Materials Science in Semiconductor Processing 21 (2014) 167–179
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Nano sized ZnO composites: Preparation, characterization and application as photocatalysts for degradation of AB92 azo dye Neda Mohaghegh a, Mahboubeh Tasviri b,n, Esmail Rahimi c, Mohammad Reza Gholami a,nn a b c
Department of Chemistry, Sharif University of Technology, Azadi Avenue, Tehran, Iran Department of Chemistry, Shahid Beheshti University, Evin, P.O. Box 19839-63113, Tehran, Iran Department of Mining Engineering, Islamic Azad University, South Tehran Branch, Tehran, Iran
a r t i c l e i n f o
abstract
Available online 31 January 2014
ZnO and Mordenite zeolite (MOR) nanoparticles were prepared by precipitation process using ultrasonic irradiation and hydrothermal method, respectively. Supported ZnO catalysts were prepared and the effect of different supports on the photocatalytic activity of ZnO nanoparticles was investigated. All prepared samples were characterized by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Fourier Transform-Infra Red Spectroscopy (FTIR), UV–vis spectroscopy (UV–vis) and BET surface area technique. The photocatalytic activity of the synthesized catalysts was elucidated using the photooxidation of Acid Blue 92 (AB92) as a hazardous pollutant under UV light. The effect of different parameters such as catalyst concentration, initial dye concentration, pH, UV irradiation and amount of ZnO loaded on the nanocomposites have been examined on the yield and the rate of photocatalytic degradation process. The photodegradation results of AB92 in aqueous medium under UV irradiation revealed that nanocomposite of ZnO and mordenite zeolite exhibit much higher photocatalytic activity than the other nanocomposites and pure ZnO. It was found that the type of support plays an important role in photocatalytic oxidation of AB92 and significantly improved the photocatalytic activity of ZnO. Chemical Oxygen Demand (COD) for dye solutions were examined at regular intervals and gave a good idea about mineralization of the dye. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Photocatalysis ZnO nanoparticles Mordenite zeolite Nanocomposites Acid Blue 92
1. Introduction Toxic wastewater effluents from textile and other dyestuff industries contain a large variety of toxic residual organic dyes [1–3]. Azo dyes are the largest group of widely used synthetic dyes and one of the major hazards for human health in dye wastewaters [4]. Acid Blue 92 (AB92) arising from dyestuff industries is highly toxic and comparatively more refractory to natural degradation. It is a monoazo dye bearing three sulfonic groups (Fig. 1(a)) [5]. n
Corresponding author. Tel.: þ98 21 29902895; fax: þ98 21 22431661. Corresponding author. Tel.: þ98 21 66165314; fax: þ 98 21 66029165. E-mail addresses:
[email protected] (M. Tasviri),
[email protected] (M.R. Gholami). nn
1369-8001/$ - see front matter & 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2013.12.023
This dye is used in huge quantities in Iran and produces a lot of environmental problems. When AB92 solution is incorporated into the body, cleaved into aromatic amines by liver enzymes and can cause various cancers in human. Therefore, the removing of AB92 dye from wastewaters is more important [4]. The UV–vis absorption spectrum of AB92 in aqueous medium is shown in Fig. 1(b). The peak at λ¼ 572 nm was used to monitor the dye concentration. The elimination of hazardous dyes from textile wastewater effluents is vital for human being. In recent years, heterogeneous metal oxide semiconductor assisted photocatalysis has become an important area among various Advanced Oxidation Processes (AOPs) for wastewater treatment [4–7]. ZnO is one of the most promising photocatalyst for degradation and complete mineralization
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Fig. 1. (a) Chemical structure of AB92 (C.I. 13390) and (b) UV–vis absorption spectrum of AB92 in an aqueous solution.
