Mineralization of Azo Dyes under Integrated Anaerobic/Aerobic Conditions

Abstract

Integration of anaerobic and aerobic conditions is a good strategy for the complete mineralization of azo dyes. The success of the strategy relies on anaerobic azo dye reduction occurring inside biofilms exposed to oxygen and subsequent aerobic mineralization of the formed aromatic amines. First, the effect of oxygen on the azo dye reduction by methanogenic granular sludge was studied using Mordant Orange 1 (MOI, Color Index no. 14030) as a model azo dye and ethanol as a cosubstrate. Cosubstrate is required for the formation of anaerobic microniches and supply of reducing equivalents for the reduction. Aromatic amines released from MOI reduction were not mineralized due to the absence of appropriate aromatic amine degrading bacteria in the methanogenic sludge utilized. Second, the mineralization of 4-phenylazophenol (4-PAP, Color Index no. 11800) was studied under integrated anaerobic/aerobic conditions. Aniline and 4-aminophenol (4-AP) were formed during the anaerobic reduction of 4-PAP. It was examined whether aerobic microorganisms from aerobic sludge and sediment from the Rhine were capable to degrade these aromatic amines. Aniline was readily degraded but 4-AP autoxidized easily under aerobic conditions. No anaerobic degradation could be found for either aromatic amines, indicating the necessity of an aerobic step for removal of these aromatic amines. The bacterial cocultures, anaerobic granular sludge together with the aerobic enrichment culture, were able to degrade the azo dye under integrated anaerobic/aerobic conditions.