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Effect of Ag loading on activated carbon doped ZnO for bisphenol A degradation under visible light
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Original Research Paper
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Effect of Ag loading on activated carbon doped ZnO for bisphenol A degradation under visible light
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Khanitta Intarasuwan a, Pongsaton Amornpitoksuk a,b,⇑, Sumetha Suwanboon c, Potchanapond Graidist d,e, Saowanee Maungchanburi d, Chamnan Randorn f,g a
Department of Chemistry, Faculty of Science, Prince of Songkla University, Hat-Yai, Songkhla 90112, Thailand Center of Excellence for Innovation in Chemistry, Faculty of Science, Prince of Songkla University, Hat-Yai, Songkhla 90112, Thailand Department of Materials Science and Technology, Faculty of Science, Prince of Songkla University, Hat-Yai, Songkhla 90112, Thailand d Department of Biomedical Science, Faculty of Medicine, Prince of Songkla University, Hat-Yai, Songkhla 90112, Thailand e The Excellent Research Laboratory of Cancer Molecular Biology, Prince of Songkla University, Hat-Yai, Songkhla 90112, Thailand f Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand g Materials Science Research Center, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand b c
a r t i c l e
i n f o
Article history: Received 26 March 2018 Received in revised form 16 June 2018 Accepted 13 July 2018 Available online xxxx Keywords: ZnO Bisphenol A Photocatalytic property Cytotoxicity
a b s t r a c t Silver modified activated carbon doped zinc oxide (Ag/AC-ZnO) was synthesized via a calcinationelectroless deposition route. The samples were characterized by X-ray powder diffractometry, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectrometry, Fourier transform infrared spectroscopy and UV–vis diffuse reflectance spectroscopy. The photocatalytic activity of the Ag/AC-ZnO was evaluated for bisphenol A degradation in the presence of H2O2 under visible light irradiation. The archived results showed that the photocatalytic activity of the Ag/AC-ZnO was higher than that of AC-ZnO and pure ZnO. The cytotoxicity of the bisphenol A after photocatalysis under visible light irradiation was tested using L929 mouse fibroblast cells and the obtained results indicated that the treated bisphenol A solution exhibited no cytotoxicity against normal cells. Ó 2018 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.
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1. Introduction
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Bisphenol A (BPA) is commonly used as the starting material in the production of polycarbonate plastics and epoxy resins. It is emitted into the environment during manufacturing process and leaches from products. BPA exhibits moderate acute toxicity to vertebrates [1] and when BPA discharged into the river as a result of industrial activity, it may affect aquatic animals and whoever drinks the contaminated water. BPA is a stable compound and does not readily degrade in the ecosystem so it must be treated before being released into the environment. Nowadays, there are many treatment processes to remove BPA from wastewater such as microfiltration, ozonation and biological methods [2]. One of the most effective treatments is an advanced oxidation process that the pollutant is removed by oxidation through the reaction with
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⇑ Corresponding author at: Department of Chemistry, Faculty of Science, Prince of Songkla University, Hat-Yai, Songkhla 90112, Thailand. E-mail addresses: [email protected] (K. Intarasuwan), pongsaton. [email protected] (P. Amornpitoksuk), [email protected] (S. Suwanboon), gpotchan@ medicine.psu.ac.th (P. Graidist), [email protected] (S. Maungchanburi).
reactive species such as the hydroxyl radical or the superoxide anion radical. This treatment has many advantages over conventional processes, for example, it produces harmless degradation products, does not use expensive or hazardous oxidizing chemicals and takes advantage of natural sunlight. In the field of photocatalysis, zinc oxide (ZnO) is a good candidate because of its good photocatalytic property, high stability and eco-friendliness. In the literature, ZnO based photocatalysts such as pure ZnO [3], carbon doped ZnO [4], Ag/ZnO [5] and Ce-ZnO [6] showed a high degradation potential for BPA in aqueous media under UV irradiation but there are only a few reported investigations of these photocatalysts under visible light irradiation. Qui et al. [7] reported that 93% of BPA was degraded after 4 h under visible light irradiation using N-doped ZnO and the visible-light response of this photocatalyst was attributed to the narrow band gap of the N 2p state isolated above the valence band of ZnO. However, modification of the band gap energy of the ZnO, in order to increase the visible light absorption capacity, is too complicated compared to a coupling method. Among the many coupling materials, it is well known that Ag is a good candidate sensitizer to make a wide band gap photocatalyst that functions in visible light. After exposure to
https://doi.org/10.1016/j.apt.2018.07.006 0921-8831/Ó 2018 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.
Please cite this article in press as: K. Intarasuwan et al., Effect of Ag loading on activated carbon doped ZnO for bisphenol A degradation under visible light, Advanced Powder Technology (2018), https://doi.org/10.1016/j.apt.2018.07.006
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visible light, according to the energy allowed transition, electrons from Ag can transfer to the conduction band of the ZnO and these injected electrons further react with O2 to produce the superoxide anion radical (O 2 ). Furthermore, the photocatalytic activity of the Ag modified ZnO can be enhanced by inhibited charge recombination in Ag due to the electron injection. Another important factor in the photocatalytic process is the substance or pollutant adsorption on the surface of the photocatalyst. It has been demonstrated that the adsorption of pollutant molecules on the photocatlyst surface can significantly enhance the photocatalytic efficiency [8]. This property would be increased by the addition of porous materials to the photocatalyst. Activated carbon (AC) is a very cheap material that can increase the adsorption ability of many semiconductorbased photocatalysts [4]. Based on the above considerations, silver modified carbon doped zinc oxide (Ag/AC-ZnO) was prepared by a calcinationelectroless deposition route. Furthermore, this work also examined the influence of Ag loading on AC-ZnO on the photocatalytic degradation of BPA under visible light irradiation. The overall cytotoxicity of the treated BPA solution after photocatalysis was assessed on L929 cells.
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2. Experimental
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2.1. Synthesis of Ag/AC-ZnO
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AC-ZnO was prepared by the precipitation-calcination method. In a typical procedure, 20 mL of 0.25 M of Zn(NO3)26H2O containing 0.01 g of activated carbon (QRëC) were added dropwise into 60 mL of 0.166 M H2C2O4. In the preparation of the ZnO, the activated carbon was not used in this step. Next, this mixture was heated under vigorous stirring at 70 °C for 1 h in a water bath. After cooling to room temperature, the precipitant was filtered, washed with distilled water and dried at 100 °C for 1 h in a hot air oven. After that, the powders were calcined at 500 °C for 1 h in a muffle furnace. Ag/AC-ZnO was prepared by a reduction from [Ag(NH3)2]+ ions. This reagent was prepared by the addition of 3 mL of conc NH3 to 40 mL of AgNO3 in various concentrations. One gram of AC-ZnO powders (1 g of ZnO was used for the preparation of the Ag/ZnO) was stirred in a solution of [Ag(NH3)2]+ for 15 min followed by the addition of a 1 M glucose solution and this mixture was continuously stirred for 1 h at room temperature. After that, the solid powder was filtered, rinsed with distilled water and dried at 100 °C for 1 h in an oven.
