Surface Reactions of Apatite Dissolution - Science Direct

JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.

191, 489–497 (1997)

CS974942

Surface Reactions of Apatite Dissolution Sergey V. Dorozhkin Research Institute of Fertilizers and Insectofungicides, Kudrinskaja sq. 1—155, 123242 Moscow D-242, Russia Received March 12, 1997; accepted April 18, 1997

New experimental data about surface processes of interaction between natural apatite and phosphoric acid solutions were obtained by scanning electron microscopy, Auger electron spectroscopy, and IR reflection spectroscopy. The interaction was found to occur nonstoichiometrically (incongruently) on the very thin surface layer of apatite. The experimental data obtained were compared and extended with results taken from literature. The following sequence of ionic detachment from the surface of apatite to a solution was suggested: first fluorine for fluorapatite or hydroxyl for hydroxyapatite, next calcium, and afterward phosphate. A new chemical mechanism of apatite dissolution was proposed as a result. The mechanism for the first time described the surface irregularity of the dissolution process at the nanolevel. A comparison between this new dissolution mechanism and earlier mechanisms described in the literature was made. q 1997 Academic Press Key Words: apatite; surface chemistry; dissolution; mechanism; nanolevel.

INTRODUCTION

Calcium apatite (fluorapatite Ca10 (PO4 )6F2 and hydroxyapatite Ca10 (PO4 )6 (OH)2 ) turns out to be the main source of inorganic phosphorus in nature (1). Phosphorus-containing compounds appear to be very important both for plants and for animals and people: phosphorus fertilizers are necessary for agriculture to produce high-yielding crops (2), while hydroxyapatite (more exactly, nonstoichiometric hydroxycarbonate apatite) happens to be the main inorganic component of bones and teeth in animals and humans (3). For these reasons, the chemistry of apatite dissolution and crystallization (4) is being widely investigated both for fertilizer production industry (2) and medicine (4), including problems of artificial bone creation (biomineralization (5–7) and bioceramics (8)), not to mention dental caries protection (9). Each of the recent books (1–9) includes hundreds references therein. The latter points to importance of apatite and other calcium phosphates for the human being. If a specific influence of biological molecules is omitted, such different processes like phosphorus fertilizer production (2), dental caries (9), and demineralization of bones as a result of some diseases (5) appear to be rather similar: each of them can be simulated by the acidic dissolution of apatite

(1–9). So, both natural and artificial apatites are widely used for experimental investigations in the fertilizer production industry (the former), as well as in medicine instead of the real bones and teeth (the latter). That is why various results devoted to apatite dissolution are taken from literature and discussed in this article. A number of investigations devoted to apatite dissolution kinetics and mechanisms in water, buffers, and acidic media have already been made (e.g., 10–48 and references therein). Different models for apatite dissolution have been proposed as a result. Some of them are listed below: diffusion-controlled and kinetic (or surface)-controlled models (e.g., 18–25, 36, 37, 41, and references therein), self-inhibited calcium-rich layer formation (e.g., 14, 15, and references therein), and similar permselective membrane models (e.g., 16 and 17), polynuclear mechanisms (e.g., 10–12 and references therein), congruent/incongruent (or stoichiometric/nonstoichiometric) mechanisms for apatite (e.g., 25, 33, 36–38, 41, 45, and 46) and dental enamel (e.g. 47 and 48), etch pit formation (e.g., 13, 27–32, and references therein), and rather similar to it two-site (42–44) mechanisms, ion exchange mechanisms (26), etc. The catalytic effect of H / ions chemisorbed on an apatite surface to the dissolution process was also discussed several times (11, 12, 18–24). The latter can be probably described as a hydrogen catalytic mechanism. All experimental and theoretical results above (1–48) were obtained by different experimental techniques under various experimental conditions. This is why a large number of mechanisms have been proposed. It turns out that no dissolution mechanism for apatite has been created completely. As a result, each mechanism appeared to be able to describe the dissolution process rather well under a specific set of experimental conditions only. Nobody tried to summarize the different data obtained to create more or less universal dissolution mechanism. The brief analysis below proves that surface processes of apatite dissolution are not completely understood either. For example, diffusion-controlled and kinetic-controlled mechanisms (18–25, 36, 37, 41), as well as models of permselective membrane and self-inhibition calcium-rich layer formation (14–17), are mainly devoted to transport of chemical

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0021-9797/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved.

