Microporous Chitosan–Bentonite Beads as Reusable Adsorbents for Orange II Removal

The adsorption and desorption behavior of the azo dye Orange II (OII) was investigated using composite beads prepared from shrimp shell–derived chitosan (50 wt%) and montmorillonite-rich clay. The structural and morphological properties of the synthesized beads were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and FT-IR (Fourier Transform Infrared Spectroscopy). Batch adsorption experiments were performed to evaluate the removal efficiency of OII from aqueous solutions under various conditions, revealing that a low adsorbent dosage (0.5 g L−1) and an acidic medium (pH 4) provided optimal adsorption performance. Adsorption kinetics and equilibrium isotherms were analyzed to elucidate the adsorption mechanism. Thermodynamic parameters indicated that the adsorption process was spontaneous (ΔG° < 0) and endothermic (ΔH° > 0). Equilibrium data were fitted to both Langmuir and Freundlich isotherm models, with the Freundlich model providing the best correlation (R2 = 0.99), suggesting multilayer adsorption on a heterogeneous surface. The adsorption capacity increased significantly with temperature, rising from 98.35 mg g−1 at 298 K to 182.57 mg g−1 at 318 K, further confirming the endothermic nature of the process. Kinetic analysis revealed relatively rapid adsorption, with maximum adsorption capacities increasing from approximately 100 mg g−1 at 25 °C to 123 mg g−1 at 45 °C. Regeneration and reusability tests demonstrated that the composite beads could be reused through adsorption–desorption cycles; however, a gradual decline in removal efficiency was observed, decreasing from 97% in the first cycle to 25% after the fifth cycle. This decrease is likely associated with partial structural degradation or the detachment of bead components during repeated regeneration. Overall, the results highlight the potential of chitosan–clay composite beads as promising and sustainable adsorbents for the removal of azo dyes from aqueous media.