Waste-derived hydroxyapatite as a functional adsorbent: mechanistic insights into metal ion uptake

Waste-derived hydroxyapatite (HAp) represents a sustainable functional material platform for investigating interfacial adsorption phenomena. In this study, HAp synthesized from bonefish waste via calcination was employed to examine the adsorption behavior and underlying mechanisms of divalent metal ions (Cu2+, Ni2+, and Co2+). Structural characterization using X-ray diffraction, Fourier-transform infrared spectroscopy, and electron microscopy confirmed the formation of crystalline HAp with well-defined morphology and surface features. Batch adsorption experiments conducted at pH 5.5 and ambient temperature revealed ion-specific performance, with removal efficiencies of approximately 62% for Cu2+, 31% for Ni2+, and 23% for Co2+ at an initial concentration of 50 mg L−1. Kinetic analysis followed a pseudo-second-order model, indicating surface-controlled adsorption processes. At the same time, equilibrium data were best described by the Langmuir model for Cu2+ and the Freundlich model for Ni2+ and Co2+, reflecting differences in surface homogeneity and interaction strength. Mechanistic interpretation indicates that Cu2+ adsorption proceeds via coupled Ca2+ ion exchange and inner-sphere complexation, suggesting strong interfacial binding and localized charge redistribution. In contrast, Ni2+ and Co2+ adsorption is governed by weaker, heterogeneous interactions consistent with outer-sphere complexation. These findings establish clear structure–property relationships and demonstrate the potential of waste-derived HAp as an efficient functional material for metal ion capture.