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Catalytic performance of hematite nanostructures prepared by N2 glow discharge plasma in heterogeneous Fenton-like process for acid red 17 degradation
Journal of Industrial and Engineering Chemistry 50 (2017) 86–95
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Catalytic performance of hematite nanostructures prepared by N2 glow discharge plasma in heterogeneous Fenton-like process for acid red 17 degradation Alireza Khataeea,b,* , Peyman Gholamia , Behrouz Vahidc a Research Laboratory of Advanced Water and Wastewater Treatment Processes, Department of Applied Chemistry, Faculty of Chemistry, University of Tabriz, 51666-16471 Tabriz, Iran b Department of Materials Science and Nanotechnology Engineering, Near East University, 99138 Nicosia, North Cyprus, Mersin 10, Turkey c Department of Chemical Engineering, Tabriz Branch, Islamic Azad University, 51579-44533 Tabriz, Iran
A R T I C L E I N F O
Article history: Received 17 September 2016 Received in revised form 27 January 2017 Accepted 27 January 2017 Available online 3 February 2017 Keywords: N2 glow discharge plasma Plasma-treated hematite Nanostructures Heterogeneous Fenton-like
A B S T R A C T
Plasma-treated hematite (PTH) nanostructures were produced from natural hematite (NH) using N2 plasma considering its cleaning and sputtering effects which lead to larger surface area. The NH and PTH were characterized by XRD, FT-IR, SEM, EDX, XPS, and BET methods. The catalytic activity of the PTH in heterogeneous Fenton-like process was higher than the NH for degradation of Acid Red 17. The GC–Mass technique was used to recognize some of the intermediates and a possible degradation pathway was proposed. Environment-friendly preparation of the catalyst, insignificant leaked iron concentration and successive usages at milder pH were the substantial advantages of the PTH. © 2017 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction Discharging various organic materials into the environment by different industries has caused concerns owing to their high toxicity and stability [1]. For instance, immense consumption of organic dyes (particularly azo dyes) in many industrial activities including leather, textile, plastic, and paper factories generates significant amounts of wastewaters carrying these pollutants [2]. Dyes generally affect human health and aquatic life. Accordingly, careful treatment should be implemented for the dye-containing effluents before their release into lakes and rivers. Traditional wastewater treatments such as biological, chemical, and physical processes are not efficient enough to remove all persistent organic pollutants (POPs) from polluted effluents [3,4]. Advanced oxidation processes (AOPs) produce reactive oxygen species (ROS) which can attack and degrade a broad range of POPs. Among AOPs, Fenton process has attracted considerable attention as a simple and effective method, which is widely used for the removal of various soluble organic compounds from the aqueous solutions [5]. Conventional Fenton process is confined by low pH levels (around 3) to prohibit iron precipitation due to the generation of
* Corresponding author. Fax: +98 41 33340191. E-mail address: [email protected] (A. Khataee).
ferric hydroxide sludge. Furthermore, the difficulty in separation and recycling of the catalyst from the solution after the process limits the homogeneous Fenton application. These major disadvantages can be conquered by means of heterogeneous iron sources, which are applied at milder pHs and can be easily separated from the solution [6]. The ability of iron minerals including magnetite [7], ferrihydrite [8], goethite [9], and pyrite [10] to catalyze the Fenton and Fentonlike processes have been confirmed by the previous studies. Hematite (a-Fe2O3) is the most stable Fe (III) oxide mineral. Therefore, it is the most prevalent ore in the natural sediments and soils [11]. The utilization of hematite has been investigated for the degradation of various organic and inorganic contaminants through the Fenton-based and adsorption processes [12,13]. However, the use of heterogeneous catalysts has some restrictions compared to the homogeneous one such as limited number of active reaction sites and low mass transfer rate. One of the efficient techniques to overcome these shortcomings is to use nano-scaled catalysts [14]. Therefore, different synthetic methods have been used for production of nanosized catalysts with diverse morphologies. It should be noticed that a number of the main drawbacks of these methods such as (i) toxicity of the metal-organic precursors, (ii) complexity of the procedures, and (iii) high costs have confined their wide utilizations [15,16]. Plasma consists of almost the same number of electrons and positively charged ions. Plasma-assisted techniques are simple,
http://dx.doi.org/10.1016/j.jiec.2017.01.035 1226-086X/© 2017 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
A. Khataee et al. / Journal of Industrial and Engineering Chemistry 50 (2017) 86–95
environment-friendly and low cost, and are utilized for production of a diverse nanomaterials with various applications [17,18]. Recently, non-thermal plasma techniques have attracted notable attention for the catalyst development. For example, radio frequency plasma treatment was used to prepare Pd/TiO2 catalyst for selective hydrogenation of acetylene [19]. The plasma treated Pd/HZSM-5 was used as a more stable and active catalyst in methane combustion process [20]. Dielectric-barrier discharge plasma method was used to modify Ni/MgO catalyst for CO2 reforming of methane [21]. Furthermore, our previous studies show that the prepared nanostructured magnetite and pyrite from nature ores using N2 glow discharge plasma are efficient catalyst for the heterogeneous Fenton process [22,23]. In the present research, the nanostructured hematite was prepared via N2 glow discharge plasma treatment of hematite ore. The physical and chemical properties of the natural hematite (NH) and the plasma-treated hematite (PTH) were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and Brunauer–Emmett–Teller (BET). Subsequently, the performance of the PTH was compared with NH for the Acid Red 17 (AR17) degradation through hematite/H2O2 process. Moreover, the effect of the solution pH, initial H2O2 concentration (mg/L), PTH dosage (g/L), initial dye concentration (mg/L), and presence of peroxydisulfate and inorganic salts on the degradation efficiency (DE%) of the AR17 were evaluated. The GC–Mass method was performed on the treated dye solution to recognize the degradation intermediates. Experimental Reagents and materials H2SO4 (98%), HCl (37%), NaOH (99%), H2O2 (30%), HNO3 (65%), and NaNO3 (99%) were purchased from Merck (Germany) and applied without purification. The natural hematite was purchased from Morvarid iron mine in north-west of Iran. The mono azo dye, Acid Red 17 (molecular formula = C20H12N2Na2O7S2,lmax = 510 nm, and Mw = 502.435 g/mol) was provided from Shimi Boyakhsaz Co., Iran. Preparation of hematite nanostructures Firstly, the NH was crushed using a milling instrument (Kian Madan Pars Co, Iran) to the range of 125–177 mm. Then, the micrograined particles were washed by the distilled water and dried at 50 C for 3 times. Fig. 1 illustrates a schematic design of the instrument used for plasma treatment. The plasma set-up was constructed from a horizontal Pyrex tube (diameter of 5 cm and length of 40 cm) and two aluminum electrodes on the both sides of the tube. During the plasma treatment, dried hematite particles (3 g) were laid on the Pyrex plate fixed in the plasma reactor. The
