Decolorization and degradation of Ponceau S azo-dye in aqueous solutions by the electrochemical advanced Fenton oxidation

Desalination 264 (2010) 143–150

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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l

Decolorization and degradation of Ponceau S azo-dye in aqueous solutions by the electrochemical advanced Fenton oxidation Hanaa S. El-Desoky ⁎, Mohamed M. Ghoneim, Naglaa M. Zidan Analytical and Electrochemistry Research Unit, Chemistry Department, Faculty of Science, Tanta University, 31527-Tanta, Egypt

a r t i c l e

i n f o

Article history: Received 8 May 2010 Received in revised form 2 July 2010 Accepted 7 July 2010 Available online 7 August 2010 Keywords: Ponceau S azo-dye Decolorization Degradation Mineralization Electro-generated Fenton's reagent

a b s t r a c t Oxidation (decolorization/degradation) of Ponceau S azo-dye in aqueous solutions by electro-generated Fenton's reagent (Fe2+/H2O2) was optimized. This was carried out in a reactor containing 0.05 M Na2SO4 solution of pH 2.5 and a catalytic quantity of 0.1 mM FeSO4 while a cathode potential of − 1.0 V (vs. SCE) was applied. The reactor was an undivided glass electrochemical cell with a reticulated vitreous carbon (RVC) cathode and a platinum gauze anode. Progress of oxidation of various concentrations of Ponceau S with time of electro-Fenton reaction was monitored by UV–visible absorption measurements. Mineralization of the dye solutions was examined by estimation of the chemical oxygen demand removal and HPLC analysis at various times of electro-Fenton oxidation. The complete color removal of 0.05, 0.1 and 0.3 mM Ponceau S was achieved by electro-Fenton oxidation for 30, 60 and 90 min, respectively. However, approximately 98% mineralization of 0.05, 0.1 and 0.3 mM Ponceau S was achieved due to electro-Fenton oxidation for 40, 60 and 120 min, respectively. HPLC analyses showed also that almost no aromatic compounds were remaining in the treated solutions indicating the efficiency of the electro-generated Fenton's reagent for complete decolorization and significant mineralization of the Ponceau S azo-dye in aqueous solutions. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Synthetic azo-dyes are widely used in textile, printing, cosmetic, food colorants and pharmaceutical industries. Some azo-dyes are also used in laboratories as either biological stains or pH indicators. The strong electron withdrawing character of the azo-group stabilizes the aromatic substances against conversion by oxygenases. The durability of azo-dyes, however, causes pollution of surface water and consequently the soil and groundwater once the dye discharged into the environment as effluent. Most of the synthetic azo-dyes and their biodegradation products, sulfonated and unsulfonated aromatic amines, are toxic against aquatic organisms and suspicious of being carcinogenic and mutagenic for humans [1–8]. Therefore, these dyes necessitate proper treatment before discharge into the environment. Various methods for removal of synthetic azo-dyes from wastewaters have been reported in the literature. These include adsorption on inorganic or organic matrices, biological activation, coagulation, chemical oxidation and electrochemical oxidation methods [1–3,7– 9]. Nevertheless, some of these methods are usually non-destructive, inefficient, costly, and resulted in production of secondary waste products such as sludge which may need further disposal. ElectroFenton oxidation method as an indirect electrochemical advanced oxidation process was developed and widely applied for oxidation of various organic pollutants [9–25]. In this method, H2O2 is continu⁎ Corresponding author. Tel.: +20 40 2241143; fax: +20 40 3350804. E-mail address: [email protected] (H.S. El-Desoky). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.07.018

ously generated by reduction of the dissolved molecular O2 in mildly acidic aqueous medium (Eq. (1)) using different cathodes (e.g. reticulated vitreous carbon [14], mercury pool [15], carbon-felt [16] and O2-diffusion [17,18]): þ

−

o

O2 þ 2H þ 2e →H2 O2

ðE ¼ 0:69V=NHEÞ

ð1Þ

The oxidizing power of H2O2 is enhanced in presence of Fe2+ ions (as catalyst) in mildly acidic solution. Hydroxyl radical (OH•) and Fe3+ ions are then generated from the classical Fenton's reaction between Fe2+ ions and H2O2 [14–19,26,27]: 2þ

Fe

þ

þ H2 O2 þ H →Fe

3þ

0 þ H2 O þ OH•ðFenton s reactionÞ

ð2Þ

Fe2+ ions consumed by Fenton's reaction in the homogeneous medium (Eq. (2)) are regenerated at the cathode by reduction of Fe3+ ions (Eq. (3)) which would induce the Fenton chain reaction efficiently [16]. 3þ

