Hydroxyapatite (HAp) generated as a waste product in the food manufacturing process was dissolved in acidic aqueous solution, and subsequently re-precipitated in the presence of magnetic γ-Fe_2O_3 nanoparticles by the addition of alkaline to form HAp/γ-Fe_2O_3 composites. The synthesized composites were characterized by XRD, SEM, BET and the adsorption properties for cadmium ions in aqueous solutions were evaluated. When ultrasound was irradiated during the re-precipitation process, the products exhibited superior adsorption properties compared to those prepared under mechanical stirring, which is attributable to high specific surface area of the sonicated HAp. Because the re-precipitated HAp under ultrasound irradiation was homogeneously combined with γ-Fe_2O_3, the sonochemical composites adsorbing cadmium ions were readily collected by the application of the external magnetic field.