ZnO nanocomposite for dye removal from aqueous medium: Optimization by response surface methodology

Accepted Manuscript Title: Photocatalytic process by immobilized carbon black/ZnO nanocomposite for dye removal from aqueous medium: Optimization by response surface methodology Author: R. Darvishi Cheshmeh Soltani A. Rezaee A.R. Khataee M. Safari PII: DOI: Reference:

S1226-086X(13)00421-8 http://dx.doi.org/doi:10.1016/j.jiec.2013.09.003 JIEC 1538

To appear in: Received date: Revised date: Accepted date:

27-5-2013 3-9-2013 5-9-2013

Please cite this article as: R.D.C. Soltani, A. Rezaee, A.R. Khataee, M. Safari, Photocatalytic process by immobilized carbon black/ZnO nanocomposite for dye removal from aqueous medium: Optimization by response surface methodology, Journal of Industrial and Engineering Chemistry (2013), http://dx.doi.org/10.1016/j.jiec.2013.09.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Photocatalytic process by immobilized carbon black/ZnO nanocomposite for dye removal from aqueous medium: Optimization by response surface

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methodology

Department of Environmental Health Engineering, School of Public Health, Arak University of

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R. Darvishi Cheshmeh Soltani 1, A. Rezaee 2, A. R. Khataee 3, M. Safari 4

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Medical Sciences, Arak, Iran.

Department of Environmental Health Engineering, Faculty of Medical Sciences, Tarbiat

Research Laboratory of Advanced Water and Wastewater Treatment Processes, Department of

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Modares University, Tehran, Iran.

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Applied Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran Department of Environmental Health Engineering, School of Health, Kurdistan University of

Medical Sciences, Sanandaj, Iran.

Graphical_Abstract

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Abstract A rectangular photo-reactor equipped with carbon black (CB)/ZnO nanocomposite film was used

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for photodegradation of methyl orange. Central composite design was used for evaluation of the effects of initial dye concentration, reaction time, CB/ZnO ratio and initial pH. The high

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correlation coefficients (R2=0.954 and adjusted R2=0.912) obtained by analysis of variance (ANOVA) demonstrated close fit between the predicted and experimental values. The initial dye

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concentration, reaction time, CB/ZnO ratio and initial pH were 13 mg/L, 95 min, 0.05 and 5,

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respectively for 80% decolorization efficiency. The result of TOC analysis showed that the

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process allowed 55.79% mineralization under optimized conditions.

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Keywords: Photocatalysis; ZnO nanoparticles; Carbon black; Experimental design.

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1. Introduction Photocatalytic processes based on semiconductors function as photocatalyst have been widely

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used as promising techniques in ecosystem protection due to their high efficiency for degradation of various organic pollutants in aqueous environments [1-3]. Among different conventional

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catalysts, ZnO nanoparticles have been widely used because of wide band gap, large volumearea ratio, low cost, large initial rate of activities and UV adsorption potential [4-8]. It seems

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that, like TiO2 that has been modified with carbon materials in several studies [9-13], ZnO can be

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modified with a suitable carbon material to enhance its photocatalytic activity. According to the literature, modification of the photocatalyst with carbon material can be an efficient way for

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improving photocatalytic activity [14-17]. As an alternative, carbon black (CB) has been applied in photocatalytic processes due to its advantages including high electrical conductivity for

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hindering the speed of electron-hole recombination and large pores for easy diffusion of the

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pollutants [18, 19]. In our previous study, we used CB for the preparation of gas diffusion

