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Ultrasonics Sonochemistry 14 (2007) 615–620

Treatment of dye wastewater by using photo-catalyticoxidation with sonication

Akinori Maezawa a,*, Hiroya Nakadoi a, Keiko Suzuki a, Tsubasa Furusawa a,Yasuyuki Suzuki b, Shigeo Uchida c

a Department of Materials Science and Chemical Engineering, Shizuoka University, 3-5-1, Johoku, Hamamatsu 432-8561, Japanb Suzuki Pump Mfg. Co. Ltd., 13-6, Shimoikegawa, Hamamatsu 430-0905, Japanc Polytechnic College Hamamatsu, 693 Norieda-cho, Hamamatsu 432-8053, Japan

Received 10 August 2006; received in revised form 26 October 2006; accepted 6 November 2006Available online 2 February 2007

Abstract

The degradation of Acid Orange 52 in aqueous solutions was investigated by using three processes (photocatalysis, sonolysis, andphotocatalysis with sonication). In the case of photocatalysis, although the concentration of Acid Orange 52 decreased to 35% in480 min, the color of the solution was not disappeared. In the case of sonolysis, it was decomposed completely in 300 min, but the totalorganic carbon concentration decreased down by only about 13% in 480 min. In the case of photocatalysis with sonication, the concen-tration of Acid Orange 52 reached to 0 in 240 min and the total organic carbon concentration decreased by about 87% in 480 min. Theseresults indicate that the ultrasonic irradiation enhanced the photocatalytic degradation. The addition of chloride ion (50 ppm) into AcidOrange 52 solution decreased the decomposition efficiency for photocatalysis. In the cases of sonolysis and photocatalysis with sonica-tion, the decomposition efficiency did not change significantly by the addition of chloride ion. These results indicate that chloride iondisturbs the photocatalysis of dye, but the decomposition of dye using the irradiation of ultrasound is not influenced by chloride ion.From these results, it is considered that the photocatalysis with sonication is most effective for the decomposition of dye in the threeprocesses in this study.� 2007 Published by Elsevier B.V.

Keywords: Waste water treatment; Decomposition of dye; Photocatalysis; Sonolysis

1. Introduction

The treatment of waste water containing the organiccarbon has been one of the most important subjects asthe environmental protection. The effluent of dye worksis often colored. This indicates that the dye is a persistentchemical substance and it is not decomposed by the con-ventional method such as the activated sludge method.Therefore, there is an image that the colored water is con-siderably polluted though the concentrations of polluting

1350-4177/$ - see front matter � 2007 Published by Elsevier B.V.

doi:10.1016/j.ultsonch.2006.11.002

* Corresponding author.E-mail addresses: tcamaez@ipc.shizuoka.ac.jp (A. Maezawa), sha-

cho@suzukipump.com (Y. Suzuki), s2.uchida@ehdo.go.jp (S. Uchida).

materials in wastewater are below the regulation values.The decomposition of organic pollutants such as dye andthe recycle of purified waste water are also desirable forenvironmental conservation.

Photocatalysis is one of the harmless waste water purifi-cation methods. The ultraviolet and titanium oxide arecommonly used as the light source and the photocatalystin the photocatalytic process, respectively. Ultraviolet hasthe disinfectant effect and decomposes the organic com-pounds to inorganic materials such as carbon dioxide andwater. Some researchers have studied on the decompositionof dye by photocatalysis [1,9]. However, the decompositionefficiency is low when the organic concentration in thewastewater is high, and some kinds of organic compoundsare hardly decomposed.

616 A. Maezawa et al. / Ultrasonics Sonochemistry 14 (2007) 615–620

The oxidation by the irradiation of the ultrasound(called sonolysis) is another method which decomposesthe organic compounds to inorganics. This attracts theattention as a new harmless treatment process. Severalresearchers have studied on the sonolysis [3,5,11,4]. Oneof the problems with sonolysis is the low decompositionefficiency. However, it has been reported by severalresearchers that the ultrasonic irradiation enhances photo-catalysis. Stock et al. [10] carried out the decomposition ofnaphthol blue black. They have reported the improvementof mass transfer rates of reactants and products betweenthe bulk solution and the catalyst surface. Davydov et al.[2] have reported that the effect of ultrasound irradiationon the decomposition rate of salicylic acid is attributed tothe catalyst deaggregation and the ulilization of speciesproduced by ultrasound. Ragaini et al. [6] have reportedthat the surface area of the photocatalyst increased afterultrasonic irradiation for 6 h. This means that the ultra-sound irradiation decreases the particle size. Theron et al.[12] have reported that the rate constant of rhenyltrifluo-romethylketone decomposition by the photocatalysis withsonication is higher than the summation of the reactionrate constants in the photocatalysis and the sonolysis andthis attribute to the formation of OH radical from H2O2

produced by the ultrasound on the surface of photocata-lyst. Seli [7] has also reported that the enhancement effectof the ultrasound on the photocatalysis of Acid Orange 8is due to an increased concentration of radicals. The reasonof the enhancement effect of ultrasound irradiation on thephotocatalysis has been unable to come to an agreement.These researchers used the titan oxide particles asphotocatalyst.

