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A remarkably efficient, catalytic oxidative degradation of Orange II in aqueous solution was observed in the presence of simple Mn(II) salts used in combination with the green oxidant peracetic acid (PAA) under mild reaction conditions at pH 9.5 and 25 1C. In order to gain further insight into the complex reaction network responsible for the fast and efficient degradation of Orange II, detailed kinetic studies on the catalytic degradation reaction were performed. The influence of the catalyst, oxidant, and buffer concentrations on the reaction course was studied in detail by in situ UV/Vis spectroscopy. The degradation rate increased with increasing catalyst and PAA concentration, decreased with increasing carbonate buffer concentration, and the reaction showed maximum reactivity at pH E 9.4. Studies performed in the absence of substrate revealed the successive formation of various high valence manganese intermediates under the selected experimental conditions. The UV/Vis spectral changes observed upon addition of PAA to a Mn(II) ion containing solution at pH 9.5 showed biphasic behavior, and led to the formation of Mn VII O 4À and colloidalMn IV O 2 as final products. Through the combination of EPR and UV/Vis spectroscopy, the presence of a Mn(IV) species during the first phase of the reaction was observed. If the addition of a dye to the Mn(II)/PAA containing reaction mixture was carried out after different delay times during the course of the reaction, significant changes observed in the degradation performance pointed to changes in the catalyst composition with time. Thereby, the highest reactivity was reached just before the formation of the high valence intermediates and colloidal Mn IV O 2 occurred. Selected catalytic experiments with different in situ formed intermediates showed the essential role of a small amount of hydrogen peroxide (omnipresent in commercial peracetic acid, 6 wt% H 2 O 2 ) for efficient catalytic dye decomposition. H 2 O 2 was shown to play a crucial role as a reducing agent in the catalytic cycle, avoiding the over-oxidation of the catalyst, and thereby extending the lifetime of the catalytic system. The data were evaluated to gain mechanistic insight into the complex chemical reactions occurring during the Mn(II) catalyzed oxidative dye degradation with PAA. A comparison with earlier results concerning the use of H 2 O 2 /HCO 3 À as oxidant was made.

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The kinetics of the hydrogen peroxide induced oxidative degradation of the azo dye Orange II in aqueous carbonate buffered solution were studied for the oxo-bridged [Mn(2)(III/IV)(mu-O)(2)(bpy)(4)](ClO(4))(3) complex and its mononuclear analogue [Mn(II)(bpy)(2)Cl(2)] as catalysts to reveal the underlying reaction mechanism and reactive intermediates participating in the catalytic cycle. Both catalysts show identical oxidative reactivity when used at equimolar manganese concentration. If a simple Mn(II) salt and a 1 : 2 concentration of bipyridine are added to the substrate and oxidant containing reaction mixture, the same oxidative reactivity as found for both readily prepared catalysts was observed for several investigated substrates. This demonstrates the in situ accessibility of a reactive intermediate and its precursor complex. The crucial role of bicarbonate as co-catalyst was studied. The distinct dependence of the observed rate constant for the oxidation reaction on the total carbonate concentration can be accounted for in terms of in situ generation of peroxycarbonate. EPR and rapid scan UV/Vis measurements of the reaction of hydrogen peroxide in carbonate buffered solution with [Mn(2)(III/IV)(mu-O)(2)(bpy)(4)](ClO(4))(3) and [Mn(II)(bpy)(2)Cl(2)], revealed for both catalysts the presence of monomeric Mn(II) and Mn(IV)-oxo species as the main intermediates. The proposed reaction mechanism involves two-electron oxidation of a mononuclear Mn(II) precursor complex to a high-valent Mn(IV)=O intermediate as catalytically active species. Differences in the activity of in situ prepared catalyst precursors of different metal to ligand ratios are reported. The 1 : 2 complex was found to be the catalytically more active precursor for the oxidation of the selected substrates, whereas the 1 : 3 complex rather catalyzed the disproportion of hydrogen peroxide.

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