# Kinetic Study of the Removal of Methyl Orange Dye by Coupling WO3/H2O2

Authors: N’goran Sévérin Eroi; Aimé Serge Ello; Donourou Diabaté; Konan Roger Koffi
Journal: Journal of Chemistry
 (2022)

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## Abstract

In the present work, the heterogeneous Fenton-like process was employed to investigate the kinetic models of the degradation of methyl orange (MO) using tungsten oxide/hydrogen peroxide couple. Tungsten oxide particles were successfully synthesized by reflux without surfactant and characterized by using XRD, SEM, TEM, and FT-IR techniques. The influence of parameters such as temperature and concentration of MO was studied and pseudo first-order and second-order models were applied. WO3/H2O2 showed high efficiency in the removal of methyl orange and attained more than 92.8% in 180 min. The first-order kinetic model was described by the removal process with the correlation coefficient of R2 = 0.99.


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## Body

## 1. Introduction

Advanced oxidation processes based on UV, photo catalysis, Fenton, and Fenton-like oxidation were suggested for the removal of pollutants from an aqueous media [1–6]. Among these methods, Fenton reactions attracted much attention and showed high removal efficiency of different dyes from wastewater [7–10]. The classical Fenton reaction using iron salts (Fe2+ or Fe3+) and H2O2 can produce powerfully oxidizing radical species such HO• for decomposition of dyes:(1)Fe2++H2O2⟶Fe3++HO•+OH−,(2)Fe3++H2O2⟶Fe2++HO2•+H+.However, the implementation of homogeneous Fenton oxidation based on ferrous or ferric salts has several limitations, including the conversion of Fe (OH)3 into sludge at pH above 4 [11]. To improve the oxidation efficiency, heterogeneous Fenton-like reaction has been developed. Heterogeneous Fenton-like process was established by replacing Fe2+ in the Fenton reagent with a solid catalyst including metal oxide. It was shown that metal oxide decomposes hydrogen peroxide H2O2 to hydroxyl radical HO• as in classical Fenton reaction [11–13] and then successfully used as heterogeneous catalysts for degradation of organic pollutants. The following mechanism generating HO• from H2O2 was proposed for Mn3O4/H2O2 [14] and Cr (VI)/H2O2 [15].(3)MVIsurf+H2O2⟶MVsurf+HO•+HO−,(4)MVsurf+H2O2⟶MVIsurf+HO•+HO−.M is a metal oxide and VI and V are the oxidation states, respectively. The reduction of M (VI) on the surface generates radical hydroxyl species (3), and the reactive M (V) species are capable of generating HO• from H2O2 (4). Heterogeneous Fenton-like oxidation process has an advantage over the classical Fenton process. The recovery of catalysts or metal oxides is easily and prevents precipitate of iron hydroxide (sludge) as shown in classical Fenton process [11]. Several works for the green elimination of dyes were investigated with the heterogeneous Fenton-like methods using different metal oxides. Thus, hematite (α-Fe2O3) nanostructures and iron oxide supported on carbon nanotube (Fe3O4/MWCNTs) were used as heterogeneous Fenton catalysts and showed 76.6% [16] and 91.9% [17], respectively, to the removal of methyl orange (MO). Iron oxide (Fe2O3) coated on the surface of sand showed 60% of methyl orange removal and increased to 74% with heterogeneous photo Fenton system [18]. In the strong acid media, it was possible to oxidise more than 97.8% of methyl orange [19] using Fenton reaction. According to previous work, tungsten oxide used as a heterogeneous Fenton-like catalyst resulted in a good efficiency in the degradation of methyl orange and indigo carmine [20, 21]. However, these works involving this heterogeneous system for the removal of recalcitrant dyes did not elaborate on the kinetic models of their studies. The aim of this project is to investigate kinetic models of the heterogeneous Fenton-like process using WO3 as a new metal oxide catalyst for the removal of methyl orange, which is an azo dye widely applied in various industries. Additionally, tungsten oxide particles were characterized, and the optimal removal parameters such as temperature and pollutant concentrations were investigated. These methods are very simple and exhibit very high oxidation potential.


## 2. Materials and Methods

### 2.1. Preparation of Tungsten Oxide Particles

Tungsten oxide particles were prepared by a simple dispersion of metallic tungsten powder (1 g) (APS 1-5μm, purity 99.9%, Alfa Aesar Co) in deionized water (10 ml), glacial acetic acid (1 ml, ACS reagent, purity ≥99.7%) followed by addition of H2O2 (10 ml, aqueous solution; 30%, Fisher Scientific Co) drop wise. The solution was heated at 358 K for 3 h to form tungsten oxide particles. The tungsten oxide particles were washed with water and then dried at 333 K overnight (16 h) and annealed at 373 K for 4 h.


