Sequestration of toxic metals (e.g., Pb) by hydroxyapatite (HAP) has attracted wide attentions 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 (FTIR) and high-resolution transmission electron microscopy (HRTEM). Results revealed that Pb uptake mechanism involves not only surface adsorption, precipitation but also incorporation, depending on initial Pb concentration. At the 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 concentration of 0.5–5.7 mM, formation of lead phosphate precipitates was evidenced by the XRD analysis that new peaks at 2θ of ~30.3o and can be indexed to crystalline hydroxypyromorphite (HPY). This was also consistent to the ratios (0.9–1.0) of Pbsorbed/Cadissolved at Pb level of 2.5–5.7 mM. At higher Pb concentration (≥ 6.6 mM), larger ratios (1.2–1.9) of Pbsorbed/Cadissolved was observed; and surprisingly the XRD signal denoting to HPY was weakened. This may imply that Pb has incorporated through substituting the Ca sites into HAP crystal lattice, different from the dissolution-precipitation process occurring at the medium Pb concentration range. Direct observation of PbxCa5-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.

中文翻译：

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纳米羟基磷灰石对铅吸收的表征：分子尺度的观点

羟基磷灰石（HAP）隔离有毒金属（例如，Pb）在土壤修复中引起了广泛关注。在这项研究中，使用混合间歇法，X射线衍射（XRD），傅立叶变换红外（FTIR）和高分辨率透射电子显微镜（HRTEM）对HAP吸收铅进行了全面研究。结果表明，铅的吸收机制不仅涉及表面吸附，沉淀，而且还涉及掺入，这取决于铅的初始浓度。在低Pb浓度（例如0.1 mM）下，由于XRD分析中未观察到变化，表面吸附可能对总Pb吸收有很大贡献。在0.5–5.7 mM的中等浓度下，X射线衍射分析表明在2θ的〜30处出现了新的峰，从而形成了磷酸铅沉淀。3o，并且可以被索引为结晶羟基焦晶石（HPY）。这也与在2.5–5.7 mM的Pb水平下Pb吸收/ Ca溶解的比率（0.9–1.0）一致。在较高的Pb浓度（≥6.6 mM）下，观察到较大比例的Pb吸收/溶解（1.2-1.9）；令人惊讶的是，表示HPY的XRD信号被削弱了。这可能意味着，Pb通过将Ca位置代入HAP晶格而掺入，这与在中等Pb浓度范围内发生的溶解-沉淀过程不同。从HRTEM直接观察到高Ca含量的PbxCa5-x（PO4）3OH固溶体进一步支持了这一论点。全面的，