A low-cost adsorbent deriving from eggshell modified with Mg(II)-doped hydroxyapatite for removal of Cd2+ from water.

The industrial by-product cadmium can pollute the environment and be harmful to humans after being discharged from wastewater. In this study, a low-cost adsorbent deriving from kitchen waste eggshells (BC), was modified by magnesium-doped hydroxyapatite in order to form a new adsorbent (MHBC) capable of removing Cd²⁺ from water. When compared to BC, the adsorption capacity of MHBC was found to be greatly improved, while their equilibrium adsorption capacities were 4.82 and 49.41 mg·g^–1, respectively. Kinetic, isothermal, and environmental factors’ analyses suggested that chemisorption played a major role in the Cd²⁺ adsorption process delivered by MHBC. Particularly, chemical complexation, ion exchange, and electrostatic interaction could be the main mechanisms involved. Both the pseudo-second-order kinetics equation and the Sips isotherm model were found to fit the adsorption process well. Sips model revealed that the maximum adsorption capacity of MHBC at 25°C was 68.898 mg·g^–1. Acidic conditions and the presence of Mg²⁺, Pb²⁺, Cu²⁺, and Zn²⁺ were found to reduce the adsorption capacity of MHBC for Cd²⁺. After three cycles, the removal percentage of Cd²⁺ by MHBC was still able to reach 78.75%. Therefore, our study suggests that MHBC is a highly efficient, green, and sustainable Cd²⁺-removing adsorbent. [ABSTRACT FROM AUTHOR]