Journal of Chemistry
Volume 2017 (2017), Article ID 4619159, 13 pages
https://doi.org/10.1155/2017/4619159

Research Article

Characterization, Dissolution, and Solubility of Zn-Substituted Hydroxylapatites [(ZnxCa1−x)₅(PO₄)₃OH] at 25°C

Xin Zhao,¹ Yinian Zhu,² Zongqiang Zhu,³ Yanpeng Liang,³ Yanlong Niu,² and Ju Lin²

¹College of Light Industry and Food Engineering, Guangxi University, Nanning 530004, China
²College of Environmental Science and Engineering, Guilin University of Technology, Guilin 541004, China
³Guangxi Key Laboratory of Environmental Pollution Control Theory and Technology, Guilin University of Technology, Guilin 541004, China

Correspondence should be addressed to Yinian Zhu; zhuyinian@glut.edu.cn and Zongqiang Zhu; zhuzongqiang@glut.edu.cn

Received 21 March 2017; Revised 2 May 2017; Accepted 10 May 2017; Published 15 June 2017

Academic Editor: Henryk Kozlowski

Copyright © 2017 Xin Zhao et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

A series of Zn-substituted hydroxylapatites [(ZnxCa1−x)₅(PO₄)₃OH, Zn-Ca-HA] with the Zn/(Zn + Ca) molar ratio () 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 (). For the Zn-Ca-HA dissolution at 25°C with an initial pH of 2.00, the final solution pH increased, while the final solution calcium and phosphate concentrations decreased with the increasing . The final solution zinc concentrations increased with the increasing when and decreased with the increasing when = 0.08~0.16. The mean values for (ZnxCa1−x)₅(PO₄)₃OH at 25°C decreased from 10−57.75 to 10−58.59 with the increasing from 0.00 to 0.08 and then increased from 10–58.59 to 10–56.63 with the increasing from 0.08 to 0.16. This tendency was consistent with the dependency of the lattice parameter on . The corresponding free energies of formation increased lineally from −6310.45 kJ/mol to −5979.39 kJ/mol with the increasing from 0.00 to 0.16.

1. Introduction

Phosphate apatites form an enormous mineral group due to their huge isomorphic capacity [1, 2], which play an important role in many research areas such as biomaterials and environmental science [3–7].

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, 8–11]. 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 [10–12]. 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 [8–10]. 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 Zn²⁺, Pb²⁺, Cd²⁺, Cu²⁺, Ni²⁺, and U²⁺ by transforming these toxic metal ions into some new forms having low solubility and high geochemical stability [1, 5, 14–18]. Heavy metal cations can easily substitute for Ca²⁺ 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 [(ZnxCa1−x)₅(PO₄)₃OH] 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 [Ca₅(PO₄)₃OH, Ca-HA] and Zn-substituted hydroxylapatites [(ZnxCa1−x)₅(PO₄)₃OH, 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 (Zn²⁺, Ca²⁺, and ) were studied, and the solubility product () and the corresponding free energy of formation () of the Zn-Ca hydroxylapatites were determined.

2. Experimental Methods

2.1. 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: 5M²⁺ + + OH⁻ = M₅(PO₄)₃OH, 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 NH₄H₂PO₄ into ultrapure water, and a series of the mixed aqueous solutions with [Zn + Ca] = 0.4 mol/L were then prepared by dissolving Zn(CH₃COO)₂·2H₂O and Ca(CH₃COO)₂·H₂O into ultrapure water. The moles of Zn(CH₃COO)₂·2H₂O and Ca(CH₃COO)₂·H₂O 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 Zn²⁺ and Ca²⁺ mixture solution was added to 250 mL of 4.4 mol/L CH₃COONH₄ buffer solution and then 500 mL of the NH₄H₂PO₄ solution was also added with vigorous stirring at °C, which resulted in the forming of white suspension (Table 1). The NH₄OH 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.

Table 1: Summary of synthesis and composition of the Zn-substituted hydroxylapatites.

