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Ultrasonics Sonochemistry 14 (2007) 531–537

Visible light induced sonophotocatalytic degradation of ReactiveRed dye 198 using dye sensitized TiO2

Sumandeep Kaur a, Vasundhara Singh b,*

a School of Chemistry and Biochemistry, Thapar Institute of Engineering and Technology, Patiala 147 004, Indiab Basic and Applied Sciences, University College of Engineering, Punjabi University, Patiala 147 002, India

Received 15 July 2006; received in revised form 5 September 2006; accepted 30 September 2006Available online 6 February 2007

Abstract

In this paper we are reporting the accelerated sonophotocatalytic degradation of Reactive Red (RR) 198 dye under visible light usingdye sensitized TiO2 activated by ultrasound. The effect of sonolysis, photocatalysis and sonophotocatalysis under visible light has beenexamined to study the influence on the degradation rates by varying the initial substrate concentration, pH and catalyst loading to ascer-tain the synergistic effect on the degradation techniques. Ultrasonic activation at 47 kHz contributes through cavitation leading to thesplitting of H2O2 produced by both photocatalysis and sonolysis. This results in the formation of oxidative species, such as singlet oxygen(1O2) and superoxide (O��2 Þ radicals in the presence of oxygen. Sonication increases the amount of reactive radical species, inducing fasteroxidation of the substrate and degradation of intermediates and also the deaggregation of the photocatalyst which are responsible for theobserved synergy. Further, the photocatalytic activity of RR 198 dye sensitized TiO2 is demonstrated by the degradation of phenol undervisible light and ultrasound. A comparative study using TiO2, Hombikat UV 100 and ZnO was also carried out.� 2006 Elsevier B.V. All rights reserved.

Keywords: Sonophotocatalysis; Dye sensitized TiO2; Ultrasound; Reactive Red dye 198; Visible light

1. Introduction

Textile mills are the major consumers of water and con-sequently one of the largest groups of industries causingintense water pollution [1]. It is estimated that from 1% to15% of the dye lost during the dyeing process is releasedin wastewaters [2]. Color in dyehouse effluent has often beenassociated with the application of reactive dyestuffs, duringwhich up to 50% of the dyes may be lost to the effluent [3].Reactive dyes are present in a hydrolyzed state in theexhausted dye bath or wash water, a form that cannot bereused in the dyeing process. A variety of physico-chemicalmethods are presently available for the treatment of textilewastewater. It has been reported that majority of dyes areonly adsorbed on the sludge and are not degraded [4].

1350-4177/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.ultsonch.2006.09.015

* Corresponding author.E-mail address: vasun7@yahoo.co.in (V. Singh).

Recent experimental investigations have revealed thatreactive dyes can be decolorized by advanced oxidationprocesses [5,6] involving the generation of powerful oxidiz-ing agent like hydroxyl radicals (�OH), which completelydestroy the pollutants in wastewater. Heterogeneous pho-tocatalysis through illumination of UV [7] or solar light[8] on a semiconductor surface is an attractive advancedoxidation process. Titanium dioxide is a wide band gapsemiconductor (3.2 eV) used successfully as a photocatalystfor the degradation of organic [9] and dye pollutants[10,11]. However TiO2 absorbs a small portion of solarspectrum in the UV region (4–6%). Hence, to reap maxi-mum solar energy, it is necessary to shift the absorptionthreshold towards visible region. Conceptually, dye sensiti-zation [12–14] seems to be a viable alternative method todeal with this issue.

Environmental sonochemistry [15–17] is a rapidly grow-ing area and is an example of an AOP that deals with thedestruction of organic species in aqueous solution. The

532 S. Kaur, V. Singh / Ultrasonics Sonochemistry 14 (2007) 531–537

chemical effects of high intensity ultrasound result primar-ily from acoustic cavitation: the formation, growth, andimpulsive collapse of bubbles in liquids [17]. In sonophot-ocatalysis, due to the generation of sites of high tempera-ture and pressure by acoustic cavitation, overall watersplitting was proposed to proceed by a two-step reactionas reported in literature [18].

