Enhanced sonochemical degradation of azure B dye by the electroFenton process

Ultrasonics Sonochemistry 19 (2012) 174–178

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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultsonch

Enhanced sonochemical degradation of azure B dye by the electroFenton process Susana Silva Martínez a,⇑, Edgar Velasco Uribe b a

Centro de Investigación en Ingeniería y Ciencias Aplicadas (CIICAp), Universidad Autónoma del Estado de Morelos (UAEM), Av. Universidad 1001, Col Chamilpa, Cuernavaca, Mor. C.P. 62209, Mexico b Posgrado en Ingeniería y Ciencias Aplicadas FCQI-CIICAp, UAEM, Av. Universidad 1001, Col Chamilpa, Cuernavaca, Mor. C.P. 62209, Mexico

a r t i c l e

i n f o

Article history: Received 2 March 2010 Received in revised form 13 May 2011 Accepted 14 May 2011 Available online 25 May 2011 Keywords: Azure B Fenton Sonolysis Sono-electroFenton

a b s t r a c t The degradation of azure B dye (C15H16ClN3S; AB) has been studied by Fenton, sonolysis and sono -electroFenton processes employing ultrasound at 23 kHz and the electrogeneration of H2O2 at the reticulated vitreous carbon electrode. It was found that the dye degradation followed apparent first-order kinetics in all the degradation processes tested. The rate constant was affected by both the pH of the solution and initial concentration of Fe2+, with the highest degradation obtained at pH between 2.6 and 3. The first-order rate constant decreased in the following order: sono-electroFenton > Fenton > sonolysis. The rate constant for AB degradation by sono-electroFenton is 10-fold that of sonolysis and 2-fold the one obtained by Fenton under silent conditions. The chemical oxygen demand was abated 68% and 85% by Fenton and sono-electroFenton respectively, achieving AB concentration removal over 90% with both processes. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction The textile and dyestuff industries are the two major sources of dye compounds released into the environment. Wastewater polluted with dye compounds has recently become a major concern because the dyestuff is not readily degradable and it is not removed from the aqueous effluents by conventional sewage plant treatments because of their high biochemical stability, their relatively high molecular weight and the presence of aromatic rings [1–3]. Colored wastewaters impose serious aesthetic and environmental problems because of their color and high chemical oxygen demand. Furthermore, direct discharge of such effluents can cause the formation of toxic aromatic amines under anaerobic conditions in waters, and contaminate the soil and groundwater. An estimation of about 15% of the total world production of dyes is lost in wastewater stream during the dyeing and finishing operations [4]. Recently, several advanced oxidation processes have been developed and applied specifically to wastewater treatment. In order to remove efficiently the dye compounds from wastewaters, the combination of two or more individual processes have been developed. Some of these include ultrasound/H2O2 or ultrasound /ozone [5–8], UV light/H2O2 or UV/ozone [5,9], sonophotocatalysis [5,10,11], Fenton, electro-Fenton, photo-Fenton processes [12– 17], photocatalytic oxidation and electrochemical processes [1,3,9,18–20]. The use of Fenton, photo-Fenton and electro-Fenton processes has attracted the attention of different research groups.

⇑ Corresponding author. Tel./fax: +52 777 329 70 84. E-mail address: [email protected] (S.S. Martínez). 1350-4177/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2011.05.013

Peternel et al. [21] studied the degradation of Reactive Red 45 by several advanced oxidation processes, including the photo-Fenton process. They reported that the decolorization and mineralization extents were influenced by the initial pH, hydrogen peroxide dosage and initial dye concentration and that the photo-Fenton process was the most efficient. Guivarch et al. [13] reported that the kinetic analysis of the data obtained from the degradation of the azo dyes azobenzene, p-methyl red and methyl orange in aqueous solution by electro-Fenton showed a pseudo first-order degradation reaction for all azo dyes. These authors also reported that the degradation of dyes and intermediates proceeds by oxidation of azo bonds and aromatic rings by hydroxyl radicals. The azure B dye (AB), also called Methylene Azur B (trimethylthionine chloride compound, C15H16ClN3S), with large molecular structure (shown in Scheme 1) is a metachromatic basic dye soluble in water. AB is a recalcitrant compound and it has been found in wastewaters [22]. The degradation of AB compound has been studied by other research groups. Ferreira-Leitao et al. [23] investigated the oxidation of AB by horseradish peroxidase (HRP) of plant origin. These authors reported that the HRP was unable to achieve aromatic ring cleavage from the AB and that the azure C, which was formed by sequential oxidation of AB, was a major reaction product in HRPmediated reactions. Rodríguez et al. [24] studied the photocatalytic degradation of AB employing ZnO immobilized in alginate gel beads. These authors reported that total discoloration of AB was achieved with a 52% reduction of the organic carbon content. zAartih et al. [18] also studied the photocatalytic degradation of AB employing combustion-synthesized TiO2 and Degussa P-25 TiO2 and reported that the presence of metal ions significantly

