Decolorization of Congo red via hydrodynamic cavitation in combination with Fenton’s reagent

Chemical Engineering & Processing: Process Intensification 150 (2020) 107874

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Decolorization of Congo red via hydrodynamic cavitation in combination with Fenton’s reagent

T

Zahra Askarniya, Mohammad-Taghi Sadeghi*, Soroush Baradaran Department of Chemical Engineering, Iran University of Science and Technology (IUST), Tehran, Iran

A R T I C LE I N FO

A B S T R A C T

Keywords: Hydrodynamic cavitation (HC) Fenton’s reagent Response surface methodology (RSM) Residence time distribution (RTD) Congo red

Congo red is a highly toxic diazoic dye used in textile industries. In this work, the decolorization of Congo red was performed using hydrodynamic cavitation (HC) individually and in combination with H2O2, FeSO4, and Fenton process. Response surface methodology (RSM) was used to design experiments and model the influence of parameters and interactions. Solution initial pH (3–10), H2O2 concentration (0−1000 mg L−1), and FeSO4 concentration (0−50 mg L -1) were selected as the parameters of the experimental investigation. A maximum decolorization of 70 % was achieved using HC in combination with Fenton’s reagent at a H2O2 concentration of 1000 mg L−1, a FeSO4 concentration of 25 mg L−1, and an initial pH of 3 within 60 min. A synergistic coefficient of 3.22 and a yield efficiency of 8.3 × 10-3 mg kJ-1 were obtained for the combined process of HC and Fenton’s reagent. The flow behavior of the HC reactor was investigated using the residence time distribution (RTD) analysis for the first time, and the results demonstrate that the tank-in-series model with two tanks almost fits the HC reactor.

1. Introduction Release of textile effluents into the environment is extremely hazardous for aquatics and other living organisms because of the toxicity and turbidity induced by most of synthetic dyes [1,2]. Almost 70 % of colorants annually used in industries are in the class of azoic dyes [3]. The molecules of azoic dyes are made of one or more groups of (-N = N) linking aromatic rings [4]. Treatment of industrial effluents has been a serious challenge because of the mentioned problems and therefore the development of efficient processes incorporating the concept of green chemistry is necessary [5]. Using conventional biological procedures for the decolorization and the decontamination of textile effluents usually results in low efficiency because of the complexity and toxicity of azoic dyes [6,7]. Congo red (C32H22N6O6S2Na2) is a synthetic diazoic dye with a high molecular weight and complex molecular structure. The dye is utilized widely in textile and plastic industries and has severe environmental impacts [8,9]. This dye is known as an extremely toxic and carcinogenic dye with high resistance to conventional wastewater treatment methods [10]. Problems created by this carcinogenic dye highlight the necessity of developing new efficient methods for the removal of it from wastewater. Many studies have been devoted to developing methods which are able to mitigate the hazardous effects of the dye. Shen et al. [9] investigated the degradation of Congo red using cobalt-cooper oxalate nanofibers and reported that the complete ⁎

degradation occurred within 100 min. Chowdhury et al. [11] studied the decolorization of Congo red by catalyst wet air oxidation and reported that 90 % decolorization was obtained within 250 min. Schmidt et al. [4] investigated the biodegradation of Congo red and reported that 93.6 % decolorization was achieved within 96 h. In recent years, research on the exploitation of HC has been an interesting topic, and HC combined with various advanced oxidation processes (AOP) have been considered to be a promising process for the treatment of industrial effluents [12–14]. The degradation of pollutants occurs based on the generation of free radicals using HC, AOP, and the combination of HC and AOP. When wastewater flows through the throttle of a cavitation device such as an orifice, if its pressure drops below the vapor pressure, cavitation happens and bubbles are generated. [15–17]. Collapse of bubbles releases a considerable amount of energy, which can lead to the pyrolysis of water molecules and pollutants molecules [18–20]. In many cases, applying AOP and HC individually results in high cost or low efficiency, while HC in combination with oxidizing additives such as hydrogen peroxide [21], ozone [22], photo catalyst [23], and Fenton’s reagent [24] are usually much more efficient. Rajoriya et al. [6] studied the treatment of textile dyeing industry using HC in the presence of oxidizing agents and demonstrated that 98 % decolorization was achieved by use of HC combined with Fenton’s reagent, while the individual processes resulted in the lower amounts of the decolorization. In the other work, Rajoriya et al. [25]

Corresponding author. E-mail address: [email protected] (M.-T. Sadeghi).

https://doi.org/10.1016/j.cep.2020.107874 Received 15 November 2019; Received in revised form 7 February 2020; Accepted 24 February 2020 Available online 25 February 2020 0255-2701/ © 2020 Elsevier B.V. All rights reserved.

