Synthesis of Nano Hydroxyapatite/Stilbite Composite for Defluoridation of Drinking Water

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Addis Ababa Universty


The presence of high fluoride in groundwater used for drinking has become a matter of great concern in the Rift Valley of Ethiopia due to its serious health hazard, flourosis. Hence, developing easily accessible and socially acceptable low cost removal methods capable of reducing the fluoride concentration below the limit established by WHO (1.5 mg/L) is nowadays a crucial target. In this study, the defluoridation capacity of locally available cheap natural stilbite zeolite and its modified version of high-performing and low energy demanding nanohydroxyapatite/stilbite, nHAST composite adsorbent for fluoride removal have been investigated. While modifying, stilbite zeolite plays a dual role: as a support material and a reagent for calcium source, because this particular stilbite from Ethiopia is rich in calcium. Based on this fact, the modification procedure was carried out simply by the addition of phosphate source, in this case (NH4)2HPO4 to stilbite. In the reaction, Ca2+ from STI are first ion-exchanged by NH4 + ions, thus released to the solution. At a favourable pH, the Ca2+ions react with PO4 3- forming hydroxyapatite that precipitates on the zeolite surface in the form of nanocrystals, nHAST composite (nanoHydroxyApatiteSTilbite) In order to manage the growth of significantly smaller crystals of hydroxyapatite with higher fluoride removal capacities, the effect of synthesis time, crystallization pH and crystallization time are carefully optimized based on powder XRD, ICP-OES, TGA, IR-ATR, 31P MAS-NMR, TEM (Cs-STEM/EELS) and defluoridation capacity. The nHAST composite, which is found to have highest efficiency and capacity was successfully synthesized using crystallization time of 144 hours at autogenous crystallization pH of 8, and 2 hours synthesis time at room temperature conditions. Batch adsorption studies were performed as a function of solution pH effect, co-ion effect, contact time, adsorbent dose and initial fluoride concentration. Interestingly, unlike most adsorbents, nHAST composite shows high defluoridation capacity in a wider pH range with maximum of 86 % and minimum of 79 % fluoride removal at pH 3 and 10, respectively. Similarly, nHAST composite shows no effect due to chloride and sulfate concentrations ranging from 0 to 500 mg/L. In real situation as well, nHAST was found to be efficient. From real water with fluoride concentration of 8.2 mg/L, at a dose of 10 g/L nHAST resulted in a final fluoride concentration of 1.40 mg/L, below the limit established by WHO. Finally, the fluoride removal performance of nHAST composite was compared with that of Bone Char, an applied technology as adsorbent in the field in Ethiopia. It is worth noting that preparation of bone char involves prolonged high-temperature charring treatments, which should be performed under carefully controlled conditions, and of course involves consumption of energy. On the other hand the synthesis of nHAST composite is very easily performed, and does not require energy or skilled manpower. The reaction mechanism involving the fluoride adsorption is analyzed based on kinetics and isotherm studies. In both nHAST and BC, the kinetic data fitted well to a pseudo-second order kinetic model of similar characteristics. In contrast, the adsorption isotherm on the nHAST composite fitted best with the Freundlich model, whereas on BC, it correlated well with the Langmuir model, suggesting a different mechanism: adsorption of fluoride on BC was homogeneous, whereas on composite, it was heterogeneous, possibly related to the higher load of fluoride on the nHAp component in the composite. At low concentrations, both adsorbents behave similarly, the maximum adsorption capacity, measured at high concentrations, is higher in the nHAST composite than in BC. The intrinsic HAp capacity of nHAST, normalized to the amount of HAp on the adsorbent, is significantly higher (9.15 mg F-/g HAp) than that of BC (1.08 mg F-/g HAp) (measured at low F- initial concentrations where the OH-/F-–exchange mechanism predominates), showing a much higher fluoride removal efficiency of nHAp on the composite. These results foresee a high potential of nHAST composite towards its application for defluoridation of potable water



Nano Hydroxyapatite/Stilbite