Controlled growth of nano-hydroxyapatite on stilbite: Defluoridation performance

Abstract

In Ethiopia, fluoride-contaminated groundwater used for drinking has become a matter of great concern, particularly in the Rift Valley region, due to its serious health hazard (fluorosis). In the present work, we present a systematic study related to the optimization of the synthesis of a composite material having a high defluoridation capacity, consisting of nanohydroxyapatite crystals growth on the external surface of a natural Stilbite zeolite (nHAST) of sizes between 74 and 105 µm. In order to control the growth of significantly small crystals of hydroxyapatite with higher fluoride removal capacity, the most important synthesis parameters, crystallization pH and crystallization time, have been carefully optimized. The obtained materials have been characterized by X-Ray Diffraction, Elemental Analysis, Thermogravimetric Analysis, Infrared Spectroscopy, 31 P Nuclear Magnetic Resonance, and Transmission Electron Microscopy; in addition, the defluoridation capacity of the materials has been determined. Interestingly, unlike most adsorbents, nHAST-144h (prepared after 144 hours of crystallization) shows significantly high fluoride-removal efficiency in a wide range of pH with maximum F-removal of 86 % and minimum of 79 % fluoride removals at pH 3 and 10, respectively. Besides, nHAST-144h adsorbent maintains its high defluoridation capacity in the presence of other ions, in particular chloride, sulfate and bicarbonate. In a real situation, nHAST-144h was found to be efficient as well; from real groundwater from the Ethiopian Rift Valley, with a fluoride concentration of 8.2 mg/L and a dose of 10 g/L, nHAST-144h reduced the fluoride concentration down to 1.40 mg/L, below the limit established by World Health Organization (WHO).

Environmental uses of zeolites in Ethiopia

“…, allows the precipitation of hydroxyapatite on the zeolite surface with very small particle sizes. Further optimization of the Ca-NH 4 exchange through variations of temperature and time provided smaller nHAp crystals with increasingly higher Fuptake capacities [31]. This adsorbent has proven to be efficient in elimination of Ffrom real ground waters of Ethiopia.…”