Characterization , Dissolution , and Solubility of Zn-Substituted Hydroxylapatites [ ( Zn x Ca 1 − x ) 5 ( PO 4 ) 3 OH ] at 25 ∘ C

A series of Zn-substituted hydroxylapatites [(ZnxCa1−x)5(PO4)3OH, Zn-Ca-HA]with the Zn/(Zn +Ca)molar ratio (XZn) of 0∼0.16 was prepared and characterized, and then the dissolution of the synthesized solids in aqueous solution was investigated by batch experiment.The results indicated that the aqueous zinc, calcium, and phosphate concentrations greatly depended on the Zn/(Zn + Ca) molar ratio of the Zn-Ca-HA solids (XZn). For the Zn-Ca-HA dissolution at 25C with an initial pH of 2.00, the final solution pH increased, while the final solution calcium and phosphate concentrations decreased with the increasingXZn. The final solution zinc concentrations increased with the increasing XZn when XZn ≤ 0.08 and decreased with the increasing XZn when XZn = 0.08∼0.16. The mean Ksp values for (ZnxCa1−x)5(PO4)3OH at 25C decreased from 10 to 10 with the increasingXZn from 0.00 to 0.08 and then increased from 10–58.59 to 10–56.63 with the increasing XZn from 0.08 to 0.16. This tendency was consistent with the dependency of the lattice parameter a onXZn.The corresponding free energies of formation (ΔG f) increased lineally from −6310.45 kJ/mol to −5979.39 kJ/mol with the increasingXZn from 0.00 to 0.16.

As the main inorganic constituent of bone and dental enamel of vertebrates, calcium hydroxylapatite (HA) has been broadly used in osteoinductive coatings, bone replacement and repair, dental orthopaedics, and so forth [3,6,[8][9][10][11].The substitution of trace ions in hydroxylapatite can affect not only its lattice parameters  and , crystallinity, and morphology, but also its dissolution mechanism and other physicochemical properties [10][11][12].Zinc is one of the most important essential trace elements for the growth of humans and its incorporation in Ca-hydroxylapatite can significantly improve the bioactivity of Ca-hydroxylapatite [8][9][10].The slow release of zinc substituted in an implant Ca-hydroxylapatite material can also promote bone metabolism and growth around the implant.Thus, the zinc-substituted hydroxylapatite can be a novel biomaterial for bone tissue engineering [10,13].
Calcium hydroxylapatite (HA) can also be applied to immobilize dangerous metallic compounds in metal-contaminated soils and industrial wastewaters due to its huge ion substitution capacity, which can considerably decrease the mobility and bioavailability of Zn 2+ , Pb 2+ , Cd 2+ , Cu 2+ , Ni 2+ , and U 2+ by transforming these toxic metal ions into some new forms having low solubility and high geochemical stability [1,5,[14][15][16][17][18]. Heavy metal cations can easily substitute for Ca 2+ in the hydroxylapatite structure and form zinc-calcium hydroxylapatite (Zn-Ca-HA), lead-calcium hydroxylapatite (Pb-Ca-HA), or cadmium-calcium hydroxylapatite (Cd-Ca-HA) through dissolution-precipitation, ion-exchange, or adsorption process [15].Therefore, a fundamental knowledge of the apatite physicochemical properties, especially the solubility, stability, and water-mineral interaction, is required to understand mineral evolution and natural phenomenon or to optimize the industrial processes concerning apatite [5,19,20].
However, the thermodynamic data for Zn-substituted hydroxylapatites are now lacking, regardless of the fact that its dissolution and elemental release from solid to aqueous solution exert a great influence on the cycling of zinc, calcium, and phosphate.So far, no experiment on the dissolution and stability of the Zn-substituted hydroxylapatite [(Zn x Ca 1−x ) 5 (PO 4 ) 3 OH] has been carried out, for which little information has been reported in literatures.Hence, no thermodynamic data can be obtained to assess the bioactivity and bioavailability of an implant Zn-Ca-hydroxylapatite biomaterial or the environmental risk of zinc concerning the Zn-substituted hydroxylapatite.Additionally, the previous data and results about the effects of the Zn substitution for the Ca sites on the apatite structure and properties are still ambiguous and rather inconsistent [8-11, 21, 22].
In this work, calcium hydroxylapatite [Ca 5 (PO 4 ) 3 OH, Ca-HA] and Zn-substituted hydroxylapatites [(Zn x Ca 1−x ) 5 (PO 4 ) 3 OH, Zn-Ca-HA] with various Zn/(Zn + Ca) atomic ratios were prepared and the influences of zinc replacement on the hydroxylapatite properties were investigated with XRD, FT-IR, FE-SEM, and FE-TEM instruments.Then, the dissolution of the synthesized solids and the release of components (Zn 2+ , Ca 2+ , and PO 4 3− ) were studied, and the solubility product ( sp ) and the corresponding free energy of formation (Δ   ) of the Zn-Ca hydroxylapatites were determined.

