Sustainable reuse of mineral waste: Synthesis and comprehensive characterization of hydroxyapatite (HAP)

The study explores an eco-efficient synthesis method for hydroxyapatite (HAP) from mineral residues rich in calcium carbonate (CaCO3) and monopotassium phosphate (KH2PO4). The process is based on a chemical reaction in an aqueous medium at ambient temperature, promoting the progressive precipitation of calcium phosphate phases. The pH evolution, monitored over 48 h, revealed the formation of transient intermediate phases such as brushite, prior to the appearance of the HAP phase. Multimodal characterization using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), thermal analysis (TGA-DTA), scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS), and BET surface area analysis confirmed the formation of a poorly crystalline, carbonated hydroxyapatite with a specific surface area reaching 81 m2/g and a Ca/P atomic ratio close to the ideal stoichiometry (1.67). The observed mesoporosity, the significant increase in specific surface area, and the low apparent density (1.24 g/cm3) indicate a favorable textural architecture. The proposed approach offers a promising, low environmental impact route for the valorization of calcium-rich residues into functional calcium phosphate materials, with potential applications in catalysis, environmental remediation, or biomedicine.