Environmental Engineering Reference
In-Depth Information
6 Mechanism of Dye Decolorization
Although for a long time, it was thought that the bacterial degradation of azo dyes
in either anaerobic or aerobic conditions were associated with symmetric cleavage
of the azo group (
N=N). This cleavage may be mediated through different
mechanisms, such as enzymes, low molecular weight redox mediators, chemical
reduction by biogenic reductants like sul
-
de, or a combination of both. However,
the location of such type of reactions could be either intracellular or extracellular.
The recent reports on azo dye decolorization have indicated involvement of
peroxidase, laccase, tyrosinase, NADH-DCIP reductase, and MG reductase in
decolorization of azo dyes.
6.1 Enzymatic Dye Decolorization
6.1.1 Azo Reductase
The presence of extracellular oxygen sensitive azo reductases in anaerobic bacteria,
such as Clostridium and Eubacterium, were decolorized sulfonated azo dyes during
their growth on solid or complex media (Ra
i et al. 1990 ). Anaerobic azo reduc-
tases were
avin oxidoreductase), localized at intracellular
or extracellular site of cell membrane, required NADH, NADPH and FADH as
electron donors for reduction of azo bonds (Ra
avoprotiens (NAD(P)H:
i and Cerniglia 1995 ; Russ et al.
2000 ). Co-factors like FADH 2 , FMNH 2 , NADH and NADPH, which provide
'
for reduction, were located at cytoplasm (Russ et al. 2000 ). Lysis of cells would
release co-factors at extracellular environment could be the possible reason for
higher azo dye reduction rates using cell extracts or starving or lysed cells than
intact or resting cells (Roxon et al. 1967 ; Walker 1970 ; Dubin and Wright 1975 ;
Wuhrmann et al. 1980 ; Mechsner and Wuhrmann 1982 ; Russ et al. 2000 ).
Intracellular sulfonated azo dye reduction required the speci
'
H
c transport system
that allows the uptake of the sulfonated azo dye into the cells (Russ et al. 2000 ). The
role of
avins to resting
cells of strain Sphingomonas strain BN6, which resulted in no signi
avin dependent azo reductase was shown by adding of
cant increase
in azo dye reduction. These
ndings suggest that in living cells with intact cell
membranes, other enzyme systems and/or other redox mediators are responsible for
reduction of azo dyes. In bacteria that possess electron transport systems in their
membranes, as in the case of aerobic or facultative anaerobic bacteria, such as
Sphingomonas strain BN6, the transfer of electrons from the respiratory chain to
appropriate redox mediators could take place directly. If intracellular reductases are
involved in the process, it is assumed that mediators, different from
avin cofactors
with a higher ability to pass through the membranes, must be involved. Although
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