### 2.2. Characterization

Powder X-ray diffraction (XRD) patterns of the tungsten oxide particles were obtained on a Bruker D8 Advance X-ray diffractometer equipped with a CuKα radiation source (λ = 1.5418 Å). Scanning electron microscopy (SEM) image was taken on a JEOL 6360 instrument at an accelerating voltage of 3 kV. Transmission electron microscopy (TEM) images and energy dispersive X-ray spectroscopy (EDS) were taken on a JEOL/JEM-2100F operated at 200 kV. Fourier transform infrared (FT-IR) absorption spectra were measured with an FTS 45 infrared spectrophotometer using KBr pellet technique.


### 2.3. Fenton-like Experiments

The removal of methyl orange dye was carried out in a reactor capacity of 250 ml previously heated and maintained at various temperatures (303–338 K). The reactor was loaded with 100 ml of various concentrations of MO aqueous solution and 250 mg of tungsten oxide particles under stirring. The heterogeneous Fenton-like degradation process was initiated by adding 2.5 mL of H2O2 carefully. The mixture was continuously stirred using a magnetic stirrer, and tungsten metal oxide particles were separated from aqueous solution at the end of the reaction time.The residual concentration of MO from the aqueous solution was determined via UV-vis spectrophotometer at 464 nm. The rate of removal of MO was obtained by Eq. (5) below.(5)% removal=C0−CtC0×100%,where C0 and Ct are the initial and residual concentrations of MO, respectively.Based on the MO degradation profile, first-and second-order kinetic models are expressed by Eq. (6-7).(6)lnC0Ct=k1t,(7)1Ct−1C0=k2t,where C0 is the initial MO concentration, Ct is the MO concentration at various times (t) and temperatures, and k1 and k2 are the rate constants for pseudo zero, first-and second-order reactions, respectively, at different temperatures.


## 2.1. Preparation of Tungsten Oxide Particles

Tungsten oxide particles were prepared by a simple dispersion of metallic tungsten powder (1 g) (APS 1-5μm, purity 99.9%, Alfa Aesar Co) in deionized water (10 ml), glacial acetic acid (1 ml, ACS reagent, purity ≥99.7%) followed by addition of H2O2 (10 ml, aqueous solution; 30%, Fisher Scientific Co) drop wise. The solution was heated at 358 K for 3 h to form tungsten oxide particles. The tungsten oxide particles were washed with water and then dried at 333 K overnight (16 h) and annealed at 373 K for 4 h.


## 2.2. Characterization

Powder X-ray diffraction (XRD) patterns of the tungsten oxide particles were obtained on a Bruker D8 Advance X-ray diffractometer equipped with a CuKα radiation source (λ = 1.5418 Å). Scanning electron microscopy (SEM) image was taken on a JEOL 6360 instrument at an accelerating voltage of 3 kV. Transmission electron microscopy (TEM) images and energy dispersive X-ray spectroscopy (EDS) were taken on a JEOL/JEM-2100F operated at 200 kV. Fourier transform infrared (FT-IR) absorption spectra were measured with an FTS 45 infrared spectrophotometer using KBr pellet technique.


## 2.3. Fenton-like Experiments

The removal of methyl orange dye was carried out in a reactor capacity of 250 ml previously heated and maintained at various temperatures (303–338 K). The reactor was loaded with 100 ml of various concentrations of MO aqueous solution and 250 mg of tungsten oxide particles under stirring. The heterogeneous Fenton-like degradation process was initiated by adding 2.5 mL of H2O2 carefully. The mixture was continuously stirred using a magnetic stirrer, and tungsten metal oxide particles were separated from aqueous solution at the end of the reaction time.The residual concentration of MO from the aqueous solution was determined via UV-vis spectrophotometer at 464 nm. The rate of removal of MO was obtained by Eq. (5) below.(5)% removal=C0−CtC0×100%,where C0 and Ct are the initial and residual concentrations of MO, respectively.Based on the MO degradation profile, first-and second-order kinetic models are expressed by Eq. (6-7).(6)lnC0Ct=k1t,(7)1Ct−1C0=k2t,where C0 is the initial MO concentration, Ct is the MO concentration at various times (t) and temperatures, and k1 and k2 are the rate constants for pseudo zero, first-and second-order reactions, respectively, at different temperatures.


