Synthesis Of Nano Hydroxyapatitestilbite Composite For Defluoridation Of Drinking Water

The presence of high fluoride in groundwater used for drinking has become a matterrnof great concern in the Rift Valley of Ethiopia due to its serious health hazard,rnflourosis. Hence, developing easily accessible and socially acceptable low costrnremoval methods capable of reducing the fluoride concentration below the limitrnestablished by WHO (1.5 mg/L) is nowadays a crucial target. In this study, therndefluoridation capacity of locally available cheap natural stilbite zeolite and itsrnmodified version of high-performing and low energy demandingrnnanohydroxyapatite/stilbite, nHAST composite adsorbent for fluoride removal havernbeen investigated. While modifying, stilbite zeolite plays a dual role: as a supportrnmaterial and a reagent for calcium source, because this particular stilbite fromrnEthiopia is rich in calcium. Based on this fact, the modification procedure was carriedrnout simply by the addition of phosphate source, in this case (NH4)2HPO4 to stilbite. Inrnthe reaction, Ca2+ from STI are first ion-exchanged by NH4rn+ ions, thus released to thernsolution. At a favourable pH, the Ca2+ions react with PO4rn3- forming hydroxyapatiternthat precipitates on the zeolite surface in the form of nanocrystals, nHAST compositern(nanoHydroxyApatiteSTilbite) In order to manage the growth of significantly smaller crystals of hydroxyapatite withrnhigher fluoride removal capacities, the effect of synthesis time, crystallization pH andrncrystallization time are carefully optimized based on powder XRD, ICP-OES, TGA,rnIR-ATR, 31P MAS-NMR, TEM (Cs-STEM/EELS) and defluoridation capacity. ThernnHAST composite, which is found to have highest efficiency and capacity wasrnsuccessfully synthesized using crystallization time of 144 hours at autogenousrncrystallization pH of 8, and 2 hours synthesis time at room temperature conditions.rnBatch adsorption studies were performed as a function of solution pH effect, co-ionrneffect, contact time, adsorbent dose and initial fluoride concentration. Interestingly,rnunlike most adsorbents, nHAST composite shows high defluoridation capacity in arnwider pH range with maximum of 86 % and minimum of 79 % fluoride removal at pHrn3 and 10, respectively. Similarly, nHAST composite shows no effect due to chloridernand sulfate concentrations ranging from 0 to 500 mg/L. In real situation as well,rnnHAST was found to be efficient. From real water with fluoride concentration of 8.2rnmg/L, at a dose of 10 g/L nHAST resulted in a final fluoride concentration of 1.40rnmg/L, below the limit established by WHO.rnFinally, the fluoride removal performance of nHAST composite was compared withrnthat of Bone Char, an applied technology as adsorbent in the field in Ethiopia. It isrnworth noting that preparation of bone char involves prolonged high-temperaturerncharring treatments, which should be performed under carefully controlled conditions,rnand of course involves consumption of energy. On the other hand the synthesis ofrnnHAST composite is very easily performed, and does not require energy or skilledrnmanpower. The reaction mechanism involving the fluoride adsorption is analyzed based on kinetics and isotherm studies. In both nHAST and BC, the kinetic data fittedrnwell to a pseudo-second order kinetic model of similar characteristics. In contrast, thernadsorption isotherm on the nHAST composite fitted best with the Freundlich model,rnwhereas on BC, it correlated well with the Langmuir model, suggesting a differentrnmechanism: adsorption of fluoride on BC was homogeneous, whereas on composite,rnit was heterogeneous, possibly related to the higher load of fluoride on the nHAprncomponent in the composite. At low concentrations, both adsorbents behave similarly,rnthe maximum adsorption capacity, measured at high concentrations, is higher in thernnHAST composite than in BC. The intrinsic HAp capacity of nHAST, normalized tornthe amount of HAp on the adsorbent, is significantly higher (9.15 mg F-/g HAp) thanrnthat of BC (1.08 mg F-/g HAp) (measured at low F- initial concentrations where thernOH-/F-–exchange mechanism predominates), showing a much higher fluoride removalrnefficiency of nHAp on the composite. These results foresee a high potential ofrnnHAST composite towards its application for defluoridation of potable water

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