Physical and Chemical Characterization, Adsorption Kinetics, Thermodynamic Analysis, and the Mechanism Involved in the Removal of Methylene Blue Dye by a Biosorbent from Pecan Nutshells

The search for low-cost and sustainable adsorbents for dye removal has driven the valorization of agro-industrial residues. In this study, pecan nutshells (PNSs) were evaluated as a biosorbent for methylene blue (MB) removal. The material was prepared by drying, grinding, and alkaline treatment with NaOH, which led to an increase in the apparent surface area (0.974 m²·g^-1) and pore volume (0.0014 cm³·g^-1). Surface characterization revealed the involvement of hydroxyl, carboxyl, and aromatic functional groups in the interaction with the dye molecules. The adsorption of MB by PNSs exhibited high efficiency, even at low dosages, and optimal performance was observed at 1.5 g·L^-1. The process remained essentially pH-independent across the 4-10 range. Equilibrium data were analyzed using the Langmuir, Freundlich, Dubinin-Radushkevich, and Sips isotherm models. Among them, the Sips model provided the best fit to the experimental data, yielding a maximum adsorption capacity of 317.5 mg·g^-1 at 25 °C, which is consistent with a heterogeneous surface presenting finite saturation. The thermodynamic parameters (Δ<i>H</i>° = +12.32 kJ·mol^-1; Δ<i>G</i>° = -13.43 kJ·mol^-1 at 25 °C; Δ<i>S</i>° = +86.27 J·mol^-1·K^-1) indicate that the adsorption process is spontaneous and endothermic, with an enthalpy change characteristic of weak to moderate interactions. Kinetic data were best described by the pseudo-second-order model, with equilibrium reached within approximately 180 min. Intraparticle diffusion analysis revealed a multistep adsorption involving an initial boundary-layer diffusion followed by a slower pore diffusion. Overall, the adsorption mechanism is interpreted as multifactorial, governed by hydrogen bonding, π-π stacking, and van der Waals forces, consistent with a physisorption-dominated process strengthened by structural heterogeneity. These results demonstrate that PNS combines high capacity with operational simplicity, representing a scalable and sustainable alternative for the treatment of dye-contaminated wastewater.