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Photocatalytic activity of spinel ZnFe2−xCrxO4 nanoparticles on removal Orange I azo dye from aqueous solution
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JTICE-800; No. of Pages 6 Journal of the Taiwan Institute of Chemical Engineers xxx (2013) xxx–xxx
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Photocatalytic activity of spinel ZnFe2xCrxO4 nanoparticles on removal Orange I azo dye from aqueous solution A.I. Borhan a, P. Samoila b, Vasile Hulea c, A.R. Iordan a, M.N. Palamaru a,* a
Alexandru Ioan Cuza University of Iasi, Faculty of Chemistry, 11 Carol 1 Boulevard, R-700506, Iasi, Romania Petru Poni Institute of Macromolecular Chemistry, 41A, Gr. Ghica Voda Alley, 700487 Iasi, Romania c Institut Charles Gerhardt, UMR 5253, CNRS-UM2-ENSCM-UM1, Mate´riaux Avance´s pour la Catalyse et la Sante´, 8 rue de l’Ecole Normale, 34 296 Cedex 5, France b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 11 June 2013 Received in revised form 25 November 2013 Accepted 1 December 2013 Available online xxx
In this study the photodegradation of aqueous solution of recalcitrant organic pollutant Orange I was investigated by using chromium substituted zinc ferrite, ZnFe2xCrxO4 (x = 0, 0.25, 0.5, 1.0, 1.50, 2.0) catalysts, for the first time, under both UV and visible light irradiation. The photocatalytic process was promoted when Fe3+ cations were substituted with Cr3+ cations. Therefore, the dye removal efficiency was enhanced up to 92.8% for x = 1.50 composition when Cr3+ cations were introduced in octahedral lattice of ZnFe2O4, showing that this material contains chromium optimal dosage for photocatalytic degradation of Orange I. Further, we studied photocatalytic activity under both UV and visible light irradiation of ZnFe0.50Cr1.50O4 above Orange I dye solution. The investigation of removal kinetics of Orange I indicates that the removal processes, under both UV and visible light, obeys the rate of firstorder kinetic equation. The best photocatalytic performance was achieved after 45 min of UV light irradiation, with first order kinetic rate constant k of 5.85 102/min. Anyway, the decolorization rate under UV light is two times higher that afforded under visible light. ß 2013 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords: Heterogeneous photocatalysis Orange I dye Photodegradation Spinel ferrite UV light Visible light
1. Introduction The pollution of water resources by the dyes from industries such as textile, paper, plastic, leather, cosmetics and photographic, has become a serious environmental problem because of their toxicity, affecting the quality of life for generation to come. Most of the dyes used in textile industry usually have a synthetic origin and complex aromatic molecular structures which make them very stable and relatively difficult to be biodegraded. Consequently, the effective removal of these kinds of pollutants from wastewaters represents a great challenge to scientists [1]. Among various treatment methods employed to remove these refractory pollutants from contaminated water, photocatalytic processes have generated a great interest in the last decade [2]. In this respect, the photocatalytic processes of dye degradation mediated by TiO2-based materials under ultraviolet irradiation were widely studied [3]. Nevertheless, the main drawback toward practical application of these kind of photocatalysts is that photocatalytic processes can be activated by ultraviolet light
* Corresponding author. Tel.: +40 232201341; fax: +40 232201313. E-mail addresses: [email protected], [email protected] (M.N. Palamaru).
exclusively, which accounts for only 4% of the incoming solar energy [4]. Hence, another class of oxides, the spinel ferrites (MFe2O4, where M is a divalent metal), capable of utilizing a largest portion of the solar spectrum, studied mostly due to their electrical and magnetic properties, has gained much interest in recent years as potentials catalysts for different processes. The general formula of spinel type oxides is (A)[B2]O4, where the tetrahedral sites (A) are occupied by divalent cations (Zn2+, Ni2+, Co2+, Cu2+, etc.) and octahedral sites (B) by trivalent cations (such as Cr3+, Al3+, In3+, La3+, etc.) in a cubic structure lattice. The band gap capable of absorbing visible light, as well as the spinel crystal structure, makes these materials attractive for photocatalytic applications [2]. Among the spinel ferrites, ZnFe2O4, has received much more consideration due to its relatively small band-gap (around 1.9 eV) [5]. Also, spinel-type compounds such as ZnCr2O4 [6] or ZnAl2O4 [7] were successfully employed for the removal of dyes from wastewater via photocatalytic processes. Nevertheless, to the best of our knowledge, little research has been paid to photocatalytic properties of substituted zinc ferrites, obtained by sol–gel autocombustion method. Also, another advantage of using spinel ferrites as photocatalysts is due to their magnetic properties, making these materials recovery from the catalytic systems, in order to reuse them in other degradation processes, less expensive than for the TiO2.
