Study of Adsorption Characteristics of Co(II) on Eggshell Residue

A comparative study was conducted to evaluate the adsorption performance of cobalt(II) ions using untreated chicken eggshell powder (ES) and sulfuric acid-treated eggshell (EST). X-ray diffraction (XRD) analysis revealed characteristic 2θ values of 29.578° for ES and 29.483° for EST, corresponding to CaCO3. Following Co(II) adsorption, a reduction in crystal spacing and an increase in d-spacing were observed, suggesting enhanced repulsion between native Ca(II) and adsorbed Co(II) ions, along with reduced crystal density and increased bond lengths. Scanning electron microscopy (SEM) images revealed irregular particle morphologies and the formation of spherical or ovoid clusters following adsorption. Adsorption equilibrium was reached within 90 minutes. The Co(II) removal efficiency improved with increasing adsorbent dosage for both ES and EST. Surface modification with H2SO4 and NaHCO3 expanded the effective pH range for Co(II) removal from 2–4 (ES) to 2–8 (EST), attributed to a shift in the point of zero charge (pHpzc) toward weakly basic values. Thermodynamic analyses confirmed that the adsorption process was endothermic and governed by physical interactions. Adsorption isotherms were evaluated using Freundlich, Langmuir, and Temkin models, with the results indicating that the Freundlich and Temkin models provided the best fit to the experimental data.