In the present study, ZnO nanostructures were ultrasonically synthesized and immobilized on the surface of diatomite and used for visible light photocatalysis of Acid Red 88 (AR88) in the aqueous phase. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses were employed for characterization of the samples. The process was optimized via response surface methodology (RSM) on the basis of central composite design (CCD). Based on numerical optimization, for the maximized color removal of 96% the initial dye concentration, the catalyst dosage, the reaction time, and the initial pH were 13 mg/L, 1.5 g/L, 85 min, and 4, respectively.
Turk J Chem (2016) 40: 454 466 ă ITAK ˙ c TUB ⃝ Turkish Journal of Chemistry http://journals.tubitak.gov.tr/chem/ doi:10.3906/kim-1504-20 Research Article Visible light photocatalysis of a textile dye over ZnO nanostructures covered on natural diatomite Reza DARVISHI CHESHMEH SOLTANI∗, Zohreh HAGHIGHAT Department of Environmental Health Engineering, School of Health, Arak University of Medical Sciences, Arak, Iran Received: 09.04.2015 • Accepted/Published Online: 27.10.2015 • Final Version: 17.05.2016 Abstract: In the present study, ZnO nanostructures were ultrasonically synthesized and immobilized on the surface of diatomite and used for visible light photocatalysis of Acid Red 88 (AR88) in the aqueous phase Scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses were employed for characterization of the samples The process was optimized via response surface methodology (RSM) on the basis of central composite design (CCD) Based on numerical optimization, for the maximized color removal of 96% the initial dye concentration, the catalyst dosage, the reaction time, and the initial pH were 13 mg/L, 1.5 g/L, 85 min, and 4, respectively The initial pH produced the largest effect, while the adsorbent dosage represented the lowest individual effect on the photocatalysis of AR88 The reusability test showed a 20% reduction in decolorization efficiency (%) within four consecutive experimental runs Overall, ZnOdiatomite nanocomposite can be applied as an efficient photocatalyst for the visible light photocatalysis of target organic dyes Key words: Organic dye, photocatalysis, visible light, nanocrystalline ZnO, immobilization Introduction Many industries such as textile, plastics, clothing, dyestuff, leather, food processing, paper, and pulp produce effluents, containing significant amounts of organic dyes 1−3 These organic molecules or their metabolites have carcinogenic and mutagenic properties, posing a serious threat to human health 1,4 Therefore, the purification of colored effluents must be considered to avoid the aforementioned problems Various treatment techniques have been employed for the decolorization of colored wastewaters, including biological degradation, adsorption, 6,7 membrane technologies, coagulation–flocculation, electrochemical processes, 10 and advanced oxidation processes (AOPs) 11−13 In recent years, the application of AOPs has gained much more attention due to the generation of hydroxyl radical (OH · ) as one of the most powerful oxidants, decomposing refractory organic dyes Nowadays, photocatalytic processes are proposed as one of the most effective AOPs for the remediation of colored effluents 14−18 Among the semiconductors being used for photocatalysis, ZnO, in nanosize, has attracted extensive interest because of its availability, low cost, wide band gap (E g = 3.37 eV), high photosensitivity, large area-to-volume ratio, and large excitation binding energy (60 meV) 11,12,19,20 However, several difficulties using suspended photocatalysts have also been observed as follows: (1) the aggregation of fine particles in the bulk solution, (2) difficulty in the separation of nanoparticles from the solution after the ∗ Correspondence: 454 darvishi@arakmu.ac.ir DARVISHI CHESHMEH SOLTANI and HAGHIGHAT/Turk J Chem