Conversion of Natural Clinoptilolite Microparticles to Nanorods by

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Conversion of Natural Clinoptilolite Microparticles to Nanorods by Glow Discharge Plasma: A Novel Fe-Impregnated Nanocatalyst for the Heterogeneous Fenton Process Alireza Khataee,*,† Soghra Bozorg,† Sirous Khorram,‡ Mehrangiz Fathinia,† Younes Hanifehpour,§ and Sang Woo Joo*,§ †

Research Laboratory of Advanced Water and Wastewater Treatment Processes, Department of Applied Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, 51666-14766, Iran ‡ Research Institute for Applied Physics and Astronomy, University of Tabriz, Tabriz, 51666-14766, Iran § School of Mechanical Engineering, Yeungnam University, Gyeongsan 712-749, South Korea ABSTRACT: In the present study, the properties of natural clinoptilolite were successfully modified by a glow discharge plasma technique. A thorough characterization was performed to clarify the morphology, chemical composition, and porosity of untreated and plasma-treated clinoptilolites. The scanning electron microscopy images of untreated and plasma-treated clinoptilolites demonstrated that the morphology of clinoptilolite was changed from microparticle to nanorod after plasma treatment. The Brunauer−Emmett−Teller results showed that the plasma treatment has enhanced the specific surface area of clinoptilolite up to 2-fold (treated clinoptilolite 45.16 m2/g compared to the untreated one 23.92 m2/g). The catalytic performance of untreated and plasma-treated Fe-impregnated clinoptilolites was compared in the heterogeneous Fenton process for the decolorization of a textile dye solution. Results indicated that the decolorization efficiency was significantly increased from 36.93%, in the presence of untreated catalyst, to 97.66% with the use of the plasma-treated catalyst after 35 min of reaction.

1. INTRODUCTION In most of industrial factories, including the textile, cosmetic, paper, leather, pharmaceutical, and food industries, large amounts of dyes are annually produced and applied in many different sections. The introduction of these huge amounts of pollutants, including synthetic dyes, from factories to the environmental wastewater streams leads to chronic pollution of water sources.1−3 Also, various problems related to their carcinogenicity and toxicity to aquatic life could appear. Moreover, dyeing effluents are difficult to treat because of their resistance to biodegradability and their stability to light, heat, and oxidizing agents.4,5 So the development of new techniques for the management of wastewater resources is crucial. The simplicity and high efficiency of advanced oxidation processes (AOPs) make them a proper choice for removing toxic and recalcitrant chemicals from wastewater in recent years.6−10 Among various AOPs, the use of H2O2 and Fenton reagent (homogeneous Fenton process) has been extensively reported as a homogeneous catalytic process for the removal of various pollutants.6,11−13 However, the homogeneous Fenton process has some well-known disadvantages including a narrow pH range, the production of iron-containing sludge, and the catalyst deactivation by the produced intermediate.7,14 These drawbacks can be removed with the use of heterogeneous Fenton-type catalysts such as iron-substituted synthetic and natural zeolites,15 laponite,16 and pillared clays.17 Among mentioned catalysts, iron-substituted zeolites have been efficiently used within heterogeneous Fenton-type processes because of their unique physical and chemical properties including crystallinity, stability in thermal and chemical © 2013 American Chemical Society

processes, regular cage structure in molecular size, and ability to ion-exchange.18−21 Zeolites are porous crystalline hydrated aluminosilicates.22,23 Their structure consists of an infinitely expanded threedimensional network of AlO 4 5− tetrahedral and SiO 4 4− tetrahedral units connected by shared oxygen atoms.24−26 Each aluminum ion in the zeolite framework gives a net negative charge, which is balanced by an extra framework cation, usually from group IA or IIA. Their structure consists of regular channels or cages of size ranging between 3 and 20 Å occupied by the charge balancing ions and water molecules.26−28 As it was mentioned above, both natural-ironsubstituted and synthetic-iron-substituted zeolites were used within the heterogeneous Fenton process. Clinoptilolite, the most abundant natural zeolitic mineral, has been widely used in research studies owing to its abundance and considerable low cost.24,29 In this context, clinoptilolite exhibits no toxicity and makes no environmental pollution, so these properties spark our interest to investigate the substitution of the homogeneous catalyst with clinoptilolite in this work.18,20,30 Despite the mentioned novel properties for this natural zeolite, still some matters remain. One of them is related to the surface area of the clinoptilolite. Its low specific surface area considerably influences the reaction rate in many catalytic reactions. This subject not only decreases its catalytic efficiency, but also limits its application. In terms of catalysts or Received: Revised: Accepted: Published: 18225