Reports on heterogeneous sonophotocatalysis under dif-ferent experimental conditions [18–21] are available in liter-ature. The simultaneous use of two techniques wasreported to be more effective than their sequential combi-nation [21] though leading to just additive effects, and tobe more effective than sonolysis only when employing rela-tively low ultrasound intensity [20]. For the practical appli-cation of dye wastewater treatment by photocatalysis andsonophotocatalysis, there is a need to determine the opti-mal conditions of experimental parameters.

In continuation with our study in photodegradation oforganic pollutants [22,23], we have undertaken the degrada-tion of mono azo dye Reactive Red 198 [24] under visiblelight via dye sensitized TiO2. In order to exploit the synergis-tic effect between sonolysis and photocatalysis, and to ascer-tain whether it is a consequence of processes occurring at thewater–semiconductor interface, or in the aqueous phase,a systematic study was undertaken on the influence ofthe degradation rate of substrate concentration, optimumphotocatalyst concentration, pH and effect of quenchers.

2. Materials and methods

2.1. Materials

Commercially available Reactive Red dye 198 (RR 198)was obtained from Nahar Fabrics, Derabassi, India andwas used as such without any purification. Stock solutionsof the dye containing the desired concentration were pre-pared in double distilled water. Degussa P25 titanium diox-ide (80% anatase, 20% rutile, surface area 50 m2/g andprimary particle size of 30 nm), Hombikat UV 100 (Sach-tleben Chemie, Germany) and ZnO (Merck) wereemployed as semiconductor photocatalysts. The otherchemicals used were NaN3 (extra pure, Merck), DABCO(C6H12N2, 98%, Aldrich), 1,4-benzoquinone (prepared inthe laboratory [25] and recrystallised twice with pet. ether),NaOH, HCl and phenol were obtained from S.D. FineChemicals Limited, India.

2.2. Apparatus

An immersion well photochemical reactor made ofPyrex glass equipped with a water-circulating jacket main-taining a temperature of 25 �C and an opening for supplyof oxygen was used [24]. To study the effect of ultrasoundthe whole assembly was immersed in the Branson Ultra-sonic cleaning bath model 3150DTH (47 kHz, 130 W).Irradiations were carried out using a 50 W halogenlamp (Philips). Samples (5 ml) were collected and filtered

through a syringe filter (0.45 lm, Millipore). The pH ofthe solution was varied by adding HCl or NaOH solutionas per requirement and measured using ELICO, India LI120-pH meter.

2.3. Analysis

For the determination of concentration of dye, HitachiU-2001 UV–Vis spectrophotometer was used atkmax = 515 nm. For identification of intermediate prod-ucts, aqueous solution of RR 198 containing TiO2 was irra-diated for 2 h and the filtrate was extracted usingchloroform and was subsequently dried over anhydroussodium sulphate. The solvent was removed under reducedpressure to give the residual mass, which was analyzed byGC-MS. GC-MS was done using Hewlett Packard HP6890 series, GC system with FID detector, 5973 MassSelective Detector and auto sampler was used. ColumnDB-5 MS (5% phenyl polysiloxane (30 · 250 · 0.25 lm),ID – 0.25 mm, film lm – 0.25) with temperature limit of�60 to 325 �C purchased from J&W scientific was used.Operating temperature was programmed (100-5-200-15-280) in splitless mode. An injection volume of 0.5 ll withhelium as a carrier gas was used.

2.4. Procedure

Stock solution of dye at different concentrations wereprepared in distilled water with a natural pH 4.6 and mixedwith TiO2 and dispersed in a batch of 200 ml aqueous sus-pension in the photoreactor. The solution was stirred andbubbled with molecular oxygen for at least 2 h in the darkto allow equilibration of the system so that the loss of com-pound due to adsorption can be taken into account [24].The dye sensitized TiO2 was subjected to visible light irra-diation for the degradation of dye in solution. The sono-chemical experiments were performed at the point ofmaximum cavitation. An aliquot of 5 ml was taken fromthe reactor at regular interval of time. The catalyst was fil-tered from the solution by Millipore filter (0.45 lm) and thefiltrate was analyzed for determining concentration of dyeat kmax = 515 nm. The filter was washed every time toensure that no residual dye remained on the microfilter.Experiments for photolysis, photocatalysis, sonolysis andsonophotocatalysis were carried out.

