Microwave-assisted hydrothermal synthesis of hydroxyapatite from eggshell: Evaluation of initial changing pH levels, energy consumption, and economic variables

This study optimizes the microwave-assisted hydrothermal synthesis of hydroxyapatite (HA) using eggshells as a sustainable biogenic precursor, offering a low-cost and energy-intensive method for widespread use in the biomedical industry. The study aimed to systematically analyze the impact of initial solution pH on HA yield, phase identity, and its morphology, as well as the environmental, economic, and energy efficiency of the process. An initial pH of 9 provided optimal control over HA precipitation, achieving a total mass yield of 96%. Characterization of the resulting HA powder revealed that at pH 9, the precipitate contained the highest HA content (87.77%wt) with minimal impurities. Additionally, the crystallite size was at a minimum of 8.27 nm, and the agglomerate morphology was more regular and spherical at this pH. A comprehensive energy usage assessment showed that the microwave-assisted method consumes significantly less energy in the HA synthesis phase (12.4 kWh/kg HA) compared to the conventional hydrothermal method (52.5 kWh/kg HA). Techno-economic assessment confirms the financial viability of the proposed 428 kg/year pilot-scale plant, featuring an Internal Rate of Return (IRR) of 30.4% and a payback period of approximately 5.16 years. Environmental impact assessment using LCA methodology suggested that low direct emission could be obtained using this synthesis route. This study goes beyond the material-centric focus of previous research, offering a comprehensive assessment that confirms the technical and financial potential of this sustainable synthesis method.