Solid Preparation and Characterization
2.1.1.Synthesis.The synthesis of the Ca-HA and Zn-Ca-HA solids was carried out by the precipitation method after the following precipitation reaction: 5M 2+ + 3PO 4 3− + OH − = M 5 (PO 4 ) 3 OH, where M = Ca for Ca-HA and (Zn + Ca) for Zn-Ca-HA.An aqueous solution with [P] = 0.12 mol/L was first prepared by dissolving NH 4 H 2 PO 4 into ultrapure water, and a series of the mixed aqueous solutions with [Zn + Ca] = 0.4 mol/L were then prepared by dissolving Zn( were varied in each preparation to get the mixed aqueous solutions with different [Zn]/[Zn + Ca] molar ratios of 0, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14, 0.16, 0.18, and 0.20.250 mL of the Zn 2+ and Ca 2+ mixture solution was added to 250 mL of 4.4 mol/L CH 3 COONH 4 buffer solution and then 500 mL of the NH 4 H 2 PO 4 solution was also added with vigorous stirring at 23 ± 1 ∘ C, which resulted in the forming of white suspension (Table 1).The NH 4 OH solution was used to adjust the pH of the resulting suspension to 7.5.The suspension was aged at 100 ∘ C for 48 h and then subjected to suction filtration.Finally, the white precipitates obtained were cleaned cautiously with ultrapure water and dried at 70 ∘ C for 16 h.
2.1.2.Characterization.10 mg of each synthetic solid was digested in 20 mL of 1 mol/L HNO 3 solution and then diluted to 100 mL with ultrapure water.It was measured for zinc, calcium, and phosphate using an inductively coupled plasmaoptical emission spectrometer (ICP-OES, Perkin-Elmer Optima 7000 DV) to calculate the solid compositions.The solids were measured using a powder X-ray diffractometer (XRD, X'Pert PRO) that was set to 40 kV and 40 mA with a Cu K radiation at a scan speed of 0.2 ∘ /min.Phase identifications were made by comparing the recorded XRD patterns of the solids with the reference code 00-024-0033 for calcium hydroxylapatite and the reference code 00-020-1427 for ammonium zinc phosphate (NH 4 ZnPO 4 ) from the ICDD standards.A field emission scanning electron microscope (Hitachi FE-SEM S-4800) was used to observe the morphology of each solid.

XRD.
The XRD results proved that Ca-HA and Zn-Ca-HA before dissolution (Figure 1  (Figure 1(b)) were the apatite group minerals that belong to the hexagonal crystal system P6 3 /m.The precipitate of  Zn = 0 was identified to be Ca-HA (ICDD reference code 00-024-0033).The XRD patterns of the Zn-Ca-HA precipitates of  Zn ≤ 0.16 differed from each other only in their peak location, peak intensity, and peak width.The (002), ( 211), (102), and (210) reflection peaks of the solid samples shift regularly and slightly to the high-angle direction with the increasing  Zn due to the replacement of Ca 2+ (0.099 nm) by Zn 2+ (0.074 nm), which indicated that the Zn-Ca-HA solids were a continuous solid solution when  Zn ≤ 0.16 [25].When  Zn > 0.16, the characteristic diffraction peaks for ammonium zinc phosphate (NH 4 ZnPO 4 ) were also observed and the peaks for Zn-Ca-HA weakened, which showed that  NH 4 ZnPO 4 gradually became the main product; when  Zn > 0.20, the diffraction peaks of HA disappeared in our preliminary experiment.The XRD examination showed that the characters of the Ca-HA and Zn-Ca-HA samples before and after dissolution were not obviously distinguishable (Figure 1).No secondary solid phases formed in the Ca-HA and Zn-Ca-HA dissolution.
The continuous Zn-Ca-HA solid solution could be formed within limited  Zn [8,10,25].The solids prepared can be examined for their compositional homogeneity by considering the broadening of the powder XRD peaks of the major reflections [26].The XRD peak width significantly increased with the increasing  Zn from 0.00 to 0.16, which indicated that the crystallinity of Zn-Ca-HA considerably decreased with the increasing  Zn .On the other hand, the NH 4 ZnPO 4 phase formed when  Zn > 0.16.The peak intensity of NH 4 ZnPO 4 increased and the peak intensity of apatite decreased with the increasing  Zn from 0.16 to 0.20.No parascholzite (CaZn 2 (PO 4 ) 2 ⋅2H 2 O) phase was observed when  Zn = 0.16∼0.20 [27].
The cell parameter  decreased with the increasing  Zn from 0.00 to 0.08, increased with the increasing  Zn from 0.08 to 0.12, and then decreased with the increasing  Zn from 0.12 to 0.16 (Figure 2), which had also been confirmed by some previous researchers [8,10,25].The cell parameter  decreased up to  Zn = 0.05 and began to increase over  Zn = 0.05 [25].The cell parameter  decreased with the increasing  Zn up to 0.10 and increased over  Zn = 0.10 [8,10].The cell parameter  decreased with a lower zinc substitution in the Ca-HA lattice ( Zn = 0.00∼0.08)because the ion radius of Ca 2+ (0.099 nm) is larger than that of Zn 2+ (0.074 nm).The increase in the cell parameter  for higher  Zn (0.08∼0.12) was attributed to the increasing amount of lattice H 2 O that could incorporate in OH sites in the apatite structure [25].

