Oxidation of direct dyes with hydrogen peroxide using ferrous ion as catalyst

Abstract

The oxidative degradation of two direct dyes, Blue 2B (B54) and Red 12B (R31) in aqueous solution has been studied using Fenton's reagent (Fe2+ and H2O2). Results show that dyes are decomposed in a two-stage reaction. In the first stage, dyes are decomposed rapidly, and somewhat less rapidly in the second stage. The effects of different system variables like initial pH of the medium, initial concentrations of the dye, Fe2+ and H2O2, reaction temperature, and added Cl− and SO4−2 ions have been investigated. The degradation rate is strongly dependent on the initial concentrations of the dye, Fe2+ and H2O2. The results indicate that the B54 and R31 can be most effectively oxidised in an aqueous medium of pH 3 at an initial [Fe2+]:[H2O2]:[dye] ratio of 1:32.9:2.4 and 1:16.5:1.8 (mM), respectively. At the optimum initial ratio of [Fe2+]:[H2O2]:[dye], 97% degradation can be achieved in 30 min at a temperature of 30 °C, and 70% removal of initial chemical oxygen demand (COD) is achieved after 60 min. The degradation of both the dyes obeys first order rate kinetics with respect to the concentration of the dye in the second stage of oxidation. The results will be useful in designing wastewater treatment plants.