October 2, 2013 November 21, 2013 November 28, 2013 November 28, 2013 dx.doi.org/10.1021/ie403283n | Ind. Eng. Chem. Res. 2013, 52, 18225−18233

Industrial & Engineering Chemistry Research

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Table 1. Characteristics of C.I. Acid Red 17

were purchased from Merck Co. (Germany). The natural clinoptilolite tuffs were obtained from the Mianeh region in the northwest of Iran (Kan Azar Co., Tabriz, Iran). AR17 was purchased from Alvan Saabet Co. (Tehran, Iran). The characteristics of AR17 are given in Table 1. 2.2. Plasma Treatment Procedure. The experimental apparatus for plasma treatment procedure is shown in Figure 1.

reagents for water purification, the surface area of the catalyst will affect how fast the reaction occurs. This is because the reaction between the different types of molecules such as dye, H2O2, and •OH molecules, in the case of the heterogeneous Fenton process, takes place on the surface of a solid catalyst. Therefore, a larger surface area of the solid catalyst allows for a faster reaction. In this context, nanomaterials offer a high specific surface area. Increasing the catalyst surface area of clinoptilolite should increase the quantity of substance available to react and thus increase the rate of the reaction which might positively influence the process efficiency.31 So in comparison with traditional zeolites, nanosized zeolites are significantly the proper choice for catalysis applications due to the mentioned features.32 The discussed novel applications and properties of nanosized clinoptilolite depend on its structure and the composition of its surface, both of which can be changed in response to variations in the environment. Recently various synthetic methods have been developed to produce the nanosized zeolites with various morphologies.33 However, most of the used synthesis processes generally require highly sophisticated equipment and toxic metal− organic precursors.7 More recently, nonthermal plasma techniques including glow discharge, silent discharge, and radio frequency (RF) discharge have been used for surface modification34−36 and improvement of the stability, acidity, and activity of catalysts.37,38 Nevertheless, there have been no reports regarding the production of nanosized clinoptilolite particles under glow discharge plasma. So, in this work a novel method to prepare clinoptilolite nanorods using the glow discharge plasma technique is presented. Then, the obtained nanorods were modified with iron to be used as a heterogeneous Fenton-type catalyst for decolorization of C.I. Acid Red 17 (AR17) solution. The physical and chemical properties of the produced samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopic (EDX), Brunauer−Emmett−Teller (BET), Fourier transform infrared spectroscopy (FT-IR) and inductively coupled plasma (ICP) analysis. Also, the possible mechanism for the formation of nanorods was investigated. To the best of our knowledge, there is no previous literature report concerning the use of Feimpregnated clinoptilolite nanorods in the heterogeneous Fenton process.

Figure 1. Schematic diagram of the glow discharge plasma system used in this study.

The glow discharge plasma reactor is made of a pyrex tube reactor with size of 40 cm × 5 cm. The plasma is generated by the two electrodes connected to a DC high-voltage (1200− 1300 V) power supply (Tabriz, Iran). During the plasma treatment, the dried 1 g clinoptilolite powder sample was laid on the pyrex plate and located in the positive column region of the tube. Before the generation of discharge, the tube was evacuated, and N2 was applied as the plasma-forming gas in this work. During the plasma treatment, feed gas was introduced into the reactor at different pressure ranges of 20−30 Pa. Also, the plasma treatment time ranged from 15 to 60 min. Then, the catalytic efficiency of the obtained samples (under different feed gas pressures and various plasma treatment times) was assessed for decolorization of AR17 in the heterogeneous Fenton process. The achieved results confirmed that the sample treated under 26 Pa with the plasma treatment time of 60 min showed the highest catalytic activity. So, the feed gas of 26 Pa and the plasma treatment of 60 min were selected as the optimum operating pressure and time. 2.3. Preparation of Fe-Impregnated Untreated and Plasma-Treated Clinoptilolite Samples. The Fe-impregnation process was performed on the separate samples of untreated and plasma-treated clinoptilolite as follows: the Feimpregnated untreated clinoptilolite was prepared by mixing 50