Fe

−

2þ

o

þ e ⇌Fe

E ¼ 0:77V=NHE

ð3Þ

On the other hand, molecular oxygen necessary for production of H2O2 (Eq. (1)) is also regenerated at the anode by oxidation of water in the reactor (Eq. (4)): þ

−

H2 O⇌1=2 O2 þ 2H þ 2e

o

E ¼ 1:23V=NHE

ð4Þ

144

H.S. El-Desoky et al. / Desalination 264 (2010) 143–150

The sum of the above cathodic and anodic processes gives the overall reaction: 1=2 O2 þ H2 O → 2OH•

ð5Þ

The indirect electro-Fenton oxidation method is environmentally friendly since it does not involve the use of harmful chemical reagents due to the fact that Fe2+ ions, H2O2 and the produced hydroxyl radicals (OH•) are non-toxic, besides the method is easy to handle and its reactor is simple. Besides, the indirect electro-Fenton oxidation method is very promising since it achieves high reaction yields with low treatment cost and has been efficiently applied to degrade many organic compounds [9–25]. Moreover, there is no production of iron sludge in the reactor and consequently no subsequent disposal problems were found. On other side, the hydroxyl radicals (OH•) are non-selective very powerful oxidizing agent that react with organics yielding dehydrogentated or hydroxylated derivatives until their overall mineralization (conversion into CO2 and H2O). Ponceau S (3-Hydroxy-4-(2-sulfo-4-[4-sulfophenylazo] phenylazo) -2,7-naphthalenedisulfonic acid sodium salt), Scheme 1, has been used in the present study as a model of sulfonated azo-dye. Ponceau S azodye (also known as Acid Red 112) is used in dying industry of textile, leather and paper [28–30] and also used in clinical laboratories as a protein-binding dye for staining of blood serum proteins [31]. However, its biotransformation products have toxic effects against aquatic organisms and suspicious of being carcinogenic for humans [3,29]. To our knowledge there is no information reported in the literature to date concerning decolorization/degradation of Ponceau S azo-dye by electro-generated Fenton's reagent. However, its complete decolorization was achieved using a developed methylene blue immobilized resin dower-11 photocatalyst [28] for 3 h treatment in solutions of pH 9. Besides, two biological methods were reported [32,33] for decolorization of Ponceau S azo-dye in aqueous solutions. The bacterium, Kerstersia sp. strain VKY1 could only decolorize Ponceau S azo-dye under aerobic condition with 100% decolorization efficiency at varying initial dye concentrations of 200, 400, 600 and 800 mg L− 1 during 9, 12, 15, and 18 h incubation [32]. On other hand, the aerobic reduction of 0.2 mM Ponceau S azo-dye by Gram-positive bacterium Enterococcus faecalis strain ATCC 19433 to about 4.1% was achieved after 20 h cultivation in brain heart infusion medium [33]. Therefore, there is a need to optimize the optimal conditions for degradation of Ponceau S azo-dye, either to less harmful products or, more desirable, to complete mineralization by an effective and environmentally friendly method. The present study was designed to optimize the oxidation (decolorization/degradation) of Ponceau S azo-dye in aqueous solutions by electro-generated Fenton's reagent (Fe2+/H2O2) in an undivided glass electrochemical cell using a reticulated vitreous carbon (RVC) cathode and a platinum gauze anode. 2. Experimental 2.1. Materials and reagents

