Factorial Experimental Design for the Optimization of β- Naphthol Photocatalytic Degradation in TiO 2 Aqueous Suspension

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1 nadian Chemical Transactions Research Article DOI: /canchemtrans Factorial Experimental Design for the Optimization of β- Naphthol Photocatalytic Degradation in TiO 2 Aqueous Suspension Samir Qourzal 1, Idriss Bakas 1, Noureddine Barka 2,*, Ali Assabbane 1 and Yhya Ait-Ichou 1 1 Equipe de Matériaux Photocatalyse et Environnement, Département de Chimie, Faculté des Sciences, Université Ibn Zohr, BP Cité Dakhla, Agadir, Morocco 2 Université Hassan 1, Laboratoire CAE, Equipe de Recherche en Gestion de l Eau et Développement Durable (GEDD), Faculté Polydisciplinaire de Khouribga, BP. 145 Khouribga, Morocco * Corresponding Author, barkanoureddine@yahoo.fr Tel.: ; fax: Received: October10, 2013 Revised: November 18, 2013 Accepted: November21, 2013 Published: November 22, 2013 Abstract: A factorial experimental design was developed to describe the photocatalytic degradation of β- naphthol in a circulating photo-reactor using TiO 2 aqueous suspension. The main and interaction factors studied were the initial concentration of β-naphthol, TiO 2 dosage, ph and circulation flow. The parameters coded as X 1, X 2, X 3 and X 4, consecutively, and were investigated at two levels ( 1 and +1). The effects of individual variables and their interaction effects for dependent variables, namely, photocatalytic degradation efficiency (%) were determined. The experimental results and statistical analysis show that increasing in the initial concentration of β-naphthol decreases the photocatalytic degradation efficiency whereas the increase of TiO 2 dosage, ph and the circulation flow have a positive effect on the photocatalytic degradation. The interaction between TiO 2 dosage and circulation flow was the most influencing interaction. However, the interaction between TiO 2 dosage and ph was the least influencing interactions. The variance analysis (ANOVA) was applied and the statistic data showed adequate model. Keywords: Photocatalytic degradation; β-naphthol; Factorial experimental design; Optimization. 1. INTRODUCTION Heterogeneous photocatalysis represents a promising alternative technology for the degradation of organic pollutants [1]. It offers a unique advantage over other alternative treatment methods because it presents a green treatment approach; since, toxic organic pollutants are converted into carbon dioxide (CO 2 ), water and mineral acids [2-5]. The process involves the generation of conduction band electrons and valence band holes by the illumination of a semiconductor, usually TiO2, with light energy greater than the band gap energy. The electron and the hole form hydroxyl radicals which are assumed to be the Borderless Science Publishing 1

2 nadian Chemical Transactions Figure 1. Schematic diagram of the photocatalytic reactor. main reactants in the degradation of the most recalcitrant molecules [6-15]. From a fundamental point, the heterogeneous photocatalytic reactions occur on the surface of semiconductor according to Langmuir-Hinshelwood mechanism [16,17]. The degradation efficiency measured either by the disappearance of the reactant or by the formation of the ultimate products, in particular CO 2, depends on kinetic parameters such as substrate concentration, ph environment and the nature of the photocatalyst. Hydrodynamic factors such as the mode of circulation of the mixture, the geometry and capacity of the reactor also influence the process. Most previous studies tested traditional one-factor-at-a-time experiments for evaluating the influence of operating factors on photocatalytic process. Design of experiments is a powerful technique used for discovering a set of process variables (or factors) which are most important to the process and then determine at what levels these factors must be kept to optimize the process performance. Statistical design of experiments is a quick and cost-effective method to understand and optimize any manufacturing processes [18]. The experiments in which the effects of more than one factor on response are investigated are known as full factorial experiments. In a full factorial experiment, both of the ( 1) and (+1) levels of every factor are compared with each other and the effects of each of the factor levels on the response are investigated according to the levels of other factors. Doing so with the factorial planning of the experiments, it was possible to investigate simultaneously the effect of all the variables [19-22]. 2-Naphthol is an organic pollutant presented in the environment as a result of dyestuffs manufactory, pharmaceutical production and some biogeochemical processes [23]. Because of its strong toxicity and low biodegradability, the removal of 2-naphthol is of great importance in water treatment. The objective of this study is to optimize the photocatalytic degradation of β-naphthol by factorial experimental design and to develop a predictive model for the degradation rate involving four independent factors. The parameters under investigation are initial concentration of β-naphthol, TiO 2 dosage, ph and circulation flow in the photocatalytic reactor. Borderless Science Publishing 2

