Surface functionalization of coconut-shell-derived activated carbon with amine and surfactant groups for selective short-chain PFAS removal

Abstract

Short-chain per- and polyfluoroalkyl substances (PFAS) pose an increasingly urgent water treatment challenge, as regulatory transitions away from long-chain compounds have accelerated the occurrence of short-chain substitutes that resist adsorption on conventional activated carbon. Here, we engineer the surface chemistry of commercially available coconut-derived activated carbon (CAC), a renewable, highly microporous sorbent (BET surface area: 836.94 m2 g−1) through amine (ethylenediamine, EDA) and cationic surfactant (cetyltrimethylammonium bromide, CTAB) functionalization to systematically overcome this limitation. Adsorption performance was evaluated for four PFAS of perfluorobutanoic acid (PFBA), perfluorohexanoic acid (PFHxA), perfluorononanoic acid (PFNA), and perfluorooctanesulfonamide (PFOSA) spanning short and long chain lengths across environmentally relevant concentrations (1–16 µg L−1), quantified by LC-MS/MS following EPA Method 1633. Pristine CAC achieved >85% removal for long-chain PFAS (PFOSA, PFNA), driven by hydrophobic partitioning and micropore filling, but showed markedly limited affinity for short-chain compounds (PFBA, PFHxA). Surface functionalization fundamentally redirected adsorption mechanisms: EDA–CAC, despite a 20% reduction in BET surface area to 671.02 m2 g−1 substantially enhanced PFBA and PFNA uptake through protonated amine-mediated electrostatic attraction and hydrogen bonding with anionic headgroups. CTAB–CAC preserved high surface area (822.30 m2 g−1) while introducing hydrophobic alkyl domains and permanently charged quaternary ammonium sites that synergistically improved PFHxA and PFOSA removal through hydrophobic chain–chain interactions and electrostatic attraction. Across all systems, adsorption generally followed Freundlich behavior more closely than Langmuir behavior (higher R2) across most, though not all, systems, consistent with energetically heterogeneous, sub-saturation interactions on functionalized carbon surfaces. For pristine CAC, removal efficiency followed the order PFOSA > PFNA > PFHxA > PFBA, reflecting chain-length-dependent hydrophobicity; among the surface-modified sorbents, CTAB–CAC achieved the greatest enhancement for PFOSA and PFHxA, while EDA–CAC achieved the greatest enhancement for PFNA and PFBA, substantially closing this gap for short-chain PFAS. The findings established a rational interfacial engineering of biomass-derived carbons rather than textural optimization alone is a scalable and mechanistically grounded strategy for next-generation PFAS sorbents targeting short-chain contamination in drinking-water systems.