Characterization , Dissolution , and Solubility of Lead Hydroxypyromorphite [ Pb 5 ( PO 4 ) 3 OH ] at 25 – 45 ∘ C

Dissolution of the hydroxypyromorphite [lead hydroxyapatite, Pb 5 (PO 4 ) 3 OH] in HNO 3 solution (pH = 2.00), ultrapure water (pH = 5.60), and NaOH solution (pH = 9.00) was experimentally studied at 25C, 35C, and 45C. 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 25C, 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 25C∼45C. The average sp values for Pb 5 (PO 4 ) 3 OH were determined to be 10 (10−10) at 25C, 10 (10−10) at 35C, and 10 (10−10) at 45C. From the obtained solubility data for the dissolution at initial pH 2.00 and 25C, the Gibbs free energy of formation [ΔG f ] for Pb 5 (PO 4 ) 3 OH was calculated to be −3796.71 kJ/mol (−3795.55∼ −3797.78 kJ/mol).


Introduction
The calcium in hydroxyapatite [Ca 5 (PO 4 ) 3 OH, Ca-HAP] can be substituted by different divalent cations such as Pb 2+ [1][2][3][4][5][6].When the toxic lead ions, which may be found in surface and underground waters, are taken into animals, it is possible that they concentrate in animals' hard tissues through the substitution of Pb 2+ for Ca 2+ and form the Pb-Ca hydroxyapatite solid solution with vital calcium hydroxyapatite and can finally cause many bone diseases, such as osteoporotic processes and dental caries [1,[7][8][9].The synthetic or natural calcium hydroxyapatite (Ca-HAP) from different sources can be used to remove toxic lead ions from industrial wastewaters [9][10][11].The reaction of the solid Ca-HAP with lead ions resulted in the forming of hydroxypyromorphite [lead hydroxyapatite, Pb 5 (PO 4 ) 3 OH, Pb-HAP], which could include the fact that the dissolution of hydroxyapatite is followed with the forming of hydroxypyromorphite [12][13][14][15].Lead apatite can form quickly in the presence of adequate phosphate and lead ions.It is recognized as the most stable lead form under a wide variety of environmental conditions.In situ immobilization of lead-contaminated systems with phosphates is now considered to be an appropriate and costeffective technology [16,17].
The substitution of Ca 2+ in calcium hydroxyapatite by toxic Pb 2+ is of considerable importance in a great variety of research areas [5,11], which therefore needs an understanding of the essential physicochemical properties, especially the dissolution mechanism, solubility, and stability of hydroxypyromorphite under different conditions.Even though a lot of experiments on the dissolution mechanism and kinetics of natural and synthetic apatite samples in pure water and acidic and alkali solutions have been conducted [18][19][20][21][22][23][24], many of them have been only focused on calcium hydroxyapatite and fluorapatite.Unfortunately, the literature data on the thermochemical properties of hydroxypyromorphite is very sparse, which are required to clarify the behavior of hydroxypyromorphite in water environments, although its dissolution and following release of the component from solid to solution play a significant role in the cycling of lead and phosphate [24].
A unique  sp value for hydroxypyromorphite (Pb-HAP) has been reported to be nearly 10 −76.8 [24,25], which has been also adopted in many subsequent studies, such as in the minteq.v4.dat database [26].But this value for the solid phase [Pb 5 (PO 4 ) 3 OH] was not measured directly through precipitation or dissolution experiment; that is, it was calculated from an investigation of the hydrolysis of secondary and tertiary lead orthophosphates [PbHPO 4 , Pb 3 (PO 4 ) 2 ] in the pH range 3.00∼10.00.In this study, lead hydroxypyromorphite was assumed to be a product in the alkaline hydrolysis of secondary lead orthophosphate without characterizing different phases of the solid [24].In addition, the dissolution kinetics of hydroxypyromorphite has never been studied.