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2.3. Materials and analysis method

studied the decolorization of reactive blue 13 using HC in combination with oxidizing additives and reported that 91 % decolorization was achieved using HC in the presence of the optimum amount of H2O2. They also attained complete decolorization of the dye using HC combined with 3 g h−1 ozone in only 15 min. these results were higher than the amounts of the decolorization attained using the individual processes. In the combined processes of HC and AOP, the efficacy of processes and the essential amounts of oxidants extremely depend on the nature of pollutants. In fact, decolorization using the combination of HC and AOP is extremely affected by the nature of dyes and therefore the case study of pollutants is necessary [26,27]. Residence time distribution (RTD) of a reactor is an informative characterization of the reactor and important for the modeling of that reactor. Flow models are helpful for representing the hydrodynamic behavior of a real reactor and scaling- up. Plug flow and mixed flow reactors are two idealized models and were assumed to represent real rectors in early practice. These models are often unsuccessful to suitably represent real reactors because there are often differences between actual rectors and ideal rectors [28]. Therefore, the tracer method was introduced to present more acceptable estimation of flow behavior. In the tracer method, a tracer enters a stream flowing within a reactor and exactly follows the fluid. Then, RTD is measured in order to find a suitable model representing the reactor [29]. Up to date, similar experimental and modelling approaches have not been used to evaluate HC reactors. In the current study, the decolorization of Congo red was performed using HC individually and in combination with Fenton’s reagent. RSM based on Box-Behnken was utilized for designing the experiments, modeling the effects of the parameters, and finding the significance of the parameters. A synergistic coefficient and also yield efficiency were evaluated. Finally, to determine the flow behavior of the HC rector, the tracer method was used and a model representing the HC reactor was selected based on RTD curve.

Congo red (C32H22N6O6S2Na2), ferrous sulfate heptahydrate (FeSO4.7H2O), and sodium hydroxide (NaOH pellets) were purchased from Samchun (Korea). Sulfuric acid (H2SO4) and ponceau 4R (C20H11N2Na3O10S3) were purchased from Merck (Germany) and a well-known commercial producer, respectively. Hydrogen peroxide (50 % w/v) was provided from a viable domestic resource. The concentration of Congo red and ponceau 4R were measured using UV–vis spectrophotometer (Hach DR/2010) at the wavelengths of 485 nm and 518 nm, respectively. 2.4. Experimental design Box-Behnken design as an efficient method which requires a minimum number of experiments was used to design the experiments, model the effects of the parameters, determine the interactions between the parameters, and find the significance of the parameters [30]. H2O2 concentration, FeSO4 concentration, and initial pH are the three levels of the variables in Box-Behnken Design. The investigation range of the parameters are shown in Table 2. Using this method, the Number of experiments are calculated according to the Eq. (1):

N = 2V (V − 1) + C

(1)

Where N , V , and C represent the number of experiments, variables, and center points, respectively. In this study, 17 experiments including 5 center points were performed. 3. Result and discussion 3.1. Statistical analysis Table 3 shows the results of experiments designed using RSM. It can be observed in the table that the decolorization range is from 1% to 70 %. The acceptability of a recommended model and the significance of the parameters affecting the decolorization of Congo red are determined by Analysis of variance (ANOVA). According to Table 4, which presents the results of analysis variance, a polynomial secondorder quadratic model (Eq. (2)) is presented in order to represent the empirical relation between the decolorization and the parameters:

2. Material and method 2.1. Experimental set- up and procedure The schematic representation of the experimental set-up employed in this study is observed in Fig. 1. A storage tank, a centrifugal pump (PENTAX CBT600/00, 5.5 HP), a HC reactor equipped with a single hole circular orifice, two pressure gauges, and four manual valves are the essentials of this set-up. The characteristics of the orifice utilized in this work are observed in Table 1. The discharge side of the storage tank is connected to the suction side of the pump and the discharge side of the pump is divided into a main line and a bypass line. The main line involves the HC reactor and the bypass line consists of a manual valve utilized for adjusting the pressure of the main line. A cooling coil installed in the storage tank is employed to maintain temperature. All the experiments were carried out with an initial dye concentration of 20 mg L−1. The initial pH was adjusted by the use of sulfuric acid (H2SO4) and sodium hydroxide (NaOH). The operating pressure and temperature were maintained at 6 bar and 40 ± 2 °C, respectively.

