Decolorization of Acid Orange 7 by an electric field-assisted modified orifice plate hydrodynamic cavitation system: Optimization of operational parameters

Ultrasonics Sonochemistry xxx (2015) xxx–xxx

Contents lists available at ScienceDirect

Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson

Short Communication

Decolorization of Acid Orange 7 by an electric field-assisted modified orifice plate hydrodynamic cavitation system: Optimization of operational parameters Kyung-Won Jung, Dae-Seon Park, Min-Jin Hwang, Kyu-Hong Ahn ⇑ Center for Water Resources Cycle Research, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, South Korea

a r t i c l e

i n f o

Article history: Received 31 October 2014 Received in revised form 10 January 2015 Accepted 25 February 2015 Available online xxxx Keywords: Decolorization Acid Orange 7 Azo dye Hydrodynamic cavitation Electrochemical Response surface methodology

a b s t r a c t In this study, the decolorization of Acid Orange 7 (AO-7) with intensified performance was obtained using hydrodynamic cavitation (HC) combined with an electric field (graphite electrodes). As a preliminary step, various HC systems were compared in terms of decolorization, and, among them, the electric field-assisted modified orifice plate HC (EFM-HC) system exhibited perfect decolorization performance within 40 min of reaction time. Interestingly, when H2O2 was injected into the EFM-HC system as an additional oxidant, the reactor performance gradually decreased as the dosing ratio increased; thus, the remaining experiments were performed without H2O2. Subsequently, an optimization process was conducted using response surface methodology with a Box–Behnken design. The inlet pressure, initial pH, applied voltage, and reaction time were chosen as operational key factors, while decolorization was selected as the response variable. The overall performance revealed that the selected parameters were either slightly interdependent, or had significant interactive effects on the decolorization. In the verification test, complete decolorization was observed under statistically optimized conditions. This study suggests that EFM-HC is a useful method for pretreatment of dye wastewater with positive economic and commercial benefits. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction In recent years, the trend toward improving quality of life has led to the development of new compounds to meet the ever-increasing requirements of human beings. At the same time, the impact of textile industry wastewater on the environment has become the source of wide concern. Reactive dyes (e.g., azo dyes) are versatile and commonly used in the textile and paper industries; however, their toxic components (e.g., heavy metals, phenolic compounds, chlorides) and resistance to biodegradation make them damaging to the environment [1,2]. For these reasons, effluents containing them need to be treated before being discharged into aquatic ecosystems. In the past few years, numerous approaches have been tried to handle targeted pollutants. These have involved the use of a variety of methods including electrochemical, electrocoagulation, ultrasonication, chemical oxidation, membrane processes, and other advanced oxidation techniques [3–6]. Even though lab-scale efforts at decolorization of dye wastewater have been highly successful, these techniques have found ⇑ Corresponding author. Tel.: +82 2 958 5832; fax: +82 2 958 6854. E-mail address: [email protected] (K.-H. Ahn).

less application at pilot and industrial scales due to high cost and due to design constraints that hinder upscaling [7]. Moreover, when sonication is involved, several obstacles hamper the overall removal efficiency of the process. These include limits on contact efficiency between the target pollutant and oxidizing agents, effective performance being achieved only in a relatively small region very near the surface of the cavitation horn, and reduced electrode efficiency due to possible erosion of surface [8]. In the last decade, an alternative technology called hydrodynamic cavitation (HC) has been studied extensively in many fields of wastewater treatment research because this technique is energy efficient, has large potential for application at industrial scale and produces effective degradation of dye wastewater economically [9–12]. In the HC system, cavitation can be generated by pressure variation in a flowing liquid. The pressure variation and the kinetic energy are caused by constrictions (e.g., venturi nozzles or orifice plates) [13]. When the liquid passes through a constriction, it flashes, generating a number of microbubbles (vaporous cavities) when the pressure falls below the vapor pressure of the liquid. Subsequently, the microbubbles collapse due to expansion of liquid jet and pressure recovery [14]. These microbubbles generate localized ‘‘hot spots’’ with transient temperatures on the order of 10,000 K, and

http://dx.doi.org/10.1016/j.ultsonch.2015.02.010 1350-4177/Ó 2015 Elsevier B.V. All rights reserved.

Please cite this article in press as: K.-W. Jung et al., Decolorization of Acid Orange 7 by an electric field-assisted modified orifice plate hydrodynamic cavitation system: Optimization of operational parameters, Ultrason. Sonochem. (2015), http://dx.doi.org/10.1016/j.ultsonch.2015.02.010

