Adsorptive Removal of Yellow Dye Using Natural and Magnetic Bentonite: Isotherm, Thermodynamic and Mechanistic Insights

Abstract

The adsorption performance of natural bentonite (B) and magnetically modified bentonite (MB) for the removal of an anionic yellow dye (YD) from aqueous solution was comparatively investigated using a batch adsorption system. Magnetic bentonite was synthesized via the co-precipitation of Fe²⁺ and Fe³⁺ ions onto the bentonite surface, and the structural modifications were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The characterization results confirmed the successful incorporation of Fe₃O₄ onto the bentonite surface while preserving the fundamental layered structure of bentonite and creating additional adsorption-active surface features. The effects of solution pH, adsorbent dosage, initial dye concentration, and temperature were systematically investigated to determine the optimum adsorption conditions. Adsorption equilibrium was evaluated using the Langmuir, Freundlich, Dubinin–Radushkevich (D–R), and Temkin isotherm models. Among the investigated models, the Langmuir isotherm provided the best fit to the experimental data, with correlation coefficients ranging from0.989 to 0.998for natural bentonite and0.985 to 0.999for magnetic bentonite, indicating predominantly monolayer adsorption. The maximum Langmuir adsorption capacities obtained at an adsorbent dosage of 0.1 g were333.3 mg g⁻¹for natural bentonite and312.5 mg g⁻¹for magnetic bentonite. The calculated separation factor (RL) values (0 < RL < 1) confirmed that adsorption was favorable under all investigated conditions. Thermodynamic analysis demonstrated that the adsorption process wasspontaneous. (ΔG < 0)andexothermic (ΔH < 0)for both adsorbents. Magnetic modification altered the surface characteristics of bentonite, promoted favorable adsorption behavior under different operating conditions, and enabled rapid magnetic separation after adsorption. Overall, both natural and magnetic bentonites exhibited effective adsorption performance, indicating their potential as economical and environmentally friendly adsorbents for the treatment of dye-contaminated wastewater.