Mineralogical transformation during hydroxyapatite dissolution in simple aqueous solutions

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Mineralogical transformation during hydroxyapatite dissolution in simple aqueous solutions ⁎

S. Boudiaa,b,c, , P. Zuddasa, F. Fernaneb, M. Fialloc, P. Sharrockc a

Institute of Earth Sciences of Paris, Sorbonne Université, Paris, France Chemistry Department, Faculty of Science, Tizi-Ouzou Université, Algeria c SIMAD, Toulouse 3 Paul Sabatier Université, Castres 81104, France b

A R T I C L E I N F O

A B S T R A C T

Keywords: Hydroxyapatite Incongruent dissolution Solubility Monetite

Hydroxyapatite (HAP) is considered to be a stable mineral under surface Earth conditions and adopted as a proxy for paleo-environmental reconstructions and in choice of remediation strategies. A clear description of the solubility properties for HAP is fundamental and can be gained by understanding the basic principals governing the solid-solution interface. In this study we investigated the early stage of HAP dissolution at 25°, 50° and 70 °C using solubility and ATRFTIR spectroscopy measurements. We found a non-stoichiometric release of calcium and total phosphorus in solution in the experimental close to neutral pH conditions. The ATR-FTIR measurements revealed the presence of new peaks corresponding to the vibrations of HPO42 − group. Comparing HAP spectra of the pristine surface to those obtained after 7 days of hydration we propose the presence of monetite (CaHPO4). This rapidly formed new phase would affect the observed low solubility of HAP and its presence could validate the use of this mineral in paleo-environmental reconstructions and ecological remediation strategies.

1. Introduction Hydroxyapatite (Ca5(PO4)3OH) is considered one of the less soluble calcium phosphate minerals found in nature (Filippelli, 2002; Adcock et al., 2013). Detailed knowledge of the mechanism controlling the stability of this mineral is fundamental in predicting phosphorus release and ecosystem regulation (Bengtsson et al., 2009) as well as in remediation strategies for polluted wastewaters (Valsami-Jones et al., 1998; Harouiya et al., 2007; Nzihou and Sharrock, 2010; Zhu et al., 2016). Besides the low solubility and enhanced radiation tolerance resulting from the structural flexibility (Jolley and Smith, 2016), hydroxyapatite is also a potential matrix for radioactive ion removal (Nishiyama et al., 2016) and for nuclear-waste storage under geological depository (Oelkers and Montel, 2008). A number of studies focused on hydroxyapatite (HAP) solubility proposing controversial interpretations on the dissolution mechanisms (Kaufman and Kleinberg, 1979; Dorozhkin, 1997; Fulmer et al., 2002; Chaïrat et al., 2007a; Harouiya et al., 2007). Because of its low solubility, the dissolution kinetics of hydroxyapatite may be controlled by reactions occurring at the mineral surface (Berner, 1981) requiring the identification of active sites at the surface (Christoffersen et al., 1999). Several authors suggested that, during HAP dissolution, a proportion of

⁎

released ions could be ‘readsorbed’ to the mineral surface forming Caphosphate coatings (Mafe et al., 1992; Thomann et al., 1993; Bertazzo and Bertran, 2006; Bertran et al., 2006). The formation of coatings was also considered to be responsible for the difference in solubility between hydroxyapatite and fluoroapatite (Gray et al., 1962; Rootare et al., 1962; La Mer, 1962; Neuman and Bareham, 1975; Kaufman and Kleinberg, 1979; Larsen and Jensen, 1989; Bertazzo et al., 2010; Chaïrat et al., 2007b). In this work, we investigated the earlier stage of the hydroxyapatite dissolution: the evolution in solution composition was analyzed and the surface modifications were investigated by ATRFTIR spectroscopy (Attenuated Total Reflectance Fourier Transform Infrared). To avoid the heterogeneity of the natural hydroxyapatites, we chose Prayon TCP308, a synthetic phosphate made industrially by a reproducible proprietary method which only contains calcium, phosphate and hydroxide ions. 2. Experimental methods 2.1. Solid preparation and characterization As above mentioned, synthetic hydroxyapatite TCP308 (Prayon, Belgium), not thermally treated, was used in this study. The initial solid

Corresponding author at: Institute of Earth Sciences of Paris, Sorbonne Université, Paris, France. E-mail address: [email protected] (S. Boudia).

https://doi.org/10.1016/j.chemgeo.2017.12.007 Received 20 July 2017; Received in revised form 8 December 2017; Accepted 9 December 2017 0009-2541/ © 2017 Published by Elsevier B.V.