2

K.-W. Jung et al. / Ultrasonics Sonochemistry xxx (2015) xxx–xxx

pressures of about 1000 atm. Subjected to these conditions, water molecules are dissociated into OH radicals, and subsequent micro-diffusion exposes organic pollutants to these radicals and decolorizes them [15]. To date, a great deal of work has been focused on using approaches based on combining ultrasonication (acoustic cavitation), chemical (photocatalytic oxidation) and/or electric field (electrochemical); however, the success of these approaches, for the treatment of dye wastewater, has been limited [16–20]. Recently, a novel hybrid process, called as OzonixÒ, has been successfully developed in commercial scale by Gogate et al. [21] using a combination of HC system, sonication, ozone, and electrochemical oxidation/precipitation. It has been reported that the synergistic effect derived from combining those techniques leads to significant enhancement of water treatment ability. In addition, a technique combining an electric field and the HC system (namely, water jet cavitation), was developed by Wang et al. [22]. These researchers found that this combination of different methods enhanced removal efficiency and showed a synergistic effect different from the other methods, used individually. For our research, Acid Orange 7 (AO-7) or Orange II, a typical azo dye, was chosen as a model azo dye because of its wide use in a variety of industries, and because of its stable characteristics with respect to biochemical oxidation. Till date, degradation of AO-7 has been studied by various methods including combined acoustic or HC system with chemical agents [1], catalytic system [23], electrocoagulation [24–26], electrochemical oxidation [27], and electro-Fenton process [28]. In the present paper, we investigate a novel electric field-assisted modified orifice plate HC (EFM-HC) system created by Jung et al. [29] was applied for the degradation of AO-7. The proposed system, based on modifications of both the modified orifice plate HC (M-HC) and EFM-HC systems had the higher decolorization performance of AO-7, when compared with a conventional orifice plate HC (C-HC) and the electric field-assisted conventional orifice plate HC (EFC-HC) systems, respectively. To optimize, statistically and mathematically, the relevant key factors in the EFM-HC system (e.g., inlet pressure, initial pH, applied voltage, and reaction time), response surface methodology (RSM) was employed because it can be used to consider simultaneously, relevant decision factors important for reaction performance. To the best of our knowledge, this was the first attempt to degrade AO-7 using a method combining electric field and HC systems at semi-pilot scale (100 L) and the first to achieve its statistical optimization. 2. Materials and methods 2.1. Materials Acid Orange 7 (AO-7) (C16H11N2NaO4S, C.I. number 15510, MW = 350.32 g mol1) dye was supplied by Tokyo Chemical Industry Co., Ltd. (Japan). Hydrogen peroxide (H2O2, 34.5%) and sodium chloride (NaCl, 99.0%) were obtained from Samchun Pure Chemical (Republic of Korea). A stock solution of AO-7 was prepared in distilled water. The initial pH of the dye solution was adjusted using 1 mol of H2SO4 and NaOH solutions. Deionized distilled water was used throughout this study (Milli Q Plus, Merck Millipore Co., Germany). 2.2. Experimental procedure 2.2.1. Feasibility test of various electric field-assisted hydrodynamic cavitation systems As a preliminary experiment, in order to determine a suitable electrode to assist the HC system for decolorization of AO-7, a comparative study was conducted using stainless and graphite

electrodes of equal size (U10  400; anode and cathode). In order to check the sole potential of each electrode for decolorization of AO-7, we conducted a comparative study without adding NaCl. Later, experiments were performed with NaCl at a concentration of 2.5 g L1 in the solution as the electrolyte, because this is often found in textile industry effluents in a range of 2.0–3.0 g L1 [30]. The initial concentration of AO-7 was fixed at 10 mg/L and the initial pH of the dye solution was controlled (pH 6.5 ± 0.7). The distance between electrodes was fixed at 15 cm. The working volume was 1.0 L and magnetic stirring was maintained at 100 rpm in order to avoid concentration gradients. A constant voltage of 30 V was applied for 300 min using a programmable DC power supply (ODA, Republic of Korea). A sample was collected after every 20 min of reaction time. In order to minimize random errors, three experiments were conducted in parallel and the average values were determined for each set. Subsequently, in order to compare and evaluate the AO-7 decolorization efficiency of various HC systems, we performed a feasibility test using C-HC, M-HC, EFC-HC, and EFM-HC. Fig. 1 gives a schematic diagram of those semi-pilot scale HC systems. The experimental system included a storage tank (100 L), a water pump (motor: HIGEN motor Co., Ltd., South Korea; pump: Fluid-O-Tech CO. Ltd., Italy) with a flow capacity of 40 L min1 with a maximum pressure head of 7.14 kgf cm2, and a stainless steel pipe. The inner diameter of the pipe was 20 mm and both HC systems contained a single-hole of 2.0 mm. The crisscrossed pipe was placed right before the orifice plate in both of the M-HC and EFM-HC systems (see the cross-section diagram (II modified) in Fig. 1. The inner diameter of the crisscrossed pipe was 20 mm, and its length was 40 mm (a tenth part of the total pipe length). In order to smooth the inner flow, the ends of the pipes were rounded. In the feasibility test, the flow rate and pressure were fixed (by controlling the bypass line) to maintain 25 L min1 and 4.0 kgf cm2, respectively. A constant voltage of 30 V was applied using an external power supply and the distance between electrodes was fixed at 15 cm. In the experiments using combined chemical oxidation with the C-HC and M-HC systems, H2O2 was added at a fixed molar ratio of 1:10 (AO-7:H2O2). The treatment was carried out for a total of 120 min and sampling was done after every 20 min of reaction time. On the other hand, in order to determine the negative effect of adding H2O2 to the EFM-HC system, H2O2 was added at varying molar ratios of 1:5 to 1:40 (AO-7:H2O2). 2.2.2. Optimization of the EFM-HC system using RSM with BBD In this study, in order to optimize the EFM-HC system for enhanced decolorization of AO-7, and to understand the effect of key parameters on the efficiency of decolorization, a 3k BBD [31] was employed using Design-Expert software (Stat-Ease, Inc., USA). In developing a regression equation, the relationship between the coded values and actual values were described according to the following equation:

