Characterization, dissolution and solubility of the hydroxypyromorphite-hydroxyapatite solid solution [(PbxCa1-x)5(PO4)3OH] at 25°C and pH 2-9.

Bin Huang,Zongqiang Zhu,Meina Liang,Yanhua Huang,Xin Zhao,Huili Liu,Yinian Zhu

doi:10.1186/s12932-016-0034-8

Abstract

BackgroundThe interaction between Ca-HAP and Pb2+ solution can result in the formation of a hydroxyapatite–hydroxypyromorphite solid solution [(PbxCa1−x)5(PO4)3(OH)], which can greatly affect the transport and distribution of toxic Pb in water, rock and soil. Therefore, it’s necessary to know the physicochemical properties of (PbxCa1−x)5(PO4)3(OH), predominantly its thermodynamic solubility and stability in aqueous solution. Nevertheless, no experiment on the dissolution and related thermodynamic data has been reported.ResultsDissolution of the hydroxypyromorphite–hydroxyapatite solid solution [(PbxCa1−x)5(PO4)3(OH)] in aqueous solution at 25 °C was experimentally studied. The aqueous concentrations were greatly affected by the Pb/(Pb + Ca) molar ratios (XPb) of the solids. For the solids with high XPb [(Pb0.89Ca0.11)5(PO4)3OH], the aqueous Pb2+ concentrations increased rapidly with time and reached a peak value after 240–720 h dissolution, and then decreased gradually and reached a stable state after 5040 h dissolution. For the solids with low XPb (0.00–0.80), the aqueous Pb2+ concentrations increased quickly with time and reached a peak value after 1–12 h dissolution, and then decreased gradually and attained a stable state after 720–2160 h dissolution.ConclusionsThe dissolution process of the solids with high XPb (0.89–1.00) was different from that of the solids with low XPb (0.00–0.80). The average Ksp values were estimated to be 10−80.77±0.20 (10−80.57–10−80.96) for hydroxypyromorphite [Pb5(PO4)3OH] and 10−58.38±0.07 (10−58.31–10−58.46) for calcium hydroxyapatite [Ca5(PO4)3OH]. The Gibbs free energies of formation (ΔGfo) were determined to be −3796.71 and −6314.63 kJ/mol, respectively. The solubility decreased with the increasing Pb/(Pb + Ca) molar ratios (XPb) of (PbxCa1‒x)5(PO4)3(OH). For the dissolution at 25 °C with an initial pH of 2.00, the experimental data plotted on the Lippmann diagram showed that the solid solution (PbxCa1−x)5(PO4)3(OH) dissolved stoichiometrically at the early stage of dissolution and moved gradually up to the Lippmann solutus curve and the saturation curve for Pb5(PO4)3OH, and then the data points moved along the Lippmann solutus curve from right to left. The Pb-rich (PbxCa1−x)5(PO4)3(OH) was in equilibrium with the Ca-rich aqueous solution.Graphical abstractLippmann diagrams for dissolution of the hydroxypyromorphite–hydroxyapatite solid solution [(PbxCa1−x)5(PO4)3OH] at 25 ˚C and an initial pH of 2.00.Electronic supplementary materialThe online version of this article (doi:10.1186/s12932-016-0034-8) contains supplementary material, which is available to authorized users.

Highlights

The interaction between calcium hydroxyapatite (Ca-HAP) and Pb2+ solution can result in the formation of a hydroxyapatite–hydroxypyromorphite solid solution [(PbxCa1−x)5(PO4)3(OH)], which can greatly affect the transport and distribution of toxic Pb in water, rock and soil
For the dissolution at 25 °C with an initial pH of 2.00, the experimental data plotted on the Lippmann diagram showed that the solid solution (PbxCa1−x)5(PO4)3(OH) dissolved stoichiometrically at the early stage of dissolution and moved gradually up to the Lippmann solutus curve and the saturation curve for Pb5(PO4)3OH, and the data points moved along the Lippmann solutus curve from right to left
The characterization with X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), SEM and TEM showed that the hydroxypyromorphite–hydroxyapatite solid solution [(PbxCa1−x)5(PO4)3(OH)] with apatite structure was not found to change obviously after dissolution except in some cases of the dissolution at the initial pH 2.00

Summary

Introduction

The interaction between Ca-HAP and Pb2+ solution can result in the formation of a hydroxyapatite–hydroxypyromorphite solid solution [(PbxCa1−x)5(PO4)3(OH)], which can greatly affect the transport and distribution of toxic Pb in water, rock and soil. Natural apatites as raw materials for the phosphate fertilizer industry contain some traces amount of various elements [10], among which lead and cadmium are predominantly risky and may be redistributed in natural waters, soil and agricultural products, especially in rice and vegetables. When these toxic heavy metals are taken into animals through food chains, they may concentrate in animals’ hard tissues through the possible substitution, which can cause osteoporotic processes and dental caries [10,11,12,13, 15]. The existence of Pb–Ca-HAP as an intermediate phase was confirmed by X-ray diffractometer and electron microscopy analysis [23]

Methods

Results

Conclusion