Hierarchically porous activated carbon derived from Lansium domesticum peel via hydrothermal-H3PO4 activation for enhanced methylene blue removal: adsorption behavior, advanced modeling and mechanistic insights

Abstract

Dye-containing wastewater remains difficult to treat due to the persistence of synthetic dyes and the limitations of conventional adsorbents. In this study, a hierarchically porous activated carbon (ACLDP) was synthesized from Lansium domesticum peel via an integrated hydrothermal-H₃PO₄ activation strategy, yielding a mesopore-dominated structure enriched with oxygen-containing functional groups. Despite a moderate BET surface area (115.12 m² g⁻¹), the material exhibited a high adsorption capacity toward methylene blue (∼345.8 mg g⁻¹), suggesting that adsorption performance is influenced not only by surface area but also by pore accessibility and surface chemistry. A statistically rigorous model discrimination approach based on the Akaike Information Criterion (AIC), which remains less commonly applied in dye adsorption studies, was employed to evaluate nonlinear isotherm and kinetic models. The Sips model provided the best description of equilibrium data (R² = 0.9975, ΔAIC = 0), suggesting the relevance of surface heterogeneity in describing adsorption behavior. Kinetic analysis further indicated that no single model adequately captured the adsorption process, supporting a multi-step mechanism involving intraparticle diffusion and heterogeneous surface interactions. Mechanistic interpretation, supported by physicochemical characterization and adsorption behavior, suggests that electrostatic attraction, π–π interactions, hydrogen bonding, and pore diffusion collectively contribute to adsorption in a coupled and condition-dependent manner. These findings highlight that rational pore structure design and surface functionality can partially compensate for relatively low surface area, offering a viable strategy for converting agricultural residues into efficient carbon-based adsorbents. This study also provides a statistically supported framework for interpreting adsorption behavior, contributing to the development of sustainable materials aligned with circular economy principles.