6

7

1

2

310

5

Fig. 1. Experimental apparatus. (1) UV lamp (20 W), (2) fixed catalyst, (3(7) ultrasonic transducer (200 kHz), (8) ultrasonic generator (600 W), (9) heat

When the catalytic particle is used, the separation pro-cess in which the particle is removed from the treatmentwater is needed in industry. The costs of the equipmentand operation become expensive. In the case of the use ofthe fixed catalyst, the separation process is no needed.

In this study, we use the fixed catalyst of titanium oxideusing sol–gel method and investigate the efficiencies ofthree processes for dye decomposition. And the effect ofthe ultrasound irradiation on the photocatalysis is dis-cussed. Not only the dye but also the dyeing auxiliariesare included in the effluent of dye works. One of the typicaldyeing auxiliaries is sodium chloride. The chromophoricgroup of dye is anion in the solution. Therefore, the influ-ence of chloride ion on the decomposition efficiency is alsodiscussed.

2. Experimental

Fig. 1 illustrates the experimental apparatus which con-sists mainly of a reactor, a low-pressure mercury lamp, anultrasound transducer, a bath and a stirrer. The reactor ismade of a Pyrex glass cylinder of 110 mm in diameter. It isplaced in the temperature constant bath at 298 K. The vol-ume of solution is 1000 mL. The ultrasonic transducer(Kaijo Co., Japan, 200 kHz, 600 W) is installed at the bot-tom of the reactor. An air pump, a flow meter and a spar-ger are set up for aeration. Air is used as the aeration gas.Fixed TiO2 catalyst was prepared on a Pyrex glass pipe(i.d. = 60 mm, height = 100 mm) which was used as a sup-port material by dip-coating method using sol–gel titaniamade from titanium isopropoxide. After the dip-coatingtreatment, the catalyst was calcined at 823 K for 1 h.

8

9

4

11

) sparger, (4) stirrer, (5) reactor (1000 mL), (6) thermometer (298 K),er (500 rpm), (10) air cylinder and (11) pump.

A. Maezawa et al. / Ultrasonics Sonochemistry 14 (2007) 615–620 617

Low-pressure mercury lamp (Sen Light Co., Japan, spec-tral median wavelength of 253.6 nm, 20 W) is placed atthe center of the reactor.

The concentration of dye is calculated from the absorp-tion intensity of 460 nm by the UV–VIS spectrometer(Shimazu Co., Japan, UV-2450) and the concentration oftotal organic carbon (TOC) is measured by using theTOC meter (Shimazu Co., Japan, TOC-VCHS).

Reagent grade Acid Orange 52 is used as a model dye.Reagent grade sodium chloride and potassium chlorideare used as additives. All chemicals are used withoutpurification.

3. Results and discussion

Fig. 2 shows the change of the color of Acid Orange 52in solution with time in three decomposition methods (pho-tocatalysis, sonolysis, and photocatalysis with sonication(called Hybrid method)). For all methods, the color ofthe solution of Acid Orange 52 became light and the peakposition of the UV spectra of the dye solution was notchanged. These results indicate that all the methods usedin this study have the ability to decompose the Acid Orange52. However, for the photocatalysis, the color of AcidOrange 52 solutions did not disappear in 480 min. Thismeans that the part of the molecular structure of AcidOrange 52 was not decomposed by the photocatalysismethod and was remained in the solution. While, in thecases of sonolysis and Hybrid, the color of the solutionwas clear in 300 min and 240 min, respectively. This meansthat Acid Orange 52 was decomposed completely.

The time dependencies of the concentration of AcidOrange 52 in three methods are shown in Fig. 3. In the caseof photocatalysis, the concentration decreased linearly andthe Acid Orange 52 of 35% was decomposed in 480 min. Inthe case of sonolysis, the concentration of Acid Orange 52

Fig. 2. Time change of solution color (initial concentration of Acid Orangereferences to colour in this figure legend, the reader is referred to the web ver

steeply decreased in the first 60 min and became to zero in300 min. It can be said that the irradiation of ultrasounddecomposed the dye molecules. The decomposition rateof Acid Orange 52 for the sonolysis was considerably fasterthan that for the photocatalysis. This result indicates thatthe sonolysis is more effective than photocatalysis. In thecase of the Hybrid, the concentration of Acid Orange 52steeply decreased and reached to zero at 240 min. Thedecomposition rate for the Hybrid was a little faster thanthat for the sonolysis. It can be said that the decompositionof Acid Orange 52 by the sonolysis is predominant in theHybrid method.

