Journal of Scientific Research and Studies Vol. 4(10), pp. 245-253, October, 2017 ISSN 2375-8791 Copyright © 2017 Author(s) retain the copyright of this article http://www.modernrespub.org/jsrs/index.htm Full Length Research Paper

Use of hydroxyapatite Ca5(PO4)3OH obtained from refuse of cattle slaughterer as a catalyst support: an alternative

material for heterogeneous photocatalysis

Fabrício V. de Andrade1*, Geraldo M. de Lima2, Rodinei Augusti2, Márcio G. Coelho3, Yola Pertence4, Isla R. Machado4 and Luciana S. Duarte2

1Universidade Federal de Itajubá-Campus Itabira, 35903-087, Itabira, MG, Brazil.

2Departamento de Química, ICEx, Universidade Federal de Minas Gerais, 31270-901, Belo Horizonte, MG, Brazil. 3Colégio Militar de Belo Horizonte, CMBH, 31255-000, Belo Horizonte, MG, Brazil.

4Centro Universitário Newton de Paiva, 30494-270, Belo Horizonte, MG, Brazil.

*Corresponding author. E-mail: fvqandrade@yahoo.com.br, Tel.: 0055 31 3839 0897.

Accepted 12 September, 2017

The composite TiO2/hydroxyapatite, prepared by the impregnation method, has been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), electronic spectroscopy, and the surface area has been evaluated by Brunauer-Emmett-Teller (BET) technique. The photocatalytic activity was tested in degradation of three different classes of organic molecules in aqueous medium: atrazine (ATR), diclofenac (DFC) and indigo carmine (IC). Electrospray ionization mass spectrometry (ESI-MS) was used to identify the main by-products generated. Based on these results, a mechanism was proposed for the degradation of each component after the photocatalytic process. The degradation rate of each compound was determined by the data acquired from high performance liquid chromatography (HPLC) measurements. Finally, eco-toxicological tests with Artemia salina showed little differences in the toxicity of diclofenac (DFC) and indigo carmine (IC), before and after the degradation process. However, the toxic effects of ATR were reduced by half using the same oxidation approach. Key words: Photocatalysis, bovine bone, titanium oxide, wastewater, organic pollutants, eco-toxicity.

INTRODUCTION In recent years, advanced oxidation processes (AOP) have proven effective in removing organic contaminants found in aqueous media which are not removed by conventional water treatment (Wang and Xu, 2012). Fenton's reagent and photocatalysis heterogeneous seems to be the most promising approaches (Wang et al., 2012).

Among the used semiconductors, the wide band-gap TiO2 is up to now the most attractive one with high photocatalytic efficiency under UV-A illumination (Méndez-Arriaga et al., 2008). Commercial TiO2 is a widespread UV-A photocatalyst for mineralizing pollutants in small scaled environmental applications because of the low price of the raw material and its chemical stability and inertia. However, its use is limited

by photo-charge recombination (Wang et al., 2012). Its lack of activity in the region of the visible light constitutes another problem that prevents its photocatalytic applications in large scale (Xia and Yin, 2013; Liu et al., 2010). So all together, these drawbacks justify the development of more efficient photocatalytic materials, and are as safe as TiO2 for human and the environment exposure. Among many approaches, adding adsorbents such as activated charcoal, alumina, silica or zeolite to TiO2 has been reported to be an interesting way for increasing efficiency (Coelho et al., 2011; Rossetti et al., 2003; Magalhães et al., 2011) due to the high surface area, usually presented by these materials. Composite materials based on TiO2 deposited in various matrices can be prepared by different methodologies. Recently,

