Sorption mechanisms of zinc on hydroxyapatite: systematic uptake studies and EXAFS spectroscopy analysis

The systematics and mechanisms of Zn uptake by hydroxyapatite (HAP) in preequilibrated suspensions open to PCO2 were characterized using a combination of batch sorption experiments, X-ray diffraction (XRD), and extended X-ray absorption fine structure spectroscopy (EXAFS) over a wide range of pH and Zn concentrations. Sorption isotherms of Zn(II) on HAP at pH 5.0 and 7.3 show an initial steep slope at low Zn(II) concentrations, followed by a plateau up to [Zn] < approximately 750 microM, suggesting Langmuir-type behavior. At [Zn] > 750 microM, a sharp rise in the pH 5.0 isotherm suggests precipitation, whereas slight continued uptake in the pH 7.3 isotherm is suggestive of an additional uptake mechanism. The sorption isotherm at pH 9.0 shows a steep uptake step at [Zn] < or = 0.8 microM, followed by an increasing linear trend up to [Zn] = 5 microM, without any indication of a maximum, suggesting that precipitation is an important uptake process at this pH. Zn K edge EXAFS results show a first oxygen shell at 1.96-1.98 +/- 0.02 A in sorption samples with [Zn]tot < or = 250 microM at pH 5.0, 7.3, and 9.0, consistent with tetrahedral coordination. EXAFS results reveal additional P and Ca neighbors that support formation of an inner-sphere Zn surface complex where the Zn is coordinated to surface P04 tetrahedra in a corner-sharing bidentate fashion, bridging a Ca atom. In contrast, EXAFS and XRD data indicate that precipitation of Zn3(PO4)2-4H2O (hopeite) dominates the mode of Zn uptake at [Zn]tot > or = 3 mM at pH 5.0. Principal component analysis and linear combination fits of EXAFS data reveal a mixture of inner-sphere Zn surface complexation and precipitation of Zn5(OH)6(CO3)2 (hydrozincite) in sorption samples for [Zn]tot = 5 mM at pH 7.3 and for [Zn]tot = 1 mM at pH 9.0.