Sorption of antimony onto hydroxyapatite.

We prepared synthetic hydroxyapatite [HAP; Ca 5 (PO 4 ) 3-x (CO 3 ) x (OH) 1+x (x = 0.3)] and then investigated this material's ability to remove trivalent antimony [Sb(III)] from water. The HAP was characterized by X-ray diffraction analysis, scanning electron microscopy, X-ray energy dispersive spectroscopy, X-ray photoelectron spectroscopy, and infrared spectroscopy. The sorption of Sb(III) to HAP was measured over an Sb(III) concentration range of 0.05-50 mg L -1 , at constant ionic strength (I = 0.01 mol dm -3 ) in equilibrated aqueous suspensions (34 g dm -3 ) for 5 < pH < 8 in vessels that were closed to the atmosphere. Under these conditions, we found that HAP particles were enriched in Ca after incongruent dissolution of the solid. More than 95% of the Sb(III) in solution adsorbed to the solid-phase HAP in <30 min. The equilibrium distribution of Sb(III) (solid vs liquid phase) was characterized by a Langmuir model with Γ max = 6.7 ± 0.1 × 10 -8 mol m -2 (1.4 ± 0.2 × 10 -4 mol dm -3 g -1 ) and K ads = 1.5 ± 0.2 × 10 3 dm 3 mol -1 . As Sb adsorption occurred, the pH of the isoelectric point (pH iep ) of the HAP suspensions declined from 7.7 to 6.9. This finding supports the idea that the negative surface potential of the HAP increased due to the adsorption of Sb as a charged species. The decline in pH iep during Sb adsorption plus the thermodynamic description of the Sb(III)-HAP-H 2 O system suggest likely surface reactions for the interaction of Sb with HAP. We discuss the efficiency of Sb removal from water by HAP in the context of phosphate enrichment.