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2.2. Characterization
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The structural identifications of all samples were carried out using an X-ray diffractometer (XRD, X’Pert MPD, Phillips) with Cu Ka radiation at a wavelength of 0.15406 nm. The morphological studies were observed using a scanning electron microscope (SEM, Quanta 400, FEI) and transmission electron microscope (TEM, JEM-2010, JEOL). Elemental compositions of samples were analyzed by an energy dispersive X-ray fluorescent spectrometer (EDX, Oxford). The Fourier transform infrared (FT-IR) spectra of the samples in transmission mode at 400–4500 cm1 were recorded by the FT-IR spectrophotometer (Spectrum BX, Perkin Elmer). The diffuse reflectance UV–Vis absorption spectra were obtained on a UV–Vis spectrophotometer (UV-2450, Shimazu) using BaSO4 as a reflectance standard. The surface areas of all samples were evaluated by the BET (Brunauer-Emmett-Teller) method using a surface area analyzer (Autosorb 1 MP, Quantachrome). X‐ ray photoelectron spectroscopy (XPS) was performed on an AXIS Ultra DLD (Kratos Analytical Ltd.) electron spectrometer and all
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binding energies were calibrated to the C 1s line at 284.8 eV. Degraded products after photocatalytic reaction was characterized by liquid chromatograph-mass spectrometry (LC-MS, 2690-LCT, Waters, Micromass) using electrospray ionization operating in the negative (ESI) mode.
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2.3. Photocatalytic study
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The photocatalytic activities for all samples were evaluated from the degradation of the BPA under visible light irradiation in the presence of H2O2 (Fig. 1). Into a 250 mL beaker containing of 150 mL of 5 ppm BPA, 0.3 g of photocatalyst was dispersed. After that, 3 mL of H2O2 (30%, Merck) was added into the BPA solution. This mixture was stirred in the dark in order to establish adsorption/desorption equilibrium between the BPA molecules and the surfaces of the photocatalyst. The suspension was then exposed to visible light (51 W Xe lamp) for the required time. After the required time had elapsed, 2 mL of BPA solution was pipetted and centrifuged to completely remove the particles of photocatalyst. The concentration of the BPA was determined by UV–vis spectrophotometer (UV2550, Shimadzu) and the degradation of BPA
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Fig. 1. Photocatalytic experimental setup.
Fig. 2. XRD of (a) ZnO, (b) AC-ZnO and (c) 0.03Ag/AC-ZnO powders. The peaks labeled for three and four indices represent Ag (JCPDS 04-0783) and ZnO (JCPDS 361451), respectively.
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depending on irradiation time was calculated using the following equation:
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Degradation ð%Þ ¼ ½ðCo Ct Þ=Co 100 ¼½ðAo At Þ=Ao 100
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where Co and Ao are the initial concentration and the initial absorbance of the BPA, and Ct and At are the concentration and the absorbance of the BPA after the irradiation at time t.
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In order to investigate the reactive species generated in the photocatalytic process, radical scavenging experiments were carried out [9–11] by adding 1 mmol of ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA-2Na) as a hole (h+) scavenger or isopropanol alcohol (IPA) as a hydroxyl radical (OH) scavenger or nitroblue tetrazolium (NBT) as a superoxide anion radical (O 2) scavenger into 150 mL of the BPA solution before the photocatalytic tests.
Fig. 3. SEM images and EDS spectra for (a) ZnO, (b) AC-ZnO and (c) 0.03Ag/AC-ZnO powders, and (d) EDS mapping images of 0.03Ag/AC-ZnO.
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2.4. In vitro cytotoxicity test
3. Results and discussion
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The overall cytotoxicity of the BPA solution before and after photocatalysis was evaluated by MTT assay [12]. Briefly, L929 (mouse fibroblast, Mus musculus) cells were seeded in a 96-well plate and cultured in a humidified incubator for 72 h. After that, the cells were rinsed with 1X PBS and incubated with 100 lL of 0.5 mg/mL MTT at 37 °C for 30 min. The dark blue crystals of formazan (MTT metabolites) were dissolved with 100 lL of dimethyl sulfoxide (DMSO). This solution was added to each well and incubated at 37 °C for 30 min. Finally, the optical density was determined by measuring the difference in absorbance at 570 and 650 nm using a microplate reader (Spectra Max M5, Molecular Devices). Viable cells were determined as a percentage of survival and calculated using the following equation (n = 3):
3.1. Characterization
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After calcination at 500 °C for 1 h, the ZnO and AC-ZnO powders showed the same diffraction pattern as presented in Fig. 2(a) and (b) and all presented peaks matched well with JCPDS card number 36–1451 (ZnO). In the literature, pure AC usually has 2 broad peaks at 2h = 15–35° and 35–50° but the absence of these broad peaks in this work was probably due to the low AC loading in the sample. This evidence has been found in many reports [13–14]. After the metallic Ag was introduced on the surface of the AC-ZnO using a reduction of the [Ag(NH3)2]+, the XRD pattern of the resulting product showed the additional diffraction peaks corresponding to metallic Ag (JCPDS card number 04-0783), as shown in Fig. 2(c). The SEM images for ZnO, AC-ZnO and 0.03Ag/AC-ZnO (Fig. 3(a)– (c)) show the same morphology built up from the agglomeration of small ZnO particles in all samples. In 0.03Ag/AC-ZnO, there are small particles on the surface of agglomerated AC-ZnO particles that were probably assigned to the metallic Ag, as these small particles were found in ZnO and AC-ZnO. The presence of the Ag in the 0.03Ag/AC-ZnO was confirmed by EDS in mapping mode, (Fig. 3 (d)). This also indicated that the Ag particles were well dispersed on the surface of agglomerated AC-ZnO particles. In the HRTEM image presented in Fig. 4, the resolved interplanar distance of 0.281 nm matched well with the interplanar spacing (d-spacing) of the (1 0 0) plane of ZnO crystallized in würtzite structure and the lattice spacing at 0.204 nm is assigned to the d-spacing of the (2 0 0) plane of metallic Ag. Fig. 5 displays the FTIR spectra of ZnO, AC-ZnO and 0.03Ag/ACZnO. For ZnO, the broad band located at 3200–3600 cm1 assigned to the OH stretch mode and a band at 1640 cm1 corresponding to the OH bending vibration. These vibrations confirm the presence of bound H2O on the surface of the sample [15–17]. The stretching vibrational mode of the ZnO can be observed at 470 cm1 [18,19]. The FTIR spectra of Ag/AC-ZnO and AC-ZnO are similar (Fig. 5(b) and (c)). The band at 1536 cm1 is related to the C@C stretching vibration in the aromatic rings [20]. A transmittance peak at 1390 cm1 may be due to the stretching vibration of CAO and bending vibration of OAH in the carboxyl carbonates or C@C aromatics and various modes of substituted rings [21,22]. Many vibrational peaks in the region of 700–900 cm1 are assigned to the out-of-plane bending vibrational mode of CAH in the aromatic rings [23,24]. The N2 adsorption-desorption isotherms of ZnO, AC-ZnO and 0.03Ag/AC-ZnO are shown in Fig. 6 and the specific surface areas
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Survival ð%Þ ¼ ½ðA570 nm A650 nm Þ=ðA570 nm A650 nm Þ 100 where A570 nm, A650 nm, A570 nm, and A650 nm are the absorbances of the test samples (at 570 and 650 nm) and the negative control (at 570 and 650 nm), respectively.
