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Photo-Fenton process in Co(II)-adsorbed admicellar soft-template on alumina support for methyl orange degradation
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Photo-Fenton process in Co(II)-adsorbed admicellar soft-template on alumina support for methyl orange degradation Prateeksha Mahamallik, Anjali Pal
⁎
Civil Engineering Department, Indian Institute of Technology Kharagpur, West Bengal, 721302, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Co(II)-modified alumina Heterogeneous photo-Fenton Methyl orange Degradation Response surface methodology
In the present work, a novel approach has been made to prepare a new catalyst for Fenton-type degradation of dye. In the first step alumina is modified with sodium dodecyl sulfate (an anionic surfactant). Under suitable conditions the surfactant forms bilayer (admicelle) on alumina surface. The surfactant-modified alumina (SMA) can adsorb metal ions such as Co(II), Fe(II), Cu(II), Mn(II) and Ni(II) on its surface. Among all the metal ions Co (II) shows the best catalytic activity when supported on SMA. The newly formed catalyst is named as Co-SMA. In this report, we described the degradation of methyl orange (MO) under heterogeneous photo-Fenton condition by application of Co-SMA as the catalyst and in the presence of H2O2 and visible light. Effect of various independent variables such as dose of catalyst, initial concentration of the dye, H2O2 concentration, intensity of visible light, pH have been studied. The process parameters were optimized by response surface methodology (RSM) approach. Under the optimum conditions of 30 mg/L MO, 29.92 g/L Co-SMA, 37.9 mM H2O2 and pH 4.31, the maximum response for MO decolorization efficiency was 94.79%.
1. Introduction The extensive use of synthetic dyes in various industries has resulted in considerable pollution in the water bodies. Textile industries discharge large volume of dye-containing water where azo dyes are most common (70% of all dyes) [1]. It is very difficult to degrade azo dyes because of their complex structures and stability. The discharge of azo dye into water bodies is unacceptable not only for its color but also for its toxicity, non-biodegradability and mutagenic nature [2]. It is hazardous for aquatic life because it reduces the reoxygenation capacity of water. Therefore, removal of harmful azo dyes from wastewater is a serious concern. Among various treatment methods, advanced oxidation processes (AOPs) are efficient technologies for removing recalcitrant organic pollutants [3,4]. Nickel loaded TiO2 was immobilized on activated carbon and applied for photocatalytic reduction of carbon dioxide to methanol under visible light irradiation [5]. Photocatalytic oxidation of benzene was experimented by TiO2 nanowall situated on activated carbon [6]. Carbon sphere TiO2-FeO was synthesized and utilized for photocatalytic degradation of toluene from water environment [7]. Photocatalytic degradation of o-chlorophenol was carried out by TiO2/ NiO/RGO system under visible light irradiation [8]. The mechanism of AOPs involves the generation of highly reactive
⁎
free radicals (mainly hydroxyl radicals, E0 = 2.80 V/SHE) [9] which attack the pollutant and mineralize it completely. Classic homogeneous Fenton is one such powerful and widely used AOP capable of oxidizing variety of organic pollutants. In homogeneous Fenton reaction, soluble iron (II) reacts with hydrogen peroxide (H2O2) under acidic condition (pH ∼3) and produce hydroxyl (˙OH) radicals and Fe3+, and at last Fe2+ gets regenerated (Eqs. (1)–(3) [10]. Fe2+ + H2O2 → Fe3+ + OH− +˙OH Fe
3+
+ H2O2 → H
FeOOH
2+
˙
+
+ FeOOH
→ HO2 + Fe
2+
2+
(1) (2) (3)
In case of photo-Fenton reaction more hydroxyl radicals are generated (Eq. (4)). Fe(OH)2+ → Fe2+ + ˙OH
(4)
However, the homogeneous Fenton method has many drawbacks. For example it works in low pH range; produces iron sludge, and shows hindrance in Fenton catalytic cycle due to the formation of stable Fe3+ complexes. To combat these difficulties, people tried to prepare various types of heterogeneous catalysts which work for Fenton and Fenton-like reactions. In that case the reaction can take place over a wide pH range, in presence of lower concentration of metal, and the catalyst can be
Corresponding author. E-mail address: [email protected] (A. Pal).
https://doi.org/10.1016/j.cattod.2019.07.045 Received 3 June 2019; Received in revised form 27 July 2019; Accepted 29 July 2019 0920-5861/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Prateeksha Mahamallik and Anjali Pal, Catalysis Today, https://doi.org/10.1016/j.cattod.2019.07.045
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recycled. Different synthetic and natural catalyst supports such as zeolites [11–13], bentonites [14,15], alumina [16], silica [17,18], clay [19,20], resin [21] are applied to develop heterogeneous catalysts for Fenton and Fenton-like reactions. In this study, surfactant-modified alumina (SMA) is used as template for adsolubilizing different metal ions viz., Co (II), Fe(II), Cu(II), Mn(II) and Ni(II). The surfactant used here is sodium dodecyl sulfate (SDS), which is an anionic surfactant. It is reported that under suitable conditions it forms a bilayer on alumina surface which has the capability to support positively charged ions. Out of all metal ions applied the activity of cobalt-loaded SMA (Co-SMA) towards methyl orange (MO) degradation was the maximum. The detail study of MO degradation by Co-SMA was explored. 2. Materials and methods Fig. 1. Schematic of M2+-SMA preparation and MO degradation.
2.1. Chemicals
2.3.3. Loading of metal ions on surfactant-modified alumina (SMA) Stock solutions of metal ions viz., Fe(II), Ni(II), Mn(II), Cu(II) and Co(II) with concentration 100 mg/L were prepared in distilled water. Then 3 g of SMA was added to 50 mL of each of these solutions, and kept for required time under ambient condition, with occasional hand shaking. The kinetic of adsorption of metal ions on SMA was monitored using AAS in each case. After maximum adsorption (which took ∼4 h), the supernatant solutions were collected to find out the remaining concentrations of metal ions. The metal-loaded SMA in each case was washed thrice with distilled water. Then it was dried in an oven at 60 °C (4 h). The dried materials (designated as M2+-SMA) were collected and kept in airtight bottles.
Alumina and sodium dodecyl sulfate (SDS) was procured from SRL (India) and Merck, respectively. Glacial acetic acid, methyl orange (MO), cobalt chloride hexahydrate, FeSO4.7H2O, CuSO4.5H2O, NiSO4.7H2O, MnCl2.4H2O and 30% H2O2 were purchased from Merck. The dye acridine orange was purchased from Loba chemicals. The water used was double distilled. 2.2. Instrumentation UV-vis spectrophotometer (Model: SPECTRASCAN UV 2600, Chemito, India) equipped with a 1-cm quartz cuvette was used. To stir the solution, a magnetic stirrer (Tarsons, India) was used. The visible light source was a normal W-filament bulb. Atomic absorption spectrometer (AAS; Varian AA240FS, Australia) was used for cobalt determination. Acetylene flame was used in the AAS. The GC–MS (Agilent Technologies, United States) analyses of the reaction mixture were done to know the products formed. The column used was DB-5 and flow of solution was 1 mL/min.
2.3.4. Determination of metal concentration Concentrations of metals (cobalt, copper, iron, manganese, nickel) were found out from the calibration graphs made using AAS. The concentration of stock solution of all metal ions was 100 mg/L. The desired concentrations (0–5 mg/L) of metal solution were prepared from the stock solution. For each metal, the calibration graphs were plotted according to the absorbance values given by AAS. The calibration equation of iron was: Absorbance = 0.065 × Concentration (mg/L) + 0.018, R2 = 0.999. The calibration equation of nickel was: Absorbance = 0.048 × Concentration (mg/L) - 0.005, R2 = 0.998. The calibration equation of manganese was: Absorbance = 0.078 × Concentration (mg/L) - 0.009, R2 = 0.992. The calibration equation of copper was: Absorbance = 0.068 × Concentration (mg/L) - 0.01, R2 = 0.993. The calibration equation for cobalt was: Absorbance = 0.0434 × Concentration (mg/L) + 0.0001 (R2 = 0.990).
2.3. Preparation of material 2.3.1. Modification of alumina with surfactant to prepare SMA The following procedure was adopted to prepare the surfactantmodified alumina (SMA) [22]. First, 20 g of SDS was dissolved in 900 mL water and 2.5 g NaCl was dissolved in 100 mL water. These two solutions were mixed together. Then the pH of the mixture was adjusted to ∼4.5 using dilute HCl. Further, 100 g of alumina was added to the solution and the mixture was shaken for 24 h. The shaking speed was 150 rpm and the temperature was 30 °C. The supernatant was taken and the SDS concentration was measured. The as-prepared alumina was named as surfactant-modified alumina (SMA). The SMA was washed with water and dried at 60 °C for 12 h. The material was stored in a closed bottle at room temperature. The SMA was loaded with metal ions and used further as a catalyst for MO degradation. The work is pictorially shown in Fig. 1.
2.4. MO degradation 2.4.1. Experimental for MO degradation on Co-SMA The activity of Co-SMA as a heterogeneous photo-catalyst was studied by the degradation of MO in water under visible light irradiation. MO in presence of Co-SMA was stirred and the adsorption-desorption equilibrium was reached in 30 min. The desired concentration of MO was prepared by diluting the stock solution (100 mg/L). After adsorption of MO reached the equilibrium condition, heterogeneous photoFenton reaction was started by addition of H2O2 and with irradiation of visible light. The distance in between the top surface of the solution and the light source was ∼20 cm. The remaining concentrations of MO during degradation were monitored time to time. The degraded products of MO were identified by GC–MS analysis. For GC–MS analysis, 10 mL of reacted MO samples (after 20 min and 60 min of reaction time) were extracted in equal volume of ethyl acetate. The extracted sample was directly injected for GC–MS analysis.