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COD. The results of the removal of AB concentration are reported as an average value obtained from three analyses of the samples during degradation.

reduced the photocatalysis rates. Other research groups have also studied the Fenton-like process to degrade AB. Aguiar and Ferraz [22] reported the degradation of AB by Fenton-like reactions in the 3-hydroxyanthranilic acid/Fe3+/H2O2 system, achieving faster discoloration than that obtained by the conventional Fenton reaction. The purposes of this work were to study the degradation of azure B dye by Fenton and compare its degradation under sonolysis and sono-electroFenton (with hydrogen peroxide electrochemically generated). The effects of solution pH, Fe2+ concentration and H2O2 concentration were investigated. The experimental results were assessed in terms of chemical oxygen demand (COD) and AB concentration reduction to determine the overall treatment efficiency of the degradation processes. COD is an index of water pollution by organics and it is a parameter used for quality discharge. COD differences along the time are exclusively related to the degree of oxidation of the organic matter as a whole.

2. Experimental 2.1. Chemicals All solutions were prepared with distilled water using AB, sodium sulfate as background electrolyte, sulfuric acid or sodium hydroxide to adjust the pH of the solution and hydrogen peroxide (30%). All chemicals (Sigma–Aldrich) were analytical grade and used as received without further purification.

3. Results and discussion 3.1. Effect of the parameters that influence the removal of azure B in Fenton reaction A series of experiments were undertaken to study the removal of azure B as a function of the concentration of Fe2+, H2O2 and pH, as depicted in Fig. 1. The amount of catalyst is an important parameter that affects the H2O2 activation in the Fenton reaction. Thus, the degradation rate of organic compounds is also affected. In order to determine the effect of the iron catalyst amount in the Fenton reaction on the removal efficiency of azure B (AB), a series of experiments were carried out where the concentration of Fe2+ varied inside the concentration range of 0–1.48 

(A) 100 Azure B removal (%)

Scheme 1. Molecular structure of azure B.

60 40 20 0

0.0

0.4 2+

Fe

(B) Azure B removal (%)

2.2. Procedures

0.8

1.2

1.6 -1

concentration (mmol L )

100 80 60 40 20 0 0.0

1.0

2.0

3.0

4.0

5.0

H2O2 concentration (mmol L-1)

(C)

100 90

Azure B removal (%)

The degradation tests by the Fenton process (under silent conditions) were performed in a chemical reactor with volume of 0.5 L. The rest of the degradation tests were performed in the sono-reactor with a volume of 1.5 L with recirculation that permitted us to investigate the effects of sonolysis, Fenton and sonoelectroFenton either separately or simultaneously. The sonoreactor was comprised by a three electrode cell and a jacketed stainless steel cylindrical ultrasound cell that maintained the reaction temperature at 25 ± 3 °C with cooling water recirculation. An ultrasound probe (13 mm diameter) was inside the ultrasound cell powered by an ultrasonic processor (Dr. Hielscher) with a power input of 91 W at a frequency of 24 kHz. A fast stream of oxygen was fed to the test solution through the three electrode cell. The test solution was flowing through the electrochemical cell and the ultrasound cell. The electrochemical cell was comprised by a reticulated vitreous carbon (RVC) cathode (4 cm  4 cm  0.5 cm), platinum gauze counter electrode and saturated calomel reference electrode (SCE) through a lugging capillary. Samples (3 mL) were withdrawn from the sono-reactor at different time intervals to measure both the absorption spectra in the wavelength interval of 1 nm from 250 to 800 nm with a spectrophotometer (DR/ 4000U HACH) and the chemical oxygen demand (COD). The maximum absorption wavelength of AB was found at 647 nm. The absorbance measurements at 647 nm were employed to calculate the dye concentration from the calibration curve (absorbance vs. AB concentration) built at the corresponding pH of the aqueous solution. The COD was analyzed using standard methods and standard tubes [25] inside the concentration range of 0–40 mg L1

