title: Characterization, dissolution and solubility of the hydroxypyromorphite-hydroxyapatite solid solution [(PbxCa1-x)5(PO4)3OH] at 25 °C and pH 2-9.
abstract: BACKGROUND
  The interaction between Ca-HAP and Pb(2+) 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.
  
  
  RESULTS
  Dissolution 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 Pb(2+) 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 Pb(2+) 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.
  
  
  CONCLUSIONS
  The 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 K sp 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 (ΔG f (o) ) 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.
authors.affiliation: College of Environmental Science and Engineering, Guilin University of Technology, Guilin, 541004 People's Republic of China.
authors.forename: Yinian
authors.initials: Y
authors.lastname: Zhu
authors.affiliation: College of Environmental Science and Engineering, Guilin University of Technology, Guilin, 541004 People's Republic of China.
authors.forename: Bin
authors.initials: B
authors.lastname: Huang
authors.affiliation: College of Environmental Science and Engineering, Guilin University of Technology, Guilin, 541004 People's Republic of China.
authors.forename: Zongqiang
authors.initials: Z
authors.lastname: Zhu
authors.affiliation: College of Environmental Science and Engineering, Guilin University of Technology, Guilin, 541004 People's Republic of China.
authors.forename: Huili
authors.initials: H
authors.lastname: Liu
authors.affiliation: College of Environmental Science and Engineering, Guilin University of Technology, Guilin, 541004 People's Republic of China.
authors.forename: Yanhua
authors.initials: Y
authors.lastname: Huang
authors.affiliation: College of Light Industry and Food Engineering, Guangxi University, Nanning, 530004 People's Republic of China.
authors.forename: Xin
authors.initials: X
authors.lastname: Zhao
authors.affiliation: College of Environmental Science and Engineering, Guilin University of Technology, Guilin, 541004 People's Republic of China.
authors.forename: Meina
authors.initials: M
authors.lastname: Liang
keywords: Calcium hydroxyapatite; Dissolution; Hydroxypyromorphite; Lippmann diagram; Solid solution