Reductive Decolorization of Azo Dye by Bacteria - Microbial Degradation of Synthetic Dyes in Wastewaters - page 110

Consequently, a great number of studies have been carried out to test different
species for decolorization and mineralization of azo dyes. It was observed that
many microbes, including bacteria, fungi, yeasts and algae could decolorize or even
completely mineralize various azo dyes under certain environmental conditions.
A great diversity of bacteria species have been isolated and identi

ed to be
capable of removing different azo dyes under varied conditions. The complete
removal of azo dye by bacteria generally takes two steps, i.e. the initial reductive
decolorization of azo dye into corresponding aromatic amines, followed by min-
eralization of the aromatic amines under aerobic conditions. The reductive cleavage
of the azo bond was believed to be the rate-limiting step and hence it has attracted
more interests. Table 1 summarized some typical bacteria species reported for azo
dye decolorization and their speci c decolorization performance under different

conditions. The diversity of these strains suggested a wide distribution of decol-
orization capacity in bacteria and offered candidates for the biological treatment of
azo dye wastewater.

3 Factors Affecting Bacterial Decolorization

Like many other biological treatment processes, various physico-chemical opera-
tional parameters, such as pH, temperature, dissolved oxygen/agitation, salinity,
dye structure/concentration, carbon/nitrogen sources etc., might directly affect the
decolorization capacity of different bacterial strains. Therefore, optimization of
these factors is essential to obtain good treatment results.

3.1 Effects of pH

The pH of the medium not only affects the adsorption capacity anddecolorization
activity of cells, but also impacts the solubility of the dye and color of the solution.
The tolerance to high pH is essential as many processes using azo dyes were
operated under alkaline conditions. The optimal pH for bacterial decolorization is
often between 6.0 and 10.0 (Saratale et al. 2011 ; Khan et al. 2013 ). A pH-inde-
pendent decolorization performance was found for the Brilliant Blue G by a
combination of Galactomyces geotrichum and Bacillus sp. (Jadhav et al. 2008a ).
Wang et al. ( 2009a ) reported the effective decolorization of Reactive Red 190 by
Citrobacter sp. CK3 under strongly acidic (pH 4) and alkaline (pH 12) conditions.
Tolerance to extreme pH values makes the bacterial strain suitable for practical
application.