Ultrasonics Sonochemistry 17 (2010) 515–520

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Ultrasonics Sonochemistry

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Electrochemical degradation of reactive dye in the presence of water jet cavitation

Xiaoning Wang, Jinping Jia *, Yalin WangSchool of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China

a r t i c l e i n f o

Article history:Received 10 June 2009Received in revised form 30 September2009Accepted 28 October 2009Available online 31 October 2009

Keywords:DecolorizationElectrolysisReactive Brilliant Red X-3BSynergistic effectWater jet cavitation

1350-4177/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.ultsonch.2009.10.023

* Corresponding author. Tel.: +86 21 5474 1065; faE-mail address: jpjia@sjtu.edu.cn (J. Jia).

a b s t r a c t

Degradation of a reactive dye, Brilliant Red X-3B, induced by electrolysis coupled with water jet cavita-tion was studied. The experiment was performed in 4.5 L of aqueous solution containing X-3B concentra-tions ranging from 40 to 120 mg/L by applying Ti–IrO2 as anode and graphite as cathode. The water jetcavitation process decreased the diffusion layer thickness and consequently increased the current den-sity. Compared to water jet cavitation and electrolysis alone, the combination of the two methodsenhanced X-3B removal and showed a synergistic effect. The azo bond of the dye molecule was brokendown and the naphthalene ring was transformed to multi-substituted benzene during the combined pro-cess. The dye degradation rate increased with increasing concentration. Acidic conditions (e.g., pH 1)favored the decolorization of the reactive dye. The use of TiO2 coated with IrO2 as anode and graphiteas cathode showed the best performance for the dye removal efficiency, compared to other electrodepairs. Addition of SO2�

4 , NO�3 , and especially Cl� ions into solution significantly enhanced the degradation.However, CO2�

3 inhibited the dye decolorization.� 2009 Elsevier B.V. All rights reserved.

1. Introduction

Approximately 10,000 different commercial dyes and pigmentsare used, and over 7 � 105 tons of dyes are produced annually overthe world. It has been estimated that about 10–15% of these dyesare released in effluents during dyeing processes [1]. Dye wastewa-ter has attracted more environmental attention because the strongcolor is not only aesthetically displeasing, but also impedes lightpenetration which resists biological processes in water [2]. Fur-thermore, the dye components are hardly degraded by physico-chemical or biological methods [3]. Among dyestuffs, reactivedyes, especially those containing azo-groups, constitute a greatportion of dye pollutants [4].

Conventional methods for treating dye wastewater includeadsorption on various sorbents, chemical decomposition by oxida-tion, photodegradation and microbiological decolorization [5].Among these methods, electrochemistry is a clean water treatmenttechnology, and is widely used in the destruction of textile dyes[6–8]. However, when the laboratory electrolytic process is scaledup for industrial exploitation, some restrictions appear such as lim-itation of mass transfer to the electrode surface, fouling of the elec-trode surface, and evolution of gases during the electrochemicalreaction [9].

Cavitation, a technology for the formation, growth and implo-sive collapse of gas or vapor-filled bubbles in liquids, has beenextensively investigated as an effective approach for destruction

ll rights reserved.

x: +86 21 5474 0825.

of large variety of organic pollutants such as aromatic compounds[10–12], chlorinated hydrocarbons [13,14] and organic (azo) dyes[15,16]. However, most of these investigations were carried outwith acoustic cavitation (i.e. sonochemistry), and the reports onhydrodynamic cavitation created during turbulent flow of liquidswere limited. The use of cavitation processing for wastewatertreatment offers several advantages over many conventional tech-niques, e.g., occurrence of localized transient high temperature andpressure without application of rigorous conditions [17,18],improvement in mass transport due to turbulence and microstrea-ming [19], and diminution of reagents required [20]. The powerfuloxidizing agent �OH, with a redox potential of +2.8 V, producedfrom the dissociation of water molecules inside the cavitation bub-bles, also plays an important role in the degradation of organic pol-lutants [21].