  xi ¼ X i  X i =DX i

ð1Þ

where xi is the coded value of the ith independent variable, Xi is the uncoded value of the ith independent variable, X i is the uncoded value of the ith independent variable at the center point, and DXi is the step change value. In order to correlate the relationship between the variables and responses, a quadratic polynomial equation was fitted. The general form of the predictive polynomial quadratic equation is as follows:

y ¼ b0 þ

Xk

bx i¼1 i i

þ

Xk

b x2 i¼1 ii i

þ

X

b xx i
ð2Þ

where y is the response, b0 is the constant, bi is the linear coefficient, bii is a quadratic coefficient, and bij is the interactive coefficient.

Please cite this article in press as: K.-W. Jung et al., Decolorization of Acid Orange 7 by an electric field-assisted modified orifice plate hydrodynamic cavitation system: Optimization of operational parameters, Ultrason. Sonochem. (2015), http://dx.doi.org/10.1016/j.ultsonch.2015.02.010

3

K.-W. Jung et al. / Ultrasonics Sonochemistry xxx (2015) xxx–xxx

Fig. 1. Schematic diagram of electric field-assisted hydrodynamic cavitation system.

In this study, decolorization efficiency was chosen as the response variable in relation to decolorization, while the inlet pressure (X1), initial pH (X2), applied voltage (X3), and reaction time (X4) were chosen as independent variables. For X1, X2, and X4, each independent variable was evaluated at three levels (between 1 and 1) in the ranges determined by the feasibility study (X1: 2–5 kgf cm2, X2: pH 2–10, X4: 5–40 min). On the other hand, in order to determine the maximum range of the applied voltage (X3), an additional batch experiment was conducted at 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 V for 40 min reaction time. The experimental conditions were the same as in the preliminary test, except for the NaCl concentration (2.5 g L1). In order to minimize random errors, three experiments were conducted in parallel and the average values were determined for each set. 2.3. Analytical method The collected samples were analyzed using a UVSpectrophotometer (DS5000, HACH Company, Netherlands) to observe the change in the absorbance of AO-7 with time at a maximum wavelength (kmax) of 485 nm. The concentration of AO-7 was then calculated using a calibration curve (0.5–50 mg L1) prepared for AO-7. The concentration of AO-7 was determined immediately after each sampling to prevent reaction.

3. Results and discussion 3.1. Feasibility test of various electric field-assisted hydrodynamic cavitation systems Prior to conducting the feasibility test for the decolorization of AO-7 using a technique combining an electric field and the HC system, a preliminary test was conducted to determine which electrodes would be suitable to assist the HC system. As shown in Fig. 2, the graphite electrode performed better than the stainless steel one, for decolorization of AO-7. It has been reported that the graphite electrode, made of carbon and widely used for H2O2 generation, performed better than the stainless steel electrode due to its capacity for reduction of oxygen, and to its potential for the evolution of hydrogen [24,32]. For this reason, graphite was chosen as the most appropriate electrode for subsequent experiments. Hence, the following experiments were performed using NaCl in solution (2.5 g L1), as the electrolyte. The possible reactions in the presence of NaCl in the solution are shown in Eqs. (3)–(6): 

2Cl ! Cl2 þ 2e Cl2 þ H2 O $ HOCl þ Hþ þ Cl

ð3Þ 

ð4Þ

Please cite this article in press as: K.-W. Jung et al., Decolorization of Acid Orange 7 by an electric field-assisted modified orifice plate hydrodynamic cavitation system: Optimization of operational parameters, Ultrason. Sonochem. (2015), http://dx.doi.org/10.1016/j.ultsonch.2015.02.010

4

K.-W. Jung et al. / Ultrasonics Sonochemistry xxx (2015) xxx–xxx

Fig. 2. Determination of suitable electrode for decolorization of AO-7 at applied voltage of 30 V.



HOCl $ Hþ þ OCl 

6OCl þ 3H2 O !

