Comparative bacterial degradation and detoxification of model and kraft lignin from pulp paper wastewater and its metabolites

Continuous discharge of lignin containing colored wastewater from pulp paper mill into the environment has resulted in building up their high level in various aquatic systems. In this study, the chemical texture of kraft lignin in terms of pollution parameters (COD, TOC, BOD, etc.) was quite different and approximately twofold higher as compared to model lignin at same optical density (OD 3.7 at 465 nm) and lignin content (2000 mg/L). For comparative bacterial degradation and detoxification of model and kraft lignin two bacteria Citrobacter freundii and Serratia marcescens were isolated, screened and applied in axenic and mixed condition. Bacterial mixed culture was found to decolorize 87 and 70 % model and kraft lignin (2000 mg/L), respectively; whereas, axenic culture Citrobacter freundii and Serratia marcescens decolorized 64, 60 % model and 50, 55 % kraft lignin, respectively, at optimized condition (34 °C, pH 8.2, 140 rpm). In addition, the mixed bacterial culture also showed the removal of 76, 61 % TOC; 80, 67 % COD and 87, 65 % lignin from model and kraft lignin, respectively. High pollution parameters (like TOC, COD, BOD, sulphate) and toxic chemicals slow down the degradation of kraft lignin as compared to model lignin. The comparative GC–MS analysis has suggested that the interspecies collaboration, i.e., each bacterial strain in culture medium has cumulative enhancing effect on growth, and degradation of lignin rather than inhibition. Furthermore, toxicity evaluation on human keratinocyte cell line after bacterial treatment has supported the degradation and detoxification of model and kraft lignin.