Desalination 279 (2011) 183–189

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Oxidation of dyes from colored wastewater using activatedcarbon/hydrogen peroxide

Shooka Khorramfar a, Niyaz Mohammad Mahmoodi b,⁎, Mokhtar Arami a, Hajir Bahrami a

a Textile Engineering Department, Amirkabir University of Technology, Tehran, Iranb Department of Environmental Research, Institute for Color Science and Technology, Tehran, Iran

⁎ Corresponding author. Tel.: +98 21 22969771; fax:E-mail addresses: nm_mahmoodi@aut.ac.ir, nm_mah

(N.M. Mahmoodi).

0011-9164/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.desal.2011.06.005

a b s t r a c t

a r t i c l e i n f o

Article history:Received 8 March 2011Received in revised form 31 May 2011Accepted 1 June 2011Available online 29 June 2011

Keywords:Dye oxidationAC/H2O2

Colored wastewaterKineticAliphatic intermediate

In this paper, the oxidation of dyes from colored textile wastewater by activated carbon (AC)/hydrogen peroxide(H2O2) was discussed. Acid Red 18 (AR18), Direct Red 80 (DR80) and Reactive Red 194 (RR194) were used asmodel dyes. The surface characteristics of AC were investigated using Fourier transform infrared (FTIR). Dyeoxidation by AC/H2O2was studied by UV–Vis spectrophotometer and Ion chromatography (IC). The effects of ACdosage, initial dye concentration, pHand salt ondyeoxidationwere evaluated. Kinetics analysis indicated that thedye oxidation rates could be approximated at pseudo-second order model. Formate anion was detected asdominant aliphatic intermediate. Results indicated that the AC/H2O2 could be used as an eco-friendly material todegrade dyes from colored wastewater.

+98 21 22947537.moodi@yahoo.com

l rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

The presence of color in textile effluent is a challenging problem.Dyeing process produces large volumes of colored wastewater [1–5].Synthetic dyes are difficult to degrade due to their complex aromaticstructure and synthetic origin. Some of them are known to be toxic orcarcinogenic [1,6]. Hence, degradation of dyes is necessary from apoint of view of public health and safety.

Severalmethods such as adsorption [7], nanofiltration [8], ozonation[9], electrochemical [10], etc. were used to remove dyes fromwastewater.

Advanced oxidation processes (AOPs) using strong oxidants such ashydroxyl radicals (HO•) are an emerging technique to degradepollutants in aqueous media. Hydroxyl radicals can be produced fromH2O2 using homogeneous (e.g. Fenton reaction) or heterogeneouscatalysts (supported metal catalysts, metal oxides, graphite andactivated carbon (AC)) [11].

AC is widely used as an adsorbent of pollutants due to its porousnature and large specific surface area [11–19]. Although AC adsorptionmethod is effective to the removal of organic compounds, the AC can getsaturated easily in the process,which requires regeneration or completereplacement. Combination of both adsorption and heterogeneouscatalysis into a single process could offer an attractive alternative inthe wastewater treatment. Due to the complex role of AC, the catalyticdecomposition of H2O2 and contaminants with AC deserves further

investigation [11]. The catalytic decomposition of H2O2 by AC involvesthe exchange of a surface hydroxyl grouponAC surfacewith a hydrogenperoxide anion (HO2

−) to generate peroxide on the surface. The ACactive site is regenerated by decomposition of H2O2molecule to oxygen[11,15,19–21]. Other than this direct decomposition reaction, it iscommonly accepted in the literature that H2O2 can be activated on theAC surface to generate hydroxyl free radicals [11,15,19]. The AC can beregarded to function as an electron-transfer catalyst just like theHaber–Weiss mechanism involving the reduced (AC) and oxidized (AC+)catalyst states (Reactions (1) and (2)) [11]:

AC + H2O2→ACþ + HO− + HO• ð1Þ

ACþ + H2O2→AC + HO•2 + Hþ ð2Þ

With a high oxidation potential (Eo) of 2.80 V, the HO• is a highlyreactive species that can nonselectively degrade and rapidly mineralizea variety of refractory compounds into smaller andmore biodegradableones such as CO2 and H2O [11,22].

Due to theHO•, an integratedH2O2 oxidation and AC adsorptionmaysynergistically enhance the catalytic degradation rate of pollutants, thusimproving their biodegradability. Therefore, the application of AOPusing heterogeneous catalysts could be one of the most promisingalternatives for wastewater treatment.

