Biodegradation Products of a Sulfonated Azo Dye in Aerobic Granular Sludge Sequencing Batch Reactors Treating Simulated Textile Wastewater

This study focused on the biodegradation of an azo dye (Acid Red 14, AR14) in two anaerobic-aerobic sequencing batch reactors (SBRs) treating synthetic textile wastewater, operated with aerobic granular sludge under different hydrodynamic regimens. The aim was to investigate the fate of the anaerobic AR14 breakdown products (aromatic amines) during the SBRs' aerobic reaction phase. Specifically, liquid chromatography coupled with electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS) was used for structural characterization of AR14 biodegradation metabolites, their molecular formulas being confirmed by accurate mass measurements. Nineteen molecules potentially related to AR14 were detected in the SBRs, and their relative abundances were followed along the aerobic stage of treatment cycles. The two SBRs shared most of the identified compounds but with differences in their metabolite profiles. Biodecolorization through AR14 anaerobic azo bond reduction was confirmed by the identification of the aromatic amine 4-amino-naphthalene-1-sulfonic acid, which was further aerobically biodegraded, involving deamination and hydroxylation of the aromatic ring. The other aromatic amine (1-naphthol-2-amino-4-sulfonic acid) was not detected, being suggested to undergo autoxidation reactions forming dimeric, stable products. A different AR14 biodegradation pathway was observed when nitrate was added to the feed, a new intermediate product being detected (naphthalene-1-sulfonate).