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reagents to apatite the surface being dissolved and products of chemical interaction backward; polynuclear (10–12), etch pit formation (13, 27–32), and two-site (42–44) mechanisms are devoted to the dissolution of apatite as if it were an ordinary solid (the first one is based on dissolution nuclei formation on crystal surface, the second and third ones are based on defects in crystal structure). Only congruent/incongruent (stoichiometric/nonstoichiometric) (25, 33, 36– 38, 41, 45, 46), hydrogen catalytic (11, 12, 18–24), and ion exchange mechanisms (26) can be considered as attempts to describe the surface processes occurring during dissolution. The congruent/incongruent (stoichiometric/nonstoichiometric) mechanism seems to be better developed among the latter ones. It appeared in the 1960s and probably earlier, when investigators found that Ca/P ratios in the equilibrium solutions with hydroxyapatite were larger than the stoichiometric ratio of 1.67 (34, 35). This effect was explained by surface precipitation of CaHPO4r2H2O. Surface precipitation of CaHPO4r2H2O at pH õ 4.5 on hydroxyapatite and dental enamel was also discussed (36, 37) and found (38– 40) later by other investigators. Moreover, the superphosphate industry (this is a kind of phosphorus containing fertilizer) is based on precipitation of both CaSO4r2H2O and Ca(H2PO4 )2rH2O onto the surface of natural apatite as it is being dissolved (2). The latter is not the surface chemistry of apatite dissolution; it is a precipitation process only, but that is another story. Nevertheless, results of refs. (34–40) can be described as the first attempts in creation of surface chemistry of apatite dissolution. The results obtained by Mika et al. (46) should be mentioned especially. The authors investigated the role of surface reactions in dissolution of stoichiometric hydroxyapatite at pH values ranging from 4.90 to 9.94. They found that ‘‘the dissolution of hydroxyapatite in aqueous medium is always nonstoichiometric,’’ but ‘‘when the solid was successively equilibrated at any given pH, the solution Ca:P ratio approached a limiting value of 1.67. Once this limiting value was reached, the solid only maintained this solution ratio by dissolving stoichiometrically’’ (46, p. 700). Moreover, ‘‘successive re-equilibration of hydroxyapatite at any given pH results in the formation of a surface whose composition is unique for that particular pH’’ (46, p. 700). In other words, on the basis of results of the Ca:P ratio in solution (chemical analysis), the authors found that during dissolution a local nonstoichiometry always appeared on the surface of hydroxyapatite (moreover, it depended on pH), while in the bulk solution nonstoichiometry appeared only at the very beginning of dissolution and was transformed into a stoichiometric ratio when dissolution progressed. The latter was explained by the presence of the previous history of the apatite used (46). To conclude the introduction, a chemical approach to apatite dissolution should be discussed. Two chemical reactions

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are widely used for description of fluorapatite and hydroxyapatite dissolution (1–9): Ca10 (PO4 )6 (F, OH)2 / 14H / Å 10Ca 2/ / 6H2PO 40 / 2HF, H2O,

[1]

Ca10 (PO4 )6 (F, OH)2 S 0 0 10Ca 2/ / 6PO 30 4 / 2F , OH .

[2]

The first describes chemical transformation of apatite into acidic calcium phosphates, while the second describes the dissolution process as a simple dissociation and reports nothing about final products of chemical transformation. Both of them ‘‘do not necessarily give the mechanism but merely express the net reaction’’ (36, p. 333). A very interesting discussion, that of which reaction is more correct and where chemical interactions between pro0 0 anions occur (on the surface tons and PO 30 4 , F , and OH of apatite (10–12) or in solution near the surface (36, 37)), was promoted by Pearce (49), followed by a response by Chow (50). Each person who is truly interested in surface phenomena of apatite dissolution should read these discussions. Both authors reported very interesting arguments, but unfortunately, an experimental setup that is able to give the complete answers for Dr. Pearce’s questions (49) has not yet been created. Currently only indirect investigations can be used to create the surface dissolution mechanism at the atomic (ionic) level instead of reactions [1] and [2]. The latter can be considered as the main goal of the present investigation. MATERIALS AND METHODS