Decolorization% = 35.40 + 14.00 × A + 9.63 × B – 18.13 × C + 2.25 × AB + 1.25 × AC – 0.5 × BC + 4.8 × A2 – 1.95 B2 – 3.45 × C2 (2) According to ANOVA, the F value, which implies the ratio of mean square to error term, is 17.02. The model F-value confirms that the proposed model is appropriate for the decolorization of Congo red. Each of the model terms with a p-value less than 0.05 is significant and the others which have p-values higher than 0.1 are considered to be insignificant [31]. According to the results in Table 4, A, B, and C (the concentration of H2O2, the concentration of FeSO4, and the solution pH, respectively) are the significant terms in this case. It can also be realized that FeSO4 concentration is less significant than the other parameters. Furthermore, the high values of R2 (correlation coefficient) and R2Adj, which are 0.9563 and 0.900, respectively, demonstrate that the model is fairly adjusted with experimental data. Fig. 2 presents diagnostic plots. In addition to R2 and R2Adj, using the normal probability plot of residuals, which represent differences between experimental and predicted results, this is another technique for determining the adequacy of a model proposed by RSM. The plot is a graphical way to evaluate normal distribution of the residuals [30,32]. According to Fig. 2 (a) however, some scatterings exist. The normal probability plot of the residuals is almost linear, which demonstrates normal distribution of the error terms and reveals that the model is

2.2. RTD measurement A pulse tracer experiment was used to determine the hydrodynamic behavior of the HC rector. Water and ponceau 4R solution were selected as the working fluid and tracer, respectively. The tracer was injected into the stream entering the HC reactor. A rotating disk operated by an electrical motor was utilized for sampling. The tracer concentration was measured at the same time intervals when the tracer was leaving the HC reactor. 2

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Fig. 1. Schematic representation of experimental set-up. Table 1 Characteristics of the orifice.

Table 3 Experimental design based on Box-Behnken and obtained results.

Item

Quantity

Run

H2O2

FeSO4

pH

Decolorization% (in 60 min)

Pipe diameter (mm) Hole Diameter (mm) Flow Area (mm2) Hole diameter to pipe (β) Flow area to pipe cross sectional flow area (β0) Total perimeter of holes to the total flow area of the plate mm−1) α)

30 4 12.6 0.13 0.02 1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

500 0 500 500 500 0 500 1000 1000 0 0 500 500 1000 500 500 1000

25 25 0 50 25 0 50 0 50 25 50 25 0 25 25 25 25

6.5 3 10 3 6.5 6.5 10 6.5 6.5 10 6.5 6.5 3 10 6.5 6.5 3

35 41 1 60 37 21 24 41 60 1 31 32 35 35 28 45 70

Table 2 Investigation range of independent variables. Factor

Unit

Min(-1)

Max(1)

Mean(0)

H2O2 FeSO4 pH

mg L-1 mg L-1 –

0 0 3

1000 50 10

500 25 6.5

reasonable [32]. Fig. 2(b) was utilized to assess the concordance between the experimental results and those predicted by the proposed model. According to the Fig. 2, the results almost spread on a straight line, which can show acceptable proximity of the actual results and predicted values. It can confirm that the model is a reliable one for predicting the effects of the parameters on the decolorization of Congo red.

result can be an indication of energy created by bubbles collapsing as a result of pressure recovery in the downstream. [5]. Decolorization by the pyrolysis of pollutants molecules usually happens near or within collapsing cavitation bubbles and therefore extremely depends on the hydrophobicity of pollutants. Since hydrophobic dyes tend to be inside bubbles, the breakage of their chromophore bonds may happen by use of energy generated by collapsing bubbles. [31]. Decolorization can also begin by the pyrolysis of water molecules and the generation of hydroxyl radicals according to Eq. (3) as a result of the extreme conditions created by the collapse of bubbles [21].