The reaction kinetics was studied by varying differentparameters like initial concentration of dye, catalyst load-ing and initial pH of the solution. All experiments were car-ried out in triplicate for reproducibility of results.

3. Results and discussion

3.1. Decolourization and kinetic analysis

Reactive Red 198 is a strongly absorbing dye in the UV–visible region with distinct bands in the UV region(k = 294 nm) and another one in the visible region

-4

-3

-2

-1

0

0 2 4 6 8

Time (h)

ln C

/Co

Fig. 2. First order kinetic plots of RR 198 degradation under (·) visiblelight (vis), (m) ultrasound in the presence of TiO2 particles (US + TiO2),(j) photocatalysis (vis + TiO2) and (d) sonophotocatalysis (US + vis +TiO2). Initial dye concentration: 50 ppm, TiO2 amount = 300 mg/l,pH = 4.6 (natural).

Table 1First order rate constants of RRed degradation under sonolysis in thepresence of dye sensitized TiO2 (US + TiO2), photocatalysis (vis + TiO2)and sonophotocatalysis (vis + US + TiO2)

Entry C0 TiO2 amt.(mg/l)

kvis+US+TiO2kvis+TiO2

kus+TiO2Synergy

1 50 300 0.0234 0.0145 0.0038 0.21792 40 300 0.0466 0.0272 0.0020 0.37333 30 300 0.1970 0.0849 0.0012 0.56294 20 300 0.3087 0.1257 0.0025 0.5847

S. Kaur, V. Singh / Ultrasonics Sonochemistry 14 (2007) 531–537 533

(k = 515 nm) as shown in Fig. 1. The latter is responsiblefor the red color arising from aromatic rings connectedby azo groups and the former is associated with benzenelike structures in the molecule. The disappearance of thevisible band in a short period is due to the fragmentationof the azo links by immediate OH radical attack (hydrox-ylation) in the oxidation process. In addition to this rapidbleaching effect, the decay of the absorbance at 294 nm isconsidered as evidence of aromatic fragment degradationin the dye molecule and its intermediates.

The degradation rates measured under different experi-mental conditions could conveniently be compared interms of first order rate constants, obtained from the slopesof plots of Fig. 2. The degradation was negligible in thepresence of only visible light without TiO2. However thepollutants underwent relatively slow degradation underultrasound in the presence of TiO2 (US + TiO2), whileunder photocatalysis (vis + TiO2) the degradation occurredat higher rate. A further increase in reaction rate wasobserved on illuminating the sample suspensions simulta-neously with visible light and ultrasound in the presenceof TiO2 (vis + US + TiO2).

Synergistic effect was seen: the combined effect of sonol-ysis and photocatalysis led to a degradation rate constant(kUS+vis+TiO2

) which was greater than the sum of thedegradation rate constants measured under photocatalysis(kvis+TiO2

) and sonolysis (kUS+TiO2). The synergy between

photocatalysis and sonolysis can be usefully quantified(Eq. (1)) as the normalized difference between the rateconstants obtained under sonophotocatalysis and the sumof those obtained under separate photocatalysis and sonol-ysis in the presence of a semiconductor and is given inTable 1

Synergy ¼ kvisþUSþTiO2� ðkUSþTiO2

þ kvisþTiO2Þ

kvisþUSþTiO2

ð1Þ

Fig. 1. Spectral changes that occur during the sonophotocatalytic degradatioC0 = 50 ppm.

It is observed that with 60% dilution, the synergy factorincreased fourfold. Both in photocatalysis and sonophot-ocatalysis the pollutants in water are degraded mainlythrough the generation of OH radical. Sonolysis furtherenhances the degradation rate by increasing the catalyticactivity of the semiconductor catalyst. This could occurthrough decrease in size of the photocatalyst and deaggre-gation of particles leading to an increase in surface areaand thus the catalytic performance [20,26].

n of aqueous solution of Reactive Red dye: pH = 4.6; (TiO2) = 300 mg/l;

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 20 40 60

Initial conc. (mg/l)

k (m

in-1

)

Fig. 4. Initial rate of RR 198 degradation, r0 under (�) US + TiO2 + vis,(j) vis + TiO2, (m) US + TiO2, as a function of the initial dye concen-tration, (TiO2) = 300 mg/l, pH = 4.6.

