Sorption of antimony onto hydroxyapatite

We prepared synthetic hydroxyapatite [HAP; Ca-5(PO4)(3-x)(CO3)(x)(OH)(1+x) (x = 0.3)] and then investigated this material's ability to remove trivalen t antimony [Sb(III)] from water. The HAP was characterized by X-ray diffrac tion analysis, scanning electron microscopy, X-ray energy dispersive spectr oscopy, X-ray photoelectron spectroscopy, and infrared spectroscopy. The so rption of Sb(III)) to HAP was measured over an Sb(III)) concentration range of 0.05-50 mg L-1, at constant ionic strength (l = 0.01 mol dm(-3)) in equ ilibrated 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 pa rticles were enriched in Ca after incongruent dissolution of the solid. Mor e 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) wa s characterized by a Langmuir model with Gamma (max) = 6.7 +/- 0.1 x 10(-8) mol m(-2) (1.4 +/- 0.2 x 10(-4) mol dm(-3) g(-1)) and K-ads = 1.5 +/- 0.2 x 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 find ing supports the idea that the negative surface potential of the HAP increa sed due to the adsorption of Sb as a charged species. The decline in pHiep during Sb adsorption plus the thermodynamic description of the Sb(III)-HAP- H2O 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 conte xt of phosphate enrichment.