Transformation of Biogenic Calcium Carbonate from Waste Eggshells into Hydroxyapatite: A Comparative Study on Crystallographic Characteristics

Abstract

Hydroxyapatite (HAp) is a calcium phosphate–based biomaterial that has been widely applied in biomedical fields due to its excellent bioactivity and biocompatibility. In this study, HAp was synthesized using a wet chemical precipitation method by utilizing eggshell waste as a calcium source, aiming to develop a sustainable and cost-effective synthesis route. The synthesized products were investigated under two different conditions, namely without calcination and with calcination at 900 °C, in order to evaluate the effect of thermal treatment on the structural and microstructural characteristics of HAp. Phase formation and crystal structure were analyzed using X-ray diffraction (XRD), while surface morphology and elemental composition were examined using scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). XRD results confirmed the formation of hydroxyapatite with a hexagonal crystal structure, and calcination treatment led to sharper diffraction peaks, indicating an improvement in crystallinity. SEM observations revealed that the synthesized HAp particles exhibited irregular shapes with a tendency to agglomerate, which is commonly observed in HAp produced via wet chemical precipitation. EDS analysis verified the presence of the main constituent elements, Ca, P, and O, with a Ca/P atomic ratio of 1.67, which is in good agreement with the theoretical stoichiometric value of hydroxyapatite. Crystallite size was evaluated using several XRD-based methods, including the Scherrer, Monshi-Scherrer, Williamson-Hall, Size-Strain Plot, and Halder-Wagner approaches. The calculated crystallite sizes were found to be within the nanometer scale, with variations depending on the assumptions and principles of each method. Overall, the results demonstrate that wet chemical precipitation using eggshell-derived calcium is an effective approach for producing hydroxyapatite with favorable structural and microstructural properties.