Carbonate substitution in lead hydroxyapatite Pb5(PO4)3OH

Accepted Manuscript Carbonate substitution in lead hydroxyapatite Pb5(PO4)3OH

M. Kwaśniak-Kominek, M. Manecki, J. Matusik, M. Lempart PII:

S0022-2860(17)30891-8

DOI:

10.1016/j.molstruc.2017.06.111

Reference:

MOLSTR 23997

To appear in:

Journal of Molecular Structure

Received Date:

17 May 2017

Revised Date:

22 June 2017

Accepted Date:

23 June 2017

Please cite this article as: M. Kwaśniak-Kominek, M. Manecki, J. Matusik, M. Lempart, Carbonate substitution in lead hydroxyapatite Pb5(PO4)3OH, Journal of Molecular Structure (2017), doi: 10.1016/j.molstruc.2017.06.111

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ACCEPTED MANUSCRIPT Carbonate substitution in lead hydroxyapatite Pb5(PO4)3OH

1 2

M. KWAŚNIAK-KOMINEK1*, M. MANECKI1, J. MATUSIK1, M. LEMPART2

3 4 5

1 AGH

6

Protection, al. Mickiewicza 30, 30-059 Krakow, Poland, e-mail: [email protected]

7

(MKK), [email protected] (MM), [email protected] (JM)

8

2

9

1, 31-002, Krakow, Poland, e-mail: [email protected]

10

- University of Science and Technology, Faculty of Geology, Geophysics and Environment

Institute of Geological Sciences, Polish Academy of Sciences, Krakow Research Centre, Senacka

Address correspondence to e-mail: [email protected], mobile: +48793382756

11 12

Abstract

13

Synthetic carbonate lead hydroxyapatite Pb5(PO4,CO3)3(OH,CO3) was precipitated from

14

aqueous solution and characterized. The maximum content of CO32- ion in lead apatites does not

15

exceed 2.25 wt%. For precipitation from aqueous solutions this is even lower and controlled by the

16

solubility of cerussite PbCO3. Carbonate substitution occurs simultaneously in two structural

17

positions: at OH− sites (A-type substitution) and at PO43− sites (B-type substitution). This is the

18

most pronounced in FTIR (Fourier Transform Infrared Spectroscopy) spectra at 865 cm-1 and within

19

the range of 1300–1500 cm−1. The substitution results in slight increase of the unit cell parameter a

20

from 9.874 to 9.904 A. The presence of CO32− in two structural positions results in two stages of the

21

release of CO2 upon heating: at 300–350 °C and at 400 °C. The presence of carbonates has little

22

effect on thermal decomposition of lead hydroxyapatite which starts at about 450 °C resulting in the

23

formation of lead pyrophosphate.

24 25

Keywords: carbonate lead apatite, hydroxylpyromorphite, thermal analysis, XRD, Raman, FTIR

26 27

1. Introduction

28

Lead apatites have recently received a lot of attention. Still, little is known about the

29

presence of carbonate ions in lead apatites (see Sternlieb et al. [1] for the literature review). Similar

30

to calcium apatite, the structure of lead apatite allows many substitutions. According to the general

31

formula of apatites M[XO4]Y, position M can be occupied by Ca and Pb (also by Zn, Cd, Fe),

32

position Y by OH, Cl, F, Br, I, while the anion [XO4] represents [PO4], [VO4], [SO4] and [AsO4]

33

[2]. Apatites usually crystallize in the P63/m space group. The crystal structure of apatite is

34

presented in Fig. 1. The metal ions (e.g., Ca or Pb) occupy nonequivalent structural positions M(1) 1

ACCEPTED MANUSCRIPT 1

and M(2). Each of these ions is bonded with three oxygen atoms. Phosphate tetrahedrons are linked

2

together to form columns of M ions and their coordinating oxygens. As a result, a three-dimensional

3

network of phosphate tetrahedrons and columnar M ions is formed with channels between them.

4

This structure provides the possibility for various substitutions in different structural arrangements

5

[2]. One of the most common and important substitutions in the apatite structure is carbonate

6

substitution. The presence and effects of CO32− in the calcium apatite structure are relatively well

7

studied and understood as the calcium hydroxyapatite is a biomineral and a major component of

8

bones and teeth [3-13].

9

Recent studies [14-15] have shown that position as well as the amount of carbonates in the

10

apatite structure depend strongly on the chemical composition (e.g. the presence of OH−, F−, Cl−)

11

and the conditions of crystallization. Generally, the carbonate ion can be incorporated into an

12

apatite structure in two positions: as a substitution for OH− group in the center of the hexagonal

13

channel (A-type substitution) and for PO43− tetrahedron (B-type substitution) [16]. The atomic

14

arrangement becomes slightly disturbed by, among others, the vacancies appearing as a result of

15

charge compensation [4,17]. A-type substitution is often observed in hydroxyapatite precipitation

16

from aqueous solutions at ambient conditions. This is due to relatively weak bonding of the

17

hydroxyl ions in the apatite structure. At higher concentrations of carbonates, the B position

18

becomes favored [18].

19

Lead apatites have become a subject of intense research during the past 25 years. This

20

results from the development of new reclamation procedure based on in situ immobilization of Pb

21

in soils, sediments, and waste [19-21]. The immobilization method is based on phosphate-induced

22

precipitation of lead apatites (pyromorphite Pb5(PO4)3Cl, hydroxylpyromorphite Pb5(PO4)3OH),

23

which are extremely stable and insoluble [22-24]. Carbonate substitution in lead apatites is of

24

interest as the presence of carbonates is known to significantly decrease the stability of lead

25

apatites. This is based on the studies of carbonate calcium apatites which show that the substitution

26

of carbonates has a destabilizing effect. The B-type substitution (a substitution for PO43−) has a

27

more significant impact on the structure in comparison to the A-type substitution [3,18,25]. Similar

28

mechanism of carbonate substitution in lead apatites may have strong effect on the efficiency of Pb

29

immobilization in the environment which is usually rich in CO2. Therefore, the knowledge on the

30

mechanisms of the carbonate substitution in the pyromorphite structure is necessary to explain the

31

CO32− effect on the stability of substituted lead apatites. The presence of CO32− ion in the structure

32

of

33

(Pb0.963Ca0.01Ba0.027)9.85(PO4)5.7(CO3)0.3[Cl0.90(OH)1.1]. Additionally, the possibility of carbonate

34

ions substitution in the structure of fluorpyromorphite was reported [20,26]. An explanation of the

lead

apatite

was

reported

by

Botto

2

et

al.

