Several sites are addressing new regulations that are bringing greater scrutiny on the impacts from use of aqueous film-forming foams (AFFFs). These major changes in the regulatory side have created the need for accurate assessment of risk to humans and ecosystems from the presence of per- and polyfluorinated substances (PFAS). While equilibrium passive samplers are increasingly well established for neutral organics, no comparable device has achieved wide acceptance for measuring amphiphilic PFAS.
The overarching goal of this one-year project was to develop and test a range of functionalized polymeric thin films and polymer-inclusion membranes for use in equilibrium passive sampling of PFAS. The intent is to move away from a kinetic sampler like the polar organic chemical integrative sampler (POCIS) to an equilibrium passive sampler similar to the current state of the art for hydrophobic organic compounds.
The project team explored four broad classes of polymers: polydimethylsiloxane, agarose, cellulose acetate and cellulose triacetate, that were cast as the thin base film in the functionalized passive sampler construction. The project team used a range of sorbent amendments to allow equilibrium sorption of a wide range of PFAS. A range of polymer and polymer composites were prepared to test sorption capacity and linearity. A wide variety of polymers were synthesized and tested. The goal was to synthesize passive sampling materials having a range of properties.
The project team demonstrated for the first time that several composite polymers could be synthesized that remain stable in the water environment and provide adequate sorption of PFAS from aqueous solution at equilibrium. The performance of seven target PFAS was analyzed, including short and long-chain perfluorinated carboxylates, short and long-chain perfluorinated sulfonates, 8:2 fluorotelomer sulfonate, and n-methyl perfluorooctanesulfonamidoacetic acid. For all target PFAS, the uptake was rapid, and seven days appeared to be sufficient for loading from the aqueous medium. Uptake kinetics were modeled using a pseudo first-order model. PFAS desorption studies from the polymers over seven days showed good reversibility in deionized water, which suggests that using isotopically labeled PFAS as performance reference compounds is feasible.
PFAS concentrations ranging from 80 ng/L to 2000 ng/L were spiked for isotherm studies. Different isotherms were fitted to the data to estimate Kd values for each PFAS in each of the four classes of polymers. Estimated Kd values were mostly within the desired range of log Kd=1.5 to 3.
This effort sought to develop a sampler with the goal of enabling reproducible measurements of bioavailable PFAS and allowing improved risk assessment and management for PFAS-impacted sites by accurately measuring the freely dissolved concentration. The sampler would also enable time integrated measurements in sediments, surface water, and groundwater, and allow efficient determination of Cfree necessary for phase partitioning calculations. A successful PFAS equilibrium passive sampler can be integrated with the current practice of passive equilibrium sampling for organic compounds, not just in the device and deployment aspects, but also in the data interpretation and framework for use in risk assessment.
The project team believes that the initial results of this project showed great promise for pursuing the concept of equilibrium passive sampling for PFAS. Further work is needed to operationalize the passive sampling system explicitly designed to quantify the time-weighted average bioavailable PFAS concentration in a reproducible fashion. (Phase I Completion - 2022)