process, (3) the small amount of absorbed photons in the visible light region, (4) the high recombination rate of the generated electron–hole pairs, and (5) difficulty in the application of fine particles in continuous flow mode reactors 14,16 Another disadvantage in using suspended nanoparticles is associated with their fine size, which can adversely affect human health and the environment 13,21 For these problems, the immobilization of fine photocatalyst particles on clay-like materials such as montmorillonite, 22,23 bentonite, 12,21 sepiolite, 24 laponite, 14 and kaolin 25 has been investigated Diatomite, which is a sedimentary rock of siliceous crusts composed of silica microfossils of aquatic plants, has been considered an alternative to the proposed clay-like materials because it has a high surface area (high porosity), high absorbability, chemical stability, thermal resistance, and suitable surficial functional groups for the adsorption of fine particles 26−28 It is mainly composed of SiO (> 80%) and widely used as a filter aid, adsorbent, and high temperature insulating material 29 Thus, in the present study, ZnO nanostructures were synthesized and immobilized on natural diatomite and subsequently used for the photocatalysis of Acid Red 18 (AR18) dye, as model organic dye The immobilization of metal oxide nanoparticles on clay-like supports can be performed by different techniques 21 In the present work, ZnO nanostructures were synthesized and uniformly immobilized on the diatomite surface via ultrasound It is demonstrated that the ultrasonically synthesized catalysts have photocatalytic activity higher than that of the catalysts synthesized via conventional methods 30 In the following, the effect of studied operational parameters on the visible light photocatalysis of AR88 was assessed via response surface methodology (RSM) based on central composite design (CCD) This statistical approach gave us the opportunity to conduct experiments with reduced experimental runs In addition, evaluation of the interaction of operational parameters and determination of optimum values for maximum efficiency are achievable via RSM based on CCD 4,31−33 To the best of our knowledge, the use of ultrasonically synthesized ZnO-diatomite nanocomposite for the visible light photocatalysis of organic dyes has not been investigated before Results and discussion 2.1 Characterization Scanning electron microscopy (SEM) analysis was performed The results are provided in Figure Figure 1(a) exhibits the SEM image of raw diatomite The image indicates that the diatomite has a porous and rugged structure suitable for the immobilization of ZnO nanostructures The SEM image of ZnO-coated diatomite shows that the formation and immobilization of ZnO nanostructures on the diatomite surface have been carried out well (Figure 1(b)) Figure exhibits the XRD pattern of ZnO-diatomite nanocomposite As shown, the XRD pattern of ZnO-diatomite nanocomposite contains peaks of both diatomite and ZnO Two sharp peaks at 2θ of 21.8 ◦ and 26.7 ◦ represent the pure phase of crystalline silica in the diatomite structure This is in accordance with the JCPDS Card no 00-001-0647 According to JCPDS Card no 36-1451, the peaks of hexagonal wurtzite ZnO (placed at 2θ of 32.0 ◦ , 34.6 ◦ , 36.3 ◦ , 47.6 ◦ , 56.7 ◦ , 62.9 ◦ , 66.3 ◦ , and 68.0 ◦ ) were shifted to 33.0 ◦ , 33.8 ◦ , 38.0 ◦ , 44.8 ◦ , 54.0 ◦ , 58.6 ◦ , 59.8 ◦ , and 68.9 ◦ after the formation and immobilization on the diatomite surface (Figure 2) These changes are significant enough to demonstrate the creation of nanostructured ZnO particles in interlayer spaces of the diatomite structure The high intensity of the peaks related to ZnO reflect the formation of highly crystalline ZnO nanostructures integrated with the diatomite lattice 22 The crystalline size of the nanocomposite was measured using the Debye–Scherrer equation 34 As a result, the average