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Structure and Properties of Hydroxyapatite/Titanium Nanocomposites for Photocatalytic Degradation of Methyl Orange
Hydroxyapatite/TiO2 nanocomposite photocatalysts were successfully synthesized by in situ precipitation condition of precursor matters from hydroxyapatite and Ti(OH)4. Composites with different HA/TiO2 ratio were studied to evaluate the influence of TiO2 on the morphology and the photocatalytic behavior of the materials. Identification of the products was achieveed by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, energy dispersive X-ray spectroscopy (EDAX), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques. The photocatalytic activity of the prepared catalysts was analyzed by monitoring photocatalytic degradation of methyl orange under UV irradiation. The effects of operational parameters such as the amount of photocatalyst, dye concentration and initial pH on photocatalytic of Methyl Orange (MO) have been examined. Experimental results illustrat that the obtained Hydroxyapatite/TiO2 nanocomposite, which combines the excellent adsorption capacity of HAp and the high photocatalytic activity of TiO2, can effectively degrade persistent organic pollutant methyl orange under UV irradiation.
Keywords:Hydroxyapatite; Titanium; Photocatalyst; Methyl orange; Degradation
Dyes and pigments are knowns one of the most environment problems, which are discharged in to wastewaters from numerous industrial branches, generally from the dye manufacturing, textile finishing and also from food coloring, cosmetics, paper and carpet industries [1, 2]. Among various dyes, azo dyes are the most important and frequently applied for colourization in textile industries. Many Studies demonstrate that the reductive cleavage of the azo bond (–N=N–) by azoreductase enzyme in the liver produces aromatic amines and can even lead to serious disease [3, 4]. The degradation of these group of dyes in order to decrease the environment contaminant to meet increasing environmental demand has continued to attract the interest of several research groups. Three physical, chemical and biological methods are now available for decolourization of azo dye effluents from industries [5-7]. However, the physical and biological methods are not valuable due to simply transfer the pollutants from one phase to another phase. Furthermore, high expensive of equipments involved in these methods limits their practical large scale application. Advanced oxidation processes (AOPs) are different process for decolorizing and reducing wastewater effluents produced by industries [8-12]. Most studies using the photo assisted decomposition of dyes have utilized either Fenton’s reagent, ozonation, H2O2 or titanium dioxide (titania, TiO2) in their treatment methods . Titanium dioxide shows good activity for oxidation of organic compounds by photo when, either oxygen or liquid oxidants are utilized [14, 15]. It is chemically stable non- toxicity and commercially available with low cost . Many attempts have been interested to the further development of the photocatalytic performance of TiO2 for the degradation of numerous pollutants [17-19]. More recently, doping TiO2 with other oxide semiconductor components has attracted considerable interest since these mixed oxide systems can provide higher photocatalytic activity and exceptional characteristics compared with pure TiO2 [20-24].
In recent years, with the developing essential for biomaterials, hydroxyapatite Ca10(-PO4)6(OH)2, abbreviated as HAp, has considered extensive attention for its use as bone filler and implant material through its excellent biocompatibility, close chemical and crystallographic structure with the mineral phase of natural bone . Hydroxyapatite is not only an essential component of hard tissues, which includes bones and teeth, but a material carried out for bioceramics and adsorbents because it has a high affinity to biomaterials such as proteins . The mechanical properties of HAp are poor, especially in wet environment, alarming their limitations for use in heavy-loaded implants, such as artificial bones or teeth. Thus, regardless of their favorable biological properties, the poor mechanical properties of HAp bioceramics can lead to instability and unsatisfactory performance of the implant or scaffold in the presence of body fluids and under local loading . The best way to overcome these mechanical limitations is to use bioactive HAp as ceramic/metal composites so as to achieve both the necessary mechanical strength and bioactive properties . In this regard, many reinforcements such as alumina [29, 30], zirconia [31, 32], bioglass  and titania [34, 35] have been used in HAp materials. Such composites are expected to have improved mechanical properties compared to pure ceramics, and better structural integrity and flexibility than brittle ceramics. It has been reported that titania and HAp represent a good combination for functionally graded materials providing a gradient of bioactivity and good mechanical properties . The photo-induced electronic excitation in HAp are similar to the phenomena of photocatalysis in TiO2, which is a well-established photocatalytic material used for the degradation of organic molecules . TiO2 has also been investigated extensively for the killing or growth inhibition of bacteria [38, 39]. Therefore, the composites of hydroxyapatite and TiO2 have the ability to adsorb bacteria and organic materials and are considered to be good in antibacterial applications and environmental purifications and also for photocatalytic decomposition of biomaterials, such as proteins and lipids [40-42].
