decolorization and mineralization of an azo reactive dye using loaded nano-photocatalysts on spacer...

Journal of the Taiwan Institute of Chemical Engineers xxx (2014) xxx–xxx

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JTICE-898; No. of Pages 11

Decolorization and mineralization of an azo reactive dye usingloaded nano-photocatalysts on spacer fabric: Kinetic study andoperational factors

Seyed Majid Ghoreishian a, Khashayar Badii b,*, Mohammad Norouzi c, Abosaeed Rashidi d,Majid Montazer e, Mahsa Sadeghi f, Maedeh Vafaee g

a Young Researchers and Elites Club, South Tehran Branch, Islamic Azad University, Tehran, Iranb Department of Environmental Researches, Institute for Color Science and Technology (ICST), Tehran, Iranc Young Researchers and Elites Club, Science and Research Branch, Islamic Azad University, Tehran, Irand Department of Textile Engineering, Science and Research Branch, Islamic Azad University, Tehran, Irane Department of Textile Engineering, Amirkabir University of Technology, Tehran, Iranf Department of Textile Engineering, South Tehran Branch, Islamic Azad University, Tehran, Irang Department of Chemical Engineering, Semnan University, Semnan, Iran

A R T I C L E I N F O

Article history:

Received 21 January 2014

Received in revised form 9 April 2014

Accepted 12 April 2014

Available online xxx

Keywords:

Decolorization

Textile wastewater

Azo reactive dye

3-Dimensional fabric

Kinetics study

A B S T R A C T

In this study, the photocatalytic decolorization and mineralization of Remazol Black B (RBB), an azo

reactive dye, in aqueous solutions was investigated using UV/H2O2/ZnO, UV/H2O2/TiO2 and UV/H2O2/

ZnO:TiO2 systems. ZnO and TiO2 nanoparticles were loaded on 3-dimensional polyethylene

terephthalate fabrics (spacer fabrics). Morphology of the spacer fabrics and the presence of the

nanoparticles were studied by scanning electron microscopy (SEM) and energy dispersive X-ray

spectroscopy (EDS), respectively. Furthermore, the effects of key operational parameters on the

efficiency of the decolorization were investigated. These parameters included initial pH value, initial

hydrogen peroxide concentration, initial dye concentration, the loaded nanoparticle ratio and the

presence of anions (sulfate, chloride and bicarbonate). Zero-, first- and second-order reaction kinetics

were evaluated. Complete decolorization and high efficient mineralization with 90% total organic carbon

(TOC) reduction were achieved at 120 min treatment in the case of ZnO:TiO2 under optimum condition.

The results proved that the novel heterogeneous photocatalytic process is capable of decolorizing and

mineralizing azo reactive dyes in textile wastewater.

� 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

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Journal of the Taiwan Institute of Chemical Engineers

jou r nal h o mep age: w ww.els evier . co m/lo c ate / j t i c e

1. Introduction

The discharge of textile wastewater containing various organicand mineral pollutants such as dyes and pigments into naturalstreams and rivers causes serious environmental problems. In fact,most of the dyes owing to their conjugated aromatic compoundsand complicated chemical structure are toxic to the aquatic livingorganisms [1–5].

Among the broad range of the dyes used in textile industry,reactive dyes due to their favorable characteristics such as lowenergy consumption in the dyeing process, color brightness andwater fastness, have found extensive application and they

* Corresponding author. Tel.: +98 2122969777/+98 2122543290.

E-mail address: [email protected] (K. Badii).

Please cite this article in press as: Ghoreishian SM, et al. Decolorizatiphotocatalysts on spacer fabric: Kinetic study and operational factj.jtice.2014.04.015

http://dx.doi.org/10.1016/j.jtice.2014.04.015

1876-1070/� 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V.

constitute almost 50% of the annual worldwide production ofthe dyes [6].

Some reactive dyes, such as RBB which are composed of one ormore azo bonds (–N55N–) and aromatic rings are categorized asxenobiotic compounds. Additionally, because the fixation efficien-cy of azo dyes is estimated to fall in the range of 60% to 90%, theirloss in the dyeing process causes environmental concerns [7–9].

The conventional methods for wastewater decolorization suchas chemical, physical or biological methods only transform dyesfrom aqueous to another phase, which consequently results insecondary pollution [10–17]. Therefore, post-treatments of thesolid wastes, which are costly operations, are required [18,19].