3 nadian Chemical Transactions Table 1. Values of operating variables used in the 2 4 factorial design study Operating variables Code Low ( 1) High (+1) Initial concentration (mol/l) X TiO 2 dosage (g/l) X ph X Circulation flow (ml/min) X Table 2. Design and experimental results of 2 4 full factorial design Experiment Coded experiments matrix X 1 X 2 X 3 X 4 Measured response Y(%) MATERIALS AND METHODS 2.1. Materials TiO 2 Degussa P25 was used for the degradation process. It consists of 80% anatase and 20 % rutile with a specific BET surface area of 50 m 2 /g and primary particle size of 20 nm. All the other chemicals used in the experiments were of laboratory reagent grade and used as received without further purification. β-naphthol was supplied by Fluka (> 99% purity). Acetonitrile, NaOH and HCl were purchased from Merck Photo-reactor and Light Source Photocatalytic degradation experiments were performed in a circulating photoreactor system. A schematic diagram of the experimental set-up is shown in Fig. 1. The reactor was cylindrical and was made of quartz glass which permits the transfer of the irradiation. The volume of the reactor was 1 L. The reactor was exposed to a luminous source (a HPK 125W Philips ultraviolet lamp with a wavelength of maximum absorption of 365 nm), placed in axial position inside a water cooling jacket. The suspension was continuously purged with molecular oxygen throughout each experiment. The recirculation of the solution Borderless Science Publishing 3

4 nadian Chemical Transactions was controlled by a peristaltic pomp. The agitation was assured by means of a magnetic stirrer placed at the reactor base. Table 3. Estimated coefficients for β-naphthol photocatalytic degradation Term Coefficient Sum of squares df Mean squares b b b b b b b b b b b b b b b b Error 0 0 total Procedure and Analysis Photocatalytic degradation experiments were carried out by loading appropriate volume of β- Naphthol solution at the desired concentration, TiO 2 dosage and ph in the photocatalytic reactor. The ph was adjusted to a given value by addition of HCl (1N) or NaOH (1N) and was measured using a JENWAY ph-meter After 60 min of irradiation, the aqueous solutions were filtered by Millipore membrane filter type 0.45 µm HA. β-naphthol degradation was monitored by HPLC using a Varian Polychrom 9065 Photodiode Array Detector and Waters 510 pump. A good separation of the products was achieved using the reversephase column ODS2-Spherisorb-Chrompack (length 25 cm; internal diameter, 4.6 mm; particle diameter, 5 μm) and the detector with a wavelength at 280 nm. The mobile phase was composed of acetonitrile and doubly distilled water. The v/v ratio CH 3 CN/H 2 O was 80/20 and the flow rate was 0.4 ml/min. The injection volume was equal to 20 μl. The photocatalytic degradation efficiency was determined by using the following equation: (Co Cr) Y(%) x 100 (1) Co where Y the photocatalytic efficiency (%), Co and Cr both in (mol/l) are respectively the initial and residual concentrations of β-naphthol in solution. Borderless Science Publishing 4