Fig. 4. HRTEM image of 0.03Ag/AC-ZnO powders.
Fig. 5. FTIR spectra of (a) ZnO, (b) AC-ZnO and (c) 0.03Ag/AC-ZnO.
Fig. 6. Nitrogen adsorption-desorption isotherms of (a) ZnO, (b) AC-ZnO and (c) 0.03Ag/AC-ZnO.
Please cite this article in press as: K. Intarasuwan et al., Effect of Ag loading on activated carbon doped ZnO for bisphenol A degradation under visible light, Advanced Powder Technology (2018), https://doi.org/10.1016/j.apt.2018.07.006
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determined by BET method for all samples are presented in Table 1. The ZnO showed a characteristic of type II isotherm (non-porous material) that had a hysteresis loop at high relative pressure (P/ P0 > 0.9) [25]. On the other hand, the nitrogen adsorptiondesorption isotherms of AC-ZnO and 0.03Ag/AC-ZnO exhibit type IV with type H3 hysteresis loop, indicating their mesoporous microstructure [26]. Fig. 7 displays the XPS spectra of 0.03Ag/AC-ZnO calcined at 500 °C for 1 h. The C 1s signal was deconvoluted into six peaks at 283.5, 284.2, 285.1, 286.2, 287.4 and 288.7 eV corresponding to the ZnAC, sp2 (CAC), sp3 (CAC and CAH), CAO, OAC@O and C@O bonds [27,28], respectively. The Zn 2p spectrum contains two sets of double peaks. The peaks at binding energies of 1021.2 and 1044.2 eV were assigned to the Zn 2p3/2 and 2p1/2, and the difference between these peaks was 23 eV that indicated that the Zn ions in this sample are of +2 states [29]. The additional peaks at 1022.8 and 1045.7 eV refer to the ZnAC bonds [30] and are in agreement with the results of XPS of C 1s spectrum. The Ag 3d spectrum showed two peaks centered at 367.3 and 373.3 eV attributed to the Ag 3d5/2 and Ag 3d3/2, respectively, and the peaks difference is 6 eV, confirming that the Ag in this sample is metallic silver [31]. The asymmetric O 1s XPS peak can be deconvoluted into four components, centered at 530.1, 531.4, 532.2 and 533.3 eV. The peak on the lower binding energy can be attributed to the Zn-O bonding while the one with the highest binding energy is due to the adsorbed water on the surface. The peak at 532.2 eV has usually been assigned to the chemisorbed or dissociated oxy-
Table 1 Surface areas evaluated by BET method for prepared photocatalysts. Sample
Surface area (m2/g)
Sample
Surface area (m2/g)
ZnO AC-ZnO 0.005Ag/AC-ZnO 0.01Ag/AC-ZnO
13.61 63.25 29.12 27.08
0.03Ag/AC-ZnO 0.07Ag/AC-ZnO 0.10Ag/AC-ZnO
20.62 16.68 16.37
5
Fig. 8. Diffuse reflection absorption spectra for (a) ZnO, (b) AC-ZnO and (c) 0.03Ag/ AC-ZnO powders.
gen or OH species on the surface of ZnO or adsorbed O2. Furthermore, this sample showed the peak at 531.4 eV that has been described in oxygen-deficient regions within ZnO structures (oxygen vacancies) [32]. The optical absorption spectra for ZnO, AC-ZnO and 0.03Ag/ACZnO are shown in Fig. 8. The absorption capacities of visible light are in the order: 0.03Ag/AC-ZnO > C-ZnO > ZnO. All samples showed the strong absorption band in the UV-region corresponding to the electron transition between the valence band and the conduction band of ZnO. The broad band in the range 425–800 nm probably comes from the photon absorption of AC, while metallic Ag showed the high photon absorption in the visible region.
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3.2. Photocatalytic activity
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The chronological change in absorbance for BPA during irradiation time is shown in Fig. 9(a). The absorbance of BPA decreased
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Fig. 7. XPS spectra of C 1s, Zn 2p, Ag 3d and O 1s for 0.03Ag/AC-ZnO.
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Fig. 10. Photocatalytic degradation of BPA in the presence of H2O2 using 0.005Ag/ AC-ZnO (}), 0.01Ag/AC-ZnO (h), 0.03Ag/AC-ZnO (4), 0.05Ag/AC-ZnO (j), 0.07Ag/ AC-ZnO (*) and 0.1Ag/AC-ZnO (s) as the photocatalysts.
Fig. 9. (a) UV–vis spectra of BPA by photocatalysis for different irradiation times over 0.03Ag/AC-ZnO under visible light irradiation and (b) photocatalytic degradation of BPA in the presence of H2O2 using ZnO, AC-ZnO, 0.03Ag/ZnO and 0.03Ag/ACZnO as the photocatalysts.