Currently, advanced oxidation processes (AOPs) have beendeveloped as alternative methods for decolorization of dyes andother contaminated wastewaters. The main purpose of AOPs is totransfer all organic compounds to water, carbon dioxide and

on and mineralization of an azo reactive dye using loaded nano-ors. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/

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NH2OH

SO3NaNaO3S

N=NN=N S

O

O

CH2CH2OSO3NaS

O

O

NaO3SOH2CH2C

Fig. 1. Chemical structure of RBB.

S.M. Ghoreishian et al. / Journal of the Taiwan Institute of Chemical Engineers xxx (2014) xxx–xxx2

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mineral acids [20]. Among different methods of AOPs, heteroge-neous photocatalytic processes which use metal oxide semicon-ductor (MOS) nanoparticles such as TiO2 and ZnO are considered asthe most effective approach in degrading and mineralizing organiccompounds in wastewater [21]. The fundamental of this techniqueis based on the irradiation of ultraviolet (UV) light on MOS toproduce reactive species such as hydroxyl radical (OH�) whichoxidize a broad range of organic pollutants non-selectively [22,23].

Although TiO2 is the most frequently used photocatalyst for awide range of organic compounds, it exhibits some disadvantages[24,25]. Among other semiconductors, ZnO represents an aptalternative candidate owing to the similarity of its photo-degradation mechanism to TiO2. Furthermore, it has beensubstantiated by some reports that ZnO is more efficient thanTiO2 under certain conditions whose reason might be attributed toits higher quantum efficiency [26]. Photocatalytic processesincorporating semiconductor nanoparticles involve two typicalapproaches including suspension of the nanoparticles in solutionand immobilization of the nanoparticles on surface. Amidst those,the immobilization method is preferred, because the catalystseparation and recycling from the treated wastewater prior todischarge are eliminated [27,28].

There has been some reports on the immobilization of nano-photocatalysts onto solid materials such as glass wool, glass beads,glass reactors, microporous cellulosic membranes, ceramic mem-branes, monoliths, zeolites, and stainless steel [29]. However, theimmobilization of nanoparticles on 3-dimensional polyethyleneterephthalate fabrics (spacer fabrics) for photocatalytic processeshas not been reported yet.

Spacer fabrics can be defined as complex 3-dimensionalconstructions composed of two separated layers of fabric whichare connected with spacer yarns for whose production weavingand knitting technologies are generally required. The 3-dimen-sional structures of these fabrics make them markedly differentfrom their conventional counterparts. Owing to the low possibilityof physical or chemical absorption of the reactive dyes onpolyethylene terephthalate (PET), spacer fabrics made of PETwere selected in the current study. Furthermore, it is noteworthyto mention that passing of wastewater through the two layers of

Fig. 2. Photograph of


the spacer requires more time which leads to provide sufficienttime for contact with the nanoparticles and consequently thephotocatalytic process [30–32].

The main purpose of this study is to investigate the photo-catalytic decolorization and mineralization of RBB using ZnO andTiO2 nanoparticles loaded on spacer fabric. The effects of theoperational parameters on efficiency of the decolorization wereevaluated to determine optimum condition. Moreover, the kineticsof the photocatalytic decolorization were investigated.

2. Experimental

2.1. Materials

RBB (C26H21N5Na4S6, MW = 991.82 g/mol) was obtained fromAlvan Sabet Co., Iran. The chemical structure of RBB is shown inFig. 1. Nano powder of TiO2 (Degussa P-25) (with an average size of21 nm and a combination of 80% anatase/20% rutile) and nanopowder of ZnO (with an average size of <50 nm, zincite) wereprovided from Evonik, Germany and Sigma-Aldrich, USA, respec-tively. Spacer fabric (a surface string of 150 den, a beneath string of150 den, a monofilament connector string of 30 den, a weight of255 g/m2 and a thickness of 9 mm) were supplied from BonyadFiber Production Co., Tehran, Iran (Fig. 2). Hydrochloric acid (37%w/w), sodium hydroxide, hydrogen peroxide (H2O2, 30% w/w),citric acid monohydrate (CA, C6H8O7�H2O) as a cross-linking agent,sodium chloride, sodium sulfate, sodium bicarbonate, polyethyl-ene glycol 400 (PEG 400) as an anticoagulant agent, were allsupplied from Merck, Germany. Sodium hypophosphite (SHP,NaH2PO2�H2O) as a catalyst was obtained from Sigma-Aldrich, USA.

