The X-ray diffraction (XRD) charact

The X-ray diffraction (XRD) characterization of the CaO catalyst was performed on a Rigaku (MiniFlex II, England) based generator X-ray diffractometer using CuKα radiation over a 2θ range from 20° to 80° with a step size of 0.04° at a scanning speed of 3°/min. The elemental chemical compositions of the samples were analyzed by X-ray fluorescence spectroscopy (XRF - Oxford, ED-2000, England) under energy dispersive mode for precise measurement of both light and heavy elements. The microstructures of the calcined eggshells were observed by a scanning electron microscope (SEM). The SEM images of the representative sample were obtained from a Camscan-MX 2000 (England) equipped with an energy dispersive spectroscope (EDS).