crystalline size of ZnO-diatomite nanocomposite was computed to be about 43 nm 455 DARVISHI CHESHMEH SOLTANI and HAGHIGHAT/Turk J Chem Figure SEM images of bare diatomite (a) and ZnO-coated diatomite (b) Figure XRD pattern of ZnO-diatomite nanocomposite 2.2 Results of CCD modeling According to the results of CCD modeling, the relationship between the color removal (as response) and studied operational parameters (dye concentration (x1 ), catalyst dosage ( x2 ) , reaction time ( x3 ) , and initial pH (x4 )) was described by an empirical equation as below: Y (Color removal (%)) = 51.20 − 8.99x1 +1.88x2 +3.48x3 −12.00x4 −0.11x1 x2 − 1.02x1 x3 +3.49x1 x4 +0.19x2 x3 +0.13x2 x4 −0.73x3 x4 +1.14x21 − 1.02x22 (1) −1.01x23 +0.79x24 The experiments were carried out according to the experimental design (Table 1) After the experiments, the experimental responses were written down, together with the predicted responses computed by Eq (1) (Table 2) To confirm the results of modeling, analysis of variance (ANOVA) was conducted and its results are given in Table As shown in Table 3, a good fit between the experimental and predicted response (color removal (%)) was achieved based on the obtained correlation coefficient (R = 0.961), indicating that the model can 456 DARVISHI CHESHMEH SOLTANI and HAGHIGHAT/Turk J Chem elucidate 96.1% of the variations in color removal by the process Nevertheless, 3.9% of the variations are not predicted by the applied model The fit between the actual (experimental) and predicted response can be clearly seen in Figure 3(a) The adequacy of the model can be evaluated by the residuals, which were calculated by Table Ranges and levels of the experimental parameters Run no Code Parameter X1 X2 X3 X4 Dye concentration (mg/L) Catalyst dosage (g/L) Reaction time (min) Initial pH Studied –2 (α) 0.5 20 ranges -1 15 25 1.0 1.5 45 70 +1 35 2.0 95 +2 (α) 45 2.5 120 11 Table Experimental and predicted results for the decolorization of AR88 Run no 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 Coded parameters X1 X2 X3 X4 0 +2 0 0 +1 +1 –1 0 –2 0 +2 0 +1 +1 +1 –1 –1 +1 –1 –1 +1 –1 –1 –1 0 0 –1 +1 –1 +1 –2 0 0 0 –1 +1 +1 –1 0 –2 +1 +1 –1 –1 –1 –1 –1 –1 –2 0 0 +2 0 0 0 0 –1 –1 +1 +1 0 0 +1 –1 +1 +1 +1 –1 –1 +1 +2 0 –1 +1 +1 +1 +1 +1 +1 +1 0 0 –1 –1 +1 –1 +1 +1 –1 +1 –1 –1 –1 +1 Actual parameters X1 X2 X3 X4 25 1.5 120 25 1.5 70 35 1.0 95 25 1.5 20 25 2.5 70 35 2.0 95 15 2.0 45 35 1.0 45 25 1.5 70 15 2.0 45 1.5 70 25 1.5 70 15 2.0 95 25 1.5 70 35 2.0 45 15 1.0 45 25 0.5 70 25 1.5 70 11 25 1.5 70 25 1.5 70 15 1.0 95 25 1.5 70 35 1.0 95 35 1.0 45 45 1.5 70 15 2.0 95 35 2.0 95 25 1.5 70 15 1.0 95 35 2.0 45 15 1.0 45 Color removal (CR(%)) Experimental CR (%) 56.2 53.8 51.5 40.2 54.1 53.5 67.4 47.9 48.6 42.6 81.2 51.3 80.3 83.5 49.8 65.3 42.2 27.3 52.1 49.3 44.5 50.8 36.4 32.1 32.4 47.3 40.2 52.5 76.7 34.2 39.6 Predicted CR (%) 54.1 51.2 52.0 40.2 50.9 55.6 72.0 46.0 51.2 42.8 73.7 51.2 82.9 78.4 48.9 68.7 43.4 30.4 51.2 51.2 46.1 51.2 33.2 30.2 37.8 50.7 37.4 51.2 78.8 33.6 38.9 457 DARVISHI CHESHMEH SOLTANI and HAGHIGHAT/Turk J Chem determining the difference between the experimental and predicted responses The plot of residuals versus run number exhibits a normal distribution of the points (Figure 3(b)) Adjusted R amends the R -value for the number of terms in the model and the sample size Hence, the R -value of 0.961 is in reasonable agreement with the adjusted R -value of 0.928 As tabulated in Table 3, an “adequate precision” of 19.34, which is greater than 4, indicated the desirability of the applied model for predicting the response 32 Moreover, a relatively low value of the coefficient of variance (CV = 7.66%) displayed reliability of the results Furthermore, the P-values less than 0.0001 suggested the significance of the CCD model for predicting color removal (Table 3) There is only a 0.0001% chance that the model occurs as a result of noise