In situ sol–gel synthesis of mesoporous HAP/PVA composites for Co(II) and Ni(II) adsorption

Industrial wastewater containing heavy metal ions such as cobalt Co(II) and nickel Ni(II) poses significant environmental and health risks. This study reports the synthesis and application of a Hydroxyapatite (HAp)/Polyvinyl alcohol (PVA) composite as an efficient adsorbent for Co(II) and Ni(II) removal. The composite was synthesized via an in situ sol–gel method using duck eggshell waste as a calcium precursor. An optimum PVA content of 20% ( HAp/PVA 20%) was identified. Structural and surface analyses (fourier transform infrared spectroscopy (FTIR), Brunauer–Emmett–Teller (BET), X-ray diffraction (XRD), and energy-dispersive X-ray spectroscopy (SEM-EDS)) confirmed the formation of a mesoporous composite with increased specific surface area, a crystalline structure consistent with ICSD #157481, and a more uniformly distributed surface morphology with reduced agglomeration. Adsorption experiments revealed optimal performance at pH 10, an initial concentration of 30 mg/L, a contact time of 120 min, and adsorbent dosages of 0.05 g for Co(II) and 0.025 g for Ni(II). Adsorption followed the Langmuir isotherm and pseudo second order kinetic models. Regeneration efficiencies of 62.41% for Co(II) and 41,63% for Ni(II) were achieved in the final cycle (5 th cycle). The adsorption mechanism involved electrostatic interactions, ion exchange, and pore filling. These results demonstrate the potential of HAp/PVA 20% as a sustainable and reusable adsorbent for heavy metal wastewater treatment.