3.2. Decolorization via HC At an initial pH of 6.5, a temperature of 40 ± 2 °C, and a pressure of 6 bar, 21 % decolorization was achieved using HC within 60 min. The 3

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decolorization at a pressure of 7 bar within 60 min. The differences in values of the decolorization can be due to the fact that in addition to reaction time, a variety of factors can influence the effectiveness of HC for decolorization. It can be extremely affected by the nature of dyes since different molecules have different reactivity towards hydroxyl radicals and also different dissociation energy [10,33]. It can also be the result of different geometrical design of cavitating devices, which can affect the intensity of HC, pressure recovery and the residence time of bubbles [25]. Another reason can be different operating conditions, which can influence on the intensity of HC, amount of hydroxyl radicals in the solution and the position of dyes molecules [7,31,34].

Table 4 Analysis of variance table (ANOVA) for Box-Behnken model. Source

Sum of Squares

df

Mean Square

Model A-H2O2 B-FeSO4 C-pH

5119.81 1568 741.13 2628.12

9 1 1 1

568.87 1568 741.13 2628.12

17.02 46.92 22.18 78.64

AB AC BC A2 B2 C2 Residual Lack of Fit Pure Error R2 R2Adj

20.25 6.25 1 97.01 16.01 50.12 233.95 72.75 161.2 0.9563 0.9001

1 1 1 1 1 1 7 3 4

20.25 6.25 1 97.01 16.01 50.12 33.42 24.25 40.3

0.61 0.19 0.03 2.9 0.48 1.5

0.0006 0.0002 0.0022 < 0.0001 0.4618 0.6784 0.8676 0.1322 0.5112 0.2604

0.6

0.6474

⎯⎯⎯⎯→ H2 O HC Ho+Oo

F Value

P-value Prob > F significant

3.3. Decolorization via HC combined with H2O2 In the similar operating conditions, using HC combined with 1000 mg L−1 H2O2 resulted in 41 % decolorization within 60 min. H2O2 is a conventional oxidant with an oxidation potential of 1.78 V. H2O2 is known as an important source of generation of hydroxyl radicals because it can be decomposed by HC and produce active hydroxyl radicals through Eq. (4) [25,31,33].

not significant

(3)

H2 O2 HC 2oOH ⎯⎯⎯⎯⎯⎯⎯→

This free radical with a high oxidation potential of 2.8 V can attack dyes molecules and result in the breakage of chromophore group [31]. In addition, the mentioned decolorization pathways are also affected by the dissociation energy of pollutants molecules as well as water molecules. The dissociation energy of water molecules is 119 kcal/mole. Therefore, the decolorization of days can mainly happen by pyrolysis of the dyes molecules containing bonds with significantly lower dissociation energy [10]. Suresh Kumar et al. [7] investigated the decolorization of Methylene blue by HC-related processes using a circular venturi as a cavitating device. They reported that 32.32 % decolorization was achieved using HC alone at pH of 2 and pressure of 5 bar within 120 min. Cai et al. [20] performed the decolorization of Orang G by HC using an orifice plate as a cavitating device. They reported 25.6 % decolorization for the dye at pH of 3 and fluid flow of 0.55 Ls−1 within 120 min. Rajoria et al. [25] studied the decolorization of Reactive blue 13 via HC using a slit venturi as a cavitating device. They achieved 47 % decolorization at pH of 2 and 4 bar pressure within 120 min. Baradaran et al. [31] investigated HC for the decolorization of Coomassie brilliant blue. They used a single hole circular orifice as a cavitating device and reported 45 %

(4)

The radicals can diffuse among fluid, react with pollutants molecules, and lead to the degradation of pollutants. In addition, H2O2 can enhance cavitation zone and therefore increase the number of cavitation events. In fact, it can act as a nuclei and lead to an increase in cavitation intensity [21,31]. Hence, the enhanced decolorization achieved using the combined process of HC and H2O2 compared to the result obtained using individual HC may be the result of synergistic effect of H2O2 and HC on the formation of hydroxyl radicals. [27,33]. 3.4. Decolorization via HC combined with Fenton’s reagent The combination of HC and Fenton’s reagent provides more hydroxyl radicals than the individual processes. This combined process has been considered to be an effective process for the treatment of industrial wastewater [35]. According to Eq. 5, hydroxyl radical and ferric ion are generated thorough reacting Fe2+ with H2O2. Fe2+ + H2O2 → Fe3+ + °OH + HO−

Fig. 2. Diagnostic plots ((a) Normal probability of studentized residual, (b) Predicted and actual results). 4

(5)

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Fig. 3. Effects of the individual parameters on the decolorization using HC + fenton’s reagent ((a) effect of FeSO4, (b) effect of H2O2, (c) effect of pH).

