Activation of persulfate by heat, FeNPs, and Fe/MnO2 for the advanced oxidation processes

In this work, we used only persulfate (S2O82−), Fe0/S2O82−, Fe/MnO2/S2O82− systems as an activator to generate reactive radical species for the chemical oxidation of Congo red azo dye. The formation of a new colored species at 363 nm and its decomposition was observed at a specific reaction condition for the first time. Congo red-S2O82− redox reaction was very slow at 22 °C. After the addition of small amount of Fe/MnO2, the red color was decolorized completely within 6 min. The maximum degradation of Congo red was achieved at optimum parametric values of [S2O82−] = 0.2 mM, [Congo red] = 0.6 mM, amount of Fe/MnO2 = 10 mg, pH = 6.5, and temperature = 22 °C. The presence of MnO2 accelerates the generation of sulfate radicals (SO4−•) during the Fenton- like reaction between Fe/MnO2 and S2O82− due to the synergistic effect. The activation energy was found to be 40.1, 48.0, and 92.2 kJ/mol for Fe/MnO2/S2O82−, Fe/S2O82−, and only S2O82−, respectively. The oxidation of Congo red follows complex order kinetics with respect to pH as well as concentrations of S2O82−. Chemical probes experiments with various radical scavengers such as ethanol, tertiary butyl alcohol (TBA), and potassium iodide showed that the SO4−• radicals participate effectively in situ chemical oxidation of Congo red. Cationic cetyltrimethylammonium bromide (CTAB) retarded the persulfate activation by Fe/MnO2 significantly. The morphology of Fe/MnO2 was determined by XRD, EDX, FTIR, SEM, and TEM.