of azo dyes because of its exceptional properties such as low cost, non toxicity, high ultraviolet (UV) absorption, lack of creation of dangerous by-products and high chemical stability [8–15]. In spite of these desirable properties, ZnO photocatalyst usually has a low specific surface area due to the crystalline grain growth, which takes place during calcinations and displays rapid deactivation because of the aggregation. The photocatalytic activity of ZnO catalyst depends heavily on its structure and physical properties. The photocatalytic reactions mostly take place on the surface of ZnO catalysts. Hence, the ZnO surface area is a key factor in the kinetic and efficiency of photocatalytic reaction. It was known that supported photocatalysts with high adsorption ability can (i) attract the pollutant substances near the reactive surface of the catalyst particles, (ii) create a lot of active sites for adsorption of intermediates, (iii) extend life time and reusability of the photocatalyst and (iv) decrease recombination rate of photogenerated electron–hole pairs. So, it can promote the degradation rate and enhance photocatalytic activity of ZnO [16–19]. Many host materials such as silica, alumina, zeolite and activated carbon have been used as a support in various preparation methods to increase the photodegradation efficiency of ZnO [18,20]. Zeolites (crystalline alumiosilicates) with uniform nanoscaled pore size and extensive surface area have been applied in the design of operative photocatalytic systems [21]. Mordenite (MOR), a zeolite, is a mineral compound with an ideal composition (Na8) [Al8Si40O96] nH2O [22]. Due to its high thermal stability and catalytic activity, MOR has been used as a catalyst in photocatalytic and adsorptive applications [22,23]. In this study, ZnO and mordenite zeolite particles (MOR) were prepared by the precipitation method using ultrasonic irradiation and hydrothermal method, respectively and then, ZnO nanoparticles were supported on mordenite zeolite, alumina and activated carbon. Deposition of ZnO on a suitable support is an important subject of the research and the effect of support on photocatalytic activity was studied. The synthesized catalysts were characterized by different techniques. The photocatalytic activity of the prepared compounds and the effect of
operational parameters were examined by degradation of AB92 under UV illumination and the experimental results were discussed. 2. Experimental 2.1. Reagents Sodium silicate (Na2SiO3) as a silica source, aluminum nitrate [Al(NO3)3 3H2O] as an aluminum source, and sodium hydroxide (NaOH) as an alkali source were obtained from Merck, which were used for MOR synthesis. Zinc acetate (ZnAc2 2H2O), isopropyl alcohol (C3H8O), ethylene glycol monobutyl ether (C6H14O2), activated carbon (AC), alumina (Al2O3) and all other chemicals were purchased from Merck. Acid Blue 92 (MW¼695.58 g mol 1, CI¼13,390), provided by Iran Color Research Center, was used without further purification and chosen as a model compound to test the photocatalytic activity. 2.2. Instruments The XRD pattern of the prepared samples was recorded on a Bruker D4 X-ray diffractometer with Cu Kα irradiation (λ¼0.15406 nm) as the X-ray source. The average particle size and morphology of photocatalysts were distinguished by Scanning Electron Microscopy (SEM, XL30 model). FT-IR spectra in the range of 4000–400 cm 1 were recorded on an ABB BOMER MB series spectrophotometer. The average particle size of a ZnO colloid and the dye concentration in each sample were determined by a UV–vis spectrophotometer (GBC Cintra 40). The specific surface area and pore volume distributions of the products were calculated from the nitrogen adsorption/desorption isotherms at 77 K, using Belsorp apparatus. A 125-W mercury lamp was used as a source of UV light. 2.3. Catalyst preparation 2.3.1. Synthesis of ZnO For the synthesis of ZnO nanoparticles, the following procedure was used. 1 mmol of zinc acetate dehydrate was
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dissolved in isopropyl alcohol under vigorous stirring at 45 1C. Then 1 g of ethylene glycol monobutyl ether was added to zinc acetate solution and sonicated for 30 min. A 0.2 M NaOH solution was prepared by adding sodium hydroxide to the pure isopropyl alcohol solvent under vigorous stirring at 45 1C. After cooling, NaOH solution was slowly added dropwise into the zinc acetate solution under magnetic stirring at 40 1C in a water bath. The white precipitate was collected by centrifugation, washed with ethanol and double-distilled water several times, dried in oven at 80 1C and calcined at 450 1C for 3 h. 2.3.2. Synthesis of MOR Hydrothermal synthesis of nanocrystalline MOR zeolite with a uniform particle size was performed according to the previous report [25]. The formation of MOR zeolite involves a two step process. First, a gel containing silica and aluminum species prepared by dissolving finely
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divided silica and aluminum in alkaline solution of 6 M NaOH. To obtain small particles and complete mixing of the gel, solution of the gel was magnetically stirred for 2 h. Second, the product was immediately transferred into the stainless steel pressure bomb with a Teflon liner under autogenous pressure and static conditions. After completing the period of synthesis, the resulting solid product was collected by filtration and repeatedly washed with doubledistilled water until the pH of the resultant solution was below 9. Finally, the obtained sample was dried at 100 1C and calcinated at 550 1C for 8 h. The prepared zeolite denoted MOR. 2.3.3. Synthesis of ZnO composites The ZnO supported catalysts were prepared by mixing synthesized MOR zeolite with zinc acetate solution in 2-propanol under vigorous stirring at room temperature for 14 h. After cooling, 0.2 M NaOH solution was slowly added dropwise into the prepared solution under magnetic stirring at 40 1C in a water bath. After completing the period of synthesis, the white precipitate was separated by centrifugation, repeatedly washed with double-distilled water, dried at 80 1C and calcined at 450 1C for 3 h. In another similar experiment, activated carbon and alumina were used as the supporting materials. The conditions for preparing ZnO nanostructures in the presence of AC and alumina were the same as MOR. 2.4. Photocatalytic experiments
Fig. 2. Schematic representation of the experimental set-up used for photocatalytic experiments.
