Surface properties of porous hydroxyapatite derived from natural phosphate

Materials Chemistry and Physics 136 (2012) 1022e1026

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Surface properties of porous hydroxyapatite derived from natural phosphate H. Rhaiti a, A. Laghzizil a, *, A. Saoiabi a, S. El Asri a, K. Lahlil b, T. Gacoin b a b

Laboratoire de Chimie Physique Générale, Faculté des Sciences, Université Mohamed V-Agdal, BP 1014 Rabat, Morocco Laboratoire de Physique de la Matière Condensée, Ecole polytechnique, 91128 Palaiseau Cedex, France

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

< The porous hydroxyapatite is synthesized through a novel and cost effective method. < Natural phosphate is used as a calcium and phosphorus precursors in the synthesis of hydroxyapatite. < The formation mechanism is a dissolutioneprecipitation reactions. < The surface properties of porous material have been tested to bind fluoride and lead species.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 April 2012 Received in revised form 24 July 2012 Accepted 24 August 2012

Porous hydroxyapatite c-HAp was prepared using a one-step process involving dissolution/precipitation of natural phosphate. This process provides a simple and flexible route to prepare porous apatite. The structure of c-HAp was investigated by combining XRD, solid-state NMR and N2 adsorption measurements. The nanoporous c-HAp with the largest BET surface area of 150 m2 g
1, exhibited much mesopores and the small micropores simultaneously. In order to investigate the surface properties of this novel material, the adsorption of fluoride, lead was evaluated. Ó 2012 Elsevier B.V. All rights reserved.

Keywords: Natural phosphate Porous hydroxyapatite Dissolutioneprecipitation Surface modification Lead and fluoride-binding

1. Introduction Porous materials with high surface area, large pore volume and tailored components have received much attention because of their wide application as biomaterials, absorbents, catalysts [1e6]. Among them, porous hydroxyapatite Ca10(PO4)6(OH)2 increasingly capture researchers attention due to its outstanding properties such as chemical stability, bioactivity, good ion-exchange properties and affinity for organic [7e9]. There were several reports on the preparation of porous apatites, mainly focused on properties of powders prepared from commercial precursors (oxides or salts) * Corresponding author. Tel./fax: þ212 5 37 77 54 40. E-mail address: [email protected] (A. Laghzizil). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.08.042

[10e12]. In this study, we presented a novel chemical wet method based dissolution/precipitation reactions using natural phosphate NP as calcium and phosphorus precursors. This method is easy and economic due to the low cost of NP raw material [13,14]. The textural properties of porous hydroxyapatite were strongly dependent on the operatory conditions (pH, temperature and type of precursors) which could be tailored by adjusting synthesis parameters [15]. It is crucial to prepare porous c-HAp material from natural phosphate as a promising strategy to obtain efficient sorbent at low cost and limited environmental impact. Compared to previous works, this paper focuses in detail the surface properties of porous c-HAp with tailored structure. In terms of surface properties, the very interesting surface affinity of this material offers lower cost for environmental applications.

H. Rhaiti et al. / Materials Chemistry and Physics 136 (2012) 1022e1026

2. Experimental section 2.1. Synthesis Converted mesoporous hydroxyapatite (c-HAp) from Moroccan natural phosphate was synthesized from phosphate rock via a dissolutioneprecipitation method that provides a simple and economic route for synthesis of mesoporous hydroxyapatite powder (Fig. 1). A complete dissolution of the natural phosphate was slowly performed in acidic medium (HNO3, pH 2) under vigorous stirring to obtain Ca2þ and H3PO4 precursors. The filtrated solution was then precipitated with concentrated ammonia solution (pH 10) at room temperature (20  C) and aged for 24 h. The product was collected by filtration, washed, dried at 100  C overnight. The washing waters were treated to regenerate NH4NO3 salt in alcoholic medium. 2.2. Techniques The crystalline phases were identified using a powder X-ray diffractometer (XRD) (Philips PW131 diffractometer). Infrared spectra were recorded from 400 to 4000 cm
1 on a Bruker (Carlsruhe, Germany) IFS66v Fourier transform spectrometer using KBr pellets. The calcium and phosphorus were chemically analyzed by inductively coupled plasma (ICP) emission spectroscopy. The N2 adsorptionedesorption isotherms for dried powders were obtained by multipoint N2 gas sorption experiments at 77 K using a Micromeritics ASAP 2010 instrument. The specific surface areas were calculated according to the BrunauereEmmetteTeller (BET) method using adsorption data in the relative pressure range from 0.05 to 0.25. The pore volume and pore size distribution were determined using the BarreteJoynereHalenda (BJH) approximation in the mesoporous range, while the average pore size was estimated using the HorvatheKawazoe (HK) model in the microporous range. Solid-state MAS NMR studies were performed on an Avance III 700 Bruker spectrometer for 31P nucleus, while 29Si, and 13 C MAS-NMR spectra were recorded on a Bruker MSL 300 spectrometer equipped with an Andrew type rotor rotating. 2.3. Lead and fluoride-binding experiments Lead and fluoride solutions with concentrations ranging from 0 to 1000 mg L
1 were prepared from PbNO3 and NaF salts

