Temporal profiling of Congo red degradation products during exposure to microalgae in synthetic wastewater

This study is focused on the degradation mechanism and decolourisation efficiency of diazo dye Congo red with time when exposed to live Scenedesmus sp. in synthetic wastewater. The maximum decolourisation efficiency in synthetic wastewater achieved was 63.1 ± 3.8% on the 15th day of the experiment. For dye degradation analysis, UV–Visible spectroscopy was performed, which showed the breakage of the azo bond in the Congo red dye molecule after treatment. Gas chromatography–mass spectroscopy analysis performed on the 5th day of the experiment in synthetic wastewater revealed the formation of degradation products like benzene sulfonic acid, benzidine, and an unstable dinitro-naphthalene-1-sulfonic acid after the cleavage of the azo bond in Congo red dye. It was also assumed that the degradation products subsequently underwent further degradation into simpler forms by the 15th day and led to the formation of simpler compounds. Further, Fourier transform infrared spectroscopy analysis elucidated the significance of functional groups, providing evidence of Congo red dye molecule attachment on the surface of algal cells. Scanning electron microscopy showed the morphological deformation in Scenedesmus sp. in the presence of dye-containing wastewater. Phytotoxicity assessment revealed a decrease in the phytotoxic effects of the treated Congo red dye.