HaP/SBA-3 Nanostructured Composite to Remove Fluoride Effectively from Contaminated Water

Nanostructured materials, based on silica (MCM, SBA [1,2]) and ordered mesoporous carbon (OMC), such as CMK-1, CMK-3 [3] and Multi-Walled Carbon Nanotubes (MWCN) [4], because of their large surface area, pore volume and possibility to vary the pore size, offer a great opportunity for their application in various fields, from industrial processes to biomedical engineering, air and water pollution decontamination and energy reservoirs. Recently, we have developed a technique of preparation of monocrystalline hydroxyapatite (HaP) , and by the same procedure, but in the presence of the respective hosts, forming in situ composites such as HaP/MCM-41 and HaP/SBA15 composites [5]. Numerous approaches for fluoride removal from contaminated water, such as precipitation-coagulation [6], membrane technology [7,8], ion exchange [9] and adsorption [10,11]. Among the above methods, fluoride adsorption on adsorbents is attracting increasing attention due to its advantages of convenient design and operation, and low cost [12]. Different types of adsorbents, such as metal adsorbents [13], carbon adsorbents [14], carbon materials [15], natural materials [16], and biosorbents [17], have been used to remove fluoride from water. MOFs have also been used as new adsorbents for fluoride adsorption due to their surface functional groups and ordered atomic arrangement [18]. Calcium phosphate apatites are compounds of the formula Ca5 (PO4)3X, where X can be an Fion (fluorapatite, FaP), OH(hydroxyapatite, HaP), or a Clion (chlorapatite). The ion is substituted by another ion of the same sign but of different charge. Neutrality is maintained by substitutions of ions with different charges or vacancies [19]. Studied the crystallization of HaP (hydroxyapatite) on polymers containing C-N groups, from supersaturated solutions of HaP has been reported [20]. The growth of hydroxyapatite on silica gels in the presence of organic additives was studied by Rivera-Muñoz et al. [21]. Laghzizil et al. [22] have correlated the results of HaP fluorination with the physicochemical properties of HaP. Recently, morphological modifications of hydroxyapatite using fatty acids as an organic modifier have been reported [23]. The goal of this work is the successful Crimson Publishers Wings to the Research Short Communication


Introduction
Nanostructured materials, based on silica (MCM, SBA [1,2]) and ordered mesoporous carbon (OMC), such as CMK-1, CMK-3 [3] and Multi-Walled Carbon Nanotubes (MWCN) [4], because of their large surface area, pore volume and possibility to vary the pore size, offer a great opportunity for their application in various fields, from industrial processes to biomedical engineering, air and water pollution decontamination and energy reservoirs. Recently, we have developed a technique of preparation of monocrystalline hydroxyapatite (HaP) , and by the same procedure, but in the presence of the respective hosts, forming in situ composites such as HaP/MCM-41 and HaP/SBA-15 composites [5]. Numerous approaches for fluoride removal from contaminated water, such as precipitation-coagulation [6], membrane technology [7,8], ion exchange [9] and adsorption [10,11]. Among the above methods, fluoride adsorption on adsorbents is attracting increasing attention due to its advantages of convenient design and operation, and low cost [12]. Different types of adsorbents, such as metal adsorbents [13], carbon adsorbents [14], carbon materials [15], natural materials [16], and biosorbents [17], have been used to remove fluoride from water. MOFs have also been used as new adsorbents for fluoride adsorption due to their surface functional groups and ordered atomic arrangement [18]. Calcium phosphate apatites are compounds of the formula Ca5 (PO4)3X, where X can be an Fion (fluorapatite, FaP), OH -(hydroxyapatite, HaP), or a Cl-ion (chlorapatite). The ion is substituted by another ion of the same sign but of different charge. Neutrality is maintained by substitutions of ions with different charges or vacancies [19]. Studied the crystallization of HaP (hydroxyapatite) on polymers containing -C-N groups, from supersaturated solutions of HaP has been reported [20]. The growth of hydroxyapatite on silica gels in the presence of organic additives was studied by Rivera-Muñoz et al. [21]. Laghzizil et al. [22] have correlated the results of HaP fluorination with the physicochemical properties of HaP. Recently, morphological modifications of hydroxyapatite using fatty acids as an organic modifier have been reported [23]. The goal of this work is the successful

