Characterization of Lead Uptake by Nano-Sized Hydroxyapatite: A Molecular Scale Perspective

Sequestration of toxic metals (e.g., Pb) by hydroxyapatite (HAP) has attracted wide attention in soil remediation. In this study, a comprehensive investigation on Pb uptake by HAP was conducted using combined batch method, X-ray diffraction (XRD), Fourier-transform infrared, and high-resolution transmission electron microscopy (HRTEM). Results revealed that the Pb uptake mechanism involves not only surface adsorption and precipitation but also incorporation depending on the initial Pb concentration. At low Pb concentrations (e.g., 0.1 mM), surface adsorption may contribute considerably to the total Pb uptake as no changes were observed in the XRD analysis. At medium concentrations of 0.5–5.7 mM, formation of lead phosphate precipitates was evidenced by XRD analysis with new peaks at 2θ of ∼30.3° and can be indexed to crystalline hydroxypyromorphite (HPY). This was also consistent with the ratios (0.9–1.0) of Pb_sorbed/Ca_dissolved at Pb level of 2.5–5.7 mM. At higher Pb concentrations (≥6.6 mM), larger ratios (1.2–1.9) of Pb_sorbed/Ca_dissolved were observed, and surprisingly, the XRD signal denoting HPY was weakened. This may imply that Pb has incorporated through substituting the Ca sites into the HAP crystal lattice different from the dissolution–precipitation process occurring at the medium Pb concentration range. Direct observation of Pb_xCa_5–x(PO₄)₃OH solid solutions with high Ca content from HRTEM further supported this argument. Overall, the molecular level understanding of the fundamental geochemistry for Pb interaction with HAP provided by this research could improve the efficiency of the application of HAP material for environmental remediation.