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reactor was vacated and after that N2 as plasma-generating gas was fed into it at a flow rate of 3 cm3/s for adjusting the pressure at 50 Pa. A high-voltage DC power supply (1200–1300 V, Tabriz, Iran) was connected to the electrodes to form the glow discharge plasma. The NH particles were in the lowest temperature region (close to the anode), with a temperature of 20 C. The highest temperature of the reactor was 45 C, located in the region near the cathode [24]. It should be mentioned that, the N2 gas was introduced into the reactor at various pressures in the plasma treatment of the NH, which ranged from 25 to 75 Pa. Then, the obtained catalysts after 45 min of the plasma treatment were washed and their catalytic performance was evaluated for AR17 degradation in the PTH/H2O2 process. The results indicated that the plasma treated sample under 50 Pa showed the best catalytic activity. It can be attributed to the plasma density, which increased with enhancement of the pressure up to the desired amount of 50 Pa and then declined due to the collisional recombination at high pressures [25]. Characterization methods of the NH and PTH To determine the crystallographic structure of the NH before and after the plasma treatment, the XRD (D8 Advance, Bruker, Germany) and XPS (XPS, K-ALPHA Thermo Scientific spectrometer, UK) analyses were applied. An FT-IR spectrometer (Tensor 27, Bruker, Germany) was used to record the FT-IR spectra of the catalysts by KBr pellet technique [26]. The morphology and size of the NH and PTH were examined using a SEM model TESCAN, MIRA3, Czech Republic equipped with an EDX microanalysis. The PTH size distribution was determined by Microstructure Distance Measurement software (Nahamin Pardazan Asia Co, Iran). The microstructural characteristics of the samples were investigated using N2 adsorption/desorption method using a PHS-1020 (PHSCHINA) instrument. The pH value in which the surface charge of the PTH sample was zero is called the point of zero charge (pHpzc) of the sample. The pHpzc of the PTH sample was determined using the salt addition method [27]. Heterogeneous Fenton-like set-up and procedure A batch cylindrical reactor with a magnetic stirring bar was used to carry out the AR17 degradation (500 mL) experiments through the Fenton-like process. To start each run, a predetermined amount of H2O2 was added to the AR17 solution containing a distinct dosage of the PTH. The NaOH (0.1 M) and H2SO4 (0.1 M) were used for adjusting the pH of the AR17 solution. The sample of 3 mL was extracted by pipette at an interval of 10 min during an hour reaction time and centrifuged to separate the suspended PTH particles from the solution. Then, the variation of the dye absorption (A) during the process was determined by optical absorption of the sample at 510 nm applying an UV–vis spectrophotometer (WPA lightwave S2000, England). The iron concentration was examined by an atomic absorption spectroscopy (Novaa 400, Analytikjena, Germany). The GC–Mass analysis was applied using an apparatus (Agilent 6890 GC and 5973 mass spectrometer, Palo Alto, Canada) to identify the main degradation intermediates of AR17, which were generated through the process by a procedure explained in our previous research [28]. Results and discussion Characterization of the catalysts and glow discharge plasma mechanism
Fig. 1. Schematic diagram of the nitrogen glow discharge plasma system.
The XRD spectra of the NH and PTH are observed in Fig. 2. In both spectra, the peaks seen at 2u of 24.1, 33.1, 35.6, 41.0, 49.4, 54.1,
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Fig. 2. XRD of (a) NH and (b) PTH.
62.5, and 64.0 were indexed as (0 1 2), (1 0 4), (11 0), (1 1 3), (0 2 4), (1 1 6), (2 1 4), and (3 0 0) planes of hematite (JCPDS card No. 330664), respectively. The proper match between standard hematite pattern and the obtained spectra of the studied samples indicated the appropriate crystallinity of the hematite after N2 plasma treatment. Hence, the glow discharge plasma method did not affect the structure of the hematite sample [29]. The FT-IR was carried out to find the variations in functional groups of the hematite after the plasma treatment. The FT-IR patterns of the NH and PTH are presented in Fig. 3. The peaks at 450.2, and 520.7 cm1 was attributed to the stretching vibration of FeO species [30,31]. The Fe O OH vibration was found at 1020.6 cm1 [32,33]. The bands around 1663.7 and 3457.2 cm1 were related to bending vibration of water molecules and the stretching vibration of OH groups, respectively [31]. The peaks at 2924.5 and 2851.1 cm1 were assigned to the asymmetric and symmetric C H bonds, respectively [34]. The SEM images of the NH and PTH are displayed in Fig. 4 with various magnifications. A bulky structure of the NH was observed in Fig. 4a and b. The Fig. 4c and d indicated fine nanostructures on
the PTH in which their average width was 20–40 nm (Fig. 5). From the FT-IR and XRD results, the N2 plasma modification did not transform the hematite to other substance. The SEM micrographs of NH and PTH revealed the hematite nanostructures generation by the plasma method. The generated nanostructures enhanced the available surface area and thus improved the PTH catalytic efficiency. The EDX results of the NH and PTH are demonstrated in Fig. 6a and b, which confirmed that the Fe and O elements were present in the both samples. Weight percent (wt%) of the surface Fe enhanced after plasma modification indicating the removing some of the impurities on NH such as C and Si [35,36]. Accordingly, the hematite nanostructures development by the plasma treatment could be attributed to the impurities removal, which were mainly located in crystallites boundaries and some superficial Fe and O of hematite crystallites; thus, the mineral purity was improved [32,33]. The XPS analysis was carried out to recognize the present elements and Fe oxidation states on the surface of the NH and PTH samples (Fig. 7). The obtained XPS data proved that the surface of the samples held iron, oxygen, and carbon. The Fe 2p peaks were more dominant than of Fe 3s and Fe 3p. The locations related to Fe 2p peaks was investigated in the binding energies of 700–740 eV (Fig. 7b). The peaks revealed the presence of the doublet Fe 2p3/2 and Fe 2p1/2 at binding energies of 710.8 and 725.1 eV, respectively. The Fe 2p3/2 peak accompanied with a satellite peak located at about 7 eV more than the main peak, was the characteristic of a-Fe2O3 [37]. The Fig. 7a also showed a peak at 537.5 eV related to O 1s. This peak could belong to the surface O2. Furthermore, the XPS results implied the oxidation states of +3 for Fe and 2 for O [38]. Fig. 8a and b illustrates the N2 adsorption-desorption isotherms determined at 77.35 K for the NH and PTH. It was observed the plasma treated hematite is of type IV, based on the IUPAC classification, which revealed that the sample contained mesopores (2–50 nm) with spherical particles [39]. According to the obtained BET results after the plasma modification, the pore volume and specific surface area of the NH were 0.566 cm3/g and 4.383 m2/g, which enhanced to 0.767 cm3/g and 11.597 m2/g, respectively. These increases verified the formation of hematite nanostructures. Fig. 9 represents DpH variation against the initial pH. Meanwhile the initial pH of the solution was 7.8, DpH was measured to be zero that implied that the surface charge of the PTH was zero at this pH. Therefore, as the pH was lower than the pHPZC, the PTH net charge was positive and it was negative as the pH was higher than the pHPZC [40]. An electronic mechanism has been suggested for the function of non-thermal plasma. First, the catalyst trapped the electrons as the sink of electrons; next, the trapped electrons generated a plasma sheath around the catalyst, which was repulsed effectively by the plasma zone electrons. Simultaneously, the trapped electrons on the particle also repulsed each other strongly. Consequently, owing to the repulsive interactions in the particle and plasma sheath about the catalyst, the bonds in the catalyst could be distorted, stretched out and split leading to the structural transformations [24,41]. Heterogeneous Fenton-like process
Fig. 3. FT-IR spectra of (a) NH and (b) PTH.