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with an increment of irradiation times, implying that the BPA can degrade under this condition. The photocatalytic activities for BPA degradation over ZnO, AC-ZnO, 0.03Ag/ZnO and 0.03Ag/AC-ZnO in the presence of H2O2 are shown in Fig. 9(b). Actually, the ZnO is a wide band gap semiconductor material and shows photocatalytic properties only under UV exposure. Although the AC-ZnO can absorb photons in the visible light region, it cannot degrade BPA. The degradation of BPA under visible light irradiation would be observed when the Ag/AC-ZnO was introduced into the BPA solution, as shown in Fig. 9(b) (the reason will be discussed later). The content of Ag loading in the AC-ZnO powders strongly influenced the photocatalytic degradation of BPA under visible light as presented in Fig. 10. It is clear to observe that, after 30 min of adsorption in the dark, a little reduction of BPA concentration was observed, indicating that some BPA can be removed from the solution by adsorption phenomena. However, most BPA molecules would be degraded by the photocatalytic reaction. After irradiation, all Ag/AC-ZnO samples showed photocatalytic properties as the concentration of BPA also decreased with an increment of the irradiation times. Overall activity for the BPA degradation is in the order: 0.03Ag/AC-ZnO > 0.01Ag/AC-ZnO > 0.005Ag/AC-ZnO > 0.05Ag/AC-ZnO > 0.07Ag/AC-ZnO > 0.1Ag/AC-ZnO > AC-ZnO. However, this order was not related to their surface areas as presented in Table 1. It is clear to see that the photocatalytic activity increased continuously with the increment of Ag contents and the maximum activity was produced by 0.03Ag/AC-ZnO. After this point, however, increasing the Ag content decreased the photocat-
alytic activity, as seen in Fig. 10. As the O 2 generation is a surface reaction and it would occur at the surface of AC-ZnO as presented in Eq. (5) so, at high Ag loading, the surface of AC-ZnO could be mostly covered with Ag and O 2 production would be reduced. To evaluate the stability of the photocatalyst, 0.03Ag/AC-ZnO was used to degrade BPA in five repeated cycles and the results are shown in Fig. 11. After recycling five times, the photocatalytic activity of this photocatalyst still remained higher than 85%, indicating that the 0.03Ag/AC-ZnO is relatively stable under visible light irradiation. In this system, the Ag/AC-ZnO did not degrade the BPA under visible light irradiation if there was no addition of H2O2, as presented in Fig. 12(d). In the presence of H2O2, when the H2O2 contacted the metallic Ag on the surface of the AC-ZnO, O2 bubbles were automatically generated in the solution [33] in the following reaction:
2H2 O2 + Ag ! 2H2 O + O2 + Ag
ð1Þ
To ensure that the BPA degradation did not result from this reaction, the photocatalyst was added into BPA solution with continuous stirring in the dark for 3 h. It found that the H2O2 was decomposed in the dark but this could not degrade the BPA as shown in Fig. 12(c). From Fig. 12(e), without photocatalyst, no degradation of BPA molecules was observed in the presence of H2O2 and light irradiation. It can be concluded that no reactive species was produced when H2O2 was exposed to visible light. Accord-
Fig. 11. Reusability of the 0.03Ag/AC-ZnO photocatalyst for photocatalytic degradation of BPA in the presence of H2O2.
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The important reactive species in BPA degradation using a photocatalyst of 0.03Ag/AC-ZnO were identified through observing the inhibitory activity produced by adding different scavengers and the results are presented in Fig. 13. The degradation of BPA decreased slightly upon addition of IPA or EDTA-2Na, suggesting that neither OH nor h+ was the main reactive species for the degradation of BPA. However, the addition of the NBT quencher greatly reduced the photocatalytic activity of the 0.03Ag/AC-ZnO. This indicated that O 2 was the main reactive species for BPA degradation in the system. From these above results, the major routes in the photocatalytic BPA degradation mechanism in the presence of H2O2 under visible light irradiation could be proposed as follows:
Fig. 12. Photocatalytic degradation of BPA in the presence of 0.03Ag/AC-ZnO with various conditions: (a) with H2O2 and light irradiation, (b) with air bubbles and light irradiation, (c) with H2O2 in the dark, (d) without H2O2 and light irradiation and (e) no catalyst (with H2O2 and light irradiation).
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ing to Eq. (1), the decomposition of H2O2 to O2 would be one of the main factors that enhanced the photocatalytic degradation of BPA. To confirm this hypothesis, the addition of H2O2 was replaced by O2 blowing. From Fig. 12(b), in the absence of H2O2, the BPA was still degraded in the presence of 0.03Ag/AC-ZnO with O2 bubbles under visible light irradiation. It probably confirms that the O2 in the solution is the important factor for BPA degradation when using 0.03Ag/AC-ZnO as the photocatalyst.
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2H2 O2 + Ag/AC-ZnO ! 2H2 O + O2 + Ag/AC-ZnO
ð2Þ
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Ag/AC-ZnO + hv ! Ag(e + hþ )/AC-ZnO
ð3Þ
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Ag(e + hþ )/AC-ZnO ! Ag(hþ )/AC-ZnO(e , CB)
ð4Þ
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Ag(hþ )/AC-ZnO(e , CB) + O2 ! O2
ð5Þ
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O2 + dye ! degraded products
ð6Þ
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Fig. 13. Photocatalytic BPA degradation of 0.03Ag/AC-ZnO with H2O2 in the absence of scavenger and in the presence of NBT, IPA and EDTA-2Na under visible light irradiation.
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To confirm the degradation of BPA, the BPA solutions before and after photocatalysis were introduced to the mass spectrometer. Initial BPA solution showed the mass peak at m/z = 227.2 (Fig. 14) corresponding to [MH] [34] but this parent peak disappeared after the photocatalytic process. As there were no detectable main fragment peaks such as 3-(4-hydroxyphenyl)-3methyl-2-oxobutanoic acid (m/z = 207), 4-vinylphenol (m/z = 133) or 4-hydroxyacetophenone (m/z = 135) [35] and no peak in the range of m/z = 100–300, the BPA probably degraded into small molecules as CO2 and H2O.
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3.3. Cytotoxicity
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The cytotoxicities of the BPA solutions before and after photocatalysis were evaluated in vitro by calculating the relative number of viable L-929 mouse fibroblast cells after exposure to the treated and untreated BPA solutions using the MTT assay (viability of control = 100.828 ± 0.354). Table 2 shows the cell viability after incubation for 72 h with treated and untreated BPA solutions. Exposure of L-929 cells to the untreated BPA solution for 72 h reduced the number of viable cells to 67.553 ± 5.138%. On the other hand, the BPA solution treated with 0.03Ag/AC-ZnO in the presence of H2O2 under visible light showed no cytotoxic effect on the cells.
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Fig. 14. Mass spectra of BPA (a) before and (b) after photocatalysis.
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Table 2 Viability of L-929 cells exposed to the treated and untreated BPA solutions for 72 h using the reverse osmosis water as a control. Substance
Survival (%)
BPA before photocatalysis BPA after photocatalysis
67.553 ± 5.138 99.640 ± 5.285
CM = 100.000 ± 0.354.
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This result is in agreement with the results of the mass spectrometry analysis that detected no toxic intermediate peak (Fig. 14).
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4. Conclusion
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The Ag/AC-ZnO powders were successfully prepared by the calcination-electroless method. This photocatalyst degraded the BPA molecules in the presence of H2O2 under visible light irradiation. During the reaction, O2 gas was produced when H2O2 contacted the Ag deposited on the surface of the AC-ZnO. The experimental results showed that O2 is a very important factor in the photocatalytic degradation of BPA by Ag/AC-ZnO and the O 2 is the main reactive species for this system. As there was no detectable toxic degraded product after photocatalysis, the treated BPA has no cytotoxic effect on L-929 mouse fibroblast cells.
