Hydroxyapatite/MCM-41 and SBA-15 Nano-Composites: Preparation, Characterization and Applications

Composites of hydroxyapatite (HaP) and highly ordered large pore mesoporous silica molecular sieves such as, Al-SBA-15 and Al-MCM-41 (denoted as SBA-15 and MCM-41, respectively) were developed, characterized by XRD, BET, FTIR, HRTEM and NMR-MAS, and applied to fluoride retention from contaminated water. The proposed procedure by a new route to prepare the HaP/SBA-15 and HaP/MCM-41, composites generates materials with aluminum only in tetrahedral coordination, according to the 27Al NMR-MAS results. Free OH- groups of HaP nanocrystals, within the hosts, allowed high capacity fluoride retention. The activity of fluoride retention using HaP/MCM-41 or HaP/SBA-15 was 1-2 orders of magnitude greater, respectively, than with pure HaP.


Introduction
Calcium phosphate apatites are compounds of the formula Ca 5 (PO 4 ) 3 X, where X can be a F − (fluorapatite, FaP), OH − (hydroxyapatite, HaP) or a Cl − ion (chlorapatite). One ion is replaced by OPEN ACCESS another of the same sign but of different charge. Neutrality is maintained by substitutions of ions with dissimilar charges or vacancies [1].
Fluoridated calcium hydroxyapatites have been studied in relation to their physico-chemical properties [2][3][4][5][6]. It is well known that fluoride is one of the elements contained in biological apatites as trace amounts, which strongly modifies their crystallinity and their solubility. Porous hydroxyapatite biomaterials have a great stability and a good biocompatibility. They can be used as composite biomaterials for their ability to form a strong chemical bond with natural bones.
Laghzizil et al. [5,6] enhanced the fluoride adsorption capacity onto hydroxyapatite (HaP) prepared in a highly porous form using a modified chemical wet method. Besides, they have also analyzed the effect of the Fions on the crystallinity and electrical properties of hydroxyapatite biomaterials. Moreover, Dalas et al. [7] have studied the crystallization of hydroxyapatite on polymers, containing -C-N groups, from supersaturated solutions of HaP. Consequently, this method was particularly useful to study the formation of new phases on the substrates in which HaP was deposited, for example the growth of hydroxyapatite on silica gels in the presence of organic additives [8]. In other research, nanosized hydroxyapatite particles have been successfully synthesized from microemulsions stabilized by a biodegradable surfactant [9,10]. These particles possess powder characteristics that make them superior in many composites applications. The microemulsion-derived hydroxyapatite powders exhibit a high specific surface area, lowered degree of particle agglomeration and narrow particle size distribution [10].
On the other hand, the synthesis of mesoporous hydroxyapatite was reported by several authors [11][12][13][14][15][16][17], e.g., Tang et al. [18] described a simple and new method for the preparation of hydroxyapatite porous biomaterials with a uniform pore size distribution by sintering the mixture of HaP powders and monodispersed polystyrene microspheres.
In a previous work, we published our first report on the activity of HaP/ MCM-41 and HaP-BEA composites for fluoride retention [19]. We developed a technique of preparation of nanocrystalline HaP (ex-situ) and in the presence of the respective hosts, forming in situ composites. We also compared the capacity of F − retention from contaminated water, with respect to a commercial sample.
In the present work, we prepare composites of hydroxyapatite (HaP) and highly ordered large pore mesoporous silica molecular sieve such as Al-SBA-15 and Al-MCM-41. We correlate fluoride retention, from contaminated water, with the physicochemical properties of HaP/MCM-41 and HaP/SBA-15 nanocomposites. Our first results concerning the development of SBA-3, SBA-15 and SBA-1 was recently reported [20].

FTIR studies
FTIR data of a pure commercial hydroxyapatite sample (CHaP), HaP/MCM-41 and HaP/SBA-15 with the assigned bands (prior to the retention of F -) are shown in Figure 3. In ther HaP spectrum, the P-O stretching IR mode appears at ~ 962 cm −1 and the PO 4 region appears as a very strong bands at ~1,029 cm −1 and at ~1,092 cm −1 , whereas the band at 3,567 cm −1 is assigned to OH stretching mode    27 Al-NMR-MAS results of the samples [20,28], showed a intense peak at 53 ppm, assigned to Al IV Td form, a very low signal at 0 ppm due to octahedral extra framework aluminum (Al VI Oc ), can be seen in Figure 4