Degradation of AR17 in various processes Fig. 10 represents the comparison of the DE% over the different studied processes. As can be seen from Fig. 10, the DE% was lower than 6% after 60 min in the presence of H2O2, exclusively. The results indicated that the only H2O2 had insignificant effect on the dye degradation. Moreover, the adsorptions of AR17 by the NH and
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Fig. 4. SEM images of (a and b) NH and (c and d) PTH.
PTH samples were less than 11% in the dark. On the other hand, 97.5% of degradation occurred applying the PTH/H2O2 process which was contrasted with NH/H2O2 system of 47.9% removal (%). In this study, the nanostructured hematite generated under the N2 plasma treatment, enhanced the NH surface area [42]. Hence, available iron increased on the hematite surface, where the ROS, specially hydroxyl radicals (OH), were produced through the heterogeneous Fenton-like process according to the following reactions [43]:
Fig. 5. The PTH size distribution.
Fe3+ hematite + H2O2 ! Fe hematite OOH2+ + H+
(1)
Fe hematite OOH2+ ! Fe2+ hematite + HO2
(2)
Fe2+ hematite + H2O2 ! Fe3+ hematite + OH + OH
(3)
In addition, during the plasma treatment, some impurities were removed from the hematite surface, which enhanced the iron
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Fig. 6. EDX analysis of (a) NH and (b) PTH.
presence on the surface of catalyst [35]. These findings demonstrated that the plasma treatment could improve the catalytic efficiency of NH. Consequently, PTH was selected as an efficient nanocatalyst and its catalytic performance was evaluated in the PTH/H2O2 process for the AR17 degradation under various experimental conditions of the solution pH, initial H2O2 concentration, PTH dosage, initial dye concentration, and presence of peroxydisulfate enhancer and inorganic salts. The apparent pseudo-first order rate constants (kapp) for the studied systems were calculated by linear regression analysis and given in Table 1. The inset of Fig. 10 demonstrates that pseudo-first order kinetic model was suitable to describe the AR17 degradation by all of the studied processes, which was confirmed by other studies [44]. Appropriate correlation coefficients (R2), that was higher than 0.95 (Table 1) verified the suggested kinetic model. Effect of pH The pH is one of the significant operational factors in the PTH/H2O2 process. Therefore, the influence of this variable on the AR17 degradation was examined, and the data are presented in Fig. 11. The DE% declined by the pH increment from 3 to 9. The higher degradation efficiencies were achieved at lower pHs, and it declined with increasing the suspension pH. This observation can be ascribed based on the following three factors: (1) The OH radicals oxidation potential is raised by reducing the pH [45]; this demonstrates that in the acidic condition, the ability of the produced hydroxyl radicals for the AR17 removal was more than that of alkaline and neutral aqueous solutions. (2) The pH of suspension affects the pollutants adsorption on the heterogeneous catalyst surface, wherein the hydroxyl radicals were generated and attacked to the contaminants. According to the obtained results
Fig. 7. XPS spectrum of NH and PTH samples in the binding energies range of (a) 0– 1200 and (b) 700–740 eV.
from the pHpzc analysis (Fig. 9), at pH values lower than 7.8, the surface of PTH became positively charged. Therefore, the adsorption of the negatively charged AR17 molecules on the PTH surface enhanced. As a result, more AR17 molecules were removed by the surface OH radicals [34,46]. The opposite was happened at higher pH values than pHpzc. (3) Moreover, the acidic condition could dissolve more PTH surface iron into the suspension (Fig. 12). The better degradation efficiencies was achieved as the pH of suspension was adjusted between 3 to 5, with approximately the same effectiveness after 60 min of the process. Hence, the pH 5 was chosen as the favorable amount. On the other hand, the leached Fe concentration was less than 0.36 mg/L after PTH/H2O2 process at various pH values (Fig. 11). Considering the minor concentrations of the dissolved Fe ions, the production of hydroxyl radicals could be significantly attributed to the heterogeneous process on the surface of the PTH when compared to the homogeneous one, in AR17 degradation [47].
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Fig. 10. Degradation efficiency of the AR17 in the various processes; [AR17]0 = 20 mg/L, [PTH] = 0.75 g/L, pH = 5 and [H2O2]0 = 2 mM. The inset represents mentioned processes follow pseudo-first order kinetic. Table 1 Effect of the various processes on the apparent pseudo-first order constants of degradation of AR17. No. 1 2 3 4 5
Process H2O2 NH PTH NH/H2O2 PTH/H2O2
kapp (min1) 2
0.09 10 0.17 102 0.23 102 1.18 102 5.43 102
Correlation coefficient (R2) 0.97 0.95 0.96 0.98 0.96
Fig. 8. Nitrogen adsorption/desorption isotherms of (a) NH and (b) PTH.
Fig. 11. The effect of solution pH on the DE% of PTH/H2O2 process; [AR17]0 = 20 mg/ L, [PTH] = 0.75 g/L and [H2O2]0 = 2 mM.
Fig. 9. Plot for calculation of PTH pHpzc.
Effect of H2O2 initial concentration Optimization of the H2O2 concentration is essential because the major cost associated with the PTH/H2O2 process, is that of hydrogen peroxide. Furthermore, an excessive amount of oxidant acts as hydroxyl radical scavenger and reduces the degradation efficiency [48]. The effect of hydrogen peroxide concentration on the AR17 degradation is shown in Fig. 13. As can be observed from Fig. 13, the DE% enhanced with of the increment in the H2O2 concentration up to 2 mM. This enhancement was resulted from the production of higher amount of ROS through the H2O2 decomposition in the presence of PTH (Eqs. (1)–(3)). However, further addition of H2O2 decreased the DE% owing to the scavenging effect on OH radicals (Eq. (4)) [49–51].