Please cite this article as: Boudia, S., Chemical Geology (2017), https://doi.org/10.1016/j.chemgeo.2017.12.007

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Fig. 1. Evolution of calcium and total phosphorus vs time at different temperature.

Scientific Inc.) software analytical procedures were used to convert the spectra from ATR to transmittance and to perform additional analysis such as peak resolution. In order to identify calcium phosphate species present on the ATR-FTIR spectra, several species were considered: Ca (H2PO4)2·H2O bis (dihydrogenphosphate) monohydrate or MCPM monocalciumphosphate monohydrate (Sánchez-Enríquez and ReyesGasga, 2013); CaHPO4 calcium hydrogenphosphate, or monetite (Petrov et al., 1967; Tortet et al., 1997; Hsu et al., 1998; Xu et al., 1999; Zavgorodniy et al., 2012); CaHPO4·2H2O, calcium hydrogenphosphate dihydrate, or brushite (Petrov et al., 1967); Ca8(HPO4)(PO4)·5H2O or octacalcium phosphate, OCP (LeGeros, 1985; Drouet, 2013); CaxHy(PO4)z·nH2O, n = 3–4.5, or amorphous calcium phosphate (ACP) (Gadaleta et al., 1996; Sinyaev et al., 2001); Ca10 − x(HPO4)y(PO4)6 − xy calcium deficient hydroxyapatite, or HA(HAP) (Domashevskaya et al., 2014).

was analyzed by powder X-ray diffraction (XRD) with a Bruker D2 X'PertPRO diffractometer using Cu Kα radiation (40 kV and 40 mA). The crystallographic identification of TCP308 was accomplished by comparing the experimental XRD pattern to card#9001233 of hydroxyapatite standard (Crystallography Open Database, COD). In our starting material we found in the typical 2θ regions only the characteristic peaks at 21–29°, 32–34°, 39–41°, 46–54° corresponding to hydroxyapatite phase (space group P63/m). The grain morphology observed by scanning electron microscopy (SEM, Zeiss Supra 55VP) revealed the presence of well-formed euhedral grains with an average size between 200 and 800 nm. The specific surface area, determined by BET method (Micrometrics Gemini Vacprep 061) was 7 m2g− 1. 2.2. ATR-FTIR spectroscopy

2.3. Dissolution experiments and solution analysis

Attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectra were obtained on a Nicolet Impact 410 FTIR. Spectra were recorded with a horizontal ATR accessory and a diamond crystal as the reflection element. All absorption spectra were obtained at 8 cm− 1 resolution covering the 4000–500 cm− 1 range and then subtracting a blank scan obtained without samples. OMNIC 8.1 (Thermo Fisher

Dissolution experiments were carried out introducing 1 g of solid and 25 ml of deionized water into 50 ml polypropylene flasks. The flasks were agitated with a Memmert shaking water bath at the constant speed of 240 rpm for a for a time range between 1 h and 22 days at a 2

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given fixed temperature. Three different temperatures of reaction were investigated: 25°, 50° and 70 °C. After the time of reaction, the flask was removed and the suspension filtered using a 0.20 μm pore size membrane filter. In the recovered solution, the pH value was rapidly measured by an ORIAM 290 pH-meter with a glass electrode, previously calibrated by NIST buffers placed in a water bath at 25°, 50° and 70 °C ( ± 0.01) to obtain uniform temperature for pH measurement due to the sensitivity of the pH-meter. Calcium was analyzed by inductively coupled plasma-optical emission ICP-OES (JobinYvon Horiba Y2000) while the total dissolved phosphate was determined by UV–visible spectroscopy (Evolution 220, Thermoscientific) using the phosphovanadomolybdic acid complex colorimetric reaction (Murphy and Riley, 1962). The solid fraction, recovered after filtration from each reactor, was dried at 60 °C overnight and characterized by XRD, SEM and ATRFTIR. 2.4. Thermodynamic calculations

Fig. 3. Variation of Ca/P ratio vs time at different temperature.