The combination of electrochemical degradation and the cavita-tion process produces a great deal of advantages. A significantenhancement of electro-oxidation of an acidic dye, Sandolan Yel-low, was achieved by the use of ultrasound [22]. De Lima Leiteet al. [23] reported the electro-oxidation of 2,4-dihydroxybenzoicacid (2,4-DHBA) assisted by ultrasound at two frequencies: at highfrequency, hydroxyl radicals were generated and directly oxidizedorganic pollutants, whereas an increase in the mass transfer rate ofthe electro-active species and degradation rate of 2,4-DHBA wasobserved at low frequency. Zhao et al. [24] indicated the degrada-tion of phenol by ultrasound assisted electrochemical oxidation onboron-doped diamond and Pt electrodes and found that the degra-dation rate and current efficiency on both electrodes were en-hanced with the help of ultrasound due to the ultrasonic positive

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a

516 X. Wang et al. / Ultrasonics Sonochemistry 17 (2010) 515–520

effect on mass transport, adsorption amount and electrochemicalreaction. A great deal of work has been done on the applicationof ultrasonic cavitation in the electrochemical processing of organ-ic contaminants. However, combination of electrolysis with waterjet cavitation for the treatment in dye wastewater has not beenreported.

Ti-based iridium dioxide (Ti–IrO2) electrode is a dimensionallystable anode and possesses high electro catalytic activity. The elec-trode has been extensively applied in treatment of wastewatercontaining biorefractory organic pollutants [25–27]. The objectiveof this work was to explore the possible synergistic effect of waterjet cavitation coupled with electrolysis employing Ti–IrO2 as theanode on degradation of a commercial reactive dye, Reactive Bril-liant Red X-3B (X-3B). The impacts of initial dye concentration,medium pH, electrode material, and inorganic anions on the dyeremoval efficiency were investigated. To explore the synergistic ef-fect, mechanical stirring coupled with the electrochemical reactionwas included as a comparison.

b

2 m

m

25.4

mm

130 mm

Fig. 2. (a) Experimental setup of water jet cavitation and electrolytic system. (1)Reactor; (2) Cooling water inlet; (3) Cooling water outlet; (4) Centrifugal pump;(5,6) Control valves; (7) Pressure gauge; (8) Cavitation tube; (9) Anode; (10)Cathode; (11) Thermometer; and (12) Direct current power (b) Geometry of thecavitation tube.

2. Experimental

2.1. Materials

Reactive Brilliant Red X-3B was chosen as a model dye and ob-tained from Jiaying Chemical Co., Ltd., Shanghai. It was used asanalytical grade without further purification. The structure of thedye molecular is shown in Fig. 1. All other chemicals were of ana-lytical reagent grade and used as received. A plate Ti–IrO2 electrodewith dimensions of 100 mm � 40 mm � 2 mm was obtained fromSchool of Chemistry and Chemical Engineering, Shanghai Jiao TongUniversity (SJTU) and used for the dye electrolysis as an anode.Graphite and stainless steel sheets were obtained from School ofEnvironmental Science and Engineering, SJTU and employed as an-ode or cathode. All electrodes were washed with water before elec-trolysis was conducted.

2.2. Experimental setup

Electrochemical degradations of X-3B coupled with water jetcavitation or mechanical stirring were performed in a 6-L bench-scale reactor (Fig. 2). The reactor is a glass vessel with an integratedcooling water jacket. The temperature of the solution was main-tained at 30 ± 3 �C throughout all runs. The visual surface area im-mersed in solution for all electrode materials was 28 cm2. Aconstant working electrode potential was supplied by a direct cur-rent power source (Model WYJ, Shanghai, China) during electro-chemical experiments.

