Ultrasonics Sonochemistry 20 (2013) 1442–1449

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Ultrasonics Sonochemistry

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Optimization of a heterogeneous catalytic hydrodynamic cavitationreactor performance in decolorization of Rhodamine B: Applicationof scrap iron sheets

1350-4177/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.ultsonch.2013.04.013

⇑ Corresponding author. Address: Department of Applied Chemistry, Faculty ofChemistry, Bu-Ali Sina University, Shahid Fahmideh St., Hamedan 65178, Iran. Tel.:+98 8118282808; fax: +98 8118257407.

E-mail address: zonouzian@basu.ac.ir (S.A. Ebrahimzadeh Zonouzian).

Jalal Basiri Parsa, Seyyed Alireza Ebrahimzadeh Zonouzian ⇑Department of Applied Chemistry, Faculty of Chemistry, Bu-Ali Sina University, Hamedan, Iran

a r t i c l e i n f o

Article history:Received 19 January 2013Received in revised form 23 April 2013Accepted 24 April 2013Available online 9 May 2013

Keywords:Hydrodynamic cavitationOrifice platesBack-pressureHeterogeneous catalystScrap iron sheetsRhodamine B

a b s t r a c t

A low pressure pilot scale hydrodynamic cavitation (HC) reactor with 30 L volume, using fixed scrap ironsheets, as the heterogeneous catalyst, with no external source of H2O2 was devised to investigate theeffects of operating parameters of the HC reactor performance. In situ generation of Fenton reagents sug-gested an induced advanced Fenton process (IAFP) to explain the enhancing effect of the used catalyst inthe HC process. The reactor optimization was done based upon the extent of decolorization (ED) of aque-ous solution of Rhodamine B (RhB). To have a perfect study on the pertinent parameters of the heteroge-neous catalyzed HC reactor, the following cases as, the effects of scrap iron sheets, inlet pressure (2.4–5.8 bar), the distance between orifice plates and catalyst sheets (submerged and inline located orificeplates), back-pressure (2–6 bar), orifice plates type (4 various orifice plates), pH (2–10) and initial RhBconcentration (2–14 mg L�1) have been investigated. The results showed that the highest cavitationalyield can be obtained at pH 3 and initial dye concentration of 10 mg L�1. Also, an increase in the inletpressure would lead to an increase in the ED. In addition, it was found that using the deeper holes (thickerorifice plates) would lead to lower ED, and holes with larger diameter would lead to the higher ED in thesame cross-sectional area, but in the same holes’ diameters, higher cross-sectional area leads to the lowerED. The submerged operation mode showed a greater cavitational effects rather than the inline mode. Also,for the inline mode, the optimum value of 3 bar was obtained for the back-pressure condition in the sys-tem. Moreover, according to the analysis of changes in the UV–Vis spectra of RhB, both degradation ofRhB chromophore structure and N-deethylation were occurred during the catalyzed HC process.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Nowadays, increasing economic constraints and stronger envi-ronmental regulations have caused the improvement of AdvancedOxidation Processes (AOPs) as effective techniques for degradationof complex organic contaminants. Introducing the individual orcombined AOPs such as cavitational based AOPs, photochemicallyinduced AOPs, Fenton and photo-Fenton processes, and etc. [1–4]are the result of taken efforts to achieve both environmental andeconomic aspects of treatment of industrial wastewaterstechnology.

One of the AOP techniques is cavitation that is the phenomenaof generation, growing and collapse of microbubbles (cavities) andlocally release of a large amount of energy through milliseconds[1,5]. Hydrodynamic cavitation (HC) that is of concern here, is re-

sulted from the pressure variation caused by the pass of liquidthrough a constriction [6]. Extremely rapid collapse of cavities,which is assumed to be done adiabatically, produces a largeamount of localized energy. This phenomenon causes the genera-tion of hot spots with temperature and pressure in scale of fewthousand Kelvins and atmospheres, in overall ambient condition[1]. If water was used as the bulk media, this severe conditionwould lead to the fragmentation of trapped water molecules inthe cavities and release of free HO� [6,7].

