<p>This study synthesized hydroxyapatite (HAP) from oyster shells using hydrothermal and mechanochemical methods for the efficient adsorption and removal of cadmium from water. Batch adsorption experiments and characterization techniques were employed to comparatively analyze the differences in cadmium adsorption performance and mechanisms between the HAP synthesized for 6 and 12 h using the hydrothermal method (H6 and H12) and mechanochemical method (M6 and M12), respectively. The results showed that H6 had a smooth surface and higher crystallinity, while M6 had smaller crystallite sizes, a more developed mesoporous structure, and a significantly higher specific surface area than H6. The pseudo-second-order kinetic model and Freundlich adsorption isotherm model best fitted the cadmium adsorption curves for both types of HAP, indicating that the adsorption process is dominated by chemical and multilayer adsorption. The theoretical maximum adsorption capacities of H6 and M6 were 302.38 mg·g^-1 and 429.63 mg·g^-1, respectively. M6 exhibited stronger pH adaptability and greater resistance to cationic interference. Adsorption thermodynamics studies revealed that cadmium adsorption by HAP is a spontaneous endothermic process, and the surface disorder degree of H6 was higher than that of M6. Based on semi-quantitative analysis of mechanism contribution, precipitation, ion exchange, and complexation were the main adsorption mechanisms for both H6 and M6, accounting for 94.46% and 93.34%, 5.41% and 6.60%, and 0.13% and 0.06% of the total adsorption capacities, respectively. In summary, the HAP synthesized by mechanochemical methods demonstrated better environmental adaptability and adsorption performance; its rich mesoporous structure and large specific surface area endowed it with significant potential.</p> Graphical Abstract <p></p>