The jet flow loop (Fig. 2) was driven by a high-pressure, self-priming stainless steel pump (Model 25FDB-65D, Shanghai QiquanPump Co., Ltd., China), with a unidirectional piston and a motorwith an electric power rating of 0.75 kw with a speed of2900 rpm. The suction side of the pump is submerged underwater.The discharge from the pump branches into the main and bypasslines. The bypass line has throttling valve (5) for adjusting the pres-

N N

NaO3S SO3Na

OH NH C

N

N

C

C

N

Cl

Cl

Fig. 1. Chemical structure of X-3B.

sure and flow rate into the main line. A pressure gauge (7) is pro-vided in the main line to measure the pressure. A venturi tubeacting as the producer of cavitation is equipped at the terminationof the main line and is submerged below the liquid level. The insidediameter of the delivery line of the closed loop system is 15 mm.The venturi tube is made of organic glass for visual observationof the movement of the fluid. The diameter of the throat of the ven-turi tube is 2 mm and the detailed information is shown in Fig. 2b.Venturi tube and 2 electrodes with 8 cm spacing were accommo-dated in the reactor.

2.3. Experimental procedure

Four and half liters of aqueous solution containing X-3B concen-trations ranging from 40 to 120 mg/L were prepared with appro-priate electrolyte in deionized water and used in eachexperiment. Samples were collected at intervals of 10 min for spec-trophotometric analyses in the UV–visible region. Control experi-ments included water jet alone and electrolysis undermechanical stirring.

In the study of water jet cavitaiton, control valve (6) was leftopen and valve (5) was totally closed to obtain the highest pressureand flow rate in the main line (Fig. 2a). When the dye solution wasdrawn from the reactor and passed through the venturi tube, thevelocity at the throat of the tube increased due to the suddenreduction in area offered for the flow, resulting in a decrease inthe pressure. If the increase in velocity was sufficient to allowthe local pressure to go below the medium vapor pressure underthe operating conditions, cavities were formed. When the solutionwas discharged from the throat, the velocity kept changing due tothe increase in the area of cross-section, giving rise to pressurefluctuations which controlled the different stages of cavitation,

Fig. 3. Changes in the UV–vis spectra of X-3B during electrolysis coupled withwater jet cavitation (initial pH = 6.35, X-3B concentration was 80 mg/L, supportingelectrolyte was 7 mmol/L Na2SO4, and Ti–IrO2 was used as anode and graphite ascathode).

X. Wang et al. / Ultrasonics Sonochemistry 17 (2010) 515–520 517

namely, formation, growth and collapse [18]. The inlet pressure ofthe main line was 0.34 MPa measured by the pressure gauge (7).The cavitation number Cv of this study was calculated to be 0.41according to the following equation [28]:

Cv ¼p2 � pv

12 qv2

0

ð1Þ

where p2 is the fully recovered downstream pressure, pv is the vaporpressure of liquid at the operating temperature, q is the liquid den-sity, and v0 is the velocity of the liquid at the throat of the constric-tion which can be estimated from the knowledge of the flow ratethrough the main line and the geometry of the cavitation tube.

2.4. Effect of initial X-3B concentration and pH

To determine the concentration effect, a series of solutions withX-3B concentrations ranging from 40 mg/L to 120 mg/L was testedunder the combined process where initial pH was 6.35, supportingelectrolyte was 7 mmol/L Na2SO4, and Ti–IrO2 was used as anodeand graphite as cathode. In the pH effect experiment, solution pHwas adjusted between 1.06 and 11.52 using minimum quantitiesof concentrated H2SO4 or NaOH solutions. The X-3B concentrationwas 80 mg/L and the supporting electrolyte and electrode pairwere the same as before.