Fig. 4. 3D response surface graph for the decolorization of Congo red (FeSO4 vs H2O2). (For interpretation of the references to colour in the Figure, the reader is referred to the web version of this article).

Fig. 5. 3D response surface graph for the decolorization of Congo red (FeSO4 vs pH). (For interpretation of the references to colour in the Figure, the reader is referred to the web version of this article).

Further reactions can result in the regeneration of Fe2+. Therefore, Fe2+ may act as a catalyst, and the cycle of generation of hydroxyl radicals can continue as long as H2O2 exists in the solution [34]. In the current work, a maximum decolorization of 70 % was achieved using HC in combination with Fenton’s regent at a H2O2 concentration of 1000 mg L−1, a FeSO4 concentration of 25 mg L−1, an initial pH of 3, and mentioned operating conditions. H2O2 concentration, FeSO4

concentration, and initial pH were studied as some of the effective parameters for the decolorization of Congo red. To evaluate the effects of the parameters and the interactions, Fig. 3–6 were plotted using design expert 10.0.7 software. Fig. 3 shows the effect of each of the parameters individually. According to the results given in Fig. 3(a), at the fixed values of H2O2 5

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appeared more effectively in a situation where a higher amount of FeSO4 was added to the solution. The trend can show that HC was not able to decompose the whole amount of H2O2 at the mentioned operating conditions. Hence, at the lower amounts of FeSO4, a portion of H2O2 remained unreacted, which resulted in a reduction in the effect of H2O2 on the decolorization. Furthermore, in the absence of H2O2, increasing FeSO4 loading from 0 to 50 mg L−1 led to an increase in the decolorization from 21 % to 31 %, while the decolorization achieved by increasing FeSO4 loading was almost double at the maximum amount of H2O2. In fact, using maximum amount of H2O2, increasing FeSO4 from 0 to 50 mg L-1 resulted in an increase in the decolorization from 41 % to 60 %, which can confirm that FeSO4 played a more efficient role in a situation where a higher amount of H2O2 was utilized. The trend can demonstrate that the amount of H2O2 generated in the solution trough was not adequate for reacting with the whole amount of FeSO4. Therefore, at the lower amounts of H2O2, a portion of FeSO4 did not contribute to the generation of hydroxyl radicals, which caused a decrease in the effect of FeSO4 on the decolorization. In fact, the results shown in Fig. 4 can confirm the synergistic effect of H2O2 and FeSO4. This synergistic effect resulted in an increase in the generation of hydroxyl radicals. Therefore, the performance of the combination of HC and Fenton’s reagent was more effective than the performances of the other combined processes (HC+H2O2 and HC + FeSO4). According to the results shown in Fig. 5, increasing FeSO4 and decreasing pH improved the decolorization achieved using the combination of HC and Fenton’s reagent. The enhanced decolorization achieved by increasing FeSO4 loading can be attributed to the fact that since FeSO4 is one of the reactants in Eq. (5), increasing it can result in the intensification of generation of hydroxyl radical. In addition, it can be observed that using the combined process of HC and H2O2 (at FeSO4 = 0), a decrease in the pH resulted in an increase in the decolorization. It is shown in Fig. 6 that increasing H2O2 and decreasing pH enhanced the decolorization attained using the combined process of HC and Fenton’s reagent. The improved decolorization obtained as a result of increasing H2O2 loading can be attributed to the fact that H2O2 as a source of hydroxyl radicals can produce these active radicals through Eq. (4) and (5) and therefore increasing it can lead to an increase in the decolorization. In addition, it is observed that by the use of HC combined with FeSO4 without using any external source of H2O2 (at H2O2 = 0), a decrease in the initial pH led to an increase in the decolorization. The effect of changing the initial pH upon the decolorization of Congo red can be attributed to the fact that the generation of hydroxyl radicals and the concentration of Fe2+ are controlled by the pH of solution [34]. At high pH values, the decomposition of H2O2 occurs slowly and the generation of Fe(Π) complexes can cause a decrease in Fe2+ concentration. [39,40]. The enhanced decolorization achieved by a decrease in the initial pH may also be because of the higher oxidation potential of hydroxyl radicals in the acidic condition [27].

