Hierarchical Microporous Miscanthus-Derived Activated Carbon Enables Entropy-Driven High-Efficiency Dye Removal

Abstract

Thermochemically treated lignocellulosic biomass represents a sustainable route to high-performance activated carbons for water purification. In this study, a miscanthus-derived activated carbon (ACM) was synthesized via one-step high-temperature KOH activation and evaluated for methylene blue (MB) removal from aqueous solutions. ACM exhibits a high specific surface area (SSABET = 1290 m²/g), dominant microporosity, and a hierarchical pore structure, enabling rapid dye diffusion. Adsorption kinetics follow a pseudo-second-order model, while intraparticle diffusion analysis reveals a two-stage mass-transfer mechanism. The weak pH dependence suggests that electrostatic interactions are not the sole controlling factor. Equilibrium data are best described by the Redlich-Peterson isotherm, indicating heterogeneous surfaces and mixed adsorption behavior. The maximum adsorption capacity increases from 410.2 to 463.8 mg/g as temperature rises from 298 to 318 K, confirming endothermic adsorption. Thermodynamic parameters indicate spontaneous, entropy-driven adsorption. FTIR analysis shows that MB uptake is predominantly governed by strong non-covalent interactions, including π–π stacking and hydrogen bonding with ACM surface functionalities, with partial contribution from pore filling rather than classical chemisorption. Reusability tests demonstrate an 80.7% removal efficiency after three adsorption–desorption cycles without detectable mass loss, highlighting ACM as a cost-effective and sustainable sorbent for wastewater treatment