Enhanced Removal of Erythrosine B Dye Using Chemically Modified Chitosan Beads: A Comparative Evaluation

Erythrosine B (ER) dye is widely used and increasingly detected in wastewater, necessitating effective removal. This study compares chitosan beads (CB) and chemically crosslinked beads, namely chitosan–tripolyphosphate (CT) and chitosan–sulphite (CS), for ER removal via batch adsorption studies. Characterisation confirmed successful crosslinking of the modified beads. Under optimised conditions, CB, CT, and CS achieved removal efficiencies of 75.27%, 91.69%, and 98.73%, respectively, at an initial concentration of 100 mg/L within 50–60 min. Kinetic analysis suggested that the rate-controlling step was not solely governed by intraparticle diffusion but also involved physisorption and chemisorption. While the Langmuir isotherm adequately described the adsorption process of CB and CT, with maximum adsorption capacities of 71.80 mg/g and 89.33 mg/g, respectively, a better fit was observed for the Freundlich and Redlich–Peterson isotherms, indicating multilayer adsorption. In contrast, CS showed moderate agreement with all isotherms, suggesting a complex removal process. CS demonstrated the highest adsorption capacity, with 120.30 mg/g, highlighting sodium metabisulphite (SM) as a promising crosslinking agent for improved dye removal. Density functional theory (DFT) analysis proposed that at the molecular level, interactions between the ionised oxygenated groups of ER and protonated amine groups of chitosan facilitated the adsorption process.