HRTEM and SEM studies
The HRTEM images illustrated in Figure 5, reveal the existence of a long-range hexagonal arrangement of nanosized mesopores. The higher order reflections are still discernable clearly in the sample HaP/MCM-41 and HaP/SBA-15 compared with the HRTEM of the hosts reported in literature [19,29]. Thus, the nanosized crystals of HaP are within the mesostructure of the hosts.  The size and shape of the samples indicate good morphology of the crystals. HaP-SBA-15 images reveal that it consists of many rope-like domains with relatively uniform sizes of 1.5-2 μm, without other phases (clusters of HaP crystals) as well as in HaP-MCM-41 microphotographs, but with micellar rod-like shape hexagonal crystals, with size of 1.5 × 2 .2 µm (see Figure 6), in agreement with HRTEM data showed in Figure 5.  Figure 7 shows the Fretention capacity of the samples. The method used for the host inclusion (not found in literature) seems to be adequate, since the OHgroups of HaP were not blocked. In the case of HaP (ex-situ), its lower crystal size has favored the Fretention, compared with the commercial sample. MCM-41 and SBA-15b act as supports to anchor the HaP crystals, on a nanometer scale (<3 nm and 10 nm, respectively), with higher fluoride retention from contaminated water, in correspondece with the data showed in FTIR studies (see Figure 3). In Figure 7, we can see that the fluoride retention by the hosts is not significant. The results demonstrated first, a fast retention of fluoride from 0 to 10 hours and then decaying to the stationary state, in about 25 hours. The final concentration of fluoride ion was 0.15 and 0.02 × 10 −3 M, for HaP/MCM-41 and HaP/SBA-15 respectively. In this way, Table 2 shows the diminution of the OHband of HaP (signal at 3567 cm -1 for CHaP and nanosized HaP prepared in this work and 3,569-3,570 cm -1 for HaP/SBA-15 and HaP/MCM-41 nanocomposites), as a function of time on stream, for the data shown in Figure 7.

Fluoride retention
The results are shown as percentage of the OHband, in absorbance units for each sample, which remains unalterable during the fluorides retention test, considering the 100% of the absorbance of this signal before the beginning of the test.
As can be seen, the nanocomposites of HaP/MCM-41 and HaP-SBA-15, retain fluoride with better performance, even after 20 h of evaluation, than the HaP crystals. The best performance of HaP/SBA-15 with respect to HaP/MCM-41, could be due to a high dispersion with lower size of HaP nanocrystals (linked to its higher surface area and pore volume). Taking account that SBA-15 material has higher amount of silanol groups than MCM-41 [20], the condensation of adjacent silanol groups (Si-(OSi) 3 -OH) forms siloxane species, which might anchor Ca 2+ , in order to make available sites for the HaP nanocrystals nucleation. Thus, as the silanol sites increase, the possibility to generate more sites for the growth of HaP crystals increases, these results are in agreement with Díaz et al. [30].

Host synthesis
Al-SBA-15 was synthesized using 15-crown-15, (PEO15, Aldrich) as a co-polymer mono block and cetylpyridinium bromide (BDH 95%) as surfactant, TEOS (Aldrich 99%) and NaAlO 2 (Aldrich 99%) , as silica and aluminum source, respectively; as described elsewhere [20]. The final Si/Al ratios determined by ICP of the samples were 50-33 (denoted as SBA-15a and SBA-15b). Al-MCM-41 was developed by a new technique [28]. An aqueous solution of NaAlO 2 was added to the mixture of silica source (Ludox TM-40 colloidal silica, Aldrich, 40% suspension in water) and aqueous TMAOH (tetramethylammonium hydroxyde). Then, both aqueous solutions of CTABr (cetyltetramethylammonium bromide) and NaH 2 PO 4 were added to the synthesis, then mixtured and stirred for 30 min at 20 ºC. The final gel mixtures were refluxed under stirring for a period of 24 h. The Si/Al molar ratio was 30, determined by ICP for the final catalytic material, denoted as MCM-41.

