Complete biodegradation of the azo dye azodisalicylate under anaerobic conditions

Azo dyes are a widespread class of poorly biodegradable industrial pollutants. In anaerobic environments, azo bonds are reductively cleaved yielding carcinogenic aromatic amines, many of which are assumed to resist further metabolism by anaerobes. Here we report for the first time that an azo dye compound is completely biodegradable in the absence of oxygen. A pharmaceutical azo dye, azo-disalicylate, constructed from two 5-aminosalicylic acid (5ASA) molecules was mineralized in an adapted methanogenic consortium to CH4 and NH3 with transient accumulation of 5ASA as a degradation intermediate in both batch assays and continuous bioreactors. The anaerobic metabolism of 5ASA was shown to provide the electrons required for the initial reductive cleavage of the azo group. Our results suggest that it is possible to design azo dyes that are fully mineralized in the environment; thereby, avoiding accumulation of notorious toxic intermediates.