Several publications dealwith the oxidation of pollutants in aqueoussolutions by AC/H2O2 [5,6,11,13–16,19,21–25]. A literature reviewshowed that the operational parameters and kinetic studies of AC/H2O2 on dye oxidation were not studied in details. In this paper, theoxidation of dyes from colored textile wastewater by activated carbon/

NHO3S N

HO3S

SO3H

OH

a

N N N N

SO3Na

OHSO

3Na

NaO3S

N N N N

NHNaO3S

OHNaO

3S

CO

NH

SO3Na

b

N

SO3H

N

HO3S SO3H

OH NH

N

N

N

Cl

NH

SO2CH2CH2OSO3H

c

Fig. 1. The chemical structure of dyes (a) AR18, (b) DR80 and (c) RR194.

184 S. Khorramfar et al. / Desalination 279 (2011) 183–189

hydrogen peroxide was investigated in details. Acid Red 18 (AR18),Direct Red80 (DR80) andReactiveRed194 (RR194)wereused asmodeldyes. The surface characteristics of AC were investigated by Fouriertransform infrared (FTIR). Dye oxidation by AC/H2O2 was studied usingUV–Vis spectrophotometer and Ion chromatography (IC). The effects ofAC dosage, initial dye concentration, pH and salt on dye oxidation wereinvestigated. In addition kinetics analysis of dye oxidation was studied.

2. Experimental

2.1. Chemicals

Acid Red 18 (AR18), Direct Red 80 (DR80) and Reactive Red 194(RR194) were used as model dyes. The dyes were obtained from CibaLtd and used without further purification. The chemical structure ofdyes was shown in Fig. 1. All other chemicals were of analytical gradeand purchased from Merck.

Fig. 2. FTIR spectrum of AC.

2.2. Surface characterization of AC

Fourier transform infrared (FTIR) spectroscopy (Perkin-ElmerSpectrophotometer Spectrum One) in the range of 4000–450 cm−1

was studied.

2.3. Dye oxidation

The dye oxidation experiments were conducted by mixing variousamounts of AC (0–0.35 g/L) in jars containing 400 mL of a dye solution(50 mg/L) and H2O2 (0.41 mM) at various pH (2.5–10) and agitationspeed of 200 rpm and 25 °C. The solution pH was adjusted by adding asmall amount of H2SO4 or NaOH. The sampleswerewithdrawn from thesolution at regular time intervals and the reaction was blocked byaddition of methanol. The suspensions were centrifuged by HettichEBA20. The change on the absorbance at maximumwavelength (λmax)of dyes (509 nm for AR18, 543 nm for DR80 and 513 nm for RR194)wasmonitored by UV–Vis spectrophotometer (Perkin-Elmer Lambda 25).

Ion chromatograph (METROHM 761 Compact IC) was used toassay the appearance of carboxylic acids formed during the oxidationof dyes using a METROSEP anion dual 2, flow 0.8 ml/min, 2 mMNaHCO3/1.3 mM Na2CO3 as eluent, temperature 20 °C, pressure3.4 MPa and conductivity detector.

To investigate the effect of AC dosage on dye oxidation, differentamounts of AC (0, 0.05, 0.15, 0.25 and 0.35 g/L) were applied to thereactor containing 400 mL dye solution (50 mg/L) and H2O2

(0.41 mM) using jar test at room temperature (25 °C) and pH 2.5for 60 min.

The effect of initial dye concentration on the percentage of dyeoxidation was studied by adding optimum amount of AC (0.1 g for AR18, 0.06 g for DR80 and 0.06 g for RR194) to 400 mL of different dyeconcentrations (25, 50, 150, 250 and 350 mg/L) and H2O2 (0.41 mM)at pH 2.5.

The effect of different pH values (2.5, 5–5.5, 7 and 10) on dyeoxidation was investigated by contacting 400 mL of dye solution(50 mg/L), H2O2 (0.41 mM) and optimum amount of AC for each dye.

Fig. 3. The effect of AC dosage on oxidation of dyes using AC/H2O2 (a) AR18, (b) DR80and (c) RR194.

Fig. 4. The effect of dye concentration on oxidation of dyes using AC/H2O2 (a) AR18,(b) DR80 and (c) RR194.

185S. Khorramfar et al. / Desalination 279 (2011) 183–189

For investigating the effect of salt on the percentage of dyeoxidation, 0.02 M of different salts (Na2SO4, NaCl, Na2CO3 andNaHCO3) was added to 400 mL of dye solution (50 mg/L) and H2O2

(0.41 mM) with optimum amount of AC at pH 2.5.For comparative purposes, experiments of adsorption on activated

carbon and oxidation with H2O2 in the absence of activated carbonwere also carried out in the batch system described above, under thesame conditions.