5 nadian Chemical Transactions Table 4. Estimated regression coefficients and ANOVA for response surface quadratic model Source Sum of Squares df Mean Square F-value P-value Status Model < Significant X < X X 1 X X 1 X X 2 X X 3 X X 1 X 2 X X 1 X 3 X Residual Cor Total R-Squared 0.98 Adj R-Squared 0.95 Pred R-Squared 0.89 Adeq Precision 19 Std. Dev. 3.2 Figure 2. Pareto chart of effects on photocatalytic degradation efficiency 2.4. Experimental Design According to preliminary experiments carried out to identify the appropriate parameters and to determine the experimental domain. Initial β-naphthol concentration factor 1 (X 1 ), TiO 2 dosage factor 2 (X 2 ), ph factor 3 (X 3 ) and circulation flow factor 4 (X 4 ) were foreseen as affecting the β-naphthol Borderless Science Publishing 5

6 Y Predicted nadian Chemical Transactions photocatalytic degradation. Minimum and maximum levels of each influential factor are reported in Table 1. The codified mathematical model employed for the 2 4 factorial design is: Y = b 0 + b 1 X 1 + b 2 X 2 + b 3 X 3 + b 4 X 4 + b 12 X 1 X 2 + b 13 X 1 X 3 + b 14 X 1 X 4 + b 23 X 2 X 3 + b 24 X 2 X 4 + b 34 X 3 X 4 + b 123 X 1 X 2 X 3 + b 124 X 1 X 2 X 4 + b 134 X 1 X 3 X 4 + b 234 X 2 X 3 X 4 + b 1234 X 1 X 2 X 3 X 4 (2) where Y is the response (photocatalytic degradation efficiency), Xi values (i = 1, 2, 3, 4) indicates the corresponding parameter in their coded forms and b i are coefficients of the respective effect. With four factors, 2 4 factorial designs require 16 runs. Meanwhile, the number of coefficients of b i,i = to be estimated is 16. The analysis of results was performed with statistical and graphical analysis software (Design Expert Software (Version ) of Stat-Ease Inc., USA) software. 50 R 2 = Y Experimental Figure 3. Predicted values versus experimental values 3. RESULTS AND DISCUSSION 3.1. Full Factorial Model Experimental results obtained in the photocatalytic degradation experiments are presented in Table 2. Regression analysis was performed to fit the response function (photocatalytic degradation efficiency) with the experimental data. The effects, regression coefficients, and the associated standard errors are shown in Table 3. The final regression equation, after putting values of all coefficients, is as follows: Y = X X X X 4 4.3X 1 X X 1 X X 1 X X 2 X X 2 X X 3 X X 1 X 2 X 3 4.5X 1 X 2 X 4 1.8X 1 X 3 X X 2 X 3 X X 1 X 2 X 3 X 4 (3) Eq (3) shows the effect of individual variables and interaction effects for β-naphthol photocatalytic degradation. According to this equation, the TiO 2 dosage, ph and circulation flow have a Borderless Science Publishing 6