purification. Solutions of H2SO4 (1 M), anhydrous Na2SO4 (1 M), NaCl (1 M), KCl (1 M), FeSO4·7H2O (5 mM), K2Cr2O7 (0.025 N) and FeSO4(NH4)2SO4·6H2O (0.025 N) (all are analytical-grade reagents) were prepared in deionized water. Desired lower concentrations were obtained by the accurate dilution of the standard solutions by deionized water. Ferrion indicator solution (which was used in analysis of chemical oxygen demand, COD, by the standard open reflux method [34]) was prepared by dissolving 1.485 g of 1,10phenanthroline monohydrate and 695 mg FeSO4·7H2O in 100 mL deionized water. 2.2. Electrochemical apparatus A Potentiostat Model 362-PAR (Princeton Applied Research, Oka Ridge, TN, USA) was used in the present study. An undivided glass electrochemical cell (reactor) of 600-mL volume with a three electrode system was used. The cathode was a reticulated vitreous carbon (RVC) sheet (60 PPI) of the dimensions 5 cm, 7 cm and thickness of 0.9 cm (ARG Materials and Aerospace Corporation, Oakland, CA, USA). The anode was a platinum gauze of an area of 3.8 cm2 placed at the center of the electrochemical cell. A reference saturated calomel electrode (SCE) was placed in a glass-luggin capillary at a position of 3 mm from the cathode surface. RVC was used as a cathode in the present electrochemical cell based on the fact that it is chemically and electrochemically inert over a wide range of potentials and with a broad variety of chemicals. It has a high specific surface area within the porous structure that is accessible to electrochemically active species. RVC has also a high fluid permeability, and it is easily shaped as required by cell design considerations. Furthermore, RVC has a good selectivity for H2O2 electrosynthesis. This is because it exhibits a range of electrochemical activities towards oxygen reduction, high overpotential for hydrogen evolution and low catalytic activity for decomposition of hydrogen peroxide. 2.3. Procedure Oxidation of Ponceau S azo-dye by electro-generated Fenton's reagent was carried out in 450 mL of 0.05 M Na2SO4 solution of pH 2.5 (the pH of solution was adjusted by addition of H2SO4 solution) in the presence of a catalytic quantity of 0.1 mM FeSO4 at a RVC cathode applied potential of −1.0 V vs. SCE using the described undivided glass electrochemical cell (reactor). Pure oxygen gas was bubbled (before starting the electro-Fenton oxidation) into the supporting electrolyte for 30 min to saturate the aqueous solution. The investigated solution was stirred magnetically in a rate of 400 rpm and the solution pH remained practically constant (2.5 ± 0.1). For monitoring the decolorization/ degradation of the examined azo-dye in aqueous solution by electroFenton oxidation, 1.5 mL of the examined solutions were withdrawn from the reactor at regular time intervals (0, 10, 20, 30, 45, 60, 90, 120, 150, 180, 240, 300, 360, 420, 480, 540 and 570 min) then analyzed. 2.4. Monitoring of oxidation reaction

A commercial Ponceau S azo-dye (Aldrich Chemical Co. Inc, St. Louis, MO, USA) was used in the present study without further

Progress of oxidation (decolorization/degradation) of Ponceau S azo-dye by electro-generated Fenton's reagent was monitored in the present study by:

Scheme 1. Chemical structure of Ponceau S azo-dye molecule.

2.4.1. UV–visible absorption measurements The UV–visible electronic absorption spectra of Ponceau S azodye in aqueous solutions were recorded at ambient temperature (25 ± 2 °C) within the wavelength range of 200–800 nm using a Shimadzu UV–visible spectrophotometer Model 160A (Kyto, Japan) with a spectrometric quartz cell (1 cm path length). Quantitative analysis of Ponceau S azo-dye in aqueous solution was monitored spectrophotometrically by measuring its absorbance at λmax. = 515 nm at different time intervals of electro-Fenton oxidation and then computing the corresponding concentration Ct of the dye from a

H.S. El-Desoky et al. / Desalination 264 (2010) 143–150

constructed calibration curve. The extent of color removal of the investigated solution can be expressed as: % Color removal ¼ ð1−Ct =C0 Þ  100

ð6Þ

where C0 is the initial concentration of the Ponceau S azo-dye and Ct is its concentration at reaction time t (min). 2.4.2. COD analysis To quantitatively characterize the extent of oxidation of Ponceau S azo-dye in aqueous solutions, 3 mL of the examined solutions were withdrawn from the reactor at regular time intervals (0, 10, 20, 30, 45, 60, — min) then the chemical oxygen demand (COD) was determined by means of the standard open reflux method [34] and then the % COD removed ratio was estimated: % COD removal ¼ ð1−CODt =COD0 Þ  100

ð7Þ

where COD0 and CODt are the measure of oxygen equivalent to the organic matter of a sample that is susceptible to oxidation by a strong chemical oxidant at reaction time 0 and t of electro-Fenton treatment of Ponseau azo-dye aqueous solutions, respectively. 2.4.3. HPLC analysis Reversed-phase HPLC was also utilized for monitoring the progress of degradation of Ponceau S azo-dye in aqueous solutions, formation and/or destruction of reaction aromatic products during the oxidation of Ponceau azo-dye. Liquid chromatograms of solutions of Ponceau S azo-dye during its oxidation by electro-Fenton's reagent were recorded using a high-pressure liquid chromatographic pump (Bischoff, Switzerland) equipped with a UV-detector (Bischoff, Lambda 1000). Data acquisition and peak integration was done with the Bischoff McDAcq integrator software v1.5. A reversed-phase column (Prontosil C18, 250 × 4.0 mm, 5 μm) was used in the present investigation at ambient temperature (25 ± 2 °C). A mixture of acetonitrile:water:acetate buffer of pH 5 (50:10:40 v/v/v) was used as a mobile liquid phase at a flow rate of 0.5 mL/min. The injection volume was 20 μL with a Rheodyne 7125 injector valve. The detection was performed by UV absorption at λmax = 349 nm. The % removed ratio of aromatic rings was calculated as: % Ar rings removal ¼ ð1−At =A0 Þ  100

