Assessment of Bio Elimination and Detoxification of Phenothiazine Dye by Bacillus firmus in Synthetic Wastewater under High Salt Conditions

High salt concentrations in textile industrial wastewater challenge the efficiency of microbial strains used for decolorization of dye pollutants. In this study, the decolorization of Methylene blue (a phenothiazine dye) under high salt conditions using Bacillus firmus was investigated. The bacterial isolate was obtained from textile effluent as a positive enrichment culture with dye using simulated saline dye wastewater at 4% salt content. The effects of chemical parameters inherent in real textile wastewater such as dye, salt (sodium chloride) and surfactant (sodium duodecyl sulphate) concentration on decolorization of Methylene Blue (MB) by the bacterium were assessed. The bacterium showed tolerance for high concentrations of the dye as it was able to decolorize different concentrations (25-125 mg L-1) of MB albeit at varying rates. Efficient Methylene blue decolorization in saline medium was at salt concentrations between 1 and 6% while, tolerance to the effect of surfactant was at concentrations between 0.1 and 5.0 mg L-1. The isolate had a broad spectrum decolorization efficiency being able to decolorize seven structurally different dyes: Safranin (82.6%); Serva blue (76.8%); Neutral red (63.7%); Methylene blue (94.6%); Crystal violet (67.2%); Nigrosine (41.5%) and Basic fuchsin (68.1%) to varying degrees under high salt conditions. Spectrophotometric analyses in the UV-visible domain of culture supernatants indicated color removal was due to biodegradation rather than adsorption of dyes on bacterial cells. Detoxification of MB following decolorization by Bacillus firmus was confirmed using phytotoxicity studies. Results demonstrate the ability of the isolate to eliminate MB and other dyes under high salt conditions.