Sample number
Volumes of the precursors (mL)
Solid composition

0.4 M
0.4 M
4.4 M
0.12 M

Zn(CH₃COO)₂·2H₂O
Ca(CH₃COO)₂·H₂O
CH₃COONH₄
NH₄H₂PO₄

Zn-Ca-HA-00
0
250
250
500
(Zn0.00Ca1.00)₅(PO₄)₃OH

Zn-Ca-HA-01
5
245
250
500
(Zn0.02Ca0.98)₅(PO₄)₃OH

Zn-Ca-HA-02
10
240
250
500
(Zn0.04Ca0.96)₅(PO₄)₃OH

Zn-Ca-HA-03
15
235
250
500
(Zn0.06Ca0.94)₅(PO₄)₃OH

Zn-Ca-HA-04
20
230
250
500
(Zn0.08Ca0.92)₅(PO₄)₃OH

Zn-Ca-HA-05
25
225
250
500
(Zn0.10Ca0.90)₅(PO₄)₃OH

Zn-Ca-HA-06
30
220
250
500
(Zn0.12Ca0.88)₅(PO₄)₃OH

Zn-Ca-HA-07
35
215
250
500
(Zn0.14Ca0.86)₅(PO₄)₃OH

Zn-Ca-HA-08
40
210
250
500
(Zn0.16Ca0.84)₅(PO₄)₃

Zn-Ca-HA-09
45
205
250
500
(Zn0.17Ca0.83)₅(PO₄)₃

Zn-Ca-HA-10
50
200
250
500
(Zn0.19Ca0.81)₅(PO₄)₃

the forming of ammonium zinc phosphate (NH₄ZnPO₄).

2.1.2. Characterization

10 mg of each synthetic solid was digested in 20 mL of 1 mol/L HNO₃ solution and then diluted to 100 mL with ultrapure water. It was measured for zinc, calcium, and phosphate using an inductively coupled plasma-optical 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 Kα 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 (NH₄ZnPO₄) from the ICDD standards. A field emission scanning electron microscope (Hitachi FE-SEM S-4800) was used to observe the morphology of each solid.

2.2. Dissolution Experiments

Each dissolution was designed by adding 2.0 g of the synthesized Ca-HA or Zn-Ca-HA to 100 mL of HNO₃ solution (pH = 2) or ultrapure water (pH = 5.6) or NaOH solution (pH = 9) in a series of 150 mL polypropylene bottles, which were then capped and soaked in the water bath of 25°C. From the bottles, 5 mL of the aqueous solutions was sampled at 23 intervals (1, 3, 6, 12, 24, 48, 72, 120, 240, 480, 720, 1080, 1440, 1800, 2160, 2880, 3600, 4320, 5040, 5760, 7200, 7920, and 8640 hours), filtered through a 0.22-μm-pore-size filter and stabilized using 0.2% HNO₃ solution. After each sampling, 5 mL of the corresponding HNO₃ solution, ultrapure water, or NaOH solution was added. The dissolved zinc, calcium, and phosphate in the aqueous solutions were measured using ICP-OES. After 8640 h dissolution, the solids were sampled, cleaned, dried, and then examined using XRD, FT-IR, FE-SEM, and FE-TEM.

2.3. Thermodynamic Calculations

The PHREEQC program (version 3) [23] was used to calculate the aqueous activities of Zn²⁺, Ca²⁺, , and OH⁻, and then the ion activity product (IAP) for Ca-HA and Zn-Ca-HA was determined after the mass-action expression. The minteq.v4.dat and llnl.dat databases were chosen in the simulation [24]. The following solution species were considered in the calculation: Zn²⁺, ZnOH⁺, , Zn, Zn,  , ZnHPO₄, and ZnH₂ for zinc; Ca²⁺, CaOH⁺, Ca, CaHPO₄, and CaH₂ for calcium; and ,  , H₂, H₃, Ca, CaHPO₄, and CaH₂ for phosphate.

3. Results and Discussion

3.1. Solid Characterizations

3.1.1. Chemical Component

The component of the precipitate was dependent on the [Zn]/[Zn + Ca] molar fraction in the starting solution (Table 1). The Zn/(Zn + Ca) molar ratio () and the (Zn + Ca)/P molar ratio of each solid sample were equal to the designed composition of the Zn-Ca hydroxylapatites [(ZnxCa1−x)₅(PO₄)₃OH] when .