## 3. Results and Discussion

The XRD analysis was applied to identify the structure of tungsten oxide synthesized and we obtained the pattern showed on Figure1.Figure 1
XRD pattern of tungsten oxide synthesized.The major peaks appeared at 2θ = 16.53, 25.63, 35.03, and 49.64 degree were assigned to (020), (111), (131), and (202) planes related to the structure of the hydrated tungsten oxide of orthorhombic structure (JCPDS card N°84-0886) [22, 23]. All the strong and sharp diffraction peaks indicated high degree of crystallization. SEM image of tungsten oxide synthesized reveals nonregular nanoplates in Figure 2(a). TEM image of tungsten oxide is shown in Figure 2(b), and the irregularly shaped particles and near to rectangular particles are in the nanosize.Figure 2
(a) SEM image of tungsten oxide synthesized, (b) TEM image of tungsten oxide synthesized, (c) EDS spectrum of tungsten oxide synthesized, and (d) FT-IR spectrum of tungsten oxide synthesized.
(a)(b)(c)(d)Figure2(c) depicts the EDS surface analysis of WO3 particles. The spectrum showed major peaks corresponding to W (tungsten) and O (oxygen). The element of Cu arises from the use of Cu grids during TEM analysis [24]. The ratio of W to O peak was in proximity of 1 : 3 as expected. Figure 2(d) shows the FT-IR spectrum of tungsten oxide synthesized. The symmetric stretching vibrations (W-OH... H2O) at 3397 cm−1 are attributed to intercalated water [25, 26]. We also observed the bending vibration peak located at 1613 cm−1, which corresponds to the W-OH plane [25, 27] in agreement with the hydrated orthorhombic structure showed with XRD. FT-IR confirmed the presence of water in the sample. The various vibration peaks at 942 and 671 cm−1 were assigned as the stretching of W=O and O-W bonds, respectively [25, 28]. According to the results of the XRD pattern, TEM-EDS images, and FT-IR spectrum, hydrated orthorhombic tungsten oxide WO3-H2O was successfully synthesized.Figure3 shows that when hydrogen peroxide is only used to remove MO, the removal rate increased to 12.5% at pH 4. This value of pH was previously observed as an optimal condition to remove MO [20]. The removal rate of MO decreased around 3% when WO3 was as used the catalyst. In fact, hydrogen peroxide has a high oxidation capacity and showed high efficiency of removal of organic dyes [29].Figure 3
Effect of catalysts on the rate of removal of MO: experimental condition: pH = 4, (OM) = 30 mg L−1, VH2O2 = 2.5 ml, T = 303 K mass of WO3 = 0.25 g.The coupling of WO3/H2O2 considerably increased the rate of removal of methyl orange to 43%. The performance of the WO3/H2O2 couple was much higher than when tungsten oxide and hydrogen peroxide are used alone. According to the literature, tungsten oxide nanoparticles with H2O2 can generate reactive oxygen species such as HO• [30] in acidic media [31–33]. The mechanism of MO degradation was studied in our previous works. The degradation of methyl orange with WO3/H2O2 leads to breaking of the azo bond (N=N) following the benzene ring which generates intermediates species. These results are consistent with the works reported in previous literature [18]. The effect of temperature was also studied. The efficiency of the couple WO3/H2O2 was shown in the Figure 4.Figure 4
(a) Effect of temperature on the rate of removal of MO. Experimental conditions [MO] = 30 mg. L−1, pH = 4, mass of WO3 = 0.25 g. (b) Effect of temperature on the first-order kinetic of MO removal. (c) Arrhenius plots of MO degradation.
(a)(b)(c)The increase of the reaction temperature from 303 to 338 K showed the enhancement of the rate of MO removal (Figure4(a)). The optimum temperature for the most effective degradation of MO was 338 K, and the efficacy reached 87%. The increase of the performance with temperature is in agreement with the degradation of organic dyes by advanced oxidation processes [34, 35]. The enhancement of the degradation with increasing temperature was also observed in black amido 10B and methylene blue [35, 36].In fact, the increase of temperature activated more the reaction between H2O2 and metal oxide and increased HO• species [35, 38]. The kinetic of MO removal was showed at different temperatures (Figure 4(b)). The pseudo first-order kinetic model fitting the experimental data was significantly affected by reaction temperature (Figure 4(b)). The correlation coefficients were high (R2 = 0.982, 0.968, 0.993, and 0.997) at 303, 313, 323, and 338 K, respectively. Thus, the results of the experiments indicated that degradation of MO dye by heterogeneous