1876-1070/$ – see front matter ß 2013 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jtice.2013.12.002
Please cite this article in press as: Borhan AI, et al. Photocatalytic activity of spinel ZnFe2xCrxO4 nanoparticles on removal Orange I azo dye from aqueous solution. J Taiwan Inst Chem Eng (2013), http://dx.doi.org/10.1016/j.jtice.2013.12.002
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This study reports, for the first time, the utilization of chromium substituted zinc ferrite under both UV and visible light to the photodegradation of the recalcitrant organic pollutant Orange I, which is a dye widely used in the textile industry. The synthesis via sol–gel autocombustion technique and the characterization of the ZnFe2xCrxO4 (x = 0, 0.25, 0.5, 1.0, 1.50, 2.0) nanoparticles was exposed extensively in the previous paper [8] studying the relation between structural features and magnetic and electrical properties of these materials. Therefore only the results important from the photocatalytic activity point of view are presented in the current paper. Orange I (Tropaeolin 000 No. 1) was selected as probe molecule because it belongs to the family of azo-dyes, which accounts for almost 80% of annual production of commercial dyes all over the world. 2. Experimental 2.1. Synthesis and characterization Pure nanocrystalline catalysts with the general formula ZnFe2xCrxO4 (x = 0, 0.25, 0.5, 1.0, 1.50, 2.0) were synthesized by sol–gel auto-combustion method using analytical grade iron nitrate [Fe(NO3)39H2O] (99.9%, Aldrich), chromium nitrate [Cr(NO3)39H2O] (99.9%, Aldrich) and zinc nitrate solution, obtained in situ from ZnO (99%, Sigma–Aldrich) and nitric acid (Merck) 20% solution, and tartaric acid as chelating and fuel agent. The synthesis procedure consisted in dissolving corresponding salts in distilled water followed by the addition of tartaric acid. Consequently, the obtained solutions were heated at 80 8C through gel phase transformation. The gels were gradually heated up to 350 8C until the autocombustion occurred. The obtained powders were heat treated at 500 8C/7 h and then at 700 8C/7 h. The protocol synthesis was extensively described in a previous study [8]. IR spectroscopy and XRD are methods commonly used to verify the success of synthesis and spinel structure formation [9,10]. Monitoring solid phase chemical reaction and the disappearance of the organic and nitric phases were performed by infrared spectroscopy using a Bruker TENSORTM27 with ATR cell, with 2 cm1 resolution. The powder phase composition was identified by X-ray diffraction (XRD; Model Shimadzu LabX6000 diffractometer) using CuKa radiation (l = 1.5405 A˚), for 2u ranging between 20 and 808, at a scanning speed of 0.028/s. The morphology and particle size were observed using a scanning electron microscope (SEM; Model Hitachi S2600 N Microscope). Textural properties including surface area and pore size were measured by nitrogen adsorption–desorption isotherms method at 78 K on a ASAP 2020 analyzer. The specific surface areas SBET were determined using the Brunauer–Emmett–Teller (BET) method. The mean pore size (Dpore) corresponds to maximum of pore size distribution curve. The optical absorption spectrum was recorded on a Perkin-Elmer, Lambda 35 UV–vis spectrometer, using H2O2 as reference sample, in the range of 200–700 nm. To study the light absorption of the catalysts, the diffusive reflectance spectra (DRS) of the catalyst samples in the wavelength range of 200–1100 nm were obtained using a UV–vis spectrophotometer (Shimadzu UV), with MgO as a reference.
Fig. 1. Chemical structure of Orange I dye obtained by the Hyperchem software.
irradiation. The photocatalytic degradation experiments were performed into a Pyrex cylindrical photo-reactor. A low pressure Hg lamp (Philips, Netherlands) emitting a wavelength of 185 nm was positioned at the center of the reactor and was used for photoreaction under UV light. The photo-reactor was surrounded by a Pyrex circulating water jacket in order to maintain the temperature constant during the reaction. The photocatalytic activities of the asobtained photocatalyst were evaluated by the decomposition of Orange I in an aqueous solution at constant temperature. In each experiment, the reaction suspension was prepared by adding 0.40 g of catalyst powder into 75 mL Orange I solution with an initial concentration of 80 mg/L and pH 3. The suspension was magnetically stirred in the dark for 30 min to reach adsorption–desorption equilibrium at constant temperature prior to irradiation, and then the solution was illuminated at different times. During illumination, stirring was maintained in order to keep the mixture in suspension. At regular time intervals (5, 10, 15, 30 and 45 min), the analytical samples were taken from the suspension to investigate the change in the concentration (absorbance) of each degraded solution by measuring the absorbance in range of 200–700 nm for Orange I. The absorbance of Orange I solutions were determined at 475 nm and it is used to calculate its concentration. For comparison purposes, the photoreaction under visible light illumination was performed using a 400 W high pressure sodium lamp (APF, IP65, Italy) with main emission in the range of 400– 800 nm, for the ZnFe0.50Cr1.50O4 sample only, using the same protocol described before. 3. Results and discussion 3.1. Optimal Cr3+ dosage First, the photocatalytic activity of ZnFe2xCrxO4 (x = 0, 0.25, 0.50, 1.0, 1.50, 2.0) powders under UV light was evaluated performing experiments on the degradation of Orange I in aqueous solution. In Fig. 2, a comparison of photocatalytic activity among all
2.2. Photocatalytic reaction procedure The photocatalytic activity of the ZnFe2xCrxO4 (x = 0, 0.25, 0.5, 1.0, 1.50, 2.0) catalysts was evaluated by degradation of recalcitrant textile dye Orange I (Tropaeolin 000 No. 1) with chemical formula C16H11N2NaO4S (d = 13.9 A˚) (Fig. 1) under UV-light
Fig. 2. Percentual dye removal with increasing Cr3+ content (x) in the presence of ZnFe2xCrxO4 powders.
Please cite this article in press as: Borhan AI, et al. Photocatalytic activity of spinel ZnFe2xCrxO4 nanoparticles on removal Orange I azo dye from aqueous solution. J Taiwan Inst Chem Eng (2013), http://dx.doi.org/10.1016/j.jtice.2013.12.002
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studied photocatalysts at the same operation conditions and after 45 min of illumination is shown. Orange I was a relatively stable azo dye and the photodegradation using ZnFe2O4 powder under Hg lamp irradiation was relatively slow and only 36.8% of Orange I could be removed in 45 min irradiation time. It is important to note that chromium substitution of nanocrystalline zinc ferrites samples could modify significantly the photocatalytic activity. Hence, the photocatalytic process was promoted when Fe3+ cations were substituted with Cr3+ cations, in any studied proportion. Therefore, the dye removal efficiency was enhanced up to 92.8% for x = 1.50 sample when Cr3+ cations were introduced in octahedral lattice of ZnFe2O4. From Fig. 1, we conclude that ZnFe0.50Cr1.50O4 compound contains chromium optimal dosage for photocatalytic degradation of Orange I. Further, we were studied photocatalytic activity under visible light irradiation of ZnFe0.50Cr1.50O4 above Orange I dye solution. Thus, we will discuss below only the photodegradation of Orange I dye under UV and visible light irradiation in the presence of ZnFe0.50Cr1.50O4 catalyst.
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Fig. 4. X-ray diffraction pattern for ZnFe0.50Cr1.50O4 powder.
