Biodegradation potential of Citrobacter cultures for the removal of amaranth and congo red azo dyes

Abstract

Azo dyes are a class of synthetic dyes with a wide variety of colors and shades with a vast application in industry. Due to their chemical structure, such dyes exert mutagenic and carcinogenic effects on living organisms. Problems caused by environmental contamination with azo dyes highlight the importance of developing new treatment methods. Thus, biological treatment appears as a low-cost and eco-friendly technology, which uses the enzymatic production of some selected microorganisms for dye decolorization/degradation. Therefore, this work aims to select new bacterial isolates efficient in the mineralization of the azo dyes amaranth and congo red. For this purpose, soil contaminated with azo dyes was collected and 10 bacterial isolates capable of decolorizing both amaranth and congo red were selected. After evaluation of ideal pH and temperature of discoloration, two bacterial isolates belonging to the genus Citrobacter (LAJ01 and LAJ02) were selected aiming to evaluate the degree of dye mineralization. The temperature selected was 35 °C, and the pH was 7.5. LAJ01 reached 91.5% of amaranth discoloration, and LAJ02 and consortium (LAJ01 + LAJ02) reached 90.1% and 90.2% of amaranth discoloration, respectively. Phenol, indole, and N-glycylglycine were the main biodegradation products identified. For congo red dye, LAJ01 reached 93.6% of discoloration, LAJ02 reached 93.0% of discoloration, and the consortium reached 90.6% of discoloration. Phenol, indole, [1,1′-biphenyl]-3-amine, and benzidine were the main biodegradation products identified. Using as reference metabolites formed after discoloration, the isolate LAJ02 achieved a complete mineralization of amaranth. However, no bacterial isolates achieved the complete mineralization of congo red.