This work focuses on the removal of a synthetic textile dye trisodium ferric complex of N-methyl-1,8-naphtalimide-4-sulfonate from contaminated aqueous solutions using two types of biosorbents: industrial-grade chitosan and iron-modified chitosan. The objective of the study was to assess and compare the adsorption performance of both materials under various experimental conditions, with particular attention to their physicochemical characteristics and adsorption behavior.

The adsorbents were characterized using Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and pH at the point of zero charge (pHpzc), confirming structural differences induced by iron incorporation. Batch adsorption experiments were conducted to evaluate the effect of several parameters including contact time, initial pH, and agitation speed on dye removal efficiency.

The results revealed a remarkable enhancement in adsorption capacity for the iron-chitosan composite, achieving up to 96% removal efficiency, with equilibrium attained within 120 minutes for both materials. Kinetic studies showed better fitting with a pseudo-second-order model, while adsorption isotherms were best described by the Freundlich model, suggesting a multilayer adsorption on a heterogeneous surface.

A significant pH-dependent behavior was observed: industrial chitosan performed optimally under alkaline conditions, whereas the iron-modified form showed superior efficiency in acidic media. Additionally, a moderate agitation speed was identified as optimal, balancing diffusion kinetics and surface contact efficiency.

These findings underline the promising potential of iron-functionalized chitosan as an effective and sustainable material for the treatment of dye-laden industrial wastewater, offering a low-cost, environmentally friendly alternative to conventional adsorbents.