is that the short-chain PFAS form stable ion pairs with HIP counterions, and the resulting PFAS-HIP ion pairs feature greater hydrophobicity for enhanced sorption on GAC. There will be four interrelated tasks to test the hypothesis and to achieve the project objectives. Task 1 will identify the performance of various biodegradable HIP cations to improve the sorption of PFAS on GAC. Task 2 will evaluate the impact of carbon properties on HIP approach. Task 3 will focus on how varying water chemistry and quality will impact the stability of the PFAS-HIP ion pairs and their sorption on GAC. Controlled batch adsorption experiments using selected PFAS will be conducted for Tasks 1 and 2 that will allow the project team to optimize the HIP selection and dosing. In Task 3, the project team will optimize the dosing of HIP for flow through column experiments. Rapid small scale column testing will be employed to test two HIP dosing strategies that will inform on the scalability of the approach and process design for practical applications. In Task 4, the project team will perform a combination of batch and column experiments to study the HIP treatment of complex PFAS mixtures representing real world conditions. Both targeted and non-targeted PFAS will be analyzed to evaluate HIP performance to treat a wide suite of PFAS in samples of varying complexity.

Successful demonstration of this approach will address a critical research need in treating PFAS that are poorly removed by current treatment approaches. Advantages of the HIP process is its flexibility for practical applications: (i) the process can be easily introduced in existing GAC (and similar carbonaceous adsorbents) systems without the need for modifying the adsorbent prior to treatment, (ii) the dosing of HIP additive can be temporary or continuous depending on the water quality and target treatment goals, (iii) the dosing could be done in lag columns in a lead-lag GAC system, thus targeting only the poorly removed PFAS that breakthrough from the lead GAC column, and (iv) the approach can be combined with other technologies as a treatment train to remove ultra-short chain and short chain PFAS that are usually reaction byproducts of treatment. The approach will provide cost savings in the treatment of PFAS by extending carbon life and performance. The HIP approach has broader implications that can inform on several other remediation strategies that utilize hydrophobic interactions as a mechanism to remove PFAS, such as (i) in situ sequestration of PFAS in impacted soils, (ii) membrane filtration processes, (iii) PFAS interaction with flocs in coagulation/flocculation systems, and (iv) extractability of short-chain PFAS during foam fractionation. The successful execution of this research will lead to a new technology for site cleanup efforts, ultimately protecting the warfighter and installation communities. (Anticipated Completion Date - 2026)