General Experimental Conditions Crystals of natural Kola fluorapatite and chemically pure phosphoric acid were used for the experiments. These crystals were chosen because they contained a very small amount ( õ0.01%) of hydroxyl, carbonate, and chlorine (2). Before experiments, the crystals used were selected from admixed minerals with a polarization microscope MFNE 1U-4.2 (LOMO, USSR) and a needle. 0.1–1 g of the selected crystals was placed into a heated glass reactor with an agitator and a reflux condenser, which contained 200–300 ml of the acid solution. The experimental conditions used were as follows: interaction time, was 3–20 s; temperature, 50– 907C; Reynolds hydrodynamics number (Re), 2000–3000; concentration of the acid solution, 2–7 M H3PO4 ; initial size of the crystals investigated, 500–2000 mm. Scanning Electron Microscopy A special technique was developed for the investigations (51). Briefly, experiments were performed in a thermostatic

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SURFACE REACTIONS OF APATITE DISSOLUTION

glass vessel with acid solution. A relatively large (1–2 mm) single crystal of natural fluorapatite, glued directly onto a microscope stub, was dipped into the vessel with phosphoric acid for 3–5 s. The stub with the crystal was then quickly ( õ0.5 s) transferred into acetone for 15–20 s to stop the dissolution process and to remove traces of acid, followed by drying the specimen in air at room temperature. Then the crystal was studied and filmed in the scanning electron microscope (SEM) JSM 35CF in the secondary electron mode (52) under the acceleration voltage of 15 kV without previous vacuum deposition of a golden film (51). After being filmed, the crystal was taken out of the SEM and the treatment in acid described above followed again. Thus, the technique made it possible to follow the dissolution process only periodically but under magnifications as great as 10,000 and even more. The complete description of the above technique was published earlier (51). Infrared and Auger-Electron Spectroscopy A flat piece (1 cm in diameter and 0.3 cm thick) was cut from a very large crystal of natural fluorapatite. After being polished, the piece was placed into a heated glass reactor with an agitator and a reflux condenser, which contained 200–300 ml of the acid solution. After being slightly dissolved (during 3–10 s), the piece was quickly removed from the acid and placed in acetone for 20–25 s, followed by drying in air at room temperature until a constant mass could be distinguished. After being washed and dried, the surface of the piece was studied with infrared (IR) reflection spectroscope Specord 75 (in the range of 2000–200 cm01 ) and Auger electron spectroscope (AES) Jump 10 with the standard measurement techniques (52), published elsewhere (51, 53). Acetone, used for washing here and in the SEM investigations described above, was distilled under low pressure; the residues of the acetone were diluted with hydrochloric acid, and determination of calcium (atomic absorption (54)) and phosphate (colorimetric procedure (54)) followed. The results obtained for the treated crystals were compared with those for initial fluorapatite. RESULTS

Auger Electron Spectroscopy The results obtained with AES indicated that some chemical changes occurred on the surface of fluorapatite during interaction with phosphoric acid (51). A very thin surface layer, equal to the Auger electrons formation and scattering (several nm), was found to have lost all fluorine. Taking into consideration a small initial amount of fluorine in fluorapatite (about 3.5% F) on the one hand and difficulties in the AES determination of the light elements (52) on the other hand, the amount of fluorine left was not determined precisely.