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246 J. Sci. Res. Stud. the preparation of various alternative materials for application in advanced oxidative processes has been reported in the literature (de Andrade et al., 2014, 2015). The studies on the degradation in water organic contaminants have usually focused on the efficiency promoted by oxidation methodologies, whereas less attention has been given, to the characterization of by-products probably formed after degradation conditions. Some works use GC-MS to identify products resulting from the degradation of some dyes (Sahoo and Gupta, 2015; Ghelardi et al., 2015). An alternative technique to be used for this purpose is electrospray ionization mass spectrometry (ESI-MS). This technique possesses remarkable capability to gently transfer species from the condensed phase to the gas phase without inducing undesirable side reactions. Thus, ESI-MS has been successfully applied in the monitoring of an increasing number of environmental methodologies (Otłowska et al., 2015). In the present work, the TiO2/hydroxyapatite composite was prepared and its photocatalytic activity was investigated against photodegradation of emerging organic contaminants. It is important to mention that the hydroxyapatite used as the support of the catalyst was obtained from the residues of a cattle slaughterer. EXPERIMENTAL PROCEDURE Materials and reagents All chemicals were purchased from Sigma–Aldrich (Milwaukee, WI) and used without further purification. Doubly distilled water was used to prepare solutions. Commercial sea salt was purchased from material suppliers. Hydroxyapatite preparation The hydroxyapatite was prepared as follows: a bovine bone (leg region) was milled and heated to 400°C in order to promote decomposition of the organic material. After this step, the powder was washed with distilled water to remove any remaining residue and then dried at 60°C. Compound preparation The TiO2/hydroxyapatite compound was prepared by the impregnation method, where the hydroxyapatite powder was added under vigorous stirring in a solution of TiCl4 in isopropanol. After stirring for 4 h, NH4OH was added to promote the formation of Ti(OH)4. Then, the solid phase was filtered and held in an oven for 24 h at 60°C for drying. The powder obtained in this process, was thermally treated at 550°C for 2 h for the formation of

TiO2 in the anatase phase. Characterization techniques XRD patterns were collected with a Siemens D5000 instrument using a Ni-filtered Cu Kα radiation (λ = 1.5418 Å) and a graphite monochromator in the diffracted beam. A scan rate of 1°/min was applied to record a pattern in the 2θ range of 20°- 80°. Silicon was used as internal standard. UV–Vis measurements were performed by using a Hitachi U-2010 spectrophotometer. The surface area and porosimetry measurements were carried out on an ASAP2010 Micrometrics using N2 as the adsorbent at liquid nitrogen temperature. Before the N2 adsorption, the material was outgassed at 200°C for 3 h in order to desorb the impurities or moisture from its surface. The total surface area was calculated from the N2 adsorption isotherms using the BET method, and the micropore surface area was derived from the t-plot curve. The morphology of the composite sample was characterized using scanning electron microscopy (SEM) analysis by means of a JSM-6700 LV electron microscope operating at 5.0 kV. Catalytic experiments The prepared composite (30 mg) was added to an aqueous solution (50 mL) of ATR (5 mg L-1), DFC (5 mg L-1) or IC (30 mg L-1). Then the system was exposed to ultraviolet radiation (UV lamp: Philips HPL–N, 36 W, wavelength ranging from 200 to 400 nm). For all assays, aliquots were taken at times of 0, 5, 15, 30, 60, 90 and 120 min, filtered using a 0.45 mm filter (Millipore, Jaffrey, NH) to eliminate solid particles and kept protected from light in a cattle slaughterer prior to the HPLC-MS and TOC analysis. The total organic carbon (TOC) was measured in a TOC-V CPH instrument (Shimadzu, Tokyo, Japan). The aliquots were kept protected from light in a cattle slaughterer at 4°C prior to the ESI-MS and UV-Vis analyses.

Analyses by direct infusion ESI were performed using a mass spectrometer (model IT-TOF, Shimadzu, Tokyo, Japan) with two analyzers in tandem: ion trap (IT) and time-of-flight (TOF). The mass spectrometer operating at high resolution and mass accuracy was optimized under the following conditions: (i) ESI voltage at -3.5 kV (negative ion mode); (ii) nebulizer gas (nitrogen) flow rate at 1.5 L min-1; (iii) curved desorption line (CDL) interface temperature at 200°C; (iv) drying gas (nitrogen) pressure at 100 kPa; and (v) octapole ion accumulation time of 100 ms. Mass spectra were obtained in the full scan mode within a m/z 100-500 range.