2.2. Preparation of functionalized spacer fabrics

The loading of the nanoparticles on surface of the fabrics usingcarboxylic acids was carried out according to the methods reportedin the earlier researches [33,34]. Aqueous solutions containing50 wt% of the nanoparticles (with respect to the weight of thefabric, with the ratio of 100% ZnO, 100% TiO2 and ZnO:TiO2 with 50/50 wt% ratio), CA and SHP (10 wt% and 6 wt%, respectively) were

the spacer fabric.




Fig. 3. Scheme of the photocatalytic reactor. (1) air pump, (2) UV lamp, (3) dye solution (4) spacer fabric (5) thermometer and (6) fan.

S.M. Ghoreishian et al. / Journal of the Taiwan Institute of Chemical Engineers xxx (2014) xxx–xxx 3

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prepared in L:G (Liquid to Good Ratio)B40:1. PEG 400 was added tothe solutions to prevent agglomeration of the nanoparticles. Then,the prepared solutions were exposed to sonication (Laborette 17,Fritsch GmbH, Germany) for 15 min. Afterward, the spacers wereimpregnated in the solutions and the sonication process wasresumed for 15 min. Then, the spacer fabrics were passed througha pad mangle (with 90% wet pick-up), were dried at 100 8C for8 min and were cured at 140 8C for 4 min in air-circulating oven(Memmert, Germany).

2.3. Photoreactor

Decolorization process was performed in a closed photoreactorcomprising a rectangular tray of 72 cm � 54 cm � 45 cm made ofborosilicate glass and three glass vessels which operated with aninitial working volume of 200 mL (Fig. 3). The total volume of thereactor was 0.6 L. Two UV lamps (TUV 15 W/G15T8, Philips, TheNetherland) as artificial UV sources which emitted UV radiationwith a peak wavelength of 254 nm [35] were located at top of thephotoreactor with a distance of 10 cm away from the solutions. Theinternal body of the photoreactor was covered with an aluminumfoil to avoid radiation losses. An aquarium air pump (Hailea, China)with the capability of blowing air (6.5 L/min) was used for mixingand bubbling the dye solution. The internal temperature wasadjusted to 25 � 1 8C.

2.4. Procedure

In the first step, the optimum pH value and H2O2 concentrationfor the decolorization experiments were determined. To adjust thepH value of the each solution, NaOH and HCl solutions with aconcentration of 0.1 M were used. After determining the optimizedpH and H2O2 concentration, the effects of the dye concentration,the loaded nanoparticle ratio and the anion presence on efficiencyof the decolorization process were investigated separately whilethe other parameters were kept constant. For the decolorizationtreatment, after pH adjusting, the solutions were transferred intothe photoreactor, were kept in darkness and were then mixed byair bubbling to obtain a homogenous solution and increase theconcentration of oxygen molecules as an electrophilic substanceand oxidant. Subsequent to addition of H2O2, the lamps wereturned on and this point was recorded as the initiation time of thereaction. At certain intervals (0, 5, 10, 20, 30, 60 and 120 min), 5 mLof the solutions were retrieved and the concentration of dye wasdetermined by spectrophotometer (Lambada 25 Perkin-Elmer,Germany). To investigate the effect of the anions, according to theinstruction of the dye producer, 10 mg/L of NaCl, Na2SO4 andNaHCO3 were used. The degradation degree of RBB was calculatedby Eq. (1).

Degradationð%Þ ¼ C0 � Ct

C0� 100 (1)

where C0 is the initial dye concentration (mg/L) and Ct is theconcentration of dye at time t (mg/L).


3. Results and discussion

3.1. SEM images

Morphology of the samples was observed by scanning electronmicroscopy (SEM, LEO 1455VP, Cambridge, UK). Fig. 4(a and d)illustrates the SEM images of the untreated spacer fabric (a), thetreated spacer with ZnO, CA and SHP (b), the treated spacer withTiO2, CA and SHP (c) and the treated spacer fabric with ZnO:TiO2,CA and SHP (d). The porous structure of the spacer (Fig. 4a) led toan increase in the capacity of the dye adsorption from wastewater.Also, Fig. 4 confirms that the surface morphology of the spacerfabrics changed after the loading process and a layer ofnanoparticles widely covered the surface of the samples whichimplies the homogeneous loading of the nanoparticles.