3.1.2. XRD

The XRD results proved that Ca-HA and Zn-Ca-HA before dissolution (Figure 1(a)) and after dissolution (Figure 1(b)) were the apatite group minerals that belong to the hexagonal crystal system P6₃/m. The precipitate of was identified to be Ca-HA (ICDD reference code 00-024-0033). The XRD patterns of the Zn-Ca-HA precipitates of 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 due to the replacement of Ca²⁺ (0.099 nm) by Zn²⁺ (0.074 nm), which indicated that the Zn-Ca-HA solids were a continuous solid solution when [25]. When , the characteristic diffraction peaks for ammonium zinc phosphate (NH₄ZnPO₄) were also observed and the peaks for Zn-Ca-HA weakened, which showed that NH₄ZnPO₄ gradually became the main product; when , 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.

(a)

(b)

Figure 1: X-ray diffractograms (XRD) of the Zn-substituted hydroxylapatites [(ZnxCa1−x)₅(PO₄)₃OH] before dissolution (a) and after dissolution at 25°C with an initial pH of 2.00 for 360 d (b).

The continuous Zn-Ca-HA solid solution could be formed within limited [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 from 0.00 to 0.16, which indicated that the crystallinity of Zn-Ca-HA considerably decreased with the increasing . On the other hand, the NH₄ZnPO₄ phase formed when . The peak intensity of NH₄ZnPO₄ increased and the peak intensity of apatite decreased with the increasing from 0.16 to 0.20. No parascholzite (CaZn₂(PO₄)₂·2H₂O) phase was observed when = 0.16~0.20 [27].

The cell parameter decreased with the increasing from 0.00 to 0.08, increased with the increasing from 0.08 to 0.12, and then decreased with the increasing 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 and began to increase over [25]. The cell parameter decreased with the increasing up to 0.10 and increased over [8, 10]. The cell parameter decreased with a lower zinc substitution in the Ca-HA lattice ( = 0.00~0.08) because the ion radius of Ca²⁺ (0.099 nm) is larger than that of Zn²⁺ (0.074 nm). The increase in the cell parameter for higher (0.08~0.12) was attributed to the increasing amount of lattice H₂O that could incorporate in OH sites in the apatite structure [25].

Figure 2: Change of the cell parameters and with the increasing for the Zn-substituted hydroxylapatites [(ZnxCa1−x)₅(PO₄)₃OH].

3.1.3. FE-SEM

Figure 3 shows the FE-SEM images of the solids with various to 0.19. The Ca-HA solid () 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 up to 0.19, which also indicated that the crystallinity of the apatite solids decreased with the increasing , as showed also in the XRD diffraction (Figure 1).

Figure 3: Field emission scanning electron micrographs (FE-SEM) of the Zn-substituted hydroxylapatites [(ZnxCa1−x)₅(PO₄)₃OH] before dissolution (a) and after dissolution at 25°C with an initial pH of 2.00 for 360 d (b).

3.2. Dissolution

3.2.1. Change of Solution pH and Elemental Concentrations with Time

Evolution trends of the solution pH and concentrations of Zn²⁺, Ca²⁺, and with time for the Zn-Ca-HA dissolution in HNO₃ solution (pH = 2.00) or ultrapure water (pH = 5.60) or NaOH solution (pH = 9.00) were showed in Figures 4(a), 4(b), and 4(c).

(a) pH = 2.00

(b) pH = 5.60

(c) pH = 9.00

Figure 4: (a) Change of the solution pH and elemental concentrations with time for the dissolution of the Zn-substituted hydroxylapatites [(ZnxCa1−x)₅(PO₄)₃OH] at 25°C with an initial pH of 2.00 for 360 d. (b) Change of the solution pH and elemental concentrations with time for the dissolution of the Zn-substituted hydroxylapatites [(ZnxCa1−x)₅(PO₄)₃OH] at 25°C with an initial pH of 5.60 for 360 d. (c) Change of the solution pH and elemental concentrations with time for the dissolution of the Zn-substituted hydroxylapatites [(ZnxCa1−x)₅(PO₄)₃OH] at 25°C with an initial pH of 9.00 for 360 d.

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 () (Figure 5). The solution Zn²⁺, Ca²⁺, and concentrations were greatly affected by (Figure 5). The solids with lower (0.00, 0.02, 0.04, 0.06, and 0.08) showed a different dissolution process from the solids with higher (0.10, 0.12, 0.14, 0.16, 0.17, and 0.19) (Figures 4(a) and 5).