2.3.2. Determination of surfactant loading on Al2O3 The calibration curve of SDS was developed where a phase separation technique was adopted to measure the concentration of SDS [22]. For this, acridine orange (5 × 10−3 M), which is a cationic dye, and glacial acetic acid were added to SDS solutions having different known concentrations (0–5 mg/L). Acridine orange acted as an ionpairing agent with SDS. The complex was soluble in toluene; so it was extracted in toluene and the absorbance was measured at 467 nm. The absorbance is proportional to SDS concentration. The calibration equation was: Abs. = 0.2 × Conc. (mg/L) + 0.008 (R2 = 0.987). The SDS loading was 115.5 mg (0.4 × 10−3 mole) per gram of alumina. 2
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cobalt was 1.57, 1.64, 1.65, 1.66, and 1.38 mg/g of SMA, respectively (Fig. 2(b)).
2.4.2. Analytical procedure to determine MO decolorization efficiency The analysis of decolorization efficiency was done by the help of spectrophotometric method. Standard solutions of MO were prepared in the concentration range of 0–30 mg/L from the prepared stock of 100 mg/L. The absorbance of MO was monitored at λmax 463 nm. Concentration of MO was found out from the calibration equation: Absorbance = 0.078 × Concentration (mg/L) + 0.017 (R2 = 0.997). The percentage decolorization of MO was calculated as follows:
3.1.2. Decolorization of MO by M2+-SMA The activities of M2+-SMA towards Fenton reaction for degradation of MO were studied. Firstly, 10 mL of MO solution with initial concentration of 20 mg/L was stirred for 30 min to get the dye adsorbed on the surface of M2+-SMA (dose: 20 g/L). Then, H2O2 at a concentration of 111.7 mM was added to the solution and it was irradiated with visible light. The adsorption of MO on M2+-SMA surface was found to be 26.7, 5.1, 5.8, 0.45, 0.38% in case of Co, Cu, Fe, Ni, Mn metal ions loaded SMA surfaces, respectively (Fig. 3(a)). The removal efficiency of MO through adsorption was highest for Co-SMA. The color removal of MO after 60 min of reaction was found to be 94.04, 44.55, 29.8, 3.27, and 4.9% by Co-SMA, Cu-SMA, Fe-SMA, Ni-SMA, and Mn-SMA (Fig. 3(b)). The decolorization efficiency was the maximum in case of Co-SMA. So, Co-SMA was selected as the working material for further study. Here the metal Co(II), embedded on SMA, was the active catalyst for Fenton reaction; hence it was important to compare, the performance of SMA to adsolubilize Co(II), with the performance of normal alumina. As reported in our earlier studies, which were carried out for the removal of other metal ions [25,26], the SMA showed > 6 times higher adsorption capacity for Co(II) as compared to that of alumina. Under similar experimental conditions, the loading of cobalt on alumina was 0.205 mg/g (3.47 × 10−6 mole/g) and that on SMA was 1.38 mg/g (2.34 × 10-5 mole/g). It is interesting to note that, the ratio of SDS / cobalt on SMA was ∼17:1 (mole / mole). Thus the prepared Co-SMA was used as the catalyst for the degradation of MO.
Dye decolorization (%) = [(C0 - Ct) / C0] × 100 where, C0 = Concentration of MO at zero time of reaction and Ct = Concentration of MO at time t.
3. Results and discussion 3.1. Selection of catalyst 3.1.1. Adsorption of metal ion (M2+) on SMA surface Alumina surface can adsorb SDS to form hemimicelle or admicelle under appropriate conditions. This micellar layer can be exploited to adsorb organic molecules [22–24] or metal ions [25,26] at a high concentration. In this work our focus was to observe the synergistic performance of such a system, where the metal ion and the dye both could co-exist within the surfactant bilayer in fully adsorbed state; or a system where the dye remains partly in solution phase and partly adsorbed and the metal ion remained in solid supported form; and finally they reacted with the H2O2 in Fenton reaction. The soft-template of admicelle on the alumina surface may thus provide some better environment for the reaction to take place. The catalysis in heterogeneous solid surface containing the micellar environment may lead to interesting behavior. This type of attempt is novel in Fenton process. The system was selected such that, the metal ion was supported on a substrate. In this regard SMA was considered as a suitable template for such reaction. Here we have selected several metal ions (M2+) viz., iron, manganese, copper, nickel, cobalt to compare the activity in the photo-Fenton process. The metal ions were adsorbed on SMA surface through the interaction of negatively charged dodecyl sulfate and positively charged metal ion (Fig. 1). To find out the optimum time for metal ion (M2+) adsorption, a kinetic study was done. As shown in Fig. 2(a), in 4 h time the adsorption for copper, manganese, nickel and iron was ∼94%, ∼98%, ∼99% and ∼99%, respectively. In contrast to this, adsorption of Co(II) on SMA was less. During the initial phase the rate was faster and ∼67% removal of Co(II) took place in initial 50 min (Fig. 2(a)). However, 240 min time was considered as the contact time for preparation of M2+-SMA. This is because during this period maximum amount of metal ions were removed (Fig. 2(a)). The loading of copper, manganese, nickel, iron and
3.2. Characterization of Co-SMA In our earlier report the characterization of Co-SMA using different methods viz., scanning electron microscopy (SEM)/electron diffraction X-ray analysis (EDAX), diffuse reflectance spectroscopy (DRS), X-ray photo electron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) was described [27]. It was confirmed that, Co-SMA contained both cobalt (0.22% by weight) and SDS. 3.3. Degradation of MO under different conditions Degradation of MO was studied under different conditions i.e. MO + Visible light (VL), MO + VL+H2O2, MO + VL + Alumina, MO +H2O2+Alumina, MO + VL+H2O2+Alumina, MO + VL + SMA, MO +H2O2+SMA, MO + VL+H2O2+SMA, MO + VL + Co-SMA, MO +H2O2+Co-SMA and MO + VL+H2O2+Co-SMA. The results are shown in Fig. 4. Out of all combinations applied, only MO+H2O2+CoSMA and MO + VL+H2O2+Co-SMA combinations were able to degrade MO. In all the systems where alumina or Co-SMA was used, MO
Fig. 2. Time dependent adsorption of metal ion (M2+) on SMA surface (a) concentration (mg/L) remained vs. time (min) and (b) loading (mg/g) vs. time (min). 3
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Fig. 3. Activity of M2+-SMA towards MO degradation (a, b) (conditions for (a): initial concentration of MO: 20 mg/L; dose of M2+-SMA: 20 g/L; H2O2 concentration: 111.7 mM; intensity of visible light: 1620 lux; time of reaction: 60 min).
3.4.2. Effect of initial MO concentration The reaction was studied using different MO concentrations (in the range of 10–30 mg/L), with a constant Co-SMA dose of 10 g/L. The reaction followed first order rate at all MO concentrations. The rate constant decreased from 0.025 to 0.017 min−1 with the increase in concentration. The kinetics of MO degradation is shown in Fig. 6. 3.4.3. Effect of H2O2 concentration In our earlier report [27] the decomposition of H2O2 in presence of Co-SMA and visible light was studied, and the decomposition rate was found to be dependent on the catalyst dose. The decomposition of H2O2 produced hydroxyl radicals which degraded the dye. Thus in the reaction, both the SMA dose and the initial concentration of H2O2 play a vital role in the oxidation of organic compounds in the heterogeneous Fenton processes. In the present work the effect of the hydrogen peroxide was analyzed by varying its initial concentration between 37.9 mM and 251 mM (Fig. 7). The reaction followed first order with all applied H2O2 concentrations. The rate constant increased from 0.013 to 0.027 min−1 as the concentration of H2O2 increased from 37.9 mM to 182.7 mM. Since the concentration of H2O2 is directly related to the amount of hydroxyl radicals produced in the catalytic reaction, this parameter influences degradation efficiency. However, excess amount of H2O2 (251 mM) was not found advantageous to the degradation because of the scavenging of ·OH radical by excess H2O2 (Eq. (5)) [28].
Fig. 4. Time dependent degradation of MO under different experimental conditions (conditions: initial concentration of MO: 20 mg/L; catalyst dose: 20 g/L; light intensity: 1620 lux; concentration of H2O2: 111.7 mM).
was initially allowed to be adsorbed on the solid surface. The degradation of MO was ∼71% in 45 min (Fig. 4). However, the same system if operated in presence of visible light (i.e. if the adopted condition is MO + VL+H2O2+Co-SMA) the efficiency was further improved up to ∼86% in 45 min (Fig. 4). So the reaction with the combination MB + VL+H2O2+Co-SMA was considered to proceed further. The reason of the increased efficiency under visible light illuminated condition is possibly because of Co(III) to Co(II) conversion by the visible light. This Co(III) to Co(II) conversion is essential for our Fentonlike process.
·OH + H2O2 → H2O + H2O·
(5)
3.4.4. Effect of the intensity of visible light To examine the impact of visible light intensity on the catalytic activity of Co-SMA, the light intensity was varied from 993 to 5560 lux. It is clearly observed from Fig. 8 that with increase in light intensity from 993 to 1620 lux, the decolorization efficiency increased from ∼74% to ∼93%. Further increase in light intensity, however, caused reduction in decolorization efficiency.
