Adsorption of cadmium ions from aqueous solutions using sesame as a low-cost biosorbent: kinetics and equilibrium studies

Sesame leaf and stem, as a kind of useless agricultural waste, was used as a sort of low-cost biosorbent for the removal of cadmium ions from aqueous solutions in batch and continuous modes. The biomass was characterized by Fourier transform infrared spectroscopy, scanning electron microscope and energy-dispersive X-ray analysis before and after adsorption. There were hydroxyl group, carboxyl group, etc. on the surface of the adsorbent from Fourier transform infrared spectroscopy. A batch study was employed as a function of the initial concentration, adsorbent particle size, adsorbent dose, contact time and the pH of the solution. Maximum sorption was found to occur at an initial pH of 6. The equilibrium process was satisfactorily described by the Langmuir isotherm model with the maximum sorption capacity of 84.74 mg g. Kinetic studies also indicated that both pseudo-first-order and pseudo-second-order kinetic models (with R values of 0.9929 and 0.9952, respectively) were able to describe the process. A continuous study was carried out, and the maximum uptake of cadmium ions in a fixed-bed adsorption column was found to be 22.88 mg g, where the initial concentration of cadmium ions, bed height, flow rate and pH were 60 mg L, 2 cm, 2.5 mL min and 5.5, respectively.