Bacterial decolorization of black liquor in axenic and mixed condition and characterization of metabolites.

The pulping byproducts (black liquor) cause serious environmental problem due to its high pollution load. In order to search the degradability of black liquor, the potential bacterial strains Citrobacter freundii (FJ581026) and Citrobacter sp. (FJ581023) were applied in axenic and mixed condition. Results revealed that the mixed bacterial culture are more effective than axenic condition and can reduce 82% COD, 79% AOX, 79% color and 60% lignin after 144 h of incubation period. Additionally, the optimum activity of lignin degrading enzyme was noted at 96 h and characterized as manganese peroxidase (MnP) by SDS–PAGE analysis. Further, the HPLC analysis of control and bacterial degraded sample has shown the reduction as well as shifting of peaks compared to control indicating the degradation as well as transformation of compounds of black liquor. The comparative GC-MS analysis of control and degraded black liquor revealed that along with lignin fragment some chlorophenolic compounds 2,4,6-trichlorophenol, 2,3,4,5-tetrachlorophenol and pentachlorophenol were detected in black liquor degraded by axenic culture whereas these chlorophenolic compounds were completely absent in black liquor degraded by mixed bacterial culture. These chlorophenol inhibit the oxidative degradation which seems a major reason behind the low degradability of axenic degradation compared to mixed culture. The innovation of this aerobic treatment of alkaline black liquor opens additional possibilities for the better treatment of black liquor along with its metabolic product.