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Acknowledgement
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This research was supported by PSU Ph.D. Scholarship under contract PSU 2557-001. The authors acknowledge the Center of Excellence for Innovation in Chemistry (PERCH-CIC), Office of the Higher Education Commission, Ministry of Education.
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References
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Original Research Paper
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Effect of Ag loading on activated carbon doped ZnO for bisphenol A degradation under visible light
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Khanitta Intarasuwan a, Pongsaton Amornpitoksuk a,b,⇑, Sumetha Suwanboon c, Potchanapond Graidist d,e, Saowanee Maungchanburi d, Chamnan Randorn f,g a
Department of Chemistry, Faculty of Science, Prince of Songkla University, Hat-Yai, Songkhla 90112, Thailand Center of Excellence for Innovation in Chemistry, Faculty of Science, Prince of Songkla University, Hat-Yai, Songkhla 90112, Thailand Department of Materials Science and Technology, Faculty of Science, Prince of Songkla University, Hat-Yai, Songkhla 90112, Thailand d Department of Biomedical Science, Faculty of Medicine, Prince of Songkla University, Hat-Yai, Songkhla 90112, Thailand e The Excellent Research Laboratory of Cancer Molecular Biology, Prince of Songkla University, Hat-Yai, Songkhla 90112, Thailand f Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand g Materials Science Research Center, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand b c
a r t i c l e
i n f o
Article history: Received 26 March 2018 Received in revised form 16 June 2018 Accepted 13 July 2018 Available online xxxx Keywords: ZnO Bisphenol A Photocatalytic property Cytotoxicity
a b s t r a c t Silver modified activated carbon doped zinc oxide (Ag/AC-ZnO) was synthesized via a calcinationelectroless deposition route. The samples were characterized by X-ray powder diffractometry, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectrometry, Fourier transform infrared spectroscopy and UV–vis diffuse reflectance spectroscopy. The photocatalytic activity of the Ag/AC-ZnO was evaluated for bisphenol A degradation in the presence of H2O2 under visible light irradiation. The archived results showed that the photocatalytic activity of the Ag/AC-ZnO was higher than that of AC-ZnO and pure ZnO. The cytotoxicity of the bisphenol A after photocatalysis under visible light irradiation was tested using L929 mouse fibroblast cells and the obtained results indicated that the treated bisphenol A solution exhibited no cytotoxicity against normal cells. Ó 2018 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.
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1. Introduction
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Bisphenol A (BPA) is commonly used as the starting material in the production of polycarbonate plastics and epoxy resins. It is emitted into the environment during manufacturing process and leaches from products. BPA exhibits moderate acute toxicity to vertebrates [1] and when BPA discharged into the river as a result of industrial activity, it may affect aquatic animals and whoever drinks the contaminated water. BPA is a stable compound and does not readily degrade in the ecosystem so it must be treated before being released into the environment. Nowadays, there are many treatment processes to remove BPA from wastewater such as microfiltration, ozonation and biological methods [2]. One of the most effective treatments is an advanced oxidation process that the pollutant is removed by oxidation through the reaction with
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⇑ Corresponding author at: Department of Chemistry, Faculty of Science, Prince of Songkla University, Hat-Yai, Songkhla 90112, Thailand. E-mail addresses: [email protected] (K. Intarasuwan), pongsaton. [email protected] (P. Amornpitoksuk), [email protected] (S. Suwanboon), gpotchan@ medicine.psu.ac.th (P. Graidist), [email protected] (S. Maungchanburi).
reactive species such as the hydroxyl radical or the superoxide anion radical. This treatment has many advantages over conventional processes, for example, it produces harmless degradation products, does not use expensive or hazardous oxidizing chemicals and takes advantage of natural sunlight. In the field of photocatalysis, zinc oxide (ZnO) is a good candidate because of its good photocatalytic property, high stability and eco-friendliness. In the literature, ZnO based photocatalysts such as pure ZnO [3], carbon doped ZnO [4], Ag/ZnO [5] and Ce-ZnO [6] showed a high degradation potential for BPA in aqueous media under UV irradiation but there are only a few reported investigations of these photocatalysts under visible light irradiation. Qui et al. [7] reported that 93% of BPA was degraded after 4 h under visible light irradiation using N-doped ZnO and the visible-light response of this photocatalyst was attributed to the narrow band gap of the N 2p state isolated above the valence band of ZnO. However, modification of the band gap energy of the ZnO, in order to increase the visible light absorption capacity, is too complicated compared to a coupling method. Among the many coupling materials, it is well known that Ag is a good candidate sensitizer to make a wide band gap photocatalyst that functions in visible light. After exposure to
https://doi.org/10.1016/j.apt.2018.07.006 0921-8831/Ó 2018 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.
Please cite this article in press as: K. Intarasuwan et al., Effect of Ag loading on activated carbon doped ZnO for bisphenol A degradation under visible light, Advanced Powder Technology (2018), https://doi.org/10.1016/j.apt.2018.07.006
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visible light, according to the energy allowed transition, electrons from Ag can transfer to the conduction band of the ZnO and these injected electrons further react with O2 to produce the superoxide anion radical (O 2 ). Furthermore, the photocatalytic activity of the Ag modified ZnO can be enhanced by inhibited charge recombination in Ag due to the electron injection. Another important factor in the photocatalytic process is the substance or pollutant adsorption on the surface of the photocatalyst. It has been demonstrated that the adsorption of pollutant molecules on the photocatlyst surface can significantly enhance the photocatalytic efficiency [8]. This property would be increased by the addition of porous materials to the photocatalyst. Activated carbon (AC) is a very cheap material that can increase the adsorption ability of many semiconductorbased photocatalysts [4]. Based on the above considerations, silver modified carbon doped zinc oxide (Ag/AC-ZnO) was prepared by a calcinationelectroless deposition route. Furthermore, this work also examined the influence of Ag loading on AC-ZnO on the photocatalytic degradation of BPA under visible light irradiation. The overall cytotoxicity of the treated BPA solution after photocatalysis was assessed on L929 cells.
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2. Experimental
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2.1. Synthesis of Ag/AC-ZnO
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AC-ZnO was prepared by the precipitation-calcination method. In a typical procedure, 20 mL of 0.25 M of Zn(NO3)26H2O containing 0.01 g of activated carbon (QRëC) were added dropwise into 60 mL of 0.166 M H2C2O4. In the preparation of the ZnO, the activated carbon was not used in this step. Next, this mixture was heated under vigorous stirring at 70 °C for 1 h in a water bath. After cooling to room temperature, the precipitant was filtered, washed with distilled water and dried at 100 °C for 1 h in a hot air oven. After that, the powders were calcined at 500 °C for 1 h in a muffle furnace. Ag/AC-ZnO was prepared by a reduction from [Ag(NH3)2]+ ions. This reagent was prepared by the addition of 3 mL of conc NH3 to 40 mL of AgNO3 in various concentrations. One gram of AC-ZnO powders (1 g of ZnO was used for the preparation of the Ag/ZnO) was stirred in a solution of [Ag(NH3)2]+ for 15 min followed by the addition of a 1 M glucose solution and this mixture was continuously stirred for 1 h at room temperature. After that, the solid powder was filtered, rinsed with distilled water and dried at 100 °C for 1 h in an oven.