3   O2 þ 6Hþ þ 4Cl þ 2ClO3 þ 6e 2

ð5Þ

ð6Þ

The chlorine generated quickly initiates hydrolysis (Eq. (4)) and ionization reactions (Eq. (5)) follow. In addition, the generated Cl can be recycled back to the anode surface and used as the reactant in Eq. (4). The strong oxidant, HOCl, can enhance the decolorization efficiency of AO-7. However, the OCl formed in Eq. (6) is consumed in Eq. (7) [33]. As shown in Fig. 2, when Cl was introduced to the solution by adding NaCl, a remarkable improvement in decolorization was observed. After 100 min of operation, the AO-7 was completely decolorized. Subsequently, a feasibility test was conducted in order to check the applicability of electric field-assisted HC systems for decolorization of AO-7. In addition, a comparative study using C-HC, M-HC, and combinations of both C-HC and M-HC with H2O2, was also performed to compare decolorization efficiency among these methods. There was negligible decolorization (0.75%) with H2O2 only, even after 120 min of reaction time with magnetic stirring. A much higher degree of decolorization was found 39.5% and 51% after 120 min of reaction time in the C- and M-HC systems, respectively, after addition of H2O2 (compared with the results from the two HC systems individually: 15.42% and 22.06%, respectively). If used alone, the efficiency of H2O2 in oxidizing pollutants is low because the dissociation of H2O2 into OH radicals, under conventional stirring conditions, is poor. However, the efficiency of H2O2 in degrading pollutants can be enhanced significantly if it is used in combination with an HC system. This is because the energy imparted by the collapsing cavities drives the dissociation of H2O2 into OH radicals, and supports their subsequent microdiffusion [1,7,34]. Furthermore, the overall performance results showed that a technique combining electric field and HC system had a significant positive effect on the decolorization of AO-7, and reduced reaction time, as shown in Fig. 3(A). This resulted in perfect decolorization after 40 and 80 min of reaction time in the EFM-HC and EFC-HC, respectively. The drastic enhancement of decolorization was caused by two main mechanisms: strong oxidants (HOCl and OCl) were created with the assistance of the electric field (electrochemical reaction), and the HC system created localized ‘‘hot spots’’ and OH radicals. Moreover, the greater decolorization in the EFM-HC (compared to the EFC-HC) was caused by mechanical effects due to the creation of turbulence, and from intense shockwaves breaking molecular bonds in the crisscrossed

Fig. 3. Feasibility study of AO-7 decolorization: (A) comparison of decolorization efficiency using various HC systems with and without H2O2; (B) effect of H2O2 molar ratio on decolorization.

pipe at the front of the pipe throat. This condition made pollutant molecules more amenable to attack by OH radicals [29]. In general, to enhance the degradation of target pollutant, H2O2 was also injected into the reactor as an oxidant [35]. The results reported in these studies were confirmed here. However, interestingly, decolorization performance decreased when EFM-HC and H2O2 were combined at the same molar ratio with C-HC and M-HC (Fig. 3(A)). Moreover, as shown in Fig. 3(B), decolorization gradually decreased with the increasing molar ratio of H2O2 from 1:5 to 1:40. Similar results have been reported that the increase of H2O2 concentration is not necessarily enhancing reactor performance because optimal condition depends on type of target pollutant and cavitational intensity [36,37]. Although this is speculative, there may be a link between the combined reactions of OCl consumption and H2O molecule generation [38]. Thus, the generation of more H2O in the solution by the chemical combination of OH and HO2 radicals, forced a reduction in the recycling reactions of Cl2 and HOCl, thereby gradually decreasing them, and increasing the H2O2 molar ratio. Therefore, we conclude that EFM-HC system applied here is a useful and efficient means for decolorization of AO-7 because it requires no additional oxidant, and because the HC system is easily applied in the field. 3.2. Optimization of the EFM-HC system using RSM with BBD As described above, the combination of an electric field and M-HC appears an effective and reasonable method for decolorization of AO-7. Therefore, a statistical optimization was undertaken

Please cite this article in press as: K.-W. Jung et al., Decolorization of Acid Orange 7 by an electric field-assisted modified orifice plate hydrodynamic cavitation system: Optimization of operational parameters, Ultrason. Sonochem. (2015), http://dx.doi.org/10.1016/j.ultsonch.2015.02.010

5

K.-W. Jung et al. / Ultrasonics Sonochemistry xxx (2015) xxx–xxx

Fig. 4. Decolorization efficiency of electric field under various applied voltages.

using RSM with a BBD. Prior to the optimization of the EFM-HC system, we determined the applied voltage range under different conditions (5–100 V at intervals of 10 V) during 40 min of reaction time. The AO-7 and NaCl concentrations were fixed at 10 mg/L and 2.5 g L1, respectively. As expected, increasing the voltage exhibited a positive effect on the decolorization performance; this resulted in complete decolorization after from 60 V was applied (Fig. 4). Even though 100% decolorization was observed at 60 V, the difference between 50 V and 60 V was small (only 0.3%). However, 1.5 times more electrical energy (W h = power of used machine (W)  reaction (h)) was consumed at 60 V. For this reason, 50 V was selected as the maximum applied voltage for the optimization study on the EFM-HC system using RSM with the BBD, for the sake