Among the techniques for producing cavitation phenomena,which are Acoustic cavitation, Hydrodynamic cavitation, Optic cav-itation and Particle cavitation, HC has the largest active zone andenergy efficiency, and also, the constrictions; such as orifice plates,can be made easily [8,9]. Such advantages of HC have introduced itas an efficient industrial method in wastewater treatment [5,9],water disinfection [10,11], synthesis of biodiesel [12] and prepara-tion of nanostructured particles [13].

The main purpose of the current study is to investigate the ef-fect of diverse operational and designing parameters on the effi-ciency of a pilot scale heterogeneous catalyzed HC reactor. Scrap

Nomenclature

C RhB concentration (mg L�1)Ci the initial dye concentration (mg L�1)Ct the dye instantaneous concentration (mg L�1)Cv cavitation numberF the flow rates across orifice platesP2 fully recovered pressure after constriction or back-pres-

sure (Pa)Pv vapor pressure of the liquid at ambient temperature

(Pa)Q flow rate in (m3 s�1)

t time (s)U liquid velocity through the throat of the constriction

(m s�1)V total volume of RhB solution (L)Y cavitational yield in (mg MJ�1)

Greek symbolsD differenceq liquid density (kg m�1)

J. Basiri Parsa, S.A. Ebrahimzadeh Zonouzian / Ultrasonics Sonochemistry 20 (2013) 1442–1449 1443

iron sheets (SISs) were chosen as heterogeneous catalyst and themeasurement of Rhodamine B (RhB) decolorization was utilizedto show the effects of the parameters. It should be mentioned thatthe combination of the HC and the individual H2O2, Fenton, and ad-vanced Fenton processes increases the extent of decolorization(ED) of RhB [14–17], and enhances the treatment process of indus-trial wastewater effluents [18]. On the other hand, these intensemethods mask the effects of the other parameters; in this regard,using external H2O2 has been deliberately omitted, and the net ef-fect of each operational parameter on heterogeneous catalyzed HCprocess can be shown more obviously.

Utilizing inexpensive and simple techniques and the applicationof SISs as a cost effective catalyst to improve the performance of HCmake this research unique. Initial dye concentration, pH, inlet andback-pressures and characteristics of orifice plates were selected asthe operational parameters. It should be mentioned that the SISswere used in both conditions of close to and far from the locationof orifice plates.

V-24 V-23

E-7

E-8

P-119

P-110

P-121

P-123 P-120

P-124P-115

P-127

V-22

8

7

9

10

11

Tap Water

P-116

P-122

V-25

V-26

P-129P-111P-112

4

Waste

6

Drain2

P-117

1

5

(a)

8 cm

14

P-114

3

V-21

20 cmP-132

Fig. 1. Schematic representation of experimental setup of

2. Materials and methods

2.1. Materials

Rhodamine B (C28H31ClN2O3) with the molecular weight of479.02 g mol�1, analytical grade of N,N-diethyl-p-phenylenedi-amine (DPD), sulfuric acid and Sodium Hydroxide were obtainedfrom the Merck company. All chemicals were utilized withoutany further purification. Deionized water was used to prepare allsolutions. Scrap iron sheets have been cut from a main single sheet,in desired sizes.

2.2. HC reactor configuration

According to the location of the applied orifice plates and dis-tance between them and SISs, the experimental setup had twooperation modes of submerged mode and inline mode, which areshown in Fig. 1a and b. As the figure shows, the apparatus was

P-143

V-27

V-281112

13

(b)

Transparent

Transparent

V-31 V-30

E-9

E-10

P-148

P-149

P-157

P-151 P-154

P-153P-147

P-159

V-32

8

7

9

10

V-33

V-34

P-158P-160P-150

4

Waste

6

2

P-156

1

5

14

3P-152

P-161Tap Water

DrainV-35

HC reactor; (a) Submerged mode and (b) Inline mode.

Table 1The characteristics of multiple-hole orifice plates.