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Thus, only qualitative results for fluorine removal were obtained by the AES. However, the same results of AES for calcium and phosphorus unambiguously indicated that surface composition of fluorapatite changed toward formation of acidic calcium phosphate(s). The atomic ratio Ca:P was found to fall down from 1.67 { 0.05 (initial fluorapatite) to 1.30 { 0.05 for crystals treated in acid (51). The latter value was close to that of octacalcium phosphate Ca8H2 (PO4 )6r5H2O (Ca:P Å 1.33) (1, 3, 4). It should be noted that octacalcium phosphate and apatite were found to be able to overgrow easily on each other (e.g. 55–59). From a first point of view, the experimental data obtained are in good agreement with literature. But the value of the Ca:P ratio only is not enough for making the precise conclusion about chemical composition of the substance(s) obtained. So, it would be better to mention intermediate acidic calcium phosphate(s) formed on the fluorapatite surface during dissolution. A layer of this substance(s) was found to be very thin (this value could have been calculated if information about penetration of Auger electrons into fluorapatite crystal lattice was available), but the exact value was not very important. What was really important was the chemical composition of the surface layer, because it undoubtedly consisted of intermediate products of chemical interactions between fluorapatite and phosphoric acid. Scanning Electron Microscopy The results of SEM (secondary electron mode) showed clear evidence for appearance of a conducting layer on the surface of fluorapatite crystals treated in acid (51). Fluorapatite appears to be dielectric (1, 3, 4); the latter prevents direct observation with the SEM (vacuum predeposition of a golden film is usually used for dielectrics (52)), but unlike initial fluorapatite, surface conductivity of the fluorapatite treated in acid was found to be good enough to be studied in the SEM directly (51). On the other hand, crystals of acidic calcium phosphates CaHPO4r2H2O and Ca(H2PO4 )2rH2O were also found to have a surface conductivity good enough to be studied in the SEM without additional preparations (60). Thus, the results of SEM also pointed to formation of some surface layer on apatite treated (51); the layer obtained more likely consisted of acidic calcium phosphates CaHPO4r2H2O and/or Ca(H2PO4 )2rH2O, because both of them appeared to have conductive properties similar to those of fluorapatite treated (60). Currently any exact properties of this layer (conductivity, thickness, chemical composition) remain unknown, but thickness appeared to be less than the resolution of modern SEM. A similar conclusion about a possibility of HPO 20 ions 4 formation on the surface of pure apatite during dissolution was recently made by Christoffersen et al. (11, 12). In dentistry a high content of HPO 20 4 ions was found in artificially

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TABLE 1 Wavenumbers (ni, cm01) of the IR Spectra Bands for the Fluorapatite Crystals n1 initial fluorapatite fluorapatite treated and washed with acetone only fluorapatite treated, washed with acetone, followed by additional washing with water

produced carious lesions at pH Å 4.0 (28), as well as in dissolution experiments of dental enamel (38). The possibility of calcium phosphates being covered by a surface coating of more acid calcium phosphate(s) was also discussed by Nancollas (20). Thus, the results of SEM are in good agreement with references. Infrared Spectroscopy The results of IR spectroscopy also confirmed formation of some layer on fluorapatite crystals, treated in acid. Table 1 presents the data on the position of some adsorption bands of fluorapatite in the IR spectra for initial fluorapatite (row 1), fluorapatite treated in acid followed by washing in acetone (row 2), and same as row 2 but with additional 1 min washings in water (row 3). Three new absorption bands at 1100, 1046, and 668 cm01 were found for the fluorapatite treated (Table 1, row 2). Two of them, at 1100 and 1046 cm01 , were found in the libration range of phosphate groups (4, 61), and, most probably, they represented some of the chemical changes that occurred with bands at 1116 and 1078 cm01 , respectively. The third band at 668 cm01 was new. It appeared in the libration range of hydroxyl groups (4, 61). Bands at 1100 and 1046 cm01 were found to be removed easily following washing in water, while the band at 668 cm01 could not be removed (Table 1, row 3). This band appeared to be removed by mechanical polishing only. To conclude experimental results, one should mention that AES and SEM methods confirmed that the surface layer obtained on fluorapatite was removed in water (51), as described above for IR spectroscopy (Table 1, row 3). In similar dissolution experiments, which were followed by additional washing in water, the results of AES pointed to increasing of the Ca:P atomic ratio from 1.30 { 0.05 to 1.65 { 0.10, while results of SEM pointed to restoration of dielectric properties of fluorapatite treated (51). DISCUSSION

According to the above, three different methods of surface state analysis pointed to some changes that occurred on the surface of fluorapatite as a result of chemical interaction with phosphoric acid. Briefly, the following changes took place: fluorine left the crystal surface of fluorapatite, most probably, completely and the Ca:P ratio simultaneously fell