Figure 5: The solution pH and elemental concentrations after the dissolution of the Zn-substituted hydroxylapatites [(ZnxCa1−x)₅(PO₄)₃OH] at 25°C with an initial pH of 2.00 for 360 d.

For the Zn-Ca apatites with lower or higher , the solution Ca²⁺ 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 Zn²⁺ 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 or lower , the solution Ca²⁺ 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 Zn²⁺ 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 Ca²⁺ concentrations. Generally, the final aqueous Ca²⁺ and phosphate concentrations decreased with the increasing of the Zn-Ca apatites (Figure 5). The final aqueous Zn²⁺ concentrations increased with the increasing when and decreased with the increasing when = 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 () decreased with increasing time and were not equal to the stoichiometric Zn/(Zn + Ca) atomic ratios of the Zn-Ca apatites () (Figure 6). During the early stage of dissolution, the solution [Zn]/[Zn + Ca] molar ratios () were nearly equal to the stoichiometric for the Zn-Ca apatites of . The solution [Zn]/[Zn + Ca] molar ratios () decreased steadily with time. The higher the , the higher the solution [Zn]/[Zn + Ca] molar ratios (). For the dissolution of the Zn-Ca apatites with = 0.10~0.19, the solution [Zn + Ca]/[P] molar ratios increased slowly with time and reached the highest values in 3~48 h; after that, the solution [Zn + Ca]/[P] molar ratios decreased steadily and became stable to the end of dissolution. For the dissolution of the Zn-Ca apatites with , the solution [Zn + Ca]/[P] molar ratios increased steadily to 2.47~3.59 after 3~48 h dissolution and reached 1.61~2.93 at the end of dissolution. The solution [Zn + Ca]/[P] molar ratios increased with the increasing when and decreased with the increasing when = 0.10~0.19. The solution [Ca]/[P] molar ratios changed with time in much the same manner as the solution [Zn + Ca]/[P] molar ratios because of the very low Zn concentrations in comparison to the Ca concentrations.

Figure 6: Change of the solution [Zn]/[Zn + Ca] and [Zn + Ca]/[P] molar ratios with time for the dissolution of the Zn-substituted hydroxylapatites [(ZnxCa1−x)₅(PO₄)₃OH] at 25°C.

3.2.2. Dissolution Mechanism

For the (ZnxCa1−x)₅(PO₄)₃OH dissolution in acidic solution (pH 2.00 or 5.60), the H⁺ consumption indicated that the adsorption of H⁺ ions to the phosphate groups on the (ZnxCa1−x)₅(PO₄)₃OH surface could transform the phosphate groups from to 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 (ZnxCa1−x)₅(PO₄)₃OH dissolution. In order to describe the H⁺ depletion in the (ZnxCa1−x)₅(PO₄)₃OH dissolution comprehensively, many processes should be considered: the stoichiometric dissolution of (ZnxCa1−x)₅(PO₄)₃OH during the early stage, the substitution of 2H⁺ for Zn²⁺ or Ca²⁺ 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 (ZnxCa1−x)₅(PO₄)₃OH dissolution in acidic solution:

(A)Diffusion of H⁺ to the solution-solid interface and adsorption of H⁺ onto the Zn-Ca-HA surface.(B)Transformation of to on the Zn-Ca-HA surface in acidic solution.(C)Desorption of Zn²⁺, Ca²⁺, and ions from the Zn-Ca-HA surface and ion complexation.(D)Readsorption of Zn²⁺, Ca²⁺, and/or ions from solution back onto the Zn-Ca-HA surface.

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), Zn²⁺, Ca²⁺, and 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 Zn²⁺ cations, Ca²⁺ 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 (ZnxCa1−x)₅(PO₄)₃OH dissolved in water, Zn²⁺ cations were transformed into ZnOH⁺, Zn, Zn, Zn,  , ZnHPO₄, and ZnH₂. Ca²⁺ cations were transformed into CaOH⁺, , CaHPO₄, and CaH₂, and   cations were transformed into , H₂, H₃, Ca, CaHPO₄, and CaH₂, and only a small portion of zinc, calcium, and phosphate existed in the dissociation forms such as Zn²⁺, Ca²⁺, and .

