Enhanced fluoride removal from water by sulfate-doped hydroxyapatite hierarchical hollow microspheres

Abstract A novel adsorbent, sulfate-doped hydroxyapatite (HAP) hierarchical hollow microspheres, was prepared and developed for fluoride removal from water. The adsorbent was characterized by using SEM, TEM, XRD, FTIR, XPS and BET analyses, respectively. Batch of adsorption experiments were investigated to study the fluoride removal performance. The sulfate-doped HAP showed enhanced fluoride removal performance with a defluoridation capacity of 28.3 mg g−1 with initial fluoride concentration of 100 mg L−1 at 25 °C under neutral conditions, which was higher than other previously reported HAP-related adsorbents. The adsorption isotherm could be better defined by Freundlich model than Langmuir model. The adsorption kinetic followed the pseudo-second-order model. And the sulfate-doped HAP had a relatively high defluoridation capacity in a wide pH range of 3.0–10.0. The study of adsorption mechanism suggested that the hydroxyl groups and the sulfate groups on the surface were all involved in the fluoride adsorption, and these active sites cooperatively enhanced the fluoride removal.