Rationally-designed organosilica adsorbents were synthesized to study the adsorption of per- and polyfluoroalkyl substances (PFAS) from water. The goals of this proof-of-concept project were to better understand mechanisms of adsorption and thus design optimal adsorbents to minimize costs of remediation. Swellable organically modified silica (SOMS) was used as a platform as it can be modified by use of different silane monomers and entrapped polymers. A unique feature of SOMS is the ability to volumetrically swell significantly, enlarging the microscale pore structure. Swelling enhances capacity and improves adsorption kinetics.
For this project, a step-wise approach was used. First, a diverse set of SOMS materials with fluoroalkyl groups and/or cationic groups added to a hydrophobic resin were synthesized. Materials were then screened for PFAS adsorption using kinetics and adsorption isotherms. Adsorbents with optimal performance were further studied using a series of bench-scale column experiments. Measurements were done in comparison to activated carbon and ion exchange resins currently used in water treatment. Finally, the best SOMS adsorbent was evaluated in a pilot test installed on a side stream at the Former Joint Reserve Base Naval Air Station Willow Grove. Reversibility of the resins also were tested.
Optimal PFAS adsorbents were found to be highly porous SOMS materials possessing hydrophobicity and cation groups. Cationic groups help to bind anionic PFAS. Data suggest that PFAS self-assemble into aggregates that enhance the adsorption of long-chain PFAS. SOMS is ideally suited to take advantage of PFAS self-aggregation since the resin has an expanded pore structure obtained via swelling. For purposes of evaluation, during the pilot test, poly-SOMS adsorbent developed in this study yielded an adsorption capacity of 2,600 μg/g of PFAS given an influent concentration of 40 μg/g total PFAS and a contact time of one minute.
SOMS-based PFAS adsorbents have improved capacity over current technologies. Scale-up of SOMS was accomplished during the study making the material an option for full scale PFAS remediation activities. SOMS-based resin has two benefits. First, it has higher capacity than ion exchange resin, especially in the presence of natural organic matter. Second, SOMS can be regenerated using a solvent rinse. Regeneration allows resins to be used and PFAS to be concentrated for residue management. The new adsorbent technology appears useful for economical remedial action to manage groundwater resources.
Stebel, E.K., K.A. Pike, H. Nguyen, H.A. Hartmann, M.J. Klonowski, M.G. Lawrence, M. Rachel, R.M. Collins, C.E. Hefner, and P.L. Edmiston. 2019. Absorption of Short-Chain to Long-Chain Perfluoroalkyl Substances Using Swellable Organically Modified Silica. Environmental Science: Water Research & Technology, 5:1854-1866.