Bacterial Degradation of Textile Dyes - Microbial Degradation of Synthetic Dyes in Wastewaters - page 241

investigators have shown that acidogenic as well as methanogenic bacteria also
contribute to dye decolorization. Reduction under anaerobic conditions appears to
be non-speci

c, as a group of azo compounds are decolorized. In such situation, the
rate of decolorization is dependent on the added organic carbon source, as well as
the dye structure (Stolz 2001 ). Furthermore, there is no correlation between
decolorization rate and molecular weight, indicating that decolorization is not a
speci c process and cell permeability is not important for decolorization. Thus,

anaerobic azo dye decolorization is the process where dye might act as an acceptor
of electrons supplied by carriers of the electron transport chain. Alternatively,
decolorization might be also attributed to non-speci

c extracellular reactions
occurring between reduced compounds generated by the anaerobic biomass (van
der Zee et al. 2001 ).

2.1.3 Decolorization Under Anoxic Conditions

Anoxic decolorization of various azo dyes by mixed aerobic and facultative
anaerobic microbial consortia has been reported by various workers (Nigam et al.
1996 ; Khehra et al. 2005 ; Moosvi et al. 2005 ). Although many of these cultures
were able to grow aerobically, decolorization was achieved only under anoxic
conditions. Pure bacterial strains, such as Pseudomonas luteola, Aeromonas hy-
drophila, Bacillus subtilis, Proteus mirabilis and Pseudomonas sp. decolorized azo
dyes under anoxic conditions (Chang et al. 2001 ; Yu et al. 2001 ; Chen et al. 2003 ).
Azo dye decolorization by mixed as well as pure cultures generally required
complex organic sources, such as yeast extract, peptone, or a combination of
complex organic sources (Chen et al. 2003 ; Khehra et al. 2005 ). Glucose is the
preferred substrate in anaerobic dye decolorization under methanogenic conditions,
but its suitability for anoxic dye decolorization by facultative anaerobes and fer-
menting bacteria seems to vary depending on the bacterial culture. Decolorization
of Mordant Yellow 3 by Sphingomonas xenophaga strain BN6 was greatly
enhanced by glucose, whereas a signi

cant decrease in azo dye decolorization in its
presence was reported for Pseudomonas luteola, Aeromonas sp. and a few other
mixed cultures (Haug et al. 1991 ; Chang et al. 2001 ). The negative effect of glucose
on anoxic decolorization has been attributed either to a decrease in pH due to acid
formation, or to catabolic repression (Chen et al. 2003 ).

2.1.4 Anaerobic Azo Dye Reduction

Anaerobic azo dye reduction is the reductive cleavage of azo linkages resulting in
the formation of aromatic amines. As aromatic amines are generally colorless, azo
dye reduction is also referred to as azo dye decolorization. The

rst study on azo
dye reduction was published as early as 1937, when the decolorization of food azo
dyes by lactic acid bacteria isolated from the human gut was reported (Brohm
and Frohwein 1937 ). Hence, as the formation of toxic aromatic amines in humans