Photocatalytic reaction of AB92 was carried out in a Pyrex reactor. The reactor was equipped with an UV lamp and covered with aluminum foil followed by a black cloth to prevent UV light leakage. This set-up of reactor was surrounded by a circulating water jacket to maintain constant temperature as demonstrated in Fig. 2. Solutions with the desired dye concentration and catalyst were fed into the reactor and pH was controlled with phosphate buffer. Prior to irradiation, the suspension was magnetically stirred for 30 min in dark and then photocatalytic
Fig. 3. UV–vis absorption spectrum of the ZnO colloids as a function of time.
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reaction was started. 2 mL of samples was withdrawn for UV–vis analysis at regular time intervals and centrifuged for 10 min at the rate of 10,000 rpm. The dye concentration in each degraded sample was determined spectrophotometrically at λ ¼572 nm. 3. Results and discussion 3.1. Characterization of synthesized samples 3.1.1. UV–vis absorption spectrum of ZnO As shown in Fig. 3, the UV–vis absorption spectrum of ZnO demonstrates the absorption onset which is significantly blue-shifted compared to the absorption onset for zinc oxide in bulk at about 385 nm. This blue shift in absorption spectrum is attributed to the quantum size effect and shows that the average particle size is in the quantum regime [24]. It is obvious from Fig. 3 that the absorption peak of ZnO shifts to higher wavelength (red shift) with time which attributed to the increasing
of average particle size. As a result, the average particle radius continues to increase with time due to diffusionlimited coarsening. The average particle radius in a colloid can be determined from the absorption onset using the effective mass model where the particle band gap En (eV) can be approximately calculated from Eq. (1) [26,27]. ! 2 h 1 1 1:8e n bulk E ¼ Eg þ þ 4πεε0 r 2er 2 mne m0 mnh m0 !1 0:124e3 1 1 þ n ð1Þ 2 n h ð4πεε0 Þ2 me m0 mh m0 bulk
bulk
where Eg is the bulk band gap energy (for ZnO, Eg ¼ n 3.4 eV), h is Plank0 s constant, r is the particle radius, me n and mh are the effective masses of the electron and hole, m0 is the free electron mass, e is the charge of the electron, ε is the relative permittivity, and ε0 is the permittivity of free space. The average particle size of prepared ZnO was calculated from the band gap energy derived from the optical
Fig. 4. XRD patterns of ZnO, 5% ZnO/MOR and 25% ZnO/MOR nanostructures calcined at 450 1C.
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Fig. 5. XRD pattern of MOR nanostructure.