80

80 70 60 50 40 1.0

1.5

2.0

2.5

3.0

3.5

4.0

pH Fig. 1. Removal of 5  104 mol L1 Azure B in 0.5 L of aqueous solution by Fenton reaction in the presence of: (A) 2.4 x 103 mol L1 H2O2 at pH 2.8, (B) 0.8  103 mol L1 Fe2+ at pH 2.8, (C) 2.4 x 103 mol L1 H2O2 and 0.8  103 mol L1 Fe2+.

S.S. Martínez, E.V. Uribe / Ultrasonics Sonochemistry 19 (2012) 174–178

103 mol L1 in a 0.5 L of solution containing 5.0  104 and 2.4  103 mol L1 of AB and H2O2 respectively at pH 2.8. Fig. 1(A) shows the results for the degradation of AB by the Fenton reaction as a function of the concentration of Fe2+ after 20 min of reaction. Negligible removal of AB concentration is observed in the absence of iron; whereas, as the Fe2+ concentration increases, the rate of azure B removal is increased until the concentration of Fe2+ is 0.8  103 mol L1 and the solution becomes colorless. Above this iron concentration no further removal of AB was observed. The lack of AB degradation after an optimal iron concentration may be explained by the competing reaction between Fe(II) and OH radicals when the iron concentration is in excess [14]. Hence, low iron concentration should be employed to avoid the scavenging effect. This behavior is in agreement with results of other workers [12,14,26]. Fig. 1(B) reports the effect of the concentration of hydrogen peroxide on the removal of AB after 20 min of reaction. The amount of H2O2 varied inside the concentration range of 0.6–4.8  103 mol L1 in 0.5 L of aqueous solutions containing 5  104 and 8  104 mol L1 of AB and Fe2+ respectively at pH 2.8. Negligible AB removal from the solution was observed without the addition of Fe2+. This behavior is because the hydrogen peroxide is unable to oxidize the AB, even though H2O2 is a strong oxidant. The H2O2 needs to be activated by the iron ions to produce the highly reactive oxidizing species that are responsible for the degradation of organic pollutants present in aqueous effluents. The oxidizing power of the hydrogen peroxide is highly enhanced by the addition of iron generating the Fenton reaction, leading to the generation of active intermediates (such as OH and OOH radicals) by the iron catalyzed decomposition of the hydrogen peroxide. The formation of a highly reactive iron-oxo complex (ferryl ion, FeO2+) has also been proposed as the oxidative intermediate in the Fenton reaction [27,28]: 2þ

Fe

2þ

þ H2 O2 ! FeO

þ H2 O

Despite the numerous studies attempting to clarify the mechanism involved in Fenton reaction, the controversy still remains whether the chemical mechanism involves radical production or ferryl ion generation as the active intermediate species. Fig. 1(B) also shows that the removal of AB increases by increasing the concentration of H2O2, reaching a maxima removal of AB concentration at 2.4  103 mol L1 H2O2. The removal of AB is due to the generation of the active intermediates (OH, OOH radicals and FeO2+) in the Fenton reaction. A further increase in the H2O2 concentration did not enhanced AB degradation. Thus, 2.4  103 mol L1 H2O2 seems to be the optimal oxidant concentration to achieve high AB degradation during Fenton reaction. The lack of AB removal after an optimal H2O2 concentration may be explained by the competing reaction between H2O2 and OH radicals when the H2O2 concentration is in excess [14]. Hence, an optimal concentration of H2O2 should be employed to avoid the scavenging effect. The pH of the solution also plays an important role in the Fenton reaction and can affect the degradation rate of AB. In order to determine the effect of the solution pH on AB removal efficiency, a series of experiments were carried out inside the pH range of 1.6– 5.8 in 0.5 L of aqueous solutions containing 5  104, 2.4  103 and 0.8  103 mol L1 of AB, H2O2 and Fe2+ respectively. Fig. 1(C) reports the results for the degradation of AB by the Fenton reaction as a function of the solution pH after 20 min of reaction. It is observed that as the pH of the solution increases, the removal rate of AB concentration increases until the pH of the solution is between 2.8 and 3.0, where the solution became colorless. At higher pH, AB removal decreases because the iron precipitates as oxyhydroxides [12]. From these results, it is observed that the best oxidative degradation of AB by Fenton reaction was obtained at pH values of 2.6–3.0.