The F-value of 28.49 also implies that the model is meaningful The estimated regression coefficient and corresponding F- and P-values obtained during the ANOVA are listed in Table The data provided in Table demonstrate that the linear effect of the initial pH ( x4 ) , the quadratic effect of the initial dye concentration (x 11 ), and the interaction effect of the initial pH and the initial dye concentration (x14 ) were significant for the visible light photocatalysis of AR88 at the specified confidence levels Table The results of analysis of variance (ANOVA) for the decolorization of AR88 Source Regression Residuals Lack of fit Pure error Total Sum of squares 6116.61 245.36 225.56 0.000 6361.97 Degree of freedom 14 16 10 30 Mean square 436.90 15.34 22.56 3.30 F-value P-value 28.49 0.0001 Significant R = 0.961, adjusted R = 0.928, adequate precision = 19.34, coefficient of variation (CV) = 7.66 (%) Figure Plot of predicted versus experimental color removal efficiencies (a) along with the plot of residuals versus run number (b) 458 DARVISHI CHESHMEH SOLTANI and HAGHIGHAT/Turk J Chem Table Estimated regression coefficient and corresponding F − and P-values for the decolorization of AR88 Coefficient x0 x1 x2 x3 x4 x12 x13 x14 x23 x24 x34 x11 x22 x33 x44 Coefficient estimate 51.20 –8.99 1.88 3.48 –12.00 –0.11 –1.02 3.49 0.19 0.13 –0.73 1.14 –1.02 –1.01 0.79 Standard error 1.48 0.80 0.80 0.80 0.80 0.98 0.98 0.98 0.98 0.98 0.98 0.73 0.73 0.73 0.73 F-value 28.49 126.42 5.53 18.94 225.21 0.012 1.08 12.74 0.039 0.018 0.56 2.43 1.95 1.90 1.17 P-value < 0.0001 < 0.0001 0.0319 0.0005 < 0.0001 0.9149 0.3135 0.0026 0.8456 0.8950 0.4659 0.1389 0.1820 0.1871 0.2963 2.3 Effect of the operational parameters The interactive effects of the operational parameters were analyzed using three-dimensional (3-D) response surface plots and corresponding contour plots The interactive effect of the initial dye concentration and the reaction time is exhibited in Figure 4, where the catalyst dosage and the initial pH were constant at 1.5 g/L and 7, respectively As depicted, decreasing the initial AR88 concentration, along with increasing the reaction time, resulted in increasing the visible light photocatalysis of AR88 The low initial dye concentration leads to the immediate photocatalysis of dye molecules on the catalyst surface in comparison with the high initial dye concentration, which requires a large number of surficial active sites In addition, increased dye concentration results in decreased path length of the generated photons entering the solution Therefore, a low number of photons reaches the catalyst surface and consequently the generation of OH · for the degradation of the target pollutant will be limited 16,35 Figure 3-D surface (a) and corresponding contour plot (b) of the interactive effect of initial AR88 concentration and reaction time 459 DARVISHI CHESHMEH SOLTANI and HAGHIGHAT/Turk J Chem Figure shows how the catalyst dosage and the reaction time affect the visible light photocatalysis of AR88 over ZnO-diatomite nanocomposite In this step, the initial dye concentration and the initial pH were constant at 25 mg/L and 7, respectively As depicted in the 3-D plot and corresponding contour plot (Figure 5), the color removal (%) increased with increasing catalyst dosage and reaction time Increasing the catalyst dosage favored the photocatalysis of AR88 due to the generation of a higher number of OH · 36 As can be observed in Figure 5, increasing the catalyst dosage up to a specified value resulted in no significant increase in color removal (%) This can be explained by the fact that higher amounts of catalyst adversely affect the transparency of the solution, reducing the adsorption of emitted photons for the generation of OH · 22 Figure 3-D surface (a) and corresponding contour plot (b) of the interactive effect of catalyst dosage and reaction time The initial pH of the