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n2

1116

n3

n5

1046

668 668

1078 1100

1116

n4

1078

down to formation of either Ca8H2 (PO4 )6r5H2O or a mixture of acidic calcium phosphates (results of AES); acidic calcium phosphates CaHPO4r2H2O and Ca(H2PO4 )2rH2O appeared to have surface conductive properties similar to those of treated fluorapatite (results of SEM); some chemical transformations of phosphate anions occurred, and a new band in the libration range of hydroxyl appeared (results of IR spectroscopy). As mentioned before, all of these events occurred in the very thin surface layer, equal to penetration of the IR radiation, Auger, and secondary electrons, respectively (approximately several nanometers). So it is hardly possible to speculate about a new surface phase precipitated on fluorapatite during dissolution. Some additional information about the surface layer obtained was extracted from literature. For example, the surface of chemically pure fluorapatite and hydroxyapatite was found to be positively charged in acidic media and negatively charged in basic media (62, 63). A more complete analysis of the interfacial properties in the apatite–aqueous solution system was given in (64, 65). According to the literature, in acidic media a positive charge on apatite appeared as a result of chemisorbtion of either protons (62, 63) or calcium cations and protons (64, 65) from the solution. In basic media apatite was found to be negatively charged because of chemisorbtion of OH 0 anions on the surface (62–65), but equilibration of apatite together with calcium hydroxide was found to result in positive charge formation on apatite surface even in basic medium. The latter was explained by specific chemosorbtion of calcium cations (64). Unfortunately, all mentioned above are hypotheses only because an experimental setup, able to verify directly if chemosorbed protons and/or calcium cations really cause positive charge formation on apatite, has not yet been created. Nevertheless, the positive charge formation on the surface of apatite in acidic media can hardly be doubted. Similar results for some natural apatites from Africa were reported recently (66). The experimental results, devoted to sequence of ionic dissolution of fluorapatite, should be mentioned especially. Fluorapatite was found to dissolve incongruently (nonstoichiometrically) (33, 38, 45–48), and the following sequence of ionic dissolution was established: fluorine, next calcium, and afterward phosphate (67, 68). This conclusion was made after determination of the specific dissolution rates

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for F 0 , Ca 2/ , and PO 30 4 ions. A nonstoichiometrically high amount of fluorine dissolved compare to that for calcium and phosphate was also measured by other investigators (33, 69, 70). Some investigators proposed the possibility of a surface coating formation on the apatite that was being dissolved; the coating was believed to consist of acidic calcium phosphate(s) (11, 12, 20, 28, 34–40). Thus, the results obtained in literature provided a lot of indirect information for the description of the surface phenomena of apatite dissolution. Surface Reactions of Apatite Dissolution

From that discussed above, surface reactions of apatite dissolution in acids can be created. As soon as all experiments above were made in acidic medium, reaction [1] might be considered as an initial basis. According to the results of AES and some references (33, 67–70), fluorine most probably dissolved first. But the results of IR spectroscopy (Table 1, row 2) pointed to hydroxyl incorporation into the crystal lattice of fluorapatite. Hydroxyl is known (1, 3, 4, 9), to be easily replaced with fluorine, and back again, in the lattice channels that are parallel to the c axis. So it would be quite logical if an initial surface reaction describes the exchange of fluorine with hydroxyl or water: Ca5 (PO4 )3F / H2O / H / Å Ca5 (PO4 )3 (H2O) / / HF.

[3]

Incorporation of water instead of hydroxyl is chosen here, because in the acidic medium investigated (2–7 M H3PO4 ) incorporation of basic hydroxyl would be very unusual. A proton (more likely as H3O / (71, 72)) in Eq. [3] is supposed to be previously chemosorbed on the surface of fluorapatite and resulted in surface positive charge formation (62–66). Moreover, according to literature (11, 12, 18–24) it can be used as a catalyst for removing of fluorine and water incorporation instead. A possibility for such substitution was discussed in literature (71–73). A molecule of water consists of two O–H bonds; each of them is believed to be able to a form liberation band at 668 cm01 (results of IR spectroscopy). Moreover, Eq. [3] gives another possible explanation of the surface positive charge formation. According to [3], positive charge is a result of replacing of fluorine anions with neutral water molecules. It does not mean that this article contradicts the previous explanation given in literature (62–66); reaction [3] describes another possible hypothesis only. Now formation of acidic calcium phosphate(s) should be described. As soon as surface positive charge on fluorapatite crystals appeared, Eq. [3], the possibility for interaction of other protons with the surface should decrease. To remove

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the positive charge obtained, dissolution of one calcium cation may be easily supposed: 2Ca5 (PO4 )3 (H2O) / Å 3Ca3 (PO4 )2 / Ca 2/ / H2O.