Solution saturation state and aqueous ion speciation were estimated using the PHREEQC (Version 3) software with the minteq.v4.dat database (Parkhurst and Appelo, 2013). The aqueous species considered in the calculation included Ca2 +, CaOH+, CaHPO4, CaPO4−, and CaH2PO4+ for the calcium species; PO43 −, HPO42 −, H2PO4−, H3PO4, CaHPO4, CaPO4−, and CaH2PO4+ were considered for the phosphate species.

Table 1 Analytical data of dissolution of hydroxyapatite at 25 °C, 50 °C and 70 °C. Temp

Reaction time

pH

(h) 25 °C

3. Results and discussion 3.1. Evolution of the solution composition The evolution of chemical composition as a function of time is reported in Fig.1, whereas pH and Ca/P molar ratios are reported in Figs. 2 and 3, respectively (all data are reported in Table 1). At 25 °C, aqueous calcium concentration increases slightly as a function of time reaching a steady state after 24 h of reaction. At this temperature, total dissolved phosphorous increases in the first 8 h, then decreases, reaching steady state after 168 h of reaction. At higher temperature, calcium concentration increases the first 6 and 4 h at 50° and 70 °C, respectively, then reaching a steady state. Moreover, at the highest temperatures, phosphorus release is faster: at steady state, concentration is one half to 5 times lower compared to 25 °C. We found that when temperature increases from 25° to 70 °C, calcium concentration is by 2–3 times lower while total phosphorous decreases by 1 order of magnitude. Fig. 3 shows that, in the first stage of the interaction, the dissolved Ca/P molar ratio corresponds to the stoichiometry of the dissolving mineral (1.67 ± 0.05) while after 24 h it increases to 2.30 ± 0.07 and 3.70 ± 0.11 at 25° and 70 °C, respectively. Under all

50 °C

70 °C

1 2 3 4 6 8 24 72 168 312 528 1 2 3 4 6 8 24 72 168 312 528 1 2 3 4 6 8 24 72 168 312 528

6.64 6.74 6.71 6.76 6.75 6.81 6.83 6.92 6.98 7.06 6.95 6.59 6.58 6.67 6.69 6.75 6.72 6.92 7.03 6.97 7.05 7.01 6.78 6.81 6.83 6.86 6.91 7.03 7.15 7.07 7.13 7.17 7.15

Analytical concentration (μmol/L) Ca

P

46.94 48.88 50.43 53.19 54.06 56.96 60.72 59.75 60.20 60.38 59.36 62.19 63.17 65.65 66.89 69.68 64.72 53.29 44.63 42.39 44.36 42.14 64.20 65.13 66.76 69.07 58.62 52.23 31.03 20.06 19.28 18.36 20.15

21.91 22.39 22.65 24.08 25.77 27.35 26.24 17.76 15.13 16.75 15.73 30.89 32.51 31.76 29.88 28.60 25.54 19.78 9.93 7.19 7.89 6.45 31.04 31.30 28.77 26.44 24.17 20.51 8.29 3.71 2.84 2.61 2.28

the experimental conditions, we found that, irrespective of the reaction temperature, the pH value slightly increases in the first hours from 6.7 to 7.3. The results of this study show that HAP dissolution can rapidly deviate from the congruency confirming previous experimental investigations (Smith et al., 1974; Mika et al., 1976; Kaufman and Kleinberg, 1979; Ingram, 1990; Pearce et al., 1995; Bengtsson et al., 2009; Zhu et al., 2009). However, our study shows that both calcium and phosphorous concentrations at steady state decrease when the temperature increases at constant pH conditions indicating the key role of the temperature in the reaction of hydroxyapatite dissolution. Fig. 2. Evolution of pH vs time at different temperature.

3

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Fig. 4. SEM of the hydroxyapatite before (A) and after (B) dissolution at 25 °C for 7 days.