Materials synthesis
The procedure designed for the synthesis of SBA-3 was as follows [24,25]: the surfactant (cetyltrimethylammonium bromide), was added to water and HCl; afterwards, 3g of TEOS was also added, stirring to obtain a mixture with a molar composition of: TEOS:H2O:HCl:CTAB=1:150:9:0.15. After 60min, the white precipitate was filtered, washed and dried, and then the surfactant was extracted using ethanol at reflux for 4h, and subsequently calcined at 550 °C in air for 2h. Nanonized HaP was obtained using solutions of varying concentration 1-0.6M CaCl2 in 2H2O (a) and 1.8-2.30 M K2HPO4 (b) and then Benzalkonium Chloride 10% Solution (Alkylbenzyldimethylammonium chloride, C6H5CH2N(CH3)2RCl, R= C18H37), in double distilled water was added to prevent the growth of HaP crystals (ex-situ). The resulting solution (pH=9) was stirred vigorously for 2h at 65 °C, obtaining nanometric HaP (4-5nm). For synthesizing HaP-Host, the same procedure was performed, with the modification that was added to the mixture of solutions (a) and (b), SBA-3 (50-70 % of HaP wt/ wt with respect to the host), after the first hour of reaction which was used to prepare HaP ex situ. The suspensions of HaP/SBA-3 were stirred vigorously for 4h, at 60 °C, filtered, washed with triple distilled carbon dioxide-free water, and then dried at 100 °C for 4h. Subsequently, the composite was activated by heating at 500 °C in N2 flow for 5h, and then calcined at 550 °C at a heating rate of 5 °C/min from 100 °C for 4h. Hap/MCM-41 and HaP-SBA-15 [4], were also used in this study to compare the effectiveness of fluoride retention.

Fluoride retention essay
Experimental conditions: 2g of HaP in 100mL of NaF solution with an initial concentration of 9 x10 -2 M were used. The weight of the composite used was normalized to HaP. Fluoride ion solutions were prepared using a Teflon device with magnetic stirring, specially designed to bubble N2 to avoid CO2 contamination at 25 ºC. The pH of the solutions was measured with a Mettler pH meter; the instrument was calibrated with buffers of pH=4 and 7.5. The concentration of Fions was determined using an Fspecific electrode, with a dynamic range between 1-300 ppm. In addition, trace amounts of Fwere followed by FTIR. Fluoride retention capacity was evaluated in successive cycles at different times.

Characterization
Samples were characterized by means of powder X-ray diffraction in a X'Pert Pro PANalytical diffractometer equipped with a CuKa radiation source (k=0.15418nm). N2 adsorption/ desorption isotherms were measured on ASAP 2020 equipment after degassing samples at 673K, determining textural properties such as surface area and pore volume; pore size distribution was estimated using Barrett, Joyner and Halenda (BJH) algorithm. TEM was recorded in a JEOL 2100F microscope operated with an accelerating voltage of 200kV (point resolution of 0.19nm). FTIR studies were performed in a JASCO 5300 Fourier transform infrared spectrometer (FTIR).