[5]

in

natural

pyromorphite

ACCEPTED MANUSCRIPT 1

mechanisms and the extent of potential substitution is lacking though. The presence of carbonate

2

ion in a synthetic pyromorphite structure was described by Sternlieb et al. [1]. These results,

3

however, are partly inconclusive and partly exaggerated as the products of their syntheses were

4

contaminated with cerussite PbCO3 in the form of inclusions. Therefore, the reported high levels of

5

carbonates in pyromorphites are strongly overestimated and the presence of undesirable phase

6

interferes with the results and interpretation of the spectroscopic data.

7

This study presents the comparative characteristics of synthetic lead hydroxyl apatite

8

(hydroxylpyromorphite) and carbonated lead hydroxyl apatite which is the closest analog of a well-

9

characterized calcium carbonate hydroxyapatite. This way, the interpretation and explanation of the

10

results could be carried out by analogy to Ca apatites. For the first time, the results of differential

11

thermal analysis of hydroxylpyromorphites are presented. Moreover, the use of hydrochemical

12

modeling allowed estimation of the maximum range of possible carbonate concentrations in

13

solutions during apatite crystallization, which is controlled by the solubility of other Pb-containing

14

solid phases, particularly cerussite.

15 16

2. Materials and methods

17

2.1. Synthetic procedure

18

Synthesis of carbonate-free hydroxylpyromorphite (sample name: HPY-P) from aqueous

19

solutions was carried out using Methom 846 Dosing Interface. The system allowed a controlled

20

slow rate (0.25 mL/min) mixing of 200 mL of 0.05 M Pb solution (in the form of Pb(NO3)2,

21

CHEMPUR, 99%) and 200 mL of 0.03 M phosphorous in the form of KH2PO4 (POCH SA, 99%)

22

with 100 mL of ultrapure water at 80 °C and at pH=8 (maintained with 2 M KOH). The elevated

23

temperature was used to minimize the dissolution of CO2 from air. After the reaction was

24

completed, the precipitates were filtered through 0.22 µm Millipore filters,. Washed with water and

25

dried in oven for 1 day at 80 °C. Before each analysis, the powder was dried at 80°C for 1 h to

26

minimize contamination by CO2 and water adsorbed from the air.

27

Carbonate-rich hydroxylpyromorphite (sample name: HPY-CO3) was synthesized by

28

mixing the aqueous solutions of 0.037 M Pb(NO3)2 and 0.022 M (NH4)H2PO4 (POCH SA, 99%).

29

Each solution contained 0.0074 M (NH4)CO3 (POCH SA, 99%). Based on solution speciation using

30

PHREEQC [27], this is the maximum concentration of carbonate ions below the saturation of

31

cerussite PbCO3 at the conditions of the experiment. The reagents were weighed into 1 L beakers

32

and dissolved in 500 mL of redistilled water at 25 °C. The lead solution was dropwise added to the

33

phosphate solution with the pH set to 9 with NH4OH. The suspension was the aged for 24 h,

3

ACCEPTED MANUSCRIPT 1

filtered, washed with redistilled water, sonicated and centrifuged. The washing was repeated five

2

times. The product was dried at 80 °C for 1 h prior to the use.

3 4

2.2. Characterization of synthesized products

5

The elemental composition of the precipitates was determined by wet chemical analysis and

6

by SEM-EDS (Scanning electron miscoscopy with energy dispersive spectroscopy). The aliquot of

7

0.5 g of each sample was dissolved in 100 mL of HNO3. Lead concentration was determined with

8

an atomic absorption spectroscopy (AAS, GBC SavantAA). The concentration of phosphates was

9

determined colorimetrically by molybdenum-blue method measuring the absorbance at 870 nm

10

using a HITACHI 1600 UV-Vis spectrophotometer and 1-cm cells. Direct determination of CO2

11

release by Leco analysis was used to determine the weight percent content of CO2 (AcmeLab,

12

Canada, detection limit ±0.02 wt%). The content of H was measured by CHNS method. The

13

morphology of the precipitated powders was characterized with FEI QUANTA 200 FEG SEM

14

microscope on gold-coated samples (AGH, Kraków, Poland).

15

Powder XRD (X-ray diffraction) patterns of the precipitates were recorded with a Philips

16

PW 3020 X’Pert diffractometer (AGH, Kraków) with curved crystal graphite monochromator, Cu

17

radiation, constant step of 0.02° 2Θ, and a rate of 2 s per step. The patterns were collected using Si

18

as an internal standard. The products were identified using XRAYAN software by comparing the

19

sample patterns with the ICDD powder diffraction patterns database. The DHN-PDS software was

20

used to refine the unit cell parameters.

21

The infrared spectra were collected by a Nicolet 6700 spectrometer (Thermo Scientific,

22

AGH Kraków, Poland) using the DRIFT technique (Praying Mantis accessory–Harrick). The

23

samples (3% wt. sample/KBr) were scanned in the mid-infrared (MIR) range of 4000–400 cm−1

24

with a resolution of 4 cm−1 (64 scans). Deconvolution of carbonate bands in the IR spectra was done

25

using Thermo Fisher Scientific Software. The processing was performed in a range of 1250–1550

26

cm−1. The band fitting was achieved using the Gaussian function with a constant baseline.

27

Raman spectra were collected with a DXR Raman microscope (Thermo Scientific) using

28

10,000 scans at 2 cm−1 with Nd laser YAG at 520 nm. The laser power was maintained at 10 mW.

29

Deconvolution of spectra was maintained using Thermo Fisher Scientific Software in a range of

30

950-1200 cm−1. The band fitting was achieved using Gaussian function with a constant baseline.

31

Primary peaks positions as well as their intensity were estimated using the methodology described

32

by Awonusi et al. [28].