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cathode because of its high electrical conductivity, fine size and subsequently high porosity for efficient generation of hydrogen peroxide [20]. In the present study, CB was incorporated into ZnO nanoparticles to gain the aforementioned advantages to improve photocatalytic oxidation of methyl orange (MO) as target pollutant. The formation of hydroxyl radicals (OH•) during a photocatalytic process over ZnO nanoparticles is shown in Eqs. 1-2 [21-23]: (1) (2) The toxicity of nanomaterials like ZnO for terrestrial and aqueous ecosystems has been demonstrated [24]. In addition, the agglomeration of slurry mixture of carbon and ZnO in 3 Page 3 of 35

aqueous solutions has been observed [17]. Therefore, in the present study, CB/ZnO nanocomposite was immobilized on glass plates to reduce its potentially negative environmental effects and agglomeration in aqueous solutions. To the best of our knowledge and on the basis of

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literature data, there has been no investigation on the degradation of dye using photocatalytic process over CB/ZnO nanocomposite. To achieve valuable results regarding the efficiency of the

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photocatalytic process for MO removal, central composite design (CCD) was used instead of

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conventional “one-factor-at-a-time” method [25-28]. Using CCD, the effect of initial dye concentration, reaction time, carbon black to ZnO ratio and initial pH on decolorization

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efficiency was evaluated.

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2.1. Chemicals

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2. Material and methods

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Methyl orange was chosen as a model organic pollutant to evaluate the photocatalytic activity of CB/ZnO nanocomposite, and its characteristics can be seen in Table 1. ZnO nanoparticles were purchased from US Research Nanomaterials, USA. All analytical grade reagents were purchased from Merck, Germany. An industrial grade NaOH solution was used for functionalization of the glass surface. The CB used in this study was Vulcan XC72R purchased from Cabot Co., USA. To adjust the pH of the solutions, NaOH and H2SO4 solutions with desirable molarity were used.

2.2. Preparation of CB/ZnO nanocomposite A mixture of ZnO nanoparticles and CB was dissolved in distilled water to achieve a 3% suspension. The resulting suspension was mixed with a magnetic stirrer (Heidolph MR 3001, 4 Page 4 of 35

Germany) at 500 rpm for 2 h. After that, it was sonicated in an ultrasonic bath (Starsonic 18-35, Italy) at a temperature of 50°C for 90 min. The homogeneous suspension was then coated on the surface of glass plates using a pipet (5 mL for each glass plate). Before coating, the bare glass

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plates were dipped in a NaOH solution (50%) for 24 h. This approach leads to the functionalization of the surfaces with hydroxyl groups to avoid detachment of the CB/ZnO

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nanocomposite. The glass plates containing nanocomposite were dried in room temperature for

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24 h and then annealed at 400°C for 3 h in an electric furnace (Exciton Co., Iran).

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2.3. Experimental reactor

The photocatalytic process was carried out in a 600-mL rectangular reactor equipped with three

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3×20 cm glass plates. Five 6-W low-pressure UVC lamps with peak intensity at 254 nm (Philips,

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Holland) were placed 2 cm from the surface of the three glass plates containing nanocomposite

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film. The solution containing MO dye is passing as a thin film onto CB/ZnO nanocomposite immobilized on glass plates. Recirculation of the solution containing MO through the reactor

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was carried out using a Heidolph peristaltic pump (Model 5001, Germany) at a constant recirculation rate of 100 rpm. The nanocomposite was regenerated after each run by recirculating solution containing a few drops of H2O2 under UV light irradiation. The solution temperature in the reactor was controlled at 25°C by placing the pipes within cooled water and was measured by a thermometer. Before each experiment, the reactor was placed in the dark and the dye solution was circulated through the reactor for 30 min to reach adsorption equilibrium.

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2.4. Analytical methods A portable pH meter was used to measure the pH of the solution (pHTestr 10, Malaysia).

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Residual MO in the solution was measured with a UV-Vis spectrophotometer (Unico 2100) at 464 nm. The concentrations of MO were calculated using a seven-point standard calibration

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curve. The decolorization efficiency was estimated through Eq. 3:

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(3)

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where Co and C are the dye concentrations at time 0 and t, respectively. Total organic carbon (TOC) was measured using a Shimadzu TOC analyzer (TOC-VCSH, Japan). Structural

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characteristics of the pure ZnO, pure CB and CB/ZnO nanocomposite were analyzed using a scanning electron microscope (Philips XL 30, the Netherlands). The X-ray diffraction (XRD)

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patterns of the samples were gained by a Philips X-ray diffractometer (Model: X’Pert MPD, the

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material of Cu.

