Reduction of azo dyes by desulfovibrio desulfuricans

Azo dyes are widely used in textile finishing, and have become of concern in wastewater treatment because of their color, bio-recalcitrance, and potential toxicity to animals and humans. Thus, wastewater with aro dyes must be decolorized and furthermore mineralized in appropriate systems combining biological and chemical processes. In this study, the potential for sulfate reducing bacteria (SRB) to decolorize azo dyes was studied, employing the pure culture of Desulfovibrio desulfuricans (D. desulfuricans) with varying sulfate levels. Under sulfate-rich conditions, the sulfide produced from sulfate respiration with pyruvate (electron donor) by D. desulfuricans chemically decolorized the azo dyes C. I. Reactive Orange 96 (RO 96) and C. I. Reactive Red 120 (RR 120). Under sulfate-depleted conditions (less than or equal to 0.1 mmol/L), the decolorization of RO 96 and RR 120 occurred in correlation with the fermentation of pyruvate by D. desulfuricans. It is suggested that the electrons liberated from the pyruvate oxidation were transferred via enzymes and/or coenzymes (electron carriers) to the dyes as alternative terminal electron accepters, giving rise to decolorization, instead of to the protons (H+), resulting in the production of H-2. Both decolorization pathways were compared in light of bioenergetics and engineering aspects.