Table 1The kinetic constants of dye oxidation using AC/H2O2 at different AC dosages (solution: 40k2 (L/mg.min)).

AC (g/L) AR18 DR80

k1 R2 k2 R2 k1 R2

0 0.0001 0.9600 0.0001 0.9620 0.0018 0.9920.05 0.0125 0.9870 0.0146 0.9950 0.0225 0.9900.15 0.0405 0.9879 0.0597 0.9969 0.0702 0.9580.25 0.0634 0.9718 0.1137 0.9923 0.0882 0.9030.35 0.0880 0.9466 0.1962 0.9913 0.1064 0.902

3. Results and discussion

3.1. Surface characteristics

The FTIR spectrum of AC (Fig. 2) shows that the peak positions are at3409, 2923, 2845, 1600, 1171, 1094 and 1042 cm−1. The band at3409 cm−1 is due to O–H and N–H stretching. The bands at 2923 and2845 cm−1 correspond to asymmetric and symmetric aliphatic C–H

0 mL, dye: 50 mg/L, H2O2: 0.41 mM, T: 25 °C and pH: 2.5 for 60 min) (k1 (1/min) and

RR194

k2 R2 k1 R2 k2 R2

4 0.0015 0.9920 0.0002 0.9941 0.0002 0.99437 0.0224 0.9989 0.0194 0.9800 0.0233 0.99566 0.1053 0.9911 0.0595 0.9431 0.0997 0.99777 0.1531 0.9984 0.0831 0.9016 0.1700 0.99510 0.2199 0.9927 0.0982 0.9010 0.2317 0.9984

Table 2The kinetic constants of dye oxidation using AC/H2O2 at different dye concentrations (solution: 400 mL, AC: 0.1 g for AR18, 0.06 g for DR80 and 0.06 g for RR194, H2O2: 0.41 mM, T:25 °C and pH: 2.5 for 60 min) (k1 (1/min) and k2 (L/mg.min)).

Dye (mg/L) AR18 DR80 RR194

k1 R2 k2 R2 k1 R2 k2 R2 k1 R2 k2 R2

25 0.1025 0.9116 0.5187 0.9950 0.0863 0.9115 0.4372 0.9981 0.0722 0.9209 0.2994 0.999450 0.0634 0.9718 0.1137 0.9923 0.0702 0.9586 0.1053 0.9911 0.0595 0.9431 0.0997 0.9977150 0.0394 0.9493 0.0195 0.9917 0.0427 0.9455 0.0189 0.9925 0.0274 0.9691 0.0128 0.9938250 0.0211 0.9687 0.0055 0.9906 0.0208 0.9725 0.0054 0.9920 0.0184 0.9760 0.0047 0.9924350 0.0102 0.9851 0.0018 0.9924 0.0110 0.9968 0.0019 0.9979 0.0087 0.9939 0.0016 0.9974

Fig. 5. The effect of pH on oxidation of dyes using AC/H2O2 (a) AR18, (b) DR 80 and(c) RR194.

186 S. Khorramfar et al. / Desalination 279 (2011) 183–189

stretchings, respectively. While the band at 1600 cm−1 reflects C_Cstretching of aromatic rings whose intensity is enhanced by thepresence of oxygen atoms as phenol or ether groups. Bands at 1300–1000 cm−1 correspond to C–O stretching [26,27].

3.2. Effects of operational parameters on dye oxidation

The effects of operational parameters such as activated carbondosage, initial dye concentration, pH and salt (Na2SO4, NaCl, Na2CO3

and NaHCO3) were investigated on dye oxidation.

3.2.1. Effect of activated carbon dosageFig. 3 shows the effect of AC dosage on dye oxidation. The oxidation

with H2O2 in the absence of activated carbon is not significant (Fig. 3).Adsorption of dyes on activated carbon is a process that also presents alow performance for dye removal from solutions (3% for AR18, 5% forDR80 and 4% for RR194 after 60 min of adsorption process), because ofits high molecular size and relatively low affinity to the surface ofactivated carbon.

Considering that the hydrogen peroxide alone is not effective tooxidize dyes in the colored wastewater samples, it needs to beactivated by AC, which played predominant roles in its integratedtreatment. It is observed that the simultaneous use of AC and H2O2 to acertain extent yielded a significant improvement of dye oxidationcompared to that of the H2O2 oxidation alone (Fig. 3). The synergisticeffects of AC and H2O2 were observed when the optimum AC dosagewas applied. AC catalyzes the decomposition of hydrogen peroxideinto free radicals, such as hydroxyl radicals, which are very active inoxidation reactions in the aqueous phase [11]. The electron transferfrom the surface of activated carbon seems to be involved in thecorresponding mechanism, accordingly to a pathway similar to theFenton reaction [5,6,11,13–16,19,21–25].