3.4. Effect of reaction parameters on the decolorization of MO 3.4.1. Effect of the catalyst dosage To study the effect of dose of catalyst on MO degradation efficiency, the dose was varied from 10 g/L to 30 g/L. To achieve the adsorptiondesorption equilibrium, first the solution was stirred and it was seen that in 30 min time the adsorption equilibrium was attained. About 30% (i.e. 6 mg/L) of MO was adsorbed on Co-SMA surface in case of all doses of SMA (Fig. 5(a)). After that the reaction was started by adding required amount of H2O2 and under exposure of visible light. Within 30 min of the reaction (after adsorption) degradation of MO increased from ∼33% to ∼61% when the dose was increased from 10 g/L to 30 g/L (Fig. 5(a)). The reaction followed first order. The graph plotting ln C/C0 vs. time (min) is shown in Fig. 5(b). The reaction rate constant increased from 0.019 to 0.074 min−1 with the increase in Co-SMA dose from 10 g/L to 30 g/L. This is obvious that, more hydroxyl radicals were produced with increase in Co-SMA dosage, which enhanced the degradation rate of MO.
3.4.5. Effect of the initial solution pH The influence of initial pH was examined by varying the pH from 4 to 12 by adding HCl or NaOH. It is clearly observed in Fig. 9 that, the decolorization efficiencies were almost same for initial pH 4, 6, and 9. But it decreased drastically for initial pH 12. MO was allowed to achieve adsorption-desorption equilibrium for 30 min then H2O2 was added and it was exposed to visible light. Removal of MO by adsorption was ∼39%, ∼31% and ∼32% in case of initial pH 4, 6, and 9, respectively. But adsorptive removal was only ∼0.12% for pH 12. This reveals that adsorption has an important role in fast decolorization of MO. The surface charge of Co-SMA plays an important role in the degradation of MO at different initial pH. The pHzpc of Co-SMA was found 4
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Fig. 5. (a) Decolorization of MO vs. Co-SMA dose (g/L) (time of reaction: 30 min) and (b) Plot of ln C/C0 vs. time (min) (after adsorption) (conditions: initial concentration of MO: 20 mg/L; H2O2 concentration: 111.7 mM; intensity of visible light: 1620 lux).
Fig. 8. Decolorization of MO vs. light intensity (conditions: initial concentration of MO: 20 mg/L; dose of Co-SMA: 25 g/L; H2O2 concentration: 111.7 mM; reaction time: 45 min). Fig. 6. Time dependent degradation of MO at different concentrations (conditions: dose of Co-SMA: 10 g/L; H2O2 concentration: 111.7 mM; intensity of visible light: 1620 lux).
Fig. 9. Plot of decolorization (%) of MO vs. time at different initial pH (conditions: initial concentration of MO: 20 mg/L; dose of Co-SMA: 25 g/L; H2O2 concentration: 111.7 mM; light intensity: 1620 lux).
Fig. 7. First order kinetic plots of MO degradation at different H2O2 concentrations (conditions: initial concentration of MO: 20 mg/L; dose of Co-SMA: 10 g/L; intensity of visible light: 1620 lux). 5
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Table 2 ANOVA results of the quadratic model for decolorization of MO by heterogeneous photo-Fenton reaction. Source
Sum of squares
Degree of freedom (df)
F-value
p-value
Model A B C D AB AC AD BC BD CD A2 B2 C2 D2 Lack of fit
9083.08 786.02 430.32 76.81 4613.06 0.72 1155.66 108.47 196.84 2.50 13.07 90.77 1.83 655.38 901.92 50.78
14 1 1 1 1 1 1 1 1 1 1 1 1 1 1 10
160.88 194.91 106.71 19.05 1143.91 0.18 286.57 26.90 48.81 0.62 3.24 22.51 0.45 162.52 223.65 3.58
< 0.0001 < 0.0001 < 0.0001 0.0006 < 0.0001 0.6785 < 0.0001 0.0001 < 0.0001 0.4445 0.0934 0.0003 0.5110 < 0.0001 < 0.0001 0.1154
R2 = 0.9938; R2adj = 0.9876; R2pred = 0.9670; CV% = 3.28; Adeq. precision = 46.458.
Fig. 10. Efficiency of Co-SMA for decolorization of MO in consecutive cycles (conditions: initial concentration of MO: 20 mg/L; dose of Co-SMA: 25 g/L; H2O2 concentration: 111.7 mM; light intensity: 1620 lux).
3.5. Recyclability of the catalyst
to be 9.2. Below pH 9.2, the surface is positively charged. So, below pH 9.2, adsorption of MO is higher because of ionic interaction between positively charged Co-SMA surface and negatively charged MO molecule. This is in agreement with the obtained results. Degradation of MO also followed the similar trend. The degradation of MO at pH 4, 6 and 9 was ∼90%, ∼84% and ∼91% respectively, whereas at pH 12 the decolorization was only ∼30%.
Recyclability tests were conducted to investigate the reusability of the catalyst in consecutive runs. After each cycle, the catalyst was easily separated from the reaction mixture by centrifugation. This was followed by washing and drying in an oven at 60 °C, and finally the catalyst was reused. As shown in Fig. 10, the decolorization efficiency was 93.37% in first cycle, whereas it was 84.68, 83.4, 69.81% in second, third and fourth cycle, respectively. The adsorption of MO on Co-SMA was increased significantly from 31% to 54.7% after first cycle.
Table 1 BBD matrix of independent variables and their corresponding experimental and predicted values for RSM model. Sl. No.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
Initial concentration of MO (A, mg/L)
30 20 30 30 20 20 20 20 20 20 30 20 30 10 20 10 20 20 10 20 20 20 10 10 10 20 20 20 30
Dose of Co-SMA (B, g/L)
H2O2 Concentration (C, mM)
30 20 20 20 20 30 10 20 20 30 20 10 10 20 20 30 20 20 10 30 20 30 20 20 20 20 10 10 20
144.45 251 251 144.45 251 251 144.45 144.45 144.45 144.45 144.45 144.45 144.45 251 37.9 144.45 144.45 348.8 144.45 144.45 144.45 37.98 37.9 144.45 144.45 37.9 37.9 251 37.9
6
pH (D)
8 4 8 4 12 8 12 8 8 4 12 4 8 8 12 8 8 8 8 12 8 8 8 12 4 4 8 8 8
Color removal (%) Experimental
Predicted
70.37 66.39 39.19 77.91 32.01 59.06 31.28 68.53 68.73 83.83 26.91 71.92 58.91 92.48 22.89 84.94 67.78 70.97 71.78 45.82 68.87 66.9 55 52.87 83.04 64.5 42.21 62.43 69.7
69.66 67.46 40.10 77.65 31.86 59.90 30.27 68.98 68.98 81.46 28.02 71.06 58.53 90.29 23.19 86.69 68.98 68.98 73.87 43.82 68.98 68.87 51.53 54.62 83.42 66.02 42.86 61.95 69.04
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Fig. 11. (a) Comparison of experimental and predicted response for each run and (b) Predicted vs. actual data.
3.6. Comparative study on homogeneous and heterogeneous Fenton-like reaction It is essential to compare the efficiency of homogeneous and heterogeneous cobalt mediated photo-Fenton degradation of MO. For this purpose, the degradation efficiency of MO under homogeneous Fentonlike reaction condition was studied with the Co(II) concentration of 27.6 mg/L in 20 mg/L of MO solution. The concentration was chosen by the assumption that, the whole amount of Co adsorbed on SMA took part in the reaction. It was interesting to note that no degradation in solution phase took place at that Co(II) concentration when homogeneous photo-Fenton reaction was carried out. So, the solution phase concentration of cobalt was increased gradually by 5, 150, 350, 700, 900, 1100 times as that of the Co(II) present in Co-SMA. To our surprise it was noted that, for homogeneous catalysis conditions even at 1100 times higher concentration of Co(II) present in Co-SMA, the degradation of MO was found to be only ∼39% after 60 min reaction. On the other hand, in case of Co-SMA, the MO degradation was ∼90% in 45 min with 20 g/L Co-SMA dose. It revealed that, the heterogeneous photo-Fenton reaction was much more efficient compared to the homogeneous one. From the above experimental results, the possibility of MO degradation due to the leached cobalt was ruled out.
Fig. 12. Pareto chart showing the percentage effect of variables on MO decolorization.
3.7. Cobalt leaching from Co-SMA during MO degradation It was pertinent from environmental point of view to study the leaching of cobalt for the adopted photo-Fenton process. The concentration of cobalt in the reacting solution after 60 min was very low (1.81 mg/L), and this was only 2.15% of the loaded cobalt on SMA. This small leaching might be due to the bilayer structure of surfactant, which helped to attach cobalt firmly. This speaks in favor of adsolubilizationbased approach, which is beneficial for water treatment. 3.8. Turn over number (TON) and turn over frequency (TOF) Turn over number (TON) and turn over frequency (TOF) gives the idea about the performance of a catalyst. The catalysts, which have higher TON and TOF are considered as better catalysts. Here the TON and TOF of the catalyst were calculated for the reaction of MO having concentration 20 mg/L and Co-SMA having dose 20 g/L in presence of 111.7 mM H2O2 and visible light having intensity of 1620 lux. The TON and TOF values for the above condition were found to be 18.4 × 1023 molecules/g and 6.8 × 1020 molecules/g/second, respectively.
Fig. 13. Plot of normal probability and internally studentized residuals for color removal of MO dye.