33

Thermogravimetric (TGA) analyzes were conducted using a NETZSCH STA 449 F3 Jupiter

34

apparatus (AGH, Kraków, Poland) in argon atmosphere. Samples of 50 mg were heated up to 950 4

ACCEPTED MANUSCRIPT 1

°C at 10°C/min rate. The evolved gas was analyzed for NH3, CO2, and H2O content using a mass

2

spectrometer and argon atmosphere (NETZSCH QMS 403 Aëolos).

3 4

3. Results and discussion

5

At the conditions of the experiment, the products of the synthesis precipitate as a result of

6

two competing reactions: formation of hydroxylpyromorphite and formation of cerussite. At

7

equilibrium, the reactions are:

8

5Pb2+ + 3PO43− + OH− ↔ Pb5(PO4)3OH

(1)

9

Pb2+ + CO32− ↔ PbCO3

(2)

10

The molar concentrations of dissolved substrates were carefully calculated to maintain

11

simultaneously an oversaturation with respect to Pb5(PO4)3OH and slight undersaturation with

12

respect to PbCO3 at the conditions of experiment. This was achieved by equilibrium modeling using

13

PHREEQC computer software [27]. The solubility constants equal to Ksp = 10−25.75 for HPY [29]

14

and Ksp = 10−13.13 for cerussite [30] as well as pH=8 open to the air were used for calculations. This

15

way the concentration of carbonates was high enough to induce ionic substitution in HPY but to

16

avoid coprecipitation of cerussite obscuring the analyses.

17

Both syntheses resulted in formation of a very fine white precipitate identified with XRD as

18

crystalline hydroxylpyromorphite. Wet chemical analysis as well as SEM/EDS indicate that the

19

elemental composition is close to theoretical (Tab. 1). Molar ratio of Pb to P is close to

20

stoichiometric, 1.76 for HPY-P. Slight deficiency of phosphorous results in higher Pb/P ratio equal

21

to 1.96 for HPY-CO3. This is expected in the case of carbonate substitution for phosphate (B-type

22

substitution). The content of H, however, is also slightly lower in HPY-CO3 indicating

23

simultaneous partial substitution at A position. Substitution in both position is often observed for

24

apatites synthesized from aqueous solutions [2].

25

Carbonate content determined in calcium hydroxyapatites reaches 6 wt% [10]. This is an

26

equivalent of 0.5 mol CO3 per formula unit. Assuming similar substitution mechanisms in Pb

27

apatites, one can expect 0.5 mol CO3 per formula unit as a maximum theoretical content of

28

carbonate ion in HPY-CO3. Since the molar mass of Pb-apatite is much higher than that of Ca-

29

apatite, this is an equivalent of 2.25 wt% CO3 in lead apatite. Molar content of carbonates in the

30

synthetic HPY-CO3 in this study equals to 0.11 which is an equivalent of 0.5 wt% (Tab. 1). This is

31

a maximum substitution at the conditions of the synthesis: any higher concentration of CO32− in the

32

solution would result in coprecipitation of lead carbonates.

33

Botto et al. [5] determined 1.32 wt% of carbonates (0.3 mol CO3 per formula unit) in natural

34

substituted Cl-pyromorphite, which is within the theoretical range. In contrast, Sternlieb et al. [1] 5

ACCEPTED MANUSCRIPT 1

report unrealistic values of up to 4.2 wt% carbonate content in synthetic HPY (an equivalent of 0.94

2

mol CO3 per formula unit). This is overestimated probably due to the presence of cerussite which

3

was detected in the products of their synthesis.

4

Scanning electron micrographs of the synthetized products are presented in Fig. 2. Both

5

samples reveal features typical for the precipitates of pyromorphite obtained by dropwise synthesis

6

from aqueous solutions [31]. The morphology of crystals is typical for lead apatites and includes

7

hexagonal rods and needles. The precipitate of HPY-CO3 is finer than HPY-P. Two size

8

populations of crystals are apparent which is, at least partially, a result of the aging of precipitate in

9

suspension. The aging and recrystalization through Ostwald ripening [32] results in coarser crystals

10

similar in both samples. Different synthesis procedure and maturation time can introduce changes in

11

morphology and size of the HPY crystals, hovewer does not change the mechanism of the

12

maturation and further consideration. Observed slight differences in size but not in shape of the

13

HPY crystals indicates that that the presence of carbonate ions in the system does not affect

14

significantly the morphology of hydroxylpyromorphite precipitating from aqueous solutions.

15

The products of both syntheses were identified as HPY by comparing the position of XRD

16

peaks with the patterns of pure and well crystalline form of hydroxylpyromorphite (International

17

Centre for Diffraction Data card: ICDD 87–2477) reported in the Joint Committee on Powder

18

Diffraction Standards (JCPDS). This particular standard has been chosen for comparison because it

19

has been synthesized at 300°C at hydrothermal conditions [33] which allows to assume that there

20

are no carbonate substitutions in the structure. No impurities resulting from the precipitation of

21

cerussite PbCO3, hydrocerussite Pb3(CO3)2(OH)2, lead nitrate Pb(NO3)2 or amorphous compounds

22

were noticed within the detection limits of XRD. The positions of diffraction peaks derived from

23

HPY-P are in perfect agreement with the standard while a very small systematic shift of the peak

24

positions towards the lower angles was observed for selected diffraction peaks of HPY-CO3 (Fig.

25

3). The greatest shift (~0.1 Å) is observed for the crystalline planes with Miller indices (100), (200),

26

(111), and (110) (Table 2). This indicates that the presence of CO3 affects unit cell dimensions

27

parallel to the a axis stronger than these parallel to the c axis. The comparison of unit cell

28

parameters of a carbonate-free standard 87–2477 (a = 9.8828 and c = 7.4406 [33]), the synthetic no-

29

carbonate HPY-P (this work: a = 9.87(4) and c = 7.42(7)), and the synthetic carbonated HPY-CO3

30

(this work: a = 9.90(4) and c = 7.43(6)) indicates that the substitution of CO32− ion in the structure

31

of hydroxylpyromorphite results in slight increase of the parameter a while the parameter c remains

32

the same within the experimental error. This can be interpreted by the analogy to carbonated

33

calcium hydroxyapatites where A-type substitution (for OH anion in the channel) results in an

34

increase in the a-axis and a slight decrease in the c-axis, whereas B-type substitution (in the 6

ACCEPTED MANUSCRIPT 1

phosphate site) results in a decrease in the a-direction and a slight increase in the c-axis [34-36].