2. EXPERIMENTAL SECTION 2.1. Reagents and Materials. All chemicals used in this study were of analytical grade and were used without further purification. FeCl3, 6H2O, NaOH, H2SO4, and H2O2 (30%) 18226

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g of untreated sample with 150 mL of 0.125 M FeCl3.6H2O solution, and 200 mL of 2 M NaOH solution in a pyrex flask with the total volume of 500 mL. The addition of NaOH solution was rapid with stirring. The flask was kept in a water batch at 60 °C for 6 h. Then, the obtained sample was washed several times with distilled water and dried in an oven in air at 60 °C overnight. This procedure was repeated for three times in order to get the Fe-impregnated untreated clinoptilolite sample. The Fe-impregnation process for the plasma-treated clinoptilolite was the same as that of the untreated sample as mentioned earlier. 2.4. Heterogeneous Fenton Process. All reactions were performed in a batch glass reactor equipped with a magnetic stirrer at atmospheric pressure and ambient temperature. In a typical run, 2 g/L of Fe-impregnated catalyst was placed into 500 mL of an aqueous dye solution (20 mg/L). In all the experiments, the pH value was adjusted to 5 by the addition of 0.1 M H2SO4 to solution. Then, 3 mM H2O2was added. At different time intervals during 35 min, 3 mL of the sample was taken and the concentration of the dye in the sample was obtained by measuring absorbance at maximum wavelength (λmax = 510 nm) using UV−vis spectrophotometer, Lightwave S2000 (England). The color removal efficiency (CR (%)) was expressed as the percentage ratio of decolorized dye concentration to that of the initial one. 2.5. Characterization Instruments. To prepare the clinoptilolite sample for XRD analysis, first the initial sample was powdered. Then, the particles were sorted according to their size distribution. After treatment of the sample with plasma, the untreated and plasma-treated samples were analyzed by XRD analysis (PANalytical-X’Pert PRO diffractometer, Eindhoven, The Netherlands). XRD analysis was used to identify the crystal structure and phase purity of the untreated and plasma-treated clinoptilolite samples with Cu Kα radiation (40 kV, 30 mA) and a PIXcell solid state detector. The patterns were recorded at room temperature with stepsizes of 0.02°. The surface morphology, the structure, and the chemical composition of the prepared samples were obtained with the aid of a SEM (S-4200, Hitachi, Japan) equipped with an EDX microanalysis at an acceleration voltage of 10 kV. Moreover, the obtained SEM image was analyzed using Manual Microstructure Distance Measurement software (Nahamin Pardazan Asia Co., Iran) to determine the diameter and length size distribution of the obtained samples. Nitrogen sorption analyses were obtained with a sorptometer (Micromeritics, ASAP-2010) using standard continuous procedures at 77.35 K on calcined samples that had been degassed at 363 K for 1 h and then at 403 K under high vacuum for at least 10 h. The surface area was calculated according to the BET model over a relative pressure range of 0.009−0.90. The FT-IR spectra in the 4000−400 cm−1 range were recorded for the both untreated and plasma-treated samples using a FT-IR spectrometer (Tensor 27, Bruker, Germany) by the KBr pellet technique. For each sample, four scans in the 4000−400 cm−1 spectral ranges were recorded with a resolution of 4 cm−1. Fe concentration in the bulk of the untreated and plasma-treated samples was determined by ICP (GBC Integra XL, Australia).