3.9. Application of statistical experimental design to heterogeneous photoFenton decolorization of MO using Co-SMA as catalyst A three-level four-variable Box-Behnken design (BBD) of experiment 7
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Fig. 14. (a1) Response surface plot and (a2) the contour showing effect of H2O2 concentration and initial concentration of MO (conditions: dose of Co-SMA: 20 g/L; pH: 8), (b1) response surface plot and (b2) contour showing effect of hydrogen peroxide concentration and dose of Co-SMA (conditions: initial concentration of MO: 20 mg/L; pH: 8), (c1) response surface plot and (c2) contour showing effect of dose of Co-SMA and initial concentration of MO (conditions: concentration of H2O2: 144.45; pH: 8), (d1) response surface plot and (d2) contour showing effect of pH and initial concentration of MO (conditions: dose of Co-SMA: 20 g/L; concentration of H2O2: 144.45), (e1) response surface plot and (e2) contour showing effect of pH and dose of Co-SMA (conditions: initial concentration of MO: 20 mg/L; concentration of H2O2: 144.45), (f1) response surface plot and (f2) contour showing effect of pH and H2O2 concentration (conditions: initial concentration of MO: 20 mg/ L; dose of Co-SMA: 20 g/L). 8
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Fig. 14. (continued)
Fig. 15. Degradation pathway of methyl orange during Co-SMA mediated photo-Fenton process.
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with increase in concentration of MO (Fig. 14 (c1 and c2)). Decolorization of MO is maximum in the pH range of 4–8. However in this range also there is decrease in color removal with increase in initial concentration of MO with fixed dose 20 g/L and 144.45 mM H2O2 concentration (Fig. 14 (d1 and d2)). Again from Fig. 14 (d2) it is clear that, pH 4–8 is favorable for MO decolorization. The decolorization efficiency is higher at 30 g/L dose than that of 10 g/L for 20 mg/L MO and 144.45 mM H2O2 concentration because of enhanced production of hydroxyl radical (Fig. 14 (e1 and e2)). The interactive effect of pH and H2O2 concentration shows that, the efficiency is maximum in the pH range of 4–6 and 144.45 mM H2O2 concentration (Fig. 14 (f1 and f2)). Any process is optimized to find out the best possible output using modeling tools and analytical procedures. The optimum conditions for the dye decolorization was found to be 30 mg/L MO, 29.92 g/L CoSMA, 37.9 mM H2O2 and 4.31 pH. Under these conditions the decolorization efficiency of MO was 94.79%. The model was validated in batch reactor to check the accuracy. The decolorization of MO in the optimized condition was found to be 94.2% which is close to the predicted value.
was considered to analyze the photo-Fenton decolorization of MO by Co-SMA. In the present study, four significant process variables viz. initial concentration of MO (A, mg/L), dose of Co-SMA (B, g/L), H2O2 concentration (C, mM) and pH (D) that primarily influenced the decolorization efficiency of MO dye were considered as independent variables. Total number of experiments was 29 with 5 replicates at central point. The batch experiments were conducted to analyze the interactive effect of independent variables on the decolorization efficiency of MO and the results are listed in Table 1. The response of photo-Fenton-like reactions was fitted to the quadratic model and the significance and adequacy were tested by ANOVA (analysis of variances) using Design Expert Software (Version 7.0.0, Stat Ease Inc., USA) (Table 2). The coefficient of determination (R2) is 0.9938 which implies the regression model fits well to the experimental value. The predicted R2pred (0.9670) is in reasonable agreement with adjusted R2adj (0.9876). The decolorization efficiency explained by independent variables in terms of coded factors is expressed by the following second-order equation (Eq. (6)), Y = 68.98 - 8.09A + 5.99B + 2.53C - 19.61D - 0.42AB - 17AC - 5.21AD - 7.02BC + 0.79BD + 1.81CD + 3.74A2 - 0.53B2 - 10.05C2 - 11.79D2 (6)
3.10. Degradation pathway of MO dye The reaction intermediates of MO degradation by photo-Fenton reaction using Co(II)-SMA were analyzed using GCeMS, and the mechanism of degradation was suggested. The sample after 20 min reaction showed peak at m/z 327 which was due to the parent molecule [30]. Therefore, from this peak it was confirmed that MO was not fully degraded at 20 min. At 16.7 min retention time, the peak found at m/z 137 represented N,N-dimethyl p-phenylenediamine. This proved the symmetric cleavage of azo bond [30,31]. The molecule further degraded to smaller molecules showing peaks at m/z 94 and 95. After 60 min reaction, at 13.1 min retention time, a peak was noticed at m/z 181. This was due to the other half of the symmetric cleavage of methyl orange molecule [32]. The compound further degraded by the hydroxyl radicals (generated during Fenton reaction), to smaller molecules showing peaks at m/z 159 and 129. The plausible degradation pathway of MO is shown in Fig. 15.
In Eq. (6), Y is the decolorization efficiency of MO; A, B, C and D is the corresponding coded variables of initial dye concentration, catalyst dose, H2O2 concentration and pH respectively. In this model, the linear coefficients of A, B, C and D, interaction coefficients of AC, AD and BC, and quadratic coefficient of A2, C2 and D2 are significant terms. The process variables possessing higher Fisher's Ftest values have greater influence on the decolorization efficiency of MO. The higher model F-value (i.e. 160.88) and lower p-value (i.e. < 0.0001) have corroborated that the developed model is highly significant and there is only a 0.01% chance of occurrence of model Fvalue due to noise (Table 2). The experimental and RSM predicted results for decolorization of MO is shown in Fig. 11(a). Both the data are close to each other in each run. The model predicted and experimental fitted well with R2 equal to 0.9938 (Fig. 11(b)). Additionally, the influence of each parameter on MO decolorization was calculated by Pareto chart analysis [29] and represented in Fig. 12. The extent of each bar in the Pareto chart represents the percentage effect of that variable on the response. The maximum effect on decolorization of MO was contributed by initial pH of the solution, i.e. ∼34.5%, followed by interaction between initial concentration of MO and H2O2 concentration which is ∼26%, followed by quadratic effect of initial pH (∼12.5%). The normal probability plot of the residuals is an important diagnostic tool to detect and explain the deviation of assumptions that errors are normally distributed and they are independent to each other. The normal probability plot of the residuals has been shown in Fig. 13, which indicates that, there is almost no serious violation of the assumptions underlying the analyses and it confirms the normality assumptions and independence of the residuals. The effect of the independent variables (initial concentration of MO, catalyst dose, concentration of H2O2 and pH) and their interaction on the decolorization of MO are graphically represented by three dimensional surface plots and two dimensional contour plots. The interaction of initial MO concentration and H2O2 concentration is shown in Fig. 14 (a1 and a2). The decolorization is maximum in the range of 144.45–251 mM, as evident from the contour plot. Color removal goes on decreasing with increase in initial concentration of MO. The interactive effect of H2O2 concentration and dose of Co-SMA is shown in Fig. 14 (b1 and b2). Decolorization of MO goes on increasing with increase in dose. However, the efficiency is maximum within 144.45–197.72 mM. With increase in dose from 10 to 30 g/L, the decolorization efficiency increases whereas the color removal decreases
4. Conclusions Heterogeneous Fenton-like reaction for MO degradation was studied in presence of Co-SMA, H2O2 and visible light. The degradation rate of MO decreased with the increase in initial concentration of MO. In case of H2O2 concentration and light intensity, the rate constant increased up to a certain point then it decreased. This heterogeneous Fenton-like reaction has several advantages over the others. The material used in the present study has significant recycle ability. The process operates efficiently at neutral pH. The catalyst material can be separated from the reaction system easily. The process parameters are optimized by response surface methodology (RSM). Under the optimum conditions of 30 mg/L MO, 29.92 g/L Co-SMA, 37.9 mM H2O2 and pH 4.31, the maximum response for MO decolorization efficiency is 94.79%. The reaction intermediates are identified by GC–MS analysis. Acknowledgements The authors are thankful to Indian Institute of Technology, Kharagpur, India for the financial support. The authors would like to thank Mr. Ashish Kumar Nayak for helpful discussions. References [1] S. Yang, P. Wang, X. Yang, L. Shan, W. Zhang, X. Shao, R. Niu, J. Hazard. Mater. 179 (2010) 552–558. [2] C.E. Clarke, F. Kielar, H.M. Talbot, K.L. Johnson, Environ. Sci. Technol. 44 (2010) 1116–1122.