3.2. EDS pattern

The presence of ZnO and TiO2 nanoparticles on the structure ofthe spacer fabrics was investigated by energy dispersive X-rayspectroscopy (EDS) (Oxford Instruments, UK). Fig. 5 shows EDSspectra of the nanoparticles (a and b), untreated fabric (c) andtreated fabrics (d–f). The obtained results also confirm thepresence of nano-ZnO, nano-TiO2 and ZnO:TiO2 on the surfaceof the spacer fabrics and other elements such as O and C were alsodetected while no signs of Zn and Ti peaks were seen in the rawspacer fabric (Fig. 5c).

3.3. Effect of initial pH value

Textile wastewaters have a broad range of pH and therefore, theeffect of pH on the efficiency of decolorization process is animportant parameter [36–40]. Fig. 6 shows the efficiency of thedecolorization in 120 min of the treatment as a function of pHvariation in the range of 2 to 12.

Several mechanisms have been suggested to explain the effectsof pH on the efficiency of photocatalytic decolorization of the dyes.However, the effect of pH of zero point charge (pHzpc) and itsrelation to the ionic form of the organic dye (anionic or cationic)seems to be more preferable. pHzpc is an effective factor indetermining the optimum range of pH in photocatalyticdecolorization processes. At pHzpc, surface of ZnO and TiO2

nanoparticles is uncharged (pHzpc = 9 for ZnO and pHzpc = 6.7 forTiO2) [18]; hence, the surface charge of the catalyst is changedat lower and higher values of pHzpc (Eqs. (2) and (3), M representsTi or Zn element).

MOH þ Hþ! MOHþ2 ðpH < pHzpcÞ (2)

MOH þ OH�! MO� þ H2O ðpH > pHzpcÞ (3)

At high pH value, the negative charge on the surface of thenanoparticles increases preventing the absorption of the dye.However, in acidic pH, there is an electrostatic force between the




Fig. 4. SEM images of the (a) untreated spacer fabric (35�), (b) treated spacer with ZnO, CA and SHP (1000�), (c) treated spacer with TiO2, CA and SHP (1000�) and (d) treated

spacer with ZnO:TiO2, CA and SHP (300�).


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negative charge of the anionic dye and the positive charge of thecatalyst which results in the adsorption of the dye on surface of thenanoparticles and beginning of the degradation process. [19].

Furthermore, at low pH values, photocorrosion of ZnO takesplace which causes a reduction in the efficiency of the dyedecolorization (Eqs. (4) and (5)). The formation of Zn2+ isattributed to the oxidation of ZnO by hþVB (Eq. (6)) [41–43].

ZnOðsÞ þ 2Hþ$ Zn2þ þ H2OðlÞ (4)

ZnOðsÞ þ Hþ$ ZnOHþ (5)

ZnO þ 2hþVB! Zn2þ þ O� (6)

RBB possess sulfuric groups in the structure, which arenegatively charged in alkaline conditions. Therefore, at high pHvalues, dye may not be adsorbed onto ZnO surface effectively (Eqs.(7) and (8)) [41].

ZnOðsÞ þ OH� þ H2OðlÞ $ ZnðOHÞ�3 (7)

ZnOðsÞ þ 2OH� þ H2OðlÞ $ ZnðOHÞ�4 (8)

It can be concluded from Fig. 6 that, although pH 2 is the mosteffective pH value for the decolorization in the case of ZnO:TiO2, pH6 was chosen as the optimum value because photocorrosion of ZnOoccurs at pH levels lower than 4.

Additionally, the effect of initial pH on lmax was alsoinvestigated. As shown in Fig. 7, by increasing the pH, lmax

shifted to higher levels of absorbance. This can be attributed to the


changes in RBB molecular structure in alkaline and acidicconditions (Fig. 8) [44,45].

