A Novel Apatite-Based Sorbent for Defluoridation: Synthesis and Sorption Characteristics of Nano-Micro Crystalline Apatite on Limestone

Abstract

Fluorosis is caused by chronic excessive ingestion of fluoride primarily through drinking contaminated water, and affects millions of people, particularly those in developing countries. The motivation for this thesis was to address the concern about fluorosis by looking for an inexpensive simple method for treating drinking water and removing fluoride. My thesis focused on designing nanocrystalline hydroxyapatite coated limestone for a defluoridation filter. Apatite coated limestone was synthesized in batch experiments under a range of experimental conditions and analyzed using scanning electron microscopy to determine the best way to obtain complete high surface area coatings. Higher temperatures, slightly acidic pH, and higher phosphate concentrations resulted in well-covered apatite-coated limestone particles with generally high surface area apatite crystal morphologies. Fluoride sorption batch experiments were conducted to see how efficiently the filter particles removed fluoride from solution. Apatite-coated limestone grains of a smaller size fraction consistently reduced fluoride concentrations to levels below the World Health Organization’s recommended fluoride drinking water limit. The resulting aqueous solutions were also studied and this analysis suggests that ion-exchange of hydroxyapatite with fluoride to form fluorapatite at the surface or solid solutions of fluoridated hydroxyapatite, dissolution and reprecipitation of hydroxyapatite to form fluorite, and adsorption are all possible mechanisms for fluoride removal.