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Netherlands) with scanning angle = 20-80°, λ = 1.54056 Å, step size = 0.02°/s and anode

2.5. Experimental design based on CCD RSM based on CCD as a widely used experimental design was used to optimize the MO removal by the photocatalytic process. Design-Expert software was used for the analysis of the obtained experimental data. The effect of four main factors (parameters) influencing the photocatalytic MO removal was assessed: the initial dye concentration (mg/L), reaction time (min), CB/ZnO ratio and initial pH. The number of experiments was estimated through Eq. 4:

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(4) where N, k and x0 are the number of required experiments (facts), the number of parameters and

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the number of central points, respectively [25, 26]. Based on Eq. 5, the total number of experiments was 30 (k =4, x0 = 6). The chosen parameters (Xi) were coded as xi according to the

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following equation:

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(5)

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where x0 and δx are the values of xi at the center point and step change, respectively [25, 27]. The mathematical relationship between the response (decolorization efficiency) and the operational

(6)

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parameters can be described through Eq. 6:

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where Y is the predicted response and b0, bi, bij and bii are constant, linear, interaction and

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quadratic coefficients, respectively. In addition, xi and xj are the coded values for the experimental parameters [25]. The ranges of the parameters influencing the MO removal are presented in Table 2.

3. Results and discussion 3.1. SEM analysis

The SEM images of the pure ZnO nanoparticles, pure CB and CB/ZnO nanocomposite attached on the glass plates are shown in Fig. 1. The obtained SEM images demonstrate uniformity in size of both unmodified-ZnO and CB/ZnO nanocomposite coated on the glass plates which is related 7 Page 7 of 35

to the fine size of the carbon black incorporated into the pure ZnO. Fig. 1 (c) shows densely packed particles with low bare spaces on glass plate as a result of tight grafting of CB to the ZnO nanoparticles. It led to homogeneous coverage of the glass plates by the CB/ZnO nanocomposite

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3.2. Effect of the presence of CB on photocatalytic MO removal

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without any tendency to co-aggregation.

To evaluate the effect of the modification of ZnO nanoparticles with CB on decolorization

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efficiency, experiments with pure ZnO, CB/ZnO and UV alone (photolysis) were carried out and

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the results showed that adsorption of MO onto pure ZnO (6.53%) and CB/ZnO (10.33%) could be responsible for a little portion of decolorization efficiency within 150 min reaction time (Fig.

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2). Additionally, photolysis process with UV lamps alone had a decolorization efficiency of

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28.53% which is insignificant for exposure time of 150 min. Fig. 2 shows that pure ZnO nanoparticles film can’t be efficient enough to remove MO in aqueous medium via

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photocatalytic process (60.53%), indicating the need for modification of pure ZnO for photocatalytic MO removal. Similar results have been reported by Wang et al. for photocatalytic MO removal using pure ZnO film [29]. As can be seen in Fig. 2, the process allowed 87.27% decolorization efficiency (%) within 150 min using CB/ZnO nanocomposite. The synergistic effect of the modification of ZnO nanoparticles with CB can be attributed to the high electrical conductivity of CB which can reduce the speed of electron-hole recombination along with enhanced UV light adsorption potential of CB-incorporated ZnO compared to the pure ZnO nanoparticles [18, 30]. Furthermore, the incorporation of CB into the ZnO nanoparticles results in long-term activity of the catalyst for the generation of hydroxyl radicals [19]. Besides, as

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stated in the previous section, ZnO modification with CB prevents aggregation of the pure ZnO nanoparticles on the glass surface and thus maximizing their photo-excitation under UV irradiation [13]. Pulido Melian et al. exhibited that photocatalytic activity of carbon/ZnO

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composite is much more than pure ZnO [17]. In agreement to our findings, Mao et al. demonstrated that the photocatalytic activity of CB/TiO2 was further than that of activated

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carbon/TiO2 composite [18].