3.4. Effect of initial H2O2 concentration

Adding oxidants such as H2O2 to the photocatalytic wastewatertreatment result in higher efficiency of the decolorization.Interpretation of the relationship between the decolorizationefficiency and H2O2 concentration can be explained on the basis of(1) prevention from recombination of e�CB=hþVB

� �pairs through an

increase in the number of surface electrons (Eqs. (9) and (10)); (2)acceleration of OH� formation, (3) increase in oxidation efficiencyof intermediates and (4) restitution of oxygen deficiency in thesolution (Eq. (11)) [46].

H2O2 þ 2hþVB! O2 þ 2Hþ (9)

H2O2 þ e�CB! OH� þ OH� (10)

H2O2 þ O��2 ! OH� þ OH� þ O2 (11)

However, in high concentrations, H2O2 can act as OH� scavengerwhich is unfavorable for the resumption of the reaction (Eqs. (12)–(14)). Therefore, it is important to determine the optimumconcentration of H2O2 [47].

H2O2 þ OH� ! HO�2 þ H2O (12)

HO�2 þ OH� ! H2O þ O2 (13)

OH� þ OH� ! H2O2 (14)




Fig. 5. EDS spectrum of (a) nano ZnO powder, (b) nano TiO2 powder, (c) untreated spacer fabric, (d) treated spacer with ZnO, CA and SHP, (e) treated spacer with TiO2, CA and

SHP and (f) treated spacer with ZnO:TiO2, CA and SHP.

Fig. 6. Effect of pH on the photocatalytic decolorization of RBB. [RBB]0 = 40 mg/L,

[H2O2]0 = 100 mg/L, ZnO:TiO2.


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In order to predict the required range of H2O2 concentration, thestoichiometric Eqs. (15) and (16) were used.

C26H21N5Na4O19S6 þ 76H2O2�!UV

MOS26CO2 þ 80H2O þ 4Naþ

þ5NO�3 þ 6SO2�4 þ 13Hþ

(15)

C26H21N5Na4O19S6 þ 39O2 þ 2H2O2�!UV

MOS26CO2 þ 10H2O þ 4Naþ

þ5NO�3 þ 6SO2�4 þ 5Hþ

(16)

According to these equations, if H2O2 is used as an oxidantagent, 76 mol of H2O2 are needed for complete degradation of1 mol of the dye (Eq. (15)). However, regarding Eq. (16), if anoxygen molecule is introduced into the solution, the requiredamount of H2O2 decreases to 2 mol for each mol of the dye.Therefore, molecular oxygen has a determinant role in thedecolorization process. Concerning Eq. (16), the efficiency ofthe process in various concentrations of H2O2 (50 to 200 mg/L) wasassessed for the spacer fabric treated by ZnO:TiO2. With the rise inH2O2 concentration from 50 to 100 mg/L, the decolorizationefficiency increased (Fig. 9). Notwithstanding, no significantdifference was observed between the decolorization resulted fromthe initial concentration of 100 and 150 mg/L of H2O2. Therefore,100 mg/L was selected as the optimum concentration of H2O2.


3.5. Effect of initial concentration of dye

The effect of initial dye concentration on the efficiency of thedecolorization is a key factor in investigating the kinetics of thereaction [48]. In order to evaluate the effects of dye concentration,the initial concentration of RBB was selected in the range of 40 to80 mg/L and the other parameters were kept constant (pH 6,




Fig. 7. UV–vis absorption spectra of RBB aqueous solution (40 mg/L) at different pH.

Fig. 10. Effect of the initial dye concentration on the photocatalytic decolorization.

pH 6, [H2O2]0 = 100 mg/L, (a) ZnO (b) TiO2 (c) ZnO:TiO2.

Fig. 8. Possible changes in RBB molecular structure.


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[H2O2] = 100 mg/L). Fig. 10(a–c) shows the percentage of thedecolorization in various concentrations of the dye using spacerfabrics treated with ZnO, TiO2 and ZnO:TiO2. It can be concludedfrom Fig. 10 that in all concentrations, the spacer fabric treatedwith ZnO:TiO2 nanoparticles demonstrated superior efficiency.According to Fig. 10 by increasing the dye concentration, thedecolorization efficiency reduced. This can be attributed to the factthat by increasing the dye concentration, a greater number ofmolecules were absorbed on active surface sites of the photo-catalyst for hydroxyl ion adsorption, which are responsible forhydroxyl radical formation, decreased.