absorption spectra using Eq. (2) and estimated to be 4–6 nm [23]. En can be calculated from En ¼ hc=λ, where λ is the absorption onset wavelength. 1243:1 8:37 0:61 ¼ 3:2 þ λ r r2
ð2Þ
3.1.2. XRD analysis The XRD pattern of samples calcined at 450 1C are demonstrated in Fig. 4. The reflection peaks in Fig. 4(a), can be indexed to the known hexagonal wurtzite structure of ZnO with lattice constants of a¼b¼3.242 A1 and c¼5.205 A1 (JCPDS89-1397). The diffraction peaks of pure ZnO are intense and narrow which reveals the highly crystalline character of ZnO. The average crystalline size of the sample calcined at 450 1C was calculated from half the width of the diffraction peaks using the Scherrer equation [28]. The mean particle size for the crystallographic planes (101), (002) and (100) was calculated and estimated to be 30 nm. The XRD patterns of 5% ZnO/MOR and 25% ZnO/MOR are depicted in Fig. 4(b) and (c), respectively. In the XRD pattern of 5% ZnO/MOR, (Fig. 4(b)) no diffraction peak belonging to the pure ZnO was observed. This suggests that either ZnO can be completely dispersed over the MOR surface or the amount of ZnO loaded on MOR is too slight to be detected by XRD. In Fig. 4(c), ZnO crystalline phases are observed. It is interesting to note that the intensity of these peaks is almost as same as that of ZnO in Fig. 4(a). This indicates that by supporting ZnO on MOR surface no change of crystalline order of ZnO has taken place and the frame structure is almost unchanged. The XRD pattern of MOR is depicted in Fig. 5. The average crystallite size of the MOR powder was calculated and estimated to be 22 nm. The zeolitic phases were identified by comparing the diffraction peaks with the data reported in the inorganic crystal structure database (ICSD) [25]. 3.1.3. BET analysis According to Table 1, the surface area and total pore volume of the samples were increased by the addition of
Table 1 Propertis of the samples. Samples
SBET (m2 g 1)
Total pore volume (cm3 g 1)
ZnO MOR AC Al2O3 5% ZnO/MOR 25% ZnO/MOR 25% ZnO/AC 25% ZnO/Al2O3
17.42 402.02 500.0 373.7 384.4 314.6 414.6 371.2
0.0430 0.2129 0.2311 0.1512 0.1891 0.1760 0.2131 0.1472
MOR and the other support materials to ZnO nanoparticles. This addition stabilizes textural structure; prevent from the particle aggregation and the collapse of the pore structure, which reduce the loss of surface area of ZnO during calcination. Since supports have high surface area, supported ZnO nanocomposites would have higher BET surface area than pure ZnO nanoparticles. 3.1.4. SEM images SEM images of the prepared photocatalysts are shown in Figs. 6 and 7. Two magnifications of ZnO/MOR are shown in Fig. 6(b) and (c). Under the same calcination temperature (450 1C), the nanoparticle size of ZnO in ZnO/MOR is smaller than pure ZnO (72 nm for 5% ZnO/MOR and 58 nm for 25% ZnO/MOR). The zinc precursor may interact with the defect sites on the MOR surface which led to the distribution of ZnO on the MOR surface. The SEM images also show that ZnO nanoparticles are distributed well on the surface of MOR. The SEM images of 25% ZnO/AC and 25% ZnO/Al2O3 are shown in Fig. 7(a) and (b), respectively. A common characteristic observed in two composites is a heterogeneous dispersion of ZnO nanoparticles on the surface of supports. 3.1.5. FT-IR spectroscopy Fig. 8 shows the FT-IR spectrum of MOR. The peaks around 420–500 cm 1 and 700–850 cm 1 and 1000– 1150 cm 1 attributed to the asymmetric and symmetric
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Fig. 6. SEM images of (a) pure ZnO, (b) 5% ZnO/MOR and (c) 25% ZnO/MOR particles calcined at 450 1C.
stretching of the T–O–T bond and stretching of the T–O bond in SiO4 and AlO4, respectively (TQSi, Al). The peak at 3588 cm 1 corresponds to the physically adsorbed water on the MOR [25]. Fig. 9 shows the FT-IR spectra of the ZnO and MOR nanocamposite samples calcined at 450 1C. The strong peak
around 492 cm 1 in ZnO spectrum belongs to Zn–O stretching modes. The bands around 1628 and 3493 cm 1 are attributed to the O–H bending and stretching, which means that ZnO nanoparticles physically adsorb water in the air [29,30]. The intensity of the band related to Zn–O is very low because the content of ZnO in 5% ZnO/MOR is slight in 5% ZnO/MOR
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Fig. 7. SEM images of (a) 25% ZnO/Al2O3 and (b) 25% ZnO/AC.