Fenton-like process was better described by the pseudo first-order kinetic model. The result was in agreement with the studies conducted by previous works reported in Table 1 on decolorization of dyes with Fenton process [16, 34, 37]. The pseudo first-order observed in our work was compared with the kinetic parameters of MO removal reported in the literature in Table 1. The results showed a consistency with the literature. The Arrhenius expression is the relationship between the reaction temperature, and the specific reaction rate constant k is expressed as follows:(8)k=A exp−EaRT,where A is the pre-exponential factor, Ea is the activation energy (J·mol−1), R is the ideal gas constant (8.314 JRTmol−1·K−1), and T is the reaction absolute temperature (K). The Arrhenius expression showed good linear relationships between the plots of ln (k) and 1/T (Figure 4(c)) because the regression coefficient was higher than 0.997. The calculated activation energy of the degradation reaction of MO was 63.4 J·mol−1. The study of various concentrations of MO was carried out at 338 K and the results are shown in Figure 5(a). We observed the removal efficiency of MO decreased from 92.8% to 76.8% when concentration of MO increased from 15 mg·L−1 to 60 mg·L−1, respectively. The increase of initial methyl orange concentrations was opposite to the degradation efficiency previously observed [19, 37]. The decrease of the rate of removal efficiency is due to the relative lower concentration of HO• species resulting from the increasing concentration of MO. The hydroxyl radical (HO•) is a nonselective oxidant and can oxidise not only target pollutants but also the intermediates generated during the reaction process. However, the high rate of removal of MO is observed in the concentration of 15 mg·L−1.Table 1
Comparison of the efficiency and kinetic parameters of MO removal.
(MO)Catalyst[H2O2]pHYieldOrderR2k (min−1)(Ref).Photo-Fenton30 mg·L−1Co-SMA 29.92 g L−137 mM4.394.79First order0.017[37]Fenton Fe2+/H2O25.4 × 10−5(Fe2+) = 1.9 × 10−42.93 × 10−3M2.7997.8Second order0.8674.9273 × 104 M−1[19]Photo-Fenton2.5 ppmαFe2O3 30 mg30μl—76.6First order0.9690.0118[16]Photo-Fenton100 mg·L−1Fe-Sand 1.5 g·L−1200 mg·L−12.560———[18]Fe-Sand 1.5 g·L−1 + UV—74Heterogeneous-Fenton50 mg·L−1Fe3O4/MWCNTs 2.9 g·L−119.38 mmol/L291.9First order——[17]Heterogeneous-Fenton15 mg·L−1WO3 2.44 mg·L−10.179 mM·L−1492.8First orderOur work60 mg·L−1WO3 2.44 mg·L−10.179 mM·L−176.8First orderFigure 5
(a) Effect of initial concentration of MO on the degradation of methyl orange. (b) First- and (c) second-order kinetic model of the removal of MO. Experimental conditions: mass of WO3 = 0.25 g and pH = 4.
(a)(b)(c)Figures5(a)-5(b) show the fitting plots of first- and second-order models at 338 K. In order to fit the best experimental data, the correlation coefficients of both models were compared. They indicated that the first-order kinetic models fitting of the experimental data showed high values of R2 as shown in Table 2. First-order kinetic model is more in agreement with the experimental data than the second-order model based on regression coefficient values (R2). The rate constants obtained from kinetic plots of the pseudo first-order model were higher than the pseudo second-order model.Table 2
Rate parameters of the removal of MO by WO3/H2O2.
Initial concentration (mg·L−1)Pseudo first-order model (min−1)Pseudo second-order model mol−1·L−1·min−1k1 × 10−2R2k2 × 10−3R2151.420.9961.160.879301.190.9950.580.953451.030.9960.310.977600.880.9942.990.983The rate constant decreased from 0.0142 to 0.0088 min−1 with increase in concentration. The results indicated that the removal of lower concentrations of MO was faster than higher concentrations.


## 4. Conclusion

Kinetic models of the heterogeneous Fenton-like process were successfully studied in the degradation of MO. Tungsten oxide particles synthesized showed particles in nanosize and orthorhombic crystal structure. The couple WO3/H2O2 was performed for the degradation of MO. The increase of the temperature of the reaction from 303 to 338 K showed the enhancement of the rate of MO removal. Temperature of 338 K showed the most effective degradation of MO with WO3/H2O2. The rate of methyl orange removal increased with decreasing the concentration of methyl orange. The kinetic studies indicated that the degradation of MO followed the first-order kinetics. Heterogeneous WO3/H2O2 system can be a promising process for the treatment of dyes in wastewater.



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