Thus, the lead hydroxypyromorphite (Pb-HAP) was first synthesized by precipitation method and examined by XRD, FT-IR, and FE-SEM in this work, and then its mechanism and dissolution rate were investigated at initial pH 2.00∼9.00 and 25∼45 ∘ C.Moreover, the aqueous concentrations were used to estimate the solubility product and free energy of forming for hydroxypyromorphite.The Pb solution was then mixed with 250 mL 4.4 mol/L CH 3 COONH 4 buffer solution in a 1 L polypropylene vessel.After that, 500 mL 0.12 mol/L NH 4 H 2 PO 4 solution was rapidly added to the vessel with stirring, resulting in white suspension.The suspension was adjusted to pH 7.50 by adding NH 4 OH solution, stirred for 10 min at room temperature, and aged at 100 ∘ C for 48 h.The obtained precipitate was then settled, washed carefully using ultrapure water, and finally dried in an oven at 70 ∘ C for 16 h.10 mg of the obtained hydroxypyromorphite solid was first dissolved in 20 mL of 1 mol/L nitric acid solution and diluted to 100 mL with ultrapure water.The Pb and P concentrations were then determined by the inductively coupled plasma-optical emission spectrometer (ICP-OES, PE Optima 7000DV).The prepared solid was also examined by the powder X-ray diffractometer (XRD, X'Pert PRO) using Cu K radiation (40 kV and 40 mA) at a scanning rate of 0.10 ∘ /min within a 2 range 10∼80 ∘ and then identified crystallographically via comparing the obtained XRD pattern with the ICDD reference code 01-087-2477 for lead hydroxyapatite.The solid was also investigated in KBr pellets within 4000∼400 cm −1 using the Fourier transform infrared spectrophotometer (FT-IR, Nicolet Nexus 470).The morphology was scanned by the field emission-scanning electron microscope (FE-SEM, Hitachi S-4800).

Thermodynamic Calculations.
The aqueous activities of Pb 2+ (aq), PO 43− (aq), and OH − (aq) were first calculated using PHREEQC Version 3 with the minteq.v4.dat and llnl.datdatabases [26], and then the ion activity products (IAPs) for Pb 5 (PO 4 ) 3 OH were determined according to the massaction expressions.The aqueous species considered for the total lead calculation included Pb

Results and Discussion
3.1.Solid Characterizations.The chemical composition of the synthetic lead hydroxypyromorphite [Pb 5 (PO 4 ) 3 OH] is dependent on the Pb/P molar ratio of the precursor solutions.
The Pb-HAP precipitation should be conducted by mixing the lead solution with the phosphate solution very slowly.The obtained crystal was proved to be the intended component of Pb 5 (PO 4 ) 3 OH with the atomic Pb/P ratio of 1.67.The obtained crystal was proved to be the intended component of Pb 5 (PO 4 ) 3 OH with the atomic Pb/P ratio of 1.67, which decreased to 1.43∼1.50,1.58, and 1.58 after the dissolution at initial pH values 2.00, 5.60, and 9.00, respectively.As illustrated in the XRD, FT-IR, and FE-SEM figures, the component and characteristics of the Pb-HAP solids were almost indistinguishable before and after the dissolution at different initial pH values and 25 ∘ C (Figures 1-3).No secondary precipitates had been identified in the experiment at different initial pH values and 25 ∘ C. The XRD patterns of the hydroxypyromorphite solids showed that all the samples were recognized as lead phosphate hydroxide (reference code 01-087-2477), which were well crystallized and showed the apatite structure of the hexagonal system P6 3 /m with the estimated lattice parameters of  = 0.989 nm and  = 0.748 nm (Figure 1).But after the dissolution at initial pH 2.00 and 35 ∘ C (Figure 1(c)) or at initial pH 2.00 and 45 ∘ C (Figure 1(d)), the peaks of Pb 3 (PO 4 ) 2 [lead phosphate, reference code 00-025-1394] were also recognized, which means that Pb 3 (PO 4 ) 2 as secondary precipitate formed during the Pb-HAP dissolution at higher temperature.In the FT-IR spectra of the hydroxypyromorphite solids (Figure 2), the normal vibrational modes of phosphate tetrahedra of lead hydroxypyromorphite (Pb-HAP) were witnessed within the range around at 938.24 cm −1 (] 2 ), 985.01 and 1031.77cm −1 (] 3 ), and 536.62∼ 573.74 cm −1 (] 4 ).The phosphate peak at 471.53 cm −1 (] 1 ) was not observed.The FT-IR spectra of all hydroxypyromorphite solids showed the very sharp bands at 3553.83 cm −1 for the stretching vibrations of the bulk OH − .The 668.72 cm −1 bands represented the vibrational motion of OH − [9].The bands at 3650∼3680 cm −1 for the surface P-OH groups [9] and the bands at 1455 cm −1 for the CO 3 2− vibration [28] were also not identified in the spectra.The band at 871 cm −1 , which was assigned to HPO 4 2− ions present in