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[19] N.H.M. Azmi, V.M. Vadivelu, B.H. Hameed, Desal. Water Treat. 52 (2014) 5583–5593. [20] E.G. Garrido-Ramírez, M.L. Mora, J.F. Marco, M.S. Ureta-Zañartu, Appl. Clay Sci. 86 (2013) 153–161. [21] J. Ma, L. Zhou, W. Dan, H. Zhang, Y. Shao, C. Bao, L. Jing, J. Colloid Interface Sci. 446 (2015) 298–306. [22] A. Adak, M. Bandyopadhyay, A. Pal, Colloid Surf. A 254 (2005) 165–171. [23] A. Adak, A. Pal, M. Bandyopadhyay, Colloid Surf. A 277 (2006) 63–68. [24] A.K. Das, S. Saha, A. Pal, S.K. Maji, J. Environ. Sci. Health, Part A: Toxic/Hazard. Subst. Environ. Eng. 44 (2009) 896–905. [25] M.U. Khobragade, A. Pal, J. Environ. Chem. Eng. 2 (2014) 2295–2305. [26] M.U. Khobragade, A. Pal, J. Environ. Sci. Health Part A: Toxic/Hazard. Subst. Environ. Eng. 50 (2015) 385–395. [27] P. Mahamallik, A. Pal, RSC Adv. 6 (2016) 100876–100890. [28] J. Fernandez, J. Bandara, A. Lopez, P. Buffat, J. Kiwi, Langmuir 15 (1999) 185–192. [29] A.K. Abdessalem, N. Oturan, N. Bellakhal, M. Dachraoui, M.A. Oturan, Appl. Catal. B Environ. 78 (2008) 334–341. [30] L.G. Devi, S.G. Kumar, K.M. Reddy, C. Munikrishnappa, J. Hazard. Mater. 164 (2009) 459–467. [31] G.K. Parshetti, A.A. Telke, D.C. Kalyani, S.P. Govindwar, J. Hazard. Mater. 176 (2010) 503–509. [32] H. Lee, Y.-K. Park, S.-J. Kim, B.-H. Kim, H.-S. Yoon, S.-C. Jung, J. Ind. Eng. Chem. 35 (2016) 205–210.
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Photo-Fenton process in Co(II)-adsorbed admicellar soft-template on alumina support for methyl orange degradation Prateeksha Mahamallik, Anjali Pal
⁎
Civil Engineering Department, Indian Institute of Technology Kharagpur, West Bengal, 721302, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Co(II)-modified alumina Heterogeneous photo-Fenton Methyl orange Degradation Response surface methodology
In the present work, a novel approach has been made to prepare a new catalyst for Fenton-type degradation of dye. In the first step alumina is modified with sodium dodecyl sulfate (an anionic surfactant). Under suitable conditions the surfactant forms bilayer (admicelle) on alumina surface. The surfactant-modified alumina (SMA) can adsorb metal ions such as Co(II), Fe(II), Cu(II), Mn(II) and Ni(II) on its surface. Among all the metal ions Co (II) shows the best catalytic activity when supported on SMA. The newly formed catalyst is named as Co-SMA. In this report, we described the degradation of methyl orange (MO) under heterogeneous photo-Fenton condition by application of Co-SMA as the catalyst and in the presence of H2O2 and visible light. Effect of various independent variables such as dose of catalyst, initial concentration of the dye, H2O2 concentration, intensity of visible light, pH have been studied. The process parameters were optimized by response surface methodology (RSM) approach. Under the optimum conditions of 30 mg/L MO, 29.92 g/L Co-SMA, 37.9 mM H2O2 and pH 4.31, the maximum response for MO decolorization efficiency was 94.79%.
1. Introduction The extensive use of synthetic dyes in various industries has resulted in considerable pollution in the water bodies. Textile industries discharge large volume of dye-containing water where azo dyes are most common (70% of all dyes) [1]. It is very difficult to degrade azo dyes because of their complex structures and stability. The discharge of azo dye into water bodies is unacceptable not only for its color but also for its toxicity, non-biodegradability and mutagenic nature [2]. It is hazardous for aquatic life because it reduces the reoxygenation capacity of water. Therefore, removal of harmful azo dyes from wastewater is a serious concern. Among various treatment methods, advanced oxidation processes (AOPs) are efficient technologies for removing recalcitrant organic pollutants [3,4]. Nickel loaded TiO2 was immobilized on activated carbon and applied for photocatalytic reduction of carbon dioxide to methanol under visible light irradiation [5]. Photocatalytic oxidation of benzene was experimented by TiO2 nanowall situated on activated carbon [6]. Carbon sphere TiO2-FeO was synthesized and utilized for photocatalytic degradation of toluene from water environment [7]. Photocatalytic degradation of o-chlorophenol was carried out by TiO2/ NiO/RGO system under visible light irradiation [8]. The mechanism of AOPs involves the generation of highly reactive
⁎
free radicals (mainly hydroxyl radicals, E0 = 2.80 V/SHE) [9] which attack the pollutant and mineralize it completely. Classic homogeneous Fenton is one such powerful and widely used AOP capable of oxidizing variety of organic pollutants. In homogeneous Fenton reaction, soluble iron (II) reacts with hydrogen peroxide (H2O2) under acidic condition (pH ∼3) and produce hydroxyl (˙OH) radicals and Fe3+, and at last Fe2+ gets regenerated (Eqs. (1)–(3) [10]. Fe2+ + H2O2 → Fe3+ + OH− +˙OH Fe
3+
+ H2O2 → H
FeOOH
2+
˙
+
+ FeOOH
→ HO2 + Fe
2+
2+
(1) (2) (3)
In case of photo-Fenton reaction more hydroxyl radicals are generated (Eq. (4)). Fe(OH)2+ → Fe2+ + ˙OH
(4)
However, the homogeneous Fenton method has many drawbacks. For example it works in low pH range; produces iron sludge, and shows hindrance in Fenton catalytic cycle due to the formation of stable Fe3+ complexes. To combat these difficulties, people tried to prepare various types of heterogeneous catalysts which work for Fenton and Fenton-like reactions. In that case the reaction can take place over a wide pH range, in presence of lower concentration of metal, and the catalyst can be
Corresponding author. E-mail address: [email protected] (A. Pal).
https://doi.org/10.1016/j.cattod.2019.07.045 Received 3 June 2019; Received in revised form 27 July 2019; Accepted 29 July 2019 0920-5861/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Prateeksha Mahamallik and Anjali Pal, Catalysis Today, https://doi.org/10.1016/j.cattod.2019.07.045
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recycled. Different synthetic and natural catalyst supports such as zeolites [11–13], bentonites [14,15], alumina [16], silica [17,18], clay [19,20], resin [21] are applied to develop heterogeneous catalysts for Fenton and Fenton-like reactions. In this study, surfactant-modified alumina (SMA) is used as template for adsolubilizing different metal ions viz., Co (II), Fe(II), Cu(II), Mn(II) and Ni(II). The surfactant used here is sodium dodecyl sulfate (SDS), which is an anionic surfactant. It is reported that under suitable conditions it forms a bilayer on alumina surface which has the capability to support positively charged ions. Out of all metal ions applied the activity of cobalt-loaded SMA (Co-SMA) towards methyl orange (MO) degradation was the maximum. The detail study of MO degradation by Co-SMA was explored. 2. Materials and methods Fig. 1. Schematic of M2+-SMA preparation and MO degradation.
2.1. Chemicals
2.3.3. Loading of metal ions on surfactant-modified alumina (SMA) Stock solutions of metal ions viz., Fe(II), Ni(II), Mn(II), Cu(II) and Co(II) with concentration 100 mg/L were prepared in distilled water. Then 3 g of SMA was added to 50 mL of each of these solutions, and kept for required time under ambient condition, with occasional hand shaking. The kinetic of adsorption of metal ions on SMA was monitored using AAS in each case. After maximum adsorption (which took ∼4 h), the supernatant solutions were collected to find out the remaining concentrations of metal ions. The metal-loaded SMA in each case was washed thrice with distilled water. Then it was dried in an oven at 60 °C (4 h). The dried materials (designated as M2+-SMA) were collected and kept in airtight bottles.
Alumina and sodium dodecyl sulfate (SDS) was procured from SRL (India) and Merck, respectively. Glacial acetic acid, methyl orange (MO), cobalt chloride hexahydrate, FeSO4.7H2O, CuSO4.5H2O, NiSO4.7H2O, MnCl2.4H2O and 30% H2O2 were purchased from Merck. The dye acridine orange was purchased from Loba chemicals. The water used was double distilled. 2.2. Instrumentation UV-vis spectrophotometer (Model: SPECTRASCAN UV 2600, Chemito, India) equipped with a 1-cm quartz cuvette was used. To stir the solution, a magnetic stirrer (Tarsons, India) was used. The visible light source was a normal W-filament bulb. Atomic absorption spectrometer (AAS; Varian AA240FS, Australia) was used for cobalt determination. Acetylene flame was used in the AAS. The GC–MS (Agilent Technologies, United States) analyses of the reaction mixture were done to know the products formed. The column used was DB-5 and flow of solution was 1 mL/min.
2.3.4. Determination of metal concentration Concentrations of metals (cobalt, copper, iron, manganese, nickel) were found out from the calibration graphs made using AAS. The concentration of stock solution of all metal ions was 100 mg/L. The desired concentrations (0–5 mg/L) of metal solution were prepared from the stock solution. For each metal, the calibration graphs were plotted according to the absorbance values given by AAS. The calibration equation of iron was: Absorbance = 0.065 × Concentration (mg/L) + 0.018, R2 = 0.999. The calibration equation of nickel was: Absorbance = 0.048 × Concentration (mg/L) - 0.005, R2 = 0.998. The calibration equation of manganese was: Absorbance = 0.078 × Concentration (mg/L) - 0.009, R2 = 0.992. The calibration equation of copper was: Absorbance = 0.068 × Concentration (mg/L) - 0.01, R2 = 0.993. The calibration equation for cobalt was: Absorbance = 0.0434 × Concentration (mg/L) + 0.0001 (R2 = 0.990).