XRD, BET, SEM and HRTEM
The surface area of the nanonized HaP measured by BET (N2) method was 96m 2 /g. The surface area of SBA-3 was 1064m 2 /g, and 750m 2 /g for Hap/SBA-3 respectively. The pore diameter of the host was: 3.1nm for SBA-3 and a 0 of 3.56 (lattice parameter

RDMS.000883. 16(2).2021
The XRD pattern of the nanonized HaP prepared ex-situ and the HaP/SBA-3 composite are illustrated in Figure 1b & 1c. Composite pattern diffraction peaks confirm a high crystallinity or long-range order structure in the nanostructured host SBA-3.
SEM studies (Figure 2a) show the size and shape of the HaP/ SBA-3 composite, which indicate a good morphology of the crystals, with an absence of other phases caused by HaP. The shape of the HaP/SBA-3 crystals is practically spherulitic with a mean crystal size in the range of 1.5 to 2μm. The HRTEM image illustrated in Figure  2b, detail the long-range hexagonal arrangement of HaP/SBA-3. Higher level reflections remain clearly detectable. Consequently, the nanostructured HaP crystals is within the nanostructure of the host.

FTIR studies
The FTIR analysis of HaP/SBA-3 ( Figure 3) reveals bands assigned to SBA-3 and HaP in the composite. The peak at 3570cm -1 is associated with the OH stretching mode of HaP and at 3415 with Si-OH streching. The signals at 1080, 1227 and 1669cm -1 (asymmetric T-O, internal and external stretching respectively) are due to SBA-3. PO4 appeared as a very strong band at ~1030cm -1 . P-O stretching is found at ~970cm -1 . The clearly defined bands at 650 and 564cm -1 are ascribed to the O-P-O asymmetric deformation components. The band at 800cm -1 is caused by SI-O symmetric stretching of SBA-3. The typical band followed for Fretention is that of OHat 3570cm -1 from HaP in the composite.

Flouride retention
To study the fluoride retention capacity of HaP/SBA-3 nanocomposite and its comparison with HaP, HaP/MCM-41 and HaP/SBA-15 two strategies were employed. a: All fresh materials were evaluated in the function of contact time with a solution containing fluorides and the decrease of fluorides from 20%, 40%, 60%, 80%, and 100%. Thus, Table 1 shows a high Fretention capacity at short contact times with respect to the rest of the materials used, even with the mesoporous silica nanocomposites. The prepared nanosized HaP did not retain 100% Feven after several hours in contact with the solution.

Reusability
HaP/SBA-3 was taken and its reusability was studied, in different cycles with a duration of 2h and the percentage of retained Fand the amount free OHbands of HaP, in % Absorbance Units, in the composite are reported. In Figure 4 it can be observed that after 5 cycles of 4h duration the nanocomposite still retains 100% of Ffrom fresh solutions. Inter-cycle, the nanocomposite was filtered, washed only with bi-distilled water and dried at 100 °C, without

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any reactivation. The results suggest that in the fresh HaP/SBA-3 composite, the OHof the HaP has remained intact because of high dispersion with the smaller size of the nanocrystals (less than 2nm) of HaP and the higher proportion of free OHper gram of the active material. The results also indicated that HaP/SBA-3 nanocomposite developed is not only active in terms of the time in contact with the solution to achieve 100% Fretention, but also that it remains active after several cycles, making it a highly attractive material for Fretention of contaminated water.

Conclusion
SBA-3 nanostructured materials have good structural and textural properties, useful to acts as hosts for incorporating nanocrystals of hydroxyapatite, forming active HaP/SBA-3 composite. By HRTEM. FTIR, XRD, we found that the HaP nanocrystals are within the hosts, and not on the external surface, indicating good incorporation of nanocrystals in the host with sizes lower than 2 nm. The superior efficiency of HaP/SBA-3 relative to HaP/MCM-41 and HaP/SBA-15 could also be because SBA-3 has a higher abundance of Si-OH groups than the other reservoirs. Thus Ca2+ of HaP became anchored onto the large quantity of siloxane species composed of adjacent silanol groups contained in SBA-3, enhancing the chance of more growth sites for the smaller HaP nanocrystals, because of the small pore size of SBA-3, than the other mesoporous silica supports studied (4 and 6 nm of MCM-41 and SBA-15 respectively).