[4]

Chemical reaction [4] corresponds completely with the results of (67, 68) (calcium dissolves ahead of phosphate) and partly with those of AES, because for the intermediate Ca3 (PO4 )2 obtained, the Ca:P ratio is 1.50 (i.e. the ratio is already less than 1.67 { 0.05, but it is still higher than 1.30 { 0.05). A hypothesis about surface formation of intermediate Ca3 (PO4 )2 during dissolution of apatite becomes more reasonable after considering that amorphous Ca3 (PO4 )2 was often discussed as a possible precursor of apatite formation during crystallization (e.g. 3, 4, 74, and 75). If Ca3 (PO4 )2 could be discussed as a precursor of apatite formation during crystallization, why could not it be supposed as a virtual surface product of apatite dissolution? After the positive charge was removed, further interaction between protons and calcium phosphate obtained on the surface could be described as follows: Ca3 (PO4 )2 / 2H / Å Ca 2/ / 2CaHPO4 .

[5]

Here another calcium cation was replaced with two protons on the surface. Equation [5] completely corresponds with the results of (67, 68) (calcium dissolves ahead of phosphate), SEM (a conductive layer appears because an acidic calcium phosphate has been formed), and IR spectroscopy (some changes with phosphate bands at 1116 and 1078 cm01 have occurred) and partly with the results of AES, because for CaHPO4 the Ca:P ratio is already 1:1 (i.e. below 1.30 { 0.05). Two reasonable explanations for the results of AES are proposed. The first one supposes that an intermediate mixture of CaHPO4 and Ca3 (PO4 )2 is formed on the surface of fluorapatite crystals treated. The second one supposes that octacalcium phosphate Ca8H2 (PO4 )6r5H2O (Ca:P Å 1.333) is formed. No results have been found in the literature to make a choice between the two hypotheses. The only information available, that octacalcium phosphate and apatite were found to overgrow easily on each other (55–59). Octacalcium phosphate on apatite in dissolution experiments was probably found during determination of metastable equilibrium solubility of carbonate apatite powders (76, 77). Unfortunately, both articles contradict each other: ‘‘when a surface complex with octacalcium phosphate stoichiometry was assumed in the calculations, the agreement between the experiments and predictions was almost as good as with the hydroxyapatite stoichiometry’’ (76, p. 356) and ‘‘the entire data analysis was repeated using CaHPO4 stoichiometry and also using the stoichiometry of octacalcium phosphate. Both of these alternative stoichiometries were

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found to be markedly inferior to the assumption of hydroxyapatite stoichiometry for the surface complex (77, p. 243). Thus for now a reasonable choice between surface formation of a mixture of CaHPO4 and Ca3 (PO4 )2 or that of Ca8H2(PO4 )6r5H2O cannot be made. It should be emphasized especially that reaction [5] describes a process of acidic calcium phosphate formation on the surface of apatite being dissolved. From the first point of view, it is rather similar to the results of previous investigators, who found (34–37) or discussed (28, 38–40) the possibility of re-precipitation of dicalcium phosphate in the surface region. But this picture is wrong completely! Reaction [5] does not describe precipitation, it describes formation of the intermediate surface substance CaHPO4 . At last, phosphate anion(s) also detach from the surface according to the following surface reactions: CaHPO4 / H / Å Ca 2/ / H2PO 40 ,

[6]

CaHPO4 / 2H / Å Ca 2/ / H3PO4 .