of economy. In this section, the effect of operating variables (i.e., inlet pressure, initial pH, applied voltage, and reaction time for the AO-7 decolorization) was investigated using RSM with a BBD. The variables, experimental design, and corresponding values are presented in Table 1. A total of twenty-seven experimental sets were determined, and three replicates were applied at the center point. The batch runs were conducted under the BBD designed experimental conditions, to visualize the effects of independent factors on responses, and on the results, under each experimental condition. As a general trend, increased inlet pressure, applied voltage, and reaction time; as well as decreased initial pH, enhanced AO-7 decolorization in the test runs. The maximum decolorization of 95.39% was achieved at inlet pressure of 3.5 kgf cm2, initial pH 2.0, applied voltage of 50 V, and reaction time of 22.5 min. In order to decide an adequate type of regression model for further optimization steps, the sequential model sum of squares was carried out and the results are summarized in Table 2. As results, a quadratic type model was suggested because it had lower standard deviation of 8.50, which means that the predicted values would be more accurate and closer to the actual value. In addition, ‘‘Adequate Precision’’ measures the signal to noise ratio and a ratio greater than 4 is desirable. As shown in Table 2, the ratio of 16.441 indicates that adequate signals for the models can be applied to navigate the design space. In this study, the cubic type model was found to be aliased due to the lacking in runs to support a full cubic model in case of a BBD. Subsequently, the experimental results were subjected to an analysis of variance (ANOVA) and the results are provided in Supplementary Table 1. As can be seen, the calculated model F-value of 25.81 implies that the model was significant; a model P-value less than 0.0001, indicates that the model terms were significant. Based on the ANOVA results, the final quadratic regression model for the decolorization of AO-7 (Eq. (7)) was obtained using Eq. (2), to fit the experimental data of the decolorization of AO-7:

Table 1 The Box–Behnken experimental design for EFM-HC with four independent variables and the corresponding experimental results. Trial No.

1 2 3 4 5 6 7a 8a 9a 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 a

Inlet pressure (kgf cm2)

Initial pH

Applied voltage (V)

Reaction time (min)

Decolorization (%)

X1

Code

X2

Code

X3

Code

X4

Code

Predicted

Actual

2 2 2 2 2 2 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 5.0 5.0 5.0 5.0 5.0 5.0

1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1

6 6 6 10 2 6 6 6 6 6 6 10 2 10 2 6 6 2 10 2 10 10 2 6 6 6 6

0 0 0 1 1 0 0 0 0 0 0 1 1 1 1 0 0 1 1 1 1 1 1 0 0 0 0

50 27.5 27.5 27.5 27.5 5 27.5 27.5 27.5 5 50 27.5 27.5 50 27.5 50 5 5 27.5 50 5 27.5 27.5 5 27.5 50 27.5

1 0 0 0 0 1 0 0 0 1 1 0 0 1 0 1 1 1 0 1 1 0 0 1 0 1 0

22.5 40 5 22.5 22.5 22.5 22.5 22.5 22.5 40 40 40 40 22.5 5 5 5 22.5 5 22.5 22.5 22.5 22.5 22.5 5 22.5 40

0 1 1 0 0 0 0 0 0 1 1 1 1 0 1 1 1 0 1 0 0 0 0 0 1 0 1

44.48 37.23 11.61 31.20 33.59 7.50 68.67 68.67 68.67 7.07 87.36 28.44 78.80 30.10 20.52 22.38 8.54 6.87 6.13 95.59 7.61 25.54 87.90 13.20 21.26 87.43 76.23

51.57 30.79 8.08 30.17 41.58 3.41 69.21 68.10 68.71 6.98 79.34 41.43 78.37 25.51 7.35 24.19 1.20 9.93 6.37 95.39 6.28 19.27 90.66 5.92 26.16 91.33 78.21

Center point.

Please cite this article in press as: K.-W. Jung et al., Decolorization of Acid Orange 7 by an electric field-assisted modified orifice plate hydrodynamic cavitation system: Optimization of operational parameters, Ultrason. Sonochem. (2015), http://dx.doi.org/10.1016/j.ultsonch.2015.02.010

6

K.-W. Jung et al. / Ultrasonics Sonochemistry xxx (2015) xxx–xxx

Table 2 Model summary statistics tested for the response.

a

Model

Std. Dev.a

R2

R2Adja

R2Preda

PRESS

Remark

CVa (%)

Adeq. Precisiona

Mean

Linear 2FI Quadratic Cubic

18.95 16.59 8.50 1.68

0.7071 0.8366 0.9679 0.9996

0.6539 0.7345 0.9304 0.9973

0.5855 0.6682 0.8150 0.9429

11177.35 8946.82 4988.93 1538.59

Suggested Aliased

21.53

16.441

39.46

Std. Dev. = standard deviation; R2Adj = adjected-R2; R2Pred = predicted-R2; CV = coefficient of variation; Adeq. Precision = adequate precision.

Fig. 5. Predicted values versus actual values of decolorization.