534 S. Kaur, V. Singh / Ultrasonics Sonochemistry 14 (2007) 531–537

3.2. Effect of catalyst loading

The amount of catalyst is one of the main parametersfor the degradation studies from economical point of view.In order to avoid the use of excess catalyst it is necessary tofind out the optimum loading for efficient removal of dye.Several authors have investigated the reaction rate as afunction of catalyst loading in photocatalytic oxidationprocess. First order rate constants of sonolytic, photocata-lytic and sononophotocatalytic degradation of ReactiveRed 198 in the presence of different amounts of titaniumdioxide (100–500 mg/l) are reported in Fig. 3.

The degradation rate under sonolysis was low and wasnot influenced by the amount of photocatalyst. Progres-sively higher kvis+TiO2

and kUS+vis+TiO2values under photo-

catalysis and sonophotocatalysis respectively weremeasured with increasing TiO2 concentration. The resultsclearly show that the optimum catalyst loading for degra-dation of RR 198 is 300 mg/l. The reasons for the decreasein degradation rate are (i) aggregation of TiO2 particles athigh concentrations causing decrease in the number of sur-face active sites and (ii) increase in opacity and light scat-tering of TiO2 particles at high concentration leading todecrease in the passage of irradiation through the sample.Ultrasound induces the increased reaction rate due to theproduction of more number of photoactive species anddue to the water splitting leading to the formation ofH2O2. Moreover sonication also leads to deaggregationof the TiO2 particles leading to the increase in the surfacearea.

3.3. Effect of initial substrate concentration

The effect on the degradation rate of different initial sub-strate amounts was investigated in suspensions containing300 mg/l of TiO2 at a normal pH of 4.6. The degradationrate of dye measured under different experimental condi-tions, are shown in Fig. 4.

Under sonolysis all substrates underwent slow degrada-tion. Under both photocatalysis and sonophotocatalysisthe reaction proceeded much faster, however the degrada-tion rate decreases with increase in the initial concentrationof the substrate. Our results are in good agreement withthose reported in literature [19]. The combined action of

0

0.01

0.02

0.03

0.04

0.05

0.06

0 200 400 600Catalyst loading (mg/l)

k (m

in-1

)

Fig. 3. Rate constants of RR 198 degradation under (m) US + TiO2, (j)vis + TiO2, (d) US + vis + TiO2, as a function of the TiO2 concentration.Initial concentration = 40 ppm, pH = 4.6.

photocatalysis and sonolysis produced synergistic effectsin all the investigated range of substrate concentration;however, the effect was more prominent at lower concen-trations. A decrease in reaction rate with increase in sub-strate concentration is due to the constancy of theamount of substrate adsorbed on the semiconductor.Simultaneous sonolysis did not induce any modificationin this trend, indicating that under photocatalytic andsonophotocatalytic conditions the reaction system exhibitsthe same dependence on the amount of dye, which deter-mines the water–semiconductor interface phenomena.During the course of reaction due to formation of interme-diates, competition starts between the intermediates andthe dye molecules for the surface active sites of TiO2 lead-ing to the decrease in the degradation rate.

3.4. Effect of pH

The pH is a complex parameter since it is related to theionization state of the surface as shown in Eqs. (2) and (3)as well as to that of reactants and products such as acidsand amines:

TiOHþHþ ¡ TiOHþ2 ð2ÞTiOHþOH� ¡ TiO� ð3Þ

Three possible mechanisms can contribute to dye degra-dation: hydroxyl radical attack, direct oxidation by thepositive hole and direct reduction by the electron in theconducting band depending on the nature of the substrateand pH [11]. In our experiments, any changes in the initialdegradation rate with varying pH values must be ascribedto variations of the acid/base properties of the TiO2 parti-cle surface. Since the photoxidation of dyes is accompaniedby the release of protons [27], its efficiency may then changebecause of the reversible protonation of the TiO2 surface.Photocatalytic activity reached a maximum in acidic condi-tions, followed by a decrease of r0 in the pH range from 7to 9. Same trend is seen in both photocatalytic and sono-photocatalytic conditions.