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3.3. Model results for decolorization

According to the RSM based on CCD, an empirical mutual relationship between the dependent

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parameter (decolorization efficiency) and independent parameters involved in the photocatalytic

(7)

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process was attained using Design-Expert software and is expressed in Eq. 7:

According to the above relationship, the experimental and predicted results for decolorization efficiency are presented in Table 3. To test the significance and adequacy of the quadratic model, an analysis of variance (ANOVA) was conducted [25, 26]. As shown in Table 4, the regression model has a high coefficient of determination (R2=0.954). The value of the adjusted R2 also demonstrates the significance of the model [31]. The value of the adjusted R2 was found to be 0.912. Therefore, it seems that there is not a significant difference between R2 and adjusted R2. In addition, low P-values in the present study demonstrate the significance of the model (Table

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5). The preferred “Adequate precision” value is greater than 4 [25]; thus, the ratio of 17.71 in the present work implies the suitability of the model (Table 4). Finally, a relatively low coefficient of variation (9.17%) indicates the good reliability of the model. Fig. 3 (a) shows a good

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agreement between the predicted and experimental decolorization efficiency (R2=0.954), indicating the adequacy and significance of the model. In addition, the adequacy of the model

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can be evaluated by the residuals calculated by determining the difference between the

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experimental and the predicted decolorization efficiency. Fig. 3 (b) depicts the internally studentized residuals and normal probability plot for decolorization efficiency. As is evident

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from the Fig. 3 (b), the obtained data points consistently appear on a straight trend line, demonstrating that there is no obvious dispersal. Furthermore, the residuals are plotted against

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the predicted decolorization efficiency (Fig. 3 (c)) and the run number (Fig. 3 (d)). Random

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dispersal of the residuals can be seen in the Fig. 3 (c) and (d).

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3.4. Interactive influences of operational parameters on decolorization efficiency Response surface and contour plots can be applied for various interactions of any two parameters [26, 32]. These plots can be used to assess any changes in the response surface based on a polynomial function. In this approach, two parameters are constant and the other two parameters will be varied within the experimental ranges [25-27]. The effect of the initial dye concentration on decolorization efficiency is depicted in Fig. 4 at a CB/ZnO ratio of 0.1 and initial pH of 7.5. Fig. 4 shows that decreasing initial dye concentration from 30 to 5 mg/L results in increasing decolorization efficiency. As the initial dye concentration decreases, more hydroxyl radicals (OH•) are available to degrade the dye, such that decolorization efficiency increases [27].

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Furthermore, it can be stated that OH• concentrations decreases as a result of decreasing path length of the photons entering the dye solution to excite the surface of the photocatalyst, while the initial dye concentration increases [33, 34]. The same result has been reported by Zhu and

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coworkers during the photocatalytic removal of MO using TiO2/ZnO/chitosan nanocomposite film [34]. Fig. 5 shows the effect of CB/ZnO ratio ranging from 0.01 to 0.18 on decolorization

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efficiency at an initial dye concentration of 18 mg/L and initial pH of 7.5. As depicted in Fig. 5,

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decolorization efficiency increased as the CB/ZnO ratio increased from 0.01 to 0.05 and then decreased as the ratio increased from 0.05 to the highest value of 0.18. The possible reason is

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that when the amount of CB incorporated into ZnO nanoparticles increases, the light-shielding effect due to black-colored CB increases [13, 35]. As is obvious from Fig. 6, the optimum

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CB/ZnO ratio is necessary for enhancing decolorization efficiency and avoiding the use of

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excess catalyst. Mao et al. in their study on photocatalytic MO removal in an aqueous slurry

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solution using TiO2/CB composite demonstrated that the ratio of TiO2 to CB largely affected the decolorization efficiency [18]. At optimum CB/ZnO ratio, the enhanced photocatalytic MO

