Mn(II) oxidation and manganese-oxide reduction on the decolorization of an azo dye

The development of biological processes for treating azo-dye-contaminated waters is an important objective. Herein, we investigated the effects of Mn(II) oxidation and manganese-oxide reduction on the removal of the azo dye, Bordeaux S (BS), under aerobic and anaerobic conditions using two reactors, one with installed abiotic-MnO₂, and one devoid of abiotic-MnO₂. Under aerobic conditions, very little BS was removed, even at the highest Mn(II) oxidation rate (0.22 kg Mn(II) m⁻³ d⁻¹), indicating that Mn(II) oxidation has essentially no effect on azo-dye removal. In contrast, under anaerobic conditions, the reduction and/or presence of MnO_x was found to have an effect. Although BS decolorization increased with increasing K-medium loading, and the decolorization rate of 4.75 kg m⁻³ d⁻¹ was achieved using both reactors, the reactor with abiotic-MnO₂ performed better at low K-medium loading rates. The presence of MnO_x induced higher secretion of extracellular polymeric substances, which serve as electron donors for the decolorization of BS under K-medium-deficient conditions. BS decolorization is caused by the simple cleavage of the azo bond to the 4-amino-1-naphthalenesulfonic acid moiety, and further degradation did not occur under anaerobic conditions. Although methane was produced even in the presence of MnO_x, we conclude that Methanosaeta were inhibited.