# 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

## TL;DR

This study explores using pecan nutshells as a low-cost, sustainable biosorbent to remove methylene blue dye from water.

## Contribution

The study introduces a novel method for preparing and characterizing pecan nutshells as an effective biosorbent for dye removal.

## Key findings

- Pecan nutshells showed a maximum adsorption capacity of 317.5 mg·g–1 for methylene blue at 25 °C.
- Adsorption was found to be spontaneous, endothermic, and governed by physisorption mechanisms.
- The process reached equilibrium within 180 minutes and was effective across a wide pH range.

## Abstract

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
(ΔH° = +12.32 kJ·mol–1; ΔG° = −13.43 kJ·mol–1 at 25 °C; ΔS° =
+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.