In our present work, HAp/TiO2 nanocomposite has been successfully synthesized by in situ precipitation method. This new catalyst can efficiently degrade methyl orange (MO). X-ray diffraction (XRD), energy dispersive X-ray analysis (EDAX), transmission electron microscopic (TEM), transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) techniques were used for the characterizations of photocatalysts. The nanocomposite photocatalysts appear high photodegradation efficiency of methyl orange (MO) under UV irradiation.
2. Material and methods
All reagents used were of analytical grade. Calcium hydroxide (Ca(OH)2), orthophosphoric acid (H3PO4), titanium tetrachloride (TiCl4) and sodium hydroxide (NaOH) were purchased from Merck chemicals and used as-received. Test solutions of MO dyes were prepared by serial dilution of stock MO solution (1000 mg L−1) using deionized distilled water.
2.2. Synthesis of nano HAp powder
Hydroxyapatite nanopowders were prepared according to a reported procedure . Briefly, a 0.4 M suspension of calcium hydroxide Ca(OH)2 was prepared and stirred using a magnetic stirrer. Then, a certain amount of H3PO4 solution (0.24 M) (the Ca/P ratio was kept the same as the stoichiometric value for HAp, 1.67) was added dropwise to the Ca(OH)2 dispersed medium. Following that, the pH of mixture suspension was adjusted to 11 by adding NaOH solution (2 M). After thoroughly mixing the reactants, the mixed solution was stirred at room temperature overnight. Subsequently suspension was centrifuged at 4500 rpm for 15 min and repeatedly washed three times with distilled water. The prepared precipitate was dried in an oven at 100 ºC for 24 h.
2.3. Synthesis of HAp/TiO2 nanocomposite
The synthesis of HAP/TiO2 nanocomposite has been done through the chemical precipitation technique. In order to prepare colloidal Ti(OH)4, the NH4OH solution has been added into TiCl4 solution. The products were also filtered and washed with distilled water. In the production of the HAp/TiO2 nanocomposites, 1 M of aqueous Ti(OH)4 solution and 0.24 M of H3PO4 solution were added dropwise to 0.4 M of calcium hydroxide Ca(OH)2 suspension. The pH was monitored at pH 11 by adding NaOH. After the mixture was stirred at room temperature for 24 h, the products were filtered and washed with distilled water, finally dried in an air oven at 90 ºC overnight. Fallowing that, homogenous precipitation was calcined at 400ºC for 2 h.
3. Results and discussion
3.1 Characterization of HAP/TiO2 nanocomposite
FTIR spectra of the HAP/TiO2 nanocomposite with different amounts of Ti4+ ions content (10, 15 and 20 wt.% TiO2) are shown in Fig. 1. These are typical spectra of HAp showing
PO43-derived bands at 569, 605, 875, and 1030–1090 cm−1 . Broad band’s attributed to absorbed water on surfaces were observed at 3416 cm−1 . The split peaks at 1425 and 1475 cm−1, are attributed to
CO32-ions which might be the result of the absorption of atmospheric CO2 on the surface of HA particles . With increasing the Ti4+ concentration, it can be observed that the intensity of the peaks associated with phosphate bands was reduced. These effects can be explained by decrease of crystallinity due to increased Ti substitution in the HAp lattice.