Y ¼ 68:67 þ 12:16X 1  16:19X 2 þ 27:80X 3 þ 20:15X 4  14:99X 1 X 2 þ 9:31X 1 X 3 þ 7:34X 1 X 4  16:56X 2 X 3  8:99X 2 X 4 þ 12:34X 3 X 4  10:50X 21  13:61X 22  20:02X 23  21:59X 24

ð7Þ

where X1, X2, X3, and Y are the inlet pressure (kgf cm2), initial pH, applied voltage (V), reaction time (min), and corresponding decolorization (%), respectively. Values of ‘‘P > F’’ less than 0.05 indicate that the model terms are significant, while values greater than 0.1 indicate model terms that are not significant. In this study, X1, X2, X3, X4, X1X2, X1X3, X2X3, X3X4, X21, X22, X23, and X24 were significant model terms. However, X1X4 and X2X4 cannot be eliminated to support the hierarchy of the model, because the predicted values versus the actual values of decolorization clearly revealed that this model could explain up to 96.7% of the variability in response (Fig. 5). The ‘‘Lack of Fit F-value’’ of 282.19 implies that there was only a 0.35% chance that this result could occur due to noise. Therefore, this model within the specified design range can be used effectively to predict and optimize the EFM-HC system for the decolorization of AO-7. As shown in Table 2, the coefficient of variation values of 21.53%, indicating that this model can be considered as reasonable model for reproducibility. In addition, the adjusted R2 (R2Adj) was calculated to be 0.9304 (reasonable agreement with the predicted R2 (R2pred = 0.8150), which indicates that the proposed equation ensures an appropriate approximation for the illumination of the relationship between the independent variables and the response variable. Based on this model, maximum decolorization (100%) was predicted with a desirability of 1.0, under the following optimum conditions: inlet pressure of 3.9 kgf cm2, initial pH of 3.0, applied voltage of 37.7 V, and reaction time of 30 min.

Fig. 6. Three dimensional contour plots for decolorization: fixed at each optimum condition. (A) Voltage of 37.7 V and reaction time of 30 min; (B) initial pH of 3.0 and reaction time of 30 min; (C) initial pH of 3.0 and voltage of 37.7 V; (D) inlet pressure of 3.9 kgf cm2 and reaction time of 30 min; (E) inlet pressure of 3.9 kgf cm2 and voltage of 37.7 V; (F) inlet pressure of 3.9 kgf cm2 and initial pH of 3.0.

Please cite this article in press as: K.-W. Jung et al., Decolorization of Acid Orange 7 by an electric field-assisted modified orifice plate hydrodynamic cavitation system: Optimization of operational parameters, Ultrason. Sonochem. (2015), http://dx.doi.org/10.1016/j.ultsonch.2015.02.010

K.-W. Jung et al. / Ultrasonics Sonochemistry xxx (2015) xxx–xxx

In order to highlight the interactive effect of the key parameters (inlet pressure, initial pH, applied voltage, and reaction time) in the EFM-HC, three-dimensional response surfaces derived from Eq. (7) are presented in Fig. 6(A)–(F) with the independent variables being kept constant at the statistically optimized. The other two independent variables were varied within the experimental range. It is clear that a rounded ridge running diagonally along the plot suggests that the key parameters had either a slightly interdependent or significantly interactive effects on the decolorization of AO-7. As shown in Eq. (7) and Fig. 6(B), (C), and (F), the positive values of the inlet pressure (X1), applied voltage (X3), and reaction time (X4) indicate that the higher the operating conditions are, the higher the decolorization efficiency (Fig. 6(B), (C), (E) and (F)). In case of inlet pressure, the higher conditions lead to an increase in decolorization because an increase in inlet pressure forces an increase in the cavitation intensity [39,40]. Similarly, the higher conditions of applied voltage and reaction time lead to an increase in decolorization because those factors are closely related with electron transfer intensity and contact time between target pollutant and oxidizing agents, respectively. In contrast, a negative value of the initial pH (X2) shows that a decrease in the pH condition leads to an increase in decolorization (Fig. 6(A), (E) and (D)). It has been reported that the ratio of HOCl to OCl strongly depends on the pH condition of the solution (Eq. (6)) and that the former has 100 times more oxidation potential. Below pH 5.0, HOCl is the predominant form; however, its effectiveness declines with increasing pH because the OCl form then becomes predominant [41]. Finally, based on the experimental optimization results, a verification test was conducted three times in order to confirm the validity of the statistically optimized conditions, and of the corresponding decolorization efficiency. This result, as shown in Supplementary Fig. 1, confirms complete decolorization, which corresponds to 100% of the predicted value. In conclusion, the EFM-HC developed here is a suitable way to degrade AO-7 efficiently without any oxidizing chemicals (normally H2O2) in the HC system. This simplifies the whole operation and enhances the applicability of the system in the field. 4. Conclusion We studied the decolorization of AO-7 in aqueous solution by an electric field-assisted HC system and concluded that the application of the EFM-HC system described here was highly suitable for the successful treatment of dye wastewater. As a preliminary step, the applicability of the proposed system for decolorization was investigated, and subsequently, its operational conditions were statistically optimized using RSM with a BBD. Complete decolorization was monitored under optimized conditions (inlet pressure = 3.9 kgf cm2, initial pH = 3.0, applied voltage = 37.7 V, and reaction time = 30 min). According to these results, it was concluded that the combination of a modified HC system and an electric field was effective for complete decolorization of AO-7. However, even though this is a promising approach that has high potential for application at commercial scale, further study for the identification of intermediates or final byproducts (e.g., chlorinated compounds) and the evaluation of their toxic effect on living organisms are necessary. Acknowledgements This work was supported by grants from the Korea Research Council of Fundamental Science and Technology (Project No. 2N38090) and the KIST Institutional Program (Project No. 2E24562).