H2O2 + OH ! H2O + HO2
(4)
Effect of the PTH dosage Fig. 14 shows the impact of the catalyst dosage on the PTH/H2O2 function in the dye degradation process. An increase in the PTH dosage up to 0.75 g/L led to an enhanced degradation efficiency. The reason for this phenomenon was that high catalyst dosage caused an extensive active sites for decomposition of H2O2 to ROS [39,52]. However, an inverse trend was observed with further increase in the PTH dosage. This trend was because of the scavenging effect of Fe (II) on OH radicals as described by Eq. (5) [53]. OH + Fe2+ ! OH + Fe3+
(5)
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Fig. 12. Leached iron concentration in the solution after 60 min; [PTH] = 0.7 g/L. Fig. 15. The effect of dye concentrations on the AR17 degradation efficiency by PTH/ H2O2 process; [PTH] = 0.75 g/L, pH = 5 and [H2O2]0 = 2 mM.
Effect of inorganic ions on the AR17 degradation Salting-out effect of inorganic salts such as Na2SO4, Na2CO3 and NaCl was investigated on the DE%. Sulfates, carbonates and chlorides are the commonly available anions in the wastewaters. These anions act as OH radical scavengers that produce other radicals with less oxidation potentials (Eqs. (6)–(8)). Furthermore, the catalyst positively charged sites at pH 5, were occupied by the anions and prohibite the treatment process. Thus, the DE% declined in the presence of the scavengers in the solution (Fig. 16) [55–58]. Cl + OH ! ClOH
Fig. 13. The effect of H2O2 concentration on the DE% of PTH/H2O2 process; [AR17]0 = 20 mg/L, [PTH] = 0.75 g/L and pH = 5.
Effect of dye initial concentration The influence of AR17 initial concentrations on the DE% was studied. The results are displayed in Fig. 15. The degradation of AR17 was decreased with an increase in its initial concentration because the same amounts of generated ROS at the identical operational conditions have to decompose more AR17 molecules and the by-products produced by its degradation [53,54]. Furthermore, the pseudo-first-order kinetics was fitted to the obtained data and kapp was calculated from the inset plots of Figs. 11, 13–15 and shown in Table 2.
(6)
CO32 + OH ! CO3
+ OH
SO42 + OH ! SO4
+ OH
(8) 2
The peroxydisulfate (S2O8 ) influence was also studied on the DE%. Sulfate radical (SO4) was formed in the presence of the PTH (Eqs. (9)), which had higher oxidation potential (2.6 V) than S2O82 Table 2 Effect of the operational parameters on the apparent pseudo-first order constant of degradation for PTH/H2O2 process. Operational parameters and amounts pH 3 5 7 9 H2O2 concentration (mM) 0 0.5 1 2 3 PTH dosage (g/L) 0 0.25 0.5 0.75 1
Fig. 14. The effect of PTH dosage on the DE% of PTH/H2O2 process; [AR17]0 = 20 mg/ L, pH = 5 and [H2O2]0 = 2 mM.
(7)
AR17 concentration (mg/L) 20 30 40 50
kapp (min1)
Correlation coefficient (R2)
6.54 102 5.43 102 1.92 102 0.50 102
0.99 0.96 0.99 0.98
0.14 102 1.04 102 1.91 102 5.43 102 2.58 102
0.94 0.98 0.97 0.96 0.99
0.13 102 0.53 102 1.23 102 5.43 102 3.39 102
0.94 0.98 0.99 0.96 0.97
5.43 102 2.23 102 1.48 102 0.98 102
0.96 0.97 0.97 0.97
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AR17 degradation intermediates The formed intermediate compounds during the degradation of AR17 through PTH/H2O2 process were determined using the GC– Mass method. The retention time (tR) and main fragments of the by-products after 15 min of process are given in Table 3. However, It was not feasible to identify the all present compounds due to their little accumulation and restriction of the GC–Mass method. Subsequently, a possible degradation pathway was suggested based on the identified intermediate compounds, (Fig. 17). The degradation process could occur by the cleavage of N¼N, C S, C C and C N bonds and finally, mineralizing the intermediates to H2O and CO2 [60,61].
Fig. 16. The effect of peroxydisulfate and OH radical scavengers on the AR17 degradation by PTH/H2O2 process; [AR17]0 = 20 mg/L, [PTH] = 0.75 g/L, pH = 5, [H2O2]0 = 2 mM and [Scavenger]0 = 20 mg/L.
(2.01 V) [59]:
2 ! Fe3þ þ SO2 Fe2þ 4 surf þ S2 O8 surf þ SO4
ð9Þ
Stability of PTH in successive PTH/H2O2 process Catalyst stability in the repeated usages is one of the substantial properties from practicable viewpoint [62]. Thus, the PTH was used in five successive Fenton-like processes, and then, its performance was determined. In each run, 20 mg/L AR17, 2 mM H2O2 and 0.75 g/ L PTH in 500 mL of solution were stirred together at pH of 5 for 60 min. The results demonstrated that the DE% was approximately unchanged after the applications (Fig. 18). Besides, the leaked iron determinations data (Fig. 12) revealed that the released iron concentration into the solution was 0.19 mg/L per run, wherein the
Table 3 Identified by-products during degradation of AR17 by PTH/H2O2 process. No.
Compound name
Retention time (min)
Main fragments
1
a-Nitronaphthalene
11.358
115, 127, 145, 173
2
a-Naphthol
26.384
58, 89, 115, 144
3
a-Naphthoquinone
16.427
76, 102, 130, 158
4
1,2-Benzenedicarboxylic acid
9.464
50, 76, 104
5
1,8-Octanediol
8.475
31, 41, 55
6
n-Pentanoic acid
4.782
27, 41, 45, 60, 73
7
2-Propenoic acid
5.449
27, 45, 55, 72
Structure
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which were characterized using XRD, FT-IR, SEM, EDX, XPS, and BET analyses. The sputtering effect of N2 plasma leds to the formation of the PTH nanostructures with a large surface area. Moreover, the plasma treatment could eliminate impurities from the surface of NH. As a result, the PTH function in the heterogeneous Fenton-like process enhanced remarkably compared to the NH for the AR17 degradation as a model textile dye. Degradation efficiency of 97.7% was obtained at pH of 5, initial H2O2 concentration of 2 mM, PTH dosage of 0.75 g/L, and initial AR17 concentration of 20 mg/L after 60 min of the reaction. The increase in peroxydisulfate ion and OH radical scavengers declined the AR17 degradation, verifying the key role of hydroxyl radicals in the PTH/H2O2 process. The GC–Mass analysis was applied to identify a number of the degradation by-products. The other considerable benefits of the stable PTH were low leaked iron in the solution and its application over milder pH values. Acknowledgments We thank the University of Tabriz for the support provided. The authors acknowledge Dr. Shima Rahim Pouran for language edit. We also acknowledge the support of Iran Science Elites Federation. References
Fig. 17. Proposed pathway for degradation of AR17 by PTH/H2O2 process.
Fig. 18. Reusability of the PTH for five repeated runs; [AR17]0 = 20 mg/L, [PTH] = 0.75 g/L, pH = 5 and [H2O2]0 = 2 mM.
