Kinetic Study of the Effect of Methylene Blue on the Decolorization of Other Dyes by Fenton Processes

Abstract

The textile industry can cause serious environmental impacts due to the wastewater generated in its operations involving dyes, many of which have carcinogenic and mutagenic properties. The advanced oxidative processes, especially those based on the Fenton reaction (Fe2+ + H2O2 → Fe3+ + HO- + HO●), have been highlighted as an alternative for the treatment of textile effluents. These processes are based on the generation and action of free radicals, such as the radical HO●, for the degradation of organic pollutants. As these effluents can present more than one dye, the present work evaluated the influence of Methylene Blue dye in the decolorization of other dyes by Fenton processes (Fe2+/H2O2, Fe3+/H2O2). The decolorization tests were carried out over 60 min and monitored by spectrophotometry. With basis on the decolorization data, kinetic models (zero-, first-, second-orders, BMG) were analyzed to verify if the Methylene Blue has pro- or antioxidant properties. It was found that Methylene Blue promoted an increase in the decolorization of Phenol Red and Safranin T, little influence on Methyl Orange, while the decolorization of Cromotrope 2R was diminished. In general, the kinetic model that best fit the experimental data was the 2ª order. The BMG model fitted well to Fe2+-catalyzed reactions, whereas those containing initially Fe3+ the zero- and 1st-order models also showed good adjustments. Except for Cromotrope 2R, increases in kinetic parameter values were observed due to the addition of AM mediator dye. These results demonstrate that a dye can enhance, or even decrease, the degradation of other dyes by Fenton-based processes.