Optimization of Oxidative Decolorization of Acid Violet 7 by Peroxydisulfate using Response Surface Methodology, Kinetic, and Thermodynamic Evaluation

Abstract

Response surface methodology based on central composite design was successfully applied to the optimization of the operating conditions in the Fe²⁺ catalyzed decolorization of acid violet 7 using peroxydisulfate. The impact of four process variables, initial dye concentration, concentrations of Fe²⁺, temperature, and pH on the decolorization of acid violet 7 was evaluated. Predicted values of decolorization efficiency were found to be in good agreement with experimental values (R² = 98.18%), which indicated the suitability of the central composite design model employed. The interaction of the parameters is also studied based on its P-value with the help of analysis of variance. The experimental data were examined employing kinetics of both first and second order. The decolorization kinetics of acid violet 7 in the peroxydisulfate process adhered to the principles of second-order reaction kinetics. Initial dye concentration of 2.5 µM, pH of 2.0, temperature 313 K, and Fe²⁺ concentration of 0.6 mM were the best conditions. The predicted decolorization rate under the optimum conditions determined by response surface methodology was 100%. Confirmatory tests were carried out under the optimum conditions and the decolorization rate of 99.6% was observed, which closely agreed with the predicted value. Fe²⁺ activates \({{{\text{S}}}_{{\text{2}}}}{\text{O}}_{8}^{{2 - }}\) to generate sulfate radical ion, amplifying acid violet 7 decolorization. Peroxydisulfate decolorized acid violet 7 with a low activation energy of 45.32 kJ mol^–1 at 2.25 mM starting concentration. The positive ΔG value indicates the non-spontaneity of the decolorization process.