Catalytic activity of polyaniline/MnO2 composites towards the oxidative decolorization of organic dyes

The kinetics of the oxidative reaction of the textile dyes: direct red 81 (DR-81), indigo carmine (IC), and acid blue 92 (AB-92) with H2O2 in the presence of polyaniline (PANI)/MnO2 composites has been investigated. Experiments were carried out in batch reactor under pseudo-first order conditions with respect to the [dye]. Effects of the preparation conditions of the composites on the reaction rate have been investigated. Among these conditions are [aniline], [acid], type of acid, mass of β-MnO2, polymerization time, and annealing temperatures. At fixed [dye], [H2O2], and mass of composite, the rate constants increased upon increasing (i) the doping ratio of PANI, (ii) the percentage of PANI content, (iii) the degree of crystallinity of PANI, and (iv) the molecular weight of PANI. The presence of MnO2 counterpart kept the medium at pH range (3–4) via adsorption of H+. The oxidation rate was enhanced with the increase in the initial concentration of H2O2, attained a maximum at 0.01 M and thereafter decreased. In contrast, the rate was inhibited with the increase in the initial dye concentration, and the pH. The activation parameters were determined. From the isokinetic relationship it was concluded that the catalytic reactions are entropy controlled. A reaction mechanism involving electron transfer from H2O2 to the PANI counterpart has been proposed. These results demonstrate that the PANI/MnO2–H2O2 catalytic system may be envisaged as a method for the treatment of colored wastewaters.