Influence of calcination temperature on equine bone hydroxyapatite structure and lead adsorption efficiency

Lead (Pb²⁺) contamination in aquatic environments represents a serious global threat due to its toxicity, persistence, and non-biodegradable nature. In this study, hydroxyapatite (HA) was synthesized from equine scapula bone and investigated as a low-cost, sustainable adsorbent for Pb²⁺ removal from aqueous media. HA was thermally treated at 100 °C, 500 °C, and 900 °C to assess the effect of calcination on physicochemical and adsorption properties. Characterization techniques including XRD, FTIR, SEM/EDX, TGA and XRF confirmed structural and compositional evolution with temperature. Among the samples, HA-500 exhibited the highest adsorption performance, achieving 99% Pb²⁺ removal and a maximum capacity of 50 mg/g under optimal conditions (0.25 g dose, 20 mg/L initial Pb²⁺ concentration). Kinetic studies followed a pseudo-second-order model (R² > 0.999), and equilibrium data were best described by the Freundlich isotherm (R² = 0.9839), indicating multilayer adsorption on heterogeneous surfaces. Thermodynamic analysis revealed that Pb²⁺ adsorption is spontaneous and exothermic, with negative Gibbs free energy values and positive entropy changes, confirming increased randomness at the solid-liquid interface. Furthermore, regeneration studies showed that HA-500 retained 73% of its efficiency after three cycles. These findings demonstrate the potential of equine bone as a novel bio-based source of hydroxyapatite for efficient and sustainable heavy metal remediation.