7

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ultsonch.2015.02. 010. References [1] P.R. Gogate, G.S. Bhosale, Comparison of effectiveness of acoustic and hydrodynamic cavitation in combined treatment schemes for degradation of dye wastewaters, Chem. Eng. Process. 71 (2013) 59–69. [2] S.H. Tian, Y.T. Tu, D.S. Chen, Y. Xiong, Degradation of acid orange II at neutral pH using Fe2(MoO4)3 as a heterogeneous Fenton-like catalyst, Chem. Eng. J. 169 (2011) 31–37. [3] A. Alinsafi, M. Khemis, M.N. Pons, J.P. Leclerc, A. Yaacoubi, A. Benhammou, A. Nejmeddine, Electro-coagulation of reactive textile dyes and textile wastewater, Chem. Eng. Process. 44 (2005) 461–470. [4] P.R. Gogate, A.B. Pandit, A review of imperative technologies for wastewater treatment I: oxidation technologies at ambient conditions, Adv. Environ. Res. 8 (2004) 501–551. [5] M.A. Rauf, S.S. Ashraf, Fundamental principles and application of heterogeneous photocatalytic degradation of dyes in solution, Chem. Eng. J. 151 (2009) 10–18. [6] X. Wang, Z. Yao, J. Wang, W. Guo, G. Li, Degradation of reactive brilliant red in aqueous solution by ultrasonic cavitation, Ultrason. Sonochem. 15 (2008) 43–48. [7] M.M. Gore, V.K. Saharan, D.V. Pinjari, P.V. Chavan, A.B. Pandit, Degradation of reactive orange 4 dye using hydrodynamic cavitation based hybrid techniques, Ultrason. Sonochem. 21 (2014) 1075–1082. [8] P.R. Gogate, Application of cavitational reactors for water disinfection: current status and path forward, J. Environ. Manage. 85 (2007) 801–815. [9] M.V. Bagal, P.R. Gogate, Wastewater treatment using hybrid treatment schemes based on cavitation and Fenton chemistry: a review, Ultrason. Sonochem. 21 (2014) 1–14. [10] P. Braeutigam, Z.L. Wu, A. Stark, B. Ondruschka, Degradation of BTEX in aqueous solution by hydrodynamic cavitation, Chem. Eng. Technol. 32 (2009) 745–753. [11] K.P. Mishra, P.R. Gogate, Intensification of degradation of rhodamine B using hydrodynamic cavitation in the presence of additives, Sep. Purif. Technol. 75 (2010) 385–391. [12] V.K. Saharan, M.P. Badve, A.B. Pandit, Degradation of Reactive Red 120 dye using hydrodynamic cavitation, Chem. Eng. J. 178 (2011) 100–107. [13] P.R. Gogate, A.B. Pandit, Hydrodynamic cavitation: a state of the art review, Rev. Chem. Eng. 46 (2001) 1–85. [14] P.R. Gogate, A.B. Pandit, A review and assessment of hydrodynamic cavitation as a technology for the future, Ultrason. Sonochem. 12 (2005) 21–27. [15] V.K. Saharan, A.B. Pandit, P.S.S. Kumar, S. Anandan, Hydrodynamic cavitation as an advanced oxidation technique for the degradation of Acid Red 88 dye, Ind. Eng. Chem. Res. 51 (2012) 1981–1989. [16] R.H.D.L. Leite, P. Cognet, A.M. Wihelm, H. Delmas, Anodic oxidation of 2,4-dihyroxybenzoic acid for wastewater treatment: study of ultrasound activation, Chem. Eng. Sci. 57 (2002) 767–778. [17] J.P. Lorimer, T.J. Mason, M. Plattes, S.S. Phull, Dye effluent decolourisation using ultrasonically assisted electro-oxidation, Ultrason. Sonochem. 7 (2000) 237–242. [18] M. Siddique, R. Farooq, Z.M. Khan, Z. Khan, S.F. Shaukat, Enhanced decomposition of reactive blue 19 dye in ultrasound assisted electrochemical reactor, Ultrason. Sonochem. 18 (2011) 190–196. [19] G. Zhao, S. Shen, M. Li, M. Wu, T. Cao, D. Li, The mechanism and kinetics of ultrasound-enhanced electrochemical oxidation of phenol on boron-doped diamond and Pt electrodes, Chemosphere 73 (2008) 1407–1413. [20] K.P. Mishra, P.R. Gogate, Intensification of degradation of aqueous solutions of rhodamine B using sonochemical reactors at operating capacity of 7 L, J. Environ. Manage. 92 (2011) 1972–1977. [21] P.R. Gogate, S. Mededovic-Thagard, D. McGuire, G. Chapas, J. Blackmon, R. Cathey, Hybrid reactor based on combined cavitation and ozonation: from concept to practical reality, Ultrason. Sonochem. 21 (2014) 590–598. [22] X. Wang, J. Jia, Y. Wang, Electrochemical degradation of reactive dye in the presence of water jet cavitation, Ultrason. Sonochem. 17 (2010) 515–520. [23] S. Rismayani, M. Fukushima, H. Ichikawa, K. Tatsumi, Decolorization of orange-II by catalytic oxidation using iron (III) phthalocyanine-tetrasul-fonic acid, J. Hazard. Mater. 114 (2004) 175–181. [24] L. Xu, H. Zhao, S. Shi, G. Zhang, J. Ni, Electrolytic treatment of C.I. Acid Orange 7 in aqueous solution using a three-dimensional electrode reactor, Dyes Pigm. 77 (2008) 158–164. [25] H.Z. Zhao, Y. Sun, L.N. Xu, J.R. Ni, Removal of Acid Orange 7 in simulated wastewater using a three-dimensional electrode reactor: removal mechanisms and dye degradation pathway, Chemosphere 78 (2010) 46–51. [26] N. Daneshvar, H. Ashassi-Sorkhabi, A. Tizpar, Decolorization of Orange II by electrocoagulation method, Sep. Purif. Technol. 31 (2003) 153–162. [27] N. Daneshvar, S. Aber, V. Vatanpour, M.H. Rasoulifard, Electro-Fenton treatment of dye solution containing Orange II: influence of operational parameters, J. Electroanal. Chem. 615 (2008) 165–174.