Additionally, according to Beer–Lambert law, by increasing themolecules of the dye, penetration of photons into the solution

Fig. 9. Effect of the initial concentration of H2O2 on the photocatalytic

decolorization. pH 6, [RBB]0 = 40 mg/L, ZnO:TiO2.


decreases, which causes lower photonic adsorption on the nano-photocatalyst and consequently a reduction in the efficiency ofe�CB=hþVB

� �pair formation [49,50].

3.6. Effect of anions

Auxiliary chemicals such as mineral acids and salts are usuallyused for adjusting pH and improving color fastness. Therefore, thepresence of mineral anions in textile wastewater is common. Theeffect of anions on the decolorization of RBB in optimum conditionsand in the concentration of 10 mg/L of NaCl, Na2SO4 and NaHCO3

was investigated (Fig. 11). It can be concluded from Fig. 11 that thespacer fabrics treated with ZnO:TiO2 exhibited superior perfor-mance of the decolorization in the presence of sodium salts. Theobserved detrimental effect on the photocatalytic degradation ofRBB obeyed the following order: SO2�

4 < Cl�< HCO�3 .Moreover, the effects of dye and anions on the photocatalytic

decolorization of RBB have been shown in Scheme 1. The decreased




Fig. 12. Temporal change of RBB during photocatalytic degradation. pH 6,

[H2O2]0 = 100 mg/L, (a) ZnO (b) TiO2 (c) ZnO:TiO2.

Fig. 11. Effect of anion (NaCl, Na2SO4 and NaHCO3) on the photocatalytic

decolorization. pH 6, [RBB]0 = 40 mg/L, [H2O2]0 = 100 mg/L, [anion]0 = 10 mg/L,

(a) ZnO (b) TiO2 (c) ZnO:TiO2.


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efficiency of the decolorization is attributable to the trapping ofOH� and hþVB by the anions (Eqs. (17)–(23)) [51–53].

Cl� þ OH� ! Cl� þ OH� (17)

Cl� þ Cl�! Cl��2 (18)

Cl��2 þ dye ! dye�þðintermediateÞ þ 2Cl� (19)

SO2þ4 þ hþVB! SO��4 (20)

SO��4 þ dye ! SO2�4 þ dye�þðintermediateÞ (21)

HCO�3 þ OH� ! CO��3 þ H2O (22)

CO��3 þ OH�! CO2�3 þ OH� (23)


With respect to the reactions (17)–(23), it can be concluded thatthe inhibiting effects of the anions can be explained through thereaction of hþVB and OH� with anions resulting in a prolonged orinefficient dye degradation. With the consumption of OH�, itsconcentration gradually declined which resulted in the reducedefficiency of the process. In such cases, more amounts of H2O2 wererequired [54,55].

The observed effects of chloride and sulphate are mainlyattributable to the light adsorption capability of the ions whereasin the case of bicarbonate it may be a result of the combined effectsof light attenuation, competition for active sites and their tendencyto act as scavengers of hydroxyl radicals resulting in a prolongeddecolorization [56].

3.7. Degradation and mineralization of RBB

In general, complete dye decolorization does not mean thatthorough mineralization occurs [57]. Furthermore, because of the




Scheme 1. Schematic interaction of RBB, anions and nanoparticles.


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possibility of producing more toxic and carcinogenic intermediatesin photocatalytic degradation of azoic dyes, mineralization ofwastewaters is of great importance. Since the selected reactive dyecan cause skin and respiratory allergy, mineralization of the dyeshould be seriously considered [58]. Fig. 12 shows the UV–visspectrums of RBB solutions as a function of irradiation time at pH 6,[H2O2]0 = 100 mg/L where spacer fabrics treated with ZnO, TiO2

and ZnO:TiO2 were used. As it can be seen in Fig. 1, each moleculeof RBB consists of two molecules of vinyl sulfone and one moleculeof H-Acid which are connected by two –N55N– bonds. The mainpeak at 597 nm is related to the chromophoric group (azoic bonds)and the peaks in UV region are related to benzene (265 nm) andnaphthalene rings (312 nm) [52,59]. The peaks gradually de-creased and finally disappeared which indicates completedecolorization and degradation. After 120 min of the treatment,although the peak of 597 nm disappeared completely, a minorpeak was still observable around 312 nm. As depicted in Fig. 12,

Fig. 13. Decolorization and TOC reduction during the photocatalytic decolorization

of RBB. pH 6, [RBB]0 = 40 mg/L, [H2O2]0 = 100 mg/L, ZnO:TiO2.


hydroxyl radicals show a stronger tendency to attack azoic bondsin comparison to aromatic groups.