ð8Þ

145

constructed (r = 0.999) and was used as a calibration curve for monitoring concentration of the dye in aqueous solution at different time intervals during its electro-Fenton oxidation. Preliminary study of decolorization/degradation of 0.1 mM Ponceau S azo-dye in aqueous solution of 0.05 M Na2SO4 regulated with H2SO4 at pH 2.5 in presence of 0.05 mM FeSO4 was carried out by electro-Fenton oxidation while controlling the cathode bias potential at −0.5 V (vs. SCE) using the described reactor. The UV–visible absorption spectra of a treated solution of Ponceau S withdrawn from the reactor at different time intervals were recorded and depicted in Fig. 1. It was clearly observed that the intensity of characteristic band at 515 nm of Ponceau S azo-dye diminished gradually during the experiment until it disappeared totally due to electro-Fenton oxidation for 570 min. The ultraviolet bands (at 349, 311 and 270 nm) were also observed to gradually diminish but with a lower rate than that of the visible band. This behavior indicated the effective destruction of the chromophore and breaking down of the aromatic rings of the dye molecules due to their attack with OH• radicals. However, decolorization process of Ponceau S azo-dye was faster than that of destruction of the aromatic rings. This is because OH▪ radicals attack first the –N = N– group of the lowest energy [35] leading to the opening of the –N = N– double bonds, then destructing the long conjugated π systems, and consequently causing decolorization of the investigated solution. For optimizing the operational conditions for efficient oxidation of Ponceau S azo-dye in aqueous solutions by electro-generated Fenton's reagent, the effect of each of the cathode applied potential, pH of the medium, nature of supporting electrolyte, and initial concentration of Fe2+ ions as a catalyst were studied spectrophotometrically. 3.2. Effect of the cathode applied potential Effect of the RVC cathode applied potential upon the oxidation of 0.1 mM Ponceau S azo-dye by electro-generated Fenton's reagent was examined at each of −0.5, −0.7, and −1.0 V vs. SCE in 0.05 M Na2SO4 aqueous solution of pH 2.5 containing 0.05 mM FeSO4. Absorbance of Ponceau S azo-dye solution at λmax = 515, 349, 311 and 270 nm was found to decrease with the increase of the cathode applied potential from −0.5 V to −1.0 V. The efficiency of color removal of Ponceau S azodye has found to reach approximately 100% by electro-Fenton oxidation for 180, 330, and 570 min at the cathode applied potential of −1.0,

where A0 and At are the peak area at reaction time 0 and t of electroFenton's oxidation of the investigated azo-dye solutions, respectively. All experiments were performed in triplicate under ambient temperature. Reproducible results of % Color removal calculated from spectrophotometric measurements, % COD and % Aromatic ring removal were obtained under all the tested experimental conditions. 3. Results and discussion 3.1. UV–visible absorption spectral measurements The absorption spectra of various concentrations of Ponceau S azodye in acidic 0.05 M Na2SO4 aqueous solution displayed a main band in the visible region with a maximum absorption (λmax) at 515 nm in addition to three smaller bands in the ultraviolet region of λmax at 349, 311 and 270 nm. The band in the visible region (λmax = 515 nm) accounts for color of the azo-dye was attributed to the absorption of the n → π⁎ transition related to the –N = N– group [35]. While the bands at 311 and 349 nm were ascribed to the absorption of the π–π⁎ transition related to the naphthalene rings bonded to the –N = N– group of the azo-dye molecule [35]. The band at 270 nm was attributed to the adsorption of the π–π⁎ transition within the benzenoid system [35]. Plot of absorbance (at λmax = 515 nm) vs. concentration of Ponceau S azo-dye in aqueous solution was

Fig. 1. UV–visible absorption spectra of 0.1 mM Ponceau S azo-dye aqueous solution (diluted 2-folds) at different time t intervals: 0, 10, 20, 30, 45, 60, 90, 120, 150, 180, 240, 300, 360, 420, 480, 540 and 570 min (from up to down) of electro-Fenton oxidation in 0.05 M Na2SO4 aqueous solution of pH 2.5 in the presence of 0.05 mM FeSO4 at a RVC cathode applied potential of − 0.5 V.

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−0.7, and −0.5 V, respectively (Fig. 2). This behavior indicated that the electro-Fenton oxidation (decolorization/degradation) of Ponceau S azo-dye was much faster under a cathode applied potential of −1.0 V. However, for a potential more cathodic than −1.2 V vs. SCE the side reaction of hydrogen evolution commenced [36].