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2.2. Characterization
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The structural identifications of all samples were carried out using an X-ray diffractometer (XRD, X’Pert MPD, Phillips) with Cu Ka radiation at a wavelength of 0.15406 nm. The morphological studies were observed using a scanning electron microscope (SEM, Quanta 400, FEI) and transmission electron microscope (TEM, JEM-2010, JEOL). Elemental compositions of samples were analyzed by an energy dispersive X-ray fluorescent spectrometer (EDX, Oxford). The Fourier transform infrared (FT-IR) spectra of the samples in transmission mode at 400–4500 cm1 were recorded by the FT-IR spectrophotometer (Spectrum BX, Perkin Elmer). The diffuse reflectance UV–Vis absorption spectra were obtained on a UV–Vis spectrophotometer (UV-2450, Shimazu) using BaSO4 as a reflectance standard. The surface areas of all samples were evaluated by the BET (Brunauer-Emmett-Teller) method using a surface area analyzer (Autosorb 1 MP, Quantachrome). X‐ ray photoelectron spectroscopy (XPS) was performed on an AXIS Ultra DLD (Kratos Analytical Ltd.) electron spectrometer and all
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binding energies were calibrated to the C 1s line at 284.8 eV. Degraded products after photocatalytic reaction was characterized by liquid chromatograph-mass spectrometry (LC-MS, 2690-LCT, Waters, Micromass) using electrospray ionization operating in the negative (ESI) mode.
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2.3. Photocatalytic study
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The photocatalytic activities for all samples were evaluated from the degradation of the BPA under visible light irradiation in the presence of H2O2 (Fig. 1). Into a 250 mL beaker containing of 150 mL of 5 ppm BPA, 0.3 g of photocatalyst was dispersed. After that, 3 mL of H2O2 (30%, Merck) was added into the BPA solution. This mixture was stirred in the dark in order to establish adsorption/desorption equilibrium between the BPA molecules and the surfaces of the photocatalyst. The suspension was then exposed to visible light (51 W Xe lamp) for the required time. After the required time had elapsed, 2 mL of BPA solution was pipetted and centrifuged to completely remove the particles of photocatalyst. The concentration of the BPA was determined by UV–vis spectrophotometer (UV2550, Shimadzu) and the degradation of BPA
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Fig. 1. Photocatalytic experimental setup.
Fig. 2. XRD of (a) ZnO, (b) AC-ZnO and (c) 0.03Ag/AC-ZnO powders. The peaks labeled for three and four indices represent Ag (JCPDS 04-0783) and ZnO (JCPDS 361451), respectively.
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depending on irradiation time was calculated using the following equation:
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Degradation ð%Þ ¼ ½ðCo Ct Þ=Co 100 ¼½ðAo At Þ=Ao 100
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where Co and Ao are the initial concentration and the initial absorbance of the BPA, and Ct and At are the concentration and the absorbance of the BPA after the irradiation at time t.
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In order to investigate the reactive species generated in the photocatalytic process, radical scavenging experiments were carried out [9–11] by adding 1 mmol of ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA-2Na) as a hole (h+) scavenger or isopropanol alcohol (IPA) as a hydroxyl radical (OH) scavenger or nitroblue tetrazolium (NBT) as a superoxide anion radical (O 2) scavenger into 150 mL of the BPA solution before the photocatalytic tests.
Fig. 3. SEM images and EDS spectra for (a) ZnO, (b) AC-ZnO and (c) 0.03Ag/AC-ZnO powders, and (d) EDS mapping images of 0.03Ag/AC-ZnO.
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2.4. In vitro cytotoxicity test
3. Results and discussion
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The overall cytotoxicity of the BPA solution before and after photocatalysis was evaluated by MTT assay [12]. Briefly, L929 (mouse fibroblast, Mus musculus) cells were seeded in a 96-well plate and cultured in a humidified incubator for 72 h. After that, the cells were rinsed with 1X PBS and incubated with 100 lL of 0.5 mg/mL MTT at 37 °C for 30 min. The dark blue crystals of formazan (MTT metabolites) were dissolved with 100 lL of dimethyl sulfoxide (DMSO). This solution was added to each well and incubated at 37 °C for 30 min. Finally, the optical density was determined by measuring the difference in absorbance at 570 and 650 nm using a microplate reader (Spectra Max M5, Molecular Devices). Viable cells were determined as a percentage of survival and calculated using the following equation (n = 3):
3.1. Characterization
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After calcination at 500 °C for 1 h, the ZnO and AC-ZnO powders showed the same diffraction pattern as presented in Fig. 2(a) and (b) and all presented peaks matched well with JCPDS card number 36–1451 (ZnO). In the literature, pure AC usually has 2 broad peaks at 2h = 15–35° and 35–50° but the absence of these broad peaks in this work was probably due to the low AC loading in the sample. This evidence has been found in many reports [13–14]. After the metallic Ag was introduced on the surface of the AC-ZnO using a reduction of the [Ag(NH3)2]+, the XRD pattern of the resulting product showed the additional diffraction peaks corresponding to metallic Ag (JCPDS card number 04-0783), as shown in Fig. 2(c). The SEM images for ZnO, AC-ZnO and 0.03Ag/AC-ZnO (Fig. 3(a)– (c)) show the same morphology built up from the agglomeration of small ZnO particles in all samples. In 0.03Ag/AC-ZnO, there are small particles on the surface of agglomerated AC-ZnO particles that were probably assigned to the metallic Ag, as these small particles were found in ZnO and AC-ZnO. The presence of the Ag in the 0.03Ag/AC-ZnO was confirmed by EDS in mapping mode, (Fig. 3 (d)). This also indicated that the Ag particles were well dispersed on the surface of agglomerated AC-ZnO particles. In the HRTEM image presented in Fig. 4, the resolved interplanar distance of 0.281 nm matched well with the interplanar spacing (d-spacing) of the (1 0 0) plane of ZnO crystallized in würtzite structure and the lattice spacing at 0.204 nm is assigned to the d-spacing of the (2 0 0) plane of metallic Ag. Fig. 5 displays the FTIR spectra of ZnO, AC-ZnO and 0.03Ag/ACZnO. For ZnO, the broad band located at 3200–3600 cm1 assigned to the OH stretch mode and a band at 1640 cm1 corresponding to the OH bending vibration. These vibrations confirm the presence of bound H2O on the surface of the sample [15–17]. The stretching vibrational mode of the ZnO can be observed at 470 cm1 [18,19]. The FTIR spectra of Ag/AC-ZnO and AC-ZnO are similar (Fig. 5(b) and (c)). The band at 1536 cm1 is related to the C@C stretching vibration in the aromatic rings [20]. A transmittance peak at 1390 cm1 may be due to the stretching vibration of CAO and bending vibration of OAH in the carboxyl carbonates or C@C aromatics and various modes of substituted rings [21,22]. Many vibrational peaks in the region of 700–900 cm1 are assigned to the out-of-plane bending vibrational mode of CAH in the aromatic rings [23,24]. The N2 adsorption-desorption isotherms of ZnO, AC-ZnO and 0.03Ag/AC-ZnO are shown in Fig. 6 and the specific surface areas
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Survival ð%Þ ¼ ½ðA570 nm A650 nm Þ=ðA570 nm A650 nm Þ 100 where A570 nm, A650 nm, A570 nm, and A650 nm are the absorbances of the test samples (at 570 and 650 nm) and the negative control (at 570 and 650 nm), respectively.
