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 HNO3 solution (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 209.85597.72 and then decreased to 74.76237.26 for the dissolution at initial pH 2.00 and 25°C45°C. The average Ksp values 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).