Characterization, Dissolution, and Solubility of Lead Hydroxypyromorphite [Pb₅(PO₄)₃OH] at 25–45°C

Abstract

Dissolution of the hydroxypyromorphite [lead hydroxyapatite, Pb5(PO4)3OH] in HNO3solution (pH = 2.00), ultrapure water (pH = 5.60), and NaOH solution (pH = 9.00) was experimentally studied at 25°C, 35°C, and 45°C. The XRD, FT-IR, and FE-SEM analyses indicated that the hydroxypyromorphite solids were observed to have indistinguishable change during dissolution. For the hydroxypyromorphite dissolution in aqueous acidic media at initial pH 2.00 and 25°C, the aqueous phosphate concentrations rose quickly and reached the peak values after 1 h dissolution, while the aqueous lead concentrations rose slowly and reached the peak values after 1440 h. The solution Pb/P molar ratio increased constantly from 1.10 to 1.65 near the stoichiometric ratio of 1.67 to 209.85~597.72 and then decreased to 74.76~237.26 for the dissolution at initial pH 2.00 and 25°C~45°C. The averageKspvalues for Pb5(PO4)3OH were determined to be 10−80.77(10−80.57−10−80.96) at 25°C, 10−80.65(10−80.38−10−80.99) at 35°C, and 10−79.96(10−79.38−10−80.71) at 45°C. From the obtained solubility data for the dissolution at initial pH 2.00 and 25°C, the Gibbs free energy of formation [ΔGfo] for Pb5(PO4)3OH was calculated to be −3796.71 kJ/mol (−3795.55~−3797.78 kJ/mol).

Pb Mineral Precipitation in Solutions of Sulfate, Carbonate and Phosphate: Measured and Modeled Pb Solubility and Pb2+ Activity

“…Specifically, new peaks appeared near 673 and 1370 cm −1 at higher pH, indicating the greater presence of carbonate relative to that of sulfate in the precipitates. A band near 3447 cm −1 indicative of structural OH (not shown) was also present at higher pH [42]. Although carbonate ions were not initially added to the SO 4 and SO 4 -PO 4 systems, FTIR detected CO 3 2− ions in the precipitates formed from the sulfate solutions, as evidenced by a broad peak centered at 1370 cm −1 and a sharp peak at 673 cm −1 that had greater intensity at higher pH.…”

“…It is important to note that the solubility products of many of these minerals were determined decades ago by dissolution experiments that require free Pb 2+ activity to be estimated at equilibrium. This was generally attempted by the process of modeling soluble Pb speciation [21,42], but substantial uncertainties in soluble Pb ion-pair (Pb-hydroxy, Pb-carbonate, Pb-phosphate) stabilities presented a serious obstacle to calculating accurate K SP values by this method [43].…”

Theoretical equilibrium lead(II) solubility revisited: Open source code and practical relationships

“…In addition to maintaining the original functionality of LEADSOL (Schock et al, 1996), TELSS includes the following updates: (i) addition of laurionite, PbClOH (s), (Nasanen & Lindell, 1976) as additional Pb(II) solid (Table 1), (ii) implementation of (chloro)pyromorphite, Pb 5 (PO 4 ) 3 Cl (s), using a recent equilibrium constant estimate (Topolska et al, 2016), and (iii) inclusion of alternative equilibrium constants (Table 2) that have been published since Schock et al (1996) for hydroxypyromorphite, Pb 5 (PO 4 ) 3 OH (s), (Zhu et al, 2015) and pyromorphite (Xie & Giammar, 2007) or are available in the literature for laurionite (Lothenbach et al, 1999). In addition, the TELSS code is setup in a flexible manner such that additional possible selections for equilibria and/or equilibrium constants can be implemented with minor modifications to the TELSS code if the user desires (see Supplementary Information (SI) Relevant Code Chunks for a Solid (Anglesite Example) and Relevant Code Chunks for an Aqueous Complex (PbSO 4 [aq] Example) ).…”

Apatite Biominerals

“…This can lead, for moderate fluoride intoxications, to hypocalcaemia [129]. Other ions such as Pb 2+ are also retained in bone for long periods, which is possibly related to their effect on apatite solubility [130]. On the contrary, ions like Sr 2+ , which have been found to increase the solubility of Ca-Sr hydroxyapatites [131], can be easily removed from bone mineral within a few weeks after interruption of intake [132].…”