Environmental Engineering Reference
In-Depth Information
6.3 Azo Dye Decolorization by Organic and Inorganic
Compounds
Azo dye decolorization can occur from purely chemical reactions with inorganic
compounds, such as sul
de and ferrous ion which are formed as end products of
metabolic reactions under anaerobic condition. It has been shown that H
2
S gen-
eration by SRB resulted in the extracellular decolorization of azo dyes (Yoo et al.
2000
; Diniz et al.
2002
). Sulfate-in
uenced dye reduction was correlated with
biogenic sul
de formation under methanogenic conditions. In the absence of sulfur
compounds, dye decolorization readily occurred in the presence of granular sludge,
demonstrating the importance of enzymatic mechanisms. An analysis of decolor-
ization kinetics, in batch reactor and in laboratory scale anaerobic sludge bed
reactors, indicated that the relative importance of chemical dye reduction mecha-
nisms in high rate anaerobic bioreactors was small due to the high biomass in the
reactors (van der Zee et al.
2003
). The various inducers of oxidative enzymes, such
as CaCO
3
, indole, o-tolidine, veratrole, and vanillin, enhanced the dye decolor-
ization (Dawkar et al.
2008
). The electron donors, viz. sodium acetate, sodium
formate, sodium succinate, sodium citrate, and sodium pyruvate enhanced the azo
dye decolorization rate by Bacillus sp. ADR. Sodium acetate, sodium formate,
sodium citrate, and sodium pyruvate might act as arti
cial electron donors for
reductases involved in azo dye decolorization. 3,4-dimethoxy benzyl alcohol and o-
tolidine increased the azo dye decolorization rate of Bacillus sp. ADR by stabilizing
the extracellular phenol oxidase enzyme (Telke et al.
2009
).
7 Decolorization of Textile Dyes by Puri
ed Bacterial
Oxidoreductase Enzymes
7.1 Azoreductase Mediated Dye Decolorization
The azoreductase enzyme was puried from Exigubacterium sp. RD3 which had
molecular weight of 95 Kda (Dhanve et al.
2008a
,
b
). The puri
ed azoreductase
was studied for its ability for azo dye decolorization. Azoreductase mediated azo
bond cleavage in Methyl Red produced intermediates i.e., N, N-dimethyl benzene-
1,4 diamine and 2-amino benzoic acid (Fig.
8
) whereas, Congo red produced
diamino naphthyl sulfonic acid and benzedine by azoreductase action (Fig.
9
).
7.2 Laccase Mediated Dye Decolorization
Laccase was puri
ed from Bacillus sp. ADR (Telke et al.
2009
). Puri
ed laccase
was studied for decolorization of Methyl Red and Methyl Orange.
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