1023

dissolved in distilled water at pH 5 (adjusted with HNO3). 0.2 g of cHAp material was added to 100 mL of these solutions and stirred at 25  C over 3 h to reach sorption equilibrium. The Pb content of the supernatant was determined by inductively coupled plasma (ICP) emission spectroscopy (ICPS-7500, Shimadzu, Japan), while the fluoride concentration was determined with an ion-selective electrode connected to a pH/mV meter (Orion Research) carefully calibrated. The test solution was buffered at pH 6 by addition of a trisodium citrate solution. 3. Results and discussions During the NP dissolution reaction with nitric acid, calcium and phosphorus were instantly analyzed and the final solid residue was characterized by X-ray diffraction (Fig. 2). The crystalline phases were constituted by SiO2equartz and fluorineeCaF2 that cannot be dissociated at pH ¼ 2. On the other hand, the species Ca2þ and H3PO4 contained in the filtered solution are transformed into precipitate by the addition of concentrated NH3(aq). In order to regenerate NH4 þ and NO3
ions from washing waters for a green chemistry or as fertilizer compounds, an appropriate alcohol treatment has been used to recuperate the NH4NO3 salt, confirmed with XRD (Fig. 2). At the end, the XRD patterns of as-synthesized cHAp exhibited broad diffraction peaks indicating the presence of small crystallites. Moreover, after heating at 800  C, XRD patterns indicate that crystalline apatite structure was obtained with no secondary phase. For more information on phosphorus, silicon and carbon environments, solid state NMR was done. The c-HAp gives a single 31P peak at 2.9 ppm with (1H / 31P CP) cross-polarizing (Fig. 3); indicating the presence of OH channel structure or water located on crystal surfaces and is loosely bound or strongly adsorbed. No 29Si NMR peak was observed in converted c-HAp material contrary that in NP (Fig. 3), indicating that all silicon containing in NP is transferred to solid residue such as it proved by XRD (Fig. 2). However, only a little residual inorganic carbonates were detected by 13C NMR compared to that in the NP case (Fig. 3). From chemical analysis, it is worth noting that the molar ratio Ca/P of converted cHAp equals to 1.95, larger than that of stoichiometric apatite (Ca/ P ¼ 1.67), but close to that of NP (Ca/P ¼ 1.97) (Table 1), suggesting partial substitution of phosphate groups by carbonate ions. This is supported by infrared technique with no other phosphate phases, than apatite, are detected using XRD and 31P NMR. No fluorine is present in c-HAp, despite its presence in NP sample (3.07%). The

Fig. 1. Overview of the synthetic route to porous c-HAp material from natural phosphate.