Orifice plate (A) (B) (C) (D)

Number of holes 32 32 16 64Diameter of holes (mm) 1 1 2 1Thickness of orifice plates (mm) 1 2 1 1Total cross-sectional area (mm2) 25.1 25.1 50.2 50.2Total cross-sectional perimeter (mm) 100.5 100.5 100.5 200.9

1444 J. Basiri Parsa, S.A. Ebrahimzadeh Zonouzian / Ultrasonics Sonochemistry 20 (2013) 1442–1449

consisted of the following parts: (1) holding tank, (2) cooling watercontrol valve, (3) cooling steel coils, (4) ball valves, (5) centrifugalpump (2900 rpm, 5.5 kW, TYPE 3 LM/A 32-200/5.5, EBARA, Italy),(6) strainer, (7) bypass diaphragm valve, (8) Inlet pressure control-ling diaphragm valves, (9) rotameters, (10) inlet pressure measur-ing gauges, (11) multiple-hole orifice plates accommodated withflanges, (12) back-pressure measuring gauges, (13) back-pressurecontrolling diaphragm valves, (14) 4 SISs pieces with dimensionsof 1.5 cm � 5 cm � � 0.1 cm (20 g). SISs were used after properwashing with no more surface preparation and new SISs were em-ployed for each test. The properties of the orifice plates have beenintroduced in Table 1. Fig. 2 shows a schematic view of the onetype of orifice plates as an example. All used orifice plates weresharp edge and were prepared by a high speed drill. Drillingswas carried out from the exit faces to avoid burrs at entry of theorifices [19]. The orifice plates, flanges, screws and cooling coilswere made of stainless steel (grade 316). Holding tank, pipeline,valves and their fittings were made of unplasticized poly vinylchloride (UPVC). To observe the behavior of flow after orifice platesin the inline mode, transparent UPVC pipes were used.

2.3. Analysis

UV–Vis Spectrophotometer (JAS.CO V-630) was used to analyzeRhB aqueous solutions and H2O2 (DPD method) liberation at kmaxs

of 554 nm and 551 nm, respectively. After proper dilution of sam-ples, the FerroVer Iron reagent was used to determine the total dis-solved iron concentration by DR-2800 Spectrophotometer (Hach).A DENVER INSTRUMENT UB10 model pH meter was used to mea-sure the pH of solutions. Corresponding calibration curves wereobtained using the standard RhB solutions. DPD method [20] wasutilized to measure the H2O2 concentration.

2.4. Experimental method

Considering the widely reports for the application of RhB decol-orization as a model reaction in the case of AOPs [21–23], this xan-

Fig. 2. Schematic representation of char

thene dye was chosen as the target molecule. (1) shows the extentof decolorization (ED) percentage:

ED ð%Þ ¼ 100� ðCi � CtÞCi

ð1Þ

where Ci and Ct are initial and instant concentration of RhB duringthe process, respectively. Each experimental run was repeated atleast two times; hence, each data point represents the mean valueof the ED with a standard deviation of 0.31–2.60% and a mean devi-ation value of 1.07%.

In addition, the effect of using SISs, as the heterogeneous cata-lyst, on designed HC reactor was investigated. Two experimentswere conducted, in the absence of RhB, to confirm the generationof H2O2 and dissolved iron within the HC process. It was shownthat they were generated during the process; therefore, the in-duced advanced Fenton process (IAFP) was proposed as the mech-anism of the process. In addition, the effects of operatingparameters on efficiency of designed HC reactor were investigated.

In each run, the holding tank was filled with 30 L of the aqueoussolution containing desired amount of RhB. The pump was turnedon to recycle the solution through bypass line for 5 min to elimi-nate any unknown effects of turbulence circulation. The requiredamounts of inlet pressure and back-pressure across the orificeplates were adjusted by the main and bypass controlling valves.The solutions were passed through the orifice plates up to fourhours and samples were collected at regular time intervals.

All of the influencing parameters were investigated in constanttemperature of 25 ± 2 �C. The initial pH was justified by H2SO4 andNaOH solutions and the temperature was controlled via adjust-ment of the cooling water flow rate.

3. Result and discussion

3.1. Catalytic effect of the SISs on performance of the HC reactor

3.1.1. Effect of the SISsFig. 3 shows the ED of RhB by both sole HC and catalyzed HC

process using SISs. The experiments were done using 30 L of5 mg L�1 solution of RhB at pH 3 during 120 min. The orifice platetype (C) had the maximum flow rate; thus, it could obtain the max-imum number of cycles among the others. Therefore, it was used asthe constriction in this step of experiments. Also, since maximumcavitational effects of HC has been reported at the pH value of 3[24–26], and the inlet pressure of about 5 bar [5,14,27], the pH 3and the maximum pump power of 5.3 bar, which could reach theflow rate of 68 L min�1, were chosen as the presupposed propercondition to investigate the effect of SISs. According to the Fig. 3,

acteristics of orifice plate type (A).

