Hydroxyapatite as an Effective Adsorbent for Cu²⁺ and Ni²⁺: Study on Adsorption and Metal Recovery by Electrochemical Deposition in Deep Eutectic Solvent

Heavy metal pollution is a global environmental challenge. Removal of heavy metal ions by recyclable absorbents and metal recovery by environmentally friendly methods are of high importance. In this study, hydroxyapatite (HAp) was synthesized and employed for the removal of copper (Cu²⁺) and nickel (Ni²⁺) ions from aqueous solutions. The adsorption behavior of both metal ions was systematically investigated under various conditions, including changes in pH, contact time, initial concentration, and adsorbent dosage. The adsorption capacity and efficiency for Cu²⁺ were measured at 49.53 mg.g^-1 and 99.06%, respectively, while for Ni²⁺, the values were 12.15 mg.g^-1 and 80.99%. Adsorption isotherm analysis indicated a good fitting with Redlich-Peterson models, while kinetic studies confirmed that the process followed the pseudo-second-order models, suggesting chemisorption involvement. Furthermore, desorption of metal ions and recovery of elemental Cu and Ni were conducted via electrodeposition in a choline chloride-urea (reline) deep eutectic solvent. Under optimal electrolysis conditions (current density of 7.5 mA.cm^-2, 10-hour duration, 0.1 g metal-loaded HAp, and 60°C), recovery efficiencies reached 94.63% for copper and 94.80% for nickel. The regenerated HAp retained significant adsorption performance, with capacities of 39.25 mg.g^-1 for Cu²⁺ and 11.18 mg.g^-1 for Ni²⁺, corresponding to removal efficiencies of 78.49% and 74.60%, respectively. Synthesized HAp could be potentially an effective, recyclable adsorbent for the treatment and recovery of heavy metals from wastewater.