2.5. Effect of electrode materials and inorganic anions

Three groups of electrode were tested: (1) Ti–IrO2 as anode andgraphite as cathode; (2) Ti–IrO2 as anode and stainless steel ascathode; (3) graphite as anode and stainless steel as cathode. Theinitial pH and concentration of X-3B was 6.37 and 80 mg/L, respec-tively. The supporting electrolyte employed was 7 mmol/L Na2SO4.The effect of the anions needs to be considered because real indus-trial effluents contain various inorganic salts. In order to simulatethese conditions, Na2SO4, NaCl, NaNO3 and Na2CO3 were used assupporting electrolytes and added separately to the dye solution.The concentrations of these salts were 7 mmol/L. The initial con-centration of X-3B was 80 mg/L and the initial pH was 6.35. Ti–IrO2 and graphite was used as anode and cathode, respectively.

2.6. Analytical method

The absorbance spectra of the untreated and treated sampleswere scanned using UV–vis Spectrophotometry (UV-2102, Unico,USA). The color of X-3B solution was referenced as the maximumabsorbance measured at the band of 538 nm. Absorbance measure-ments were performed with a 1 cm optical path length quartz cell.Prior to injection into the cell, the samples were passed through0.45 lm Millipore filters. All experiments were carried out at leastin duplicate. Percentage of the dye removal was calculated asfollows:

Removal ð%Þ ¼ ðAbsi � Absf ÞAbsi

� 100 ð2Þ

where Absi and Absf are the absorbance at 538 nm of the pre- andpost-treated solution, respectively.

3. Results and discussion

3.1. UV–vis spectra of X-3B solution

The UV–vis spectra of X-3B is characterized by four principleabsorptions (Fig. 3): one at the visible band (513 and 538 nm) cor-responding to the whole conjugated structure, which is caused bythe p–p* transition of electrons in the azo-group connecting phe-

nyl and naphthyl; second in near UV region (331 nm) relevant tothe naphthalene ring; and the other two at short UV range (285and 235 nm) resulting from the unsaturated structure of triazineand benzene rings, respectively [29].

Upon the treatment by electrolysis coupled with water jet cav-itation, the absorption at 538 nm declined over time (Fig. 3), indi-cating that the azo-groups were attacked and N = N bonds wereopened, which resulted in the destruction of long conjugated p sys-tem and decolorization of the dyestuff. Since naphthalene ringswere more difficult to destroy than the N = N bonds, the UV bandat 331 nm disappeared more slowly than the visible band538 nm (Fig. 3). The absorption intensity at 285 nm changed little,showing that the triazine structures may not be destroyed by thecoupling process. The peak at 235 nm gradually increased withincreasing reaction time, which is attributed to the breaking downof the naphthalene rings and transforming them to multi-substi-tuted benzene. All of these observations indicated that the com-bined process of electrolysis and water jet cavitation cantransform X-3B molecules to other smaller organic substancesrather than mineralize them to CO2 and water.

3.2. Effect of water jet cavitation on the X-3B electrolysis

The current intensity increased slightly within initial 2 min byelectrolysis under mechanical stirring and leveled off afterwards.However, coupling electrolysis with water jet cavitation signifi-cantly elevated the current intensity (Fig. 4), indicating that waterjet cavitation can not only enhance the mass transfer behaving asmechanical stirring, but also reduce the diffusion layer thicknessand keep the surface of the electrode fresh. As X-3B degradationproceeded, more and more intermediates were produced duringthe combined treatments, resulting in increase in the current den-sity (Fig. 4).

Water jet cavitaiton alone showed a limited capacity for remov-ing X-3B. For example, only 5% of X-3B degraded within 90 min(Fig. 5). However, coupling water jet cavitation with electrolysissignificantly increased X-3B degradation and the removal effi-ciency was larger than the simple linear combination of the waterjet cavitation and electrolysis (Fig. 5). Obviously, a synergistic ef-fect occurred during the combined process. This is probably dueto a decrease in the diffusion layer thickness and consequentlyan increase in the current density induced by water jet cavitation

10

15

20

25

30

35

40

0 5 10 15 20 25 30Time (min)

Cur

rent

den

sity

(m

A/c

m2 )

Fig. 4. Effect of water jet cavitation on current density (initial pH = 6. 35, X-3Bconcentration was 80 mg/L, supporting electrolyte was 7 mmol/L Na2SO4, and Ti–IrO2 was used as anode and graphite as cathode) � electrolysis coupled with waterjet cavitation; j electrolysis coupled with mechanical stirring.