3.2. Catalyst characterization XRD patterns, IR spectra and SEM micrographs of studied ZnFe2xCrxO4 (x = 0, 0.25, 0.50, 1.0, 2.0) samples were not included because they were reported in a previous work [8]. For the sake of simplicity, only the characterizations of ZnFe0.50Cr1.50O4 sample are presented in this work. Nevertheless, we can remind that XRD and IR analysis revealed pure spinel cubic structure for all powders heated at 700 8C. Also, SEM micrographs showed that spherical grains with uniform size distribution and average particle size varying between 72.2 and 56.5 nm were obtained. The IR spectra, in the wavenumber range 4000–300 cm1, for ZnFe0.50Cr1.50O4 powder, heated at 700 8C, shows the presence of two main absorption peaks characteristic of spinel structure (Fig. 3). These two peaks suggest the formation of the spinel-type structure. Absorption peak observed at 602 cm1 is assigned to y1 wavenumber and that of 488 cm1 is attributed to y2 wavenumber [11]. The strongest absorption peak y1 is caused by intrinsic vibration of bonds between metal ions and oxygen ions in tetrahedral positions, whereas the weakest absorption peak y2 is assigned to the stretching vibrations of bonds between octahedral metal ions and oxygen ions [12]. Fig. 4 shows the XRD pattern of ZnFe0.50Cr1.50O4 powder proving that the catalyst structure was dominated exclusively by the spinel structure. All reflections peaks were identified and
indexed to cubic phase of spinel in good agreement with the referred database of International Center for Diffraction Data [891009 ICDD, J. Alloys Compds. 1998] [87-1164 ICDD, Mater. Res. Bull. 1996] [84-0314 ICDD, J. Appl. Phys. 1980] [89-1397 ICDD, Solid State Commun. 1979]. The average crystallite size was estimated by applying the Scherrer formula on the highest intensity peak and was obtained a size of 24.50 nm. Fig. 5 shows the SEM image of ZnFe0.50Cr1.50O4 powder. SEM image show that the spinel nanoparticles consist of uniform and homogeneous quasi-spherical grains with an average size of 45 nm. This value is significantly lower than obtained for x = 0, 0.25, 0.50, 1.0, 2.0 samples (between 56.5 and 72.2 nm) [8] and could explain the best performance in photocatalytic activity. Nitrogen adsorption–desorption isotherm for ZnFe0.50Cr1.50O4 and the corresponding average pore size distributions (insert) are shown in Fig. 6. Although this compound ZnFe0.50Cr1.50O4, had not a large specific surface area (around 60.8 m2/g) it showed a remarkably photocatalytic performance for Orange I degradation (92.80%). According to IUPAC classification, the isotherm is of type IV with a H1 hysteresis loop, representing predominantly mesoporous structure, which was confirmed by the analysis of pore size distribution [13]. The H1 type hysteresis is characteristic for pores with uniform size and shape formed from aggregates and agglomerates particles [14]. The pore size distribution curve
Fig. 3. IR spectrum for ZnFe0.50Cr1.50O4 powders.
Fig. 5. SEM micrograph of ZnFe0.50Cr1.50O4 nanoparticles.
Please cite this article in press as: Borhan AI, et al. Photocatalytic activity of spinel ZnFe2xCrxO4 nanoparticles on removal Orange I azo dye from aqueous solution. J Taiwan Inst Chem Eng (2013), http://dx.doi.org/10.1016/j.jtice.2013.12.002
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Fig. 6. Nitrogen adsorption–desorption isotherm and pore size distribution (insert) of ZnFe0.50Cr1.50O4.
displays a bimodal distribution [15] with pore sizes of 4.3 nm and 11.4 nm. The maximum diagonal length of Orange I dye molecule is 1.39 nm (see Fig. 1), therefore the ratios between the pore diameters of the catalyst and the maximum diagonal length of Orange I dye are 3.1 for the smaller pores and 8.2 for the larger pores. This implies each particle pore of ZnFe0.50Cr1.50O4 catalyst could accommodate a certain number Orange I dye molecules which was diffused from the dye solution to the pores of the catalyst particle, in both small and large pores. These results indicate that the photocatalytic activity probably depends on many factors, besides the chemical composition, such as specific surface area value [16], the pore sizes and number, or grain size (see SEM section). Fig. 7 shows the room temperature UV–vis absorption spectra of ZnFe2xCrxO4 (x = 0, 1.50) catalysts derived from the diffuse reflectance (R) spectrum using Kubelka–Munk function [17] F(R) = (1 R)2/2R, which is plotted as a function of energy (eV). From the DRS spectra, a shoulder centered around 1.96 eV corresponds to 6A1g ! 4T2g excitation which arises due to the d–
d crystal field excitations of Fe3+ ions (3d5 – high spin configuration; t2g3 eg2) in spinel lattice [18]. Above 2 eV, absorption increases significantly with two shoulders centered at 2.5 eV and near 2.96 eV. These two features were assigned to the charge transfer excitations Fe3+–O–Fe2+ [18]. As can be seen, in the case of ZnFe0.50Cr1.50O4, these two shoulders are lower, because the chromium ions do not participate in the conduction process, but decrease the number of electron hopping (Fe3+–O–Fe2+) between adjacent B sublattices. It also clearly observed the appearance of an absorption maximum at 4.5 eV, which can be attributed to 2E (t2g3) ! 4T2g (t2g2 e) excitation which arises due to the d–d crystal field excitations of Cr3+ ions [19]. A shift in this transition bands clearly indicates that substitution of Cr3+ in ZnFe2O4 leads to changes in octahedral lattice, as a consequence of contraction in unit cell volume. The Tauc plot is a method that is widely used for the determination of band gap. The estimated band-gaps of the ZnFe2O4 and ZnFe0.50Cr1.50O4 nanoparticles are found to be 1.6 and 1.3 eV, respectively. Compared with the ZnFe2O4 band-gap value of 1.6 eV, the optical absorption edge shifted slightly to the red direction for ZnFe0.50Cr1.50O4 catalysts. That is helpful to the improvement of photocatalytic activity under visible light. Note that the value of the indirect band gap decreases with the increase in the Cr content, because is affected by the decrease of various factors such as crystallite size, structural parameter (lattice constant) [8]. 3.3. Evaluation of photocatalytic activity 3.3.1. Dye removal by ZnFe0.50Cr1.50O4 powder The photocatalytic activity of ZnFe0.50Cr1.50O4 catalyst was examined by the photodegradation (decolorization) of Orange I in aqueous solution under UV light and visible light irradiation. The Orange I absorption spectra is dominated by a strong absorbance band at 475 nm due to the n–p* transition of the azo, –N5 5N–, group. Also, characteristic bands are usually observed around 240, 270, 290 and at 330 nm assigned to the p–p* transitions related to aromatic rings. The discoloration reaction occurs once the azobond disappear, meaning the disappearing of the characteristic band at 475 nm [20]. Fig. 8(a) and (b) shows the changes of the optical densities of 475 nm (–N5 5N–), 330 nm, 290 nm (naphthalene ring) and 240 nm
Fig. 7. Plots of (F(R)hy)2 versus the incident photon energy (hy) of ZnFe2O4 and ZnFe0.50Cr1.50O4 nanoparticles.
