Low-temperature transformation of mechanochemically treated oyster shells into nanocrystalline apatites

A technique combining an extended mechanochemical treatment of biogenic calcium carbonate (bCC) with a one-pot hydrothermal method was used for the first time to prepare nanocrystalline apatite. When calcitic bCC from oyster shell waste was subjected to dry milling for 1 hour (DM) the crystallite size of calcite was decreased from 92 to 14 nm, and the minimum temperature to achieve the complete conversion to apatite (Tmin) decreased from 160 °C to 80 °C. In contrast, wet milling (18 h) induced polymorphism and amorphization, yielding calcite, aragonite, and amorphous calcium carbonate, with crystallite sizes of 7 nm for calcite and 13.7 nm for aragonite. The Tmin decreased from 160 °C to 40 °C. Both transformations occurred via brushite as an intermediate metastable phase. Kinetic experiments evidenced that DM-bCC transformed faster than WM-bCC at Tmin, achieving 98% versus 82% after 4 days, even though the complete transformation took 7 days. Both bCCs and the derived Ap nanoparticles demonstrated cytocompatibility with MS1 endothelial cells and m17.1 ASC murine mesenchymal stem cells. This synthetic approach offers a cost-effective, eco-friendly (without releasing CO2), sustainable, and scalable (by using already established glass reactor technology rather than costly autoclaves) solution for valorising shells waste.