Fig. 2 shows the XRD patterns of samples with different compositions of HAp/TiO2 nanocomposites calcined at 400 ºC for 2 h. The diffraction peaks with 2θ values of 26.5, 32.1, 32.5, 33.8, 40.0, 46.8 and 49.2 are assigned to hexagonal phase HAp (JCPDS 09-0432) . Also, the diffraction peaks with 2θ values of 25.8, 39.3, 48.3, 53.8 and 56.21 can be exactly indexed to anatase phase TiO2 (JCPDS 21-1272) . The peak intensity of anatase phase increaseswith the increase of TiO2 concentration in the HAp/TiO2 nanocomposites. All the XRD patterns show either HAp or TiO2 peaks and there are no impurity peaks. This clearly confirms the high chemical and thermal stability of the HAp/TiO2 nanocomposites.
Fig. 3 shows the changes observed in the morphology of HAp/TiO2 nanocomposites after calcination. When the TiO2 concentration was increased, the surface topography shows the presence of a few plateaus of different heights scantily distributed on the TiO2 matrix, as shown in Fig. 3.
To get a clear insight into the nanostructures of the composite and also to confirm the morphology, TEM investigations were also performed. As seen in Fig. 4, as amount of titania increased from 10 wt.% to 20 wt.%, the grain size of particles increased and the crystallinity of the rod-shaped particles decreased. It seems that under in situ precipitation conditions, hydroxyapatite is stable and keeps its rod-like morphology while Ti-complex is easily hydrolyzed, then nucleated and grown as anatase elongated plate like crystals in the HAp/TiO2 nanocomposite . It can be believed that when the amount of titania is low (10 wt.%), the effect of anatase crystals on the HAp/TiO2 nanocomposites morphology can be considered as small and the theirs morphology are rod-like (like HAp crystals). But further increase amount of titania, leading to increase formation of anatase crystals, so that thin elongated plate-shaped are derived.
Fig. 5 shows the EDAX pattern for 10 wt.% TiO2 nanocomposite which confirms the presence of Ti, Ca, P and O. There is no other element observed from the EDAX analysis. This confirms the nanocomposites have only titania and hydroxyapatite. The Ca/P ratio determined from the semiquantitative analysis of the atomic concentration (At%) is 1.7, a little higher than the stoichiometric Ca/P ratio of 1.67. This phenomenon could be attributed to calcination treatment, because phosphorus atoms are able to disengage from the material surface and volatilize as oxides during heating process .
3.2. Photocatalytic activity
The photocatalytic activity of the HAp/TiO2 nanocomposites was evaluated under UV-A light (365 nm) illumination with intensity 10 W/m2. 1 L of MO with the appropriate amount of catalyst (1-3 g) was stirred for 30 min in the dark, prior to illumination in order to achieve maximum adsorption of MO onto the nanocomposite surface. During the illumination time no volatility of the solvent was observed. At specific time intervals 2–3 mL of sample was withdrawn and catalyst was removed by centrifugation. The changes in the concentration of MO were monitored from their characteristic absorption at 464 nm using UV–visible spectrophotometer (Perkin Elmer, model Lambda 25). The absorbance at 464 nm represents the aromatic part of MO and its decrease of absorbance indicates the degradation of dye. The percentage removal was calculated using the following equation:
Percentage removal%=[C0-Ca]C0 (1)
Where C0 is the initial concentration of dye solution, Ca is the dye concentration after adsorption by sonocatalysts.
3.2.1. Effect of initial solution pH
The degradation of MO was conducted in a pH range of 3–11 with the initial concentrations of MO (5 mg/L), catalyst loading (1 g/L), TiO2 loading (10 wt.%), and the results are illustrated in Fig. 6. As can be seen, decolorization depended strongly on the solution pH: it was substantially reinforced by acidic conditions, while hampered by alkaline conditions. The initial concentration of MO was 5 mg/L and the amount of catalyst was 1 g/L. After 90 min of irradiation, the decolorization efficiency was 56.4%, 42.4%, 38.5%, 36.9% and 32.4%, at pH values of 3, 5, 7, 9 and 11, respectively.