solution can influence the visible light-driven photocatalysis of AR88, because it can change the characteristics of dissolved species and surficial characteristics of the catalyst The 3-D plot and corresponding contour plot of the interactive effect of the initial pH and the reaction time showed a sharp increase in color removal (%) with decreasing initial pH from 11 to (Figure 6) AR88 is an anionic dye with p K a of lower than because of high acidic properties, and its sulfonic groups will be negatively charged at pH values higher than 1 Thus, when the surface of the nanocomposite is protonated under acidic conditions, the electrostatic attraction between the surface of nanocomposite and the sulfonic groups of the dye molecules (with negative charge) can occur as displayed in Eqs (2)–(4): 1,37 + Dye−SO3 Na → Dye−SO− +Na (pH > 1) (2) Photocatalyst+H+ ↔ Photocatalyst−H+ (acidic conditions) (3) − + Photocatalyst−H+ +Dye−SO− ↔ Photocatalyst−H SO3 −Dye (4) Accordingly, the adsorption of dye molecules on the catalyst surface leads to an increase in the interaction of photogenerated OH · with the adsorbed dye molecules 37 Despite the acidic conditions, the electrostatic repulsion between sulfonic groups and negatively charged surface of the catalyst occurs under basic conditions, resulting in a significant reduction in the visible light photocatalysis of AR88 In this case, hydroxyl radicals 460 DARVISHI CHESHMEH SOLTANI and HAGHIGHAT/Turk J Chem Figure 3-D surface (a) and corresponding contour plot (b) of the interactive effect of initial pH and reaction time generated on the catalyst surface hardly attack the dye molecules 37 The data provided in Table shows that the initial pH (F-value = 225.2) yielded the highest individual effect on the visible light photocatalysis of AR88 (%) in comparison with the initial dye concentration (F-value = 126.4), the reaction time (F-value = 18.9), and the catalyst dosage (F-value = 5.5) The interactive effect of the initial dye concentration and the catalyst dosage is displayed in Figure As shown, decreasing the initial dye concentration, together with increasing the catalyst dosage, resulted in a rapid increase in color removal (%) This is in accordance with the results in Figures and As shown, the role of the catalyst dosage in the visible light photocatalysis of AR88 is insignificant in comparison with the dye concentration This fact was confirmed by the plot represented for the interactive effect of the catalyst dosage and the initial pH (Figure 8) Figure shows a rapid increase in the color removal (%) with decreasing initial pH, while increasing the catalyst dosage led to a gentle increase in Figure 3-D surface (a) and corresponding contour plot (b) of initial AR88 concentration and catalyst dosage 461 DARVISHI CHESHMEH SOLTANI and HAGHIGHAT/Turk J Chem color removal (%) Numerical optimization was applied to optimize the operational parameters involved in the photocatalysis of AR88 In this approach, the independent operational parameters were set to “in range” and the response (color removal (%)) was set to “maximize” Accordingly, for maximized color removal of 96.0%, the initial dye concentration, the catalyst dosage, the reaction time, and the initial pH were 13 mg/L, 1.5 g/L, 85 min, and 4, respectively Figure 3-D surface (a) and corresponding contour plot (b) of the interactive effect of initial pH and catalyst dosage 2.4 Role of each process The efficiency of each process involved in the visible light photocatalysis of AR88 over ZnO-diatomite nanocomposite was determined and compared As shown in Figure 9, the removal efficiencies of AR88 by the adsorption onto ZnO alone (5.7%), diatomite alone (17.1%), and ZnO-diatomite (20.5%) were insignificant The efficacy of visible light irradiation for color removal was also negligible In addition, the visible light photocatalysis of AR88 over pure ZnO was not efficient