Environmental Engineering Reference
In-Depth Information
7 Mechanism of Bacterial Dye Degradation
It has been demonstrated recently that the rate of anaerobic reduction of azo dyes by
Sphingomonas xenophaga BN6 could be signi
cantly increased by the addition of
different quinines, such as anthraquinone-2-sulfonate or 2-hydroxy-1,4-naphtho-
quinone. Thus, it was demonstrated that the addition of naphtha quinone and natural
organic matter could signi
cantly enhance the reduction rate of nitro aromatic
compounds and hexachloroethane in the presence of bulk reductant (e.g. H 2 S).
Furthermore, it has also been seen that strictly anaerobic Fe (III)-reducing bacteria
use the reduction of quinine moieties of humic substances (and also sulfonated
anthraquinones) to transfer the reduction equivalents released during the anaerobic
oxidation of organic substances. Thus, it becomes evident that in the presence of
redox mediators, many heterotrophic aerobic bacteria decolorize azo dyes under
anaerobic conditions. Quinones may undergo one-electron reduction processes to
the corresponding hydroquinone radicals or two-electron reduction to the corre-
sponding hydroquinones. Therefore, one-electron potentials can be used to compare
the relative tendency of different quinines to take up reduction equivalents. The
understanding on microbial degradation and decolorization of azo and reactive dyes
is still limited and has been studied by only a few workers.
Kulla et al. ( 1983 ) employed strains of Pseudomonas in chemostat culture for
removal of dyes. Some anaerobic bacteria and Streptomyces have been character-
ized for decolorization of chromogenic dyes. A bacterium, Proteus mirabilis, iso-
lated from acclimated sludge from a dyeing wastewater treatment plant, rapidly
decolorized a deep Red azo dye solution (RED RBN). Features of decolorizing
process related to biodegradation and biosorption were also studied. Although
P. mirabilis displayed good growth in shaking culture, color removal was best in
anoxic static cultures and found very effective for color removal under optimum
conditions (pH 6.5
°
C). The organism showed a
remarkable color removal capability, even at a high concentration of azo dye. More
than 95 % of azo dye was reduced within 20 h at a dye concentration of 1.0 g l 1 .
Decolorization appears to proceed primarily by enzymatic reduction associated with
a minor portion (13
-
7.5 and temperature 30
-
35
17 %) of biosorption to inactivated microbial cells.
-
8 Main Factors Effecting Dye Degradation
Biodegradation of xenobiotics depends upon the physical, chemical and biological
processes which are also governed by some environmental factors. The fungal
growth and enzyme production or secretion, and consequent decolorization and
degradation are in
uenced by numerous factors, e.g. media composition, pH value,
agitation and aeration, temperature and initial dye concentration. Thus, depending
on the culture characteristics, the degradation potential for dyes also varies upon the
environmental conditions (Robinson et al. 2001 ). The structure of dyes strongly
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