Coconut shell activated carbon engineered for triphasic adsorption and multimechanistic removal of emerging...

A novel substitute for perfluorooctane sulfonate (PFOS), 6:2 Chlorinated polyfluoroalkyl ether acid (6:2 Cl-PFAES, F-53B), has been widely used in the electroplating and firefighting industries, raising concerns due to increasing environmental prevalence. This study systematically investigated the adsorption characteristics of F-53B by coconut shell activated carbon (CSAC) and elucidated the underlying mechanisms through adsorption kinetics, isotherm modeling, and multi-scale characterization. Results show that the maximum adsorption capacity of CSAC for F-53B reached 261.64 mg/g, with the process best described by the pseudo-second-order kinetic model (R² > 0.97) and the Langmuir isotherm (R² > 0.94). Intraparticle diffusion modeling revealed a three-stage adsorption sequence: surface adsorption (0-2 h), pore diffusion (2-8 h), and dynamic equilibrium (8-48 h). Solution pH influenced adsorption efficiency by regulating surface charge. Under acidic conditions (pH < point of zero charge, pHzpc = 4.49), strong electrostatic interactions between protonated CSAC surfaces and anionic F-53B resulted in an 85% removal efficiency. In contrast, alkaline conditions (pH > 7) led to a 70% reduction in adsorption efficiency (p < 0.05) due to electric repulsion. The microporous-dominated structure of CSAC (specific surface area: 857.69 m²/g; pore volume: 0.34 cm³/g) facilitated synergistic adsorption mechanisms. Oxygen-containing functional groups (C-O, -OH) promoted hydrogen bonding and hydrophobic interactions, enhancing F-53B adsorption. Besides, CSAC is optimized for emergency scenarios, it achieves 99.9% removal efficiency for 1 mg/L F-53B within 8 h at room temperature, with a maximum capacity of 261.64 mg/g; also, it maintains > 85% removal efficiency under wide pH conditions and tolerates coexisting ions (e.g., Cl^-, SO₄^2-, Ca²⁺).This study highlights the multi-mechanistic synergy of CSAC in theremoval of F-53B, providing theoretical foundations and technical insights for biochar-based remediation of emerging contaminants.