Performance of N-functionalized activated carbons for enhanced removal of short- and ultrashort-chain PFAS

Short- and ultrashort-chain per- and polyfluoroalkyl substances (PFAS) pose persistent challenges for stormwater management due to their poor removal by conventional systems. This study developed two nitrogen-functionalized activated carbons (ACs) by covalently grafting quaternary ammonium (QA) and pyridinium (Py⁺) groups onto a commercial AC (F400). Adsorption and transport behavior of eight PFAS (C1-C5) on the modified ACs were evaluated and benchmarked against pristine AC. A one-dimensional intraparticle pore-diffusion model was evaluated for its applicability to sorbents with additional electrostatic interactions. Sorption kinetics and capacity were strongly dependent on PFAS structure, with short-chain PFAS showing greater uptake than ultrashort-chain analogs and perfluoroalkane sulfonates (PFSAs) exhibiting more favorable adsorption than perfluorocarboxylates. Both modified ACs enhanced short-chain PFAS adsorption, particularly for PFSAs, and improved sorption of selected ultrashort-chain PFAS. QA-modified AC showed a 60 to 290-fold increase in sorption capacity for short-chain PFSAs relative to pristine AC at aqueous concentrations of 50-100 μgL^-1. Constant-flow column experiments using field-collected stormwater showed 11- to 81-fold delays in short-chain PFAS breakthrough and pronounced improvement for the ultrashort-chain sulfonate perfluoropropanesulfonic acid (PFPrS). The one-dimensional intraparticle pore-diffusion model successfully described PFAS-specific transport behavior and provided a framework for predicting filter lifetime using column-derived transport parameters. These findings demonstrate that nitrogen-functionalized carbons improve retention of short-chain and selected ultrashort-chain PFAS through cooperative electrostatic and apolar interactions, while providing a predictive framework for evaluating adsorbent performance and filter lifetime.