Characterization, Dissolution, and Solubility of Lead Hydroxypyromorphite [Pb-5(PO4)(3)OH] at 25-45 degrees C

Dissolution of the hydroxypyromorphite [lead hydroxyapatite, Pb-5(PO4)(3)OH] in HNO3 solution (pH = 2.00), ultrapure water (pH = 5.60), and NaOH solution (pH = 9.00) was experimentally studied at 25 degrees C, 35 degrees C, and 45 degrees 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 degrees 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 similar to 597.72 and then decreased to 74.76 degrees 237.26 for the dissolution at initial pH 2.00 and 25 degrees C similar to 45 degrees C. The average K-sp values for Pb-5(PO4)(3)OH were determined to be 10-(80.77) (10-(80.57)-10-(80.96)) at 25 degrees C, 10-(80.65) (10-(80.38)-10-(80.99)) at 35 degrees C, and 10-(79.96) (10-(79.38)-10-(80.71)) at 45 degrees C. From the obtained solubility data for the dissolution at initial pH 2.00 and 25 degrees C, the Gibbs free energy of formation [Delta G(f)(0)] for Pb-5(PO4)(3)OH was calculated to be -3796.71 kJ/mol (-3795.55 similar to -3797.78 kJ/mol).