PTH amount was 0.75 g/L. Therefore, the major amount of iron remained in the PTH structure, thus the catalyst could be considered as stable. Conclusions The environment-friendly nitrogen glow discharge plasma technique was applied successfully to form a modified hematite,
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Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Catalytic performance of hematite nanostructures prepared by N2 glow discharge plasma in heterogeneous Fenton-like process for acid red 17 degradation Alireza Khataeea,b,* , Peyman Gholamia , Behrouz Vahidc a Research Laboratory of Advanced Water and Wastewater Treatment Processes, Department of Applied Chemistry, Faculty of Chemistry, University of Tabriz, 51666-16471 Tabriz, Iran b Department of Materials Science and Nanotechnology Engineering, Near East University, 99138 Nicosia, North Cyprus, Mersin 10, Turkey c Department of Chemical Engineering, Tabriz Branch, Islamic Azad University, 51579-44533 Tabriz, Iran
A R T I C L E I N F O
Article history: Received 17 September 2016 Received in revised form 27 January 2017 Accepted 27 January 2017 Available online 3 February 2017 Keywords: N2 glow discharge plasma Plasma-treated hematite Nanostructures Heterogeneous Fenton-like
A B S T R A C T
Plasma-treated hematite (PTH) nanostructures were produced from natural hematite (NH) using N2 plasma considering its cleaning and sputtering effects which lead to larger surface area. The NH and PTH were characterized by XRD, FT-IR, SEM, EDX, XPS, and BET methods. The catalytic activity of the PTH in heterogeneous Fenton-like process was higher than the NH for degradation of Acid Red 17. The GC–Mass technique was used to recognize some of the intermediates and a possible degradation pathway was proposed. Environment-friendly preparation of the catalyst, insignificant leaked iron concentration and successive usages at milder pH were the substantial advantages of the PTH. © 2017 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction Discharging various organic materials into the environment by different industries has caused concerns owing to their high toxicity and stability [1]. For instance, immense consumption of organic dyes (particularly azo dyes) in many industrial activities including leather, textile, plastic, and paper factories generates significant amounts of wastewaters carrying these pollutants [2]. Dyes generally affect human health and aquatic life. Accordingly, careful treatment should be implemented for the dye-containing effluents before their release into lakes and rivers. Traditional wastewater treatments such as biological, chemical, and physical processes are not efficient enough to remove all persistent organic pollutants (POPs) from polluted effluents [3,4]. Advanced oxidation processes (AOPs) produce reactive oxygen species (ROS) which can attack and degrade a broad range of POPs. Among AOPs, Fenton process has attracted considerable attention as a simple and effective method, which is widely used for the removal of various soluble organic compounds from the aqueous solutions [5]. Conventional Fenton process is confined by low pH levels (around 3) to prohibit iron precipitation due to the generation of
* Corresponding author. Fax: +98 41 33340191. E-mail address: [email protected] (A. Khataee).
ferric hydroxide sludge. Furthermore, the difficulty in separation and recycling of the catalyst from the solution after the process limits the homogeneous Fenton application. These major disadvantages can be conquered by means of heterogeneous iron sources, which are applied at milder pHs and can be easily separated from the solution [6]. The ability of iron minerals including magnetite [7], ferrihydrite [8], goethite [9], and pyrite [10] to catalyze the Fenton and Fentonlike processes have been confirmed by the previous studies. Hematite (a-Fe2O3) is the most stable Fe (III) oxide mineral. Therefore, it is the most prevalent ore in the natural sediments and soils [11]. The utilization of hematite has been investigated for the degradation of various organic and inorganic contaminants through the Fenton-based and adsorption processes [12,13]. However, the use of heterogeneous catalysts has some restrictions compared to the homogeneous one such as limited number of active reaction sites and low mass transfer rate. One of the efficient techniques to overcome these shortcomings is to use nano-scaled catalysts [14]. Therefore, different synthetic methods have been used for production of nanosized catalysts with diverse morphologies. It should be noticed that a number of the main drawbacks of these methods such as (i) toxicity of the metal-organic precursors, (ii) complexity of the procedures, and (iii) high costs have confined their wide utilizations [15,16]. Plasma consists of almost the same number of electrons and positively charged ions. Plasma-assisted techniques are simple,
http://dx.doi.org/10.1016/j.jiec.2017.01.035 1226-086X/© 2017 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
A. Khataee et al. / Journal of Industrial and Engineering Chemistry 50 (2017) 86–95
environment-friendly and low cost, and are utilized for production of a diverse nanomaterials with various applications [17,18]. Recently, non-thermal plasma techniques have attracted notable attention for the catalyst development. For example, radio frequency plasma treatment was used to prepare Pd/TiO2 catalyst for selective hydrogenation of acetylene [19]. The plasma treated Pd/HZSM-5 was used as a more stable and active catalyst in methane combustion process [20]. Dielectric-barrier discharge plasma method was used to modify Ni/MgO catalyst for CO2 reforming of methane [21]. Furthermore, our previous studies show that the prepared nanostructured magnetite and pyrite from nature ores using N2 glow discharge plasma are efficient catalyst for the heterogeneous Fenton process [22,23]. In the present research, the nanostructured hematite was prepared via N2 glow discharge plasma treatment of hematite ore. The physical and chemical properties of the natural hematite (NH) and the plasma-treated hematite (PTH) were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and Brunauer–Emmett–Teller (BET). Subsequently, the performance of the PTH was compared with NH for the Acid Red 17 (AR17) degradation through hematite/H2O2 process. Moreover, the effect of the solution pH, initial H2O2 concentration (mg/L), PTH dosage (g/L), initial dye concentration (mg/L), and presence of peroxydisulfate and inorganic salts on the degradation efficiency (DE%) of the AR17 were evaluated. The GC–Mass method was performed on the treated dye solution to recognize the degradation intermediates. Experimental Reagents and materials H2SO4 (98%), HCl (37%), NaOH (99%), H2O2 (30%), HNO3 (65%), and NaNO3 (99%) were purchased from Merck (Germany) and applied without purification. The natural hematite was purchased from Morvarid iron mine in north-west of Iran. The mono azo dye, Acid Red 17 (molecular formula = C20H12N2Na2O7S2,lmax = 510 nm, and Mw = 502.435 g/mol) was provided from Shimi Boyakhsaz Co., Iran. Preparation of hematite nanostructures Firstly, the NH was crushed using a milling instrument (Kian Madan Pars Co, Iran) to the range of 125–177 mm. Then, the micrograined particles were washed by the distilled water and dried at 50 C for 3 times. Fig. 1 illustrates a schematic design of the instrument used for plasma treatment. The plasma set-up was constructed from a horizontal Pyrex tube (diameter of 5 cm and length of 40 cm) and two aluminum electrodes on the both sides of the tube. During the plasma treatment, dried hematite particles (3 g) were laid on the Pyrex plate fixed in the plasma reactor. The