For the chromatographic analyses, an ACE C18 column (2.1 x 50 mm x 3 mm diameter of a particle) was used. Water (A) and methanol (B) was employed as

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Figure 1. X-ray powder diffraction spectrum of the TiO2/Ca5(PO4)3OH composite.

mobile phase at a flow rate of 0.2 mL min-1. The gradient program started with 30% B, rising to 50% B in 4 min, then to 100% B in 3 min, which was held until 10 min. At the end of the chromatography run, the column was re-equilibrated to the initial conditions and stabilized for 4 min, which led to a total run time of 14 min. The injection volume was 5 µL. The same conditions as described above were employed for the mass spectrometer. Biological tests Toxicity tests were carried out by exposing the solutions arising from the degradation processes against brine shrimp (Artemia salina). The ecotoxicity tests were carried out following a protocol present in the literature (Meyer et al., 1982). Hence, an aqueous solution of sea salt (at 38 g. L-1) was prepared, filtered and added to a small (15 cm diameter) round plastic container. Subsequently, many A. salina eggs were added in only one half of this container, which was kept protected from light for 24 h, whereas the opposite half was continuously irradiated by a 100 W incandescent lamp. After the eggs hatched, the A. salina organisms migrated to the lit side, then a small portion of this solution with adult individuals could be collected and transferred to a cylindrical glass vial (3 cm diameter) and the volume completed to 1.0 mL by adding the above-mentioned sea salt solution. Afterwards, 4.0 mL of a reaction aliquot, collected from the degradation experiments, were put into the vial, and the volume was completed with water to 5.0 mL. The vial was then left to stand under light for 48 and 96 h and the percentage of immobilized organisms determined. The

assays with each aliquot were carried out in triplicate to estimate the toxicity of the target molecules (ATR, DFC and IC) and by-products. RESULTS AND DISCUSSION Characterization of the TiO2/hydroxyapatite composite The X-ray powder diffraction was performed to investigate the crystalline phases present in the composite. According to the results, it was verified that the material obtained after thermal treatment had two phases of TiO2 (Anatase and Rutile) and one hydroxyapatite phase (Figure 1).

The identification of the crystal phases obtained by a powder diffraction X-ray was made by comparing the XRD pattern of sample analyzed by X-ray, using the database PDF2 of ICDD- International Center for Diffraction Data/JCPDS- Joint Committee on Powder Diffraction Standards, and with the help of software search-match. The crystallographic records of the identified crystalline phases are: hydroxyapatite (34-10), TiO2 anatase (1-562) and TiO2 rutile (1-1292).

Comparing the electronic spectra of the TiO2/Ca5(PO4)3OH composite and the support (Ca5(PO4)3OH), it is observed a higher sensitivity of the composite to UV light than that of Ca5(PO4)3OH (Figure 2). In addition, a band-gap of 3.3 eV was found for the TiO2/hydroxyapatite composite. It is worth mentioning that the value obtained for the gap of this material is very close to the reported value for TiO2 with photocatalytic

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Figure 2. Spectroscopy UV-vis- absorption in the UV-vis region of the composite X support, band gap values obtained by UV–Vis absorption measurements for the composite samples (b).

Figure 3. Scanning electron microscopic (SEM), (a) 100 μm and (b)10 μm.

properties.

The surface area measurement revealed that the prepared composite has a specific surface area of about 25 m2.g-1 and mean volume of pores of 16.29 nm, suggesting the presence of mesoporous at the material

prepared (Rossetti et al., 2003; Órfão et al., 2006). Scanning electron microscopy was used to investigate

the morphology of the composite. Figure 3a reveals the existence of several particles of the composite covered with small amounts of TiO2. One of these particles was

de Andrade et al. 249

Figure 4. ESI(-)-MS of the aliquots collected at assorted times from the reaction vessel containing an aqueous solution of IC exposed to the TiO2/Ca5(PO4)3OH/UV photocatalytic system: (a) 0 min, (b) 60 min, (c) 120 min. Emerging ions are indicated in the mass spectra of the reaction aliquots.

arbitrarily chosen and the image was enlarged in this region as indicated by the arrow in Figure 3b.

Measurements by atomic absorption spectroscopy indicated that the TiO2/Ca5(PO4)3OH material has approximately 34% by weight of TiO2. Thus the hydroxyapatite obtained from bovine bone is really the support of the material. Photocatalytic activity of the composite ESI-MS was used to monitor the degradation of the target molecules exposed to the TiO2/hydroxyapatite/UV system and also for identification of by-products formed under these conditions. Hence, the ESI(-)-MS of the aliquots collected in the reaction systems of IC, DCF and ATR are shown in Figures 4, 5 and 6, respectively.