The effect of pH on the photocatalytic reaction can belargely explained by the surface charge of TiO2 (pzc of

Table 2Langmuir–Hinshelwood constants for the photodegradation of RRed dyeat different pH values

pH Photocatalysis Sonophotocatalysis

Ke (l/mg) kv (min�1) Ke (l/mg) kv (min�1)

3.5 0.006 11.14 0.004 17.704.6 0.008 5.95 0.009 5.987.0 0.016 1.27 0.035 1.319.0 0.004 2.13 0.018 1.10

S. Kaur, V. Singh / Ultrasonics Sonochemistry 14 (2007) 531–537 535

TiO2 � 6.8) and its relation to the acid dissociation con-stants of dye. Below pH 6, as pH decreases, strong adsorp-tion of dye on the TiO2 particles as a result of electrostaticattraction of the positively charged TiO2 with the ionizeddye is observed. On the other hand, above pH 6, a decreasein the reaction rate has been observed (Fig. 5) reflecting thedifficulty of anionic dye in approaching the negativelycharged TiO2 surface when increasing the solution pH.

The influence of the initial concentration of the soluteon the photocatalytic degradation derived from the kineticdata can be rationalized in terms of Langmuir–Hinshel-wood model (Eq. (4)) modified to accommodate reactionsoccurring at a solid–liquid interface [27]. A linear expres-sion can be conveniently obtained by plotting the recipro-cal initial rate against the reciprocal initial concentration(Fig. 6)

r0 ¼ �dC=dt ¼ kvKeC0

1þ KeC0

ð4Þ

where kv reflects the limiting rate of the reaction at maxi-mum coverage under the given experimental conditions.Ke represents the equilibrium constant for adsorption ofdye on to illuminated TiO2.

In Eq. (4) kv represents the apparent rate constantbecause it is also dependent on the source of visible lightand the radiation field inside a photocatalytic reactor.

00.20.40.60.8

11.21.41.61.8

0 1 2 3 4 5 6 7 8 9 10pH

r o(m

g/l

min

-1)

Fig. 5. Effect of pH on degradation of RR 198 under (j) US + TiO2 +vis, (m) vis + TiO2, initial dye concentration 50 mg/l, (TiO2) = 300 mg/l.

y = 20.041x + 0.168 R2 = 0.9988

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.02 0.04 0.061/C0

1/r o

0

0

0

0

1

1/r o

Photocatalytic

Fig. 6. Representation of Langmuir–Hinshelwood equa

These results are reflected by the kv and Ke values shownin Table 2.

3.5. Effect of various photocatalysts

The influence of three different photocatalysts (TiO2-P25, ZnO, UV-100) on the degradation kinetics of ReactiveRed 198 was investigated and results are shown in theFig. 7. TiO2 and ZnO are found to be more efficient thanUV-100. The order of activities of the photocatalysts are

y = 17.609x + 0.1671 R2

0

.2

.4

.6

.8

1

.2

0 0.02 0.04 0.061/Co

= 0.9973

Sonophotocatalytic

tion: (dye) = 50 ppm, pH = 4.6, (TiO2) = 300 mg/l.

Fig. 7. Effect of various photocatalysts on degradation of reactive red dyeunder photocatalytic conditions (PC) and sonophotocatalytic conditions(SPC) (dye) = 50 ppm, pH = 4.6, (catalyst) = 300 mg/l.

0

10

20

30

40

50

60

0Time (h)

Co

nce

ntr

atio

n (

mg

/l)

2 4 6 8

Fig. 8. Effect of radical quenchers on the degradation rate undersonophotocatalytic conditions: (·) 1,4 Benzoquinone, (j) NaN3, (m)DABCO, (�) no quencher, (dye) = 50 ppm, (TiO2) = 300 mg/l , (quench-er) = l00 mg/l.

