Recycling of eggshell waste into low-cost adsorbent for Ni removal from wastewater

The disposal of about 8 million ton/year of eggshell waste produced worldwide is an environmentally challenging issue. However, this waste can be converted to new materials useful for several industrial applications. In this study, hydroxyapatite was synthesized from eggshell waste and used as adsorbent for Ni2+ removal from aqueous solutions. Eggshell-derived hydroxyapatite was characterized using X-ray diffraction, scanning electron microscopy with the energy dispersion spectrum analysis and Fourier transform infrared spectroscopy. Batch experiments were conducted to investigate the effects of contact time and initial Ni2+ concentration on the adsorption process. Adsorption equilibrium was achieved within 80 min. The kinetics of the adsorption process was studied applying pseudo-first-order, pseudosecond-order and intraparticle diffusion kinetics models. The kinetics study showed that the pseudosecond-order kinetic model described well the adsorption process with high correlation coefficient. The maximum Ni2+ adsorption capacity was found to be 109 mg g-1 which is generally higher than other waste-derived adsorbents. The adsorption of Ni2+ on eggshell-derived hydroxyapatite was well fitted by the Langmuir equation as compared to Freundlich, Temkin and Dubinin-Radushkevic isotherm models. The removal mechanism of Ni2+ by eggshell-derived hydroxyapatite could involve ion exchange with Ca2+ and dissolution/precipitation of the starting hydroxyapatite resulting in the formation of a Ni-bearing phosphate. The results of the present study showed that hydroxyapatite synthesized from eggshell waste represents an excellent low-cost adsorbent for treatment of Ni2+ contaminated waters.