Microbial Strategies for Decolorization of Anthraquinone Based Dyes in Batch and Continuous Systems

Abstract

Environmental pollution and human exposure to dyes have increased significantly in recent years due to their growing use in industries like textiles, paints, plastics, paper, and tannery. The textile industry in India is regarded as one of the significant sources of toxic waste. The effluent produced from the textile industry generates large quantities of unfixed dyes that are generally present in wastewater that poses a potential environmental risk. The Physico-chemical effluent treatment strategies are ineffective in degrading the recalcitrant synthetic dyes completely from effluents due to their light and colorfastness, stability, and resistance to degradation. Dye decolorization by biological treatment methods has recently achieved popularity, as these are cost-effective, environmentally friendly and thus can be applied to a wide variety of dyes. The isolation of microorganisms from contaminated sites is thought to become more effective in treating both recalcitrant and xenobiotic pollutants because they have been acclimatized to the toxic effect of the contaminants. Therefore, it is significantly necessary to have a comprehensive knowledge of the biological treatment strategies and various factors required to accomplish the desired performance. In the first part of the study, ten morphologically distinguishable bacterial strains were isolated from the textile wastewater sample collected from textile processing units outlet and dye polluted soil with a high possibility of contamination by anthraquinone vat dyes. Three potential isolates significantly varied from the other seven isolates were selected for further studies based on their high tolerance and maximum decolorization efficacy. Based on the morphological and biochemical characterization, 16S rRNA analysis, the three strains were identified as Bacillus flexus TS8, Proteus mirabilis PMS, and Pseudomonas aeruginosa NCH and the 16S rRNA gene sequence was deposited in the NCBI Gene bank database. The pure bacterial strains were evaluated for decolorization of four model anthraquinone vat dyes (Indanthrene Blue RS, Vat Green 1, Vat Brown R, and Vat yellow 5G). The three strains were found to be tolerant of high dye concentrations resulting in maximum decolorization for all the four model dyes. The effect of various Physico-chemical parameters on decolorization efficiency of the isolated strains shown that alkaline pH, ambient temperature, shaking, or aerobic incubation condition favors the growth and decolorization ability of the microorganisms. The second, third, and fourth part of the research work focused on the decolorization of different anthraquinone vat dyes by isolated strains. Response surface methodology was applied for the optimization of the process parameters such as pH, temperature, and inoculum size. Degradation kinetic studies were carried out using three different models, like Michaelis-Menten, Lineweaver-Burk, and Eadie-Hofstee model. Decolorization was validated through UV-vis spectroscopy and FT-IR analysis. The fifth part of the study focuses on enhanced decolorization of anthraquinone vat dye Indanthrene Blue RS by a developed bacterial consortium-BP. The physicochemical parameters were optimized to attain maximum decolorization efficacy. The agricultural residual wastes were supplemented to increase the decolorization efficiency of consortium-BP. The oxidoreductive enzymes involved in the decolorization was studied extensively. The phytotoxicity study of the original dye and its degraded metabolites formed on mineralization by consortium-BP was performed. The final part of the study focuses on the biodegradation of simulated textile wastewater containing Indanthrene Blue RS dye in an immobilized continuous packed bed bioreactor using corn-cob biochar. The adsorption studies were performed without microbes to evaluate the adsorption efficiency in the removal of the dye. The kinetic parameters were evaluated by using linear plots of pseudo-first-order, pseudo-second-order kinetic models. The Langmuir and Freundlich isotherm model was studied at 30°C. During the continuous operation, the effect of flow rate, initial substrate concentration, inlet loading rate on the elimination capacity and removal efficiency of Indanthrene Blue RS in the bioreactor was studied. The overall results of this research work suggested that bacterial strains isolated from dye contaminated textile wastewater can be used for the degradation of organic contaminants in the environment. The information gained from the biodegradation process and the mechanisms involved in the degradation of reactive dyes provide better knowledge of the transformation of anthraquinone based dyes in the environment.