Metal removal using chemically modified eggshells: Preparation, characterization, and statistical analysis

In this study, a novel adsorbent was prepared from eggshells using inexpensive single-step modification method. Chemical surface modification of eggshell samples that were collected from local homes was investigated using three modifying agents, namely, NaOH, HNO₃ and KMnO₄ at different concentrations and contact times. The produced adsorbents were investigated for lead removal from aqueous solution. The obtained results on removal efficiencies were statistically analyzed with the response surface method. The obtained results revealed that samples modified with KMnO₄ showed high affinities for lead ions, when compared with those modified with NaOH and HNO₃. The statistical analysis demonstrated that the modifying agent has a significant role in the modification process. Poor statistical models were developed for samples modified with NaOH and HNO₃. However, a reliable model was developed for samples modified with KMnO₄, which was adequately used to predict the responses. Modified and unmodified eggshell samples were characterized using standard methods that included X-ray diffraction, Fourier transform infrared, scanning electron microscope/energy dispersive spectrometer. The characterization outcome indicated that manganese oxides were successfully loaded on the eggshell surfaces. Moreover, the porous structure of the modified eggshell was noticeably enhanced and the BET surface area of the modified eggshell at the best conditions was found to be 90 m²/g. The obtained results showed that eggshell waste could be used efficiently as a promising low-cost adsorbent for metal removal using simple direct modification process. Effects of operational parameters showed that lead removal efficiency increased with pH and adsorbent dosage, while it was decreased with increasing the initial concentration. The experimental data at the equilibrium was best fitted to Langmuir isotherm with a maximum adsorption capacity of 700 mg/g and removal efficiency of 98%. Kinetic studies indicated that the pseudo-second-order model best fitted the experimental data. The thermodynamic study revealed that the free energy, ΔG, was found to be negative, which indicates that the adsorption was spontaneous and favorable. Furthermore, the positive value of enthalpy, ΔH, (55 kJ/mole) indicated that the adsorption was endothermic. Chemisorption mechanism of lead removal using K-ES was confirmed since the activation energy, Ea, which was found to be more than 40 kJ/mole.