Fig. 4. HRTEM image of 0.03Ag/AC-ZnO powders.
Fig. 5. FTIR spectra of (a) ZnO, (b) AC-ZnO and (c) 0.03Ag/AC-ZnO.
Fig. 6. Nitrogen adsorption-desorption isotherms of (a) ZnO, (b) AC-ZnO and (c) 0.03Ag/AC-ZnO.
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determined by BET method for all samples are presented in Table 1. The ZnO showed a characteristic of type II isotherm (non-porous material) that had a hysteresis loop at high relative pressure (P/ P0 > 0.9) [25]. On the other hand, the nitrogen adsorptiondesorption isotherms of AC-ZnO and 0.03Ag/AC-ZnO exhibit type IV with type H3 hysteresis loop, indicating their mesoporous microstructure [26]. Fig. 7 displays the XPS spectra of 0.03Ag/AC-ZnO calcined at 500 °C for 1 h. The C 1s signal was deconvoluted into six peaks at 283.5, 284.2, 285.1, 286.2, 287.4 and 288.7 eV corresponding to the ZnAC, sp2 (CAC), sp3 (CAC and CAH), CAO, OAC@O and C@O bonds [27,28], respectively. The Zn 2p spectrum contains two sets of double peaks. The peaks at binding energies of 1021.2 and 1044.2 eV were assigned to the Zn 2p3/2 and 2p1/2, and the difference between these peaks was 23 eV that indicated that the Zn ions in this sample are of +2 states [29]. The additional peaks at 1022.8 and 1045.7 eV refer to the ZnAC bonds [30] and are in agreement with the results of XPS of C 1s spectrum. The Ag 3d spectrum showed two peaks centered at 367.3 and 373.3 eV attributed to the Ag 3d5/2 and Ag 3d3/2, respectively, and the peaks difference is 6 eV, confirming that the Ag in this sample is metallic silver [31]. The asymmetric O 1s XPS peak can be deconvoluted into four components, centered at 530.1, 531.4, 532.2 and 533.3 eV. The peak on the lower binding energy can be attributed to the Zn-O bonding while the one with the highest binding energy is due to the adsorbed water on the surface. The peak at 532.2 eV has usually been assigned to the chemisorbed or dissociated oxy-
Table 1 Surface areas evaluated by BET method for prepared photocatalysts. Sample
Surface area (m2/g)
Sample
Surface area (m2/g)
ZnO AC-ZnO 0.005Ag/AC-ZnO 0.01Ag/AC-ZnO
13.61 63.25 29.12 27.08
0.03Ag/AC-ZnO 0.07Ag/AC-ZnO 0.10Ag/AC-ZnO
20.62 16.68 16.37
5
Fig. 8. Diffuse reflection absorption spectra for (a) ZnO, (b) AC-ZnO and (c) 0.03Ag/ AC-ZnO powders.
gen or OH species on the surface of ZnO or adsorbed O2. Furthermore, this sample showed the peak at 531.4 eV that has been described in oxygen-deficient regions within ZnO structures (oxygen vacancies) [32]. The optical absorption spectra for ZnO, AC-ZnO and 0.03Ag/ACZnO are shown in Fig. 8. The absorption capacities of visible light are in the order: 0.03Ag/AC-ZnO > C-ZnO > ZnO. All samples showed the strong absorption band in the UV-region corresponding to the electron transition between the valence band and the conduction band of ZnO. The broad band in the range 425–800 nm probably comes from the photon absorption of AC, while metallic Ag showed the high photon absorption in the visible region.
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3.2. Photocatalytic activity
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The chronological change in absorbance for BPA during irradiation time is shown in Fig. 9(a). The absorbance of BPA decreased
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Fig. 7. XPS spectra of C 1s, Zn 2p, Ag 3d and O 1s for 0.03Ag/AC-ZnO.
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Fig. 10. Photocatalytic degradation of BPA in the presence of H2O2 using 0.005Ag/ AC-ZnO (}), 0.01Ag/AC-ZnO (h), 0.03Ag/AC-ZnO (4), 0.05Ag/AC-ZnO (j), 0.07Ag/ AC-ZnO (*) and 0.1Ag/AC-ZnO (s) as the photocatalysts.
Fig. 9. (a) UV–vis spectra of BPA by photocatalysis for different irradiation times over 0.03Ag/AC-ZnO under visible light irradiation and (b) photocatalytic degradation of BPA in the presence of H2O2 using ZnO, AC-ZnO, 0.03Ag/ZnO and 0.03Ag/ACZnO as the photocatalysts.