Please cite this article in press as: Borhan AI, et al. Photocatalytic activity of spinel ZnFe2xCrxO4 nanoparticles on removal Orange I azo dye from aqueous solution. J Taiwan Inst Chem Eng (2013), http://dx.doi.org/10.1016/j.jtice.2013.12.002
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Fig. 8. The change of absorption spectra of Orange I in presence of ZnFe0.50Cr1.50O4 powder under (a) UV light irradiation and (b) visible light irradiation.
(benzene ring) bands of orange I at different illumination time under UV light and visible light in the presence of ZnFe0.50Cr1.50O4 powder. It must be noticed that the dye concentration decreases quickly, hence, adsorption peaks of Orange I became weaker with the irradiation time, which indicates that both azo groups and aromatic parts of the dye molecule decompose under both UV light and visible light. Therefore, the photodegradation rate of Orange I over ZnFe0.50Cr1.50O4 nanoparticles increase rapidly with increasing UV irradiation time, and reaches 92,80% in 45 min, while the decolorization rate under visible light is two times lower, i.e. 49% in 45 min. Nevertheless, the behavior of ZnFe0.50Cr1.50O4 sample under visible light recommend this material as an effective photocatalyst, compared to the results previously reported in the literature concerning different azo dyes photodegradation [2]. 3.3.2. Chemical kinetic removal The degradation kinetics of Orange I on ZnFe0.50Cr1.50O4 catalyst under UV light and visible light could be analyzed by the first order kinetic model: Ct (1) ln ¼ kt C0 where k is the apparent reaction rate constant, and C0 and Ct are the initial concentration and the concentration of Orange I at reaction time t, respectively [21]. On the basis of the experimental data, the
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Fig. 9. (a) Photocatalytic degradation and (b) photodegradation kinetics of Orange I over ZnFe0.50Cr1.50O4 powder under UV irradiation and visible light.
plots of (Ct/C0) and ln(Ct/C0) versus time are depicted in Fig. 9(a) and (b), respectively. There can be observed that the photocatalytic activity was significantly higher under UV irradiation than under visible light. The best photocatalytic performance was achieved after 45 min of UV light irradiation, with first order kinetic rate constant k of 5.85 102/min. Under visible light, the first order kinetic rate constants k was about 4 times lower, i.e. k = 1.54 102/min. These results are in agreement with literature. Thus, the catalytic performance of ZnFe0.50Cr1.50O4 catalyst under visible light is identical to that obtained by Liang et al. [22] using TiO2 for degradation of Orange I. Moreover, ZnFe0.50Cr1.50O4 catalyst shows a performance twice higher than TiO2, but comparable with those of RE3+ (Sm3+, Nd3+, Pr3+) doping in TiO2 for Orange I dye degradation under UV light. According to the literature, for a high degradation efficiency of Orange I, are required an optimal dosage of trivalent cation Cr3+ in zinc ferrite and high band gap energy [21], besides surface area, the adsorption capacity and the surface acid–base properties [23]. There can be observed that Cr3+ substitution with x = 1.50 is the optimal dosage in zinc ferrite for the most efficient separation of photo-induced electron hole pairs. Therefore, by substituting Fe3+ with Cr3+ in zinc ferrite, the surface barrier becomes higher, and the space charge region becomes narrower. This variation occurs because Fe3+ ions substitution by Cr3+ ions no limits the Fe3+ $ Fe2+ electronic
Please cite this article in press as: Borhan AI, et al. Photocatalytic activity of spinel ZnFe2xCrxO4 nanoparticles on removal Orange I azo dye from aqueous solution. J Taiwan Inst Chem Eng (2013), http://dx.doi.org/10.1016/j.jtice.2013.12.002
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exchange, such as normal, and thus a variable number of electron exchanges are free to interact with the large electrical field. This increases the probability that electrons reach the surface barrier, resulting in a decrease in the space charge polarization. The electron hole pairs within the region are efficiently separated by the large electric field before recombination which led to the higher photocatalytic activity [21]. However, degradation was more effective under UV irradiation, which means that the band gap energy was much lower in the case of irradiation with visible light. 4. Conclusions The photocatalytic activity of pure spinel phase ZnFe2xCrxO4 powders (x = 0, 0.25, 0.5, 1.0, 1.50, 2.0) was examined by the degradation of Orange I in aqueous solution under UV light irradiation. The photocatalytic process was promoted when Fe3+ cations were substituted with Cr3+ cations. The best dye removal efficiency was obtained for x = 1.50 sample, especially due to his smaller grain size as observed from SEM analysis, showing that this material contains chromium optimal dosage for photocatalytic degradation of Orange I. Also, the photocatalytic performance under visible light of ZnFe0.50Cr1.50O4 material above Orange I dye solution was successfully studied. Hence, nanoparticles of ZnFe0.50Cr1.50O4 can act as photocatalyst of the degradation of azo dyes in an aqueous solution. Anyway, the decolorization rate under visible light is two times lower that afforded under UV light. The best photocatalytic performance was achieved after 45 min of UV light irradiation, with first order kinetic rate constant k of 5.85 102/min. As stated above, a very huge advantage of using these spinel ferrites as photocatalysts is because these materials can be recovery from the catalytic systems, in order to reuse them in other degradation processes, less expensive than the TiO2. Acknowledgements This work was supported by the European Social Fund in Romania, under the responsibility of the Managing Authority for the Sectorial Operational Programme for Human Resources Development 2007-2013 [grant POSDRU/107/1.5/S/78342]. Also, P. Samoila kindly acknowledge the European Union’s Seventh Framework Programme (FP7/2007–2013) under Grant Agreement No. 264115–STREAM. References [1] Sharma R, Singhal S. Structural, magnetic and electrical properties of zinc doped nickel ferrite and their application in photo catalytic degradation of methylene blue. Physica B 2013;414:83–90.