2.3. Preparation of material 2.3.1. Modification of alumina with surfactant to prepare SMA The following procedure was adopted to prepare the surfactantmodified alumina (SMA) [22]. First, 20 g of SDS was dissolved in 900 mL water and 2.5 g NaCl was dissolved in 100 mL water. These two solutions were mixed together. Then the pH of the mixture was adjusted to ∼4.5 using dilute HCl. Further, 100 g of alumina was added to the solution and the mixture was shaken for 24 h. The shaking speed was 150 rpm and the temperature was 30 °C. The supernatant was taken and the SDS concentration was measured. The as-prepared alumina was named as surfactant-modified alumina (SMA). The SMA was washed with water and dried at 60 °C for 12 h. The material was stored in a closed bottle at room temperature. The SMA was loaded with metal ions and used further as a catalyst for MO degradation. The work is pictorially shown in Fig. 1.
2.4. MO degradation 2.4.1. Experimental for MO degradation on Co-SMA The activity of Co-SMA as a heterogeneous photo-catalyst was studied by the degradation of MO in water under visible light irradiation. MO in presence of Co-SMA was stirred and the adsorption-desorption equilibrium was reached in 30 min. The desired concentration of MO was prepared by diluting the stock solution (100 mg/L). After adsorption of MO reached the equilibrium condition, heterogeneous photoFenton reaction was started by addition of H2O2 and with irradiation of visible light. The distance in between the top surface of the solution and the light source was ∼20 cm. The remaining concentrations of MO during degradation were monitored time to time. The degraded products of MO were identified by GC–MS analysis. For GC–MS analysis, 10 mL of reacted MO samples (after 20 min and 60 min of reaction time) were extracted in equal volume of ethyl acetate. The extracted sample was directly injected for GC–MS analysis.
2.3.2. Determination of surfactant loading on Al2O3 The calibration curve of SDS was developed where a phase separation technique was adopted to measure the concentration of SDS [22]. For this, acridine orange (5 × 10−3 M), which is a cationic dye, and glacial acetic acid were added to SDS solutions having different known concentrations (0–5 mg/L). Acridine orange acted as an ionpairing agent with SDS. The complex was soluble in toluene; so it was extracted in toluene and the absorbance was measured at 467 nm. The absorbance is proportional to SDS concentration. The calibration equation was: Abs. = 0.2 × Conc. (mg/L) + 0.008 (R2 = 0.987). The SDS loading was 115.5 mg (0.4 × 10−3 mole) per gram of alumina. 2
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cobalt was 1.57, 1.64, 1.65, 1.66, and 1.38 mg/g of SMA, respectively (Fig. 2(b)).
2.4.2. Analytical procedure to determine MO decolorization efficiency The analysis of decolorization efficiency was done by the help of spectrophotometric method. Standard solutions of MO were prepared in the concentration range of 0–30 mg/L from the prepared stock of 100 mg/L. The absorbance of MO was monitored at λmax 463 nm. Concentration of MO was found out from the calibration equation: Absorbance = 0.078 × Concentration (mg/L) + 0.017 (R2 = 0.997). The percentage decolorization of MO was calculated as follows:
3.1.2. Decolorization of MO by M2+-SMA The activities of M2+-SMA towards Fenton reaction for degradation of MO were studied. Firstly, 10 mL of MO solution with initial concentration of 20 mg/L was stirred for 30 min to get the dye adsorbed on the surface of M2+-SMA (dose: 20 g/L). Then, H2O2 at a concentration of 111.7 mM was added to the solution and it was irradiated with visible light. The adsorption of MO on M2+-SMA surface was found to be 26.7, 5.1, 5.8, 0.45, 0.38% in case of Co, Cu, Fe, Ni, Mn metal ions loaded SMA surfaces, respectively (Fig. 3(a)). The removal efficiency of MO through adsorption was highest for Co-SMA. The color removal of MO after 60 min of reaction was found to be 94.04, 44.55, 29.8, 3.27, and 4.9% by Co-SMA, Cu-SMA, Fe-SMA, Ni-SMA, and Mn-SMA (Fig. 3(b)). The decolorization efficiency was the maximum in case of Co-SMA. So, Co-SMA was selected as the working material for further study. Here the metal Co(II), embedded on SMA, was the active catalyst for Fenton reaction; hence it was important to compare, the performance of SMA to adsolubilize Co(II), with the performance of normal alumina. As reported in our earlier studies, which were carried out for the removal of other metal ions [25,26], the SMA showed > 6 times higher adsorption capacity for Co(II) as compared to that of alumina. Under similar experimental conditions, the loading of cobalt on alumina was 0.205 mg/g (3.47 × 10−6 mole/g) and that on SMA was 1.38 mg/g (2.34 × 10-5 mole/g). It is interesting to note that, the ratio of SDS / cobalt on SMA was ∼17:1 (mole / mole). Thus the prepared Co-SMA was used as the catalyst for the degradation of MO.
Dye decolorization (%) = [(C0 - Ct) / C0] × 100 where, C0 = Concentration of MO at zero time of reaction and Ct = Concentration of MO at time t.
3. Results and discussion 3.1. Selection of catalyst 3.1.1. Adsorption of metal ion (M2+) on SMA surface Alumina surface can adsorb SDS to form hemimicelle or admicelle under appropriate conditions. This micellar layer can be exploited to adsorb organic molecules [22–24] or metal ions [25,26] at a high concentration. In this work our focus was to observe the synergistic performance of such a system, where the metal ion and the dye both could co-exist within the surfactant bilayer in fully adsorbed state; or a system where the dye remains partly in solution phase and partly adsorbed and the metal ion remained in solid supported form; and finally they reacted with the H2O2 in Fenton reaction. The soft-template of admicelle on the alumina surface may thus provide some better environment for the reaction to take place. The catalysis in heterogeneous solid surface containing the micellar environment may lead to interesting behavior. This type of attempt is novel in Fenton process. The system was selected such that, the metal ion was supported on a substrate. In this regard SMA was considered as a suitable template for such reaction. Here we have selected several metal ions (M2+) viz., iron, manganese, copper, nickel, cobalt to compare the activity in the photo-Fenton process. The metal ions were adsorbed on SMA surface through the interaction of negatively charged dodecyl sulfate and positively charged metal ion (Fig. 1). To find out the optimum time for metal ion (M2+) adsorption, a kinetic study was done. As shown in Fig. 2(a), in 4 h time the adsorption for copper, manganese, nickel and iron was ∼94%, ∼98%, ∼99% and ∼99%, respectively. In contrast to this, adsorption of Co(II) on SMA was less. During the initial phase the rate was faster and ∼67% removal of Co(II) took place in initial 50 min (Fig. 2(a)). However, 240 min time was considered as the contact time for preparation of M2+-SMA. This is because during this period maximum amount of metal ions were removed (Fig. 2(a)). The loading of copper, manganese, nickel, iron and
3.2. Characterization of Co-SMA In our earlier report the characterization of Co-SMA using different methods viz., scanning electron microscopy (SEM)/electron diffraction X-ray analysis (EDAX), diffuse reflectance spectroscopy (DRS), X-ray photo electron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) was described [27]. It was confirmed that, Co-SMA contained both cobalt (0.22% by weight) and SDS. 3.3. Degradation of MO under different conditions Degradation of MO was studied under different conditions i.e. MO + Visible light (VL), MO + VL+H2O2, MO + VL + Alumina, MO +H2O2+Alumina, MO + VL+H2O2+Alumina, MO + VL + SMA, MO +H2O2+SMA, MO + VL+H2O2+SMA, MO + VL + Co-SMA, MO +H2O2+Co-SMA and MO + VL+H2O2+Co-SMA. The results are shown in Fig. 4. Out of all combinations applied, only MO+H2O2+CoSMA and MO + VL+H2O2+Co-SMA combinations were able to degrade MO. In all the systems where alumina or Co-SMA was used, MO
Fig. 2. Time dependent adsorption of metal ion (M2+) on SMA surface (a) concentration (mg/L) remained vs. time (min) and (b) loading (mg/g) vs. time (min). 3
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Fig. 3. Activity of M2+-SMA towards MO degradation (a, b) (conditions for (a): initial concentration of MO: 20 mg/L; dose of M2+-SMA: 20 g/L; H2O2 concentration: 111.7 mM; intensity of visible light: 1620 lux; time of reaction: 60 min).
3.4.2. Effect of initial MO concentration The reaction was studied using different MO concentrations (in the range of 10–30 mg/L), with a constant Co-SMA dose of 10 g/L. The reaction followed first order rate at all MO concentrations. The rate constant decreased from 0.025 to 0.017 min−1 with the increase in concentration. The kinetics of MO degradation is shown in Fig. 6. 3.4.3. Effect of H2O2 concentration In our earlier report [27] the decomposition of H2O2 in presence of Co-SMA and visible light was studied, and the decomposition rate was found to be dependent on the catalyst dose. The decomposition of H2O2 produced hydroxyl radicals which degraded the dye. Thus in the reaction, both the SMA dose and the initial concentration of H2O2 play a vital role in the oxidation of organic compounds in the heterogeneous Fenton processes. In the present work the effect of the hydrogen peroxide was analyzed by varying its initial concentration between 37.9 mM and 251 mM (Fig. 7). The reaction followed first order with all applied H2O2 concentrations. The rate constant increased from 0.013 to 0.027 min−1 as the concentration of H2O2 increased from 37.9 mM to 182.7 mM. Since the concentration of H2O2 is directly related to the amount of hydroxyl radicals produced in the catalytic reaction, this parameter influences degradation efficiency. However, excess amount of H2O2 (251 mM) was not found advantageous to the degradation because of the scavenging of ·OH radical by excess H2O2 (Eq. (5)) [28].