Please cite this article in press as: K.-W. Jung et al., Decolorization of Acid Orange 7 by an electric field-assisted modified orifice plate hydrodynamic cavitation system: Optimization of operational parameters, Ultrason. Sonochem. (2015), http://dx.doi.org/10.1016/j.ultsonch.2015.02.010

8

K.-W. Jung et al. / Ultrasonics Sonochemistry xxx (2015) xxx–xxx

[28] A. Fernandes, A. Morão, M. Magrinho, A. Lopes, I. Gonçalves, Electrochemical degradation of C.I. Acid Orange 7, Dyes Pigm. 61 (2004) 287–296. [29] K.W. Jung, M.J. Hwang, Y.M. Yun, M.J. Cha, K.H. Ahn, Development of a novel electric field-assisted modified hydrodynamic cavitation system for disintegration of waste activated sludge, Ultrason. Sonochem. 21 (2014) 1635–1640. [30] B.K. Körbahti, Response surface optimization of electrochemical treatment of textile dye wastewater, J. Hazard. Mater. 145 (2007) 277–286. [31] G.E.P. Box, D.W. Behnken, Some new three-level designs for the study of quantitative variables, Technometrics 2 (1960) 455–475. [32] M. Panizza, G. Cerisola, Removal of organic pollutants from industrial wastewater by electrogenerated Fenton’s reagent, Water Res. 35 (2001) 3987–3992. [33] C.J. Israilides, A.G. Vlyssides, V.N. Mourafetti, G. Karouni, Olive oil wastewater treatment with the use of an electrolysis system, Bioresour. Technol. 61 (1997) 163–170. [34] S. Merouani, O. Hamdaoui, F. Saoudi, M. Chiha, Sonochemical degradation of Rhodamine B in aqueous phase: effects of additives, Chem. Eng. J. 158 (2010) 550–557.

[35] J. Wang, X. Wang, P. Guo, J. Yu, Degradation of reactive brilliant red K-2BP in aqueous solution using swirling jet-induced cavitation combined with H2O2, Ultrason. Sonochem. 18 (2011) 494–500. [36] R.K. Joshi, P.R. Gogate, Degradation of dichlorvos using hydrodynamic cavitation based treatment strategies, Ultrason. Sonochem. 19 (2012) 532–539. [37] A.G. Chakinala, D.H. Bremner, P.R. Gogate, K.C. Namkung, A.E. Burgess, Multivariate analysis of phenol mineralization by combined hydrodynamic cavitation and heterogeneous advanced Fenton processing, Appl. Catal. B 78 (2008) 11–18. [38] Y.L. Pang, A.Z. Abdullah, S. Bhatia, Review on sonochemical methods in the presence of catalysts and chemical additives for treatment of organic pollutants in wastewater, Desalination 277 (2011) 1–14. [39] N.P. Vichare, P.R. Gogate, A.B. Pandit, Optimization of hydrodynamic cavitation using a model reaction, Chem. Eng. Technol. 23 (2000) 683–690. [40] P.R. Gogate, A.B. Pandit, Engineering design methods for cavitation reactors II: hydrodynamic cavitation, AIChE J. 46 (2000) 1641–1649. [41] H. Duckhouse, T.J. Mason, S.S. Phull, J.P. Lorimer, The effect of sonication on microbial disinfection using hypochlorite, Ultrason. Sonochem. 11 (2004) 173– 176.

Please cite this article in press as: K.-W. Jung et al., Decolorization of Acid Orange 7 by an electric field-assisted modified orifice plate hydrodynamic cavitation system: Optimization of operational parameters, Ultrason. Sonochem. (2015), http://dx.doi.org/10.1016/j.ultsonch.2015.02.010