TY - JOUR
T1 - Nano-scale reaction processes at the interface between apatite and aqueous lead
AU - Kamiishi, Eigo
AU - Utsunomiya, Satoshi
N1 - Funding Information:
The authors are grateful to Dr. Watanabe who provided assistance on SEM and XRD at the Center of Advanced Instrumental Analysis, Kyushu University. We are also grateful to Dr. K. Shimada who provided assistance on EMPA at the Laboratory of Inorganic Geochemistry for the Biosphere, Department of Earth and Planetary Sciences, Kyushu University. This work was supported by the Science Grant of the Ministry of Education, Science and Culture .
PY - 2013/2/24
Y1 - 2013/2/24
N2 - Synthetic hydroxyapatite (Ca5(PO4)3OH, HAP) powder and a bulk single crystal of natural fluorapatite ((Ca4.915Na0.014)(P3.029Si0.010)O12(F0.930Cl0.098), FAP), which was mechanically polished normal to the c axis, have been reacted with Pb nitrate solutions (2×10-3moll-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 (Pb5(PO4)3OH, 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 or b 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 ~10μm for 30days 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 nanoparticles. 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 (Pb5(PO4)3Cl), 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.
AB - Synthetic hydroxyapatite (Ca5(PO4)3OH, HAP) powder and a bulk single crystal of natural fluorapatite ((Ca4.915Na0.014)(P3.029Si0.010)O12(F0.930Cl0.098), FAP), which was mechanically polished normal to the c axis, have been reacted with Pb nitrate solutions (2×10-3moll-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 (Pb5(PO4)3OH, 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 or b 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 ~10μm for 30days 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 nanoparticles. 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 (Pb5(PO4)3Cl), 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.
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U2 - 10.1016/j.chemgeo.2012.12.019
DO - 10.1016/j.chemgeo.2012.12.019
M3 - Article
AN - SCOPUS:84873802967
SN - 0009-2541
VL - 340
SP - 121
EP - 130
JO - Chemical Geology
JF - Chemical Geology
ER -