The first and second order kinetics of dye oxidation by AC/H2O2 atdifferent AC dosages were studied (Table 1). It was possible to fit thecurves using first or second-order models, respectively according tothe well-known expressions:

C = C0 = exp −k1tð Þ ð3Þ

1= C–1= C0 = k2t ð4Þ

where C0 (mg/L) and C (mg/L) are the initial dye concentration anddye concentration at time t, respectively. The k1 (1/min) and k2(L/mg.min) are the first and second order rate constants, respectively.

The oxidation rate constants (k1 and k2) and correlation coefficient(R2) of dye for the various activated carbon dosages were shown inTable 1.

The results showed that the kinetics of dye oxidation using AC/H2O2

at different AC dosages followed secondorder kineticmodel. Thismeansthat dye oxidation depends on both hydrogen peroxide and AC.

3.2.2. Effect of initial dye concentrationTo study the effect of dye concentration on the rate of dye oxidation,

the dye concentration was varied 25 to 350 mg/L while the othervariables were kept constant (Fig. 4).

With the increase in the dye concentration, the possible cause isthe interference from intermediates formed upon oxidation of theparental dyemolecules. Such suppression would bemore pronouncedin the presence of an elevated level of oxidation intermediates formedupon an increased initial dye concentration [28].

The first and second order kinetics of dye oxidation by AC/H2O2 atdifferent dye concentrations were studied. The oxidation rateconstants and correlation coefficient of dye for the various dyeconcentrations were shown in Table 2. The results showed that thekinetics of dye oxidation using AC/H2O2 at different dye concentra-tions followed second order kinetic model.

Table 3The kinetic constants of dye oxidation using AC/H2O2 at different pH values (solution: 400 mL, Dye: 50 mg/L, AC: 0.1 g for AR18, 0.06 g for DR80 and 0.06 g for RR194, H2O2: 0.41 mM,T: 25 °C for 60 min) (k1 (1/min) and k2 (L/mg.min)).

pH AR18 DR80 RR194

k1 R2 k2 R2 k1 R2 k2 R2 k1 R2 k2 R2

2.5 0.0634 0.9718 0.1137 0.9923 0.0702 0.9586 0.1053 0.9911 0.0595 0.9431 0.0997 0.9977Natural 0.0500 0.9896 0.0803 0.9911 0.0603 0.9334 0.0867 0.9917 0.0542 0.9722 0.0874 0.99677 0.0361 0.9724 0.0510 0.9925 0.0483 0.9446 0.0617 0.9911 0.0098 0.9860 0.0109 0.993210 0.0011 0.9916 0.0012 0.9919 0.0058 0.9947 0.0051 0.9961 0.0007 0.9913 0.0007 0.9916

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3.2.3. Effect of solution pHIn this paper, comparative experiments were performed at

different pH values (2.5, natural pH (5.5 for AR18, 5 for DR80 and 5for RR194), 7 and 10). The pH of solution is an important parameter inreaction taking place on material surface, since it influences thesurface charge properties of the catalyst. The effect of pH on theoxidation of dyes using AC/H2O2 is shown in Fig. 5.

The results showed that the dye oxidation using AC/H2O2 increasedby decreasing pH. It can be attributed to the superposition of severaleffects such as the increase in the rate of the H2O2 decomposition (viaOOH−) with increasing pH, the increase in the redox potential of H2O2

(and the resulting reactive oxygen species) with decreasing pH as well

Fig. 6. The effect of salt on oxidation of dyes using AC/H2O2 (a) AR18, (b) DR80 and (c)RR194.

as possible changes in the chemical properties of the AC surface[5,13–15].

The kinetics of dye oxidation by AC/H2O2 at different pH valueswas studied. The oxidation rate constants and correlation coefficientof dye for the various pH values were shown in Table 3. The resultsshowed that the kinetics of dye oxidation using AC/H2O2 at differentpH values followed second order kinetic model.

3.2.4. Effect of saltThe occurrence of dissolved inorganic ions is rather common in

dye-containing industrial wastewater. These substances may competefor the active sites on the AC surface or deactivate the AC and,subsequently, decrease the oxidation rate of the target dyes [28]. Amajor drawback resulting from thehigh reactivity andnon-selectivity ofHO• is that it also reacts with non-target compounds present in thebackground water matrix, i.e. dye auxiliaries present in the dye bath. Itresults higher HO• demand to accomplish the desired degree ofoxidation, or complete inhibition of advanced oxidation rate andefficiency [29].