FE-SEM.
Figure 3 shows the FE-SEM images of the solids with various  Zn = 0.00 to 0.19.The Ca-HA solid ( Zn = 0.00) was an aggregate of fine rod-like particles with 20∼50 nm in width and 100∼150 nm in length.The apatite particle size decreased with the increasing  Zn up to 0.19, which also indicated that the crystallinity of the apatite solids decreased with the increasing  Zn , as showed also in the XRD diffraction (Figure 1).For the Zn-Ca-HA dissolution at 25 ∘ C with an initial pH of 2.00 (Figure 4(a)), the solution pH increased from 2.00 to 4.42∼4.90 in 1 h dissolution and became stable (pH = 4.88∼ 6.43) after 5040∼5760 h dissolution.Commonly, the solution pH increased with the increasing Zn/(Zn + Ca) molar ratios of the Zn-Ca apatites ( Zn ) (Figure 5).The solution Zn 2+ , Ca 2+ , and PO 4 3− concentrations were greatly affected by  Zn (Figure 5).The solids with lower  Zn (0.00, 0.02, 0.04, 0.06, and 0.08) showed a different dissolution process from the solids with higher  Zn (0.10, 0.12, 0.14, 0.16, 0.17, and 0.19) (Figures 4(a) and 5).

Dissolution
For the Zn-Ca apatites with lower  Zn or higher  Ca , the solution Ca 2+ concentrations increased quickly with time and reached the highest values after 24∼480 h dissolution.Then the concentrations decreased slowly and were stable after 1800∼2160 h dissolution.The solution Zn 2+ concentrations increased rapidly with time and reached the highest values in 1 h dissolution and then decreased progressively and attained a stable state after 4320∼5040 h dissolution.For the Zn-Ca apatites with higher  Zn or lower  Ca , the solution Ca 2+ concentrations increased steadily with increasing time and reached the highest values after 720 h dissolution and then decreased and became stable after 5760 h dissolution.The solution Zn 2+ concentrations increased quickly with increasing time and achieved the highest values after 1∼24 h and then decreased slowly and became stable after 4320∼5040 h dissolution.The solution phosphate concentrations had an evolution trend similar to the solution Ca 2+ concentrations.Generally, the final aqueous Ca 2+ and phosphate concentrations decreased with the increasing  Zn of the Zn-Ca apatites (Figure 5).The final aqueous Zn 2+ concentrations increased with the increasing  Zn when  Zn ≤ 0.08 and decreased with the increasing  Zn when  Zn = 0.10∼0.19(Figure 5).
For the Zn-Ca-HA dissolution at 25 ∘ C with an initial pH of 5.60 and 9.00, the solution pH, zinc, and phosphate concentrations became stable after 5040-5760 h (Figures 4(b) and 4(c)).The solution zinc and phosphate concentrations were significantly lower than those for the Zn-Ca-HA dissolution at 25 ∘ C with an initial pH of 2.00; that is, the solubility of the Zn-Ca apatites at pH 5.60 or 9.00 was considerably smaller than that at pH 2.00 (Figure 4).