Figure 2. Powder X-ray diffraction patterns of (a) untreated and (b) plasma-treated clinoptilolite samples.

heulandites framework with agglomerated sheet-like crystals. The XRD peaks at 2θ of 10.9°, 17.53°, 22.7°, and 27.63 are found to be in good agreement with the data of clinoptilolite, JCPDS card (83-1260).18,27 In addition, quartz, feldspar, and biotite phases are detected in minor quantities in X-ray diffraction patterns.21 Figure 2b demonstrates the XRD peaks of the plasma-treated clinoptilolite sample. The sharpness of the peaks after modification with plasma, confirms good crystallinity and structural stability of the sample. Therefore, the applied treatment did not simulate the destruction of the clinoptilolite structure significantly. However, as can be seen in Figure 2b, the intensities of some peaks are decreased and also some peaks are slightly shifted to the lower 2 theta values. These results are relevant with the previously reports in the field of modification of zeolite by plasma.18,39 In some of them, the authors attributed the intensity and position variations of the peaks to a less ordered zeolite framework due to the further modification of zeolite structure with plasma.18,40,41 Moreover, it can be seen from Figure 2b that the characteristic peak of plasma-treated clinoptilolite is much broader than that of untreated one. Peak broadening in the XRD pattern depends on the crystallite size, microstrains, and diffractometer characteristics. Herein, it suggests that the particle size of plasma-treated clinoptilolite is smaller compared to that without plasma treatment, which is confirmed by our further analysis. 3.2. SEM and EDX Analysis. The comparative SEM images of the untreated and plasma-treated clinoptilolite samples along with the diameter and length size distribution plots of plasmatreated clinoptilolite samples are presented in Figures 3 and 4.

3. RESULTS AND DISCUSSION 3.1. XRD Analysis. XRD patterns of the untreated and plasma-treated clinoptilolite samples are depicted in Figure 2a,b, respectively. Figure 2a shows that the XRD pattern of the untreated clinoptilolite is similar to the XRD pattern of the 18227



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the diameter and length size distribution of 35% and 25% of the nanorods are in the range of 30−40 nm and 500−1000 nm, respectively. Table 2 shows the elemental composition of untreated and plasma-treated clinoptilolite samples obtained from the EDX analysis. This table confirms the existence of the major elements such as Al, Si, and other extant ions in the structure of zeolite. Also, as it can be seen from this table, the mole ratio of Si/Al was not changed significantly after treatment; however the Na/Al and K/Al mole ratio was increased for plasmatreated samples. This shows that the crystal structure of the parent clinoptilolite was not destroyed or changed significantly after plasma treatment. So, any changes in the morphology of the samples after plasma treatment can be related to the etching and oxidation effects of plasma. Also, the EDX data of the plasma-treated clinoptilolite sample (Table 2) exhibits the same major components of clinoptilolite in comparison to the untreated one. This clarifies that the plasma treatment did not obviously change the composition of the clinoptilolite sample, and thus the alteration of the morphology from microparticles to nanorods was not initiated by the compositional variation of the clinoptilolite structure.42 Moreover, as it was described, this alteration is related to the oxidation and engraving effects of plasma. The interaction of the plasma with the clinoptilolite could be hypothesized in the following paragraph. It is well-known that plasma is a collection of energetic electrons and reactive species.43 These species can cause unique and diverse chemical reactions in the contact compound. Lui et al.40 confirmed that the observed characteristic changes in the catalyst structure after plasma treatment were not due to the thermal effect during plasma treatments. They related the obtained changes both in the structure and performance of the catalyst to other effects. In the response to what exactly happened to the catalyst powder during the nonthermal plasma treatment, an electronic mechanism as follows has been proposed.40,44 When the catalyst powder is placed in the nonthermal plasma region, the particles of the catalyst perform as electron sinks. Each catalyst particle can be charged up to thousands of electrons. Also, in another work by Liu et al,44,40 it was confirmed that the density of electrons in glow discharge plasma was dramatically decreased because of the presence of catalyst powders which led to the trapping of electrons. These trapped electrons form a plasma sheath around the catalyst particles. The electron flow in the plasma exposes a strong repulsive force on the sheath. At the same time, strong Coulomb repulsions exist between the electrons trapped on the same particle. According to the above mechanism, herein, it is believed that upon the encounter of clinoptilolite particles to the plasma species, potent repulsive forces were produced inside the structure of the clinoptilolite. The powerful forces caused the bonds between the main elements such as Al, Si, and O to possibly be elongated or distorted. Finally, under this condition, the deformed bonds are easily split if other energetic species collide with them, resulting in a drastic change in the morphology and crystal structure. 3.3. N2 Absorption−Desorption Isotherms. Figure 5 shows the nitrogen adsorption isotherms measured at 77.35 K for the untreated and plasma-treated clinoptilolite samples. A simple inspection proposes that the experimental isotherms are of Type IV, according to the IUPAC classification.45 However, a more detailed observation of the first region reveals that it is a combination of Type IV and Type V isotherms. Therefore, the isotherms can be suitably defined by the BET surface area

