Preparation of magnetic hydroxyapatite whisker and its adsorption capacity to cadmium ion

Hydroxyapatite (HA) is a safe and stable adsorbent that can be used for the removal of Cadmium ions (Cd2+), and non-generation of secondary pollution. In the study, plate-like HA whiskers with well-defined crystallinity, controllable dimensions, and high aspect ratios are synthesized by the hydrothermal method. Magnetic hydroxyapatite (HA@Fe3O4) is then formed by uniformly encapsulating the HA whiskers by nano Fe3O4. With a comparison of HA whiskers prepared with 10 mmol/L Ca2+ concentration, the hydrothermally synthesized HA whiskers with 33 mmol/L Ca2+ concentration exhibit uniform shape and high crystallinity with an aspect ratio of 40–50 and length of 60–100 µm. Not only modification capacity but also aggregation of Fe3O4 on HA whiskers increases with the increase of Fe3O4 content. The addition of 4% Fe3O4 has a higher modification efficiency to HA, low aggregation of Fe3O4 and excellent magnetic responsiveness. HA@Fe3O4 with 4% Fe3O4 demonstrates a strong capacity for cadmium ion immobilization. At an initial Cd2+ concentration of 30 ppm, the Cd2+ removal efficiency of HA@Fe3O4 with 4% Fe3O4 reaches 59% within 20 min, and the Cd2+ concentration remains relatively stable in 48 h. A substantial removal efficiency is 22% after 10 min in the solution of 3000 ppm Cd2+. Importantly, XPS and XRD results indicate that Ca2+ in HA crystals is displaced by Cd2+ and can effectively transform Cd2+ to a more stable state during the removal process. This study provides a simple and convenient method with stable adsorption and magnetic separating, which would be suitable for tackling Cd2+ contamination with different concentrations and have little impact on the environment.