Fig. 4. Time dependent degradation of MO under different experimental conditions (conditions: initial concentration of MO: 20 mg/L; catalyst dose: 20 g/L; light intensity: 1620 lux; concentration of H2O2: 111.7 mM).
was initially allowed to be adsorbed on the solid surface. The degradation of MO was ∼71% in 45 min (Fig. 4). However, the same system if operated in presence of visible light (i.e. if the adopted condition is MO + VL+H2O2+Co-SMA) the efficiency was further improved up to ∼86% in 45 min (Fig. 4). So the reaction with the combination MB + VL+H2O2+Co-SMA was considered to proceed further. The reason of the increased efficiency under visible light illuminated condition is possibly because of Co(III) to Co(II) conversion by the visible light. This Co(III) to Co(II) conversion is essential for our Fentonlike process.
·OH + H2O2 → H2O + H2O·
(5)
3.4.4. Effect of the intensity of visible light To examine the impact of visible light intensity on the catalytic activity of Co-SMA, the light intensity was varied from 993 to 5560 lux. It is clearly observed from Fig. 8 that with increase in light intensity from 993 to 1620 lux, the decolorization efficiency increased from ∼74% to ∼93%. Further increase in light intensity, however, caused reduction in decolorization efficiency.
3.4. Effect of reaction parameters on the decolorization of MO 3.4.1. Effect of the catalyst dosage To study the effect of dose of catalyst on MO degradation efficiency, the dose was varied from 10 g/L to 30 g/L. To achieve the adsorptiondesorption equilibrium, first the solution was stirred and it was seen that in 30 min time the adsorption equilibrium was attained. About 30% (i.e. 6 mg/L) of MO was adsorbed on Co-SMA surface in case of all doses of SMA (Fig. 5(a)). After that the reaction was started by adding required amount of H2O2 and under exposure of visible light. Within 30 min of the reaction (after adsorption) degradation of MO increased from ∼33% to ∼61% when the dose was increased from 10 g/L to 30 g/L (Fig. 5(a)). The reaction followed first order. The graph plotting ln C/C0 vs. time (min) is shown in Fig. 5(b). The reaction rate constant increased from 0.019 to 0.074 min−1 with the increase in Co-SMA dose from 10 g/L to 30 g/L. This is obvious that, more hydroxyl radicals were produced with increase in Co-SMA dosage, which enhanced the degradation rate of MO.
3.4.5. Effect of the initial solution pH The influence of initial pH was examined by varying the pH from 4 to 12 by adding HCl or NaOH. It is clearly observed in Fig. 9 that, the decolorization efficiencies were almost same for initial pH 4, 6, and 9. But it decreased drastically for initial pH 12. MO was allowed to achieve adsorption-desorption equilibrium for 30 min then H2O2 was added and it was exposed to visible light. Removal of MO by adsorption was ∼39%, ∼31% and ∼32% in case of initial pH 4, 6, and 9, respectively. But adsorptive removal was only ∼0.12% for pH 12. This reveals that adsorption has an important role in fast decolorization of MO. The surface charge of Co-SMA plays an important role in the degradation of MO at different initial pH. The pHzpc of Co-SMA was found 4
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Fig. 5. (a) Decolorization of MO vs. Co-SMA dose (g/L) (time of reaction: 30 min) and (b) Plot of ln C/C0 vs. time (min) (after adsorption) (conditions: initial concentration of MO: 20 mg/L; H2O2 concentration: 111.7 mM; intensity of visible light: 1620 lux).
Fig. 8. Decolorization of MO vs. light intensity (conditions: initial concentration of MO: 20 mg/L; dose of Co-SMA: 25 g/L; H2O2 concentration: 111.7 mM; reaction time: 45 min). Fig. 6. Time dependent degradation of MO at different concentrations (conditions: dose of Co-SMA: 10 g/L; H2O2 concentration: 111.7 mM; intensity of visible light: 1620 lux).
Fig. 9. Plot of decolorization (%) of MO vs. time at different initial pH (conditions: initial concentration of MO: 20 mg/L; dose of Co-SMA: 25 g/L; H2O2 concentration: 111.7 mM; light intensity: 1620 lux).
Fig. 7. First order kinetic plots of MO degradation at different H2O2 concentrations (conditions: initial concentration of MO: 20 mg/L; dose of Co-SMA: 10 g/L; intensity of visible light: 1620 lux). 5
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Table 2 ANOVA results of the quadratic model for decolorization of MO by heterogeneous photo-Fenton reaction. Source
Sum of squares
Degree of freedom (df)
F-value
p-value
Model A B C D AB AC AD BC BD CD A2 B2 C2 D2 Lack of fit
9083.08 786.02 430.32 76.81 4613.06 0.72 1155.66 108.47 196.84 2.50 13.07 90.77 1.83 655.38 901.92 50.78
14 1 1 1 1 1 1 1 1 1 1 1 1 1 1 10
160.88 194.91 106.71 19.05 1143.91 0.18 286.57 26.90 48.81 0.62 3.24 22.51 0.45 162.52 223.65 3.58
< 0.0001 < 0.0001 < 0.0001 0.0006 < 0.0001 0.6785 < 0.0001 0.0001 < 0.0001 0.4445 0.0934 0.0003 0.5110 < 0.0001 < 0.0001 0.1154
R2 = 0.9938; R2adj = 0.9876; R2pred = 0.9670; CV% = 3.28; Adeq. precision = 46.458.
Fig. 10. Efficiency of Co-SMA for decolorization of MO in consecutive cycles (conditions: initial concentration of MO: 20 mg/L; dose of Co-SMA: 25 g/L; H2O2 concentration: 111.7 mM; light intensity: 1620 lux).
3.5. Recyclability of the catalyst
to be 9.2. Below pH 9.2, the surface is positively charged. So, below pH 9.2, adsorption of MO is higher because of ionic interaction between positively charged Co-SMA surface and negatively charged MO molecule. This is in agreement with the obtained results. Degradation of MO also followed the similar trend. The degradation of MO at pH 4, 6 and 9 was ∼90%, ∼84% and ∼91% respectively, whereas at pH 12 the decolorization was only ∼30%.
Recyclability tests were conducted to investigate the reusability of the catalyst in consecutive runs. After each cycle, the catalyst was easily separated from the reaction mixture by centrifugation. This was followed by washing and drying in an oven at 60 °C, and finally the catalyst was reused. As shown in Fig. 10, the decolorization efficiency was 93.37% in first cycle, whereas it was 84.68, 83.4, 69.81% in second, third and fourth cycle, respectively. The adsorption of MO on Co-SMA was increased significantly from 31% to 54.7% after first cycle.
Table 1 BBD matrix of independent variables and their corresponding experimental and predicted values for RSM model. Sl. No.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
Initial concentration of MO (A, mg/L)
30 20 30 30 20 20 20 20 20 20 30 20 30 10 20 10 20 20 10 20 20 20 10 10 10 20 20 20 30
Dose of Co-SMA (B, g/L)
H2O2 Concentration (C, mM)
30 20 20 20 20 30 10 20 20 30 20 10 10 20 20 30 20 20 10 30 20 30 20 20 20 20 10 10 20
144.45 251 251 144.45 251 251 144.45 144.45 144.45 144.45 144.45 144.45 144.45 251 37.9 144.45 144.45 348.8 144.45 144.45 144.45 37.98 37.9 144.45 144.45 37.9 37.9 251 37.9
6
pH (D)
8 4 8 4 12 8 12 8 8 4 12 4 8 8 12 8 8 8 8 12 8 8 8 12 4 4 8 8 8
Color removal (%) Experimental
Predicted
70.37 66.39 39.19 77.91 32.01 59.06 31.28 68.53 68.73 83.83 26.91 71.92 58.91 92.48 22.89 84.94 67.78 70.97 71.78 45.82 68.87 66.9 55 52.87 83.04 64.5 42.21 62.43 69.7
69.66 67.46 40.10 77.65 31.86 59.90 30.27 68.98 68.98 81.46 28.02 71.06 58.53 90.29 23.19 86.69 68.98 68.98 73.87 43.82 68.98 68.87 51.53 54.62 83.42 66.02 42.86 61.95 69.04
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Fig. 11. (a) Comparison of experimental and predicted response for each run and (b) Predicted vs. actual data.
3.6. Comparative study on homogeneous and heterogeneous Fenton-like reaction It is essential to compare the efficiency of homogeneous and heterogeneous cobalt mediated photo-Fenton degradation of MO. For this purpose, the degradation efficiency of MO under homogeneous Fentonlike reaction condition was studied with the Co(II) concentration of 27.6 mg/L in 20 mg/L of MO solution. The concentration was chosen by the assumption that, the whole amount of Co adsorbed on SMA took part in the reaction. It was interesting to note that no degradation in solution phase took place at that Co(II) concentration when homogeneous photo-Fenton reaction was carried out. So, the solution phase concentration of cobalt was increased gradually by 5, 150, 350, 700, 900, 1100 times as that of the Co(II) present in Co-SMA. To our surprise it was noted that, for homogeneous catalysis conditions even at 1100 times higher concentration of Co(II) present in Co-SMA, the degradation of MO was found to be only ∼39% after 60 min reaction. On the other hand, in case of Co-SMA, the MO degradation was ∼90% in 45 min with 20 g/L Co-SMA dose. It revealed that, the heterogeneous photo-Fenton reaction was much more efficient compared to the homogeneous one. From the above experimental results, the possibility of MO degradation due to the leached cobalt was ruled out.
Fig. 12. Pareto chart showing the percentage effect of variables on MO decolorization.