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removal is ascribed to improved UV light adsorption behavior of CB/ZnO which is imperative for better exciting the photocatalyst to generate hydroxyl radicals [30]. The effect of initial pH on decolorization efficiency (%) was depicted in Fig. 7 at an initial dye concentration of 18 mg/L and CB/ZnO ratio of 0.1. It is evident that with the decrease in pH values from alkaline to acidic conditions along with increasing reaction time, decolorization efficiency increases. Whereas the zero-point charge of pure ZnO nanoparticles is about 9.0 [36], lower pH values lead to a positive charge on ZnO nanoparticles surface, which favors adsorption of MO anions on to ZnO surface and subsequent increase in the photocatalytic dye removal [10, 34]. Increasing the pH from acidic to alkaline conditions results in the electrostatic repulsion between the MO anions and the

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surface of the catalyst due to the adsorption of hydroxyl anions onto the catalyst [37]. Decolorization efficiency at reaction time of 65 min and initial dye concentration of 18 mg/L as the function of CB/ZnO ratio and initial pH is depicted in Fig. 8. As shown, decolorization

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efficiency increases with increasing CB/ZnO ratio up to about 0.05 then decreases. In the case of the effect of initial pH, a low increase in decolorization efficiency was observed with the

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decrease in initial pH from 12 to 3 (Fig. 8). A low color removal of nearly 20% was obtained

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with increasing pH value in the range of 3-12. This behavior may be interpreted by the fact that the adsorption of OH¯ onto the surface of ZnO at alkaline conditions favors the formation of

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hydroxyl radicals, resulting in increasing decolorization efficiency [38, 39]. This mechanism

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hinders the sharp decrease in decolorization efficiency at high pH values.

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3.5. Decolorization optimization and confirmation To optimize the operational parameters for maximum decolorization efficiency, the operational

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variables were set to values within the studied range, whereas the response (decolorization efficiency) was set to achieve a maximum value. With this approach, the maximum decolorization efficiency was 80% at an initial pH of 5, initial dye concentration of 13 mg/L, reaction time of 95 min and CB/ZnO ratio of 0.05. To validate the obtained optimal parameters, additional experiments were carried out to confirm the decolorization efficiency. The experiment showed a decolorization efficiency of 83.79% under optimal conditions compared with the decolorization efficiency of 80% obtained by the model. This indicates the suitability and accuracy of the model.

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3.6. Mineralization Apart from decolorization process, mineralization of MO and its photocatalytic degradation to

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carbon dioxide, water and mineral ions was studied by measuring TOC value of the solution as the function of reaction time. Fig. 9 shows the removal efficiencies for color and TOC by the

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photocatalytic process consisting CB/ZnO composite as catalyst under optimized operational parameters (initial dye concentration = 13 mg/L, reaction time = 95 min, CB/ZnO ratio = 0.05

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and initial pH = 5). Trend observed in Fig. 9, shows a reasonable decrease in TOC with respect

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to the decolorization efficiency. As can be seen in Fig. 9, after reaction for 95 min decolorization and TOC removal efficiencies were 83.79% and 55.79%, respectively. It can be stated that not

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4. Conclusions

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only decolorization but also mineralization of the dye was significant.

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In the present study, photocatalytic dye removal over immobilized CB/ZnO nanoparticles composite in an aqueous medium has been studied in the context of RSM. We found that modification of ZnO nanoparticles with a suitable portion of CB significantly affected the physicochemical properties of the pure ZnO and subsequently, increased photocatalytic dye removal using an optimal CB to ZnO ratio of 0.05. The results showed that the decolorization efficiency increased with decreasing initial pH and initial dye concentration. Optimization of the process by RSM based on CCD showed an 80% decolorization efficiency under optimized operational conditions (initial dye concentration = 13 mg/L, reaction time = 95 min, CB/ZnO = 0.05 and initial pH = 5). The results obtained indicated applicability of the present process for the efficient removal of dye from aqueous media. 13 Page 13 of 35

Acknowledgement The authors thank Tarbiat Modares University and Arak University of Medical Sciences for their

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supports.