The Zn-Ca apatites dissolved in the acidic solution stoichiometrically during the early stages and then nonstoichiometrically to the end of dissolution.Commonly, the solution [Zn]/[Zn + Ca] molar ratios ( Zn,aq ) decreased with increasing time and were not equal to the stoichiometric Zn/(Zn + Ca) atomic ratios of the Zn-Ca apatites ( Zn ) (Figure 6).During the early stage of dissolution, the solution [Zn]/[Zn + Ca]    and enhanced the dissolution [28].Besides, the coexisting replacement of H + for metallic cations on the solid surface could also cause a H + consumption in the (Zn x Ca 1−x ) 5 (PO 4 ) 3 OH dissolution.In order to describe the H + depletion in the (Zn x Ca 1−x ) 5 (PO 4 ) 3 OH dissolution comprehensively, many processes should be considered: the stoichiometric dissolution of (Zn x Ca 1−x ) 5 (PO 4 ) 3 OH during the early stage, the substitution of 2H + for Zn 2+ or Ca 2+ on the solid surface, and the adsorption/desorption of H + on the solid surface [29].Additionally, the experimental conditions could significantly affect the apatite dissolution [30].Various dissolution models for apatite have been proposed in literatures, but they consider only some specific dissolution aspects and cannot describe the dissolution process comprehensively [30].
Derived from the results of the present experiment and some previous works [30], the following coexisting steps or processes are considered in the (Zn x Ca 1−x ) 5 (PO 4 ) 3 OH dissolution in acidic solution: In Steps (A) and (B), the solution pH was increased from 2.00 to 4.42∼4.90 in 1 h due to the diffusion and adsorption of H + ions onto the Zn-Ca-HA surface for the dissolution at 25 ∘ C with an initial pH of 2.00.In Step (C), Zn 2+ , Ca 2+ , and PO 4 3− ions were removed from the Zn-Ca-HA surface to water solution.Many chemical reactions could happen in the apatite dissolution because of the structural complexity [7].In comparison to Zn 2+ cations, Ca 2+ cations could be preferentially removed from the Zn-Ca-HA surface.Reaction (1) for the Zn-Ca-HA dissolution in aqueous acidic media could be significantly affected by the solution pH and together with the reactions (2)∼( 5) of protonation and complexation, which had caused an increase in the solution pH.As (Zn x Ca 1−x ) 5 (PO 4 ) 3 OH dissolved in water, Zn 2+ cations were transformed into ZnOH + , Zn(OH) 2 0 , Zn(OH)  ( = 0, 1, 2) (6) In Step (D), Zn 2+ and Ca 2+ cations were partly readsorbed from solution onto the Zn-Ca-HA surface as an initial portion of Zn-Ca-HA dissolved and the solution Zn 2+ and Ca 2+ concentrations decreased as the dissolution progressed.In comparison to Ca 2+ ions, Zn 2+ ions were preferentially readsorbed from solution onto the Zn-Ca-HA surface, which resulted in an obvious decrease in the solution [Zn]/[Zn + Ca] molar ratios ( Zn,aq ) with time.Finally, adsorption and desorption of Zn 2+ and Ca 2+ reached a stable state.The solution Zn 2+ , Ca 2+ , and phosphate concentrations were nearly invariable from 7200 h to 8640 h for the (Zn x Ca 1−x ) 5 (PO 4 ) 3 OH dissolution at 25 ∘ C with an initial pH of 2.00.