2900 cm− 1reflect the OeH stretching of HPO42 − species (Zavgorodniy et al., 2012) while both bands centered at around 1400 cm− 1 (1407, 1399, 1388 cm− 1) and 1243 cm− 1 (1253, 1234 cm− 1) reflect the PeOeH in-plane bending of the same group (Tortet et al., 1997). These new bands can be related to the presence of PeOeH covalent bonds on the HAP surface. Fig. 5 indicates also the presence of a multiplicity in the PeOeH stretching region. Since hydrogen of the HPO42 − unit is distributed between more than one pair of oxygen atoms in the crystal structure of CaHPO4, the observed multiplicity can be related to the presence of intermolecular and intramolecular hydrogen bonds in hydrogen-phosphate moieties (Petrov et al., 1967). Several compounds contain the hydrogen-phosphate moiety: we exclude the eventual formation of octacalcium phosphate and brushite, as they do not vibrate at energy corresponding to 1400 and/or 1300 cm− 1 bands (Table 2). The vibration energy values observed in the IR spectra are similar to those reported for calcium hydrogen phosphate (CaHPO4), attributed to monetite in the PeOeH in-plane-bending region (Petrov et al., 1967; Bishop et al., 1994; Tortet et al., 1997; Hsu et al., 1998; Xu et al., 1999; Zavgorodniy et al., 2012).

3.2. Characterization of hydroxyapatite surface After 7 days of interaction with water the grain surface observed by SEM reveals that particles appear ‘agglomerated’ with the presence of possible ‘amorphous’ layers partially covering the hydroxyapatite grains, suggesting the possible development of new phases (Fig.4). However, the XRD spectra, collected after 7 days of reaction, do not show any evidence of different calcium phosphate phases even if we cannot exclude an eventual presence of new phases with low abundance and/or poor crystallinity. Fig. 5 shows the ATR-FTIR spectra of the HAP sample before and after the dissolution reaction at the different temperatures of investigation. In all IR spectra, we found the characteristic vibration modes of HAP (Bhatnagar, 1971; Elliot, 1994): (1) the bands related to OH− stretching and bending mode (3570 and 628 cm− 1), (2) the PeO stretching mode (1091, 1024, and 971 cm− 1), and (3) bands corresponding to the OePeO bending mode (601 and 563 cm− 1). After 7 days of reaction, new peaks appear at around 2900 and in the 1230–1400 cm− 1 region in all spectra. Bands at around

Fig. 5. ATR-FTIR spectra of the initial HAP and HAP following its dissolution for 7 days at 25°, 50° and 70 °C.

4

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1407–1403–1388–1253 1089–1014–962

629 600–564

1092–1040–962

The changes of hydroxyapatite surface observed in this study by infrared spectroscopy, could be explained by a chemical equilibrium involving PO43 −, HPO42 −, and H2PO4− ions and the given pH values. Phosphorous ion speciation in aqueous solution at 25 °C corresponds to the following set of acid-base pairs: H3PO4/H2PO4−, H2PO4−/HPO42 −, and HPO42 −/PO43 − with pK1 = 2.19, pK2 = 7.18 and pK3 = 12.3 at 25 °C (Lide, 2006). Given the constancy of pH in our experimental conditions, HPO42 − is expected to be the most abundant phosphate species in solution. The calculated concentration of the different aqueous phosphate and calcium species as a function of temperature (Fig.6) shows that PO43 − is constantly very low while H2PO4− and HPO42 − are predominant, while total calcium is constantly in the Ca2 + form. Fig. 6 also shows that the concentration of H2PO4− increases as the temperature increases while an inverse tendency is found for the HPO42 − species. This matches the observed decreases of the HPO42 − peak intensity as a function of the temperature found in the ATR-FTIR spectra. The experimental data obtained in our study, showing that both calcium and phosphorous decrease when temperature increases and that HPO42 − decreases when temperature rises, reflect the known hydroxyapatite retrograde solubility (Mengeot et al., 1973; McDowell et al., 1977; Arends et al., 1979; Verrecke and Lemaitre, 1990; Stumm and Morgan, 1996). However, when calculating the product of the ion activity at steady state (Πai = {Ca2 +}5 {PO43 −}3{OH−}), we found very close values of Πai (Πai calculated after 7 days of reaction is equal to 10− 60.21 at 25°, 10− 60.54 at 50 °C and 10− 61.52 at 70 °C) indicating that this parameter does not significantly illustrate the retrograde solubility of this very low soluble mineral. In view of the high concentration of H2PO4−, HPO42 − and Ca2+, but also of the changes in infrared spectra, we can expect the formation of CaHPO4 and/or CaHPO4·2H2O on the surface of the hydroxyapatite. Assuming that the reaction of HAP dissolution in aqueous solutions is described by the following reactions (Dorozhkin, 1997, 1999):

631 601–567 585–578–568

790

627 575–560–525

1295–1193 1077–1055–1037–1023–962 917–865 1730–1660 1410–1390–1356–1265 1124–1060–988–967–950 910–890–868–849

