Kinetic Modeling, Equilibrium Behavior and Adsorption Mechanisms of Cd(II) on Lemon‐Activated Carbon From Phragmites australis

ABSTRACT Cadmium contamination poses a critical threat to environmental and human health, while the invasive spread of Phragmites australis disrupts native ecosystems worldwide. This study converts P. australis biomass into sustainable lemon‐activated carbon (SPLAC) using natural lemon extract as a renewable green activating agent instead of conventional corrosive chemicals. SPLAC prepared at 700°C exhibited abundant oxygen‐containing functional groups (–COOH, –OH, and C═O), a mesoporous structure, and a BET surface area of 76.69 m 2 ·g − 1 . Comparison with non‐activated carbon (SP700) confirmed that lemon activation significantly enhanced Cd(II) removal (91.3% vs. 76.0%) and Langmuir adsorption capacity (61.24 vs. 43.47 mg·g − 1 ). Under the optimum conditions (initial pH 6, 55°C, and 90 min), SPLAC achieved 91.3% Cd(II) removal. Adsorption followed the Langmuir isotherm (Qmax ═ 61.24 mg·g − 1 , R 2 ═ 0.9942) and pseudo‐second‐order kinetics ( R 2 > 0.99), while the Weber–Morris model revealed a three‐stage diffusion process involving film diffusion, intraparticle diffusion, and equilibrium adsorption. Combined FTIR, point‐of‐zero‐charge (pHpzc ═ 7.92), ionic‐strength, diffusion, and thermodynamic analyses demonstrated that Cd(II) uptake is governed predominantly by inner‐sphere surface complexation, supported by ion exchange, whereas electrostatic interactions play only a secondary role. Thermodynamic analysis confirmed that the adsorption process was spontaneous (ΔG° < 0), endothermic (ΔH° ═ +12.29 kJ·mol − 1 ), and entropy‐driven (ΔS° > 0). SPLAC maintained a Cd(II) removal efficiency of 74.0 ± 1.3% after three regeneration cycles, with only 2.3% mass loss. These findings demonstrate that lemon‐activated carbon derived from invasive biomass provides a regenerable, low‐cost, and environmentally sustainable adsorbent for Cd(II) removal, supporting both circular‐economy principles and green chemistry