Decolorization of azo dyes by Shewanella sp. under saline conditions

Abstract

Wastewaters from textile processing and dye-stuff manufacture industries contain substantial amounts of salts in addition to azo dye residues. To examine salinity effects on dye-degrading bacteria, a study was carried out with four azo dyes in the presence of varying concentrations of NaCl (0–100 g l⁻¹) with a previously isolated bacterium, Shewanella putrefaciens strain AS96. Under static, low oxygen conditions, the bacterium decolorized 100 mg dye l⁻¹ at salt concentrations up to 60 g NaCl l⁻¹. There was an inverse relationship between the velocity of the decolorization reaction and salt concentration over the range between 5 and 60 g NaCl l⁻¹ and at dye concentrations between 100 and 500 mg l⁻¹. The addition of either glucose (C source) or NH₄NO₃ (N source) to the medium strongly inhibited the decolorization process, while yeast extract (4 g l⁻¹) and Ca(H₂PO₄)₂·H₂O (1 g l⁻¹) both enhanced decolorization rates. High-performance liquid chromatography analysis demonstrated the presence of 1-amino-2-naphthol, sulfanilic acid and nitroaniline as the major metabolic products of the azo dyes, which could be further degraded by a shift to aerobic conditions. These findings show that Shewanella could be effective for the treatment of dye-containing industrial effluents containing high concentrations of salt.