Mechanism of Decolorization of Azo Dyes in Anaerobic Mixed Culture

To determine the role of diverse groups of bacteria in the decolorization of a monoazo dye C. I. Reactive Orange 96 (RO 96), batch experiments with an anaerobic mixed culture were made using the selective inhibitors molybdate and 2-bromoethanesulfonic acid (BES). In the presence of acetate and sulfate, inhibition of sulfate reducing bacteria (SRB) by molybdate (8 mmol L) caused a significant decrease in the decolorization rate of RO 96. Increases in the SRB inhibitor concentration (20, 38 mmol L) led to slower decolorization of RO 96, confirming the contribution of SRB to the decolorization. Inhibition of methane producing bacteria (MPB) with BES (10, 20 mmol L), however, did not significantly influence the decolorization, indicating that MPB took no part in the decolorization. In the presence of lactate and sulfate, SRB also contributed to the decolorization of RO 96. The mechanism of decolorization of azo dyes based on extracellular chemical reduction with sulfide was postulated. Sulfide produced via sulfate respiration by SRB chemically decolorizes azo dyes.