Figure 3. (a, b and c) SEM images of untreated clinoptilolite samples.

The SEM micrograph of natural clinoptilolite presents the agglomerated, irregular, and rough surface with lamellar-like texture of clinoptilolite according to the literature data. Also according to Figure 3 panels a−c, its diameter is more than 0.5 μm. The SEM micrographs of plasma-treated clinoptilolite sample with different magnification are shown in Figure 4a− c. In these figures, highly uniform and rod-shaped nanocrystals which are distributed spatially in regular direction can be observed. These figures confirm that the morphology of clinoptilolite particles is completely changed to nanorods after plasma treatment. Additionally, the diameter and length size distribution plots of nanorods in Figure 4d−e show that 18228



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Figure 4. SEM images (a, b, and c) along with the diameter and length size distribution plots (d, e) of plasma-treated clinoptilolite sample.

Table 2. Elemental Composition of Untreated and PlasmaTreated Clinoptilolite Samples Obtained by EDX Analysis weight (%)

Table 3. Surface area Characteristics of Untreated and Plasma-Treated Clinoptilolite Samples

mole ratio

sample

untreated

plasma-treated

K/Al

specific surface area (m2/g) external surface area (m2/g)

23.9257 9.9756

45.1640 10.8801

element

Na

Al

Si

K

Si/Al

Na/ Al

untreated sample plasma-treated sample

3.58

7.07

60.33

0.72

8.26

1.25

0.15

8.86

4.81

44.27

10.94

8.88

4.52

3.23

species and excited species (including vibrationally, rotationally, and translationally excited species) which can cause powerful repulsive forces inside the crystal structure and cause the bonds between the main elements such as Al, Si, and O to be deformed. It is believed that this bond deformation will lead to the further splitting of the bonds and restructuration of the crystal structure. Also, as it was mentioned in section 3.2, it is hypothesized that, this procedure modifies the surface adsorption properties of the samples (see Table 3), which leads to the increase of the specific surface area. 3.4. FT-IR Spectroscopy. The FT-IR spectra of untreated and plasma-treated clinoptilolite samples are shown in Figure 6. For the untreated clinoptilolite sample, Figure 6a, the peak at 445.10 cm−1 is attributed to the bending of the bonds inside TO4 (T = Si and Al) and symmetric stretching of the free tetrahedral group TO4. The other peak at 611.82 cm−1 is assigned to the asymmetric stretching vibrations of T−O. Also the peaks at 796.53 and 1046.09 cm−1 are attributed to asymmetric O−T−O stretching vibration and are sensitive to the content of the framework Si and Al.18,27,46 The FT-IR spectrum of the plasma-treated clinoptilolite sample (Figure 6b) shows that some of the FT-IR vibrations have been replaced by others which are less intense and broader, while some have disappeared. As an example, the intensity of the peaks at 796.53 and 1046.09 cm−1 are reduced, and the intensity of the peak at 611.82 cm−1 is increased due to the plasma treatment. Also, noticeable changes were also detected

Figure 5. Adsorption−desorption isotherm of N2 for untreated and plasma-treated clinoptilolite samples.

method. Table 3 shows the changes in the microstructural properties of the samples. It is obvious that the specific surface area of the plasma-treated clinoptilolite sample have been improved from 23.93 to 45.16 m2/g in comparison with the untreated sample. As it was mentioned previously, plasma is a complex mixture that contains electrons, ions, photons, neutral 18229



