Synthesis of carbon nano-onion embedded metal–organic frameworks as an efficient adsorbent for cadmium ions: kinetic and thermodynamic studies

Carbon nano-onions (CNOs), metal–organic frameworks (MOF-199), and carbon nano-onion embedded metal–organic frameworks (CMOF-199) were synthesized from garlic peels as a green source of carbon atoms while MOF-199 was prepared by solvothermal interaction between 1,3,5-benzenetricarboxylic acid and copper nitrate trihydrate. All the prepared solid materials were characterized by nitrogen adsorption/desorption isotherm, transmission electron microscopy (TEM), energy-dispersive X-ray (EDX), X-ray diffraction (XRD), thermogravimetric analysis (TGA), point of zero charge (pH_PZC), and Fourier transform infrared spectroscopy (FTIR). Adsorption of cadmium ions from aqueous solution was investigated onto all prepared solid materials considering different application conditions such as adsorbent dosage, pH, contact time, initial concentration of Cd⁺², and temperature. Adsorption of Cd⁺² was investigated by Langmuir, Freundlich, Temkin, and Dubinin–Radhushkevich adsorption isotherm models. Maximum adsorption capacity (113.3 mg g⁻¹) was achieved by CMOF-199 at 40 °C. The adsorption of Cd⁺² obeys pseudo-second-order kinetic model. Thermodynamic studies confirmed that the adsorption process is spontaneous, favorable, endothermic, and physisorption. Adsorption results proved that carbon nano-onion embedded metal–organic frameworks are promising solid adsorbents for cadmium ion adsorption.