These data indicated a drastic reduction in the concentration of the three target molecules after 120 min of exposure to the TiO2/Ca5(PO4)3OH/UV system. In addition, the relative intensities of the anions originating from the by-products (indicated in each mass spectrum) in solution enhanced concomitantly with the increase in the reaction time. Based on these data, routes for the degradation of each substrate could therefore be proposed (IC: Figure 7-i; DCF: Figure 7-ii; ATR: Figure 7-iii).

Hence, in the ESI (-)-MS of the initial solution of IC (1) the anion [1 - 2 H]2- (IC doubly deprotonated) of m/z 210 is prominent (structure 1 in Figure 7). Several other

anions, i. e. [2 - H]- (of m/z 226) and [3 - H]- (of m/z 216), arose because the formation of by-products under these oxidation conditions. It is important to mention that similar by-products than those detected herein (Figure 7a) were previously described in the literature (de Andrade et al., 2014, 2015).

Figure 5 displays the ESI (-)-MS of the original solution of DFC (4). An anion of m/z 294 [4 - H]- (DFC in its deprotonated form) and its isotopologue of m/z 296 are clearly observed in this mass spectrum. The abundance ratio of 1/0.66 for these isotopologues, respectively, is that expected for a species containing two chlorine atoms. The presence of an additional anion of m/z 250 in this mass spectrum, ascribed to be [5 - H]-, is likely due to a product (5) naturally formed by the spontaneous loss of CO2 from the DFC molecule. The isotopologue distribution (relative abundance ratio of 1/0.66 for the anions of m/z 250 and 252, respectively) also indicates that this product (5) still holds two chlorines in its structure. Two other anions were detected in the mass spectrum of the subsequent aliquots: [6 - H]- (of m/z 258) and [7 - H]- (of m/z 214). The first one is proposed to be associated with a by-product (6) generated by the loss of HCl from DFC (4). The [7 - H]- anion, on the other hand, is detectable because of the formation of a by-product (7) that arose from the compound 5 by the replacement of Cl by OH. The isotopologue distribution for the anions [6 – H]- (relative abundance ratio of 1/0.33 for the anions of m/z 258 and 260, respectively) and [7 - H]- is consistent with the presence of one or zero chlorine atoms in their

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Figure 5. ESI(-)-MS of the aliquots collected at assorted times from the reaction vessel containing an aqueous solution of DCF exposed to the TiO2/Ca5(PO4)3OH/UV photocatalytic system: (a) 0 min, (b) 60 min, (c) 120 min. Emerging ions are indicated in the mass spectra of the reaction aliquots.

Figure 6. ESI(-)-MS of the aliquots collected at assorted times from the reaction vessel containing an aqueous solution of ATR exposed to the TiO2/Ca5(PO4)3OH/UV photocatalytic system: (a) 0 min, (b) 60 min, (c) 120 min. Emerging ions are indicated in the mass spectra of the reaction aliquots.

structures, respectively. Both experimental information therefore support the hypothetic mechanism as displayed in Figure 7ii.

Figure 6 displays the mass spectrum obtained for the initial solution of ATR. Two isotopologue anions of m/z214 and 216 with a relative abundance ratio of 1/0.33,

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Figure 7. Proposed route for the degradation of IC, DFC and ATR in water as induced by the TiO2/Ca5(PO4)3OH/UV photocatalytic system.

respectively, refers to ATR (8) in its deprotonated form, i. e. [8 - H]-. Only the anion of m/z 196 is detected in the mass spectra of the later aliquots. Its isotopic pattern indicates the absence of chlorine atoms in its structure. This anion corresponds to the deprotonated form of a by-product (9) which was proposed to be formed directly from ATR (8) by the replacement of Cl by OH. The compound 9 (hydroxiatrazine) is a well-known degradation product of ATR as previously reported in the literature (Panshin et al., 2000).

High performance liquid chromatography (HPLC) was used to calculate the half-life time (t1/2) for the degradation of the target molecules (IC, DFC and ATR) in

aqueous solution promoted by the TiO2/Ca5(PO4)3OH/UV photocatalytic system (Table 1). By the chromatographic data, it was also possible to verify that the deterioration of all systems adjusted well to a first order kinetic model. As can be seen in Table 1, for the systems IC/UV and IC/UV/TiO2/Ca5(PO4)3(OH), the difference in the removal rate was significant. In this system, it becomes evident of the photocatalytic activity of the composite, the system containing TiO2/Ca5(PO4)3(OH), which was able to remove a much larger amount of the indigo carmine dye than the UV light solely.

