Sustainable nitrogen-self-doped coffee ground-derived activated carbon for efficient adsorption of short- and long-chain PFAS: Mechanistic insights and practical applications

Per - and polyfluoroalkyl substances (PFAS), persistent and toxic pollutants, demand innovative, sustainable water treatment. This study introduces a steam-activated carbon from waste coffee grounds (CGAC) utilizing a chemical-free, nitrogen self-doping strategy for efficient PFAS removal. Raw coffee grounds, inherently rich in nitrogen (2.70 wt%), underwent pyrolysis and steam activation, yielding CGAC with high surface area (~1200 m 2 /g), hierarchical porosity, and retained nitrogen (1.76 wt%). Batch experiments showed that CGAC achieved nearly 100 % PFOS and > 75 % PFBS removal at a low dose of 0.025 g/L, outperforming commercial coconut and bamboo activated carbons, with adsorption capacities of 286.1 mg/g (PFOS) and 32.4 mg/g (PFBS). CGAC exhibited excellent reusability, maintaining >90 % PFOS removal over four ethanol regeneration cycles. Water matrix studies revealed that divalent cations (Ca 2+ , Mg 2+ ) enhanced PFAS uptake via cation bridging, while humic acid reduced adsorption through competitive interactions. CGAC maintained high PFOS selectivity even under anionic competition (Cl − , NO₃ − ), and competitive adsorption further confirmed suppressed short-chain PFBS uptake in the presence of PFOS. Mechanistic analyses showed that PFOS and PFBS follow fundamentally distinct adsorption pathways on the same surface: PFOS adsorption (Langmuir, pseudo-first-order, entropy-driven) was dominated by hydrophobic interactions facilitated by low desolvation energy and exhibited minimal pH dependence. Conversely, PFBS adsorption (Freundlich, pseudo-second-order, enthalpy-driven) relied on electrostatic attraction and hydrogen bonding, consistent with its pH sensitivity and cation-mediated enhancement. By explicitly demonstrating this dual-mechanism capability on a single waste-derived carbon surface, CGAC synergistically resolves the hydrophobic-electrostatic trade-off in PFAS adsorption. This leverages CG's intrinsic chemistry for a chemical-free, cost-effective, and scalable PFAS remediation solution, while simultaneously valorizing coffee waste.