cation-deficient hydroxyapatite, was not detected in the spectra [9].The FE-SEM results (Figure 3) confirmed that all hydroxypyromorphite solids were the typical hexagonal columnar crystals with a pinacoid as a termination, which elongated along the  axis.The mean particle length and width of Pb-HAP were obtained to be 7.00 m (3.43∼10.43m) and 3.76 m (1.83∼4.88)before dissolution, while these were 6.85 m (3.81∼11.87m) and 4.04 m (2.96∼5.14m), 5.49 m (2.72∼ 10.06 m) and 3.44 m (2.34∼5.58m), and 5.62 m (2.54∼ 10.44 m) and 3.55 m (2.22∼4.99m) after the dissolution at the initial pH values 2.00, 5.60, and 9.00, respectively.Dissolution of lead hydroxypyromorphite (Pb-HAP) at initial pH 2.00 appeared to be nearly stoichiometric in the early period and then nonstoichiometric to the end of the dissolution experiments.For the hydroxypyromorphite dissolution at initial pH 2.00 and 25 ∘ C (Figure 4(a)), the solution lead concentration rose continuously and reached a stable state after 720 h.The phosphate was rapidly released and the peak solution concentration was reached within the first hour of dissolution, and then the aqueous phosphate concentration decreased and reached a stable state after 720 h.The solution pH rose from 2.00 to 2.96 within 360 hours of the dissolution and then varied between 2.58 and 3.16 (Figure 4(a)).In addition, the release of lead and phosphate from solid to solution could be affected by the dissolution temperature.After dissolution for 7200 h, the solution phosphate concentrations at 45 ∘ C were lower than those at 25 ∘ C, while the solution lead concentrations at 45 ∘ C were higher than those at 25 ∘ C (Figures 4(a), 4(b), and 4(c)).The hydroxypyromorphite solids dissolved slowly while the solution Pb/P molar ratios increased constantly from 1.10-1.65 near the stoichiometric ratio of 1.67 to 210, 554, and 598 for the dissolution at initial pH 2.00 and 25 ∘ C, 35 ∘ C, and 45 ∘ C, respectively (Figure 4(f)).
Within the first 12 hours of the hydroxypyromorphite dissolution, lead and phosphate were released from the hydroxypyromorphite solid surface to the aqueous solution according to the stoichiometric Pb/P ratio of 1.67 (Figure 4(f)).The aqueous Pb/P molar ratio increased with the time increasing, which showed that the lead ions were preferentially released from the solid surface in comparison with phosphate.At the end of our experiment (7200 h), the solution Pb/P molar ratios were 74.76, 86.15, and 237.26 for the hydroxypyromorphite dissolution at initial pH 2.00 and 25 ∘ C, 35 ∘ C, and 45 ∘ C, respectively (Figure 4(f)).The aqueous Pb/P molar ratios for the dissolution at 45 ∘ C were considerably greater than the values at 25 ∘ C, which showed that the solution temperature could noticeably affect the solubility and dissolution mechanism of hydroxypyromorphite.As indicated in Figures 1(c) and 1(d), the surface phase of the lead hydroxypyromorphite particles dissolved under pH 2.00 and 35 ∘ C or initial pH 2.00 and 45 ∘ C condition, and Pb 3 (PO 4 ) 2 phase with the Pb/P molar ratio of 1.50 was formed on the surface of the particle.Moreover, some Pb 2+ sites in the lead hydroxypyromorphite structure are possibly vacant [20].Consequently the excess Pb 2+ ions were released to the solution.
For the hydroxypyromorphite dissolution in pure water (pH = 5.60) and the solution of initial pH 9.00, the solution pH values, lead and phosphate concentrations reached a stable state after 2880 h, which indicated a possible attainment of a steady state between the hydroxypyromorphite solid and the aqueous solution (Figures 4(d  of about 6.19∼6.40 after dissolution for 120 h.For the dissolution at initial pH 2.00 or 5.60, all solution pH values were higher than the initial pH values.The H + consuming showed that the H + adsorption onto negatively charged oxygen ions of phosphate groups of lead hydroxypyromorphite (Pb-HAP) may result in the transforming of PO at the solid surface and promote the dissolution process.In addition, the coexistence of dissolution and exchange processes indicated that the H + ions depleted during the hydroxypyromorphite dissolution were derived not only from the H + sorption/desorption reactions but also from various processes at the hydroxypyromorphite surface.As a result, a comprehensive elucidation of the H + consuming during the dissolution should cover all the following reactions: stoichiometric dissolution of the bulk hydroxypyromorphite solid, stoichiometric exchange of 2H + for one Pb 2+ at the hydroxypyromorphite surface, and H + adsorption/ desorption at the hydroxypyromorphite surface [20,29].
