Sustainable Synthesis of Biogenic Hydroxyapatite from Eggshells: Effect of Processing Conditions and Comparative...

Although eggshell-derived calcium sources have been widely explored for hydroxyapatite (HAp) synthesis, the minimum processing conditions required to obtain crystalline and high phase-purity HAp under comparable wet-chemical routes remain insufficiently defined. In this study, six wet-chemical synthesis methodologies were systematically compared using CaO derived from chicken eggshells as the calcium source, while maintaining equivalent chemical conditions and varying only key processing parameters, particularly the presence or absence of moderate heating. The synthesized materials were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction with Rietveld refinement (XRD), and scanning electron microscopy. Quantitative XRD analysis revealed that nonheated routes resulted in incomplete conversion, with the persistence of intermediate calcium phosphate phases, whereas methodologies employing moderate heating promoted enhanced crystallinity and phase purity. Among the investigated routes, Methodology 6, conducted at 70 °C, exhibited the most advanced structural development, yielding predominantly hydroxyapatite, with approximately 88% hexagonal HAp and 11% monoclinic HAp, nanometric crystallite sizes (≈12-33 nm), and minimal residual Ca-(OH)₂.