The presence of per- and polyfluoroalkyl substances (PFAS) in water partially stems from the use of aqueous film-forming foam (AFFF) at locations around the country. Improved field sampling methodologies are critical to improving management of these impacted sites. The benefits of passive samplers include not only improved sample collection, handling, and analysis protocols that provide inherent PFAS enrichment and sample cleanup, but also enhanced understanding of PFAS occurrence, fate, and transport through calculation of time-averaged PFAS concentrations. We envision that passive samplers would be part of a multi-pronged strategy for PFAS monitoring at sites of concern.

The overall goal of this project is to develop new strategies for passive sampling of chemically-diverse PFAS with ion-exchange membranes. The research was conducted in two phases. Phase I focused on (i) evaluating the potential for ion-exchange membranes and fibers as passive samplers for short- and long-chain PFAS with different head groups, (ii) identifying the uptake mechanism for PFAS in select ion-exchange membranes, and (iii) confirming passive sampler performance in select water quality conditions. Phase I was successful and provided a strong foundation for further development, validation, and testing of the ion-exchange membrane-based passive samplers in PFAS-impacted waters.

The main goal of Phase II is to develop and validate a universal calibration for PFAS in the ion-exchange membrane-based passive samplers. This goal will be achieved through the following specific research tasks: 

• Task 1: Characterize the effects of water quality parameters on the uptake of 29 anionic PFAS by anion-exchange membranes through laboratory studies conducted with variable solution pH, salt type and concentration, temperature, and dissolved organic matter (DOM). 
• Task 2: Ensure that the passive samplers provide consistent performance for single- and multi-sorbate scenarios involving a large suite of 29 anionic PFAS of concern. 
• Task 3: Confirm that the approach can be extended to 2 cationic and 2 zwitterionic PFAS using cation-exchange membranes and evaluate the preferred uptake mechanism, sorption kinetics, and selectivity coefficients. 
• Task 4: Validate the calibration, resistance to biofouling, required deployment time, and time-averaging period of passive sampler prototypes in laboratory-based, flow-through reactors with PFAS spiked waters. 
• Task 5: Deploy and validate the passive samplers in real waters (using the flow-through reactor) collected from at least six DoD facilities to justify adoption of the technique at PFAS-impacted sites.

The main component of the samplers is the ion-exchange membrane. In Phase I, 10 commercial ion-exchange membranes were evaluated with variable chemistry, ion-exchange capacity, and thickness, among other properties. That research mostly involved measurement of the adsorption isotherms, calculation of selectivity coefficients, and membrane performance in different water quality conditions; however, the ease of PFAS desorption and analysis was also considered to ensure effective deployment of the passive samplers. The aggregate results suggested that FAD-PET-75 was the most promising ion-exchange membrane for further consideration in Phase II activities.

The initial tasks in Phase II will focus on developing a universal calibration for up to 29 PFAS in the FAD-PET-75 membrane. In particular, first principles-based corrections will be developed to enable accurate calculation of the selectivity coefficients of PFAS anions over chloride for solutions with variable pH, salt type and concentration, temperature, and DOM content. A similar approach will be taken for cationic and zwitterionic PFAS using cation-exchange membranes with similar attributes as FAD-PET-75. The later tasks in Phase II involve validating the universal calibration and overall performance of the passive samplers in real, PFAS-impacted water from DoD facilities.

The field-ready passive samplers will involve placing the FAD-PET-75 membrane into a stainless-steel ring (protective housing) with copper mesh screens on both sides to limit potential biofouling. The samplers will be deployed for 1-3 weeks before being retrieved. Then, the membranes will be removed from the sampling device, placed into a methanolic ammonium acetate solution, which will dissolve the membranes and release the adsorbed PFAS. The extracts will undergo cleanup and analysis for PFAS of concern. The measured concentrations will be used with the universal calibration (selectivity coefficients) to back-calculate the average PFAS concentrations in the water source.

Phase I Results

Preliminary results from Phase I are available in the webinar and the Phase I Final Report. Some of the key findings from Phase I were as follows: 

• Commercially-available anion-exchange membranes were successfully evaluated as passive samplers for PFAS. Fairly rapid uptake was achieved under well-mixed (within 2 days) and static (within 2 weeks) conditions.
• Ion exchange was identified as the primary uptake mechanism for PFAS, enabling back-calculation of aqueous-phase PFAS concentrations with the selectivity coefficient. The magnitude of selectivity coefficients increased with PFAS chain length until C10 and then decreased. We hypothesized that this phenomenon was caused by (i) hydrophobic interactions between PFAS tails and the base polymer of the ion-exchange membranes and (ii) interference of the aforementioned reactions by fixed positive charges in the membrane for longer-chain PFAS.
• The best combination of capacity, selectivity, and quantitation (desorption) was achieved with the FAD-PET-75 membrane, which will be further evaluated in Phase II.
• The selectivity coefficients were robust for all PFAS in the presence of background anions and DOM. Ionic strength effects were corrected using Setschenow constants. Comprehensive relationships between water quality parameters and selectivity coefficients will be further developed in Phase II.
• The FAD-PET-75 membranes can be dissolved in methanol for quick and easy PFAS analysis (with high extraction efficiency).
• Bench- and prototype-scale passive samplers were successfully deployed in real water sources.

Given the increased importance of PFAS to ongoing cleanup and remediation efforts at DoD facilities, new strategies are required to enable monitoring of PFAS. This project will develop, evaluate, and test innovative ion exchange-based materials for passive sampling of PFAS. Unlike other PFAS techniques, which will be adversely affected by the wide-ranging chemical properties of short-chain, long-chain, and substituted PFAS, the IEM and IEF materials developed here will offer robust solutions for the full range of PFAS of interest. The results of this project will contribute new scientific understanding to the use of ion exchange passive samplers, which may be useful for other DoD-relevant contaminants, and PFAS monitoring in various environmental matrices. These combined benefits will assist DoD with management of PFAS-contaminated sites. (Anticipated Phase II Completion - 2026)