The apatite dissolution mechanism is particularly dependent on the experimental conditions [20].Many models for the apatite dissolution have been proposed depending upon the available experimental results, but all of them take only certain aspects of the apatite dissolution into consideration and cannot describe the dissolution mechanism completely [20].Depending upon the experimental results of earlier researches [20] and this work, the lead hydroxypyromorphite (Pb-HAP) dissolution in water is thought to include the following steps or processes: (I) stoichiometric dissolution coupled with protonation and complexation reactions; (II) nonstoichiometric dissolution with Pb 2+ release and PO 4 In Process I, for the hydroxypyromorphite dissolution in solution at initial pH 2.00 and 25 ∘ C, Pb 2+ and PO 4 3− in the hydroxypyromorphite structure can be released from solid to solution stoichiometrically according to reaction (1); the solution lead and phosphate concentrations rose with time simultaneously with the solution Pb/P molar ratio of 1.67 at the early stage of hydroxypyromorphite dissolution (0∼120 h).Various probable reactions should be taken into account in the apatite dissolution because of its complicated structure [30].Reaction (1) for the lead hydroxypyromorphite dissolution can be considerably affected by the solution pH and coupled with protonation and complexation reactions (2), (3), and (4), which caused the increasing of the solution pH for the hydroxypyromorphite dissolution in aqueous acidic media or the decreasing of the solution pH for the hydroxypyromorphite dissolution in aqueous alkali media.Consider The lead and phosphate speciation results based on the simulation with PHREEQC showed that, for the hydroxypyromorphite (Pb-HAP) dissolution at initial pH 2.00 and 25 ∘ C, the solution lead species existed in the order of Pb . At the early stage of the Pb-HAP dissolution at initial pH 9.00 and 25 ∘ C, the solution lead species existed in the order of PbOH At the dissolution time >120 h, the solution lead species existed in the order of Pb ; the solution phosphate species occurred in  .In Process II, lead and phosphate were removed nonstoichiometrically from the hydroxypyromorphite structure with the solution Pb/P molar ratio > 1.67.Consequently, a surface layer, which had a chemical composition different from that of the bulk solid, could be formed [20].After a first portion of the hydroxypyromorphite had been dissolved, some phosphate species could be adsorbed from the solution backwards onto the hydroxypyromorphite surface.The aqueous phosphate concentration started to decrease progressively after 1 h dissolution at initial pH 2.00 and 25 ∘ C, whereas the solution lead concentration rose continuously with time.The solution Pb/P molar ratio increased from about 1.67 to 209.85 after 1440 h dissolution.
In Process III, both of lead and phosphate species were adsorbed at the same time from the solution backwards onto the hydroxypyromorphite surface; the solution lead and phosphate concentrations declined from 1440 h to 5040 h with the decreasing solution Pb/P molar ratio (209.85 to 76.74), which might lead to the formation of a solid surface layer with a composition differing from the bulk of hydroxypyromorphite [20]; that is, the final solution reached a stable state with the solid surface layer having a composition different from hydroxypyromorphite [Pb 5 (PO 4 ) 3 OH].Because of the very low solubility, it was proposed that the apatite dissolution at the atomic level was ever nonstoichiometric [20], and its mechanism might include several chemical reactions taking place simultaneously at the apatite surface [30,31].
In Process IV, desorption and adsorption of lead and phosphate species attained a stable state.The solution lead and phosphate concentrations were nearly constant for the hydroxypyromorphite dissolution in the acidic solution (pH 2.00) at 25 ∘ C from 5040 h to 7200 h in the present work, and the solution Pb/P molar ratios were 74.76∼79.59.
The solubility products ( sp ) obtained in this and earlier researches [24,25] indicate the stability sequence for the hydroxyapatites to be Pb-HAP ≫ Cd-HAP > Ca-HAP, which may be related with the crystal radii for Pb 2+ , Cd 2+ , and Ca 2+ of 0.119 nm, 0.095 nm, and 0.100 nm, respectively.Some workers have studied the influence of various phosphate amendments (synthetic hydroxyapatite, phosphate rock, calcium hydrogen phosphate, phosphoric acid, and phosphate

3. 2 .
Dissolution Mechanism.The solution element concentrations and ratios for the lead hydroxypyromorphite (Pb-HAP) dissolution at different pH values and temperatures versus time are showed in Figures 4(a)∼4(f).
) and 4(e)).The solution lead and phosphate concentrations decreased with the increasing solution pH values.With the early release of Pb 2+ and PO 4 3− into the alkaline solution was the rapid rising of the solution pH from 9.00 to 9.72 within the 1 h dissolution, and then the solution pH values declined and attained a stable state with the pH values

Table 1
lists the pH, Pb, and P analyses at different initial solution pH values and temperatures, as well as the calculated solubility products ( sp ) for lead hydroxypyromorphite [Pb 5 (PO 4 ) 3 OH].The aqueous activities of Pb 2+ (aq), PO