0

10

20

30

40

0 10 20 30 40 50 60 70 80 90Time (min)

Rem

oval

eff

icie

ncy

(%)

Fig. 5. Effect of water jet cavitation on the X-3B electrolytic removal (initial pH = 6.35, X-3B concentration was 80 mg/L, supporting electrolyte was 7 mmol/L Na2SO4,and Ti–IrO2 was used as anode and graphite as cathode). � water jet cavitaitonalone; j electrolysis coupled with mechanical stirring; d simple linear combina-tion of � and j; N simultaneous application of electrolysis and water jet cavitation.

518 X. Wang et al. / Ultrasonics Sonochemistry 17 (2010) 515–520

(Fig. 4). The compression and rarefaction cycles in a water jet sys-tem induced the growth and implosion of cavitation bubbles whichgenerated high-pressure/temperature hot spots and in turn re-duced the diffusion layer thickness [30]. Thus, the transport ofthe dye in solution was facilitated, resulting in increased removalefficiency over conventional standard electrochemical cell. The re-moval efficiency of X-3B in the combined treatment increased upto 36%, much higher than 21% in electrolysis alone and 5% in waterjet cavitation alone (Fig. 5).

3.3. Effect of initial X-3B concentration

Many studies indicated that electrochemical degradation of or-ganic matter is fitted with first-order reaction [24,25]. So, we as-sumed that electrolysis of X-3B with mechanical stirring or withwater jet cavitation in this study also followed pseudo-first-orderkinetics. Table 1 shows their pseudo-first-order rate constantsand the enhancement factor. The rate constants of electrolysis with

stirring, k0, decreased with increasing initial dye concentration,whereas an increase was observed for the constants k00 in the elec-trolysis with water jet cavitation. The combined process of elec-trolysis and jet cavitation enhanced the electrolytic degradationof X-3B under mechanical stirring by a factor of 1.03–1.67(Table 1).

The decrease of k0 with increasing concentration was probablydue to the increasing coverage of the active sites on the electrodesby the dye and its intermediates. Water jet cavitation may allowfor a decrease of the diffusion layer, allowing for continuous clean-ing and activation of the electrode surface and formation of ions,radicals and other high energy intermediates, resulting in an in-crease in the rate constant k00 with initial concentration. It shouldbe noted that the enhancement factor E was increased with an in-crease of dye initial concentration. At the lower concentration of40 mg/L, both rate constants k0 and k00 were similar (Table 1). Themass transfer induced by mechanical stirring was comparable withthat by jet cavitation; and electrochemical passivation may not beformed on the electrodes due to the low concentration. From thispoint of view, it was estimated that the rate constant of electrolysiscoupled with jet cavitation k00 would increase to a maximum valuewith increasing the dye initial concentration due to the finite acti-vate sites on the electrode surface.

We concluded that electrolysis coupled with water jet cavita-tion is more effective in degrading X-3B at higher concentrationsthan at lower concentrations. This conclusion disagreed with theobservation of Farooq et al. [30], which probably can be explainedin that mass transfer of the metal ions from the aqueous to the so-lid phase of the electrode in the Farooq study may be the primarylimiting factor for dilute solution.

