Removal of the heavy metal cadmium (Cd) from aqueous solution by sulfur-doped graphitic carbon nitride nanosheets: a study of adsorption mechanism, kinetics, and thermodynamics

Among environmental pollutants, cadmium is considered a highly dangerous metal ion due to its high toxicity to humans and the environment, and its removal from contaminated water is crucial. In this research, sulfur-doped graphitic carbon nitride (S/g-C3N4) nanosorbent with a very high adsorption capacity (710.65 m²/g) was employed for the removal of cadmium metal ions. Various characterization methods, including X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), and BET analysis, were used to charactrization the S/g-C3N4 nanosheets. The highest removal percentage (39%) was obtained at pH = 5, with an adsorbent amount of 0.5 grams per liter of solution and an initial concentration of 10 mg/L. Optimal parameters included a stirring time of 45 minutes and an initial concentration of 10 mg/L. Isotherm studies revealed that the Langmuir isotherm showed better agreement with the data, indicating monolayer adsorption. Also, Kinetic studies demonstrated that the pseudo-second-order kinetic equation better matched the experimental data. To investigate the effect of temperature on cadmium ion adsorption, thermodynamic parameters such as standard Gibbs free energy change ( ), standard enthalpy change ( ), and standard entropy change ( ) were evaluated. The positive values indicated that the adsorption process of cadmium ions on the adsorbent surface is endothermic. Furthermore, the negative values at all studied temperatures suggest that the adsorption process is spontaneous, and higher temperatures are more favorable.