Bacterial degradation of kraft lignin, its metabolites analysis, and toxicity assessment

Background Kraft lignin (KL) generated from the pulp and paper industry is brown in color, and it causes damage to the aquatic environment and contributes to the toxicity of the effluent. However, through biodegradation, the KL structure can be modified, leading to more sustainable usage. Complex degradation techniques pose economic and environmental challenges. Hence, microbial systems are used as they are cost-effective and leave low residues. Previous studies have demonstrated that Serratia liquefaciens could be used for lignin biodegradation, but its complex structure posed challenges. To tackle this issue, process optimization was undertaken in the present study to enhance the conditions and increase degradation efficiency. Methods In this study, the biodegradation potential of the ligninolytic bacteria Serratia liquefaciens was evaluated. Kraft lignin was characterized before and after bacterial treatment by instrumental analysis. Its biodegradation was optimized by Response Surface Methodology using Box-Behnken design. Toxicity evaluation was assessed in terms of phytotoxicity using Vigna radiata L. Results S. liquefaciens , with lignin peroxidase enzyme activity (22 IU ml − 1 ), degraded kraft lignin by 53% in 120 h. The characterization of kraft lignin confirms its biodegradation. Toxicity results show that sprout length was reduced by 85% when exposed to undiluted KL solution (1000 mg/L), compared to the control. However, the sprout length inhibition was reduced to 60% after bacterial treatment, demonstrating toxicity reduction. Enzyme-treated KL exhibited weaker toxicity compared to untreated KL as the enzymes degraded lignin’s complex structure, reducing the amount of toxic phenolic compounds. Due to depolymerization and loss of chemical bonds, the toxicity was reduced. Conclusion Kraft lignin can be toxic to the environment if it is discharged without proper treatment. Wastewater treatment is unable to reduce KL toxicity, as its structure is resistant to chemicals. Fungal systems have been used to degrade lignin. Fungi are not effective as they become unstable in harsh environments. Hence, the present study used a ligninolytic bacteria. Bacteria have environmental adaptability and provide a cost-effective solution. The study concluded that S. liquefaciens have significant potential to treat KL. It may be employed as a potential bioremediation application in the paper industry. This study opens up avenues to increase lignin utilization.