[7]

As soon as the apatite described above dissolves in water medium, all ions and molecules mentioned before must be hydrated, but the hydration effect (as well as an influence of acidic anions) is omitted everywhere for simplicity.

cium cation(s) around the nearest phosphate group(s) [5]. A very thin surface layer of acidic calcium phosphates (11, 12, 20, 38) is formed as a result (Fig. 1d,e). When all (or almost all) of the nearest calcium cations have been replaced with protons according to reactions [6] and [7], phosphate anion(s) (as H2PO 40 , CaH2PO 4/ , or H3PO4 —it is not yet clear) also detach (Fig. 1f). As a result, the dissolution step moves forward jump-wise over a distance, equal to the dimensions of the phosphate anion, of ˚ (4). The detachment of phosphate anions approximately 3 A and calcium cations results in hole formation (Fig. 1f). Dimensions of this hole should be close to the lattice parameters of apatite. This hole, most probably, is a dissolution nucleus on which the polynuclear dissolution mechanism is based (10–12). The description above probably appears rather speculative, but for the first time it is able to describe such surface phenomena, as dissolution nuclei formation (in this case 0 ions, ‘‘collections of vacant sites for Ca 2/ , PO 30 4 , and OH are formed on the crystal surface’’ (10)) and an elementary act of dissolution step movement (a ‘‘jump’’). Certainly, this mechanism should be experimentally proved and veri˚ , is found, it will fied. If a jump, equal to approximately 3 A confirm the main part of the surface dissolution mechanism proposed in this article.

Comparison with the Crystal Structure of Apatite

Comparison with the Previous Dissolution Mechanisms

The crystal structure of hydroxyapatite was established in 1964 (78) and since that time has often been reproduced (1–9). Two different types of calcium are usually separated: Ca(I) and Ca(II). The former are rather far from channels, where fluorine for fluorapatite or hydroxyl for hydroxyapatite is present, while the latter construct walls of these channels. According to the literature, structures of hydroxyapatite and fluorapatite are similar; there are small differences in positions of hydroxyl and fluorine in the channels along the c axis (1–4, 79). Those differences are not taken into consideration in the surface mechanism above, except for reaction [3]. That is why everything described below is valid for dissolution of both fluorapatite and hydroxyapatite. According to the above, fluorine for fluorapatite or hydroxyl for hydroxyapatite most probably dissolves first (33, 67–70). This is explained by their positions in the channels of crystal lattice (1–9, 78, 79). Dissolution starts with replacement of fluorine for water [3]. Proton(s), chemisorbed on the nearest phosphate group(s), most probably catalyze this process (11, 12, 18–24). A local positive charge on apatite (62–66) is formed as a result (Fig. 1b). The local positive charge obtained is removed by detachment of one of the nearest calcium cations [4]: Ca(II) is more likely to be detached first (Fig. 1c). Acidic anions (not shown in the picture) presented in solution most probably take part in this process. Later, proton(s) from the bulk solution replace other cal-

It is now necessary to briefly compare the new surface mechanism, Eqs. [3] – [7], and the previous dissolution mechanisms taken from the literature. According to the introduction, only congruent/incongruent (stoichiometric/nonstoichiometric) (25, 33, 41, 45, 46), hydrogen catalytic (11, 12, 18–24), and ion exchange mechanisms (26) can be considered as attempts in understanding the surface reactions of dissolution. All other mechanisms mentioned in introduction do not describe surface phenomena at the nanolevel. Moreover, some of them are based on unproved hypotheses. For example, the polynuclear mechanism (10–12) is based on a hypothesis of surface nuclei formation. Strangely enough, the authors of this mechanism have made no attempts to discover these nuclei experimentally, in spite of the fact that atomic force microscopy was invented 10 years ago. The same is valid for the self-inhibited calcium-rich layer formation (14, 15) and permselective membrane models (16, 17). No real proofs of calcium-rich layer formation have been demonstrated except in dissolution kinetics measurements. One can notice, that no direct confirmation has been made in the present article to prove surface the dissolution mechanisms [3] – [7] either. This is correct, but no experimental setup that can follow the dissolution of single ions is available. So, this mechanism is based on indirect information only and should be experimentally verified in future. To decrease this drawback, a great number of references have been used to create this mechanism.

