Biodegradation and detoxification of Congo red by thermophilic Bacillus smithii AMPNK

Congo red (CR) is a widely used azo dye in the textile industry and is known for its carcinogenic properties. The discharge of CR-containing textile effluents necessitates efficient and sustainable treatment strategies before environmental release. In this study, the ability of the thermophilic bacterium Bacillus smithii AMPNK to biodegrade CR under different physicochemical conditions was evaluated. A maximum CR decolorization of approximately 95% was achieved at 55 °C when maltose was used as the carbon source, while beef extract was the most effective nitrogen source, yielding approximately 87.17% decolorization. The optimum pH for dye degradation was 7.0, with approximately 90% decolorization. UV-Vis, FT-IR, and GC-MS analyses confirmed the occurrence of azo bond cleavage and the formation of lower-molecular-weight degradation metabolites, indicating efficient biodegradation of CR. Laccase activity and total protein analysis further demonstrated that enzymatic degradation was closely associated with the active growth phase of B. smithii AMPNK. Phytotoxicity assays using Vigna radiata and Trigonella foenum-graecum revealed that untreated CR markedly inhibited seedling growth, reducing root length to 2.2 ± 1.23 cm and 2.1 ± 0.81 cm, respectively, whereas treatment with biodegraded metabolites restored root growth to 5.4 ± 1.50 cm and 4.7 ± 1.34 cm, respectively, indicating substantial detoxification. Overall, these findings demonstrate that thermophilic B. smithii AMPNK is a promising biocatalyst for the biodegradation and detoxification of Congo red in high-temperature textile wastewater.