Nano-scale reaction processes at the interface between apatite and aqueous lead

Synthetic hydroxyapatite (Ca-5(PO4)(3)OH, HAP) powder and a bulk single crystal of natural fluorapatite ((Ca4.915Na0.014)(P3.029Si0.010)O-12(F0.930Cl0.098), FAP), which was mechanically polished normal to the c axis, have been reacted with Pb nitrate solutions (2 x 10(-3) mol l(-1)) at an initial pH of 5.0 at room temperature and examined by high-resolution transmission electron microscopy (HRTEM). In the experiment using HAP powder crystals, secondary Pb phosphate, hydroxypyromorphite (Pb-5(PO4)(3)OH, HPY), forms epitaxially on the tip of the HAP crystal parallel to the a or b axis. Two types of mechanisms are observed in the HPY formation: (i) Whisker formation, which refers to an epitaxial growth of HPY needle-like nanocrystallites on the tip of the HAP crystal parallel to the a orb axis from the solution supersaturated with respect to HPY, and (ii) pseudomorphism, which is a direct replacement of HAP by HPY concurrent with the HAP dissolution at the interface. The HPY pseudomorph further grows as large as similar to 10 mu m for 30 days by consuming the HPY whisker nanocrystallites following the Ostwald step rule. Nucleation of HPY on the specific crystalline faces, which presumably have lower interfacial energy, predominantly governs the kinetics and mechanisms of Pb immobilization by HAP nanopartides. Such mechanisms can be applicable to nanoscale pseudomorphisms occurring on submicron-sized particles. In the experiment using bulk FAP, cross-sectional TEM demonstrates preferential leaching of Ca from the surface, and a secondary Pb-phosphate, chlorpyromorphite (Pb-5(PO4)(3)Cl), precipitates without growing epitaxially on the FAP surface. There is no evidence, under the experimental conditions of this study, of a cation exchange mechanism between Pb and Ca maintaining apatite structure (i.e., bulk diffusion of Pb into HAP crystal). The results obtained in the present study underscore the importance of the interface-driven mechanism in the interaction between aqueous toxic metals and apatite structured minerals. (C) 2013 Elsevier B.V. All rights reserved.