Determination of Solubility.
The dissolution experiments had been carried out until the analytical uncertainty for the ion activity products calculated from the last two or three samples was less than ±0.25 log units [31].To obtain the solubility products ( sp ) of the (Zn x Ca 1−x ) 5 (PO 4 ) 3 OH solids, the aqueous activities of the zinc, calcium, and phosphate species for the last two or three solution samples (7200 h, 7920 h, and 8640 h) were considered in the calculation.The PHREEQC simulation results indicated that the final equilibrated solutions were unsaturated with respect to any potential secondary minerals (e.g., portlandite The (Zn x Ca 1−x ) 5 (PO 4 ) 3 OH dissolution and the release of Zn 2+ , Ca 2+ , and PO 4 3− can be expressed using reaction (1).The equilibrium constant ( sp ) for reaction (1) can be expressed as follows: where {} is the activity of the solution species.
The standard free energy of reaction (Δ   ) can be calculated from its  sp by For the dissolution reaction (1), or Table 2 lists the calculated solubility products ( sp ) for Ca-HA and Zn-Ca-HA, together with the solution pH, zinc, calcium, and phosphate analyses for the dissolution at 25 ∘ C with an initial pH of 2.00.By using the Gibbs free energies of formation for Zn 2+ , Ca 2+ , PO  2).For calcium hydroxylapatite [Ca 5 (PO 4 ) 3 OH, Ca-HA], the average  sp value was determined to be 10 -57.75±0.12(10 −57.63 ∼ 10 −57.85 ) at 25 ∘ C and the Gibbs free energy of formation (Δ   ) was calculated to be −6310.45kJ/mol in the present work, which were consistent with the results of many previous researches.The  sp value for Ca 5 (PO 4 ) 3 OH has been reported to be 10 -57.65 [14], 10 -57 [32], 10 −58±1 [33], 10 -59 [34], and 10 -58.3 [35].

Summary
Examination by using XRD and FE-SEM confirmed that no obvious variation of the Zn-Ca apatites was observed in the dissolution.The cell parameter  of the Zn-Ca apatites decreased with the increasing  Zn from 0.00 to 0.08, increased with the increasing  Zn from 0.08 to 0.12, and then decreased with the increasing  Zn from 0.12 to 0.16.When  Zn > 0.16, ammonium zinc phosphate (NH 4 ZnPO 4 ) was also observed in the precipitates.No apatite phase formed when the [Zn]/[Zn + Ca] molar ratio in the mixed aqueous solution was greater than 0.20.
The solution concentrations of zinc, calcium, and phosphate were greatly correlated to the Zn/(Zn + Ca) molar ratios of the Zn-Ca apatites ( Zn ).For the dissolution at 25 ∘ C with an initial pH of 2.00, the solids of  Zn ≤ 0.08 showed a different dissolution process from the solids of  Zn = 0.10∼ 0.16.The final solution pH values increased, and the final solution Ca 2+ and phosphate concentrations decreased with the increasing  Zn .The final solution Zn 2+ concentrations increased with the increasing  Zn when  Zn ≤ 0.08 and decreased with the increasing  Zn when  Zn = 0.08∼0.16.For the dissolution at 25 ∘ C with an initial pH of 5.60 and 9.00, the solution zinc and phosphate concentrations were significantly lower than those for the dissolution at 25 ∘ C with an initial pH of 2.00.Generally, the solution [Zn]/[Zn + Ca] molar ratios ( Zn,aq ) were lower than the  Zn values of the corresponding solids.Ca 2+ ions were preferentially removed from solid to solution in comparison to Zn 2+ ions, while Zn 2+ ions were preferentially readsorbed from solution onto the Zn-Ca-HA surface, which resulted in a significant decrease of the solution [Zn]/[Zn + Ca] molar ratios ( Zn,aq ) with time.
The (Zn x Ca 1−x ) 5 (PO 4 ) 3 OH dissolution is considered to include four coexisting steps: diffusion and adsorption of H +

Figure 2 :
Figure 2: Change of the cell parameters  and  with the increasing  Zn for the Zn-substituted hydroxylapatites [(Zn x Ca 1−x ) 5 (PO 4 ) 3 OH].

Figure 5 :Figure 6 :
Figure 5: The solution pH and elemental concentrations after the dissolution of the Zn-substituted hydroxylapatites [(Zn x Ca 1−x ) 5 (PO 4 ) 3 OH] at 25 ∘ C with an initial pH of 2.00 for 360 d.

(
A) Diffusion of H + to the solution-solid interface and adsorption of H + onto the Zn-Ca-HA surface.(B) Transformation of PO 4 3− to HPO 4 2− on the Zn-Ca-HA surface in acidic solution.(C) Desorption of Zn 2+ , Ca 2+ , and PO 4 3− ions from the Zn-Ca-HA surface and ion complexation.(D) Readsorption of Zn 2+ , Ca 2+ , and/or PO 4 3− ions from solution back onto the Zn-Ca-HA surface.

Figure 7 :
Figure 7: Change of the solubility product and the free energy of formation with the increasing  Zn for the Zn-substituted hydroxylapatites [(Zn x Ca 1−x ) 5 (PO 4 ) 3 OH] at 25 ∘ C.