Fig. 6. 3D response surface graph for the decolorization of Congo red (H2O2 vs pH). (For interpretation of the references to colour in the Figure, the reader is referred to the web version of this article).

concentration (500 mg L−1) and initial pH (6.5), increasing FeSO4 resulted in an increase in the decolorization of Congo red. It may be because of the fact that FeSO4 is one of the reactants in the reaction of generation of hydroxyl radical. Therefore, increasing FeSO4 can lead to the intensification of generation of this radical [36]. This trend is in agreement with the results achieved by Bagal et al. [37], who studied the degradation of 2,4-dinitrophenol using HC combined with Fenton’s reagent and reported that increasing FeSO4 concentration led to an increase in the degradation of the pollutant. Also, it is demonstrated in Fig. 3(b) that increasing H2O2 concentration improved the decolorization of Congo red at the fixed values of FeSO4 concentration (25 mg L−1) and initial pH (6.5). This trend can be attributed to the generation of hydroxyl radicals through Eq. (4) and also Eq. (5) [15,38]. In other words, H2O2 is one of the reactants in Eqs. (4) and (5) and therefore increasing this substance can have a positive influence on the generation of hydroxyl radicals and enhance the decolorization [15]. The achieved trend can be confirmed with some of performed works. Gogate et al. [27] studied the degradation of triazophos using the combination of HC and Fenton’s reagent and reported that the degradation of the pollutant was enhanced as a result of increasing H2O2 concentration. Bagal et al. [37] investigated the degradation of 2,4dinitrophenol and reported the same trend. In addition, the initial solution pH was investigated as another agent which has a major effect on the decolorization of dyes. Generally it has been reported that acidic condition is the appropriate condition for the degradation of pollutant, though the proper pH for degradation using HC also depends on the state of pollutants molecules [27]. According to the results given in Fig. 3 (c), at the fixed values of FeSO4 concentration (25 mg L−1) and H2O2 concentration (500 mg L−1) a decrease in the initial pH led to an increase in the decolorization. This trend can be attributed to the higher oxidation potential of hydroxyl radicals in the acidic condition, which leads to the intensification of the decolorization [26]. The interactions between the parameters are shown in Fig. 4–6. Fig. 4 illustrates a 3D response surface graph to assess the interaction between H2O2 and FeSO4 at a constant initial pH of 6.5, pressure of 6 bar, and temperature of 40 ± 2 °C. According to the results given in Fig. 4 and Table 3, in the absence of FeSO4, increasing H2O2 loading from 0 to 1000 mg L−1 resulted in a continuous increase in the decolorization from 21 % to 41 %. At the maximum amount of FeSO4, increasing H2O2 loading from 0 to 1000 mg L-1 led to an increase in the decolorization from 31 % to 60 %. These results can confirm that H2O2

3.5. Synergistic coefficient The decolorization achieved using HC, Fenton’s reagent and the combination of HC and Fenton’s reagent are shown in Fig. 7. Based on the observations, the lowest amount of the decolorization was obtained using Fenton’s reagent. This low percentage of the decolorization may be because of mass transfer limitation, which is a severe problem of advanced oxidation processes, including Fenton’s reagent [24]. The higher percentage of the decolorization achieved using HC can be attributed to the high mass transfer rate, which is the result of extreme turbulent phenomena and shock waves created by the collapsing bubbles [41]. It is observed that the combined process of HC and Fenton’s reagent was much more efficient than each of the individual processes. The high amount of the decolorization attained using this combined process may be because of the synergistic effect of HC and Fenton’s 6

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Fig. 7. Decolorization of Congo red (at a pressure of 6 bar, temperature of 40 ± 2 °C and pH of 6.5). (For interpretation of the references to colour in the Figure, the reader is referred to the web version of this article).

Fig. 8. Yield efficiency of different processes.