3.7. Cobalt leaching from Co-SMA during MO degradation It was pertinent from environmental point of view to study the leaching of cobalt for the adopted photo-Fenton process. The concentration of cobalt in the reacting solution after 60 min was very low (1.81 mg/L), and this was only 2.15% of the loaded cobalt on SMA. This small leaching might be due to the bilayer structure of surfactant, which helped to attach cobalt firmly. This speaks in favor of adsolubilizationbased approach, which is beneficial for water treatment. 3.8. Turn over number (TON) and turn over frequency (TOF) Turn over number (TON) and turn over frequency (TOF) gives the idea about the performance of a catalyst. The catalysts, which have higher TON and TOF are considered as better catalysts. Here the TON and TOF of the catalyst were calculated for the reaction of MO having concentration 20 mg/L and Co-SMA having dose 20 g/L in presence of 111.7 mM H2O2 and visible light having intensity of 1620 lux. The TON and TOF values for the above condition were found to be 18.4 × 1023 molecules/g and 6.8 × 1020 molecules/g/second, respectively.
Fig. 13. Plot of normal probability and internally studentized residuals for color removal of MO dye.
3.9. Application of statistical experimental design to heterogeneous photoFenton decolorization of MO using Co-SMA as catalyst A three-level four-variable Box-Behnken design (BBD) of experiment 7
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Fig. 14. (a1) Response surface plot and (a2) the contour showing effect of H2O2 concentration and initial concentration of MO (conditions: dose of Co-SMA: 20 g/L; pH: 8), (b1) response surface plot and (b2) contour showing effect of hydrogen peroxide concentration and dose of Co-SMA (conditions: initial concentration of MO: 20 mg/L; pH: 8), (c1) response surface plot and (c2) contour showing effect of dose of Co-SMA and initial concentration of MO (conditions: concentration of H2O2: 144.45; pH: 8), (d1) response surface plot and (d2) contour showing effect of pH and initial concentration of MO (conditions: dose of Co-SMA: 20 g/L; concentration of H2O2: 144.45), (e1) response surface plot and (e2) contour showing effect of pH and dose of Co-SMA (conditions: initial concentration of MO: 20 mg/L; concentration of H2O2: 144.45), (f1) response surface plot and (f2) contour showing effect of pH and H2O2 concentration (conditions: initial concentration of MO: 20 mg/ L; dose of Co-SMA: 20 g/L). 8
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Fig. 14. (continued)
Fig. 15. Degradation pathway of methyl orange during Co-SMA mediated photo-Fenton process.
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with increase in concentration of MO (Fig. 14 (c1 and c2)). Decolorization of MO is maximum in the pH range of 4–8. However in this range also there is decrease in color removal with increase in initial concentration of MO with fixed dose 20 g/L and 144.45 mM H2O2 concentration (Fig. 14 (d1 and d2)). Again from Fig. 14 (d2) it is clear that, pH 4–8 is favorable for MO decolorization. The decolorization efficiency is higher at 30 g/L dose than that of 10 g/L for 20 mg/L MO and 144.45 mM H2O2 concentration because of enhanced production of hydroxyl radical (Fig. 14 (e1 and e2)). The interactive effect of pH and H2O2 concentration shows that, the efficiency is maximum in the pH range of 4–6 and 144.45 mM H2O2 concentration (Fig. 14 (f1 and f2)). Any process is optimized to find out the best possible output using modeling tools and analytical procedures. The optimum conditions for the dye decolorization was found to be 30 mg/L MO, 29.92 g/L CoSMA, 37.9 mM H2O2 and 4.31 pH. Under these conditions the decolorization efficiency of MO was 94.79%. The model was validated in batch reactor to check the accuracy. The decolorization of MO in the optimized condition was found to be 94.2% which is close to the predicted value.
was considered to analyze the photo-Fenton decolorization of MO by Co-SMA. In the present study, four significant process variables viz. initial concentration of MO (A, mg/L), dose of Co-SMA (B, g/L), H2O2 concentration (C, mM) and pH (D) that primarily influenced the decolorization efficiency of MO dye were considered as independent variables. Total number of experiments was 29 with 5 replicates at central point. The batch experiments were conducted to analyze the interactive effect of independent variables on the decolorization efficiency of MO and the results are listed in Table 1. The response of photo-Fenton-like reactions was fitted to the quadratic model and the significance and adequacy were tested by ANOVA (analysis of variances) using Design Expert Software (Version 7.0.0, Stat Ease Inc., USA) (Table 2). The coefficient of determination (R2) is 0.9938 which implies the regression model fits well to the experimental value. The predicted R2pred (0.9670) is in reasonable agreement with adjusted R2adj (0.9876). The decolorization efficiency explained by independent variables in terms of coded factors is expressed by the following second-order equation (Eq. (6)), Y = 68.98 - 8.09A + 5.99B + 2.53C - 19.61D - 0.42AB - 17AC - 5.21AD - 7.02BC + 0.79BD + 1.81CD + 3.74A2 - 0.53B2 - 10.05C2 - 11.79D2 (6)
3.10. Degradation pathway of MO dye The reaction intermediates of MO degradation by photo-Fenton reaction using Co(II)-SMA were analyzed using GCeMS, and the mechanism of degradation was suggested. The sample after 20 min reaction showed peak at m/z 327 which was due to the parent molecule [30]. Therefore, from this peak it was confirmed that MO was not fully degraded at 20 min. At 16.7 min retention time, the peak found at m/z 137 represented N,N-dimethyl p-phenylenediamine. This proved the symmetric cleavage of azo bond [30,31]. The molecule further degraded to smaller molecules showing peaks at m/z 94 and 95. After 60 min reaction, at 13.1 min retention time, a peak was noticed at m/z 181. This was due to the other half of the symmetric cleavage of methyl orange molecule [32]. The compound further degraded by the hydroxyl radicals (generated during Fenton reaction), to smaller molecules showing peaks at m/z 159 and 129. The plausible degradation pathway of MO is shown in Fig. 15.
In Eq. (6), Y is the decolorization efficiency of MO; A, B, C and D is the corresponding coded variables of initial dye concentration, catalyst dose, H2O2 concentration and pH respectively. In this model, the linear coefficients of A, B, C and D, interaction coefficients of AC, AD and BC, and quadratic coefficient of A2, C2 and D2 are significant terms. The process variables possessing higher Fisher's Ftest values have greater influence on the decolorization efficiency of MO. The higher model F-value (i.e. 160.88) and lower p-value (i.e. < 0.0001) have corroborated that the developed model is highly significant and there is only a 0.01% chance of occurrence of model Fvalue due to noise (Table 2). The experimental and RSM predicted results for decolorization of MO is shown in Fig. 11(a). Both the data are close to each other in each run. The model predicted and experimental fitted well with R2 equal to 0.9938 (Fig. 11(b)). Additionally, the influence of each parameter on MO decolorization was calculated by Pareto chart analysis [29] and represented in Fig. 12. The extent of each bar in the Pareto chart represents the percentage effect of that variable on the response. The maximum effect on decolorization of MO was contributed by initial pH of the solution, i.e. ∼34.5%, followed by interaction between initial concentration of MO and H2O2 concentration which is ∼26%, followed by quadratic effect of initial pH (∼12.5%). The normal probability plot of the residuals is an important diagnostic tool to detect and explain the deviation of assumptions that errors are normally distributed and they are independent to each other. The normal probability plot of the residuals has been shown in Fig. 13, which indicates that, there is almost no serious violation of the assumptions underlying the analyses and it confirms the normality assumptions and independence of the residuals. The effect of the independent variables (initial concentration of MO, catalyst dose, concentration of H2O2 and pH) and their interaction on the decolorization of MO are graphically represented by three dimensional surface plots and two dimensional contour plots. The interaction of initial MO concentration and H2O2 concentration is shown in Fig. 14 (a1 and a2). The decolorization is maximum in the range of 144.45–251 mM, as evident from the contour plot. Color removal goes on decreasing with increase in initial concentration of MO. The interactive effect of H2O2 concentration and dose of Co-SMA is shown in Fig. 14 (b1 and b2). Decolorization of MO goes on increasing with increase in dose. However, the efficiency is maximum within 144.45–197.72 mM. With increase in dose from 10 to 30 g/L, the decolorization efficiency increases whereas the color removal decreases
4. Conclusions Heterogeneous Fenton-like reaction for MO degradation was studied in presence of Co-SMA, H2O2 and visible light. The degradation rate of MO decreased with the increase in initial concentration of MO. In case of H2O2 concentration and light intensity, the rate constant increased up to a certain point then it decreased. This heterogeneous Fenton-like reaction has several advantages over the others. The material used in the present study has significant recycle ability. The process operates efficiently at neutral pH. The catalyst material can be separated from the reaction system easily. The process parameters are optimized by response surface methodology (RSM). Under the optimum conditions of 30 mg/L MO, 29.92 g/L Co-SMA, 37.9 mM H2O2 and pH 4.31, the maximum response for MO decolorization efficiency is 94.79%. The reaction intermediates are identified by GC–MS analysis. Acknowledgements The authors are thankful to Indian Institute of Technology, Kharagpur, India for the financial support. The authors would like to thank Mr. Ashish Kumar Nayak for helpful discussions. References [1] S. Yang, P. Wang, X. Yang, L. Shan, W. Zhang, X. Shao, R. Niu, J. Hazard. Mater. 179 (2010) 552–558. [2] C.E. Clarke, F. Kielar, H.M. Talbot, K.L. Johnson, Environ. Sci. Technol. 44 (2010) 1116–1122.
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