The time dependency of the normalized TOC (totalorganic carbon) concentration is also shown in Fig. 4. Inthe case of the photocatalysis, about 10% TOC decom-posed to inorganic compounds in 480 min. In the case ofthe sonolysis, about 30% TOC was decomposed. Takingaccount of this result with above results, it is concludedthat most of the dye decomposed but the organic interme-diates were remained without being decomposed to inor-ganic compounds. In the case of the Hybrid method, theinitial degradation rate of TOC was slow and was similarto those for the photocatalysis and the sonolysis. And then,the degradation rate became rapid after reaction time of240 min. The TOC of about 35% was decomposed in480 min. It is thought that the molecular weights of theorganic intermediates as decomposition product of dyegradually decrease but the complete mineralization hardlyoccurs when the reaction time is short. Hence, the degrada-tion rate of TOC is slow. However, after 120 min, theorganic intermediates which have small molecular weightdecompose to the inorganic compounds. Therefore, thedegradation rate becomes higher. Stock et al. [10] havereported that the irradiation of ultrasound promotes therapture of azo bond and the oxidation of reaction interme-diate is promoted by photocatalysis.

52 = 25 ppm, 500 rpm, air 50 mL/min, 298 K). (For interpretation of thesion of this article.)

0 120 240 360 480

25

20

15

10

5

Time / min

Co

nce

ntr

atio

n o

f A

cid

Ora

ng

e 52

/ p

pm

Photocatalysis

Sonolysis

Hybrid

Fig. 3. Time dependency of concentration of Acid Orange 52 (500 rpm,air 50 mL/min, 298 K).

0

100

80

60

40

20

120 240 360 480Time / min

No

rmal

ized

TO

C C

on

cen

trat

ion

[T

OC

] /[

TO

C] 0

PhotocatalysisSonolysisHybrid

Fig. 4. Time dependency of TOC concentration (500 rpm, air 50 mL/min,298 K).

Table 1Decomposition efficiency of Acid Orange 52 in 240 min

Photocatalysis(%)

Sonolysis(%)

Hybrid(%)

Withoutchloride ion

20 99 100

With chlorideion

NaCl,[Cl�] = 25 ppm

12 98 99

NaCl,[Cl�] = 50 ppm

9 99 99

KCl,[Cl�] = 25 ppm

6 98 99

KCl,[Cl�] = 50 ppm

7 98 98

Table 2Decomposition efficiency of TOC in 480 min

Photocatalysis(%)

Sonolysis(%)

Hybrid(%)

Withoutchloride ion

9 30 65

With chlorideion

NaCl,[Cl�] = 25 ppm

6 24 64

NaCl,[Cl�] = 50 ppm

5 26 62

KCl,[Cl�] = 25 ppm

4 22 64

KCl,[Cl�] = 50 ppm

4 29 56

618 A. Maezawa et al. / Ultrasonics Sonochemistry 14 (2007) 615–620

From these results, it is said that the ultrasonic irradia-tion with the photocatalysis enhances the decomposition ofdye. The some reasons of this effect are reported by severalresearchers.

(1) Catalyst particle’s physical dispersion by ultrasonicirradiation [2,6,8].

(2) Enhancement of mass transfer between the bulkliquid and the surface of the catalyst and the renewalof fluid film near the surface of catalyst [8,10].

(3) Formation of OH radicals from hydrogen peroxidewhich is produced by photocatalyst ([12,7]).

It is unable to come to an agreement on these reasons.However, in this study, the fixed catalyst was used andthe enhancement effect of ultrasonic irradiation wasobserved. Therefore, it is at least cleared that the enhance-ment by the irradiation of ultrasound is not due to thephysical dispersion of catalyst particle.

Table 1 shows the decomposition efficiency of AcidOrange 52 in 240 min with and without chloride ion. Thedecomposition efficiency of TOC in 480 min is shown inTable 2. In the case of the photocatalysis, by the additionof chloride ion, the decomposition efficiency of AcidOrange 52 was decreased from 20% to 12–6% and the effi-ciency of TOC was decreased from 10% to about 5%. Thedecomposition efficiency of Acid Orange 52 with the addi-tion of potassium chloride was lower than that with theaddition of sodium chloride at the same chloride ion con-centration. While, in the case of the sonolysis, the additionof chloride ion hardly affects the efficiency. Furthermore, inthe case of the Hybrid method, the similar result wasobtained.