(9). It is used to determine pump energy which is required to remove a specific amount of a dye [21]:

reagent. Apparent rate constants were calculated according to Eq. (6).

ƞ=

C

k=

ln( C0 )

Where k, C0, C, and t represent the apparent rate constant, the initial concentration of the dye, the concentration at time t, and the time when the decolorization was performed, respectively. According to Table 5, apparent rate constants of 4 × 10−3 min-1, 0.75 × 10−3 min-1, and 15.3 × 10−3 min-1 were obtained using HC, Fenton’s reagent, and the combined process of HC and Fenton’s reagent. It is demonstrated that the combined process was significantly more effective than each individual processes. The effectiveness can become more obvious by using a synergistic coefficient. The synergistic coefficient was calculated to determine the efficacy of the combined process according to Eq. (7).

synergetic coefficient =

k (HC + Fenton) k (HC ) + k (Fenton)

(7)

A synergistic coefficient of 3.22 achieved by the combined process can be attributed to an increase in the generation of hydroxyl, which is the result of regeneration of Fe2+ according to Eq. 8, and a decrease in the mass transfer resistance, which is the result of high turbulence created by the collapse of bubbles.

Fe−OOH2+ HC Fe2++ HOOo

3.7. Reactor hydrodynamic modeling RTD is extensively utilized in the modeling and design of equipment in a variety of processes. It is usually used for analyzing the characteristics of equipment to enhance the performance and design [32]. To study the flow behavior of the HC reactor, the tracer concentration was measured at the same time intervals. The results of this experiment are shown in Fig. 9. Next, the experimental RTD curve, which represents the time when different particles spent in the HC reactor, was plotted based on Eq. (10):

(8)

⎯⎯⎯⎯⎯⎯⎯→

(9)

Where ƞ, ΔC , and Epump represent yield efficiency in mg kJ−1, the amount of removal attained during a specific time in mg, and pump energy consumed at that specific time in kJ (Epump = pomp power (kW ) × time (second) ). According to the results shown in Fig. 8, the combined process of HC and Fenton’s reagent is more efficient than each of the individual processes. The yield efficiency attained using the combined process was 8.3 × 10−3 mg kJ-1 in 60 min, while the yield efficiency obtained by the individual processes of HC and Fenton’s reagent were 2.9 × 10−3 mg kJ-1 and 6 × 10-4 mg kJ-1, respectively. Using the combined process of HC and Fenton’s reagent, the yield efficiency reached a peak at 10 min and then continuously dropped. This trend may be because of the high concentration of the oxidant in the initial period of the experiment. Using HC, the yield efficiency had almost the same trend despite the small peak at 30 min. This trend can be attributed to the high probability of the dye molecules exposure to the hydroxyl radicals, which was provided by the high concentration of the dye in the initial period of the reaction time.

(6)

t

ΔC Epump

The result are in overall agreement with Gogate et al. [27], who investigated the synergistic effect of the combined process of HC and Fenton’s reagent and reported a synergistic coefficient of 3.34. 3.6. Yield efficiency The yield efficiency of a process is calculated according to the Eq. Table 5 Kinetics of decolorization of Congo red. Type of process

H2O2 concentration (mg L−1)

FeSO4 concentration (mgL−1)

Pseudo-first order rate constant (k × 10−3 min-1)

R2

HC Fenton HC + Fenton

0 1000 1000

0 50 50

4.0 0.75 15.3

0.901 0.940 0.995

7

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Fig. 10. Comparison between E(t)experiment and E(t)model.

Fig. 9. Concentration of the tracer.

E (t )experiment =

model curves [29]. By this comparison, the number of tanks which results in the fittest model for representing the HC reactor can be determined. Therefore, the tank-in-series model RTD curves with one tank, two tanks, and three tanks were obtained using Eq. (15), which is based on the tracer mass balance [28].

