Eggshell waste as a valuable resource for toxic metals remediation

About 8 million tons of eggshell waste are annually produced worldwide. Most of this material is disposed at landfills with odor production and microbial growth, while only occasionally it is used as fertilizer, soil conditioner or additive for animal feed. However, as eggshell is composed of calcium carbonate (94%), calcium phosphate (1%), magnesium carbonate (1%) and organic matter (4%), the conversion of this waste to new useful products is of high economic and environmental interest. In this study, hydroxyapatite was synthesized from eggshell waste by using a simple and inexpensive protocol and applied for Co2+ and Ni2+ removal from aqueous solutions. The effects of contact time and initial metal concentration were investigated in batch experiments. Eggshell-derived hydroxyapatite (ESHA) before and after the adsorption experiments was characterized by X-ray diffraction and scanning electron microscopy. The Co2+, Ni2+ and Ca2+ concentrations were analyzed by inductively coupled plasma atomic emission spectrometry. The adsorption process was rapid and reached equilibrium within 80 min. The kinetics of the adsorption process was studied applying pseudo-first-order, pseudo-second-order and intraparticle diffusion kinetics models. Numerous isotherm models were applied to represent the experimental data. The metal removal efficiency was in the range 70-80% which is generally higher than other waste-derived adsorbents. After metal ions adsorption the starting ESHA particles of sheet or flake-like structure were modified showing a smooth and homogeneous surface morphology. These morphological changes suggest dissolution-precipitation phenomena play an important role in the metal removal mechanism. The removal of metal ions from the solution is accompanied by the increase of Ca2+ concentrations in the solutions suggesting that the ion exchange mechanism is partly involved in the adsorption process. The overall mechanism for Co2+ and Ni2+ removal by ESHA involves rapid surface adsorption of metal ions on the ESHA surface, ion exchange with Ca2+ of ESHA and partial dissolution/precipitation resulting in the formation of a metal-bearing phosphate. The results of this study showed that the conversion of eggshell waste to a low-cost adsorbent for the treatment of metal contaminated waters could contribute to the environmentally friendly disposal of this waste.