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Figure captions Fig. 1. Scanning electron microscopy (SEM) images of (a) pure ZnO nanoparticles, (b) pure CB

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and (d) CB/ZnO nanocomposite immobilized on glass plates taken at 30,000X magnification. Fig. 2. Decolorization efficiency (%) of methyl orange as the function of the presence of carbon

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black within pure ZnO nanoparticles. Initial dye concentration = 15 mg/L, reaction time = 150

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min, CB/ZnO ratio = 0.1, initial pH = neutral, recirculation rate = 100 rpm.

Fig. 3. (a) A plot of the predicted versus the experimental decolorization efficiency (%) and (b, c

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and d) corresponding residual plots.

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Fig. 4. The response surface plot and corresponding counter plot of the methyl orange removal as the function of initial dye concentration (mg/L) and reaction time (min). CB/ZnO ratio = 0.1,

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initial pH = 7.5.

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Fig. 5. The response surface plot and corresponding counter plot of the methyl orange removal

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as the function of CB/ZnO ratio and reaction time (min). Initial dye concentration = 18 mg/L, initial pH = 7.5.

Fig. 6. The response surface plot and corresponding counter plot of the methyl orange removal as the function of initial pH and reaction time (min). CB/ZnO ratio = 0.1, initial dye concentration = 18 mg/L.

Fig. 7. The response surface plot and corresponding counter plot of the methyl orange removal as the function of initial dye concentration (mg/L) and CB/ZnO ratio. Initial pH = 7.5, reaction time = 65 min.

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Fig. 8. The response surface plot and corresponding counter plot of the methyl orange removal as the function of initial pH and CB/ZnO ratio. Initial dye concentration = 18 mg/L, reaction time

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= 65 min. Fig. 9. Decolorization efficiency (%) and TOC removal efficiency (%) by photocatalytic process

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over CB/ZnO nanocomposite. Initial dye concentration = 13 mg/L, reaction time = 95 min,

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CB/ZnO ratio = 0.05 and initial pH = 5.

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Table 1. Characteristics of methyl orange.

Density (g/cm3)

Molar mass (g/mol)

λmax (nm)

C14H14N3O3SNa

1.28

327.33

464

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Formula

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Structure

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Table 2. Ranges of the operational parameters for experimental design through central composite design -2 5 10 0.01 3

Range 0 18 65 0.1 7.5

+1 24 93 0.14 9.8

+2 30 120 0.18 12

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cr

Initial dye concentration (mg/L) Reaction time (min) CB/ZnO ratio Initial pH

-1 11 38 0.05 5.3

ip t

Parameter

21 Page 21 of 35

Table 3. Experimental and predicted results of the applied model for photocatalytic methyl orange removal over CB/ZnO nanocomposite.

58.29 35.62 29.70 72.65 77.15 47.68 28.22 29.42 74.36 46.10 50.07 52.87 39.34 54.26 28.05 38.42 49.14 57.12 82.91 52.87 21.72 52.87 50.67 62.48 52.87 68.43 52.87 77.10 52.87 48.61

Residual -1.08 -2.30 -2.36 3.16 -0.42 1.76 -0.75 -0.19 1.05 -8.88 0.19 4.34 1.89 -1.09 4.37 -0.14 -0.75 -0.70 -3.58 2.58 5.39 0.47 2.80 9.67 -0.54 3.94 3.85 -3.61 -2.63 2.71

ip t

57.21 33.32 27.34 75.81 76.73 49.44 27.47 29.23 75.41 37.22 50.26 57.21 41.23 53.17 32.42 38.28 48.39 56.42 79.33 55.45 27.11 53.34 53.47 72.15 52.33 72.37 56.72 73.49 50.24 51.32

cr

5.3 5.3 7.5 7.5 7.5 9.8 7.5 5.3 9.8 12.0 7.5 7.5 9.8 5.3 9.8 7.5 5.3 5.3 5.3 7.5 9.8 7.5 9.8 3.0 7.5 9.8 7.5 5.3 7.5 9.8

