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Currently tons of dye produced per year, about the one sixth tons are converted into wastewater in various industries such as textiles and dyeing, which are among the toxic, carcinogenic and mutagenic wastes due to the presence of aromatic rings in their structure. This issue has attracted a lot of attention to the purification of such compounds. Ozone is one of the strong oxidizers which produces non-toxic compounds due to its decomposition. Ozone can convert many organic materials into simpler compounds through both direct and indirect oxidation mechanisms, including degradation of wastewater that contains double bonds components such as aromatic compounds and dyes. The purpose of this study was to investigate modeling, optimization and the interactions between parameters affecting the ozonation process in removal of Acid Orange 7 in order to achieve the highest removal efficiency for the highest possible initial dye concentration under the lowest ozone injection rate, no change in initial wastewater pH and the shortest reaction time by the use of response surface method. The RSM was performed using 4 parameters pH, initial dye concentration, ozone injection rate and time with 5 levels which ends up in 30 experimental tests.
The results showed that correlation coefficients and adjusted correlation coefficients were 96.85 and 94.92, respectively, and p-value for model (less than 0.0001) and lack of fit (0.0507) were obtained as significant and non-significant, respectively. These results indicate the consistency and high reliability of the modeling results. Normal error, error independence and variance stability control were also checked which showed that the closeness between the actual and predicted values and the uniform distribution of the results obtained on the normal line indicates the uniform distribution of the error. The results and predictions of the software, the random distribution and distribution of the results indicate the suitability of the assumption considered by the software regarding the stability of the variance. Based on variance stability control, the effect of the experiments on the responses provided by the software. If one of the experiments is outside the range, this experiment will have a negative impact on the overall results of the software. In the case of experiments performed, this control was also well performed.
Based on model equation the most important parameters are the injection rate of ozone (Q(O₃)), pH, reaction time (T) and initial dye concentration [Dye], respectively, in which all parameters except the initial dye concentration have a positive effect on dye removal efficiency. After the related tests the optimum condition were the initial dye concentration of 480 mg/L, pH of 7.7, ozonation rate of 0.6 L/min and ozonation duration of 60 min which resulted in 90% dye removal efficiency. It was also found that the most effective factors were injectable ozone rate, time, pH, and dye concentration, respectively. The results showed that determining the appropriate domains can be of great importance in achieving the desired results from the response surface method. Also, the ozonation process is able to purify the dye from high initial concentrations to high removal efficiency, indicating the high strength of this applied process in the decomposition of complex organic compounds. Ozonation kinetic rate is based on pseudo first order which was increased from 0.3 to 0.6 by enhancing injected ozone rate from 0.2 to 0.6 L/min respectively and further increase of ozone injection rate didn’t had any effect on its kinetic rate.

Keywords: Ozonation, Acid Orange 7, CCD method, RSM, Modeling.

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