In Step (D), Zn²⁺ and Ca²⁺ cations were partly readsorbed from solution onto the Zn-Ca-HA surface as an initial portion of Zn-Ca-HA dissolved and the solution Zn²⁺ and Ca²⁺ concentrations decreased as the dissolution progressed. In comparison to Ca²⁺ ions, Zn²⁺ 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 () with time. Finally, adsorption and desorption of Zn²⁺ and Ca²⁺ reached a stable state. The solution Zn²⁺, Ca²⁺, and phosphate concentrations were nearly invariable from 7200 h to 8640 h for the (ZnxCa1−x)₅(PO₄)₃OH dissolution at 25°C with an initial pH of 2.00.

3.3. 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 of the (ZnxCa1−x)₅(PO₄)₃OH 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 [Ca(OH)₂], lime [CaO], CaHPO₄·2H₂O, CaHPO₄, Ca₄H(PO₄)₃·3H₂O, Ca₃(PO₄)₂(beta); Zn(OH)₂, Zn(OH)₂(am), and Zn₃(PO₄)₂·4H₂O).

The (ZnxCa1−x)₅(PO₄)₃OH dissolution and the release of Zn²⁺, Ca²⁺, and can be expressed using reaction (1). The equilibrium constant () for reaction (1) can be expressed as follows:

where is the activity of the solution species.

The standard free energy of reaction () can be calculated from its by

For the dissolution reaction (1),

or

Table 2 lists the calculated solubility products () 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 Zn²⁺, Ca²⁺, , and OH⁻ from literatures [32], that is, , , , and , the Gibbs free energies of formation for (ZnxCa1−x)₅(PO₄)₃OH, [(ZnxCa1−x)₅(PO₄)₃OH], were also calculated (Table 2).

Table 2: Analytical data and solubility determination of the Zn-substituted hydroxylapatites (25°C and initial pH 2.00).

Sample
Dissolution time (h)
pH
Concentration (mmol/L)

Average

(kJ/mol)
Average 
(kJ/mol)

Zn
Ca
P

(Zn0.00Ca1.00)₅(PO₄)₃OH
7200
4.95
0.0000
6.8916
3.5385
−57.85
−57.75
−6311.58
−6311.03

7920
4.98
0.0000
6.8941
3.5578
−57.63
−6310.34

8640
4.96
0.0000
6.9015
3.5207
−57.77
−6311.17

(Zn0.02Ca0.98)₅(PO₄)₃OH
7200
4.96
0.0055
6.4824
3.2091
−58.30
−58.21
−6273.53
−6273.00

7920
5.00
0.0052
6.4624
3.1978
−58.03
−6271.98

8640
4.96
0.0061
6.4774
3.2075
−58.29
−6273.48

(Zn0.04Ca0.96)₅(PO₄)₃OH
7200
4.95
0.0442
6.2927
2.8960
−58.67
−58.47
−6234.97
−6233.84

7920
4.99
0.0428
6.3302
2.8905
−58.38
−6233.33

8640
4.99
0.0443
6.3376
2.9283
−58.36
−6233.21

(Zn0.06Ca0.94)₅(PO₄)₃OH
7200
5.09
0.0797
5.4544
2.1479
−58.42
−58.60
−6192.92
−6193.93

7920
5.05
0.0800
5.4494
2.1599
−58.69
−6194.45

8640
5.05
0.0803
5.4643
2.1631
−58.68
−6194.41

(Zn0.08Ca0.92)₅(PO₄)₃OH
7200
5.20
0.1025
4.5511
1.6278
−58.43
−58.59
−6152.30
−6153.22

7920
5.16
0.1043
4.6260
1.6223
−58.68
−6153.72

8640
5.16
0.1048
4.6434
1.6304
−58.66
−6153.63

(Zn0.10Ca0.90)₅(PO₄)₃OH
7920
5.56
0.0541
2.5026
0.8662
−57.99
−58.05
−6109.13
−6109.47

8640
5.54
0.0566
2.4951
0.8717
−58.11
−6109.81

(Zn0.12Ca0.88)₅(PO₄)₃OH
7920
5.55
0.0536
2.7671
1.0083
−57.88
−58.00
−6067.83
−6068.52

8640
5.51
0.0554
2.7446
1.0331
−58.12
−6069.22

(Zn0.14Ca0.86)₅(PO₄)₃OH
7920
5.91
0.0142
1.7239
0.6199
−57.36
−57.48
−6024.22
−6024.89

