Mechanistic studies on the oxidative degradation of Orange II by peracetic acid catalyzed by simple manganese(ii) salts. Tuning the lifetime of the catalyst

Abstract

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 °C. 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 situUV/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 ≈ 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^VIIO₄⁻ and colloidal Mn^IVO₂ 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^IVO₂ 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% H₂O₂) for efficient catalytic dye decomposition. H₂O₂ 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 H₂O₂/HCO₃⁻ as oxidant was made.