2

Simultaneous substitution in both structural positions of HPY-CO3 can produce the effect observed

3

here [37,38]. The unit cell of calcium apatite, however, is smaller and more sensitive to this type of

4

substitution. The content of carbonates in HPY-CO3 is also lower than that observed in carbonated

5

Ca-apatites. It can be concluded that the effect of carbonate presence on the unit cell parameters and

6

subsequently on the structure of hydroxylpyromorphite is relatively small. It is possible that CO32−

7

ion substitutes in both available structural positions but the application of other methods is needed

8

to avoid overinterpretation of this small distortion of the unit cell.

9

FTIR spectroscopy allows not only detection of the presence of carbonate ions but also

10

enables the determination of their structural positions. Figure 4 shows the vibrational spectra of the

11

studied HPY. Due to scarce knowledge regarding the carbonate substitution in HPY, the absorption

12

bands were attributed to carbonate vibrations in the analogy to pyromorphite CPY and carbonate-

13

substituted pyromorphite [5,39] as well as to hydroxyapatite HAP and carbonate-substituted

14

hydroxyapatite [6,8,16,34,37-42]. The summary of the absorption bands positions for HAP, HPY,

15

and CPY is presented in Table 3. The comparison allows identification of several effects resulting

16

from the ionic substitution in apatites.

17

In calcium apatites, the phosphate bands are found at 1090-1032 cm−1 (v3), 962 cm−1 (v1),

18

473 cm−1 (v2), 420 cm−1, and 630-565 cm−1 (v4) [42]. These bands appear at frequencies higher by

19

30 cm−1 in HPY spectrum. This is probably the mass effect of Pb which is much heavier than Ca.

20

Similarly, the presence of Cl heavier than OH causes the shift of v1 and v3 phosphate vibrations

21

toward the lower values of wavenumber in CPY than in HPY spectrum.

22

The least sensitive to the molecular weight is a v4-PO4 band which stays within 473 cm−1 in

23

HAP and 453 cm−1 in HPY and CPY. The v3-PO4 band shifts slightly in the presence of carbonate

24

substitutions in HAP, from 1090, 1046, and 1032 cm−1 for pure HAP to 1089, 1046, and 1040 cm−1

25

for carbonated HAP [42]. The v1 and v4 bands are not shifted. Similarly, carbonate substitution

26

affects the position of v3-PO4 phosphate bands in the spectrum of HPY: from 1038 and 984 cm−1 for

27

HPY-P to1044 and 983 cm−1 for HPY-CO3. The v1-PO4 band of HPY is at 926 cm−1 for both

28

samples. These shifts are small and can be explained by the deformation (decrease in the symmetry)

29

of the structure of HPY as a result of CO3 substitution. A similar effect was observed by Yi et al.

30

[15] in the structure of partially carbonate-substituted fluorapatite Pb5(PO4)3F.

31

The presence of carbonate ions in the structure of synthetic HPY-CO3 results in the

32

appearance of CO32− bands in the regions of 865–875 cm−1 and mostly at 1300–1500 cm−1. The

33

complex experimental curve resulting from overlapping bands in the latter range was modeled by

34

computer decomposition into the bands expected for carbonate ions in the lattice. This spectrum 7

ACCEPTED MANUSCRIPT 1

was modeled based on the amount and position of carbonate bands in natural pyromorphite [5]. The

2

deconvolution of IR bands (Fig. 4) shows four carbonate v3-CO32− bands in HPY-CO3 at lower

3

wavenumbers than it was observed in natural CPY [5]. This confirms the presence of carbonates in

4

the HPY-CO3 structure.

5

Fleet et al. (2004) [16] describes in more details up to three possible structural positions of

6

carbonate ions in the apatite structure: two type-A positions and one type-B position. The A1

7

substitution is in “close” apatite channel configuration which means that a bisector of carbonate ion

8

is normal to c-axis. The A2 substitution is an “open” configuration where a bisector of carbonate

9

ion is parallel to c-axis. B position is in a sloping face of the PO43- ion (inclined to c-axis). The

10

extent of substitution in particular sites depends on the amount of carbonates and the condition of

11

crystallization [16]. Three potential crystallographic sites suitable for CO3 substitution generate

12

three different sets of absorption bands. According to Fleet et al. [16], the high frequency of each

13

carbonate substitution in carbonated calcium HAP are in the following areas: 1571–1506 cm−1 for

14

A2 type, 1540–1451 cm−1 for A1 type, and 1475–1416 cm−1 for B type (as v3 stretching mode).

15

Recent studies on HAP relate the presence of v3-CO3 band at 1546 cm−1 to A-type substitution and

16

1465 cm−1 to the B-type [28]. Numerous reviews have shown that the precise position of these

17

bands may vary significantly [2,3,7,34,36,39-48]. In the case of calcium HAP, v3-CO3 band occurs

18

at 1464 and 1419 cm−1 while v2-CO3 at 875 cm−1. The occurrence of the second band is considered

19

as an indicator of the presence of carbonates in the structure of apatite [42]. Similarly to calcium

20

apatites, carbonate bands in lead apatites occur within the range 1500–1300 cm−1 but at slightly

21

lower frequencies: v3-CO3 band in HPY-CO3 can be observed at 1460, 1422, 1388 and 1334 cm−1

22

while v2-CO3 is observed at 867 cm−1 (Fig. 4). For the heavier CPY, the above mentioned bands

23

appear at 1470, 1429, and 1329 cm−1 [5]. The variation in the position of carbonate bands are

24

caused by the difference in molar mass between CPY [5] and HPY (this study). Similar trend of

25

shifts caused by the variation in molar mass of As substituting for P was also observed for a

26

Pb5(PO4)3OH - Pb5(AsO4)3OH solid solution series [49]. In the spectra of lead apatites containing F-

27

instead of OH-, the carbonate bands occur at the lowest position: at 1330 cm−1 and 1382 cm−1 for A-

28

type substitution and at 1420–1460 cm−1 for substitution at both positions [43].

