Passive sampling devices for environmental water testing reduce the time and volumes required for sampling and analysis, thereby decreasing monitoring costs. Measurement of per- and polyfluoroalkyl substances (PFAS) using passive samplers is challenging due to the chemical diversity of analytes. Research (e.g., SERDP Project ER18-1300) has demonstrated that non-ion-exchange polymeric adsorbents (commercially available as Osorb®) have excellent adsorption capacity for tested PFAS, and therefore may be useful for passive samplers. The objective of this project was to design, calibrate, and field-pilot a small, easy-to-use passive sampler that would measure a wide variety of PFAS. Specific objectives were to:

• Develop passive samplers that yield representative spatial and temporal interrogation of site chemicals
• Establish physical-chemical properties of passive sampler media
• Develop and field-calibrate passive sampling methods applicable to a wide variety of environmental waters
• Develop analytical methods for the passive sampler compatible with existing PFAS methods (e.g., Environmental Protection Agency [EPA] 537.1, EPA 1633) using liquid chromatography–mass spectrometry to facilitate adoption by commercial laboratories

Several passive sampler prototypes (both equilibrium and integrative) were developed and tested in the laboratory, and an integrative passive sampler design was carried forward to field testing. Modified Osorb® resin was developed and tailored to possess high capacity for PFAS via the addition of amine groups as a weak ion-exchange resin in combination with copper ion to increase charge density. The hydrophobic resin allows interactions with longer chain PFAS while short-chain PFAS bind via ion-ion interactions. The design incorporates a membrane-free system where sorbent resin is in direct contact with water to maximize uptake rates. The adsorbent is placed behind mesh in a 2.5 cm wide x 4.5 cm long x 0.2 cm thick polyethylene housing, sized for many applications.

Sampler response was evaluated across a range of water chemistries in laboratory testing and results demonstrated high sampling rates (R_s) for PFAS that were largely unaffected by ionic strength, pH, temperature, oxidation-reduction potential, and humic acid content. Flow rate and temperature were found to have the largest effect on R_s and were accounted for in calculations. For quality control, laboratory analytical methods incorporated isotopically labeled surrogate binding prior to extraction for use with isotopic dilution methods. Field testing was performed in four study areas: groundwater (Colorado), surface water (South Dakota), the Santa Ana River basin (California), and the Ohio River. Passive samplers were deployed at 33 sample locations, measured for 19 PFAS, and compared to grab samples. Correlations were observed (typically within 2X difference) between aqueous PFAS concentrations measured by passive versus discrete grab samples across over five orders of magnitude in concentration (0.5-150,000 ng/L). Overall relative percent difference between grab and passive results were a median of 18% and detection limits were ~1 ng/L. An additional field deployment was performed in conjunction with the EPA in Minnesota testing surface water and sediment. Although slightly higher variability in field duplicates was noted for sediment porewater samples than for surface water, the results were promising and the sediment sampler will be further tested as part of ESTCP project ER23-7696.

The integrative nature of the passive sampler is advantageous for long-term monitoring of average concentrations over 3 to 45-day timeframes. The Sentinel™ passive sampler may also assist in sampling of stormwater systems with sporadic flows and variable chemistry. Sediment pore water applications are also being explored. Overall, the Sentinel™ passive sampler combines the practicality of design and analysis with broad accuracy of measurement, resulting in a flexible tool for sampling a variety of dynamic aqueous environments. The Sentinel™ PFAS passive sampler is now being manufactured commercially by Aquanex Technologies, LLC. (Project Completion - 2024)