3.3. Effect of pH of the medium Previous studies [37,38] revealed that the solution pH can dramatically influence the degradation of synthetic azo-dyes in water by Fenton oxidation and the optimal solution pH values for effective Fenton oxidation were achieved over the pH range of 2 to 5. At lower pH values, the reaction according to Eqs. (1) and (2) could be slowed down because hydrogen peroxide can stay stable probably by solvating a proton to form an oxonium ion, H3O+ 2 (Eq. (9)). The oxonium ion makes hydrogen peroxide electrophilic to enhance its stability and presumably to reduce substantially the reactivity with Fe2+ ions [39,40]. In addition, the scavenging effect of the OH• radicals by H+ is severe (Eq. (10)) [41,42]. þ

þ

H2 O2 þ H →H3 O2

OH• þ H þ e →H2 O þ

−

ð9Þ

Fig. 3. % Color removal with time (t) of electro-Fenton oxidation of 0.1 mM Ponceau S in 0.05 M Na2SO4 aqueous solution of different pH values, containing 0.05 mM FeSO4 at a cathode applied potential of − 1.0 V.

ð10Þ

respectively (Fig. 3). Since the oxidation reaction was faster in the solution of pH 2.5, it was used in the rest of the present study.

On the other hand, in solutions of pH values higher than 5 the oxidation efficiency was rapidly decreased, not only by decomposition of H2O2 leading to lose its oxidation power [43,44] but also by deactivation of the catalytic action of Fe2+ ions due to the formation of ferric hydroxide complexes and consequently leading to reduce the production of OH• radicals [45]. Hence, experiments were conducted to study the effect of pH on oxidation of 0.1 mM Ponceau S azo-dye by electro-generated Fenton's reagent in 0.05 M Na2SO4 aqueous solution of different pH values (2.5–4.5) in the presence of 0.05 mM FeSO4 at a cathode applied potential of − 1.0 V. The pH of the examined solution remained almost constant because the protons consumed at the cathode according to Eq. (1) are continuously balanced by water oxidation reaction that occurred at the anode (Eq. (4)). The results (Fig. 3) showed that approximately 100% color removal of Ponceau S azo-dye was achieved in solutions of pH values 2.5, 3.5 or 4.5 by electro-Fenton oxidation for 180, 390 or 510 min,

Electro-Fenton oxidation of 0.1 mM Ponceau S azo-dye was examined in different supporting electrolytes (0.05 M and 0.1 M of each of Na2SO4, NaCl, or KCl) of pH 2.5 in the presence of 0.05 mM FeSO4 at a cathode applied potential of −1.0 V vs. SCE. From Fig. 4, it could be concluded that approximately 100% color removal of the examined azodye was achieved by electro-Fenton oxidation for 270, 330 and 360 min (for 0.1 M Na2SO4, NaCl and KCl) or 180, 220 and 250 min (for 0.05 M Na2SO4, NaCl and KCl), respectively. This behavior indicated that the oxidation of Ponceau S azo-dye by electro-generated Fenton's reagent was faster in the solutions of Na2SO4 than that in the solutions containing Cl− ions. In practical applications, Na2SO4 is generally used as the supporting electrolyte. Such electrolyte improves the solution conductivity, and accelerates the electron transfer, thus benefits the

Fig. 2. % Color removal with time (t) of electro-Fenton oxidation of 0.1 mM Ponceau S in 0.05 M Na2SO4 aqueous solution of pH 2.5 containing 0.05 mM FeSO4 at different RVC cathode applied potentials.

Fig. 4. % Color removal of 0.1 mM Ponceau S with time (t) of electro-Fenton oxidation in different aqueous supporting electrolytes of pH 2.5, containing 0.05 mM FeSO4 at a RVC cathode applied potential of − 1.0 V.

3.4. Effect of nature of the supporting electrolyte

H.S. El-Desoky et al. / Desalination 264 (2010) 143–150

electro-Fenton reaction. Furthermore, there are reports available in the literature [46,47] which shows that in the presence of Cl− ions, the efficiency of Fenton oxidation is lowered whereas in the presence of SO−2 ions the effect is marginal. The less efficient degradation of 4 pollutants in Cl− ions containing solutions may be due to their interaction with OH• radicals (Eq. (11)). − − Cl þ OH•→ClOH•

ð11Þ

where Cl− ions scavenge some of the OH• radicals in a nearly diffusioncontrolled way and the radical anion, ClOH•− may then decay through reaction (12), ClOH• þ Fe →Cl þ OH þ Fe −

2þ

−

–

3þ

ð12Þ

2 ions with fully However, highly negatively charged species SO− 4 oxidized sulfur is not expected to interact considerably with the strong oxidant OH• radicals [48]. On the other hand, it is also advisable to carry out degradation of dyes in sulfate media (of slower decomposition rate) instead of chloride-containing media (of much faster decomposition rate) to avoid the generation of toxic and carcinogenic chlorinated byproducts. Therefore, since the oxidation reaction was faster in solution of 0.05 M Na2SO4, and to avoid generation of toxic and carcinogenic chlorinated species, a solution of 0.05 M Na2SO4 was chosen as a supporting electrolyte for the rest of the present study.

