Adsorptive performance of green synthesized zinc oxide nanoparticles for the removal of cadmium (II) and lead (II) ions

This study has Zinc oxide (ZnO) nanoparticles (NPs) synthesized from the leaf extract of Costus Afers via a green approach. The synthesized zinc oxide nanoparticle (ZnO NPs) showed excellent adsorption capabilities towards Cd (II) and Pb (II) ions. ZnO NPs were characterized by Fourier Transform Infrared (FT-IR), Scanning Electron Microscopy (SEM), and X-ray Diffraction (XRD). The formation of ZnO NPs was confirmed by the absorption band at 825cm⁻¹ and 747cm⁻¹. The XRD and SEM analysis show the high purity and hexagonal structure of ZnO NPs with a crystallite size of 83.56 nm. A batch adsorption experiment was employed to examine the process parameters such as the effect of the initial pH of the solution, initial metal ion concentration, adsorbent mass, and contact time on the sequestration efficiency of Cd (II) and Pb (II) ions. The equilibrium isotherm models were studied using Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich isotherm models. The Langmuir adsorption isotherm provided the best fit for the adsorption process with coefficient of determination (R²) values of 0.988 and 0.986 for cadmium and lead (II) ions respectively, indicating a monolayer surface on ZnONPs. The pseudo-second-order kinetics model presented a good fit for the adsorption of Cd (II) and Pb (II) ions on zinc oxide nanoparticles giving a correlation coefficient of 0.989 for Cd and 0.996 for Pb suggesting that chemisorptions are the rate-determining step.