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Please cite this article in press as: Borhan AI, et al. Photocatalytic activity of spinel ZnFe2xCrxO4 nanoparticles on removal Orange I azo dye from aqueous solution. J Taiwan Inst Chem Eng (2013), http://dx.doi.org/10.1016/j.jtice.2013.12.002
JTICE-800; No. of Pages 6 Journal of the Taiwan Institute of Chemical Engineers xxx (2013) xxx–xxx
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Photocatalytic activity of spinel ZnFe2xCrxO4 nanoparticles on removal Orange I azo dye from aqueous solution A.I. Borhan a, P. Samoila b, Vasile Hulea c, A.R. Iordan a, M.N. Palamaru a,* a
Alexandru Ioan Cuza University of Iasi, Faculty of Chemistry, 11 Carol 1 Boulevard, R-700506, Iasi, Romania Petru Poni Institute of Macromolecular Chemistry, 41A, Gr. Ghica Voda Alley, 700487 Iasi, Romania c Institut Charles Gerhardt, UMR 5253, CNRS-UM2-ENSCM-UM1, Mate´riaux Avance´s pour la Catalyse et la Sante´, 8 rue de l’Ecole Normale, 34 296 Cedex 5, France b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 11 June 2013 Received in revised form 25 November 2013 Accepted 1 December 2013 Available online xxx
In this study the photodegradation of aqueous solution of recalcitrant organic pollutant Orange I was investigated by using chromium substituted zinc ferrite, ZnFe2xCrxO4 (x = 0, 0.25, 0.5, 1.0, 1.50, 2.0) catalysts, for the first time, under both UV and visible light irradiation. The photocatalytic process was promoted when Fe3+ cations were substituted with Cr3+ cations. Therefore, the dye removal efficiency was enhanced up to 92.8% for x = 1.50 composition when Cr3+ cations were introduced in octahedral lattice of ZnFe2O4, showing that this material contains chromium optimal dosage for photocatalytic degradation of Orange I. Further, we studied photocatalytic activity under both UV and visible light irradiation of ZnFe0.50Cr1.50O4 above Orange I dye solution. The investigation of removal kinetics of Orange I indicates that the removal processes, under both UV and visible light, obeys the rate of firstorder kinetic equation. The best photocatalytic performance was achieved after 45 min of UV light irradiation, with first order kinetic rate constant k of 5.85 102/min. Anyway, the decolorization rate under UV light is two times higher that afforded under visible light. ß 2013 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords: Heterogeneous photocatalysis Orange I dye Photodegradation Spinel ferrite UV light Visible light
1. Introduction The pollution of water resources by the dyes from industries such as textile, paper, plastic, leather, cosmetics and photographic, has become a serious environmental problem because of their toxicity, affecting the quality of life for generation to come. Most of the dyes used in textile industry usually have a synthetic origin and complex aromatic molecular structures which make them very stable and relatively difficult to be biodegraded. Consequently, the effective removal of these kinds of pollutants from wastewaters represents a great challenge to scientists [1]. Among various treatment methods employed to remove these refractory pollutants from contaminated water, photocatalytic processes have generated a great interest in the last decade [2]. In this respect, the photocatalytic processes of dye degradation mediated by TiO2-based materials under ultraviolet irradiation were widely studied [3]. Nevertheless, the main drawback toward practical application of these kind of photocatalysts is that photocatalytic processes can be activated by ultraviolet light
* Corresponding author. Tel.: +40 232201341; fax: +40 232201313. E-mail addresses: [email protected], [email protected] (M.N. Palamaru).
exclusively, which accounts for only 4% of the incoming solar energy [4]. Hence, another class of oxides, the spinel ferrites (MFe2O4, where M is a divalent metal), capable of utilizing a largest portion of the solar spectrum, studied mostly due to their electrical and magnetic properties, has gained much interest in recent years as potentials catalysts for different processes. The general formula of spinel type oxides is (A)[B2]O4, where the tetrahedral sites (A) are occupied by divalent cations (Zn2+, Ni2+, Co2+, Cu2+, etc.) and octahedral sites (B) by trivalent cations (such as Cr3+, Al3+, In3+, La3+, etc.) in a cubic structure lattice. The band gap capable of absorbing visible light, as well as the spinel crystal structure, makes these materials attractive for photocatalytic applications [2]. Among the spinel ferrites, ZnFe2O4, has received much more consideration due to its relatively small band-gap (around 1.9 eV) [5]. Also, spinel-type compounds such as ZnCr2O4 [6] or ZnAl2O4 [7] were successfully employed for the removal of dyes from wastewater via photocatalytic processes. Nevertheless, to the best of our knowledge, little research has been paid to photocatalytic properties of substituted zinc ferrites, obtained by sol–gel autocombustion method. Also, another advantage of using spinel ferrites as photocatalysts is due to their magnetic properties, making these materials recovery from the catalytic systems, in order to reuse them in other degradation processes, less expensive than for the TiO2.