Different salts (Na2SO4, NaCl, Na2CO3 and NaHCO3) were used toconsider the effect of dissolved inorganic anions on the oxidation rateof dyes using AC/H2O2. The same amount (0.02 M) of these salts wasused. Fig. 6 shows the effect of salts on the oxidation rate of dyes usingAC/H2O2. Inorganic anions inhibit H2O2 oxidation through HO•

radicals scavenging. In addition, the carbonate and bicarbonate ionscould prevent the catalytic oxidation by increasing the pH (Re-actions (5) and (6)). For generation of a sufficient levels of HO• foroxidation of dyes in the presence of high levels of bicarbonate, ahigher oxidant dose should be used [28–30].

HO• + CO2−3 →OH− + CO•−

3 ð5Þ

HO• + HCO−3 →OH− + HCO•

3 ð6Þ

The kinetics of dye oxidation by AC/H2O2 at different salts wasstudied. The oxidation rate constants and correlation coefficient of dyefor the various salts were shown in Table 4. The results showed thatthe kinetics of dye oxidation using AC/H2O2 at different salts followedsecond order kinetic model.

3.3. UV–Vis spectrum studies

Changes of the dye absorption at 250 nm≤λ≤650 nm during theoxidation at different time intervals were shown in Fig. 7. Withoxidation time elapse of 60 min, the maximum absorbance in visibleregion of UV–Vis spectra decreased, which indicates that chromo-phore in dyes are the most active sites for oxidation attack.

3.4. Carboxylic acid intermediates of dye oxidation

During the oxidation of dyes, various organic intermediates wereproduced. Consequently, destruction of the dye should be evaluatedas an overall oxidation process, involving the oxidation of both theparent dye and its intermediates.

Table 4The kinetic constants of dye oxidation using AC/H2O2 at different salts (solution: 400 mL, Dye: 50 mg/L, salt: 0.02 M, AC: 0.1 g for AR18, 0.06 g for DR80 and 0.06 g for RR194, H2O2:0.41 mM, T: 25 °C, pH:2.5 for 60 min) (k1 (1/min) and k2 (L/mg.min)).

Salt AR18 DR80 RR194

k1 R2 k2 R2 k1 R2 k2 R2 k1 R2 k2 R2

Blank 0.0634 0.9718 0.1137 0.9923 0.0702 0.9586 0.1053 0.9911 0.0595 0.9431 0.0997 0.9977NaCl 0.0337 0.9621 0.0478 0.9935 0.0043 0.9972 0.0038 0.9977 0.0175 0.9824 0.0210 0.9939Na2CO3 0.0036 0.9936 0.0040 0.9950 0.0006 0.9908 0.0005 0.9912 0.0003 0.9974 0.0004 0.9975NaHCO3 0.0028 0.9937 0.0031 0.9941 0.0005 0.9974 0.0005 0.9976 0.0004 0.9936 0.0004 0.9938Na2SO4 0.0593 0.9680 0.1055 0.9929 0.0509 0.9651 0.0668 0.9926 0.0535 0.9578 0.0862 0.9947

188 S. Khorramfar et al. / Desalination 279 (2011) 183–189

Further hydroxylation of aromatic intermediates leads to thecleavage of the aromatic ring resulting in the formation of oxygen-containing aliphatic compounds [28]. Formate was detected asimportant aliphatic carboxylic acid intermediate during the oxidationof dyes. Carboxylic acids can react directly with reactive spices such ashydroxyl radicals generating CO2.

4. Conclusions

In this paper, the oxidation of dyes from colored textile wastewaterby activated carbon/hydrogen peroxide was investigated in details. Theoxidation with H2O2 in the absence of activated carbon and theadsorption of dyes on activated carbon present a low performance fordye removal from solutions. The synergistic effects of AC andH2O2were

Fig. 7. The changes of UV–Vis spectra of dyes during the dye oxidation process using AC/H2O2 (solution: 400 mL, Dye: 50 mg/L, AC: 0.1 g for AR18, 0.06 g for DR80 and 0.06 g forRR194, H2O2: 0.41 mM, T: 25 °C, pH:2.5 for 60 min) (a) AR18, (b) DR80 and (c) RR194.

observed because of the catalytic performance of AC for decompositionof hydrogen peroxide into hydroxyl radicals. The results showed thatthe kinetics of dye oxidation at different operational parametersfollowed second order kinetic model. This means that dye oxidationdependsonbothhydrogenperoxideandAC. It canbe concluded that theAC/H2O2 could be used as an eco-friendly material to remove dyes fromcolored wastewater.

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