enough for the complete removal of AR88 under the specified reaction time (59.7%) However, the application of ZnO-diatomite nanocomposite as photocatalyst resulted in enhanced visible light photocatalysis of AR88 within the same reaction time (95.1%) Therefore, the formation and immobilization of nanostructured ZnO on the diatomite surface improve the potential of the catalyst to absorb further visible light photons, producing higher amounts of OH · and consequently enhanced color removal (%) Moreover, the enhanced decolorization could be ascribed to the porous structure of the diatomite The porous structure improves the reaction area, thereby exhibiting highly efficient catalytic activity 20 The formation of OH · during the photocatalysis of AR88 over ZnO-diatomite nanocomposite is shown in the following equations: 11,15,20 ZnO/diatomite+visible light → h+ + e− (5) e− + O2 → O2· (6) h+ + H2 O → OH · H + (7) The ZnO-diatomite nanocomposite was more efficient than pure ZnO to adsorb the visible light irradiation, generating higher amounts of OH · for the degradation of AR88 dye 462 DARVISHI CHESHMEH SOLTANI and HAGHIGHAT/Turk J Chem 2.5 Stability of the catalyst The reusability potential of the catalyst is a key issue concerning its long-term application Figure 10 depicts the results of the reusability test of the applied catalyst in four consecutive operational runs As depicted in Figure 10, color removal (%) decreased from 95.1% to 76.5% within four consecutive experimental runs As a result, only a 20% reduction in the photocatalytic activity of the ZnO-diatomite nanocomposite was observed during the four experimental runs, indicating a relatively high reusability potential of the nanocomposite The reduction in color removal can be ascribed to the adsorption of some by-products of the target pollutant in the pores and cavities of the catalyst, reducing the active sites for the adsorption and generation of OH · 37 Figure Role of each process involved in the visible light photocatalysis of AR88 Figure 10 Reusability of the ZnO-diatomite nanocomposite within four sequential experimental runs Experimental section 3.1 Photocatalyst preparation All analytical grade chemicals and reagents were supplied from Merck (Germany), except for AR88 dye, which was purchased from Nasaj Sabet (Iran) The characteristics of AR88 dye are provided in Table The growth and immobilization of ZnO nanostructures on the diatomite surface were carried out as follows: 1.362 g of ZnCl , as precursor of ZnO, was added to 50 mL of deionized water Then M NaOH solution was added dropwise to the above solution under magnetic stirring until the pH reached 10 Afterwards, 2.724 g of diatomite was added to the resulting white precipitate The precipitate was sonicated in an ultrasonic bath (Games, Model: Ultra 8060D-H, UK) for 120 to achieve uniformly distributed ZnO particles on the diatomite surface Finally, the suspension was filtered and washed with deionized water and ethanol and dried in an oven at 80 ◦ C for days The ZnO to diatomite mass ratio was fixed to 0.3 The ratio was determined according to our preliminary experiments 3.2 Experimental procedure A quartz-made cylindrical reactor (working volume of 700 mL) surrounded by four visible light lamps (6 W) was used for the visible light photocatalysis of AR88 over ZnO-diatomite nanocomposite The distance between the reactor and visible lamps was kept constant For the photocatalysis of the target pollutant, the ZnO463 DARVISHI CHESHMEH SOLTANI and HAGHIGHAT/Turk J Chem diatomite nanocomposite was inserted in the dye-containing solution under batch flow mode The content of the reactor was mixed using a magnetic stirrer at 300 rpm to keep the suspension homogeneous In addition, the chemical regeneration of the spent catalyst was performed using H SO solution to assess the reusability potential of the as-prepared catalyst For this, dye-loaded