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By comparison of our achieved results with the literature reports, novel consequences of the plasma treatment on the physical and chemical properties of the clinoptilolite can be observed in this work. In previous studies,39,47,48 the structural properties of the synthetic zeolites such as the acidity, basicity, activity, and surface structure were changed after the plasma treatment. Liu et al.39 modified the Pd/HZSM-5 catalyst using glow discharge plasma. They reported that the catalytic activity and stability increased as a result of plasma treatment. Yagodovskaya and co-workers49,50 reported the preparation of the Fe2O3/ZSM-5 catalyst for hydrogenation of carbon monoxide using the glow discharge of oxygen and argon. They attributed the higher activity and selectivity of plasmamodified samples to the total decomposition of iron nitrate into the amorphous Fe2O3 and NO2 as a result of treatment by oxygen or argon glow discharge. With the incorporation of our achieved results from XRD, SEM, EDX, BET, and FT-IR, the probable influence of plasma treatment on the clinoptilolite structure is proposed as follows. By locating the clinoptilolite particles in the plasma zone, first the catalyst particles trap the electrons, and then a plasma sheath is formed around each particle. Finally due to the electron flow in the plasma zone intensive repulsive forces are produced both inside the catalyst particles and plasma sheath around the same particle. The bonds between the main elements of the clinoptilolite structure are possibly stretched out, distorted, and split, leading to the morphological and structural transformations. Figure 7 shows the schematic representation of plasma treatment sequences. It should be considered that how plasma affects the catalyst powder is a challenge, and further theoretical and experimental investigation should be conducted to propose the exact mechanism. Herein, we have proposed a possible mechanism according to our achieved results. It is worth noting that the utilization of Feimpregenated clinoptilolite nanorods is a novel concept and might open more doors toward application of such products in the field of water treatment. 3.5. Activity of the Catalysts in the Heterogeneous Fenton Process. Fe-impregnated clinoptilolite is commonly used as a catalyst in the heterogeneous Fenton process for degradation of organic dyes from contaminated solutions.22,51−54 Therefore, the performance of the Fe-impregnated-untreated (sample (a)) and Fe-impregnated-plasmatreated clinoptilolite samples (sample (b)) was evaluated in the removal of AR17 from aqueous solution. The amount of iron in the samples was determined by ICP analysis. The results from ICP analysis indicated that the amount of iron in samples (a) and (b) were 7.5 and 14.5 mg/g, respectively. Particularly,

Figure 6. FT-IR spectra of (a) untreated and (b) plasma-treated clinoptilolite samples.

in the spectrum of the plasma-treated clinoptilolite sample which are relevant to the structural Si−OH−Al vibrations in the region between 3234.91 and 3551.63 cm−1.27 The appearance of these new peaks demonstrates the formation of new Si− OH−Al bonds during plasma treatment. Consequently, we can conclude that there are powerful Coulombic repulsions between the electrons trapped both in the clinoptilolite particles and plasma sheath; these strong Coulombic repulsions maybe the main reason for any breaking and formation of the bonds, which were confirmed by the FTIR results. The finding from this section confirms the result obtained in sections 3.1 and 3.2, in which the structural and morphological changes after plasma treatment were discussed and justified.

Figure 7. Schematic representation of plasma-treatment sequences. 18230



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taking the BET results into account, the enhanced specific surface area should have a strong influence on the iron impregnation capacity of sample (b). The untreated clinoptilolite sample possesses the lower specific surface area in comparison with the treated sample, which leads to the decreased iron impregnation capacity. Figure 8 shows the

Figure 8. (a) Evaluating the catalytic properties of Fe-impregnateduntreated (sample (a)) and Fe-impregnated-plasma-treated clinoptilolite (sample (b)) in the heterogeneous Fenton process; and (b) the normalized color removal (%) to the amount of iron in the samples for decolorization of 20 mg/L AR17 solution in the presence of 3 mM H2O2, 2.0 g/L catalyst, and pH = 5.