29

A characteristic band resulting from v1 vibration of OH group appears in the IR spectrum of

30

HAP at 3572 cm−1. This hydroxyl band is observed at 3560 cm−1 in the spectra of both the samples

31

of HPY synthesized in this study, which is consistent with the previous reports [5,50]. The

32

difference between the samples is apparent only in the shape of the band. In the spectrum of HPY-

33

P, a small band at 667 cm−1 is also observed which is attribute to librational vibration of OH−. This

34

can be explained by the fact that carbonate ion partially substitutes OH in apatite column disturbing 8

ACCEPTED MANUSCRIPT 1

the OH stretching mode [13]. This indicates that the position of OH band is not sensitive to the

2

carbonate content in lead hydroxyapatites.

3

Raman spectroscopy is commonly used in the studies on carbonate substitutions as a method

4

complementary to FTIR [17,47,48,51]. In the case of pyromorphites, however, the literature data is

5

fragmentary and sparse. A good decomposition and explanation of phosphate vibration apparent on

6

Raman spectra of HPY was described by and Giera et al. [52] and that of CPY by Bajda et al. [53].

7

No attention was paid to the carbonate bands, though. A comparison of the positions of the Raman

8

bands for the selected apatites and pyromorphites are shown in Table 4. For pyromorphite, v3-PO4

9

phosphate bands occur within the range 1090–1000 cm−1, v1-PO4 approximately at 930 cm−1, v4-

10

PO4 between 440 and 600 cm−1, and v2-PO4 approximately at 390-430 cm−1 [50]. The carbonate

11

bands are found in apatites at ~1100 cm−1 (v1-CO32−) and at 630–800 cm−1 (v4-CO32−) [48,50]. In

12

the spectra of natural pyromorphite (Ca,Pb)5(PO4,CO3)3(OH,Cl), only one very weak band assigned

13

to v1-CO3 at 1116 cm−1 is present [5]. Similarly to IR spectra, the majority of Raman bands resulting

14

from phosphate ions in HPY spectrum occur at slightly lower wavenumbers than the phosphate

15

bands in HAP spectrum.

16

The phosphate and carbonate bands apparent in Raman spectra of synthetic

17

hydroxylpyromorphites are complex and require deconvolution of overlapping v3-PO4 and v1-CO3

18

at a range of 1000-1150 cm-1. The processing of each spectrum was done based on the methodology

19

described by Awonusi et al. [28]. In HPY-P spectrum, the phosphate vibrations are: v3-PO4 at 1085,

20

1045, 1020, 1006 and 959 cm−1; v1-PO4 at 927 cm−1; v4-PO4 at 578, 556, 540 cm−1, and v2-PO4 at

21

423, 421,413, and 390 cm−1 (Fig. 5). For carbonate-rich sample HPY-CO3, the phosphate bands are

22

observed at 1072, 1045, 1021, 1006, and 958 cm−1 for v3-PO4, at 925 cm−1 for v1-PO4, at 579, 551,

23

and 540 cm−1 for v4-PO4, and at 423, 413, and 390 cm−1 for v2-PO4. Similarly to carbonate HAP

24

[29], the intensity of phosphate band at 1006 cm−1 decreases with the increase of the content of

25

carbonates in the structure.

26

The presence of a v1-CO3 band at w 1070 cm−1 in Raman spectrum of HAP is considered an

27

evidence for the presence of carbonates [28,54]. This band is relatively insensitive and doesn’t

28

change the position with substitutions. Similar band is apparent at 1072 cm−1 in the spectrum of

29

HPY-CO3. It is absent in the spectrum of HPY. Most likely, this band results from the presence of

30

carbonate ions in apatite structure. However, the band at 1051 cm−1 reported by Sternlieb et al. [1]

31

does not show in our Raman spectra of HPY-CO3. This might indicate that it resulted from

32

contamination by cerussite. The assignments of 1099 cm−1 band for HPY-P and 1102 cm−1 band for

33

HPY-CO3 are still unclear. They can be considered either as v1-CO3 or as a phosphate band

34

resulting from degeneration of lead apatite structure [28]. 9

ACCEPTED MANUSCRIPT 1

The presence of carbonate substitutions was also identified and partially quantified using

2

differential thermal analysis and evolved gas analysis (TG/DTA/EGA). The patterns of calcium

3

apatites usually show thermal effects resulting from the release of water, release of CO2 (if present),

4

and from the decomposition of the mineral structure [13,55-63]. Two types of water can contribute

5

to the pattern: the adsorbed water (at lower temperatures) and the lattice water (which is released at

6

higher temperatures as a result of the decomposition of apatite structure). The adsorbed water is

7

released between 30 °C and 200 °C. The lattice water (and some CO2, if present) may evolve at

8

200–600 °C. In a CO3-free apatite, the loss of water takes place at higher temperature because

9

carbonated apatites are thermally less stable [57]. A majority of carbonates are lost in the

10

temperature ~450 °C, in one to three steps depending on the type of substitution [9,58,61-63].

11

Decomposition of the structure of HAP consists of few steps and depends strongly on

12

stoichiometry of the crystals. Generally, in stoichiometric apatites decomposition starts with the

13

loss of OH at temperatures just below 500 °C. The removal of OH groups results in rearrangement

14

of the structure to calcium oxides and Ca3(PO4)2. In Ca deficit apatites crystallize Ca2P2O7 These

15

appear as exothermic effect on the DTA curve and coexist with the opposite, endothermic effect

16

resulting from the release of carbonates. At temperatures 400-700 °C a Ca2P2O7 or Ca3(PO4)2 with

17

CaO forms CaO below 900 °C. These effects appear on TG curve as a weight loss [61]. The

18

presence of other metal ions in the apatite lowers the temperatures of these effects [62].