1024

H. Rhaiti et al. / Materials Chemistry and Physics 136 (2012) 1022e1026

Δ CaF

2

* SiO

2

Regenetate NH NO 4

3

Dried Residue

c-HAp (800°C)

c-HAp (100°C)

* * 20

Δ

25

Δ

30

* 35 40 2 Theta (degree)

NP (Brut)

45

50

Fig. 2. XRD diagrams of as-prepared and calcined c-HAp, solid residue and NP raw material.

nitrogen sorption isotherms of c-HAp and NP are shown on Fig. 4a. Both c-HAp and NP materials exhibit a type IV curve [16,17] (Fig. 4a), which indicates that the samples act as mesoporous materials and a weak interaction between nitrogen gas and the apatite surface. The c-HAp shows a significant mesoporosity as indicated by the widening of the hysteresis. Estimation of the specific surface area based on the BET model led to 150 m2 g
1 for cHAp higher than that of raw material NP (20 m2 g
1) with also higher porous volume of ca. 0.55 cm3 g
1. BJH distribution indicates that the c-HAp structure has a pore size range below 10 nm, whereas NP has a two pore population centered at ca. 2 nm and 50 nm attributed to residual clay and apatite mineral, respectively (Fig. 4b). Indeed, application of the BJH model to these systems suffers from two limitations. First, it is not fully adapted when microporosity is present. To overcome this problem, the sorption isotherms were also analyzed using the HK model [18]. This method gives access to the average pore size distributions in the microporous range Dm that is reported in Table 1. This datum clearly indicates that c-HAp exhibits a small amount of microporosity of 0.52 nm. Another limitation of the BJH model is that it considers a network of cylindrical pores that may not correspond to the structural complexity of the here-studied systems. To check this, we have used the model developed by Innes [19,20], that can be applied to parallel plate morphology. As indicated in Table 1, the results are very similar to those obtained through the BJH model, therefore validating our analyses of the c-HAp material in the mesoporous range. To investigate the surface reactivity of c-HAp in detail, two species were selected, Pb2þ and F
separately, not only due to their environmental relevance but also to evaluate their

Fig. 3. Solid-state NMR spectra for c-HAp and NP materials: (a)

31

P NMR, (b)

13

C NMR, (c)

29

Si NMR.

H. Rhaiti et al. / Materials Chemistry and Physics 136 (2012) 1022e1026

1025

Table 1 Chemical composition, specific surface area (SBET), porous volume (Vp) and average pore size (Dp) from BJH model, average micropore size (Dm,HK) from the HK model and average mesopore size (Dm,I) from the Innes model of mesoporous material.

38.66 37.84

%F

15.33 15.02

0 3.07

SBET (m2 g
1)

Ca/P

1.95 1.97

BJH model

150 20

Vp (cm3 g
1)

Dp (nm)

0.53 0.40

12 20e100

a

80

N adsorbed (cm .g , STP)

70

Innes model Vp (cm3 g
1)

Dm,I (nm)

0.52 e

11 e

0.51 e

400

Pb / c-HAp

320

240

160 Pb / NP 80

c-HAp 60

F / c-HAp

0

-1 3

2

HK model Dm,HK (nm)

the Ca-excess. In fact, when c-HAp is placed in water, surface dissolution occurs leading to calcium and hydroxide ions release and therefore to an increase in pH (Fig. 6). Here, c-HAp resuspension leads to an equilibrium pH of 7.8. In the presence of 10
2 M [F
] as initial concentration, the equilibrium pH equals 8.2, higher than 7.8 for unfluoridated sample dispersed only in water solution. This corresponds to the release of alkaline species such as

e

relative affinities toward this low-cost material. Indeed, synthetic hydroxyapatite prepared using frequent commercial precursors is well-known sorbent for metal or fluoride ions [21]. In our case, fluoride and lead sorption isotherms are shown in Fig. 5, indicating that the maximum capacity of Pb was around 352 mg g
1 for c-HAp three times higher than that of NP (98 mg g
1), despite the fact that the latter contains a large organic matter, such an improvement is related to surface area SBET is the key factor determining the lead sorption capacity. The sorption of fluoride followed a trend similar to Pb2þ. Indeed, by basing itself on our previous study on the another synthetic c-HAp using Ca(OH)2 and NH4H2PO4 [22] and with higher surface area of 235 m2 g
1, its fluoride capacity of 42 mg g
1 is close to that of the studied c-HAp (40 mg g
1). Herepresented data indicate that the two, i.e. F
and Pb2þ, ions sufficiently adsorb onto c-HAp surface, similar or sometimes higher to that cited in the literature [23e26]. Several parameters improve the surface affinity toward F
ions, such as the presence of CO3 2
and

-1

c-HAp NP

%P

q (mg.g )

%Ca

0

50

200

NP

400

600

800

1000

-1

C (mg.L )

40

0

30

Fig. 5. Effect of initial concentration of F
and Pb2þ on their adsorption processes.