C (t ) tMax

∫ C (t ) dt 0

(10)

Where E (t )experiment and C (t ) are the experimental RTD and the tracer concentration at the time of t , respectively. Proper models which are developed based on RTD are useful for representing the hydrodynamic behavior of real reactors and scalingup. The flow patterns of ideal reactors are described with mixed flow or plug flow, but the flow patterns of real reactors are usually between these two ideal situations. Tank-in-series-model, which is based on the idea that fluid flows through a series of N ideal mixed flow reactors, is a simple model and can be utilized with all kinetics. The primary parameter of the model is the number of the equal tanks (N) [28,29]. If N = 1 or σm2 = 1, the reactor is an ideal mixed flow reactor, and if N=ꝏ or σm2 = 0, it is an ideal plug flow reactor. Hence, a high value of N shows that the flow regime within the reactor approaches to the plug flow, and a low value of N indicates that the flow behavior within the reactor approaches to the mixed flow [32]. The mean residence time (tm) , which represents the average time spent by the tracer in the HC reactor, the variance of the residence time (σm2 ) , which represents the broadness of the RTD curve, the dimensionless variance (σ θ2) , and finally the number of tanks (N ) were calculated according to following Eqs. [28,29].

E (t )modlel =

nexperiment

SSE =

(11)

∫ t 2C (t ) dt =

0 tmax

∫ C (t ) dt 0

− tm2 (12)

nexperiment

nexperiment

∑

(E (t )experiment − E (t )model)2

i

4. Conclusion

σm2 tm2

(13)

1 N= 2 σθ

(14)

σ θ2 =

1

Where SSE , nexperiment , and E (t )model represent the sum of the square errors, the number of experimental samples, and the tank-in-series model RTD, respectively. Using Eq. (16) and Fig. 10, it is confirmed that the tank-in-series model with two tanks has a lower value of SSE than the model with one tank or three tanks and better fits the experimental results. According to the Fig. 10, the nonsymmetrical RTD curve of the HC reactor and its broadness can show its approximation to the mixed flow. As expected, increasing the number of tanks resulted in a decrease in the broadness of the model curve. In addition, since the flow behavior of the HC reactor can be estimated by the behavior of the fluid passing through a serious of two ideal mixed flow tanks, it can be determined that the flow behavior within the reactor approaches to the mixed flow. RTD is affected by various factors and therefore it is difficult to determine the reasons why the RTD of the HC reactor is different from the RTD of the ideal reactors. However one possible reasons for the differences can be eddy diffusion and molecular diffusion, which lead to deviation from ideal flows, especially from ideal plug flow [42].

tmax

σm2

Erri2 =

(16)

0 tMax 0

∑ i

∫ tC (t ) dt ∫ C (t ) dt

(15)

The experimental RTD curve and the model RTD curves are shown in Fig. 10. Eq. (16) was also used to accurately compare the experimental RTD curve and the model RTD curves with one tank, two tanks, and three tanks.

tMax

tm =

t N −1 t 1 exp(− N ) tmN (N − 1)! tm

The hybrid process was utilized for the decolorization of Congo red. Experimental investigation was carried out utilizing RSM based on BoxBehnken method. A maximum decolorization of 70 % was attained using the combination of HC and Fenton’s reagent within 60 min. The decolorization of the dye through the combination of HC and Fenton’s reagent was improved with increasing FeSO4 concentration and H2O2 concentration and decreasing initial pH. FeSO4 concentration was the

The number of tanks (N ) achieved according to Eq. (14) is two. One method to evaluate reliability of the number of tanks (N) attained from Eq. (14) is to make a comparison between the real RTD curve and 8

Chemical Engineering & Processing: Process Intensification 150 (2020) 107874

Z. Askarniya, et al.

least significant parameter affecting the decolorization of Congo red. A synergistic coefficient of 3.22 demonstrated the effectiveness of the combined process of HC and Fenton’s reagent as opposed to the individual processes. The yield efficiency obtained for HC, Fenton’s reagent and the combined process of HC and Fenton’s reagent were 6 × 10−4 mg kJ-1, 2.9 × 10-3 mg kJ-1, and 8.3 × 10-3 mg kJ-1, respectively. The flow behavior of the HC reactor was investigated using impulse tracer method and the results show that the tank-in-series model with two tanks almost fits the experimental results. Such a finding will be a key to scale up HC reactors.

[16] [17]

[18]

[19]

[20]

CRediT authorship contribution statement [21]

Zahra Askarniya: Writing - original draft, Investigation, Software. Mohammad-Taghi Sadeghi: Conceptualization, Methodology, Supervision, Writing - review & editing. Soroush Baradaran: Methodology, Data curation, Validation.

[22]

[23] [24]

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

[25]

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