19

To date, the results of thermal analyses of lead apatites are reported only for the synthetic

20

solid solution series pyromorphite–vanadinite [56]. In this study, however, little attention was paid

21

to the presence of carbonate ions. Similar to calcium apatite, thermal effects of several processes

22

can be observed upon heating of lead apatites including release of water, release of carbon dioxide,

23

and decomposition of the structure. The TG DTG, DTA, and EGA curves of the synthesized

24

hydroxylpyromorphites are shown in Fig. 6. Three groups of features are apparent:

25

(1) Low temperature desorption of moisture and CO2 from the sample surface at temperatures

26

below 100°C. A slight decrease in weight (0.5 wt%) is reported in both samples.

27

(2) Release of structural water, substituted carbonates, and reorganization of the structure at the

28

temperatures between 100 and 400 °C. At 300°C, a quite long, exothermic effect of HPY

29

recrystallization into Pb2P2O7 is apparent. This is overlapped with the effects resulting from the

30

presence of water and carbonates. A small effect derived from water and weak effect from the

31

release of CO2 can be observed at 200–400°C for HPY-P (Fig.6c). No significant thermal effect on

32

DTA curve is associated with that indicating that this is still CO2 adsorbed from the air on the

33

surface of the powder. In contrast, the effects observed in the HPY-CO3 curves are considerably

34

higher (Fig. 6d). The CO2 is released from hydroxylpyromorphite in two stages: at 300–350 °C, a 10

ACCEPTED MANUSCRIPT 1

rapid weight loss occurs and approximately 90% CO2 is released, while at 400°C the weight loss is

2

negligibly small and accompanying emission of CO2 is low. The evolved gas analysis shows that at

3

temperatures of 280–450°C a partial release of OH and CO2 occurs. This is associated with a mass

4

loss of 1%. Moreover, a second (small) episode of CO2 release is observed at 400 °C. A two-stage

5

release of CO2 indicates the carbonate ion substitution at two nonequivalent structural positions.

6

Similar phenomenon was observed for calcium apatite [58]. Clear exothermic peak on DTA curve

7

and the associated effect in DTG curve at a temperature of 350 °C are related both, to the release of

8

carbonate ions and to the release of structural water. This is associated with a mass loss of 1%. The

9

process results in thermal conversion of HPY into Pb2P2O7 pyrophosphate. The presence of

10

Pb2P2O7 as a reaction product after heating to 500°C was confirmed by XRD analysis (data not

11

shown).

12

(3) Above the 500 °C, a total decomposition of the mineral structure is observed. Lead

13

pyrophosphate melts above 900 °C and after termination of the analysis products crystallize in the

14

form of lead oxides and phosphorous oxides. No mass loss is associated with this stage.

15

The presence of carbonates does not alter the thermal properties of HPY significantly. The

16

overall mass loss is relatively small and equals to 1.5 wt% for HPY-P and 3 wt% for HPY-CO3.

17

The greatest weight loss (approx. 1 wt%) is observed for carbonated sample at 300-450 °C. The

18

evolution of carbonates is partially connected with evolution of structural water so the weight loss

19

associated solely with the release of carbonates may be overestimated.

20 21 22 23

4. Summary and conclusion The results of carefully designed experiments allow to conclude that substitutions of

24

carbonate ions into the structure of hydroxylpyromorphite are possible. By analogy to calcium

25

apatites, the maximum theoretical content of carbonates probably doesn’t exceed ca. 2.25 wt% CO3

26

(0.5 mol CO3 per formula unit). A considerably smaller amount of carbonate ions in lead apatites

27

than in calcium apatites precipitated from aqueous solutions can be observed. This is a result of

28

saturation of the solution relative to cerussite and competitive precipitation of PbCO3. As the

29

solubility of cerussite is lower compared to calcite, this phenomenon reduces the maximum

30

concentration of HCO3− ions available in the solution during the precipitation of lead apatite.

31

Carbonate substitution occurs simultaneously in two structural positions: at OH− sites (A-

32

type substitution) and at PO43− sites (B-type substitution). This affects slightly a crystalline structure

33

of lead apatite altering the unit cell parameters. The presence of CO32− ions is the most apparent in

34

FTIR spectra. Particularly, the presence of absorption bands at 865 cm−1 and within the range of

35

1300–1500 cm−1 in FTIR spectra can be used to demonstrate the presence of CO32− ions in the 11

ACCEPTED MANUSCRIPT 1

hydroxylpyromorphite structure. In contrast, Raman spectroscopy is far less useful to detect such a

2

low carbonate content as the symmetry of phosphate ion is much less disturbed by the presence of

3

carbonates than it is observed in calcium apatites.

4

For the first time the results of thermal differential analysis of hydroxylpyromorphites are

5

presented. The lead apatite structure undergoes thermal decomposition at about 450oC resulting in

6

the formation of lead pyrophosphate followed by transformation into a mixture of lead oxides and

7

phosphorous oxides. The presence of CO32− substitution in two structural positions was confirmed:

8

two portions of CO2 are released at two distinct temperatures. The presence of carbonates has little

9

effect on thermal stability of lead apatite.

10

Studies that have been conducted so far have demonstrated that the mechanism of ions’

11

substitution into the structure of calcium and lead apatites are similar. therefore, it can be postulated

12

that the effect of CO32− on the solubility of lead apatites is similar to that of calcium apatites. The

13

presence of carbonate ions in the structure of calcium apatite significantly increases their solubility

14

[64]. The substitution of carbonate ion into hydroxyapatite structure in two structural positions

15

results in slight destabilization of the structure [12] partly by affecting the lengths of metal-oxygen

16

bonds [41,42] and partly by necessary charge compensation through the formation of various types

17

of complexes [64]. Preliminary results of the experiments with hydroxyl lead apatites show that

18

they dissolve incongruently in acid and alkaline environments. Therefore, the effect of CO32− ion on

19

the solubility reported in [1] without taking into account the formation of secondary phases should

20

be treated qualitatively rather than quantitatively. Determination of the exact solubility constant of

21

HPY and carbonated HPY may be possible only by direct calorimetric measurements, which is the

22

subject of our further study.