20 10

H O / NP

9

2

0 0

0.2

0.4

0.6

0.8

1

F / c-HAp

Relative pressure P/P

0

b

8

4

H O/ c-HAp 2

pH

3.5 3

7

2

Pb / c-HAp

3

-1

-1

V/ D (cm .g nm ) (a.u)

c-HAp 2.5

1.5

6 Pb / NP

1

NP

0.5

5

0

1

10

100

1000

Pore diameter (nm) Fig. 4. (a) N2-sorption isotherms and (b) pore size distribution from BJH calculation for c-HAp and NP materials.

0

40

80 120 Time (min)

160

Fig. 6. The variation of equilibrium pH of solutions in the presence of c-HAp with and without adding F
(50 mg L
1) and Pb2þ (100 mg L
1) as initial concentrations.

H. Rhaiti et al. / Materials Chemistry and Physics 136 (2012) 1022e1026

In fact, the presence of carbonates in as-received c-HAp facilitates the partial dissolution and can increase the release of Ca2þ ions, as precipitant of fluoride ions after sorption process, including hydroxyl substitution. In the case of lead, the sorption process on c-HAp and NP appears more complex than fluoride. A progressive increase in pH occurs upon contact of the apatite with Pb, but less than when the c-HAp or NP are placed in water, indicating a proton liberation into solution from ^POH to complex Pb2þ to form a stable phases such as pyromorphite Pb10(PO4)6(OH)2 as previously reported [28,29].

OH...F

1026

F /c-HAp (b)

F / c-HAp (a)

OH

unfluoridated c-HAp

4. Conclusions

4000 3500 3000 2500 2000 1500 1000

500

-1

Frequency (cm ) Fig. 7. IR spectra for fluoridated c-HAp using (a) 10
3 M and (b) 10
2 M as initial fluoride concentrations.

OH
. Taken together, these data suggest that fluoride sorption occurs via OH / F exchange mechanism. To confirm this, the IR band involving the hydrogen bonded OH/F appears at 760 cm
1 (Fig. 7) indicative of the formation of Ca10(PO4)6(OH)2
xFx solid solution [22,27], while OH bands disappeared in fluoridated solids under high initial concentration. In order to examine the surfacefluorine bound, the fluoridated products are heated at 800  C then 1000  C and characterized by XRD (Fig. 8). No secondary phase was obtained for less initial fluoride concentrations; only improved crystallinity. However, for high concentration as of 10
2 M, numerous peaks have appeared attributed to b-Ca3(PO4)2 and CaF2 when the fluoridated sample is treated at 800  C, which give Ca10(PO4)6F2 after a re-treatment at 1000  C for several hours, indicating that there is a following chemical reaction at this high temperature: 3b-Ca3(PO4)2 þ CaF2 / Ca10(PO4)6F2

4 2

Ca (PO ) F 10

4 6 2

ο

3

ο

Δ CaF * β-Ca (PO ) 2

ο ο ο

ο ο ο ο

ο ο

* Δ

*

*

Δ

ο

ο ο

ο

ο

ο ο

F / c-HAp-1000°C (b)

Δ

Δ

F / c-HAp-800°C (b) F/ c-HAp-800°C (a)

unfluoridated c-HAp-800°C 25

30

35

40

45

50

2 Theta (degree) Fig. 8. XRD diagrams for calcined fluoridated samples using c-HAp: (a) 10
3 M and (b) 10
2 M as initial fluoride concentrations.