0

5

10

15

20

25

30

35

0 20 40 60 80 100 120

ED

(%

)

Time (min)

HC/SISs

HC

Fig. 3. Effect of SISs presence on decolorization of RhB.

J. Basiri Parsa, S.A. Ebrahimzadeh Zonouzian / Ultrasonics Sonochemistry 20 (2013) 1442–1449 1445

it can be found that sole cavitation has a low ED of 10% after120 min; but when SISs was used, the ED considerably enhancedto 30%.

It is reported that either sole Fe(0) or sole hydrogen peroxidehas a negligible effect on the decolorization of RhB [17]. Thus,based on in situ generation of H2O2 during the cavitation processes(Eqs. (2) and (3)) [9,28–30], the obtained ED for SISs, shown in theFig. 3, is due to the induced advance Fenton process. The IAFP reac-tions are shown in Eqs. (4)–(7) [31].

H2O�!HCH� þHO� ð2Þ

2HO� ! H2O2 ð3Þ

Fe0 þ 2Hþ ! Fe2þ þH2 ð4Þ

Fe2þ þH2O2 ! Fe3þ þHO� þHO� ð5Þ

Fe3þ þH2O2 ! ½Fe� OOH�2þ þHþ ð6Þ

½Fe� OOH�2þ �!HCFe2þ þHO�2 ð7Þ

3.1.2. In situ generation of Fenton reagentsSome experiments were performed to measure Fenton reagents

(H2O2 and total dissolved iron), which are generated within the HCreactor.

Bader’s reported DPD method [20] was used to measure andprove the H2O2 generation in the system via the HC process. Toevaluate H2O2 generation, 30 L deionized water containing noadditive was passed through the orifice plates type (C) with the in-

0

2

4

6

8

10

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0 20 40 60 80 100 120

Time (min)

Hydrogen peroxide

Total Dissolved Iron

Tot

al I

ron

(mg

L-1

)

Fig. 4. The concentration of hydrogen peroxide and total dissolved iron.

let pressure of 5.3 bar for 120 min of the process. As it is shown inFig. 4, the H2O2 generation showed an intense increasing trendduring the first 20 min of the process time, and after that, theincreasing trend became slighter. The intense increasing trendcan be explained due to the generation of H2O2 according to theEqs. (2) and (3) [9,28–30]. The slight trend is due to the fact thatat the high concentrations of H2O2, the self-scavenging effect (Eq.(8)) and pyrolysis, at the hot spotted shell region (Eq. (9)), lowerthe generation of H2O2 [32].

HO� þH2O2 ! H2OþHO�2 ð8Þ

H2O2 �!Pyrolysis

product ð9Þ

In addition, to measure the total contents of dissolved iron, theholding tank was filled by 30 L of fresh deionized water and the pHwas adjusted at 3 by appropriate amount of dilute H2SO4 solution.The pump had switched on, and the first sample was taken at zerotime of the reaction. After proper dilution of taken samples, the to-tal contents of dissolved iron were analyzed using FerroVer Iron re-agent (Hach). As it is observable in the Fig. 4, the amount of thetotal dissolved iron has been increased continuously. It is due tothe continuous corrosion of SISs in presence of water and H2O2,acidic media (Eqs. (4)–(7)) [33,34].

3.2. Effect of the inlet pressure

Effect of the inlet (upstream) pressure, as a physical operationalparameter, was investigated in 5 mg L�1 of the initial dye concen-tration using a couple of orifice plates type (A) via the submergedmode. The orifice plate type (A) was chosen, because, due to itslower total flow area rather than that of the orifice plates type(C) and (D), the obtained inlet pressure was higher; therefore,the variations of parameters such as flow rate and ED were moretangible and easy to investigate. Inlet pressures were adjustedwithin the range of 2.4–5.8 bar by means of the bypass-valve,which would results to the flow rates range of 30–40 L min�1.Fig. 5 shows improvement in the ED from 48% to 61% by increasingthe inlet pressure at the same number of cycles 420. Such improve-ment is due to the higher temperature and pressure during the col-lapse of the cavities, which is because of more pressure drop acrossthe constrictions [35]. Theoretical aspects introduce higher volumeof bubbles in the higher inlet pressure because of more intense col-lapses [36,37].