Fig. 8. FT-IR spectrum of pure MOR.
spectrum. In addition, the bending frequency of Zn–O might be overlapped with T–O stretching frequency (TQSi, Al), below 500 cm 1, and therefore the overlap peaks are observed in this region. But, in 25% ZnO/MOR the Zn–O band around 490 cm 1 confirms the formation of ZnO. Fig. 10 shows the FT-IR spectra of the 25% ZnO/AC and 25% ZnO/Al2O3 composite samples calcined at 450 1C. The intensity of the band related to Zn–O in 25% ZnO/AC spectrum is strong, which means that AC physically adsorbs ZnO and confirms the formation of ZnO. The bands around 1585 cm 1 and 1000–1185 cm 1 are attributed to CQC, C–O and C–C stretching in AC, respectively [31]. In Fig.10(b), the intensity of the band related to Zn–O is very low, because the ZnO precursor may not properly interact with the defect sites on the Al2O3 surface. 3.2. Effect of different supports on the photocatalytic degradation of AB92 of ZnO catalyst The results of dye degradation over supported catalysts are demonstrated in Fig. 11. It shows that the degradation of AB92
over 25% ZnO/MOR is higher than that over 25% ZnO/Al2O3 and 25% ZnO/AC. Degradation of AB92 over 25% ZnO/Al2O3 is the lowest, which is only 48% of dye removal after 120 min. In alkali media, Al2O3 could not achieve a high adsorption of ZnAc2 2H2O. In addition, Al2O3 is not competent due to the poor adsorption of AB92. Meanwhile, dye removal of 70% was achieved using 25% ZnO/AC in the same time. On ZnO/AC supported catalyst, both adsorption of AB92 by AC and its photodegradation by ZnO processes occur. Since the adsorption process onto AC proceeds much faster than the degradation by the photocatalysis, the AB92 removal is chiefly taken place by the former process. So, in 25% ZnO/AC particles, AC particles do not have any photoactivity for degradation and only act as an adsorbent. Although 25% ZnO/AC particles could achieve a high adsorption due to high surface area, lower photocatalysis than 25% ZnO/MOR was also observed due to the poor coverage on ZnO to hinder the mass transfer of AB92 [32–34]. When MOR was used as a support, the lower UV absorption derived from loading could be compensated by its superior adsorption of dye and its intermediates. It was known that MOR has a unique structure [23]. MOR0 s
Fig. 9. FT-IR spectrum of pure ZnO, 5% ZnO/MOR and 25% ZnO/MOR samples.
Fig. 10. FT-IR spectra of (a) 25% ZnO/AC and (b) 25% ZnO/Al2O3.
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Fig. 13. Effect of mass ratio of ZnO and MOR on the photocatalytic activity ([dye]¼ 20 ppm, [catalysts] ¼ 100 ppm, pH¼ 6).
Fig. 11. Effect of different supports on the photocatalytic degradation of AB92 of ZnO catalyst. ([dye]¼ 20 ppm, [catalysts] ¼100 ppm, pH¼ 6).
3.3. Factors influencing on the photocatalytic degradation of AB92 3.3.1. Kinetic analysis Degradation efficiency was calculated from X% ¼
C0 C 100 C0
ð3Þ
where X is the degradation efficiency of AB92, C0 is the initial concentration of dye and C is the solution concentration of dye after degradation at time (t). Experimental results showed that the photocatalytic degradation rate of AB92 in heterogeneous photocatalytic oxidation systems under UV-light illumination obeyed Langmuir–Hinshelwood (L–H) kinetics model (Eq. (4)) [35]. r¼
dC kKC ¼ dt 1 þKC
ð4Þ
where r is degradation rate (mg L 1 min 1), C is the dye concentration (mg L 1), t is the illumination time (min), k is the reaction rate constant (min 1) and K is the adsorption coefficient of dye (L mg 1). At low initial concentration of the AB92, the reaction rate is proportional to the dye concentration and the rate is pseudo-first order. Hence, Eq. (4) can be changed to Eq. (5). C0 ¼ kKt ¼ kapp t ln ð5Þ C
Fig. 12. (a) Rate constant and (b) photocatalytic degradation efficiency of AB92 in the presence of ZnO, 5% ZnO/MOR and 25% ZnO/MOR ([dye] ¼ 20 ppm, [catalysts] ¼ 100 ppm, pH¼ 6).
molecular structure is a framework containing chains of fivemembered rings of linked silicate and aluminates tetrahedral. This structure has a high propensity for adsorption of AB92 molecules and transfers them to active sites near ZnO surface, where dOH radicals are available for breaking AB92 molecules. Hence, among the ZnO supported catalysts; MOR can be an efficient support with the highest photocatalytic activity in adsorption of AB92 for ZnO in photodegradation due to the homogeneous distribution of ZnO on it. This phenomenon is consistent with the SEM images of materials where the MOR surface is homogeneously coated by ZnO nanoparticles.
