Ultrasonic-Assisted Synthesis of Hydroxyapatite from Crustacean Waste: Effects of Amplitude and H₃PO₄ Concentration

Background: This study investigates the synthesis of hydroxyapatite (HAp) from blue crab (Portunus pelagicus) shells using ultrasonic-assisted chemical precipitation, emphasizing the effects of sonication amplitude and H₃PO₄ concentration on HAp properties. Crab shells, rich in calcium (93.1% Ca content), were calcined at 1000 °C to produce CaO, serving as the calcium source. HAp was synthesized by reacting CaO with H₃PO₄ (0.5–1.4 mol L⁻¹) under various sonication amplitudes (0–100%). Results: The results confirm that all samples conform to the JCPDS standard no. 09-0432, validating the formation of crystalline HAp. Higher ultrasonication amplitudes significantly enhanced crystallinity and reduced both crystal and particle sizes due to intense cavitation effects. In contrast, increasing H₃PO₄ concentrations led to a decrease in crystallinity, attributed to reduced pH and calcium availability during synthesis, although higher phosphate concentrations promoted particle agglomeration. Optimal results characterized by small crystal size (ca 30.72 nm), high crystallinity (70%) and uniform morphology were obtained using a 1.1 mol L⁻¹ H₃PO₄ solution and 100% ultrasonication amplitude. Conclusion: This study demonstrates that by precisely controlling H₃PO₄ concentration and ultrasonication amplitude, HAp with high crystallinity, uniform morphology and confirmed phase purity can be reliably produced. Additionally, the use of crab shell waste as a raw material underscores its promise as a sustainable source for producing high-quality HAp suitable for applications in bone regeneration and implant technologies.