3.3. Effect of the distance between the orifice plates and catalyst sheets

The influence of the HC system operation mode was investi-gated based on the location of orifice plates type (A) and distance

0

10

20

30

40

50

60

0 60 120 180 240 300 360 420

ED

(%

)

Number of Cycles (time*flow-rate/volume)

5.8 bar, 80 L/min

3.6 , 70

2.4 , 60

Fig. 5. Effect of the inlet pressure variation on the ED in the submerged mode.

0

10

20

30

40

50

60

70

80

0 40 80 120 160 200 240

ED

(%

)

Time (min)

Submerged

Inline

Fig. 6. Effect of the operation modes on the ED.

1446 J. Basiri Parsa, S.A. Ebrahimzadeh Zonouzian / Ultrasonics Sonochemistry 20 (2013) 1442–1449

between them and catalyst sheets. The system’s different opera-tion modes were explained before (Section 2.2). For the case of sub-merged mode, the orifice plates were installed in the depth of 8 cmof the solution (Fig. 1a). In this section the experiments were per-formed at the maximum pump discharge capacity. The obtainedinlet pressure for the submerged and inline mode were 5.8 and6.2, respectively; this difference was due to the longer flow passwithin the pipes for the submerged mode; however, it did not influ-ence the flow rates (FSubmerged = FInline).

As it can be observed from Fig. 6, the obtained ED for the treat-ment of 5 mg L�1 the initial dye concentration for submerged andinline mode are 73% and 39%, respectively. It is attributed to thefact that in the submerged mode cavitating jets are close to the SISs;therefore, collapsing microbubbles at the reactive sites on the cat-alyst surface could clean the surface of ZIVs continuously by theimpingement of the cavitating jets that means more available Fen-ton active zones [38], which would facilitate transformation of ZVIto ZVI/iron oxides.

3.4. Effect of the back-pressure

Recovered pressure has an important role in the HC perfor-mance [39–41]; hence, five main experiments were conducted toinvestigate the influence of back-pressure in the inline mode usingorifice plates type (A) to treat 5 mg L�1 of initial dye concentration.To use full capacity of the pump, the bypass-valve was kept closed,and the inlet valves were retained completely opened. The back-pressures were adjusted within the range of 2–6 bar by valve 13(Fig. 1b). Table 2 shows the physical characteristics and the corre-sponding results of the HC system. Besides the main runs, five con-trolling tests were performed to identify the net effect of the back-pressure variation, while the flow rates were equal to the ones ofthe main experiments. In the controlling tests the inlet controllingvalves were kept completely open, and the flow rates were ad-justed with the bypass-valves. Also, back-pressure was kept con-

Table 2Characteristics of the HC system in the inline operation mode.

Characteristic Main experiments

Back-pressure (bar) 2 3 4 5Flow rate (L min�1)a 41 38.5 35 31Inlet pressure (bar) 6.2 6.5 6.9 7.2Cavitation number 0.54 0.93 1.50 2.40ED (%) (after 420 cycles) 33.5 42.5 33.4 28.7

a Flow rate through single orifice plate.

stant at the atmospheric pressure of 1 bar by the completeopening of the back-pressure controlling valves.

As presented in the Table 2, the increase in the back-pressurewould cause to the decrease in the flow rate, slight increase inthe inlet pressure and, consequently, increase in the cavitationnumber. The results were compared for the 420 cycles, which isequal to the number of cycles after 240 min for the lowest flowrate.

According to the Eq. (10) back pressure can influence the cavi-tation number (Cv). Cv is a dimensionless number that shows cav-itation potential of the flow and defined as:

Cv ¼P2 � Pv

1=2qU2 ð10Þ

where P2, Pv, q and U are fully recovered pressure after constriction(back-pressure), vapor pressure of the liquid at ambient tempera-ture, liquid density and liquid velocity through the throat of theconstriction, all in the SI units, respectively. It should be mentionedthat the cavities can be generated at the condition of Cv 6 1; so, thelower cavitation numbers obtain higher cavitational effects ideally[6,10].