The absorption experiment of Acid Orange 52 under thedark condition without the irradiation of ultrasound wasconducted with and without chloride ion. The powder oftitanium oxide (P-25, anatase type, Nihon Aerogil Co.,Japan, BET surface area: 55 m2/g) was used as catalyst.

Acid Orange 52 disassociates into Na+ and anion in theaqueous solution as follows:

25

24

24.5

23.5

22.5

22

0 20 40 60 80

Co

nce

ntr

atio

n o

f A

cid

Ora

ng

e 52

/ p

pm

Surface area of catalyst / m2

Without chloride ion With NaCl 50 ppm

With KCl 50 ppm

Fig. 6. Concentration of Acid Orange 52 vs. surface area of catalyst in thedark experiments without irradiation of ultrasound (500 rpm, 298 K).

A. Maezawa et al. / Ultrasonics Sonochemistry 14 (2007) 615–620 619

ðCH3Þ2N�C6H6�N¼N�C6H6� SO3Na

¡ ðCH3Þ2N�C6H6�N¼N�C6H6� SO�3 þNaþ ð1Þ

Fig. 5 shows the time dependency of the concentrationof Acid Orange 52. Without chloride ion, the concentrationof Acid Orange 52 decreased and became constant in about30 min. This indicates that Acid Orange 52 ion adsorbedon the catalyst surface and the adsorption equilibriumwas reached in about 30 min. Furthermore, the decomposi-tion of Acid Orange 52 was not occurred under the darkcondition without the irradiation of ultrasound. In the caseof the addition of chloride ion, the concentration of AcidOrange 52 decreased slightly. The Acid Orange 52 andchloride ion are both anions. And it must be that theadsorption sites for these anions are the same. Therefore,the chloride ion disturbs the adsorption of Acid Orange52. At the same chloride ion concentration, the amountof adsorbed Acid Orange 52 in the case of sodium chloridewas larger than that in the case of potassium chloride. Thisis due to the different influence of cation to adsorption ofAcid Orange 52.

Fig. 6 shows the influence of the amount of catalyst onthe concentration of Acid Orange 52 in 30 min with andwithout chloride ion of 50 ppm. The equilibrium concen-tration of Acid Orange 52 with chloride ion is higher thanthat without the chloride ion. Furthermore, the concentra-tion of Acid Orange 52 decreased about linearly with theincrease in the amount of catalyst.

It must be that the similar results are obtained in thecase of fixed catalyst though the above results wereobtained by using powder catalyst. Therefore, the chlorideion adsorbs on the surface of the catalyst and disturbs thecatalytic effect.

25

24.5

0 10 20 30 40

Time / min

50 60

23.5

24

23

Co

nce

ntr

atio

n o

f A

cid

Ora

ng

e 52

/ p

pm

0 ppm 25 ppm 50 ppm

NaCl

KCl

Fig. 5. Time dependency of concentration of Acid Orange 52 in the darkexperiments without irradiation of ultrasound (TiO2 particle 0.6 g/L,500 rpm, 298 K).

The hole produced on the valence band by the motion ofexcited electron reacts with the chloride ion adsorbed onthe catalyst surface and disappears [9]. Therefore, the pro-duction of OH radical to react with the organic compoundsis limited. By this reason, the decomposition efficiencies ofAcid Orange 52 and TOC in the case of the photocatalysiswere decreased.

In the sonolysis, there exist no catalyst in the reactor,that is, there are no adsorption sites for Acid Orange 52and chloride ion. Therefore, it is observed that the decom-position of dye in the sonolysis is not influenced by thechloride ion. In spite of the existence of catalyst, thedecomposition efficiency in the Hybrid method was higherthan those in the photocatalysis. Because the sonolyticdecomposition is predominant in the Hybrid method,though the decomposition rate of photocatalysis wasdecreased by the chloride ion, the dye was decomposedby the sonolysis which is not influenced by the chlorideion. Therefore, it must be that the influence of chlorideion was small in the Hybrid method.

4. Conclusion

The photocatalysis is applicable to the dye containingwaste water treatment. The irradiation of the ultrasoundincreased the dye and TOC decomposition efficiencies ofthe photocatalysis. However, the efficiency of the photoca-talysis decreased in the addition of chloride ion in order tothe absorption of that ion on the surface of the catalyst.The decomposition of dye using the ultrasonic irradiation(that is, the sonolysis and the Hybrid method) is not influ-enced by the chloride ion. And it is found that the sonolysisis the dominant decomposition in the Hybrid method.

620 A. Maezawa et al. / Ultrasonics Sonochemistry 14 (2007) 615–620

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