7 nadian Chemical Transactions positive effect, while the initial concentration has a negative effect on the photocatalytic degradation of β- naphthol in aqueous solution in the range of variation of each variable selected for the present study. The positive sign of the coefficients indicates a synergistic effect between the parameter and dependent variable, while a negative sign indicates an antagonistic effect on response. It is known that the larger the coefficient, the larger is the effect of related parameter. The most effective parameters in the photocatalytic degradation efficiency were initial concentration and TiO 2 dosage. The increase in TiO 2 dosage increases the surface area available by more photocatalyst particles. The number of active sites on the photocatalyst surface increases, which in turn increase the number of hydroxyl radicals. The increase of hydroxyl radicals leads to the increase of the photocatalytic degradation efficiency. The negative signs in the case of initial concentration are due to the increase of β- naphthol molecules in solution. Since irradiation time and the amount of catalyst are constant, the OH (primary oxidant) concentration remains practically the same. Thus, although bulk liquid concentration increases, the rate of photocatalytic degradation decreases due to a lower OH /β-naphthol ratio. The equation also seen that two-variable or three-variable interactions are significant. The interaction between TiO 2 dosage and circulation flow was the most influencing interaction. However, the interaction between TiO 2 dosage and ph was the least influencing interactions Analysis of Variance (ANOVA) After estimating the main effects, the interacting factors affecting the β-naphthol photocatalytic degradation, determination of the significant main and interaction effects of factors affecting β-naphthol photocatalytic degradation efficiency was followed by performing an analysis of variance (ANOVA). Error functions were defined to determine the significance of each parameter in model, the parameter values, and to establish the fit of the model to the experimental data. Table 4 shows the sum of squares used to estimate the F-ratios (F), which are defined as the ratio of the respective mean square effect and the mean square error. The Model F-value of implies the model is significant. There is only a 0.01% chance that a Model F-Value this large could occur due to noise. Values of P-value less than indicate model terms are significant. In this case X 1, X 2, X 1 X 2, X 1 X 3, X 2 X 4, X 3 X 4, X 1 X 2 X 4 are the significant model terms Student s T-Test Student s t-test was employed in order to determine whether calculated effects were significantly different from zero. For a 95% confidence level and 15 degrees of freedom, the t-value is equal to Figure 2 shows this evaluation as Pareto chart. The horizontal line indicates minimum statistically significant effect magnitude for a 95% confidence level. Values shown in the vertical columns are Student s t-test values for each effect. Based on F-test and Student s t-test, insignificant terms were eliminated since they did not have any statistical effect. The simplified regression equation for β-naphthol photocatalytic degradation was found as: Y = X X X 1 X X 1 X X 2 X X 3 X X 1 X 2 X X 1 X 3 X 4 (4) The predicted response values versus the actual response values are shown in Figure 3. Actual values are the experimental response data for a particular run, and the predicted values were evaluated from the model and generated by using the approximating function. As can be seen in the Figure 3, the Borderless Science Publishing 7

8 nadian Chemical Transactions Figure 4. Response surface and contour plot as a function of initial concentration of β-naphthol and TiO 2 dosage Figure 5. Response surface and contour plot as a function of initial concentration of β-naphthol and ph experimental results are in good agreement with the values calculated by the regression equation. The fit of the model was further checked by the coefficient of determination R 2. The R 2 value is always between 0 and 1. The closer its R squared value is to one, the greater the ability of that model to predict a trend. In this model, the value of R 2 was evaluated as indicating that 98.3 % of the variability in the response could be explained by the model. This indicated that the prediction of experimental data is satisfactory Analysis of RSM Figures 4-7 show three dimensional and contour plots of the significant interactions between the parameters influencing the photocatalytic degradation of β-naphthol. An interaction is effective when the change in the response from low to high levels of a factor is dependent on the level of a second factor [24]. Fig. 4 shows the effect of TiO 2 dosage and the initial concentration on the β-naphthol photocatalytic degradation. The response surface increased with increasing TiO 2 dosage from 0.1 to 2 g and initial Borderless Science Publishing 8

9 nadian Chemical Transactions Figure 6. Response surface and contour plot as a function of TiO 2 dosage and circulating flow Figure 7. Response surface and contour plot as a function of ph and Circulation flow concentration from 0.1 to 1 mmol/l. The effect of initial concentration was more significant at higher TiO 2 dosage. The effect of TiO 2 dosage was more noticeable at lower initial concentration. These results support the previous findings related to the effect of each factor on photocatalytic degradation. Similar results were found by Ray et al. for the optimization of phenol photocatalytic degradation by titanium dioxide nanoparticles [25]. Fig. 5 shows the effect of initial concentration and ph on the β-naphthol photocatalytic degradation efficiency. As can be seen, the photocatalytic degradation efficiency decreased with the individual increase in initial concentration. We observe that at low concentrations there is an inverse behavior of photocatalytic degradation when the ph level changes. The effect of initial concentration was more significant at lower ph. The interaction effect of TiO 2 dosage and circulating flow on β-naphthol photocatalytic degradation is shown in Fig. 6. From the response surface figure, it is clear that the photocatalytic degradation gradually increases with increasing TiO 2 dosage. Again, we observe that at low TiO 2 dosage there is an inverse behavior of photocatalytic degradation when the circulating flow level changes. The circulation Borderless Science Publishing 9