As it can be seen in Table 2, for the main experiments, increas-ing the back-pressure has led to a maximum value of 42.5% at3 bar. This behavior can be explained considering the opposite ef-fects of the back-pressure; an increase in the back-pressure en-hances the cavitational effects via growing the both collapsepressure and cavitation active volume [39]. On the other hand, thisincrease would lead to the decrease of the flow rate and pressuredrop which has a negative impact on the cavitational effects andthe ED [35]. With respect to the mentioned results in the Sec-tion 3.2, as it can be expected, in the controlling tests, reductionfrom 6.2 to 1.8 bar in the inlet pressure led to the decline in EDin the range of 28.1–12.5%.

In addition, the higher ED of main experiments rather than thecontrolling tests, is due to the growth of collapse pressure and cav-itation active volume, because of the increase in back-pressure[39].

As can be observed from Table 2, there are cavitational effectseven for the conditions of Cv > 1. It is due to the presence of dis-solved gases or suspended particles in the reaction media[2,10,42]; for instance, separated solid iron particles (in our case),that cause cavitational effects while the Cv is greater than one.

3.5. Effect of the orifice plate types

The applied multiple-hole orifice plates were different in thick-ness (the orifice length or holes depth), holes number and diame-ter, holes total and consequently, cross-sectional area andperimeter (Table 1). Experiments were performed in the submergedmode, at the maximum accessible inlet pressure by application of acouple of same orifice plates. The characteristic of the cavitationflows, cavitation numbers and the obtained EDs for the solutionscontaining 5 mg L�1 of the dye, after 600 cycles have been statedin the Table 3.

Controlling tests

6 1 1 1 1 127 41 38.5 35 31 277.3 6.2 4.5 4 2.7 1.8

3.79 0.27 0.31 0.36 0.46 0.6116.3 28.1 17.3 13.3 13.2 12.5

Table 3Effect of orifice plates type on characteristics of the cavitation flow and ED in thesubmerged mode.

Characteristics Orifice plate types

(A) (B) (C) (D)

Maximum inlet pressure (bar) 5.8 6.5 4.3 4Fully recovered pressure (bar) 1 1 1 1Maximum flow rate (L min�1)a 40 37.5 60 58Cavitating jets velocity (m s�1) 27 25 20 19Cavitation number 0.27 0.31 0.47 0.59ED (%) after 600 cycles 71.1 64.8 65.6 47.5

a Flow rate through a single orifice plate.

Fig. 7. Schematics for the assumed flow region in the orifice plates type (A)(separated flow) and (B) (reattachment flow after vena contracta).

0

10

20

30

40

50

60

70

80

2 3 4 5 6 7 8 9 10

ED

(%

)

pH

Fig. 8. Effect of the initial pH on the ED (%) after 240 min.

J. Basiri Parsa, S.A. Ebrahimzadeh Zonouzian / Ultrasonics Sonochemistry 20 (2013) 1442–1449 1447

3.5.1. Effect of thickness of orifice platesAs can be seen in the Table 1 and Table 3, comparing the orifice

plates type (A) and type (B), it can be found that the thicker plate(plate type (B)), has a higher inlet pressure (i.e. higher pressuredrop) and a lower flow rate. Fig. 7 suggests the flow patterns forthe orifice plates type (A) and (B). The flow pattern for the platetype (A) is separated flow and for the plate type (B) is reattachedflow [19]. It can be seen that the higher inlet pressure of plate type(B) is because of the higher frictional pressure drop, which is re-sulted from the higher thickness, and causes a lower flow rateand, consequently, higher Cv, fewer intense collapses and lowerED [9,39,43,44]. Ramamurthi and Nandakumar have reported aseparated flow region and lower pressure drop for the orifices witha unit length to diameter ratio (i.e. as same as the orifice plate type(A)) and a reattached flow region and higher pressure drop for thehigher ratios of length to diameter of orifices (i.e. orifice plate type(B)) [19].

