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In this study, a baffled photocatalytic reactor was used to treat wastewater containing azo dye. The baffles made of Plexiglas covered by TiO2 nanomaterials placed vertical in the reactor, were used. Using this reactor could enhance in the wastewater passage time, decrease in contact distance due to existence of the colored wastewater and the effect of preventing the passage of UV ray, bring about turbulence in the current, prevent from short circuit phenomenon, increase at the current length, and cause enhancement in effective surface against the relatively low occupancy level that make it possible to construct this kind of reactors in larger scales. The dimensions of the Reactor were 20 cm*25 cm*50 cm and the baffle dimensions used in the reactor were selected 20 cm*12 cm. The photocatalyst particles were fixed on baffles and then the experiments were conducted based on the experimental design by Design Expert software. In order to ensure adequate waste water to pass from the photoreactor, the rotary flow regime was used in the original design. In the research, Methyl orange one of the anionic dyes with the chemical formula C14H14N3NaO3S was used. This azo dye was the kind of amino benzene and has a functional azoic group (-N = N-) and cofactors NaSO3 and now widely used in dyeing textile, wood, paper, leather and printing applications. In order to investigate the effect of the main factors and optimizing colored wastewater treatment process by using TiO2 nanoparticles in the baffled reactor Response Surface Methodology, central composite design (CCD), was used. Based on the results, reducing the pH and initial dye concentration had synergetic effect on color and COD removal simultaneously. The effect of pH less than 5 and less than 75 mg/L concentrations are more rapidly. This phenomenon was a result of amphoteric behavior of TiO2 and the weakening of oxidation ability of the produced holes in alkaline conditions. The pH of the solution influence on how the TiO2 surface is ionized and leads to amphoteric behavior the TiO2 nanoparticles under different conditions and this behavior changes the oxidation ability of the process. Another reason for this phenomenon could be described as the reduction in light penetration due to increased dye concentration in the solution and the more dye adsorption on the surface of TiO2 causes a part of UV energy is absorbed by the molecules of the dye. Although Methyl orange is an anionic dye with the negatively charged sulfonic group thus in high pH, hydroxyl radicals lose the chances of reaction with the trapped dye quickly. At the same time reducing the pH and increasing the reaction time also increases the efficiency of COD and color removal and enhanced for the pH below 4 and after 6.5 hours for dye removal and at pH below 5 and after 8 hours for COD removal. This was due to increased opportunities for photocatalytic activity in acidic pHs reduce the initial dye concentration and increase the reaction time had amplified effect in efficiency of decolorization and reduction of COD. The rate of the phenomena was more obvious for the dye concentration less than 50 mg/L and after 8.5 hours. The results showed that the color removal efficiency was more than COD removal efficiency. The most noticeable reason for this phenomenon is the breaking of the colored azoic bond and producing colorless intermediate products that decrease removal efficiency during tests. The maximum amount of COD and color removal when the 50 mg /L initial dye concentration and at the pH= 5 were 98.81 and 69.7 percent, respectively, after 9.5 hours. The results data comply with reduced quadratic model with a correlation coefficient (R2) 94.95 and 95.30 percent for color and COD removal respectively that validate the model results. Laboratory assessment also indicated that due to the very small difference between the results of the represented model and the experimental data, the model was consistent with acceptable confidence level.

Keywords: TiO2, RSM, Azo dye acid orange, baffled photoreactor, decolorization

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