catalyst samples were placed in 0.1 M H SO for 30 Four consecutive experimental runs were carried out in order to compare the photocatalytic stability of the regenerated catalyst with that of the pristine one Table Characteristics of Acid Red 88 (AR88) dye Color index name Chemical structure Acid Red 88 (AR88) Molecular formula Mw (g/mol) λmax (nm) C20 H13 N2 O4 SNa 400.38 507 3.3 Experimental design The experimental design of the visible light photocatalysis of AR88 over ZnO-diatomite nanocomposite was accomplished using response surface methodology (RSM) on the basis of central composite design (CCD) For this, Design-Expert 7.0 software was applied to conduct the statistical procedure Four main experimental parameters, including the initial AR88 concentration (mg/L), the catalyst dosage (g/L), the reaction time (min), and the initial pH, were chosen for the modeling According to the CCD modeling, the number of experiments ( N ) was computed by Eq (8) as follows: N = 2k + 2k + x0 , (8) where k and x0 are the number of parameters (variables) and the number of center points (replications), respectively Therefore, the value of N was calculated to be 31 (k = 4, x0 = 7) The response (color removal (%)) and independent operational parameters were correlated using an empirical second-order polynomial equation as represented in Eq (9): Y = b0 + n ∑ i=1 ( bi xi + n ∑ i=1 )2 bii xi + n−1 ∑ n ∑ bij xi xj , (9) i=1 j=i+1 where Y , b0 , bi , bij , and bii are the response, constant, linear effect, interactive effect, and quadratic interaction, respectively The coded values of the experimental parameters were specified by means of xi and xj The ranges and levels of the operational parameters were selected based on the results of preliminary experiments (Table 1) 3.4 Instrumentation The samples were withdrawn from the reactor after certain time intervals The residual concentration of AR88 was measured spectrophotometrically (Hach, DR5000, USA) at λmax of 507 nm The surficial structure of 464 DARVISHI CHESHMEH SOLTANI and HAGHIGHAT/Turk J Chem the samples was characterized using a scanning electron microscope (Cambridge, S360, UK) X-ray diffraction (XRD) analysis was performed via a PANalytical diffractometer (model: X’Pert Pro MPD, the Netherlands) with anode material of Cu and scanning angle ranging from 10 ◦ to 80 ◦ Conclusions Iranian diatomite was used as support for the formation and immobilization of nanostructured ZnO to be used as photocatalyst in the visible light degradation of a textile dye After the preparation of catalyst, RSM based on CCD was applied for the experimental design According to the results of ANOVA, the applied statistical approach can be used as a valuable method for the prediction of color removal under different experimental conditions (R = 0.961 and adjusted R = 0.928) The maximized color removal of 96% was achieved at an initial dye concentration of 13 mg/L, catalyst dosage of 1.5 g/L, reaction time of 85 min, and initial pH of Finally, based on the data 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1924-1932 36 Khataee, A.; Karimi, A.; Arefi-Oskoui, S.; Soltani, R D C.; Hanifehpour, Y.; Soltani, B.; Joo, S W Ultrason Sonochem 2015, 22, 371-381 37 Nezamzadeh-Ejhieh, A.; Khorsandi, S J Ind Eng Chem 2014, 20, 937-946 466 ... suitable for the immobilization of ZnO nanostructures The SEM image of ZnO- coated diatomite shows that the formation and immobilization of ZnO nanostructures on the diatomite surface have been carried... experimental runs In addition, evaluation of the interaction of operational parameters and determination of optimum values for maximum efficiency are achievable via RSM based on CCD 4,31−33 To the best of. .. used as photocatalyst in the visible light degradation of a textile dye After the preparation of catalyst, RSM based on CCD was applied for the experimental design According to the results of ANOVA,