3.5.2. Effect of the holes’ diameterThe orifice plates type (C) and (D) were compared to study

influence of the holes’ diameter on the cavitational effects whilethe holes’ total cross-sectional areas are equal. As it can be ob-served from the Table 3, the orifice plate type (C) is 18.1% of theED more than that of the orifice plate type (D). Because of the equalholes’ total cross-sectional areas of the orifice plates type (C) and(D), they have approximately equal inlet pressure and flow rate. In-deed, increasing the orifices’ radius in constant total flow area (i.e.existence of fewer holes) can lead to a greater collapse pressure[39,45]. Some reports show similar results for the degradation ofcomplex compounds such as RhB, using holes with higher diame-ters [9,46].

3.5.3. Effect of the total cross-sectional areaTotal cross-sectional area of orifice plates was the last investi-

gated parameter, and for explaining the effect of this parameter ondye treatment, obtained EDs of orifice plates type (A) and (C) wereinvestigated. The higher ED obtained by the orifice plate type (A)rather than that of the orifice plate type (C), for the same numberof solution recycling, can be explained considering the mentionedeffects of pressure drop across orifice plates and cavitation numbers.According to the Table 3, HC process via the orifice plate type (A), re-

sults to the higher pressure drop (4.8 bar) and lower Cv (0.27) thanthose of the other one (4.3 bar and 0.47), which is due to the dou-bling of the total cross-sectional area of orifice plate type (C).

3.6. Effect of initial pH

The obtained ED for the dye concentration of 5 mg L�1 and theinitial pH range within 2–10 is shown in Fig. 8. It is notable thatpH 6 was the original pH of the solutions in the presence of SISs,without any pH adjustment. A couple of orifice plates type (A) wereused for cavitation process in the submerged mode under the inletpressure of 5.8 bar. As it can be seen from the figure, the maximumand minimum value of ED has been obtained at pH 3 and 6, respec-tively, which are in agreement with the other reports [24–26]. Theresults can be attributed to the various effects of pH on hydroxyl rad-ical’s formation via HC and advanced Fenton process. Wang et al.introduced higher hydroxyl radical oxidation potential and easierreaction of the dye due to the protonation of its amine groups inthe acidic media [15,16]. In addition, at the pH < 2.5, H2O2 moleculescan be protonated and form H3O2

+ (Eq. (11)) [26]; thus, ED will be de-creased at the pH 2 as it has been shown in the Fig. 8.

H2O2 þHþ ! H3Oþ2 ð11Þ

3.7. Effect of the initial dye concentration

The initial dye concentration was chosen within the range of 2–14 mg L�1 and the experiments were performed using a couple ofthe orifice plates type (A) in the submerged mode, while the inletpressure was 5.8 bar. As it is observable from Table 4, the efficiencyof process is inversely proportional to the initial dye concentration.As the initial concentration increased from 2 to 14 mg L�1, the EDdecreased from 87% to 34%. Observed inverse proportion is ex-pected; because, the total amount of dye molecules increases whilethe total amount of free hydroxyl radicals remains constant.

Investigation the kinetic of the reaction was another importantstudied parameter. According to the obtained results (Table 4)decolorization follows a pseudo first order kinetic reaction. Whilethe initial concentrations were increased from 2 mg L�1 to14 mg L�1, the rate constants (k) decreased from 9.0 � 10�3 min�1

to 1.6 � 10�3 min�1.Another investigated parameter was cavitational yield that was

calculated considering the total affected mass of RhB per totalhydraulic energy dissipated into the solution for 240 min of theprocess, which is shown as the following [5,43]:

Y ¼ VDCQtDP

ð12Þ

Table 4Summary of first order rate constants and ED (%) for various initial dye concentrations.

Initial dye concentration (mg L�1) 2 5 7 10 12 14

k (min�1) 9.0 � 10�3 5.7 � 10�3 4.3 � 10�3 3.2 � 10�3 2.1 � 10�3 1.6 � 10�3

R2 0.9938 0.9957 0.9922 0.9998 0.9993 0.9933ED (%) after 240 min 87 73 62 54 40 34

0

4

8

12

16

20

0 2 4 6 8 10 12 14 16

Cav

itat

iona

l Yie

ld (

mg

MJ-1

)

Initial Dye Concentration (mg L-1)

Fig. 9. Effect of the RhB initial concentration on the cavitational yield.

