Hydroxyapatite (Ca(10)(PO(4))(6)(OH)(2), HAP), both as a synthetic material and as a constituent of bone char, can serve as an effective and relatively inexpensive filter material for fluoride (F(-)) removal from drinking water in low-income countries. Fluoride uptake on HAP can occur through different mechanisms, which are, in principle, influenced by solution composition. Suspensions of HAP (2 g L(-1)) were equilibrated under controlled pH conditions (pH 6.5, 7.3, 9.5) at 25 °C for 28 d after the addition of different F(-) concentrations (0.5-7.0 mM). The reacted HAP solids were examined with Transmission Electron Microscopy (TEM), Fourier Transform Infrared Spectroscopy (FTIR), X-ray Photoelectron Spectroscopy (XPS), and Nano Secondary Ion Mass Spectroscopy (NanoSIMS). Fluoride uptake on HAP was dependent on pH, with the highest capacity at pH 6.5; the lowest uptake was found at pH 9.5. Under all experimental conditions, the thermodynamically stable mineral phase was fluorapatite, (Ca(10)(PO(4))(6)F(2), FAP). Fluoride uptake capacity was quantified on the basis of FTIR and XPS analysis, which was consistent with F(-) uptake from solution. The results of XPS and NanoSIMS analyses indicate that a fluoridated surface layer with a thickness of several nanometers is formed on nanosized HAP.
The heterogeneous precipitation of calcium phosphates on calcium hydroxyapariLe (Ca-10(PO4)(4)OH)(2) or HAP) in the presence arid absence of fluoride is important in the formation of bone and teeth, protection against tooth decay, dental and skeletal fluorosis and defluoridaLion of drinking wafer. Strontium hyclroxyapariLe (Sr-10(PO4)(6)(OH)(2) or SrHAP) and strontium carbonate (SrCO3) were used as calcium free seed templates in precipitation experiments conducted with varying initial calcium-Lophosphate (Ca/P) or calcium-Lo-phosphaLe-Lo-fluoride (Ca/P/F) ratios. Suspensions of SrHAP or SrCO3 seed templates (which were calcium limited for both templates and phosphate limited in the case of SECO3) were reacted at pH 7.3 (25 degrees C) over 3 days. The resulting solids were examined with Scanning Transmission Electron Microscopy (STEM), X-ray Diffraction (XRD), Fourier Transform Infrared (MR), and X-ray Photoelectron Spectroscopy (XPS), X-ray Absorption Near Edge Structure (XANES), and Extended X-ray Absorption Fine Structure spectroscopy (EXAFS). Calcium apatite was the predominant phase identified by all techniques independent of the added Ca/P ratios and of the presence of fluoride. It was not possible to make an unambiguous distinction between HAP and fluorapatite (Ca-10(PO4)(6)F-2, FAP). The apatite was calcium-deficient and probably contained some strontium
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