536 S. Kaur, V. Singh / Ultrasonics Sonochemistry 14 (2007) 531–537

ZnO > TiO2-P25 > UV-100. Although ZnO is the most effi-cient catalyst but it has the disadvantage of undergoingphotocorrosion under illumination in acidic conditions.The high photoreactivity of TiO2-P25 as compared toUV-100 is due to the following reasons: (i) slow recombina-tion of electron-hole pair and (ii) large surface area.

3.6. Effect of radical quenchers

The formation of oxidative intermediate species such assinglet oxygen (1O2) and superoxide (O��2 Þ radicals underphotocatalytic and sonophotocatalytic conditions and theirrole in the dye degradation process has been investigatedindirectly with the use of appropriate quenchers of thesespecies. In these experiments, a comparison is made betweenthe original decolourization curves of RR198 TiO2 disper-sions with those obtained after addition of quenchers inthe initial solution under otherwise identical conditions.Compounds used for this purpose were 1,4-diazabicyclo[2,2,2] octane (DABCO), a singlet oxygen quencher, sodiumazide (NaN3), which is also a quencher of singlet oxygenbut may also interact with OH� and 1,4-benzoquinone(BQ), which is a quencher of superoxide radical.

It is observed from Fig. 8 that in the presence of BQ,which is an O��2 quencher, photobleaching of RR dye 198is completely suppressed indicating that the superoxideradical is an active oxidative intermediate. The inhibitingeffect of NaN3, which is a 1O2 quencher but may also inter-act with OH radical, becomes significant after 1 h indicat-ing the delayed formation of singlet oxygen (and possiblyhydroxyl radical) species. Similar results were obtainedafter addition of DABCO, which is also a singlet oxygenquencher.

3.7. Identification of degraded intermediates

The GC-MS analysis of the residue after degradationshowed that several smaller organic intermediates were

Fig. 9. Sonophotocatalytic decomposition of phenol by RR dye adsorbed TiTiO2) = 300 mg/l, (phenol) = 50 ppm. Total solution = 200 ml.

formed, out of which one was the major product (18%).The structure of the major intermediate formed is sug-gested to be a phthalic acid derivative by the molecularion peak (27 8) and base peak (M+) 149 and fragmentationpattern.

3.8. Decomposition of phenol by dye-sensitized TiO2 under

visible light

To confirm the dye-sensitized mechanism the decompo-sition of phenol, a toxic organic compound, was carriedout using RR dye adsorbed TiO2 as the photocatalystunder visible light. A saturated solution of dye containingTiO2 (1.0 g) was magnetically stirred for 24 h in dark.The uptake was estimated spectrophotometrically by mea-suring free dye in the supernatant liquid obtained after fil-tration and it was found to be 374 leqiv/g. The results ofphenol degradation by sonophotocatalysis using dye sensi-tized TiO2 are shown in Fig. 9. The specific peak of phenolappeared at 270 nm, and that of the dye at 515 nm. Duringthe process of irradiation phenol peak disappears withtime. This indicates that the dye sensitized TiO2 attacks

O2 under visible light. The experimental conditions were: (dye sensitized

S. Kaur, V. Singh / Ultrasonics Sonochemistry 14 (2007) 531–537 537

phenol first and the decomposition of the dye takes placeafter the phenol had decomposed.

4. Conclusions

This study reveals that the acceleration of the degrada-tion process of dye sensitized TiO2 under visible light hasbeen achieved by ultrasonication. Mainly ultrasoundcontributes through cavitation to the scission of H2O2 pro-duced by both photocatalysis and sonolysis. This increasesthe amount of reactive radical species inducing oxidationof the substrate and degradation of intermediates and ismainly responsible for the observed synergy. The photo-degradation kinetics follows the Langmuir–Hinshelwoodmodel and depends on the TiO2 concentration and pH.These results show that a conventional dye can by usedas a photosensitizer of TiO2 functioning under visible lightand has been demonstrated by the degradation of phenol.It is also evident that the reaction takes place via formationof singlet oxygen, superoxide and hydroxyl radicals. Thismethodology has additional advantage for harnessing thevisible component of the solar energy for the degradationof organic pollutants in water. Further studies to apply thissynergistic technique using solar energy are underway.

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