The best conditions found during the study of the influencing parameters on Fenton degradation of AB have been employed to record the COD abatement as a function of time. Fig. 2 shows that 95% of COD removal was achieved after 70 min of reaction employing the Fenton process for the oxidation of 0.5 L aqueous solution containing 5  104, 2.4  103 and 0.8  103 mol L1 of AB, H2O2 and Fe2+ respectively. 3.2. Azure B removal by Fenton reaction with H2O2 electrochemically generated It has been shown that the H2O2 catalyzed with 0.8  103 mol L1 Fe2+ effectively oxidized the AB inside the optimal pH interval. The employment of H2O2 as oxidant is environmentally friendly and its electrochemical generation avoids the hazards associated with the storage and handling of this oxidant because it is generated in situ and at controlled rate. Several electrochemical experiments were carried out to observe the AB concentration abatement under different H2O2 generation rates governed by the potential applied to the RCV working electrode. It can be observed in Table 1 that AB concentration and COD abatement increase as the potential increases. This behavior is because the oxygen reduction at the RVC electrode surface increases as the potential increases, electrogenerating higher H2O2 concentration. A current efficiency of 50% during H2O2 generation is observed inside the potential range applied. It was also observed that the color removal of AB follows first-order kinetics, where the rate constants also increase with potential, as shown in Table 1. Once it was observed that the AB was removed in the electrochemical experiments, a potential of 0.7 V vs. SCE was chosen to remove the AB in combination with ultrasound. 3.3. Sono-electroFenton reaction with H2O2 electrochemically generated The ultrasound effect on AB removal was observed by combining the Fenton reaction with ultrasound, where the H2O2 was electrochemically generated with the addition of 0.8  103 mol L1 Fe2+ and 50  103 mol L1 Na2SO4 as supporting electrolyte. Fig. 3 and Table 2 reports the removal of AB concentration and COD abatement under different experimental conditions after 60 min of reaction. In the absence of both potential and ultrasound (Table 2), negligible degradation of AB is observed. The ultrasound contributed with 25% and 21% of AB concentration removal and COD abatement respectively; and the potential contributed with 82% and 67% of AB and COD removals, respectively. The sono-electroFenton process (the combination of ultrasound with

100

COD Abatement (mg L-1)

176

80 60 40 20 0 0

20

40

60

80

Time (min) Fig. 2. Removal of chemical oxygen demand of 5  104 mol L1 azure B in 0.5 L of solution containing 2.4  104 and 0.8  104 mol L1 of H2O2 and Fe2+ respectively at pH 2.8 and 25 °C.

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S.S. Martínez, E.V. Uribe / Ultrasonics Sonochemistry 19 (2012) 174–178 Table 1 Removal of AB and COD. The apparent first-order rate constants, obtained under different experimental conditions during the removal of AB concentration, are also shown. Applied potential (V) vs. SCE

0.5 0.7 0.9

Removal (%), after 60 min AB concentration

COD

59 82 85

45 67 70

k (min1)

Linear coefficient (R2)

