Adsorption-desorption of Zn2+ions using hydroxyapatite and recovery of zinc by electrochemical precipitation method into deep eutectic solvent

Hydroxyapatite has a nanostructure, the chemical formula of Ca10(PO4)6(OH)2 and a specific surface area of 91.42 m2/g. In this paper, hydroxyapatite is used to adsorb Zn2+ ions as effective adsorbent. The influence of some factors on Zn2+ adsorption capacity and efficiency has been studied. Under the suitable adsorption conditions were hydroxyapatite mass of 0.15 g in 50 mL solution, 50 mg L-1 initial Zn2+ concentration, pH 5.0, exposure time of 60 min at room temperature (25 degrees C), Zn2+ adsorption efficiency and capacity reached 94.79 % and 15.80 mg g-1, respectively. The process of desorption of Zn2+ from the loaded adsorbent and recovery of metallic Zn was also investigated. Zinc recovery efficiency reached 93.26 % under appropriate conditions such as applied current intensity of 7.5 mA, electrolysis time of 10 h, loaded hydroxyapatite mass of 0.1 g, temperature 60 degrees C. After desorption process, the adsorbent was regenerated for further studies. The adsorption isotherm was studied based on three Langmuir, Freundlich and Redlich-Peterson models. The adsorption thermodynamics were investigated and calculating the parameters. The adsorption kinetics were examined using two pseudo-firstorder and pseudo-second-order kinetic models. The negative value of Gibb's free energy change showed that the adsorption process was feasible and spontaneous in nature. The positive value of enthalpy change indicated that the process was endothermic. The positive value of entropy change showed increased randomness at the solid and solution interface.