Preparation of the composites
HaP ex-situ, was prepared using CaCl 2 •2H 2 O (a) and K 2 HPO 4 (b), and doubly distilled water. Solutions of variable concentrations were used: 1-0.51 M of CaCl 2 and 1.8-2.3 M of K 2 HPO 4 , at pH 8-9. Solution (b) is added to solution (a) in a stirred Pyrex vessel at 37 ºC, and left for 6 h, obtaining a calcium/phosphate molar ratio of 1.7 in order to have the stoichiometric ratio of HaP, with ionic strength, I = 0.16 mol·L -1 . The pH was adjusted to the required value by the slow addition of KOH solution. During the reaction, CO 2 was excluded by bubbling with presaturated N 2 gas. To prepare HaP/host (HaP-in-situ), the same procedure was followed, SBA-15 and MCM-41 were added at the first 0.5 h of the total reaction time of the preparation. The suspensions were vigorously stirred for 4 h, at 60 ºC, filtered, washed with triple-distilled carbon dioxide-free water, and then dried at 100 ºC for 4 h. The HaP ex-situ and composites were activated by heating at 500 ºC in N 2 flow for 10 h, then calcined up to 500 ºC at a heating rate of 2 ºC/min from 100 ºC for 2h. Commercial hydroxyapatite (CHaP) also was used in this study, provided by Bio-gel HTP, marketed by BIO-RAD®. The HaP content in the composite was determined by ICP following the ratio of Ca/P and Ca/Si. From Ca/P we determined the stoichiometric ratio of HaP and with Ca/Si, the amount of HaP in the composites. Thus, HaP content for HaP/MCM-41 and HaP/SBA-15 composites were 30 and 35 wt%, respectively.

Fluoride retention essay
Solutions of contaminated water with fluoride were prepared, using a Teflon device with magnetic stirring, specially designed to bubble N 2 in order to avoid CO 2 contamination at 25 ºC. The pH of the solutions was measured with a Mettler pH meter with combination glass electrodes; the instrument was calibrated with buffers of pH = 4 and 7.5. Ion Fconcentration was determined using a specific electrode for F -, dynamic range between 1 to 300 ppm. In addition, Ftraces were followed by FTIR. Experimental conditions: 2.3 g of HaP in 100 mL of NaF solution with initial concentration of 8 × 10 -3 M. The weight of the materials employed was normalized on HaP base.

Characterization
Nitrogen adsorption of the samples was measured with and ASAP 2010 Micromeritics apparatus. Elemental analysis was performed by inductively coupled plasma-atomic emission spectroscopy (VISTA-MPX) operated with high frequency emission power of 1.5 kW and plasma airflow of 12.0 L/min. The diffraction patterns were performed with with a Philips X'Pert PRO PANalytical diffractometer under Cu Kα radiation (λ = 1.5418). The diffraction data was collected by using a continuous scan mode with a scan speed of 0.02° (2 θ)/min. FTIR spectra of the samples were obtained using wafers of HaP and the composites in KBr employing a vacuum cell with special KBr windows and a JASCO 5300 Fourier Transform Spectrometer. Prior to the FTIR experiments, the samples were degassed (p < 10 -3 Pa) at 400 ºC for 4 h. 27 Al MAS-NMR spectra were taken on a BRUKER MSL300 spectrometer operating at 78.2 MHz for 27 Al. We used a BRUKER MAS 300WB CP1H-BBWH. VTN-BL4 probe with 4 mm o.d. zirconia rotors. The size and shape of the crystals were determined by SEM in a PHILIPS-SEM 501B. High-resolution transmission electron microscopy (HRTEM) images of a few representative samples were collected using a JEOL-200 CX electron microscope. Composites samples were mounted on a microgrid carbon polymer, supported on a copper grid, by placing a few droplets of a suspension of the sample in water followed by drying at ambient conditions.

Conclusions
SBA-15 and MCM-41 were successfully developed. The materials have good structural and textural properties. They are useful as hosts incorporating nanocrystals of hydroxyapatite, forming active composites of HaP/MCM-41 and HaP/SBA-15. According HRTEM studies, HaP nanocrystals are within the hosts, and not on the external surface, indicating good incorporation of nano-crystals in the host, with sizes of pores higher than 4 nm. Fluoride retention is a function of surface area and pore diameter of the hosts (SBA-15 and MCM-41), that allow the anchoring of the HaP nanocrystals, leaving OHgroups free. The capacity for fluoride retention of the HaP/hosts increases one and two order of magnitude with respect to pure HaP. Thus, we have developed useful nanocomposites of HaP/mesostructured materials, which allow efficient retention of fluoride from contaminated water.