Microbe-Mediated Degradation of Synthetic Dyes in Wastewater - Microbial Degradation of Synthetic Dyes in Wastewaters - page 215

2.2.4 Microbiological Aspects of the Reductive Decolorization of Azo
Dyes

Pure Cultures

As re ected in Table 3 , there are extensive reports for use of pure cultures, either

whole cells or speci c enzymes, for a better insight of the anaerobic azo dye

reduction mechanisms, which are not fully understood yet (Stolz 2001 ; Dos Santos
et al. 2004a , b ). Microbial decolorization requires an unspeci

c enzymatic capacity
ubiquitously found in a wide diversity of microorganisms (Chung and Stevens
1993 ). This has been mainly demonstrated with microorganisms present in the
intestine, such as, Clostridium, Salmonella, Bacillus, Eubacterium and Escherichia
coli, which are able to reduce the dyes ingested through food, drugs and cosmetics
(Rau et al. 2002b ; Chen et al. 2004 ). The understanding of azo dye reduction
mechanisms is important not only for a biotechnological approach for decoloriza-
tion, but also for a medical approach to have an insight into how the intestinal
micro ora metabolites ingested azo dyes (Semde et al. 1998 ). Azo dyes are con-

verted into aromatic amines in the presence of micro

ora and the anaerobic con-
dition in the human intestine. Aromatic amines are more mutagenic and
carcinogenic than their precursor, azo dyes (Weisburger 2002 ). Therefore, a lot of
effort has been made in the production of compounds, which are resistant to
reductive transformations. Another approach has been looked into use of azo
polymers that would be insoluble in the upper gastrointes-reduction.

Granular Sludge

Even though anaerobic azo dye reduction could be readily achieved with different
microorganisms, there is no strain reported so far that is able to decolorize a broad
range of azo dyes. Therefore, the use of a speci c strain or enzymes for reductive

decolorization does not make much sense in treating textile wastewater, which
containing many kinds of dyes (Laszlo 2000 ). The use of mixed cultures, such as
anaerobic granular sludge, which is composed of stable microbial pellets with a
high activity, is probably a more logical alternative. However, a little is known
about the microbiological aspects of the reductive decolorization of azo dyes with
anaerobic consortia, commonly found in wastewater treatment plants, although the
applicability of the cost-effective high-rate anaerobic reactors for azo dye reduction
has been well demonstrated (Cervantes et al. 2001 ; Dos Santos et al. 2004b ; Dos
Santos 2005 ). As previously explained, the reductive decolorization of azo dyes by
using methanogenic anaerobic granular sludge is likely to be controlled by a co-
metabolic reaction in the presence of different electron donors, in which the azo dye
is the terminal electron acceptor of the reduced co-factors (Dos Santos 2005 ). Thus,
the reducing equivalents are formed during the conversion of the primary electron
donor, i.e. the organic matter, during the different steps of carbon

ow under