A novel composite of chitosan and Bacillus subtilis exopolysaccharide for the removal of methylene blue from aqueous solutions.

Dye pollution in water poses serious health and ecological risks, requiring wastewater treatment before discharge and prompting increased research attention due to the widespread use of dyes in various industries. This study investigates the biosorption of methylene blue (MB) using a novel composite of chitosan and Bacillus subtilis exopolysaccharides (EPS). Fourier-transform infrared spectroscopy (FTIR) analysis confirmed the presence of essential functional groups for dye adsorption. The biosorption process was pH-dependent, with optimal removal efficiencies at pH 6 for the chitosan/EPS composite and pH 7 for chitosan alone, showing increased adsorption capacity with rising pH from 3.0 to 7.0. Contact time experiments demonstrated efficient MB removal in approximately 30 min, achieving decolorization rates of 71.6% for the composite and 60.62% for chitosan. The composite also demonstrated a higher maximum biosorption capacity (14.26 mg g−1) than chitosan (13.70 mg g−1) according to the Langmuir isotherm model, which best described the monolayer adsorption process. Kinetic analyses revealed that the pseudo-second order model best described the adsorption process, with calculated capacities closely matching experimental values (41.67 mg g−1 for chitosan and 45.87 mg g−1 for the composite). Post-adsorption FTIR results revealed involvement of –OH, –NH, –COO−, and –PO₄³− groups via electrostatic, hydrogen-bonding, and π–π interactions, confirming the interaction between MB and functional groups on the biosorbents. These findings highlight the synergistic effect of combining chitosan with EPS, resulting in a promising, efficient, and rapidly equilibrating biosorbent for mitigating dye pollution in wastewater. This work presents the first reported use of Bacillus subtilis exopolysaccharide combined with chitosan as a fully biodegradable composite biosorbent, exhibiting superior adsorption capacity (14.26 mg g−1), markedly faster kinetics (K₂ = 0.176 g mg−¹ min−¹), and equilibrium attainment within 30 min compared to pristine chitosan and most reported biopolymer-based adsorbents. Optimizing biosorption conditions can enhance dye removal efficiencies and contribute to sustainable environmental management practices.