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

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.

Transition from Endothermic to Exothermic Dissolution of Hydroxyapatite Ca5(PO4)3OH–Johnbaumite Ca5(AsO4)3OH Solid Solution Series at Temperatures Ranging from 5 to 65 °C

“…However, the enthalpy of dissolution reaction ∆H 0 r calculated for HAP based on the results by McDowell was 88 kJ/mol, also indicating endothermic dissolution [34]. Based on the dissolution reaction in Equation (1) and the thermodynamic data for reaction and ions presented in Tables 6 and 7 Based on the dissolution reaction in Equation (1) and the thermodynamic data for reaction and ions presented in Tables 6 and 7, the Gibbs free energy of formation ∆G −6310.45 [42] −6287 [43] −6255 [44] −6654.5 [44] −6762.5 [45] 390.35 [46] 347 [46] Ca 2+ −553.5 [47] −542.8 [47] −53.1 [47] −169 [48] PO 4 3− −1001.6 [47] −1259.6 [47] −222 [47] −497 [49] AsO 4 3− −648.4 [47] −888.1 [47] −162.8 [47] −486.1 [47] OH − −157.3 [50] −230 [50] −10.71 [50] −138.6 [49] The results are presented in Table 8. Only sparse literature data for JBM are available for comparison (Table 7).…”

Solubility Product of Vanadinite Pb5(VO4)3Cl at 25 °C—A Comprehensive Approach to Incongruent Dissolution Modeling

“…Thermodynamic modelling of incongruent dissolutions poses significant challenges. In this regard, few approaches for the apatite group have been proposed [71][72][73][74]. In this study, the geochemical speciation model PHREEQC (Phreeqc Interactive 3.5.0-14000) [64] was used to support the calculations of thermodynamic parameters.…”

Divalent heavy metals and uranyl cations incorporated in calcite change its dissolution process

“…Therefore, the minor/trace elements in minerals are hard to form independent secondary minerals during dissolution/weathering. During the dissolution of (Pb x Ca 1−x ) 5 (PO 4 ) 3 OH in a laboratory, both PbHPO 4 and Pb 3 (PO 4 ) 2 formed as secondary products when the Pb molar fraction in the solid solution was high enough (x ≥ 0.3) 124 .…”