3.4. Effect of pH

In the range of pH between 3.11 and 9.60, the removal effi-ciency of X-3B decreased with an increase of pH value in spite ofits increase with treatment time (Fig. 6). As pH increased to11.12 and 11.52, the removal efficiency rapidly increased within10 min and then decreased and showed little change after30 min. When the initial pH was as low as 1.06, the removal effi-ciency significantly increased and quickly reached 70% after30 min. The strong acidic condition value favored the degradationof X-3B. Solution pH affects the protonation/deprotonation of someof the basic sites present in the dye or the formation and stabilityof active intermediates responsible for the decomposition [31]. Inalkaline medium of pH greater than 11, the electrolysis of water(Eq. (3)) was the possible main competitive reaction for dye degra-dation at Ti–IrO2 anode [32]. Hydroxyl free radical is a very strongoxidant and can oxidize non-selectively a lot of chemicals. Thus,the initial degradation rate of X-3B was higher and the removal ra-tio reached to >35% within 10 min. As reaction proceeded, the aro-matic radicals formed earlier could couple to yield polymericmaterials, which made the dye removal efficiency decrease andthe color of solution stronger.

H2O! �OHþHþ þ e� ð3Þ

3.5. Effect of electrode materials

The effect of electrode materials on the degradation of X-3B in-duced by the combined electrolysis and water jet cavitation isshown in Fig. 7. The removal efficiency of X-3B was highest withTi–IrO2 as anode and graphite as cathode, decreased by 26–35%when the cathode was changed to stainless steel with the same an-ode, and by 38–46% when graphite and stainless steel were se-lected as anode and cathode, respectively. The Ti–IrO2 as anode

Table 1Effect of initial concentration of X-3B on the electrolysis with mechanical stirring or water jet cavitation (initial pH = 6. 35; supporting electrolyte was 7 mmol/L Na2SO4; Ti–IrO2

was used as anode and graphite as cathode; treatment time was 90 min).

Initial concn. C0 (mg/L) With stirring With water jet cavitation Ea

Rate constant k’ � 103 (min�1) Correlation coefficient R Rate constant k’’ � 103 (min�1) Correlation coefficient R

40 2.59 0.9997 2.68 0.9986 1.0360 2.26 0.9985 2.95 0.9950 1.3180 2.19 0.9995 3.02 0.9953 1.38100 1.93 0.9990 3.03 0.9974 1.57120 1.90 0.9996 3.18 0.9972 1.67

a Enhancement factor = k00/k0 .

0

10

20

30

40

50

60

70

80

0 10 20 30 40 50 60 70 80 90

Time (min)

Rem

oval

eff

icie

ncy

(%)

Fig. 6. Effect of initial pH on the removal of X-3B (initial X-3B concentration was80 mg/L, supporting electrolyte was 7 mmol/L Na2SO4, and Ti–IrO2 was used asanode and graphite as cathode). � pH = 1.06; j pH = 3.11; N pH = 6.35; d

pH = 9.60; � pH = 11.12; s pH = 11.52.

0

5

10

15

20

25

30

35

40

0 10 20 30 40 50 60 70 80 90

Time (min)

Rem

oval

Eff

icie

ncy

(%)

Fig. 7. Effect of electrode materials on the removal of X-3B (initial pH = 6. 35, X-3Bconcentration was 80 mg/L, supporting electrolyte was 7 mmol/L Na2SO4). � Ti–IrO2

as anode and graphite as cathode; j Ti–IrO2 as anode and stainless steel as cathode;N graphite as anode and stainless steel as cathode.

-20

0

20

40

60

80

100

0 10 20 30 40 50 60 70 80 90Time (min)

Rem

oval

eff

icie

ncy

(%)

Fig. 8. Effect of inorganic anions on the removal of X-3B (initial pH = 6.35, X-3Bconcentration was 80 mg/L, and Ti–IrO2 was used as anode and graphite ascathode). � Na2SO4; j NaCl; N NaNO3; d Na2CO3.

X. Wang et al. / Ultrasonics Sonochemistry 17 (2010) 515–520 519

had a stronger oxidation compared to graphite under the sameconditions of electrolysis. When Ti–IrO2 was used as anode, thedegradation rate for graphite cathode was much higher than thatfor stainless steel applied as cathode. This may be attributed tomore electrochemical production of hydrogen peroxide by usinggraphite cathode than that by using stainless steel cathode [33].