1876-1070/$ – see front matter ß 2013 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jtice.2013.12.002
Please cite this article in press as: Borhan AI, et al. Photocatalytic activity of spinel ZnFe2xCrxO4 nanoparticles on removal Orange I azo dye from aqueous solution. J Taiwan Inst Chem Eng (2013), http://dx.doi.org/10.1016/j.jtice.2013.12.002
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This study reports, for the first time, the utilization of chromium substituted zinc ferrite under both UV and visible light to the photodegradation of the recalcitrant organic pollutant Orange I, which is a dye widely used in the textile industry. The synthesis via sol–gel autocombustion technique and the characterization of the ZnFe2xCrxO4 (x = 0, 0.25, 0.5, 1.0, 1.50, 2.0) nanoparticles was exposed extensively in the previous paper [8] studying the relation between structural features and magnetic and electrical properties of these materials. Therefore only the results important from the photocatalytic activity point of view are presented in the current paper. Orange I (Tropaeolin 000 No. 1) was selected as probe molecule because it belongs to the family of azo-dyes, which accounts for almost 80% of annual production of commercial dyes all over the world. 2. Experimental 2.1. Synthesis and characterization Pure nanocrystalline catalysts with the general formula ZnFe2xCrxO4 (x = 0, 0.25, 0.5, 1.0, 1.50, 2.0) were synthesized by sol–gel auto-combustion method using analytical grade iron nitrate [Fe(NO3)39H2O] (99.9%, Aldrich), chromium nitrate [Cr(NO3)39H2O] (99.9%, Aldrich) and zinc nitrate solution, obtained in situ from ZnO (99%, Sigma–Aldrich) and nitric acid (Merck) 20% solution, and tartaric acid as chelating and fuel agent. The synthesis procedure consisted in dissolving corresponding salts in distilled water followed by the addition of tartaric acid. Consequently, the obtained solutions were heated at 80 8C through gel phase transformation. The gels were gradually heated up to 350 8C until the autocombustion occurred. The obtained powders were heat treated at 500 8C/7 h and then at 700 8C/7 h. The protocol synthesis was extensively described in a previous study [8]. IR spectroscopy and XRD are methods commonly used to verify the success of synthesis and spinel structure formation [9,10]. Monitoring solid phase chemical reaction and the disappearance of the organic and nitric phases were performed by infrared spectroscopy using a Bruker TENSORTM27 with ATR cell, with 2 cm1 resolution. The powder phase composition was identified by X-ray diffraction (XRD; Model Shimadzu LabX6000 diffractometer) using CuKa radiation (l = 1.5405 A˚), for 2u ranging between 20 and 808, at a scanning speed of 0.028/s. The morphology and particle size were observed using a scanning electron microscope (SEM; Model Hitachi S2600 N Microscope). Textural properties including surface area and pore size were measured by nitrogen adsorption–desorption isotherms method at 78 K on a ASAP 2020 analyzer. The specific surface areas SBET were determined using the Brunauer–Emmett–Teller (BET) method. The mean pore size (Dpore) corresponds to maximum of pore size distribution curve. The optical absorption spectrum was recorded on a Perkin-Elmer, Lambda 35 UV–vis spectrometer, using H2O2 as reference sample, in the range of 200–700 nm. To study the light absorption of the catalysts, the diffusive reflectance spectra (DRS) of the catalyst samples in the wavelength range of 200–1100 nm were obtained using a UV–vis spectrophotometer (Shimadzu UV), with MgO as a reference.
Fig. 1. Chemical structure of Orange I dye obtained by the Hyperchem software.
irradiation. The photocatalytic degradation experiments were performed into a Pyrex cylindrical photo-reactor. A low pressure Hg lamp (Philips, Netherlands) emitting a wavelength of 185 nm was positioned at the center of the reactor and was used for photoreaction under UV light. The photo-reactor was surrounded by a Pyrex circulating water jacket in order to maintain the temperature constant during the reaction. The photocatalytic activities of the asobtained photocatalyst were evaluated by the decomposition of Orange I in an aqueous solution at constant temperature. In each experiment, the reaction suspension was prepared by adding 0.40 g of catalyst powder into 75 mL Orange I solution with an initial concentration of 80 mg/L and pH 3. The suspension was magnetically stirred in the dark for 30 min to reach adsorption–desorption equilibrium at constant temperature prior to irradiation, and then the solution was illuminated at different times. During illumination, stirring was maintained in order to keep the mixture in suspension. At regular time intervals (5, 10, 15, 30 and 45 min), the analytical samples were taken from the suspension to investigate the change in the concentration (absorbance) of each degraded solution by measuring the absorbance in range of 200–700 nm for Orange I. The absorbance of Orange I solutions were determined at 475 nm and it is used to calculate its concentration. For comparison purposes, the photoreaction under visible light illumination was performed using a 400 W high pressure sodium lamp (APF, IP65, Italy) with main emission in the range of 400– 800 nm, for the ZnFe0.50Cr1.50O4 sample only, using the same protocol described before. 3. Results and discussion 3.1. Optimal Cr3+ dosage First, the photocatalytic activity of ZnFe2xCrxO4 (x = 0, 0.25, 0.50, 1.0, 1.50, 2.0) powders under UV light was evaluated performing experiments on the degradation of Orange I in aqueous solution. In Fig. 2, a comparison of photocatalytic activity among all
2.2. Photocatalytic reaction procedure The photocatalytic activity of the ZnFe2xCrxO4 (x = 0, 0.25, 0.5, 1.0, 1.50, 2.0) catalysts was evaluated by degradation of recalcitrant textile dye Orange I (Tropaeolin 000 No. 1) with chemical formula C16H11N2NaO4S (d = 13.9 A˚) (Fig. 1) under UV-light
Fig. 2. Percentual dye removal with increasing Cr3+ content (x) in the presence of ZnFe2xCrxO4 powders.
Please cite this article in press as: Borhan AI, et al. Photocatalytic activity of spinel ZnFe2xCrxO4 nanoparticles on removal Orange I azo dye from aqueous solution. J Taiwan Inst Chem Eng (2013), http://dx.doi.org/10.1016/j.jtice.2013.12.002
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studied photocatalysts at the same operation conditions and after 45 min of illumination is shown. Orange I was a relatively stable azo dye and the photodegradation using ZnFe2O4 powder under Hg lamp irradiation was relatively slow and only 36.8% of Orange I could be removed in 45 min irradiation time. It is important to note that chromium substitution of nanocrystalline zinc ferrites samples could modify significantly the photocatalytic activity. Hence, the photocatalytic process was promoted when Fe3+ cations were substituted with Cr3+ cations, in any studied proportion. Therefore, the dye removal efficiency was enhanced up to 92.8% for x = 1.50 sample when Cr3+ cations were introduced in octahedral lattice of ZnFe2O4. From Fig. 1, we conclude that ZnFe0.50Cr1.50O4 compound contains chromium optimal dosage for photocatalytic degradation of Orange I. Further, we were studied photocatalytic activity under visible light irradiation of ZnFe0.50Cr1.50O4 above Orange I dye solution. Thus, we will discuss below only the photodegradation of Orange I dye under UV and visible light irradiation in the presence of ZnFe0.50Cr1.50O4 catalyst.
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Fig. 4. X-ray diffraction pattern for ZnFe0.50Cr1.50O4 powder.