The apparent first-order rate constant (kapp) for dye degradation can be calculated by plotting ln (C0/C) versus time (t) [35]. Rate constant and the photocatalytic degradation efficiency of AB92 after 120 min in the presence of ZnO, 5% ZnO/MOR and 25% ZnO/MOR are presented in Fig. 12(a) and (b). Degradation rate of AB92 in the presence in 25% ZnO/MOR was higher than of the pure ZnO and 5% ZnO/MOR nanocomposite. 3.3.2. Effect of mass ratio on photocatalytic activity of catalysts The effect of mass ratio of ZnO and MOR on the photocatalytic activity was investigated by synthesis of composites of ZnO and MOR with mass ratios of 0.05:1 and 0.25:1. The photocatalytic activity is sharply enhanced by addition of ZnO nanoparticles. Therefore, all of the ZnO/MOR photocatalysts exhibit higher photocatalytic
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Fig. 14. Schematic diagram of photochemical generation of hydroxyl radicals and photo-generated charge carrier0 s separation by ZnO supported catalyst.
Fig. 15. Effect of catalyst concentration on (a) the photodegradation of AB92 and (b) the rate constant in the presence of 25% ZnO/MOR ([dye] ¼ 20 ppm, pH¼7).
Fig. 16. Effect of initial dye concentration on the AB92 photodegradation in the presence of (a) ZnO and (b) 25% ZnO/MOR ([catalysts] ¼100 ppm, pH ¼7).
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activity than the pure MOR under UV light irradiation. As demonstrated in Fig. 13, when the ratio of ZnO/MOR reaches 0.25:1, it exhibits the highest photocatalytic activity. The activity of 25% ZnO/MOR is obviously superior to the others. The various steps involved in photocatalytic degradation of AB92 can be summarized according to Eqs. (6)–(10):
þ
ZnOþhυ-ZnO (eCB þhVB) þ
hVB þAB92-AB92d þ -oxidation of the AB92 þ
hVB þH2O-H þ þ dOH d
e þO2-O2
d
AB92ads þO2
(6) (7) (8) (9)
or dOH-degradation products
(10)
It has been suggested that the complete removal of AB92 molecules could be due to the generation of a large number of hydroxyl radicals (dOH) and superoxide radical d anions (O2 ) (Fig. 14) with high strong oxidization properties. Moreover, it is well-known that hydroxyl radicals are nonselective, and they will directly react with the different recalcitrant pollutants [36,37]. 3.3.3. Effect of catalyst concentration The influence of catalyst concentration on the photodegradation efficiency of AB92 was investigated in the range of 0–120 ppm at constant dye amount (20 ppm) and pH of 7. As shown in Fig. 15 the more catalyst concentration the higher photodegradation rate which is related to the increasing in the number of active sites on the catalyst surface. As obviously demonstrated, the photodegradation rate reached to the maximum value (at 100 ppm) and then decreased in all catalyst concentration which can be attributed to the decreasing in UV light penetration as a result of increasing in scattering because of concentration [32,38]. 3.3.4. Effect of initial dye concentration Fig. 16 shows the effect of initial dye concentration on the degradation efficiency by changing the initial concentration of AB92 from 10 to 40 ppm with constant catalyst
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loading (100 ppm) and pH of 7. The observed results revealed that the photodegradation of AB92 increased with the increasing of dye concentration. Further, increasing of AB92 concentration resulted in decreasing in the photocatalytic degradation. The optimum initial dye concentration was obtained at 20 ppm and 30 ppm in the presence of ZnO and 25% ZnO/MOR, respectively. An explanation to this behavior is that as the dye concentration increases, the amount of dye adsorbed on the catalytic surface increases. The degradation relates to the formation of hydroxyl radicals, therefore, the generation of hydroxyl radicals will be increased. At higher concentrations of AB92, the path length of photon entering into the dye solution decreases, so the absorption of photons by the catalysts decreases, and consequently the degradation efficiency is reduced [36,38]. Another reason for the decrease in photocatalytic degradation at higher concentration of AB92 maybe is that the degradation was hard to occur at sufficiently long distance from the reaction zone close to the irradiated side due to retardation in the penetration of light [32]. 3.3.5. Effect of pH The solution pH exhibits an important role on the ZnO oxidation potential and surface charge. So, the interpretation of pH effect on the degradation efficiency is difficult since it has multiple roles. The photocatalytic degradation of AB92 was carried out at different pH values using 20 ppm of AB92 and 100 ppm of catalyst. Fig. 17 shows that by increasing of
Fig. 18. Effect of UV light and catalysts on photocatalytic degradation of AB92. ([dye] ¼ 20 ppm, [catalysts] ¼100 ppm, pH¼6).
