Optimization of the degree of deacetylation of chitosan beads for efficient anionic dye adsorption: kinetics, thermodynamics, mechanistic insights via DFT analysis, and regeneration performance

Congo red, a persistent dye widely used in the textile industry, poses significant environmental hazards if not properly treated. In this study, the effectiveness of chitosan beads for removing Congo red from textile wastewater was investigated. A Box-Behnken design was utilized to optimize the degree of deacetylation (DDA) of the chitosan beads, achieving a maximum DDA of 95.79% under the optimal conditions of 100 °C, 300 min reaction time, and 45.91% NaOH concentration. Comprehensive characterization of the synthesized adsorbent was performed using FT-IR, XRD, SEM, and BET analysis, with a BET surface area of 11.5180 m²/g, indicating a substantial surface area for effective adsorption. The adsorption process followed pseudo-second-order kinetics and was best described by the Langmuir model. At pH 6, an adsorbent dose of 0.06 g, and an optimal reaction time of 80 min, a maximum adsorption capacity of 110.37 mg/g was achieved, surpassing the performance of magnetic chitosan (40.12 mg/g) and powdered chitosan (42.48 mg/g). Thermodynamic parameters (ΔH° = 10.91 kJ/mol and ΔG° < 0) indicate that the adsorption process was endothermic and spontaneous. DFT calculations were conducted to elucidate the adsorption mechanism, focusing on the role of benzene rings and oxygen atoms in Congo red as electron donors. These findings demonstrate that chitosan beads are a promising material for the removal of Congo red from contaminated wastewater.