Overall, the natural phosphate rock has the advantage of a very low cost, low environmental impact and large abundance, but exhibits a limited porosity whereas its porous derivative c-HAp obtained from the former by dissolutionereprecipitation reaction that would introduce some processing price, but allows for much higher porosity as well as specific surface area. In terms of sorption properties, the porous c-HAp clearly exhibits better performance and can be considered as a promising low-cost sorbent. References [1] J.C. Elliott, Structure and Chemistry of the Apatites and Other Calcium Orthophosphates, Elsevier, Amsterdam-London-New York-Tokyo, 1994, p. 389. [2] R. Leboda, J. Skubiszewska-Zie˛ ba, A. Da˛ browski, V.A. Tertykh, Colloids Surf. A: Physicochemical Eng. Asp. 172 (2000) 69e77. [3] S. Zhu, N. Yang, D. Zhang, Mater. Chem. Phys. 113 (2009) 784e789. [4] Y. Wang, S. Huang, S. Kang, C. Zhang, L. Xi, Mater. Chem. Phys. 15 (2012) 1053e1059. [5] X.Z. Lin, T.Y. Ma, Z.Y. Yuan, Chem. Eng. J. 166 (2011) 1144e1151. [6] A. Bahdod, S. El Asri, A. Saoiabi, T. Coradin, A. Laghzizil, Water Res. 43 (2009) 313e318. [7] P. Melnikov, A.R. Teixeira, A. Malzac, M.B. Coelho, Mater. Chem. Phys. 117 (2009) 86e90. [8] M. Okazaki, Y. Yoshida, S. Yamaguchi, M. Kaneno, J.C. Elliott, Biomaterials 22 (2001) 2459e2464. [9] S.J. Segvich, H.C. Smith, D.H. Kohn, Biomaterials 30 (2009) 1287e1298. [10 ] L. El Hammari, H. Merroun, T. Coradin, S. Cassaignon, A. Laghzizil, A. Saoiabi, Mater. Chem. Phys. 104 (2007) 448e453. [11] W. Weng, J.L. Baptista, Biomaterials 19 (1998) 125e131. [12] S. Heinemann, T. Coradin, H. Worch, H.P. Wiesmann, T. Hanke, Compos. Sci. Technol. 71 (2011) 1873e1880. [13] R. Tahir, K. Banert, S. Sebti, Appl. Catal. A 298 (2006) 261e264. [14] S. El Asri, A. Laghzizil, A. Alaoui, A. Saoiabi, R. M’Hamdi, K. EL Abbassi, A. Hakam, J. Therm. Anal. Calorim. 95 (2009) 15e19. [15] S. El Asri, A. Laghzizil, A. Saoiabi, A. Alaoui, K. El Abassi, R. M’hamdi, T. Coradin, Colloids Surf. A 350 (2009) 73e78. [16] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemieniewska, Pure Appl. Chem. 57 (1985) 603e619. [17] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, New York, 1967. [18] P. Kowalczyk, A.P. Terzyk, P.A. Gauden, L. Solarz, Comput. Chem. 25 (2002) 26e130. [19] D.D. Do, Adsorption Analysis: Equilibria and Kinetics, Imperial College Press, London, 1998. [20] A. Corma, Chem. Rev. 97 (1997) 2373e2419. [21] J. Oliva, J. De Pablo, José-Luis Cortina, J. Cama, C. Ayora, J. Hazard. Mater. 194 (2011) 312e323. [22] L. EL Hammari, A. Laghzizil, P. Barboux, K. Lahlil, A. Saoiabi, J. Hazard. Mater. 114 (2004) 41e44. [23] T. Suzuki, T. Hatsushika, Y. Hayakawa, J. Chem. Soc. Faraday Trans. 1 (77) (1981) 1059e1062. [24] S. Gao, R. Sun, Z. Wei, H. Zhao, H. Li, F. Hu, J. Fluor. Chem. 130 (2009) 550e556. [25] S. Gao, J. Cui, Z. Wei, J. Fluor. Chem. 130 (2009) 1035e1041. [26] S.H. Jang, B.G. Min, Y.G. Jeong, W.S. Lyoo, S.C. Lee, J. Hazard. Mater. 152 (2008) 1285e1292. [27] G.C. Maiti, F. Freud, J. Chem. Soc., Dalton Trans. (1981) 949e955. [28] Y. Hashimoto, T. Sato, Chemosphere 69 (2007) 1775e1785. [29] E. Mavropoulos, C. André, J. Moreira, M. Saldanha, Environ. Sci. Technol. 36 (2002) 1625e1629.