3.5. Effect of concentration of catalytic ferrous ions Oxidation of 0.1 mM Ponceau S azo-dye was carried out by electrogenerated Fenton's reagent in 0.05 M Na2SO4 solution of pH 2.5 at a cathode applied potential of −1.0 V in the absence as well as in presence of various concentrations (0.05, 0.1, 0.5 and 1.0 mM) of FeSO4. The results indicated that decolorization/degradation of Ponceau S azodye was remarkably dependent on concentrations of Fe2+ ions. Decolorization of Ponceau S azo-dye solution was relatively limited in the absence of Fe2+ ions. Approximately 30% color removal only of Ponceau S azo-dye was achieved by its treatment in absence of Fe2+ ions for 210 min under the other optimum conditions (Fig. 5, curve a). The low oxidative ability of the Ponceau S azo-dye in absence of Fe2+ ions can be related to the poor indirect electro-oxidation reaction of the dye with the electro-generated H2O2 at the cathode (Eq. (1)) [18], hydroperoxyl radicals (HO•) that can be produced at the anode (Eq. (13)) [49] and/or slow reaction of the dye with the adsorbed

147

OH• radicals (Eq. (14)) that can be produced from water oxidation at the surface of Pt anode [17,18]. H2 O2 →HO2 • þ H þ e

ð13Þ

þ − H2 O→OH•ads þ H þ e

ð14Þ

þ

−

However, H2O2 alone is not a powerful enough oxidant and thus the dye is mainly oxidized via reaction with the adsorbed OH• but because of its low concentration at anode, there is difficulty of achieving complete oxidation [17,18], (Fig. 5, curve a). However, about 100% color removal of Ponceau S was achieved by electro-Fenton oxidation for 150, 45, 90 or 180 min in presence of 0.05, 0.1, 0.5 or 1.0 mM FeSO4, (Fig. 5, curves b, c, d, e), respectively. An obvious increase in oxidation extent was observed by increasing concentration of FeSO4 up to a critical quantity of 0.1 mM (Fig. 5, curves b, c). This is due to the fast reaction of the examined dye with enough amounts of OH• radicals generated in the medium from electro-Fenton's reaction (the main oxidation reaction) according to Eq. (2) [17,18]. At higher concentrations of Fe2+ ions than the critical one (0.1 mM), the rate of oxidation decreased (curves d, e) which may be due to amount of hydroxyl radicals scavenged by the effect of Fe2+ ions [50] as indicated by Eq. (15). The formed Fe3+ ions could react with H2O2 to produce hydroperoxyl radicals (HO2•) of less oxidation capability (Eqs. (16) and (17)) resulting in the decrease of rate of oxidation (decolorization/degradation) of the azo-dye by the electroFenton reaction [15]: 2þ

3þ − þ OH•→ Fe þ OH

3þ

þ H2 O2 → Fe–OOH

Fe

Fe

2þ

Fe–OOH → HO2 • þ Fe 2þ

2þ

ð15Þ

þH

þ

ð16Þ

ð17Þ

This means that the fast oxidation reaction (decolorization/ degradation) of Ponceau S azo-dye was achieved in the presence of 0.1 mM Fe2+ ions. On other words, the oxidation rate enhanced when the Fe2+ ions content was increased from 0.0 mM to 0.1 mM, confirming that OH• radicals are mainly produced by the Fe3+/Fe2+ catalytic system (Eq. (2)). Hence, 0.1 mM FeSO4 was considered the optimum for the cleavage of –N = N– double bonds and break down of the rest of the investigated dye molecules by electro-Fenton oxidation. According to the foregoing results, the optimal operation conditions for oxidation (decolorization/degradation) of Ponceau S azo-dye in aqueous solution by electro-generated Fenton's reagent were: 0.05 M sodium sulfate solution of pH 2.5 as a supporting electrolyte, 0.1 mM FeSO4 as a catalyst and a cathode applied potential of −1.0 V (vs. SCE).

3.6. Oxidation of various concentrations of Ponceau S azo-dye under the optimized operational conditions

Fig. 5. % Color removal of 0.1 mM Ponceau S with time (t) of electro-Fenton oxidation in 0.05 M Na2SO4 aqueous solution of pH 2.5 at a RVC cathode applied potential of − 1.0 V in the absence and presence of various concentrations of FeSO4.