These findings could be explained in terms of the amphoteric behavior of TiO2. The solution pH influenced the ionization state of the TiO2 surface according to the following reactions:
The charge on TiO2 surface was different on the basis of the zero-point charge (zpc) of TiO2; the zpc value for TiO2 was about 6.5 according to a number of previous reports . At pH˂6.5, the surface of TiO2 was positively charged and absorbed negatively charged MO molecules by electrostatic attraction, while at pH˃6.5 it became negatively charged and the absorption of MO molecules became weaker due to repulsive forces. Thus, the electrostatic attraction between the positive charged surface and the negative dye would result in increased degradation under acidic conditions.
3.2.2. Effect of catalyst loading
The amount of catalyst is one of the main parameters for the degradation studies. In order to avoid the use of excess catalyst it is necessary to find out the optimum loading for efficient removal of dye molecule. Several authors have investigated the reaction rate as a function of catalyst loading in photocatalytic degradation process [52-54]. The effect of the catalyst amount on the photocatalytic degradation MO has been carried out in the range 1–3 g of the catalyst for 1000 ml of solution. The results are shown in Fig. 7. As the amount of the catalyst is increased from 1 to 3 g, the degradation increases from 68.4% to 92.3% at 30 min of irradiation time. This is due to increase in the number of HAp/TiO2 particles, which increases the absorption of photons and adsorption of dye molecule. Further increase of HAp/TiO2 loading decreases the removal rate. Increase of the catalyst loading beyond 3 g/L (i.e.) 3.5 g/L may cause screening effect. These effects reduce the specific activity of the catalyst. At high loadings of catalyst, particle aggregation may also reduce the catalytic activity. The optimum amount of catalyst for efficient degradation is found to be 3 g/Lof MO.
The loading content of TiO2 in the range of 10-20 wt% was investigated to control the photocatalytic performance of HAp/TiO2 catalysts. The highest degradation percentage was achieved with an average loading of TiO2 in the presence of 15% HAp/TiO2 photocatalyst. For the higher loading value of 20 wt%, the formation of TiO2 aggregates shields the incident light intensity, so it reduced the adsorption capacity of the support and decreased the photocatalytic activity. And lower loading of TiO2 can not favor the generation of enough holes and hydroxyl radicals which led to a decrease in the formation of OH radicals, and also decreased the photocatalytic activity. Therefore, 15% HAp/TiO2 photocatalyst catalyst was used as a template in this study.
3.2.3. Effect of initial dye concentration
It is important from an application point of view to study the dependence of degradation efficiency on the dye initial concentration in wastewater. Hence, in this section, the effect of different concentration range of MO dye from 5 mg/L to 25 mg/L on the photocatalytic degradation was studied. As seen in Fig. 8, it can be found that, under UV irradiation the degradation ratio of dye in aqueous solution also depends on the initial concentration of the used organic dye. For all initial dye concentrations, the catalyst amount and light intensity are same. After 90 min of irradiation, the percentage values were 96.2, 89.3, 81.4, 68.2 and 51.0 % at initial concentrations of 5, 10, 15, 20 and 25 mg/L, respectively. Since the generation of hydroxyl radical remains constant, the probability of dye molecule to react with hydroxyl radical decreases. At high initial dye concentrations, the path length of photon entering into the solution also decreases. Thus, the photocatalytic degradation efficiency decreases, but at low concentration the reverse effect is observed thereby increasing photon absorption by the catalyst .
HAp/TiO2 compound had been successfully synthesized by in situ precipitation of HAp powders and colloidal Ti(OH)4 solutions. XRD, FT-IR, EDAX, TEM and SEM were used to characterize the product. This TiO2/HAp nanocomposite has both excellent adsorption performance and superior photocatalytic redox ability, and can be used as photocatalyst to effectively degrade MO under UV irradiation. The catalyst of 15% HAp/TiO2 calcined at 400 ºC showed the highest photocatalytic activity among all the samples in the degradation of Methyl Orange.
 M. Pelaez, N.T. Nolan, S.C. Pillai, M.K. Seery, P. Falaras, A.G. Kontos, P.S.M. Dunlop, J.W.J. Hamilton, J.A. Byrne, K. O’Shea, M.H. Entezari, D.D. Dionysiou, Applied Catalysis B: Environmental, 125 (2012) 331-349.