Hydrothermally Synthesized CoS ₂ Nanostructures for Efficient and Recyclable Adsorptive Removal of Methylene Blue Contributing to Water Depollution

The powder of cobalt disulfide (CoS₂) was successfully prepared using the hydrothermal method at the temperatures of 150, 180, and 210 °C for 24 h. The series of characterization techniques, including SEM, EDX, XRD, and FTIR, confirmed the formation of pure CoS₂ with a cubic crystal structure and an average crystallite size of about 14 nm, present in the form of spherical nanoparticles (NPs), whose diameters increased with increasing of the temperature. The adsorption activity of the CoS₂ NPs was evaluated by using methylene blue (MB) dye. The study showed that all samples of CoS₂ NPs are efficient adsorbents for MB under alkaline conditions (pH = 12). The kinetic study conducted on the most effective adsorbent obtained at 150 °C, representative of the synthesized CoS₂ NPs, demonstrated that the pseudo-second-order is the kinetic model that describes the adsorption phenomenon. The analysis of the adsorption isotherms revealed that the process adopts the Freundlich model with a high correlation coefficient (<i>R</i> ² = 0.99), indicating multilayer adsorption on a heterogeneous surface. The adsorption capacity increased with the temperature from 128.20 mg/g at 298 K to 222.22 mg·g^-1 at 318 K, suggesting an endothermic process. The recyclability tests demonstrated that CoS₂ NPs are stable and reusable adsorbents, showing a slight decrease in the MB removal efficiency, approximately 4%, after six successive reuses.