Citrate-Stabilized Amorphous Calcium Phosphate Nanoparticles as an Effective Adsorbent for Defluorination

Amorphous calcium phosphate (ACP), one of the most important calcium-phosphorus compounds, is widely used in dentistry, orthopedics, and medicine, but is rarely reported for fluoride removal from water. In view of this, sodium citrate-stabilized amorphous calcium phosphate (Cit-ACP) and Cit-ACP calcinated at different temperatures were successfully prepared for fluoride removal. The results showed that the adsorption data of the Cit-ACP sample could be well described by the Langmuir model, and the adsorption kinetic followed the pseudo-second-order model. The maximum adsorption capacity was 27.48 mg/g at pH 7.0 when the fluoride concentration is 100 mg/L. The thermodynamic parameters suggested that the adsorption of fluoride was a spontaneous endothermic process. The XRD, XPS, and Zeta potential analysis of the Cit-ACP sample before and after fluoride removal revealed that, owing to the core-shell structure of the Cit-ACP nanoparticles, the fluoride ions in solution and the calcium ions in shell layer of the Cit-ACP nanoparticles co-promoted the transformation of the core of the Cit-ACP nanoparticles into fluorapatite. Given the simplicity of its preparation and effectiveness of its fluoride removal properties, Cit-ACP would be a potentially economical, efficient, and biocompatible adsorbent for fluoride removal.