1448 J. Basiri Parsa, S.A. Ebrahimzadeh Zonouzian / Ultrasonics Sonochemistry 20 (2013) 1442–1449

where Y, V, C, Q, t and DP are cavitational yield in mg MJ�1, total vol-ume of RhB solution in L, RhB concentration in mg L�1, flow rate inm3 s�1, time in second and pressure drop across the constriction inPascal, respectively. As it can be observed from Fig. 9, the cavita-tional yield has increased by increasing the initial dye concentrationup to 10 mg L�1, and after this point, a decreasing trend can be seen.Since the main decolorization mechanism is IAFP (Section 3.1), itcan be resulted that the increasing trend of cavitational yield isdue to the copious amount of dye molecules; meanwhile, thedecreasing trend is due to the limited amount of generated H2O2

(Fig. 4) and shortage of hydroxyl radicals.

3.8. Spectral changes of RhB solution during heterogeneous catalyzedHC

The analysis of changes in the UV–Vis spectra of RhB is one ofthe useful techniques to obtain some insights about two possiblecompetitive mechanism of degradation of RhB, i. e. degradationof RhB chromophore structure and N-deethylation [14,17,47,48].In this regard, the changes of UV–Vis spectra of 5 mg L�1 solutionof RhB, at pH 3 during 240 min of the process were studied(Fig. 10). As can be seen in the figure, both mentioned mechanisms

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

200 250 300 350 400 450 500 550 600 650 700

Abs

orba

nce

Wave Length (nm)

0 min

60

120

180

240

556

546

Fig. 10. UV–Vis spectral changes of RhB, recorded during the dye decolorization at240 min of HC process.

were occurred. At the first, the decrease of the absorption band ofRhB at k = 556 nm indicates degradation of the xanthene structureof chromophore. Second, a blue shift of the maximum absorptionwavelength (from 556 to 546 nm at 240 min of HC time) can beattributed to the N-deethylation process [14,17,47,48]. Duringthe treatment, two new bands were appeared: a band at 210,which is because of the ring-opening reaction of N-deethylation[17], and a band at 330 nm, which is because of poly-aromaticrings destruction and the production of aromatic rings [14,48].

4. Conclusion

In the present research, an induced advanced Fenton processwas performed in a pilot scale HC reactor. The most important ob-tained results can be summarized as following:

I. Cavitational effects of the constructed HC reactor and itsability to induce the advanced Fenton process, in the pres-ence of scrap iron sheets, without any external source ofhydrogen peroxide, have been confirmed in this work.

II. A three time enhancement in the efficiency was seen as theresult of the catalytic effect of the scrap iron sheets.

III. Solution’s pH had a considerable impact on the reactor per-formance, which the best result was obtained at pH = 3.

IV. Cavitational yield was increased by increasing the dye initialconcentration up to 10 mg L�1 and then was decreased.

V. Higher inlet pressure improved the cavitational effects in theboth operation modes of submerged and inline.

VI. Geometrical characteristics of the orifice plates such asholes’ depth, diameter and number showed a significanteffect on the advanced Fenton process, induced by HC.

VII. In the inline mode, the back-pressure value could affect theHC and the optimum value was at the back-pressure of3 bar.

VIII. Proper location of the catalysts plays an important role inthe improvement of induced advanced Fenton processthrough HC based reaction. The submerged operation mode,with Lower distance between catalyst and orifice plates,showed higher ED.

IX. The best results obtained at pH 3 and the inlet pressure of5.8 bar by using a couple of the orifice plates type (A) inthe submerged mode. Under this condition the HC processshowed 71.1% of the ED for 5 mg L�1 RhB solution through600 cycles.

Acknowledgements

The authors wish to acknowledge the university authorities forproviding the financial support to carry out this work. S.A.E.Zonouzian would like to acknowledge the staff of the ‘‘TehranWater and Wastewater Company’’ and Mr. M. Moghimi for provid-ing valuable experience in the water treatment industry.

J. Basiri Parsa, S.A. Ebrahimzadeh Zonouzian / Ultrasonics Sonochemistry 20 (2013) 1442–1449 1449

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