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reactor was vacated and after that N2 as plasma-generating gas was fed into it at a flow rate of 3 cm3/s for adjusting the pressure at 50 Pa. A high-voltage DC power supply (1200–1300 V, Tabriz, Iran) was connected to the electrodes to form the glow discharge plasma. The NH particles were in the lowest temperature region (close to the anode), with a temperature of 20 C. The highest temperature of the reactor was 45 C, located in the region near the cathode [24]. It should be mentioned that, the N2 gas was introduced into the reactor at various pressures in the plasma treatment of the NH, which ranged from 25 to 75 Pa. Then, the obtained catalysts after 45 min of the plasma treatment were washed and their catalytic performance was evaluated for AR17 degradation in the PTH/H2O2 process. The results indicated that the plasma treated sample under 50 Pa showed the best catalytic activity. It can be attributed to the plasma density, which increased with enhancement of the pressure up to the desired amount of 50 Pa and then declined due to the collisional recombination at high pressures [25]. Characterization methods of the NH and PTH To determine the crystallographic structure of the NH before and after the plasma treatment, the XRD (D8 Advance, Bruker, Germany) and XPS (XPS, K-ALPHA Thermo Scientific spectrometer, UK) analyses were applied. An FT-IR spectrometer (Tensor 27, Bruker, Germany) was used to record the FT-IR spectra of the catalysts by KBr pellet technique [26]. The morphology and size of the NH and PTH were examined using a SEM model TESCAN, MIRA3, Czech Republic equipped with an EDX microanalysis. The PTH size distribution was determined by Microstructure Distance Measurement software (Nahamin Pardazan Asia Co, Iran). The microstructural characteristics of the samples were investigated using N2 adsorption/desorption method using a PHS-1020 (PHSCHINA) instrument. The pH value in which the surface charge of the PTH sample was zero is called the point of zero charge (pHpzc) of the sample. The pHpzc of the PTH sample was determined using the salt addition method [27]. Heterogeneous Fenton-like set-up and procedure A batch cylindrical reactor with a magnetic stirring bar was used to carry out the AR17 degradation (500 mL) experiments through the Fenton-like process. To start each run, a predetermined amount of H2O2 was added to the AR17 solution containing a distinct dosage of the PTH. The NaOH (0.1 M) and H2SO4 (0.1 M) were used for adjusting the pH of the AR17 solution. The sample of 3 mL was extracted by pipette at an interval of 10 min during an hour reaction time and centrifuged to separate the suspended PTH particles from the solution. Then, the variation of the dye absorption (A) during the process was determined by optical absorption of the sample at 510 nm applying an UV–vis spectrophotometer (WPA lightwave S2000, England). The iron concentration was examined by an atomic absorption spectroscopy (Novaa 400, Analytikjena, Germany). The GC–Mass analysis was applied using an apparatus (Agilent 6890 GC and 5973 mass spectrometer, Palo Alto, Canada) to identify the main degradation intermediates of AR17, which were generated through the process by a procedure explained in our previous research [28]. Results and discussion Characterization of the catalysts and glow discharge plasma mechanism
Fig. 1. Schematic diagram of the nitrogen glow discharge plasma system.
The XRD spectra of the NH and PTH are observed in Fig. 2. In both spectra, the peaks seen at 2u of 24.1, 33.1, 35.6, 41.0, 49.4, 54.1,
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Fig. 2. XRD of (a) NH and (b) PTH.
62.5, and 64.0 were indexed as (0 1 2), (1 0 4), (11 0), (1 1 3), (0 2 4), (1 1 6), (2 1 4), and (3 0 0) planes of hematite (JCPDS card No. 330664), respectively. The proper match between standard hematite pattern and the obtained spectra of the studied samples indicated the appropriate crystallinity of the hematite after N2 plasma treatment. Hence, the glow discharge plasma method did not affect the structure of the hematite sample [29]. The FT-IR was carried out to find the variations in functional groups of the hematite after the plasma treatment. The FT-IR patterns of the NH and PTH are presented in Fig. 3. The peaks at 450.2, and 520.7 cm1 was attributed to the stretching vibration of FeO species [30,31]. The Fe O OH vibration was found at 1020.6 cm1 [32,33]. The bands around 1663.7 and 3457.2 cm1 were related to bending vibration of water molecules and the stretching vibration of OH groups, respectively [31]. The peaks at 2924.5 and 2851.1 cm1 were assigned to the asymmetric and symmetric C H bonds, respectively [34]. The SEM images of the NH and PTH are displayed in Fig. 4 with various magnifications. A bulky structure of the NH was observed in Fig. 4a and b. The Fig. 4c and d indicated fine nanostructures on
the PTH in which their average width was 20–40 nm (Fig. 5). From the FT-IR and XRD results, the N2 plasma modification did not transform the hematite to other substance. The SEM micrographs of NH and PTH revealed the hematite nanostructures generation by the plasma method. The generated nanostructures enhanced the available surface area and thus improved the PTH catalytic efficiency. The EDX results of the NH and PTH are demonstrated in Fig. 6a and b, which confirmed that the Fe and O elements were present in the both samples. Weight percent (wt%) of the surface Fe enhanced after plasma modification indicating the removing some of the impurities on NH such as C and Si [35,36]. Accordingly, the hematite nanostructures development by the plasma treatment could be attributed to the impurities removal, which were mainly located in crystallites boundaries and some superficial Fe and O of hematite crystallites; thus, the mineral purity was improved [32,33]. The XPS analysis was carried out to recognize the present elements and Fe oxidation states on the surface of the NH and PTH samples (Fig. 7). The obtained XPS data proved that the surface of the samples held iron, oxygen, and carbon. The Fe 2p peaks were more dominant than of Fe 3s and Fe 3p. The locations related to Fe 2p peaks was investigated in the binding energies of 700–740 eV (Fig. 7b). The peaks revealed the presence of the doublet Fe 2p3/2 and Fe 2p1/2 at binding energies of 710.8 and 725.1 eV, respectively. The Fe 2p3/2 peak accompanied with a satellite peak located at about 7 eV more than the main peak, was the characteristic of a-Fe2O3 [37]. The Fig. 7a also showed a peak at 537.5 eV related to O 1s. This peak could belong to the surface O2. Furthermore, the XPS results implied the oxidation states of +3 for Fe and 2 for O [38]. Fig. 8a and b illustrates the N2 adsorption-desorption isotherms determined at 77.35 K for the NH and PTH. It was observed the plasma treated hematite is of type IV, based on the IUPAC classification, which revealed that the sample contained mesopores (2–50 nm) with spherical particles [39]. According to the obtained BET results after the plasma modification, the pore volume and specific surface area of the NH were 0.566 cm3/g and 4.383 m2/g, which enhanced to 0.767 cm3/g and 11.597 m2/g, respectively. These increases verified the formation of hematite nanostructures. Fig. 9 represents DpH variation against the initial pH. Meanwhile the initial pH of the solution was 7.8, DpH was measured to be zero that implied that the surface charge of the PTH was zero at this pH. Therefore, as the pH was lower than the pHPZC, the PTH net charge was positive and it was negative as the pH was higher than the pHPZC [40]. An electronic mechanism has been suggested for the function of non-thermal plasma. First, the catalyst trapped the electrons as the sink of electrons; next, the trapped electrons generated a plasma sheath around the catalyst, which was repulsed effectively by the plasma zone electrons. Simultaneously, the trapped electrons on the particle also repulsed each other strongly. Consequently, owing to the repulsive interactions in the particle and plasma sheath about the catalyst, the bonds in the catalyst could be distorted, stretched out and split leading to the structural transformations [24,41]. Heterogeneous Fenton-like process
Fig. 3. FT-IR spectra of (a) NH and (b) PTH.