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with an increment of irradiation times, implying that the BPA can degrade under this condition. The photocatalytic activities for BPA degradation over ZnO, AC-ZnO, 0.03Ag/ZnO and 0.03Ag/AC-ZnO in the presence of H2O2 are shown in Fig. 9(b). Actually, the ZnO is a wide band gap semiconductor material and shows photocatalytic properties only under UV exposure. Although the AC-ZnO can absorb photons in the visible light region, it cannot degrade BPA. The degradation of BPA under visible light irradiation would be observed when the Ag/AC-ZnO was introduced into the BPA solution, as shown in Fig. 9(b) (the reason will be discussed later). The content of Ag loading in the AC-ZnO powders strongly influenced the photocatalytic degradation of BPA under visible light as presented in Fig. 10. It is clear to observe that, after 30 min of adsorption in the dark, a little reduction of BPA concentration was observed, indicating that some BPA can be removed from the solution by adsorption phenomena. However, most BPA molecules would be degraded by the photocatalytic reaction. After irradiation, all Ag/AC-ZnO samples showed photocatalytic properties as the concentration of BPA also decreased with an increment of the irradiation times. Overall activity for the BPA degradation is in the order: 0.03Ag/AC-ZnO > 0.01Ag/AC-ZnO > 0.005Ag/AC-ZnO > 0.05Ag/AC-ZnO > 0.07Ag/AC-ZnO > 0.1Ag/AC-ZnO > AC-ZnO. However, this order was not related to their surface areas as presented in Table 1. It is clear to see that the photocatalytic activity increased continuously with the increment of Ag contents and the maximum activity was produced by 0.03Ag/AC-ZnO. After this point, however, increasing the Ag content decreased the photocat-
alytic activity, as seen in Fig. 10. As the O 2 generation is a surface reaction and it would occur at the surface of AC-ZnO as presented in Eq. (5) so, at high Ag loading, the surface of AC-ZnO could be mostly covered with Ag and O 2 production would be reduced. To evaluate the stability of the photocatalyst, 0.03Ag/AC-ZnO was used to degrade BPA in five repeated cycles and the results are shown in Fig. 11. After recycling five times, the photocatalytic activity of this photocatalyst still remained higher than 85%, indicating that the 0.03Ag/AC-ZnO is relatively stable under visible light irradiation. In this system, the Ag/AC-ZnO did not degrade the BPA under visible light irradiation if there was no addition of H2O2, as presented in Fig. 12(d). In the presence of H2O2, when the H2O2 contacted the metallic Ag on the surface of the AC-ZnO, O2 bubbles were automatically generated in the solution [33] in the following reaction:
2H2 O2 + Ag ! 2H2 O + O2 + Ag
ð1Þ
To ensure that the BPA degradation did not result from this reaction, the photocatalyst was added into BPA solution with continuous stirring in the dark for 3 h. It found that the H2O2 was decomposed in the dark but this could not degrade the BPA as shown in Fig. 12(c). From Fig. 12(e), without photocatalyst, no degradation of BPA molecules was observed in the presence of H2O2 and light irradiation. It can be concluded that no reactive species was produced when H2O2 was exposed to visible light. Accord-
Fig. 11. Reusability of the 0.03Ag/AC-ZnO photocatalyst for photocatalytic degradation of BPA in the presence of H2O2.
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The important reactive species in BPA degradation using a photocatalyst of 0.03Ag/AC-ZnO were identified through observing the inhibitory activity produced by adding different scavengers and the results are presented in Fig. 13. The degradation of BPA decreased slightly upon addition of IPA or EDTA-2Na, suggesting that neither OH nor h+ was the main reactive species for the degradation of BPA. However, the addition of the NBT quencher greatly reduced the photocatalytic activity of the 0.03Ag/AC-ZnO. This indicated that O 2 was the main reactive species for BPA degradation in the system. From these above results, the major routes in the photocatalytic BPA degradation mechanism in the presence of H2O2 under visible light irradiation could be proposed as follows:
Fig. 12. Photocatalytic degradation of BPA in the presence of 0.03Ag/AC-ZnO with various conditions: (a) with H2O2 and light irradiation, (b) with air bubbles and light irradiation, (c) with H2O2 in the dark, (d) without H2O2 and light irradiation and (e) no catalyst (with H2O2 and light irradiation).
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ing to Eq. (1), the decomposition of H2O2 to O2 would be one of the main factors that enhanced the photocatalytic degradation of BPA. To confirm this hypothesis, the addition of H2O2 was replaced by O2 blowing. From Fig. 12(b), in the absence of H2O2, the BPA was still degraded in the presence of 0.03Ag/AC-ZnO with O2 bubbles under visible light irradiation. It probably confirms that the O2 in the solution is the important factor for BPA degradation when using 0.03Ag/AC-ZnO as the photocatalyst.
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2H2 O2 + Ag/AC-ZnO ! 2H2 O + O2 + Ag/AC-ZnO
ð2Þ
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Ag/AC-ZnO + hv ! Ag(e + hþ )/AC-ZnO
ð3Þ
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Ag(e + hþ )/AC-ZnO ! Ag(hþ )/AC-ZnO(e , CB)
ð4Þ
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Ag(hþ )/AC-ZnO(e , CB) + O2 ! O2
ð5Þ
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O2 + dye ! degraded products
ð6Þ
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Fig. 13. Photocatalytic BPA degradation of 0.03Ag/AC-ZnO with H2O2 in the absence of scavenger and in the presence of NBT, IPA and EDTA-2Na under visible light irradiation.
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To confirm the degradation of BPA, the BPA solutions before and after photocatalysis were introduced to the mass spectrometer. Initial BPA solution showed the mass peak at m/z = 227.2 (Fig. 14) corresponding to [MH] [34] but this parent peak disappeared after the photocatalytic process. As there were no detectable main fragment peaks such as 3-(4-hydroxyphenyl)-3methyl-2-oxobutanoic acid (m/z = 207), 4-vinylphenol (m/z = 133) or 4-hydroxyacetophenone (m/z = 135) [35] and no peak in the range of m/z = 100–300, the BPA probably degraded into small molecules as CO2 and H2O.
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3.3. Cytotoxicity
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The cytotoxicities of the BPA solutions before and after photocatalysis were evaluated in vitro by calculating the relative number of viable L-929 mouse fibroblast cells after exposure to the treated and untreated BPA solutions using the MTT assay (viability of control = 100.828 ± 0.354). Table 2 shows the cell viability after incubation for 72 h with treated and untreated BPA solutions. Exposure of L-929 cells to the untreated BPA solution for 72 h reduced the number of viable cells to 67.553 ± 5.138%. On the other hand, the BPA solution treated with 0.03Ag/AC-ZnO in the presence of H2O2 under visible light showed no cytotoxic effect on the cells.
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Fig. 14. Mass spectra of BPA (a) before and (b) after photocatalysis.
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Table 2 Viability of L-929 cells exposed to the treated and untreated BPA solutions for 72 h using the reverse osmosis water as a control. Substance
Survival (%)
BPA before photocatalysis BPA after photocatalysis
67.553 ± 5.138 99.640 ± 5.285
CM = 100.000 ± 0.354.
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This result is in agreement with the results of the mass spectrometry analysis that detected no toxic intermediate peak (Fig. 14).
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4. Conclusion
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The Ag/AC-ZnO powders were successfully prepared by the calcination-electroless method. This photocatalyst degraded the BPA molecules in the presence of H2O2 under visible light irradiation. During the reaction, O2 gas was produced when H2O2 contacted the Ag deposited on the surface of the AC-ZnO. The experimental results showed that O2 is a very important factor in the photocatalytic degradation of BPA by Ag/AC-ZnO and the O 2 is the main reactive species for this system. As there was no detectable toxic degraded product after photocatalysis, the treated BPA has no cytotoxic effect on L-929 mouse fibroblast cells.
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Acknowledgement
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This research was supported by PSU Ph.D. Scholarship under contract PSU 2557-001. The authors acknowledge the Center of Excellence for Innovation in Chemistry (PERCH-CIC), Office of the Higher Education Commission, Ministry of Education.
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