Figure 9. Reusability behavior of sample (a) and sample (b) in the heterogeneous Fenton process for decolorization of 20 mg/L AR17 solution in the presence of 3 mM H2O2, 2.0 g/L catalyst, and pH = 5 .

4. CONCLUSIONS In the present study, natural clinoptilolite was converted to nanorods by glow discharge plasma technique. Novel results were obtained in the physical and chemical properties of the plasma-treated clinoptilolite sample based on the XRD, SEM, EDX, BET, FT-IR, and ICP analysis. XRD results confirmed that the crystallinity of the prepared sample was maintained during the plasma treatment. SEM images showed that the morphology of the natural clinoptilolite microparticles was completely changed to nanorod after plasma treatment. The EDX results demonstrated that the amount of major elements did not change significantly after plasma treatment. The increase in specific surface area and pore volume with a smaller pore diameter proved the formation of new mesopores and micropores with smaller diameters in the plasma-treated clinoptilolite sample. The possible mechanism of plasma influence on the catalyst was proposed. The catalytic performance of untreated and plasma-treated clinoptilolite samples indicated that the dye removal efficiency was significantly increased from 36.93%, in the presence of untreated catalyst, to 97.66% by using the plasma-treated catalyst at 35 min of reaction. Therefore, because of the substantially lower price of clinoptilolite as compared to other synthetic catalysts, nanorod clinoptilolite may become a viable

catalytic performance of samples (a) and (b) in decolorization of AR17 solution at 35 min of process. It can be seen that the decolorization percentage of sample (a) and (b) is 36.93% and 97.66% (Figure 8a). Also, the normalized color removal to the amount of iron in sample (a) and sample (b) was presented in Figure 8b. These Figures confirm the significance potential and high efficiency of sample (b) in the Fenton process. Figures 9 panels a and b represent the reusability behavior of samples (a) and (b) in decolorization of AR17 solution, during five cycle experiments. The experiments were done under the following conditions: 20 mg/L of AR17, 2.0 g/L of catalyst, pH = 5, and reaction time of 35 min. After each decolorization experiment, the catalyst was washed with distilled water and dried at 60 °C for 5 h and then used in new experiment. It should be stated that the pH of solution remained constant during these experiments. As can be seen in Figure 9, both samples exhibited excellent chemical stability without any considerable loss of activity during the five cycles of heterogeneous Fenton reaction. The obtained results emphasized the excellent chemical activity and high efficiency of sample (b) in the oxidation process which is an important factor for practical applications. 18231



