The sorption of lead by synthetic hydroxyapatite (HA) from solutions containing Pb2+ initial concentrations up to 1770 mg L(-1) was studied. X-ray diffractometry (XRD) associated with Rietveld methodology for refining the spectra pattern was used in order to characterize the mechanisms of lead uptake. It is shown that the dissolution of hydroxyapatite is followed by the formation of a solid solution, Pb(10-x)Ca(x)(PO4)6(OH)2, with Pb ions mostly occupying Ca(II) sites. The Ca/Pb molar ratio of this solid solution decreases continuously until it reaches the structure of a pure hydroxypyromorphite. The cell parameters and the crystallite mean size behavior of both mineral phases reinforce the hypothesis that hydroxypyromorphite, PbHA, formation is the end of a process in which Pb(10-x)Ca(x)(PO4)6(OH)2 crystallites are continuously dissolved and recrystallized producing crystals with lower calcium content. Combination of Inductively Coupled Plasma spectrometry (ICP), chemical analysis, and XRD results permitted the conclusion that lead ions are not completely immobilized by precipitating Pb(10-x)Ca(x)(PO4)6(OH)2. Additional surface mechanisms also contribute to Pb2+ uptake. During Pb2+ sorption process, pH variations of the solution phase showed a more complex pattern than previously reported. Contribution of surface mechanisms, in addition to the hydroxyapatite dissolution, could explain this behavior.

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The relationship between metal particle structure and the reactivity and selectivity of oxide−supported metal catalysts has been explored here with well-defined Pt particles on the Zn-terminated ZnO(0001)−Zn surface, using X-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy (ISS), and temperature-programmed desorption (TPD). These are model catalysts for the steam reforming and selective dehydrogenation of methanol. Vapor-deposited Pt on ZnO(0001) grows in two-dimensional (2D) islands up to ∼0.7ML. The effect of the 2D island size and 3D island height are studied. Even 0.01 ML of Pt suppresses the dissociative adsorption of methanol to methoxy on Zn sites by preferential decoration of defects. Tiny amounts of Pt also affect the selectivity of methanol conversion at the Zn sites:  the dehydration pathway of its Zn-bound formate intermediate is suppressed and its dehydrogenation pathway to formaldehyde is eliminated. Even Pt sites on 2D Pt islands catalyze the partial oxidation of methanol to CO and H2, characteristic of low-index Pt facets. When the Pt islands are 2−3 layers thick, methanol decomposition is similar to that on low-index Pt faces.

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