23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

Acknowledgements This work was supported by the NCN grant No. 2014/01/M/ST10/00355. JM was supported from AGH statutory funds No. 11.11.140.319. References 1. P.M. Sternlieb, J.D. Pasteris, B.R. Williams, K.A. Krol, C.H. Yoder, The structure and solubility of carbonated hydroxyl and chloro lead apatites, Polyhedron 29 (2010) 2364–2372. 2. J.C. Elliott, Structure and chemistry of the apatites and other calcium orthophosphates, Elsevier, Amsterdam, 1994. 3. G. Binder, G. Troll, Coupled anion substitution in natural carbon–bearing apatite, Contrib. Mineral. Petr. 101 (1989) 394–401. 4. A. Antonakos, E. Liarokapis, T. Leventouri, Micro–Raman and FTIR studies of synthetic and natural apatites, Biomaterials 28 (2007) 3043–3054. 5. I.L. Botto, V.L. Barone, J.L. Castiglioni, Characterization of a natural substituted pyromorphite, J. Material. Sci. 32 (1997) 6549–6553. 12

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49. M. Kwaśniak–Kominek, M. Matusik, T. Bajda, M. Manecki, J. Rakovan, T. Marchlewski, B. Szala, Fourier transform infrared spectroscopic study of hydroxylpyromorphite Pb10(PO4)6OH2–hydroxylmimetite Pb10(AsO4)6(OH)2 solid solution series, Polyhedron 99 (2015) 103–111. 50. H.H. Adler, Infrared spectra of phosphate minerals: symmetry and substitutional effects in the pyromorphite series, Am. Min. 49 (1964) 1002–1015. 51. R.L. Frost, S.J. Palmer, A Raman spectroscopic study of the phosphate mineral pyromorphite Pb5(PO4)3Cl, Polyhedron 26 (2007) 4533–4541. 52. A. Giera, M. Manecki, T. Bajda, J. Rakovan, M. Kwaśniak-Kominek, T. Marchlewski, Arsenate substitution in lead hydroxyl apatites: A Raman spectroscopic study, Spectrochim. Acta A 152 (2016) 370–377. 53. T. Bajda, W. Mozgawa, M. Manecki, J. Flis, Vibrational spectroscopic study of mimetitepyromorphite solid solutions, Polyhedron, 30 (2011) 2479–2485. 54. W.N. Martens, L. Rintoul, J.T. Kloprogge, R.L. Frost, Single crystal raman spectroscopy of cerussite, Am. Mineral. 89 (2004) 352–358. 55. A.V. Knyazew, N.G. Chernorukov, E.N. Bulanov, Apatite–structured compound: Synthesis and high–temperature investigation, Mater. Chem. Phys. 132 (2012) 773–781. 56. T.I. Ivanova, O.V. Frank–Kamenetskaya, A.B. Kol'tsov, V.L. Ugolkov, Crystal structure of calcium–deficient carbonated hydroxyapatite. Thermal decomposition, J. Solid State Chem. 160 (2001) 340–349. 57. D.W. Holcomb, R.A. Young, Thermal decomposition of human tooth enamel, Calcified Tissue Int. 31 (1980) 189–201. 58. M. Koel, M. Kudrjašova., K. Tõnsuaadu, M. Peld, M. Veiderma, Evolved gas analysis of apatite materials using thermochromatography, Thermochim. Acta 332 (1998) 25–32. 59. W.L. Suchanek, P. Shuk, K. Byrappa, R.E. Riman, K.S. TenHuisen, V.F. Janas, Mechanochemical–hydrothermal synthesis of carbonated apatite powders at room temperature, Biomaterials 23 (2002) 699–710. 60. K. Tõnsuaadu, M. Peld, T. Leskelä, R. Mannonen, L. Niinistö, M. Veiderma, A thermoanalytical study of synthetic carbonate–containing apatites, Thermochim. Acta 256 (1995) 55–65. 61. K. Tõnsuaadu, M. Peld, V. Bender, Thermal analysis of apatite structure, J. Therm. Anal. Calorim. 72 (2003) 363–371. 62. K. Tõnsuaadu, K.A. Gross, L. Pluduma, M. Veiderma, A review on the thermal stability of calcium apatites, J. Therm. Anal. Calorim. 110 (2012) 647–659. 63. H. Pan, B.W. Darvell, Effect of Carbonate on Hydroxyapatite Solubility, Cryst. Growth Des. 10 (2010) 845–850.

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Figure captions

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Fig. 1. General structural model of carbonate apatites (after Fleet et al., 2004). Metal (Ca or Pb) – blue balls, oxygen – red, phosphorous – green, carbon – black, hydrogen – gold. Blue line delineates a unit cell. (A) Simple model of apatite structure emphasizing the position of phosphate tetrahedrons (green). Cross section parallel to (001). (B) Ball-and-stick model of apatite structure. Hexagonal apatite channel is located between the metal cations. Cross section parallel to (001). (C) Apatite channel with OH or CO32- located along z axis. Section parallel to (120). (Color online).

22 23 24 25

Fig. 5. Raman spectra of the analyzed pyromorphites. Grey line – carbonate hydroxylpyromorphite (HPY-CO3), black line - pure hydroxylpyromorphite (HPY-P). The expanded area in the 950-1150 cm-1 region consist of deconvoluted spectra in the carbonate range. Note the lack of 1050 cm1- band diagnostic for cerussite.

26 27 28

Fig. 6. Differential thermal analysis patterns (A and B) and EGA patterns representing the presence of gaseous CO2 (C and D).

Fig. 2. SEM images of the synthetic hydroxylpyromophites: (a) HPY-P, (b) HPY-CO3. Two populations of crystals are apparent which is at least partly a result of ageing of the precipitates. Back-scattered electron images, scalebar – 5 μm. Fig. 3. XRD patterns of the synthetic hydroxylpyromorphite. Grey line (top) – carbonate hydroxylpyromorphite (HPY-CO3), black line (bottom) - pure hydroxylpyromorphite (HPY-P). The presence of CO3 in the structure results in the shift of diffraction peaks towards lower values of diffraction angle (inset). Peaks resulting from Si (internal standard) are marked by black dots. Fig. 4. Infrared spectra of the analyzed pyromorphites. Grey line – carbonate hydroxylpyromorphite (HPY-CO3), black line - pure hydroxylpyromorphite (HPY-P). Position of carbonate bands in the inset (original pattern in black, model pattern and deconvoluted bands in grey).