To evaluate the mineralization of the dye, TOC test (TOC-VCPH,Shimadzu, Japan) was conducted in the optimum conditions (pH 6,[H2O2]0 = 100 mg/L, [RBB]0 = 40 mg/L, ZnO:TiO2). In Fig. 13 theefficiency of decolorization, degradation and mineralizationprocesses are demonstrated. A0 and At are the absorbance at256, 312 and 597 nm of the reaction solution at time t = 0 and t,respectively. Moreover, TOC0 and TOCt represent the amount oftotal organic carbon at the initiation and time t of the process,respectively. By comparing them after 120 min, it can be concludedthat the reduction of TOC occurred with a lower rate in comparisonto the decolorization and degradation. In fact, highly efficientmineralization with 90% TOC reduction was achieved after120 min of the treatment. Therefore, for complete dye degradationa longer period of time is required due to the high chemicalstability of RBB.

3.8. Modeling of decolorization kinetic

Investigating the kinetics of decolorization is necessary foridentifying the parameters affecting the rate of the reaction. Thekinetics of heterogeneous photocatalytic reactions can be quitecomplex due to the number of steps involved, namely: (i) masstransfer (from the solution to the catalyst), (ii) diffusion, (iii)adsorption and (iv) competition between the different reactants atthe surface of the catalyst. Moreover, it is possible for these steps totake place simultaneously.

The rate constant (k) and their correlation coefficient (R2) ofdye oxidation by ZnO, TiO2 and ZnO:TiO2 at different RBBconcentrations were studied by zero- (Eq. (24)), first- (Eq.(25)) and second-order (Eq. (26)) kinetics with the aim of findingthe most suitable kinetic model for the nano-photocatalysts(Table 1) [60].

½C� ¼ �k0t þ ½C�0 (24)




Table 1Kinetic parameters of RBB degradation during 120 min.

Dye concentration (mg/L) R2

40 50 60 70 80

ZnO k0 0.006 0.008 0.01 0.01 0.013 0.6324

R2 0.606 0.633 0.651 0.580 0.692

k1 0.062 0.034 0.052 0.021 0.035 0.9686

R2 0.998 0.988 0.989 0.874 0.994

k2 14.83 0.483 3.522 0.068 0.314 0.863

R2 0.788 0.876 0.791 0.995 0.865

TiO2 k0 0.005 0.007 0.011 0.011 0.011 0.76

R2 0.425 0.851 0.760 0.862 0.902

k1 0.058 0.041 0.038 0.016 0.013 0.9588

R2 0.903 0.937 0.999 0.973 0.982

k2 9.237 0.942 0.508 0.031 0.018 0.9376

R2 0.913 0.947 0.850 0.987 0.991

ZnO:TiO2 k0 0.001 0.007 0.009 0.01 0.013 0.5004

R2 0.267 0.482 0.543 0.536 0.674

k1 0.065 0.065 0.069 0.068 0.054 0.9088

R2 0.741 0.912 0.917 0.978 0.996

k2 119.1 12.63 12.56 15.04 3.186 0.8626

R2 0.856 0.926 0.918 0.820 0.793

Table 2Effect of RBB concentration on the decolorization rate.

Dye concentration (mg/L)

40 50 60 70 80

ZnO kapp 0.0774 0.0692 0.0466 0.0379 0.034

R2 0.9197 0.9637 0.9956 0.9985 0.8859

r0 3.096 3.46 2.796 2.653 2.72

TiO2 kapp 0.0571 0.0434 0.0396 0.0394 0.0323

R2 0.9992 0.9889 0.9801 0.9916 0.8894

r0 2.284 2.17 2.376 2.758 2.584

ZnO:TiO2 kapp 0.0966 0.0615 0.0396 0.0218 0.0169

R2 0.9882 0.9971 0.9979 0.9975 0.9974

r0 3.864 3.075 2.376 1.526 1.352


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ln½C�0½C� ¼ k1t (25)

1

½C� �1

½C�0¼ k2t (26)

where [C]0 (mg/L) and [C] (mg/L) are the initial dye concentrationand dye concentration at time t, respectively. The k0 (mg/L.min), k1

