Chitosan/clay adsorbent for dye removal: Statistical mechanics and thermodynamic insights

The efficient removal of dyes from aqueous effluents still lacks studies that explore the adsorption mechanisms involved, especially in systems with oppositely charged dyes. This study investigates the equilibrium adsorption behavior of Methylene Blue and Acid Red 27 dyes on a chitosan/clay composite from a statistical mechanics perspective, aiming to identify differences in the removal mechanisms. Five statistical mechanics models were evaluated to identify differences in the interaction mechanisms between the adsorbent and the dyes. The models consider different numbers of layers and adsorption energies. The monolayer model with a single energy best described the experimental data for both dyes. The maximum experimental adsorption capacities were 513.98 mg g⁻¹ for Methylene Blue (pH 10) and 349.29 mg g⁻¹ for Acid Red 27 (pH 2) at 298 K and 250 min. The number of molecules adsorbed per site was greater than 1, indicating multimolecular mechanisms. The process was favored at low temperatures and characterized as physical adsorption, with adsorption energies lower than 40 kJ mol⁻¹, indicating that the mechanism occurs via electrostatic interaction and hydrogen bonds. Although they present similar removal mechanisms, the composite demonstrated a greater affinity for methylene blue, evidenced by its lower half-saturation concentration (4.79 mg L⁻¹) and higher adsorption energy (28 kJ mol⁻¹), compared to Acid red 27 (8.01 mg L⁻¹ and 22 kJ mol⁻¹, respectively). The results show the selectivity of the composite for cationic dyes and demonstrate the usefulness of statistical mechanics as a robust tool for interpreting adsorption mechanisms in hybrid adsorbents.