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FIG. 1. Schematic illustration of the surface dissolution mechanism of apatite at the nanolevel: (a) part of initial crystal surface of apatite; (b) replacement of one fluorine (or hydroxyl) anion with water molecule resulting in a local positive charge formation; (c) removal of one of the nearest calcium cations; (d) sorption of a next proton; (e) removal of another calcium cation with simultaneous formation of an acidic calcium phosphate; (f ) detachment of one phosphate anion together (or simultaneously) with the third calcium cation. A jump-wise shift of the dissolution step occurs simultaneously at stage f. ( l ) Fluorine for fluorapatite or hydroxyl for hydroxyapatite; ( ) Ca(II) on the first plane; ( ) Ca(II) on the back plane; / / / ( ) Ca(I) on the back plane; ( ❄ ( / ) ) molecule of water and a local positive charge; ( n ) PO 30 4 tetrahedra; H DH and DH represent surface tetrahedral , respectively. Chemisorbed protons, water molecules, and acidic anions are omitted for simplicity. Note that crystal structure anions of H2PO 04 and HPO 20 4 of apatite is shown very schematically: it should be hexagonal, while here it looks more or less like cubic.

The congruent/incongruent (stoichiometric/nonstoichiometric) mechanism (25, 33, 36–38, 41, 45–48, 67–70, 80) appears to be closest to the new surface mechanism, [3] – [7]. According to [3] – [7], dissolution of apatite at the ionic level occurs incongruently (nonstoichiometrically) only, similar to the results of (33, 38, 45–48, 67–70, 80). On the other hand, there are some results where the dissolution of apatite (1–4, 9, 36, 37, 41) and dental enamel (e.g. 25) was found to be congruent (stoichiometric). A reason-

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able explanation of this difference looks like this: the congruent (or stoichiometric) dissolution was found at the macrolevel only (either for dissolution of relatively large amounts of apatite (e.g. 36, 37, and 41), or at the final stages of dissolution); while, according to the new surface mechanism [3] – [7], the incongruent dissolution occurs on the very thin surface layer only (nanolevel). So the incongruence (nonstoichiometry) can be measured either in solution at the very beginning of dissolution process (46) or on

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solids by means of the modern precise methods of surface state analysis. A recent investigation made by Pearce et al. (33) is in good agreement with the above explanation. According to the results obtained, ‘‘during initial stages of dissolution the F/Ca solution ratio was lower than in the solid state’’ (i.e. a large initial incongruence) ‘‘but rose to reach a plateau higher than in the solid as dissolution progressed’’ (rather close to congruence after taking into consideration the experimental errors estimated) (33, p. 130). The similar conclusion was earlier made by Mika et al. (46) (cited in the Introduction of this paper). Now a brief discussion of about ion exchange (26) and hydrogen catalytic (11, 12, 18–24) dissolution mechanisms follows. The first one is based on the supposition about sorption of protons and acidic anions (citric acid in (26)) from solution on the surface of the apatite that is being dissolved and the removal of calcium and phosphate ions to solution instead. The second mechanism is based on a suggestion about sorption of protons on the phosphate groups of apatite. After being sorbed, protons catalyze dissolution (11, 12, 18–24). Both mechanisms are in good agreement with the surface mechanism [3] – [7] because a catalytic influence of sorbed protons to the dissolution process is assumed in all equations, except for [4]. On the other hand, reactions [4] and [5] describe removal of calcium cations to the solution. It is not necessary to explain that an interaction or exchange between acidic anions in solution and calcium cations being detached from the surface, as is suggested by the ion exchange mechanism (26), more likely occurs. Thus, the new surface mechanism [3] – [7] appears to be in good agreement with the incongruent (nonstoichiometric), ion exchange, and hydrogen catalytic dissolution mechanisms. CONCLUSIONS

A new surface mechanism for the acidic dissolution of apatite is proposed in this paper. The mechanism is based on chemical properties and crystal structure of apatite and seems to take into consideration main experimental and theoretical results currently available (1–80 and references therein). It consists of five successive chemical reactions [3] – [7] to be used instead of reaction [1] and, probably, instead of reaction [2]. The mechanism for the first time describes the main surface phenomena of apatite dissolution including nonstoichiometric (incongruent) detachment of different ions, and it appears to be able to complete and extend some other mechanisms, suggested previously. Nevertheless, further precise investigations (e.g. with atomic force microscopy) are very important to verify or correct some suggestions made in this article. REFERENCES 1. McConnell, D., ‘‘Apatite: Its Crystal Chemistry, Mineralogy, Utilization and Biologic Occurrences.’’ Springer-Verlag, New York, 1973.

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