Measurements of the Total Organic Carbon (TOC) were carried out to determine the mineralization rate of

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Table 1. Mineralization rate and half-life time obtained for each substrate (IC, DFC and ATR) in aqueous solution upon application of the photocatalytic (TiO2/Ca5(PO4)3(OH)/UV) and photolytic (UV) systems.

Substrate Mineralization rate (%) Half-life time (t1/2, min)

Photocatalysis Photolysis Photocatalysis Photolysis

IC 19.8 3.5 65 min Without determination by the plot DFC 66.4 24.6 12 min 15 min ATR 81.1 57.8 120 min 120 min

each substrate achieved by the application of the TiO2/Ca5(PO4)3OH/UV system after an exposure time of 120 min. To make a proper comparison, the experiments were also conducted in the absence of the composite (only UV light). The results are displayed in Table 1.

The results indicate that the composite not only accelerates degradation-comparing to the UV system for almost all substrates, but also leads to the attainment of higher mineralization rates. Note also that DFC and ATR are degraded by the UV radiation (photolytic system) with higher mineralization rates and shorter half-life times than IC. That is likely because the DFC and ATR molecules significantly absorb in the UV region thus competing with the semiconductor material (TiO2/Ca5(PO4)3OH) by the UV radiation. The opposite occurs with IC that absorbs in the visible region.

It is worth noting that the results obtained for the degradation of the molecules used in this work are of great interest in the environmental sciences, since diclofenac (Achilleos et al., 2010) and atrazine (Kerminen et al., 2016; Yue et al., 2016) are recognized as an environmental problem. Biological tests Ecotoxicological tests against A. salina were carried out in order to compare the toxicities of the original solutions of the substrates with those of the aliquots collected after 120 min of exposure to the TiO2/Ca5(PO4)3OH/UV system. A. salina has been chosen as probe in the tests in view of some advantages, such as: it is an organism with a wide geographic distribution, relatively simple laboratory culture and maintenance, high resistance to manipulation, short life-cycle, large offspring production and mainly a remarkable sensitivity toward xenobiotics (Nunes et al., 2006).

The results showed little differences in the toxicities among both types of solutions for the three substrates when a contact time of 48 h was evaluated. After a contact time of 96 h, invariable toxicities between the solutions (original and aliquots) of DFC and IC were observed. However, the aliquot collected from the ATZ reaction presented a toxicity of 50% smaller than the original solution. This effect can certainly be caused by the reduction in the concentration of ATZ in the reaction

medium or even by the lower toxicity of the main by-product (9, hydroxyatrazine) formed under these conditions. This result is remarkable in view of the slow degradation of ATZ in the environment and its teratogenic and estrogenic disruptor properties, even at low concentrations (Panshin et al., 2000). Conclusion The TiO2/Ca5(PO4)3OH composite prepared, acted efficiently as photocatalyst in the degradation of a number of prototype substrates in aqueous medium: Atrazine (ATZ), Diclofenac (DCF) and Indigo Carmine (IC); these results reinforce reports from the literature that show that hydroxyapatite improves the photocatalytic activity of some composites (Liu et al., 2010). In all cases, there were higher mineralization rates with conveniently low half-life times, compared to the UV system. The composite described herein presented similar activity in comparison to other material previously reported in the literature (Achilleos et al., 2010; Sriwong et al., 2010). By applying the ESI-MS technique, the structures of the main by-products and a route to the degradation of each substrate were proposed. Finally, tests against A. salina showed that the treatment of an ATZ solution by the TiO2/Ca5(PO4)3OH photocatalytic system reduced its toxicity by half.

An important aspect of the present work is the contribution to the environmental sciences, besides the preparation of a material that proved to be efficient in the photodegradation of persistent organic molecules. It was obtained from an environmental residue, the bone of cattle slaughterer. It was possible to reuse this residue, as a convenient support for a material that presents photocatalytic activity in reactions of environmental importance. Acknowledgements The authors thank the Minas Gerais State Science Foundation (FAPEMIG) and the Brazilian National Research Council (CNPq) for the financial support and the granting of research fellowships. The authors are also indebted to Dr. Rochel M. Lago (DQ-UFMG) for his

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