Fig. 17. Effect of pH on degradation efficiency of AB92 in the presence of (a) ZnO and (b) 25% ZnO/MOR. ([dye]¼ 20 ppm, [catalysts] ¼100 ppm).
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Fig. 19. Absorption spectra of textile wastewater at different times of degradation. ([dye] ¼20 ppm, [25%ZnO/MOR] ¼100 ppm, pH¼ 6).
pH, the degradation efficiency increases and reaches to the highest value at pH 6.0 which can be attributed to the more adsorption of AB92 on the catalyst surface. The surface charge of ZnO has a significant effect on the adsorption and dissociation of AB92. The zero point charge (zpc) of ZnO is 9.070.3 [32]. Hence, the catalyst surface has positive charge below this pH and negative charge above this pH. As a result, at pH 6.0 as a maximum pH, catalyst surface has positive charge. AB92 has pKa of 3.2 and for pH4pKa AB92 exists as its anion. Therefore, the optimum pH for efficient AB92 removal occurs in the pHzpc 4pH4pKa (dye) due to the anionic charge of AB92. This pH would promote the electrostatic interaction between catalyst surface with positive charge and AB92 as an anionic dye, resulting to the increasing of adsorption and photodegradation. The lower removal efficiency in high acidic pH may be related to the dissolution of ZnO.
3.3.6. Effect of UV irradiation and catalyst particles The degradation yield by catalysts in the present and the absence of UV light is demonstrated in Fig. 18. The degradation achieved by photolysis is also given for comparison. As shown in Fig. 18 degradation by photolysis was negligible and failed to treat resistant compounds. Finally, both UV light and photocatalysts are needed for the effective degradation of AB92. Hence, the combination of catalysis and UV light system is demonstrated to be an efficient process to remove organic pollutants from wastewater.
3.3.7. Photocatalytic degradation of textile wastewater In order to evaluate the photocatalytic activity of 25% ZnO/MOR, wastewater of textile industry was examined and the absorbent spectrum is shown in Fig. 19. Notably, 25% ZnO/MOR showed a decrease of absorption in all wavelengths, which displays the high activity of 25% ZnO/MOR nanocomposite.
3.3.8. Measurment of chemical oxygen demand (COD) COD is widely used as a useful technique to measure the water quality. COD of the dye solution was evaluated before and after the treatment. In this work the maximum degradation efficiency was achieved by 25% ZnO/MOR (about 88%). 4. Conclusions In conclusion, ZnO, MOR zeolite nanoparticles and ZnO/MOR, ZnO/AC and ZnO/Al2O3 nanocomposites were prepared and characterized by the various techniques. The results revealed that the insertion of ZnO on different supports decreases the particle size, increases the surface area and reduces electron–hole recombination rate in comparison to pure ZnO. Photocatalytic activity of catalysts was investigated by degradation of AB92 azo dye in water. 25% ZnO/MOR nanoparticles calcined at 450 1C showed the highest photocatalytic activity. The complete removal of AB92 model solution could be obtained in a relatively short time for 25% ZnO/MOR (as a catalyst with the highest photocatalytic activity) in optimal condition. ZnO/MOR nanocomposites demonstrate the significant increase in the AB92 photodegradation compared to the other supported catalysts. Although, ZnO/MOR nanocomposites have less surface area than ZnO/AC, they have higher photocatalytic activity. This phenomenon can be attributed to the morphology of ZnO/MOR with a unique framework that causes the highest photocatalytic activity of ZnO/MOR nanocomposites. Finally, the best catalyst, 25% ZnO/MOR, was used for photodegradation of textile wastewater where a maximum of 88% degradation efficiency was observed in 2 h.
Acknowledgment The authors are grateful to Dr. S. Rohani from Iran Color Research Center for providing dye sample (AB 92).
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