3.2. Catalyst characterization XRD patterns, IR spectra and SEM micrographs of studied ZnFe2xCrxO4 (x = 0, 0.25, 0.50, 1.0, 2.0) samples were not included because they were reported in a previous work [8]. For the sake of simplicity, only the characterizations of ZnFe0.50Cr1.50O4 sample are presented in this work. Nevertheless, we can remind that XRD and IR analysis revealed pure spinel cubic structure for all powders heated at 700 8C. Also, SEM micrographs showed that spherical grains with uniform size distribution and average particle size varying between 72.2 and 56.5 nm were obtained. The IR spectra, in the wavenumber range 4000–300 cm1, for ZnFe0.50Cr1.50O4 powder, heated at 700 8C, shows the presence of two main absorption peaks characteristic of spinel structure (Fig. 3). These two peaks suggest the formation of the spinel-type structure. Absorption peak observed at 602 cm1 is assigned to y1 wavenumber and that of 488 cm1 is attributed to y2 wavenumber [11]. The strongest absorption peak y1 is caused by intrinsic vibration of bonds between metal ions and oxygen ions in tetrahedral positions, whereas the weakest absorption peak y2 is assigned to the stretching vibrations of bonds between octahedral metal ions and oxygen ions [12]. Fig. 4 shows the XRD pattern of ZnFe0.50Cr1.50O4 powder proving that the catalyst structure was dominated exclusively by the spinel structure. All reflections peaks were identified and
indexed to cubic phase of spinel in good agreement with the referred database of International Center for Diffraction Data [891009 ICDD, J. Alloys Compds. 1998] [87-1164 ICDD, Mater. Res. Bull. 1996] [84-0314 ICDD, J. Appl. Phys. 1980] [89-1397 ICDD, Solid State Commun. 1979]. The average crystallite size was estimated by applying the Scherrer formula on the highest intensity peak and was obtained a size of 24.50 nm. Fig. 5 shows the SEM image of ZnFe0.50Cr1.50O4 powder. SEM image show that the spinel nanoparticles consist of uniform and homogeneous quasi-spherical grains with an average size of 45 nm. This value is significantly lower than obtained for x = 0, 0.25, 0.50, 1.0, 2.0 samples (between 56.5 and 72.2 nm) [8] and could explain the best performance in photocatalytic activity. Nitrogen adsorption–desorption isotherm for ZnFe0.50Cr1.50O4 and the corresponding average pore size distributions (insert) are shown in Fig. 6. Although this compound ZnFe0.50Cr1.50O4, had not a large specific surface area (around 60.8 m2/g) it showed a remarkably photocatalytic performance for Orange I degradation (92.80%). According to IUPAC classification, the isotherm is of type IV with a H1 hysteresis loop, representing predominantly mesoporous structure, which was confirmed by the analysis of pore size distribution [13]. The H1 type hysteresis is characteristic for pores with uniform size and shape formed from aggregates and agglomerates particles [14]. The pore size distribution curve
Fig. 3. IR spectrum for ZnFe0.50Cr1.50O4 powders.
Fig. 5. SEM micrograph of ZnFe0.50Cr1.50O4 nanoparticles.
Please cite this article in press as: Borhan AI, et al. Photocatalytic activity of spinel ZnFe2xCrxO4 nanoparticles on removal Orange I azo dye from aqueous solution. J Taiwan Inst Chem Eng (2013), http://dx.doi.org/10.1016/j.jtice.2013.12.002
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Fig. 6. Nitrogen adsorption–desorption isotherm and pore size distribution (insert) of ZnFe0.50Cr1.50O4.
displays a bimodal distribution [15] with pore sizes of 4.3 nm and 11.4 nm. The maximum diagonal length of Orange I dye molecule is 1.39 nm (see Fig. 1), therefore the ratios between the pore diameters of the catalyst and the maximum diagonal length of Orange I dye are 3.1 for the smaller pores and 8.2 for the larger pores. This implies each particle pore of ZnFe0.50Cr1.50O4 catalyst could accommodate a certain number Orange I dye molecules which was diffused from the dye solution to the pores of the catalyst particle, in both small and large pores. These results indicate that the photocatalytic activity probably depends on many factors, besides the chemical composition, such as specific surface area value [16], the pore sizes and number, or grain size (see SEM section). Fig. 7 shows the room temperature UV–vis absorption spectra of ZnFe2xCrxO4 (x = 0, 1.50) catalysts derived from the diffuse reflectance (R) spectrum using Kubelka–Munk function [17] F(R) = (1 R)2/2R, which is plotted as a function of energy (eV). From the DRS spectra, a shoulder centered around 1.96 eV corresponds to 6A1g ! 4T2g excitation which arises due to the d–
d crystal field excitations of Fe3+ ions (3d5 – high spin configuration; t2g3 eg2) in spinel lattice [18]. Above 2 eV, absorption increases significantly with two shoulders centered at 2.5 eV and near 2.96 eV. These two features were assigned to the charge transfer excitations Fe3+–O–Fe2+ [18]. As can be seen, in the case of ZnFe0.50Cr1.50O4, these two shoulders are lower, because the chromium ions do not participate in the conduction process, but decrease the number of electron hopping (Fe3+–O–Fe2+) between adjacent B sublattices. It also clearly observed the appearance of an absorption maximum at 4.5 eV, which can be attributed to 2E (t2g3) ! 4T2g (t2g2 e) excitation which arises due to the d–d crystal field excitations of Cr3+ ions [19]. A shift in this transition bands clearly indicates that substitution of Cr3+ in ZnFe2O4 leads to changes in octahedral lattice, as a consequence of contraction in unit cell volume. The Tauc plot is a method that is widely used for the determination of band gap. The estimated band-gaps of the ZnFe2O4 and ZnFe0.50Cr1.50O4 nanoparticles are found to be 1.6 and 1.3 eV, respectively. Compared with the ZnFe2O4 band-gap value of 1.6 eV, the optical absorption edge shifted slightly to the red direction for ZnFe0.50Cr1.50O4 catalysts. That is helpful to the improvement of photocatalytic activity under visible light. Note that the value of the indirect band gap decreases with the increase in the Cr content, because is affected by the decrease of various factors such as crystallite size, structural parameter (lattice constant) [8]. 3.3. Evaluation of photocatalytic activity 3.3.1. Dye removal by ZnFe0.50Cr1.50O4 powder The photocatalytic activity of ZnFe0.50Cr1.50O4 catalyst was examined by the photodegradation (decolorization) of Orange I in aqueous solution under UV light and visible light irradiation. The Orange I absorption spectra is dominated by a strong absorbance band at 475 nm due to the n–p* transition of the azo, –N5 5N–, group. Also, characteristic bands are usually observed around 240, 270, 290 and at 330 nm assigned to the p–p* transitions related to aromatic rings. The discoloration reaction occurs once the azobond disappear, meaning the disappearing of the characteristic band at 475 nm [20]. Fig. 8(a) and (b) shows the changes of the optical densities of 475 nm (–N5 5N–), 330 nm, 290 nm (naphthalene ring) and 240 nm
Fig. 7. Plots of (F(R)hy)2 versus the incident photon energy (hy) of ZnFe2O4 and ZnFe0.50Cr1.50O4 nanoparticles.