Experimental Predicted

us

0.05 0.05 0.10 0.10 0.10 0.14 0.10 0.14 0.05 0.10 0.01 0.10 0.14 0.14 0.05 0.18 0.14 0.05 0.05 0.10 0.14 0.10 0.05 0.10 0.10 0.14 0.10 0.14 0.10 0.05

Decolorization efficiency (%)

an

Initial pH

M

24 24 18 18 5 11 30 24 11 18 18 18 24 11 24 18 24 11 11 18 24 18 11 18 18 11 18 11 18 24

CB/ZnO ratio

d

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Reaction time (min) 93 38 10 120 65 38 65 38 93 65 65 65 93 38 38 65 93 38 93 65 38 65 38 65 65 93 65 93 65 93

te

Initial dye conc. (mg/ L)

Ac ce p

Run

22 Page 22 of 35

Table 4. Analysis of variance (ANOVA) for decolorization of methyl orange by the photocatalytic process. Sum of Degree of Mean F-value P-value squares freedom square (Prob > F) Regression 7155.13 14 511.08 22.35 0.0001 Residuals 343.03 15 22.87 Lack of Fit 306.14 10 30.61 4.15 0.0649 Pure Error 36.89 5 7.38 Total 7498.16 29 R2 = 0.954, adjusted R2 = 0.912, adequate precision = 17.71, coefficient of variation (CV) = 9.17

Ac ce p

te

d

M

an

us

cr

ip t

Source

23 Page 23 of 35

Table 5. Estimated regression coefficient and corresponding F and P values from the obtained data of central composite design experiments. P-value 0.0001 0.0001 0.0001 0.0105 0.0008 0.5389 0.5349 0.8232 0.5700 0.6672 0.9807 0.7542 0.6503 0.0264 0.7028

cr

ip t

F-value 22.35 154.70 124.22 8.53 17.37 0.40 0.40 0.052 0.34 0.19 0.0006 0.10 0.21 6.06 0.15

us

Standard error 1.95 0.98 0.98 0.98 0.98 1.20 1.20 1.20 1.20 1.20 1.20 0.91 0.91 0.91 0.91

an

Coefficient estimate 54.22 -12.14 10.88 -2.85 -4.07 -0.75 -0.76 -0.27 -0.69 -0.52 -0.029 -0.29 -0.42 -2.25 0.36

Ac ce p

te

d

M

Coefficient b0 b1 b2 b3 b4 b12 b13 b14 b23 b24 b34 b11 b22 b33 b44

24 Page 24 of 35

Table 6. Optimum values of the parameters for 80% decolorization efficiency by the process. Optimum value 13 95 0.05 5

Ac ce p

te

d

M

an

us

cr

ip t

Parameter Initial dye concentration (mg/L) (x1) Reaction time (min) (x2) CB/ZnO ratio (x3) Initial pH (x4)

25 Page 25 of 35

te

d

M

an

us

cr

ip t

Graphical Abstract.docx

Ac ce p

Comparison of decolorization efficiency of different processes. Photocatalytic process by immobilized carbon black (CB)/ZnO nanocomposite has the highest efficiency; (Experimental conditions: Initial dye concentration = 15 mg/L, reaction time = 150 min, CB/ZnO ratio = 0.1, initial pH = neutral, recirculation rate = 100 rpm).

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Fig. 1.docx

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Fig. 1

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Fig. 2

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Fig. 3

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Fig. 4

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Fig. 5

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Fig. 5.docx

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Fig. 6

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Fig. 6.docx

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Fig. 7

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Fig. 7.docx

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Fig. 8

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Fig. 8.docx

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Fig. 9.docx

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Fig. 9

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