3090 2820 2400

3.3. Relationship between solution composition and surface modification

1655 1216–1140 1128–1083–1061–1008–990 885–825 798 750–680 665 579–542

2985–2989–2904

3579 3525 3220

The results of our spectroscopic investigation show that dissolution generates a new phase attributed to the presence of monetite like precipitate. The new formed calcium phosphate is not simply new protonated phosphate mineral but may have a new crystallographic symmetry: HAP is hexagonal while monetite is triclinic with different space group (P1´).We found that the relative intensity of all new peaks decreases when the temperature increases, in agreement with the inhibiting role of temperature in the mechanism responsible for monetite formation at the HAP surface. The formation of monetite can be associated to an eventual presence of a ‘hydrated layer’ earlier reported by Wu et al., 1994; Jäger et al., 2006; and Rey et al., 2014, attributed to the presence non-apatitic domains (Rey et al., 2014) or to a “first hydration” layer (Zahn and Hochrein, 2003; Bertinetti et al., 2007). Nonapatitic domains were in fact observed in inorganic apatite (Rey et al., 2014) and bio-apatites (Wu et al., 1994). Our study shows for the first time that monetite, a non-apatitic phosphate, can be rapidly formed during the dissolution process of HAP in simple aqueous solutions.

Ca5 (PO4 )3 OH(s) + H+(aq) ⇄ Ca5 (PO4 )3 (H2 O)+(s)

(1)

2Ca5 (PO4 )3 (H2 O)+(s) ⇄ 3Ca3 (PO4 )2(s) + Ca2+(aq) + 2H2 O(aq)

(2)

Ca3 (PO4 )2(s) +

Asymmetric OeH stretching of bound water OeH stretching OeH stretching of adsorbed water (P)OeH stretching HeOeH bending and rotation of water HeOeH bending of water PeOeH in-plane-bending PeO stretching PeO(H) stretching HeOeH libration PeOeH out-of-plane bending HeOeH libration OePeO(H) bending

3530–3460 3290–3150 2900 2385

3570

3691–3675

HAP at 25 °C for 7 days (cm− 1) Hydroxyapatite HAP Ca5(PO4)3OH (cm− 1) Octacalcium phosphate OCP Ca8(HPO4)2(PO4)4·5H2O (cm− 1) Monetite DCPA CaHPO4 (cm− 1) Brushite DCPD CaHPO4 2H2O (cm− 1) Vibrations modes

Table 2 Vibration modes observed in each of the ATR-FTIR spectra shown in the initial HAP and HAP following its dissolution for 7 days at 25 °C. Characteristics and modes of vibration wavelengths observed in brushite, monetite and OCP are included for comparison (see text for explanations and references).

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2H+

⇄ 2CaHPO4(s) +

Ca2+

CaHPO4(s) + H+ ⇄ Ca2 +(aq) + H2 PO−4 (aq)

CaHPO4(s) +

⇄Ca2+

(aq)

+

HPO24−(aq)

(aq)

(3) (4) (5)

The process of hydroxyapatite dissolution may produce Ca3(PO4)2 and CaHPO4. However, given our experimental conditions it can be predicted that reactions (4) and (5) move mainly towards the formation of CaHPO4. The formation of different calcium phases previously considered as 5

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Fig. 6. Relative proportions of phosphate species vs temperature (after 7 days of dissolution).

in the crystallographic structure of the pristine mineral surface. We propose that this rapid modification of the HAP surface state controls the observed weak solubility of phosphate minerals in simple aqueous solutions. Our experimental investigation shows that HAP surface chemistry is also governed by the presence of HPO42 − units playing an unexplored role in absorption and bioactivity processes. Our results could help in interpreting the role of HAP matrix for ecological remediation strategies where heavy metals can be easily inerted as insoluble metal phosphate minerals by the presence of a significant amount to phosphate ions in solution. The formation of stable hydrogenphosphate domains should preserve the weathering of HAP under natural conditions confirming the use of this phosphate mineral as proxy of paleo-environmental reconstructions: the rapid formation of monetite passivates the surface preserving the genetic crystal information.