Degradation of AR17 in various processes Fig. 10 represents the comparison of the DE% over the different studied processes. As can be seen from Fig. 10, the DE% was lower than 6% after 60 min in the presence of H2O2, exclusively. The results indicated that the only H2O2 had insignificant effect on the dye degradation. Moreover, the adsorptions of AR17 by the NH and
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Fig. 4. SEM images of (a and b) NH and (c and d) PTH.
PTH samples were less than 11% in the dark. On the other hand, 97.5% of degradation occurred applying the PTH/H2O2 process which was contrasted with NH/H2O2 system of 47.9% removal (%). In this study, the nanostructured hematite generated under the N2 plasma treatment, enhanced the NH surface area [42]. Hence, available iron increased on the hematite surface, where the ROS, specially hydroxyl radicals (OH), were produced through the heterogeneous Fenton-like process according to the following reactions [43]:
Fig. 5. The PTH size distribution.
Fe3+ hematite + H2O2 ! Fe hematite OOH2+ + H+
(1)
Fe hematite OOH2+ ! Fe2+ hematite + HO2
(2)
Fe2+ hematite + H2O2 ! Fe3+ hematite + OH + OH
(3)
In addition, during the plasma treatment, some impurities were removed from the hematite surface, which enhanced the iron
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Fig. 6. EDX analysis of (a) NH and (b) PTH.
presence on the surface of catalyst [35]. These findings demonstrated that the plasma treatment could improve the catalytic efficiency of NH. Consequently, PTH was selected as an efficient nanocatalyst and its catalytic performance was evaluated in the PTH/H2O2 process for the AR17 degradation under various experimental conditions of the solution pH, initial H2O2 concentration, PTH dosage, initial dye concentration, and presence of peroxydisulfate enhancer and inorganic salts. The apparent pseudo-first order rate constants (kapp) for the studied systems were calculated by linear regression analysis and given in Table 1. The inset of Fig. 10 demonstrates that pseudo-first order kinetic model was suitable to describe the AR17 degradation by all of the studied processes, which was confirmed by other studies [44]. Appropriate correlation coefficients (R2), that was higher than 0.95 (Table 1) verified the suggested kinetic model. Effect of pH The pH is one of the significant operational factors in the PTH/H2O2 process. Therefore, the influence of this variable on the AR17 degradation was examined, and the data are presented in Fig. 11. The DE% declined by the pH increment from 3 to 9. The higher degradation efficiencies were achieved at lower pHs, and it declined with increasing the suspension pH. This observation can be ascribed based on the following three factors: (1) The OH radicals oxidation potential is raised by reducing the pH [45]; this demonstrates that in the acidic condition, the ability of the produced hydroxyl radicals for the AR17 removal was more than that of alkaline and neutral aqueous solutions. (2) The pH of suspension affects the pollutants adsorption on the heterogeneous catalyst surface, wherein the hydroxyl radicals were generated and attacked to the contaminants. According to the obtained results
Fig. 7. XPS spectrum of NH and PTH samples in the binding energies range of (a) 0– 1200 and (b) 700–740 eV.
from the pHpzc analysis (Fig. 9), at pH values lower than 7.8, the surface of PTH became positively charged. Therefore, the adsorption of the negatively charged AR17 molecules on the PTH surface enhanced. As a result, more AR17 molecules were removed by the surface OH radicals [34,46]. The opposite was happened at higher pH values than pHpzc. (3) Moreover, the acidic condition could dissolve more PTH surface iron into the suspension (Fig. 12). The better degradation efficiencies was achieved as the pH of suspension was adjusted between 3 to 5, with approximately the same effectiveness after 60 min of the process. Hence, the pH 5 was chosen as the favorable amount. On the other hand, the leached Fe concentration was less than 0.36 mg/L after PTH/H2O2 process at various pH values (Fig. 11). Considering the minor concentrations of the dissolved Fe ions, the production of hydroxyl radicals could be significantly attributed to the heterogeneous process on the surface of the PTH when compared to the homogeneous one, in AR17 degradation [47].
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Fig. 10. Degradation efficiency of the AR17 in the various processes; [AR17]0 = 20 mg/L, [PTH] = 0.75 g/L, pH = 5 and [H2O2]0 = 2 mM. The inset represents mentioned processes follow pseudo-first order kinetic. Table 1 Effect of the various processes on the apparent pseudo-first order constants of degradation of AR17. No. 1 2 3 4 5
Process H2O2 NH PTH NH/H2O2 PTH/H2O2
kapp (min1) 2
0.09 10 0.17 102 0.23 102 1.18 102 5.43 102
Correlation coefficient (R2) 0.97 0.95 0.96 0.98 0.96
Fig. 8. Nitrogen adsorption/desorption isotherms of (a) NH and (b) PTH.
Fig. 11. The effect of solution pH on the DE% of PTH/H2O2 process; [AR17]0 = 20 mg/ L, [PTH] = 0.75 g/L and [H2O2]0 = 2 mM.
Fig. 9. Plot for calculation of PTH pHpzc.
Effect of H2O2 initial concentration Optimization of the H2O2 concentration is essential because the major cost associated with the PTH/H2O2 process, is that of hydrogen peroxide. Furthermore, an excessive amount of oxidant acts as hydroxyl radical scavenger and reduces the degradation efficiency [48]. The effect of hydrogen peroxide concentration on the AR17 degradation is shown in Fig. 13. As can be observed from Fig. 13, the DE% enhanced with of the increment in the H2O2 concentration up to 2 mM. This enhancement was resulted from the production of higher amount of ROS through the H2O2 decomposition in the presence of PTH (Eqs. (1)–(3)). However, further addition of H2O2 decreased the DE% owing to the scavenging effect on OH radicals (Eq. (4)) [49–51].
H2O2 + OH ! H2O + HO2
(4)
Effect of the PTH dosage Fig. 14 shows the impact of the catalyst dosage on the PTH/H2O2 function in the dye degradation process. An increase in the PTH dosage up to 0.75 g/L led to an enhanced degradation efficiency. The reason for this phenomenon was that high catalyst dosage caused an extensive active sites for decomposition of H2O2 to ROS [39,52]. However, an inverse trend was observed with further increase in the PTH dosage. This trend was because of the scavenging effect of Fe (II) on OH radicals as described by Eq. (5) [53]. OH + Fe2+ ! OH + Fe3+
(5)
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Fig. 12. Leached iron concentration in the solution after 60 min; [PTH] = 0.7 g/L. Fig. 15. The effect of dye concentrations on the AR17 degradation efficiency by PTH/ H2O2 process; [PTH] = 0.75 g/L, pH = 5 and [H2O2]0 = 2 mM.