8640
5.87
0.0138
1.7079
0.6457
−57.59
−6025.57

(Zn0.16Ca0.84)₅(PO₄)₃OH
7920
6.14
0.0101
1.2296
0.7606
−56.44
−56.63
−5978.31
−5979.39

8640
6.09
0.0108
1.1927
0.7458
−56.81
−5980.46

(Zn0.17Ca0.83)₅(PO₄)₃OH
7920
6.02
0.0076
2.2766
1.0067
−56.14
−56.38
−5956.27
−5957.67

8640
5.95
0.0117
2.0809
0.9673
−56.63
−5959.06

(Zn0.19Ca0.81)₅(PO₄)₃OH
7920
5.90
0.0084
2.4043
0.9030
−45.85
−45.96
−5856.88
−5857.53

8640
5.86
0.0092
2.3978
0.9040
−46.08
−5858.19

For calcium hydroxylapatite [Ca₅(PO₄)₃OH, Ca-HA], the average value was determined to be 10–57.75±0.12 at 25°C and the Gibbs free energy of formation () was calculated to be −6310.45 kJ/mol in the present work, which were consistent with the results of many previous researches. The value for Ca₅(PO₄)₃OH has been reported to be 10–57.65 [14], 10–57 [32], 10-58±1 [33], 10–59 [34], and 10–58.3 [35].

No reports on the solubility properties of the zinc-substituted hydroxylapatites have been found in literatures. The mean values for (ZnxCa1−x)₅(PO₄)₃OH at 25°C decreased from 10–57.75±0.12 to 10–58.60±0.18 with the increasing from 0.00 to 0.06 and then increased from 10–58.60±0.18 (10−58.42–10−58.69) to 10–56.63±0.19 with the increasing from 0.06 to 0.16 (Table 2 and Figure 7). This tendency was consistent with the dependency of the lattice parameter on . On the other hand, the corresponding free energies of formation () increased lineally from −6310.45 kJ/mol to −5979.39 kJ/mol with the increasing from 0.00 to 0.16 (Table 2 and Figure 7).

Figure 7: Change of the solubility product and the free energy of formation with the increasing for the Zn-substituted hydroxylapatites [(ZnxCa1−x)₅(PO₄)₃OH] at 25°C.

4. 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 from 0.00 to 0.08, increased with the increasing from 0.08 to 0.12, and then decreased with the increasing from 0.12 to 0.16. When , ammonium zinc phosphate (NH₄ZnPO₄) 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 (). For the dissolution at 25°C with an initial pH of 2.00, the solids of showed a different dissolution process from the solids of = 0.10~0.16. The final solution pH values increased, and the final solution Ca²⁺ and phosphate concentrations decreased with the increasing . The final solution Zn²⁺ concentrations increased with the increasing when and decreased with the increasing when = 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 () were lower than the values of the corresponding solids. Ca²⁺ ions were preferentially removed from solid to solution in comparison to Zn²⁺ ions, while Zn²⁺ 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 () with time.

The (ZnxCa1−x)₅(PO₄)₃OH dissolution is considered to include four coexisting steps: diffusion and adsorption of H⁺ onto the solid surface, transformation of to on the solid surface, desorption and ion complexation of Zn²⁺, Ca²⁺, and , and readsorption of these ions from solution back onto the solid surface.

The mean values for (ZnxCa1−x)₅(PO₄)₃OH at 25°C decreased from 10–57.75 to 10–58.59 with the increasing from 0.00 to 0.08 and then increased from 10–58.59 to 10–56.63 with the increasing from 0.08 to 0.16. This tendency was consistent with the dependency of the lattice parameter on . The corresponding free energies of formation () increased lineally from −6310.45 kJ/mol to −5979.39 kJ/mol with the increasing from 0.00 to 0.16.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

Acknowledgments

The authors thank the Guangxi Collaborative Innovation Centre for Water Pollution Control and Water Safety in Karst Area for the research assistance and the financial supports from the National Natural Science Foundation of China (51638006, 41263009), the Guangxi Science and Technology Development Project (GuiKeGong14124004-3-3), and the Provincial Natural Science Foundation of Guangxi (2014GXNSFBA118054, 2012GXNSFDA053022).

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