10 nadian Chemical Transactions flow has a positive effect on the photocatalytic degradation at lower TiO 2 dosage and a negative effect at higher TiO 2 dosage. The effect of TiO 2 dosage was more noticeable at lower circulating flow. Fig. 7 shows the effect of ph and circulation flow on the photocatalytic degradation. The figure indicates that at low circulation flow, the photocatalytic degradation decreases by the increase in ph. At high circulation flow there is an inverse behavior of the photocatalytic degradation efficiency, the circulation flow has a positive effect on the photocatalytic degradation at higher ph value and a negative effect at lower ph value. 4. CONCLUSIONS This study showed that factorial experimental design approach is an excellent tool and could successfully be used to develop empirical equation for the prediction and understanding the photocatalytic degradation efficiency of β-naphthol. As observed, the most effective parameters in the photocatalytic degradation efficiency were initial concentration and TiO 2 dosage. The initial concentration has an antagonistic on the photocatalytic degradation, while TiO 2 dosage, ph and the circulation flow have a synergistic effect. The most influencing interaction was TiO 2 dosage and circulation flow. The analysis of variance (ANOVA) showed that the regression model was significant at 95% confidence level. Satisfactory values for coefficients of determination were obtained (R 2 >0.98). This means that fitting is good and that at least 98% of the variations in the response could be addressed to the obtained model equations. REFERENCES AND NOTES [1] Herrmann, J. M.; Guillard, C.; Pichat, P. Heterogenous photocatalysis: an emerging technology for water treatment. tal. Today 1993, 17, [2] Hoffmann, M.R.; Martin, S.T.; Choi, W.; Bahnemann D.W. Environmental application of semiconductor photocatalysis. Chem. Rev. 1995, 95, [3] Mills, A.; Le Hunte, S. An overview of semiconductor photocatalysis. J. Photochem. Photobiol. A: Chem. 1997, 108, [4] Ollis, D.F.; Turchi, C. Heterogeneous photocatalysis for water purification: Contaminant mineralization kinetics and elementary reactor analysis. Environ. Prog. 1990, 9, [5] Barka, N.; Qourzal, S.; Assabbane, A.; Nounah, A.; Ait-Ichou, Y. Triphenylmethane dye, patent blue V, photocatalytic degradation on supported TiO 2 : Kinetics, mineralization and reaction pathway. Chem. Eng. Com. 2011, 198, [6] Chu, W.; Choy, W. K.; So, T.Y. The effect of solution ph and peroxide in the TiO 2 -induced photocatalysis of chlorinated aniline. J. Hazard. Mater. 2007, 141, [7] Qourzal, S.; Barka, N.; Belmouden, M.; Abaamrane, A.; Alahiane, S.; Elouardi, M.; Assabbane, A.; Ait- Ichou, Y. Heterogeneous photocatalytic degradation of 4-nitrophenol on suspended titania surface in a dynamic photoreactor. Fres. Environ. Bull. 2012, 21(7), [8] Guillard, C.; Disdier, J.; Herrmann, J. M.; Lehaut, C.; Chopin, T.; Malato, S.; Blanco, J. Comparison of various titania samples of industrial origin in the solar photocatalytic detoxification of water containing 4- chlorophenol. tal. Today 1999, 54, [9] Abdennouri, M.; Galadi, A.; Barka, N.; Baâlala, M.; Nohair, K.; Elkrati, M.; Sadiq, M.; Bensitel, M. Synthesis, Synthesis, characterization and photocatalytic activity by para-chlorotoluene photooxidation of tin oxide films deposited on Pyrex glass substrates. Phys. Chem. News 2010, 54, [10] Barka, N.; Qourzal, S.; Assabbane, A.; Nounah, A.; Aît Ichou, Y. Photocatalytic degradation of an azo reactive dye, Reactive Yellow 84, in water using an industrial titanium dioxide coated media. Arab. J. Chem. 2010, 3, Borderless Science Publishing 10

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