29 30

Table captions

31 32 33 34 35 36 37 38 39 40 41 42 43

Table 1. Molar content of analyzed elements in the hydroxylpyromorphite (apfu). Table 2. Comparison of the diffraction peaks position for synthetic hydroxylpyromorphite HPY-P, carbonate hydroxylpyromorphite HPY-CO3 and a standard. The largest shifts in peak positions resulting from carbonate substitution are in bold. Table 3. Comparison of infrared band positions and their assignments for Ca5(PO4)3OH, Pb5(PO4)3OH and Pb5(PO4)3Cl. Table 4. Comparison of Raman band positions and their assignments for hydroxyapatite and lead apatites. Fig.1.

16

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1

17

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Fig.2.

2 3

Fig.3.

4

5 6 7

Fig.4.

8 18

ACCEPTED MANUSCRIPT 1

Fig.5.

2 3

Fig.6.

4 5

19

ACCEPTED MANUSCRIPT 1

Table 1.

2 3

Element Pb P O H C Pb/P ratio

theoretical 5.00 3.00 13.00 1.00 0.00 1.67

HPY-P 5.00±0.15 2.83±0.08 12.32 1.00 0.00 1.76

HPY-CO3 5.00±0.14 2.53±0.06 11.22 0.94 0.11±0.02 1.98

20

ACCEPTED MANUSCRIPT 1

Table 2. HPY-CO3 h 1 0 1 1 2 1 0 2 1 2 1 2 3 2 3 0 2 2 1 3 2 3 3 1 2 4 2 4 3 2 3 3 3 4 0 4 1 4 2 1 3 3 5 2

k 0 0 0 1 0 1 0 0 0 1 1 1 0 0 0 0 2 1 0 1 2 0 1 1 0 0 2 0 1 1 2 2 0 1 0 0 0 1 2 1 2 1 0 0

l 0 1 1 0 0 1 2 1 2 0 2 1 0 2 1 3 0 2 3 0 1 2 1 3 3 0 2 1 2 3 0 1 3 0 4 2 4 1 3 4 2 3 0 4

8.577 7.435 5.618 4.952 4.289 4.121 3.717 3.715 3.411 3.242 2.973 2.972 2.859 2.809 2.669 2.478 2.476 2.443 2.381 2.379 2.349 2.266 2.266 2.216 2.146 2.144 2.061 2.060 2.004 1.969 1.968 1.902 1.873 1.872 1.859 1.857 1.817 1.815 1.752 1.740 1.739 1.716 1.715 1.705

HPY-P d-spacing 8.554 7.429 5.609 4.939 4.277 4.113 3.714 3.707 3.407 3.233 2.969 2.965 2.852 2.804 2.662 2.476 2.469 2.439 2.379 2.373 2.343 2.262 2.260 2.214 2.143 2.139 2.056 2.055 2.000 1.966 1.963 1.897 1.870 1.867 1.857 1.853 1.815 1.810 1.749 1.738 1.735 1.713 1.711 1.704

87-2577 8.559 5.615 4.941 4.279 4.116 3.720 3.720 3.412 3.235 2.972 2.972 2.853 2.808 2.664 2.471 2.374 2.345 2.264 2.217 2.140 2.058 2.058 2.001 1.964 1.899 1.868 1.860 1.855 1.818 1.816 1.737 1.737 1.715 1.712 1.706

2 3

21

1

Table 3. FTIR

2 Assingment

[42] Ca5(PO4)3OH

[42] Ca5(PO4)3OH

This study, HPY-P Pb5(PO4)3OH

HAP

HAP with CO32-

HPY

v3 (PO4)3-

1090 1046 1032

1089 1046 1040

1038 984

v1 (PO4)3v4 (PO4)3-

962 602 567

962 603 567

v2 (PO4)3-

473

473

v1 CO32v3 CO32-

This study, HPYCO3 Pb5(PO4)3OH

[39] Pb5(PO4)3Cl

[1] Pb5(PO4)3Cl

PY

PY

1044 982

1017 950

1029 967

926 578 552 540

926 577 552 540

925 579 541

925 573 541

453

453

HPY with CO32-

1119 1464 1419

v2 CO32-

875

1460 1422 1388 1334

1470 1429

867

871

v4 CO32-

3

lib. OHOH-

473

1329

777 632 3572

667 3560

4

22

678 3560

1

Table 4. Raman Assignment

v3 (PO4)3-

[48] Ca5(PO4)3OH

[48] Ca5(PO4)3OH

[48] Ca5(PO4)3OH

This study, HPY-P Pb5(PO4)3OH

HAP

HAP-CO3 A type

HAP-CO3 B type

HPY

1077, 1064 1048, 1041 1034, 1029

1059

1070 1045 1030

1085 1045

1072? 1045

1020 1006

1021 1006

959

958

950

944

945

927

925

925

918

920

578

579

556 540

551 540

423 413 390

423 413 390

1099?

1102

v1 (PO4)3-

964

v4 (PO4)3-

v2 (PO4)3-

1031 1018

957 947

961

614 607 591 580 448

608 589 579

609 590 579 445

433

440

432

v1 CO32-

1107

v4 CO32-

765 675 630

23

This study, HPYCO3 Pb5(PO4)3OH HPY with CO32-

[1] Pb5(PO4)3OH

[5] Pb5(PO4)3Cl

[52] Pb5(PO4)3Cl

HPY with CO32-

PY

PY

1050

1045 1016 984

577 538

553 548 409 409 393

1116

ACCEPTED MANUSCRIPT Hightlights    

Carbonate substitution in the synthetic lead hydroxylapatite is confirmed The presence of carbonates can be detected by XRD, FTIR, Raman and thermal analysis The maximum content of CO32- ion in lead apatites does not exceed 2.25 wt% The CO32- occurs in two structural positions: Pb5(PO4,CO3)3(OH,CO3)