Please cite this article in press as: Borhan AI, et al. Photocatalytic activity of spinel ZnFe2xCrxO4 nanoparticles on removal Orange I azo dye from aqueous solution. J Taiwan Inst Chem Eng (2013), http://dx.doi.org/10.1016/j.jtice.2013.12.002
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Fig. 8. The change of absorption spectra of Orange I in presence of ZnFe0.50Cr1.50O4 powder under (a) UV light irradiation and (b) visible light irradiation.
(benzene ring) bands of orange I at different illumination time under UV light and visible light in the presence of ZnFe0.50Cr1.50O4 powder. It must be noticed that the dye concentration decreases quickly, hence, adsorption peaks of Orange I became weaker with the irradiation time, which indicates that both azo groups and aromatic parts of the dye molecule decompose under both UV light and visible light. Therefore, the photodegradation rate of Orange I over ZnFe0.50Cr1.50O4 nanoparticles increase rapidly with increasing UV irradiation time, and reaches 92,80% in 45 min, while the decolorization rate under visible light is two times lower, i.e. 49% in 45 min. Nevertheless, the behavior of ZnFe0.50Cr1.50O4 sample under visible light recommend this material as an effective photocatalyst, compared to the results previously reported in the literature concerning different azo dyes photodegradation [2]. 3.3.2. Chemical kinetic removal The degradation kinetics of Orange I on ZnFe0.50Cr1.50O4 catalyst under UV light and visible light could be analyzed by the first order kinetic model: Ct (1) ln ¼ kt C0 where k is the apparent reaction rate constant, and C0 and Ct are the initial concentration and the concentration of Orange I at reaction time t, respectively [21]. On the basis of the experimental data, the
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Fig. 9. (a) Photocatalytic degradation and (b) photodegradation kinetics of Orange I over ZnFe0.50Cr1.50O4 powder under UV irradiation and visible light.
plots of (Ct/C0) and ln(Ct/C0) versus time are depicted in Fig. 9(a) and (b), respectively. There can be observed that the photocatalytic activity was significantly higher under UV irradiation than under visible light. The best photocatalytic performance was achieved after 45 min of UV light irradiation, with first order kinetic rate constant k of 5.85 102/min. Under visible light, the first order kinetic rate constants k was about 4 times lower, i.e. k = 1.54 102/min. These results are in agreement with literature. Thus, the catalytic performance of ZnFe0.50Cr1.50O4 catalyst under visible light is identical to that obtained by Liang et al. [22] using TiO2 for degradation of Orange I. Moreover, ZnFe0.50Cr1.50O4 catalyst shows a performance twice higher than TiO2, but comparable with those of RE3+ (Sm3+, Nd3+, Pr3+) doping in TiO2 for Orange I dye degradation under UV light. According to the literature, for a high degradation efficiency of Orange I, are required an optimal dosage of trivalent cation Cr3+ in zinc ferrite and high band gap energy [21], besides surface area, the adsorption capacity and the surface acid–base properties [23]. There can be observed that Cr3+ substitution with x = 1.50 is the optimal dosage in zinc ferrite for the most efficient separation of photo-induced electron hole pairs. Therefore, by substituting Fe3+ with Cr3+ in zinc ferrite, the surface barrier becomes higher, and the space charge region becomes narrower. This variation occurs because Fe3+ ions substitution by Cr3+ ions no limits the Fe3+ $ Fe2+ electronic
Please cite this article in press as: Borhan AI, et al. Photocatalytic activity of spinel ZnFe2xCrxO4 nanoparticles on removal Orange I azo dye from aqueous solution. J Taiwan Inst Chem Eng (2013), http://dx.doi.org/10.1016/j.jtice.2013.12.002
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exchange, such as normal, and thus a variable number of electron exchanges are free to interact with the large electrical field. This increases the probability that electrons reach the surface barrier, resulting in a decrease in the space charge polarization. The electron hole pairs within the region are efficiently separated by the large electric field before recombination which led to the higher photocatalytic activity [21]. However, degradation was more effective under UV irradiation, which means that the band gap energy was much lower in the case of irradiation with visible light. 4. Conclusions The photocatalytic activity of pure spinel phase ZnFe2xCrxO4 powders (x = 0, 0.25, 0.5, 1.0, 1.50, 2.0) was examined by the degradation of Orange I in aqueous solution under UV light irradiation. The photocatalytic process was promoted when Fe3+ cations were substituted with Cr3+ cations. The best dye removal efficiency was obtained for x = 1.50 sample, especially due to his smaller grain size as observed from SEM analysis, showing that this material contains chromium optimal dosage for photocatalytic degradation of Orange I. Also, the photocatalytic performance under visible light of ZnFe0.50Cr1.50O4 material above Orange I dye solution was successfully studied. Hence, nanoparticles of ZnFe0.50Cr1.50O4 can act as photocatalyst of the degradation of azo dyes in an aqueous solution. Anyway, the decolorization rate under visible light is two times lower that afforded under UV light. The best photocatalytic performance was achieved after 45 min of UV light irradiation, with first order kinetic rate constant k of 5.85 102/min. As stated above, a very huge advantage of using these spinel ferrites as photocatalysts is because these materials can be recovery from the catalytic systems, in order to reuse them in other degradation processes, less expensive than the TiO2. Acknowledgements This work was supported by the European Social Fund in Romania, under the responsibility of the Managing Authority for the Sectorial Operational Programme for Human Resources Development 2007-2013 [grant POSDRU/107/1.5/S/78342]. Also, P. Samoila kindly acknowledge the European Union’s Seventh Framework Programme (FP7/2007–2013) under Grant Agreement No. 264115–STREAM. References [1] Sharma R, Singhal S. Structural, magnetic and electrical properties of zinc doped nickel ferrite and their application in photo catalytic degradation of methylene blue. Physica B 2013;414:83–90.
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Please cite this article in press as: Borhan AI, et al. Photocatalytic activity of spinel ZnFe2xCrxO4 nanoparticles on removal Orange I azo dye from aqueous solution. J Taiwan Inst Chem Eng (2013), http://dx.doi.org/10.1016/j.jtice.2013.12.002