3.6. Effect of inorganic anions

The addition of SO2�4 and NO�3 ions to the solution had a similar

effect on the degradation of X-3B, with more than 35% of X-3Bbeing removed after 90 min (Fig. 8). When Cl� was introduced tosolution, a marked improvement in the removal efficiency was ob-served (Fig. 8). The improvement may result from the competitivereaction proceeding at the anode as follow:

2Cl� ! Cl2 þ 2e� ð4Þ

The generated chlorine can be converted quickly into hypochloritein solution:

Cl2 þH2O! ClO� þ Cl� þ 2Hþ ð5Þ

The generated hypochlorite in Eq. (5) can enhance the removal effi-ciency of X-3B, while the chloride ion can be recycled and used asthe reactant in Eq. (4). Therefore, the consumption of the chlorideions is not high and the degradation rate of X-3B was consistentlyelevated. After 90-min treatment, the removal efficiency of X-3Bwas increased up to 84% (Fig. 8).

In the case of Na2CO3 addition, bicarbonate ion would be gener-ated in solution due to the hydrolysis of CO2�

3 . CO2�3 and HCO�3 are

potential competitive reagents for �OH as follows [34,35],

�OHþ CO2�3 ! OH� þ CO�3 � ð6Þ

�OHþHCO�3 ! H2Oþ CO�3 � ð7Þ

Thus, the degradation in the dye solution was inhibited, showing anegative removal efficiency (the solution getting stronger color) inthe first 30 min (Fig. 8). This phenomenon indicated that polymeric

520 X. Wang et al. / Ultrasonics Sonochemistry 17 (2010) 515–520

materials may be formed by the combination of dye intermediateswith aromatic radicals or with inorganic radicals, which may be in-duced by CO��3 generated in Eqs. (6) and (7). After 30 min, the colorof solution began to decline.

4. Conclusion

Water jet cavitation process significantly increased the currentdensity during electrochemical degradation of X-3B. Coupling elec-trolysis with water jet cavitation enhanced the X-3B degradationefficiency and showed a synergistic effect, compared to individualwater jet cavitation or electrolysis. The combined treatment trans-formed the N = N cleavage and naphthalene ring into multi-substi-tuted benzene.

The observed first-order rate constant of X-3B degradation in-creased with the increasing initial dye concentration, indicatingthat the combined process is more efficient for high initial concen-tration of X-3B. The decrease in initial pH of solution favored thedecolorization of the reactive dye. The use of Ti–IrO2 as anodeand graphite as cathode showed best performance in degradingX-3B in comparison with other electrodes (Ti–IrO2 as anode andstainless steel as cathode; graphite as anode and stainless steelas cathode). Addition of SO2�

4 , NO�3 , and especially Cl� ions intosolution significantly enhanced the degradation. However, CO2�

3

inhibited the dye decolorization.Both electrochemistry and water jet cavitation are clean water

treatment technologies. Water jet cavitation possesses the advan-tages of easier operation and maintenance, lower cost, higher en-ergy efficiency, and being more suitable for large-scaleapplication compared with acoustic cavitaiton [36]. In this study,the combined treatment of electrochemistry and water jet cavita-tion on the solution of X-3B with a large volume of 4.5 L showeda significant enhancement of electrolytic efficiency. This suggestsa possibility of electrochemistry coupled with water jet cavitationfor wastewater treatment at a practical full scale, which can beaccomplished by installing a venturi tube at the terminal of thedrainpipe into the electrochemical cell. By improving the velocityof effluent, water jet cavitation will take place and synergistic ef-fect occurs. Thus, the electrochemical degradation of pollutantswould be enhanced.

Acknowledgments

This work was partly supported by National Natural ScienceFoundation of China (No. 50878126). The authors gratefullyacknowledge Professor Xinde Cao for helpful discussion.

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