Effect of inorganic ions on the AR17 degradation Salting-out effect of inorganic salts such as Na2SO4, Na2CO3 and NaCl was investigated on the DE%. Sulfates, carbonates and chlorides are the commonly available anions in the wastewaters. These anions act as OH radical scavengers that produce other radicals with less oxidation potentials (Eqs. (6)–(8)). Furthermore, the catalyst positively charged sites at pH 5, were occupied by the anions and prohibite the treatment process. Thus, the DE% declined in the presence of the scavengers in the solution (Fig. 16) [55–58]. Cl + OH ! ClOH
Fig. 13. The effect of H2O2 concentration on the DE% of PTH/H2O2 process; [AR17]0 = 20 mg/L, [PTH] = 0.75 g/L and pH = 5.
Effect of dye initial concentration The influence of AR17 initial concentrations on the DE% was studied. The results are displayed in Fig. 15. The degradation of AR17 was decreased with an increase in its initial concentration because the same amounts of generated ROS at the identical operational conditions have to decompose more AR17 molecules and the by-products produced by its degradation [53,54]. Furthermore, the pseudo-first-order kinetics was fitted to the obtained data and kapp was calculated from the inset plots of Figs. 11, 13–15 and shown in Table 2.
(6)
CO32 + OH ! CO3
+ OH
SO42 + OH ! SO4
+ OH
(8) 2
The peroxydisulfate (S2O8 ) influence was also studied on the DE%. Sulfate radical (SO4) was formed in the presence of the PTH (Eqs. (9)), which had higher oxidation potential (2.6 V) than S2O82 Table 2 Effect of the operational parameters on the apparent pseudo-first order constant of degradation for PTH/H2O2 process. Operational parameters and amounts pH 3 5 7 9 H2O2 concentration (mM) 0 0.5 1 2 3 PTH dosage (g/L) 0 0.25 0.5 0.75 1
Fig. 14. The effect of PTH dosage on the DE% of PTH/H2O2 process; [AR17]0 = 20 mg/ L, pH = 5 and [H2O2]0 = 2 mM.
(7)
AR17 concentration (mg/L) 20 30 40 50
kapp (min1)
Correlation coefficient (R2)
6.54 102 5.43 102 1.92 102 0.50 102
0.99 0.96 0.99 0.98
0.14 102 1.04 102 1.91 102 5.43 102 2.58 102
0.94 0.98 0.97 0.96 0.99
0.13 102 0.53 102 1.23 102 5.43 102 3.39 102
0.94 0.98 0.99 0.96 0.97
5.43 102 2.23 102 1.48 102 0.98 102
0.96 0.97 0.97 0.97
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93
AR17 degradation intermediates The formed intermediate compounds during the degradation of AR17 through PTH/H2O2 process were determined using the GC– Mass method. The retention time (tR) and main fragments of the by-products after 15 min of process are given in Table 3. However, It was not feasible to identify the all present compounds due to their little accumulation and restriction of the GC–Mass method. Subsequently, a possible degradation pathway was suggested based on the identified intermediate compounds, (Fig. 17). The degradation process could occur by the cleavage of N¼N, C S, C C and C N bonds and finally, mineralizing the intermediates to H2O and CO2 [60,61].
Fig. 16. The effect of peroxydisulfate and OH radical scavengers on the AR17 degradation by PTH/H2O2 process; [AR17]0 = 20 mg/L, [PTH] = 0.75 g/L, pH = 5, [H2O2]0 = 2 mM and [Scavenger]0 = 20 mg/L.
(2.01 V) [59]:
2 ! Fe3þ þ SO2 Fe2þ 4 surf þ S2 O8 surf þ SO4
ð9Þ
Stability of PTH in successive PTH/H2O2 process Catalyst stability in the repeated usages is one of the substantial properties from practicable viewpoint [62]. Thus, the PTH was used in five successive Fenton-like processes, and then, its performance was determined. In each run, 20 mg/L AR17, 2 mM H2O2 and 0.75 g/ L PTH in 500 mL of solution were stirred together at pH of 5 for 60 min. The results demonstrated that the DE% was approximately unchanged after the applications (Fig. 18). Besides, the leaked iron determinations data (Fig. 12) revealed that the released iron concentration into the solution was 0.19 mg/L per run, wherein the
Table 3 Identified by-products during degradation of AR17 by PTH/H2O2 process. No.
Compound name
Retention time (min)
Main fragments
1
a-Nitronaphthalene
11.358
115, 127, 145, 173
2
a-Naphthol
26.384
58, 89, 115, 144
3
a-Naphthoquinone
16.427
76, 102, 130, 158
4
1,2-Benzenedicarboxylic acid
9.464
50, 76, 104
5
1,8-Octanediol
8.475
31, 41, 55
6
n-Pentanoic acid
4.782
27, 41, 45, 60, 73
7
2-Propenoic acid
5.449
27, 45, 55, 72
Structure
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which were characterized using XRD, FT-IR, SEM, EDX, XPS, and BET analyses. The sputtering effect of N2 plasma leds to the formation of the PTH nanostructures with a large surface area. Moreover, the plasma treatment could eliminate impurities from the surface of NH. As a result, the PTH function in the heterogeneous Fenton-like process enhanced remarkably compared to the NH for the AR17 degradation as a model textile dye. Degradation efficiency of 97.7% was obtained at pH of 5, initial H2O2 concentration of 2 mM, PTH dosage of 0.75 g/L, and initial AR17 concentration of 20 mg/L after 60 min of the reaction. The increase in peroxydisulfate ion and OH radical scavengers declined the AR17 degradation, verifying the key role of hydroxyl radicals in the PTH/H2O2 process. The GC–Mass analysis was applied to identify a number of the degradation by-products. The other considerable benefits of the stable PTH were low leaked iron in the solution and its application over milder pH values. Acknowledgments We thank the University of Tabriz for the support provided. The authors acknowledge Dr. Shima Rahim Pouran for language edit. We also acknowledge the support of Iran Science Elites Federation. References
Fig. 17. Proposed pathway for degradation of AR17 by PTH/H2O2 process.
Fig. 18. Reusability of the PTH for five repeated runs; [AR17]0 = 20 mg/L, [PTH] = 0.75 g/L, pH = 5 and [H2O2]0 = 2 mM.
PTH amount was 0.75 g/L. Therefore, the major amount of iron remained in the PTH structure, thus the catalyst could be considered as stable. Conclusions The environment-friendly nitrogen glow discharge plasma technique was applied successfully to form a modified hematite,
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