Microbial Enzymatic Degradation, Spectral Analysis and Phytotoxicity Assessment of Congo Red Removal by Bacillus Spp.

The textile industry is a leading contributor to water pollution. Therefore, there is a need for economic and environmental efforts to manage the textile industry’s wastewater. The present study used five bacterial strains, isolated from various Tunisian biotopes, to be characterized and screened for Congo Red (CR) decolorization, for the first time. Firstly, the isolated strains were subjected to standard cultural, microscopically, biochemical, antibiotic susceptibility and biofilm formation assays. The strain ST9 was partially identified as Bacillus spp. strain ST9. Secondly, the newly bacterial isolates crude filtrates were able to decolorize CR, generally recalcitrant to biodegradation due to its xenobiotic nature. Thus, only SM2 and ST9 strains prove their effectiveness for CR (150 mg L−1) total degradation. More than 5.5 mg L−1 h−1 of CR could be decolorized using ST9 filtrate under non-optimized conditions. UV–Visible and FT–IR analysis showed a decolorization and/or a transformation of CR, proving the role of the enzyme in dye decolorization. In fact, the azo dyes degradation was confirmed by the disparition of the characteristic peak at 499 nm and of the peaks located in the range 1610–1630 cm−1 and at 1600 cm−1 due to the azo bonds and the characteristic C=C band of the aromatic ring of CR, respectively. Combinning FTIR and UV–Vis spectra confirms the CR degradation using only SM2 and ST9 filtrates. The results showed CR has been degraded into nontoxic compounds evaluated by (1) phytotoxic assay on tomato, radish and watercress, and (2) cyto-toxicity assay in Vero cells. Furthermore, toxicity studies observed that the by-products from degradation by SM1 crude filtrate was not toxic to plants and less toxic to Vero cells as compared to pure CR. This work reported on the biodegradation of CR by bacterial crude filtrates, as green biocatalysts, which could be a potential candidate for the removal and detoxication of CR from textile wastewater.