Oxidation (decolorization/degradation) of 0.05, 0.1 and 0.3 mM of Ponceau S azo-dye in aqueous solutions by electro-Fenton's reagent was performed under the optimized operational conditions. Progress of oxidation reaction with time was monitored spectrophtometrically. The recorded UV–visible absorption spectra of the investigated solutions (e.g. Fig. 6) showed that the absorbance at each of λmax of 515, 349, 311 and 270 nm was decreased which depicted the oxidation (decolorization/degradation) of the azo-dye by electrogenerated Fenton's reagent. Approximately 100% color removal of

148

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Fig. 6. UV–visible absorption spectra of 0.1 mM Ponceau S azo-dye aqueous solution (diluted 2-folds) at different time intervals t: 0, 10, 20, 30 and 45 min (from up to down) of electro-Fenton oxidation under the optimized operational conditions.

0.05, 0.1 and 0.3 mM Ponceau S azo-dye solutions has reached by electro-Fenton oxidation for 30, 45 and 90 min, respectively (e.g. Table 1). Variation of concentration (C) of Ponceau S azo-dye in aqueous solutions (corresponding to the absorbance at λmax = 515 nm) with time t of the electro-Fenton oxidation of different initial azo-dye concentrations was exponential indicating a first-order kinetics behavior, (Fig. 7). The general elementary rate law for reaction of a target organic compound can be expressed as: −dCt =dt ¼ kOH •COH •Ct þ ∑i koxi Coxi Ct

ð18Þ

where Ct is concentration of the azo-dye during electro-Fenton oxidation for time interval t and oxi represents oxidants other than OH• that may be present. Hydroxyl radical is usually regarded as the sole or most important reactive species. Then: −dCt =dt ¼ kOH •COH •Ct

ð19Þ

Considering that the concentration of reactive species must reach quickly a stationary-state regimen during the oxidation process, and provided that COH• can be considered constant, the rate law for the oxidation process can be treated as being pseudo-first-order. In terms of consumption of the target organic compound, Eq. (19) can be written in the form: −dCt =dt ¼ kapp Ct

Fig. 7. Decay of various initial concentrations of Ponceau S azo-dye with time (t) of electro-Fenton oxidation under the optimized operational conditions.

where C0 is the initial concentration of the investigated azo-dye. Linear-fit relationships between ln (C0/Ct) and time t were obtained for the three investigated azo-dye concentrations which also indicated that the oxidation reaction of Ponceau S azo-dye followed pseudo-first-order kinetics (Fig. 8). The apparent first-order constants (kapp) were found to be 0.1860, 0.1541 and 0.0604 min− 1, for 0.05, 0.1 and 0.3 mM Ponceau S azo-dye, respectively. The decrease in value of apparent first-order constant with increasing the concentration of the azo-dye may due to greater extent of reactions between OH• and oxidation by-products compared to that involving Ponceau S azo-dye, thus leading to slow down of decay kinetics of the dye. It is known that reaction intermediates can form during the oxidation of azo-dyes and some of them could be long-lived and even more toxic than their parent compounds. Therefore, it is necessary to evaluate the degradation extent during and at the end of electro-Fenton oxidation reaction. Mineralization of the treated Ponceau S azo-dye in the investigated solutions during electro-Fenton oxidation was monitored by chemical oxygen

ð20Þ

By integration, the following equation was obtained: lnðC0 =Ct Þ ¼ kapp t

ð21Þ

Table 1 Results of oxidation of 0.1 mM Ponceau S azo-dye in aqueous solution by electrogenerated Fenton's reagent for different time intervals under the optimized operational conditions. Time (min)

Absorbance (515 nm)

% Color removal

COD mg O2/L

% COD removal

% Ar ring removal

0 10 20 30 45 60

3.32 0.45 0.17 0.07 0.01 –

0 86.5 94.9 98.1 99.9 100

173.4 117.8 74.0 35.0 13.3 4.3

0 32.1 57.3 79.8 92.3 97.5

0 58.75 73.34 88.0 95.0 99.0

Fig. 8. Plots of ln (Co/Ct) with time (t) of electro-Fenton oxidation of solutions of various initial concentrations of Ponceau S azo-dye under the optimized operational conditions.