intermediate or precursor during HAP synthesis. Francis and Web (1971) and Neuman and Bareham (1975) found the presence of CaHPO4·nH2O (where n = 0 or 2) while Eanes et al. (1966), Brecevic and Furedi-Milhofer (1972), and Termine et al. (1970) identified amorphous calcium phosphate. To test if one of these minerals may control the apparent solubility, we calculated solution saturation state with respect to these phases by aqueous species, pH and solubility products using PHREEQC (together with the minteq.v4.dat database). We found that our experimental solutions are not saturated with respect to any secondary phase (Saturation Index at 7 days of dissolution, SI, SImonetite = −2.61, − 3.27 and −4.06; SIHAP = − 1.88, − 2.96 and − 4.47 at 25°, 50° and 70 °C respectively) indicating that the formation of these new phases cannot be a classical precipitation process controlled by bulk solution oversaturation. A plausible mechanism explaining our results could be the presence of interfacial dissolutionprecipitation reaction, coupling stoichiometric release of all elements from hydroxyapatite and neoformation of a distinct secondary surface phase, as monetite. If these two processes would occur directly, a sharp dissolution front of the pristine hydroxyapatite should be in contact with a thin interfacial fluid film of water molecules. The presence of monetite corresponds to the observed decrease of the Ca/P ratio producing a secondary layer. We propose that the presence of monetite may result from a stoichiometric Ca/P release from HAP, followed by formation of a new protonated phosphate phase. The alteration identified in this study for HAP, may correspond to a more general mechanism documented in other weakly soluble salts: galena (De Giudici and Zuddas, 2001) feldspars and glass (Hellmann et al., 2015) Interaction between aqueous solutions and weakly soluble salts generally produces a passivation of reactive mineral surface with a decrease of the surface reactivity (Cailleteau et al., 2008). We believe that future investigation on the kinetics of monetite formation at the nanometric scale may explain the solution undersaturation with respect to HAP and monetite and the observed solution steady state.

Acknowledgments SB is grateful to the Algerian Ministry of Education for the fellowship. We thank Dr. Alain Person for discussion and help in the XRD analysis, Dr. Omar Boudouma for help in the SEM work and Nathalie Labourdette for running the ICP-OES. The manuscript has greatly benefited from insightful comments by three anonymous reviewers and the editor Prof Jeremy Fein who improved the quality and the clarity of the work. References Adcock, C.T., Hausrath, E.M., Forster, P.M., 2013. Readily available phosphate from minerals in early aqueous environments on Mars. Nat. Geosci., vol. 6. http://dx.doi. org/10.1038/NGEO1923. Arends, J., Schuthof, J., Van-Der-Linden, W.H., Bennema, P., Van-Den-Berg, P.J., 1979. Preparation of pure hydroxyapatite single crystals by hydrothermal recrystallization. J. Cryst. Growth 46, 213–220. Bengtsson, Å., Shchukarev, A., Persson, P., Sjöberg, S., 2009. A solubility and surface complexation study of a non-stoichiometric hydroxyapatite. Geochim. Cosmochim. Acta 73, 257–267. Berner, R.A., 1981. Kinetics of weathering and diagenesis. Rev. Mineral. 8, 111–134. Bertazzo, S., Bertran, C.A., 2006. Morphological and dimensional characteristics of bone mineral crystals. Key Eng. Mater. 309, 3–6. Bertazzo, S., Zambuzzi, W.F., Campos, D.D.P., Ogeda, T.L., Ferreira, C.V., Bertran, C.A., 2010. Hydroxyapatite surface solubility and effect on cell adhesion. J. Colloids Surf. B 78, 177–184. Bertinetti, L., Tampieri, A., Landi, E., Ducati, C., Midgley, P.A., Coluccia, S., Martra, G., 2007. Surface, structure, hydration, and cationic sites of nanohydroxyapatite: UHRTEM, IR, and microgravimetry studies. J. Phys. Chem. 111, 4027–4035. Bertran, C.A., Bertazzo, S., Faria, L.P., 2006. Surface charge of hydroxyapatite and bone mineral. Key Eng. Mater. 330, 713–716.

4. Conclusions The dissolution mechanism of hydroxyapatites is extremely complexe involving a combination of several simultaneous aqueous and surface reactions. The results of our study show that, during the early stage of HAP dissolution, the release of calcium and total phosphorus under neutral pH conditions is incongruent and monetite, a hydrogenphosphate mineral, is rapidly formed. The identified mineralogical transformation takes place by surface reactions and results in a change 6

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