Synthesis of surface coated hydroxyapatite powders for fluoride removal from aqueous solution

This contribution reports the preparation of hydroxyapatite (HAp) and cationic surfactant modified forms of HAp powder for fluoride uptake studies from aqueous solution. The structures of the synthesized sorbents were characterized by FT-IR, SEM with EDAX, XRD and mapping studies. The fluoride adsorption using HAp and modified forms of HAp powder was studied on batch mode. The maximum defluoridation capacity (DC) onto modified forms of HAp powder was 9.369 mg/g from an initial fluoride ion concentration of 10 mg/L as the dosage of 50 mg at room temperature was almost 3–4 times higher than bare HAp whose capacity was found to be 2.63 mg/g. The results indicated that the fluoride adsorption onto modified forms of HAp powder is essentially by an electrostatic attraction and ion-exchange mechanism which depend on the solution pH. Various parameters like contact time, pH, other interfering anions and temperature were optimized. The adsorption data was reasonably explained using Freundlich, Langmuir and D-R isotherms. The calculated values of thermodynamic parameters indicated that the fluoride adsorption is spontaneous and endothermic in nature. The sorption process follows pseudo-second-order and intraparticle diffusion kinetic models. The schematic sketch for fluoride adsorption mechanism by HAp and cationic surfactants modified HAp powder has been proposed. The suitability of the best modified HAp has been tested with the field samples collected in a nearby fluoride endemic area.