H.S. El-Desoky et al. / Desalination 264 (2010) 143–150

demand (COD) measurements. The chemical oxygen demand removed ratios (% COD removal) for the solutions withdrawn from the reactor at reaction times 0 and t of electro-Fenton oxidation were estimated. The results show gradual decrease in COD with time of electro-Fenton oxidation indicating the mineralization of organic matter (dyes molecules) present in aqueous solution (e.g. Table 1). These results point out also a fast COD decay, particularly at the beginning of the oxidation reaction, and consequently a high oxidative ability of electro-Fenton process. The results showed that approximately 98% destruction of Ponceau S azo-dye in the investigated solutions (initial concentrations were 0.05, 0.1 and 0.3 mM) was achieved by electro-Fenton oxidation for 40, 60 and 120 min, respectively. Also, progress of oxidation of Ponceau S azo-dye in the investigated solutions by electro-generated Fenton's reagent was monitored by recording its HPLC chromatograms before and during the electro-Fenton oxidation. HPLC chromatograms of Ponceau S azo-dye in the investigated solution before its electroFenton oxidation showed a single well-defined peak (e.g. Fig. 9, curve a) of a retention time tr = 3 min. While chromatogram of a treated solution by electro-Fenton oxidation for 10 min (curve b) showed a diminished peak (t r = 3 min) in addition to the appearance of two small new overlapped peaks at retention times tr = 3.25 and 3.60 min. The latter two small peaks may be related to the degradation products. Also, another three overlapped peaks were developed at retention times tr of 4.7 to 5 min due to electro-Fenton oxidation of Ponceau S azo-dye solution for 20 min (curve c), which may be related to some other degradation products. The HPLC chromatogram of the treated solution by electro-Fenton oxidation for 60 min (e.g. Fig. 9, curve d), showed no peaks corresponding either to the Ponceau S azo-dye itself or to any of its degradation products indicating that almost no more aromatic compounds are remaining in the solution. The aromatic ring removal of about 99% (initial concentrations were 0.05, 0.1 and 0.3 mM) was achieved by electro-Fenton oxidation for 40, 60 and 120 min, respectively. Plots of % color removal or % Aromatic (Ar) ring removal vs. % COD removal (Fig. 10) of 0.1 mM Ponceau S azo-dye solution revealed that approximately 87% color removal and 60% Ar ring removal were achieved which corresponding to approximately 32% COD removal by electro-Fenton oxidation for 10 min under the optimal operational conditions. However, approximately 100% color removal and

149

Fig. 10. Plots of % Color removal (a) and % Ar ring removal (b) of 0.1 mM Ponceau S azodye aqueous solution vs. % COD removal by electro-Fenton oxidation for 0 to 60 min under the optimized operational conditions.

99% Aromatic ring removal of the azo-dye solution was achieved which corresponding to approximately 98% COD removal by electroFenton oxidation for 60 min. This behavior indicated the effective destruction of the chromophore and breaking down of the aromatic rings with electro-generated Fenton's reagent, leading to mineralization of the dye molecules. However, Fig. 10 indicated also a faster color removal (curve a) than the aromatic ring removal (curve b) by the electro-Fenton oxidation for 10 min. This is may be due to that the OH• radicals attack first the –N = N– double bonds, which are of lowest energy absorption band assigned to the n → π* transition [35], leading to opening of the –N = N– double bonds, then destructing the long conjugated π systems, and the formation of aliphatic fragments intermediate products (e.g. carboxylic acids formed by oxidative ring opening reactions) which are more difficult to be mineralized. On the other side, the rate of decolorization/mineralization is relatively rapid at the early stages of electro-Fenton oxidation for ≤ 10 min. However slower rate of decolorization / mineralization was observed at longer oxidation time (Fig. 10 and Table 1) because carboxylic acids formed by oxidative ring opening reactions are less reactive toward hydroxyl radicals compared to the aromatics [51,52]. When oxidizeable impurities are present in the treated real azo-dye containing-wastewater, they may cause slow down, to some extent, of the rate of decolorization/mineralization of the examined azo-dye by electro-Fenton oxidation compared to that in the absence of these impurities. The results of this work demonstrate the efficiency of the electro-generated Fenton's reagent for treatments of waters polluted by persistent organic compounds. 4. Conclusion

Fig. 9. HPLC chromatograms of 0.1 mM Ponceau S azo-dye aqueous solution before oxidation (a) and during electro-Fenton oxidation for different time intervals: (b) 10 min, (c) 20 min and (d) 60 min under the optimized operational conditions.

Oxidation (decolorization/degradation) of Ponceau S azo-dye in aqueous solutions by electro-generated Fenton's reagent has been optimized using an undivided electrochemical cell with a reticulated vitreous carbon (RVC) cathode and a platinum gauze anode. The obtained results of % COD removal and the disappearance of the UV– visible absorption spectral bands and those of high performance liquid chromatograms of various concentrations of treated Ponceau S azodye aqueous solutions clearly indicated the complete decolorization and significant mineralization of the azo-dye by electro-generated Fenton's reagent. The results demonstrate the efficiency of the optimized electro-Fenton's reaction for treatment of waters polluted by persistent organic compounds

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