Azo dye removal from polluted water is vital from a sustainable viewpoint. In this study, we investigated the influence of chitosan molecular weight on the adsorption of basic blue 41. Nanocomposite (NC(M)) was prepared using magnetite nanoparticles modified by amino-silica, medium-molecular weight chitosan, diethylenetriaminepentaacetic acid and graphene oxide. FT-IR, XRD, FESEM, TEM, EDX, elemental mapping, TGA/DTG, VSM and N2 adsorption/desorption isotherm were applied to characterize NC(M). Its effectiveness for the dye adsorption was compared with the adsorbent having lowmolecular weight chitosan (NC(L)) (Asadabadi 2021). In this regard, the effect of initial pH, temperature and adsorbent concentration on the dye removal was investigated by Central Composite Design. The maximum percentage of dye removal was 31% for NC(M) at optimum conditions. An increase in the monomer number of chitosan caused to increase in the contact angle and decrease in the hydrophilic property of NC(M). Thus, it had a very low dye adsorption. However, NC(L) led to dye removal of 95%. It was proposed that NC(L) adsorbed dye by coulumbic attraction, π-stacking interactions and H-bonding. The NC(L) kinetics data were satisfactorily fitted by the modified pseudo-n-order model. About 30 min necessitated reaching the equilibrium and the rate-limiting steps changed from the film diffusion to intra-particle diffusion as time passed. The modified Langmuir–Freundlich isotherm was the best model to reproduce equilibrium data and the maximum adsorption capacity was equal to 55.87 mg·g−1. Besides, NC(L) was recovered seven times without dramatic changes in its adsorption efficiency.