0.019 ± 0.0005 0.058 ± 0.0008 0.063 ± 0.001

0.9907 0.9707 0.9560

(sono-electroFenton). AB removal profiles by the sole effect of sonolysis or Fenton are also shown in this figure. The sole effect of sonolysis on AB removal is small compared to Fenton and sono-electroFenton. Sonolysis accounts for 25% of AB removal after 60 min of ultrasonic irradiation. Whereas, the combined effect in sono-electroFenton enhances the substrate degradation attained by Fenton under silent conditions and also under the sole effect of sonolysis using the optimum concentrations of Fe2+ and pH. AB removal and COD abatement achieved by Fenton (under silent conditions) were 85% and 68% respectively. The sono-electroFenton reaction produced a colorless solution with 85% of COD removal after 60 min of reaction. Table 3 reports the apparent first-order rate constants obtained under different experimental conditions. It is observed that the rate constant for AB degradation by sono-electroFenton is 10-fold that of sonolysis and 2-fold the one obtained by Fenton under silent conditions. These results have clearly demonstrated that the combination of Fenton and sonolysis produced a synergistic effect on the degradation rate as shown by that obtained by sono-electroFenton. The enhancement in the AB degradation by the combined effect of Fenton and sonolysis can be explained by the increased yields of OH radicals by acoustic cavitation. It has been known that the primary products from the sonolysis of water are H2 and H2O2, forming H and OH; in the presence of O2, HO2 is also produced [29–31]. Fig. 4 shows the absorbance spectra recorded as a function of time during the sono-electroFenton oxidation of AB. The initial spectrum shows two bands, one at 600 nm and the other at 647 nm, with the maximum absorbance recorded for the later. It is observed that after the first 18 min of reaction the shoulder at 600 nm has almost disappeared and after 35 min of sonoelectroFenton both absorbance peaks have completely disappeared. The spectra together with the results on COD removal suggest an almost complete degradation of the dye. The color of AB solutions becomes less intense (hypsochromic effect) when all

Table 3 Apparent first-order rate constants obtained under the application of different degradation processes during the removal of AB concentration.

Fig. 3. Plot showing the removal of (A) azure B concentration and (B) chemical oxygen demand of 5  104 mol L1 azure B in 1.5 L of solution containing 0.8  103 mol L1 Fe2+ and 50  103 mol L1 Na2SO4 of supporting electrolyte at pH 2.8 during H2O2 electrochemical generation using 0.7 V vs. SCE at 25 °C. (d) Sono-electroFenton, (N) Fenton, (j) sonolysis.

Degradation process

k (min1)

Linear coefficient (R2)

Sonolysis Fenton (H2O2 generated + Fe2+ addition) Sono-electroFenton

0.0062 ± 0.0001 0.0306 ± 0.0010

0.9839 0.9770

0.0559 ± 0.0020

0.9696

Table 2 Removal of 5  104 mol L1 AB and COD in the presence and absence of ultrasound. Experimental conditions

NAP NAP AP at 0.7 V vs. SCE AP at 0.7 V vs. SCE

Removal (%), after 60 min

NAUs AUs NAUs AUs

AB concentration

COD

0 25 85 98

0 21 68 85

NAP: no applied potential. AP: applied potential. NAUs: no applied ultrasound. AUs: applied ultrasound.

Fenton) enhanced the AB and COD abatements reaching 98% and 85% removals respectively after 60 min of reaction. Fig. 3 shows the AB degradation (Fig. 3(A)) and COD abatement (Fig. 3(B)) profiles during the oxidation of AB by the electrogenerated H2O2 with iron addition (Fenton) and its combination with sonolyisis

Fig. 4. Plot showing the spectra as a function of time for the degradation of 5  104 mol L1 azure B in 1.5 L of solution containing 0.8 mM Fe2+, 50 mM de Na2SO4 at pH 2.8 during H2O2 electrochemical generation (0.7 V vs. SCE) at 25 °C by the tandem process of sono-electroFenton.

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or parts of the auxochromic groups (methyl or methylamine) degrade. This figure also reveals that the absorbance band recorded at 290 nm decreased significantly and no new bands appeared. This implies that fully oxidized forms and any intermediates produced by the degradation of AB are not present.

[10]

[11]

4. Conclusions

[12]

This study demonstrated that the concentration of azure B was removed efficiently by sono-electroFenton degradation where H2O2 was electrogenerated and COD was decreased 85%. It was found that the pH and the initial amounts of H2O2 and Fe2+ influence AB degradation by Fenton reaction. The best oxidative degradation of AB by Fenton reaction was obtained at pH between 2.6 and 3.0 using 0.8  103 mol L1 Fe2+ and 2.4  103 mol L1 H2O2. The oxidative degradation of AB followed apparent firstorder kinetics, where the rate constants decreased in the following order: sono-electroFenton > Fenton > sonolysis. The rate constant for AB degradation by sono-electroFenton is 10-fold that of sonolysis and 2-fold the one obtained by Fenton under silent conditions.

[13]

Acknowledgements

[20]

The author thank Secretaría de Educación Pública through PROMEP (Programa de Mejoramiento a Profesores) program for sponsorship this project.

[21]

[14]

[15]

[16]

[17] [18] [19]

[22]

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