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(14) Ahmed, B.; Limem, E.; Abdel-Wahab, A.; Nasr, B. Photo-Fenton treatment of actual agro-industrial wastewaters. Ind. Eng. Chem. Res. 2011, 50, 6673. (15) Tekbas, M.; Yatmaz, H. C.; Bektas, N., Heterogeneous photoFenton oxidation of reactive azo dye solutions using iron exchanged zeolite as a catalyst. Micropor. Mesopor. Mat. 2008, 115. (16) Iurascu, B.; Siminiceanu, I.; Vione, D.; Vicente, M. A.; Gil, A. Phenol degradation in water through a heterogeneous photo-Fenton process catalyzed by Fe-treated laponite. Water Res. 2009, 43, 1313. (17) Chen, Q.; Wu, P.; Dang, Z.; Zhu, N.; Li, P.; Wu, J.; Wang, X. Iron pillared vermiculite as a heterogeneous photo-Fenton catalyst for photocatalytic degradation of azo dye reactive brilliant orange X-GN. Sep. Purif. Technol. 2010, 71, 315. (18) Akgul, M.; Karabakan, A. Selective synthesis of monoolein with clinoptilolite. Microporous Mesoporous Mater. 2010, 131, 238. (19) Rache, M. L.; Garcia, A. R.; Zea, H. R.; Silva, A. M. T.; Madeira, L. M.; Ramirez, J. H. Azo-dye orange II degradation by the heterogeneous Fenton-like process using a zeolite Y-Fe catalystKinetics with a model based on the Fermi’s equation. Appl. Catal. B, Environ. 2013, 6, 43. (20) Kragovic, M.; Dakovic, A.; Sekulic, Z.; Trgo, M.; Ugrina, M.; Peric, J.; Gatta, G. D. Removal of lead from aqueous solutions by using the natural and Fe(III)-modified zeolite. Appl. Surf. Sci. 2012, 258, 3667. (21) Ozcelik, Z.; Soylu, G. S. P.; Boz, I. Catalytic combustion of toluene over Mn, Fe, and Co-exchanged clinoptilolite support. Chem. Eng. J. 2009, 155, 94. (22) Qiu, M.; Qian, C.; Xu, J.; Wu, J.; Wang, G. Studies on the adsorption of dyes into clinoptilolite. Desalination 2009, 243, 286. (23) Faghihian, H.; Talebi, M.; Pirouzi, M. Adsorption of nitrogen from natural gas by clinoptilolite. J. Iran. Chem. Soc. 2008, 5, 394. (24) Chmielewska, E.; Samajova, E.; Kozac, J. A comparative study for basic characterization of three clinoptilolite sepecimens. Turk. J. Chem 2002, 26, 281. (25) Pignatello, J. J.; Huang, L. Q. Degradation of polychlorinated dibenzo-p-dioxin and dibenzofuran contaminants in 2,4,5-T by photoassisted iron-catalyzed hydrogen peroxide. Water Res. 1993, 27, 1731. (26) Sen, T.; Jana, S.; Koner, S.; Patra, A. Efficient energy transfer between confined dye and Y-Zeolite functionalized Au nanoparticles. J. Phys. Chem. C 2010, 114, 19667. (27) Doula, M. K. Synthesis of a clinoptilolite−Fe system with high Cu sorption capacity. Chemosphere 2007, 67, 731. (28) Hernandez, M.; Rojas, F.; Lara, V. Nitrogen-sorption characterization of the microporous structure of clinoptilolite-type zeolites. J. Porous Mater. 2000, 7, 443. (29) Armbruster, T. Dehydration mechanism of clinoptilolite and heulandite: Single-crystal X-ray study of Na-poor, Ca-, K-, Mg-rich clinoptiloliteat 100 K. Am. Mineral. 1993, 78, 260. (30) Junaid, A. S. M.; Rahman, M.; Yin, H.; McCaffrey, W. C.; Kuznicki, S. M. Natural zeolites for oilsands bitumen cracking: structure and acidity. Microporous Mesoporous Mater. 2011, 144, 148. (31) Rusevova, K.; Kopinke, F.-D.; Georgi, A., Nano-sized magnetic iron oxides as catalysts for heterogeneous Fenton-like reactions Influence of Fe(II)/Fe(III) ratio on catalytic performance. J. Hazard. Mater. 2012, 241−242. (32) Wang, L.; Yang, W.; Xin, C.; Ling, F.; Sun, W.; Fang, X.; Yang, R. Synthesis of nano-zeolite IM-5 by hydrothermal method with the aid of PEG and CTAB. Mater. Lett. 2012, 69, 16. (33) Xue, Z.; Ma, J.; Zhang, T.; Miao, H.; Li, R. Synthesis of nanosized ZSM-5 zeolite with intracrystalline mesopores. Mater. Lett. 2012, 68, 1. (34) Liu, C. J.; Zou, J.; Yu, K.; Cheng, D.; Han, Y.; Zhan, J.; Ratanatawanate, C.; Jang, B. W. L. Plasma application for more environmentally friendly catalyst preparation. Pure Appl. Chem. 2006, 78, 1227. (35) Lee, J. O.; Song, Y.-H.; Cha, M. S.; Kim, S. J. Effects of hydrocarbons and water vapor on NOx using V2O5−WO3/TiO2

candidate for application in wastewater treatment processes as a novel heterogeneous Fenton catalyst.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]; [email protected]. Tel.: +98 411 3393165. Fax: +98 411 3340191. *E-mail: [email protected]. Tel.: +82 53 810 1456. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the University of Tabriz, Iran for all of the support provided. This work was funded by Grant 20110014246 of the National Research Foundation of South Korea.

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REFERENCES

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Conversion of Natural Clinoptilolite Microparticles to Nanorods by

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