(L/min) and k2 (L/mg.min) are the zero, first and second order rateconstants, respectively. The results show that the kinetics of thedecolorization using ZnO, TiO2 and ZnO:TiO2 follow the first orderkinetic model. As reported in several papers, decolorizationkinetics of most dyes by the heterogeneous photocatalytic methodfitted the Langmuir–Hinshelwood (L–H) kinetic model (Eq. (27))[61].

r0 ¼ �d½C�0dt¼ KadskL�H½C�0

1 þ Kads½C�0(27)

where r0 is the initial rate of decolorization and is time-dependent(mg/L.min), kL–H is the constant rate of the reaction (mg/L.min),Kads is the rate of dye adsorption on the surface of photocatalyst.The integrated form of Eq. (27) can be stated as follows for[C] = [C]0 at t = 0 (Eq. (28)) [62].

t ¼ 1

KadskL�Hln½C�0½C�

� �þ 1

kL�H½C�0 � ½C��

(28)

As the concentration of dye in the solution is very low (in therange of mM), by ignoring the term [C]0 � [C], Eq. (28) changed to a


pseudo-first order kinetic (Eq. (29)). By using Eq. (29) andassuming kapp = kL–HKads, Eq. (30) was obtained [56].

t ¼ 1

KadskL�Hln½C�0½C�

� �(29)

ln½C�0½C� kL�HKadst ¼ kappt (30)

With plotting ln([C]0/[C]) versus t, the apparent constant rate ofthe decolorization (kapp) was obtained (Fig. 14). For all thesamples, an increase in the dye concentration resulted in adecreased kapp and r0 (Table 2). Fig. 14 demonstrated that thephotocatalytic decolorization fitted to L–H kinetic model and in allcases, as the dye concentration increased, the rate of decolorizationdecreased. To evaluate kL–H and Kads which are both functions ofinitial dye concentration in the solution, the linear form of Eq. (27)was used (Eq. (31)).

1

r0¼ 1

kL�Hþ 1

kL�HKads½C�0(31)

Upon plotting 1/r0 versus 1/[C]0 the parameters of kL–H and Kads

were obtained. The calculated amounts of kL–H and Kads for ZnO,TiO2 and ZnO:TiO2 have been summarized in Table 3. Thedifferences between the constant rates of the decolorization inTables 1 and 2 can be attributed to differences in the dyedegradation mechanism or competition between the dye and theintermediates to occupy active sites of the nanoparticles [62].




Fig. 14. Plots of ln(C0/Ct) versus irradiation time for different initial concentrations of

RBB. (a) ZnO (b) TiO2 (c) ZnO:TiO2.

Table 3Effect of kinetic parameters on the photodegradation of RBB.

kL–H Kads

ZnO 2.2261 0.0745

TiO2 3.4211 0.0424

ZnO:TiO2 0.7858 0.0282


G Model


4. Conclusion

The decolorization of an azo reactive dye (Remazol Black B) inan aqueous solution by nano-photocatalytic systems (UV/H2O2/ZnO, UV/H2O2/TiO2 and UV/H2O2/ZnO:TiO2) was investigated.Effective parameters including initial pH value, initial H2O2


concentration, initial dye concentration, the loaded nanoparticleratio and the presence of sodium salts (NaCl, Na2SO4 and NaHCO3)were evaluated. SEM images and EDS spectra confirmed thesuccessful loading of the nanoparticles on the spacer fabrics. Thebest results were obtained by UV/H2O2/ZnO:TiO2, at pH 6 and100 mg/L of H2O2. In general, increasing the dye concentration andthe presence of Cl�, SO2�

4 and HCO�3 ions exhibited a negativeinfluence on efficiency of the decolorization. Under the optimumcondition, 100% decolorization was achieved with significantreduction in TOC (approximately 90%) within 120 min. Kineticanalysis confirmed that the decolorization followed the first orderkinetic. The results indicated that the spacer fabrics treated withthe mixture of ZnO:TiO2 nanoparticles are capable of decoloriza-tion of azoic reactive dyes in textile wastewater.

Appendix A. Supplementary data

Supplementary material related to this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jtice.2014.04.015.

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decolorization and mineralization of an azo reactive dye using loaded nano-photocatalysts on spacer...

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