Reductive Decolorization of Azo Dye by Bacteria - Microbial Degradation of Synthetic Dyes in Wastewaters - page 118

the researchers have isolated and characterized several halophilic and halotolerant
microorganisms, which belonged to genus Halomonas and could decolorize azo
dyes under high salt conditions (Asad et al. 2007 ; Guo et al. 2008 ; Amoozegar et al.
2010 ). Moreover, Khalid et al. ( 2008 ) isolated Shewanella sp. AS96 from activated
sludge and studied its impact on azo dye decolorization under saline conditions. We
recently have systematically investigated the decolorization capacities of different
Shewanella strains and found that strains, isolated from marine sources including
Shewanella algae, Shewanella aquimarina and Shewanella maris

avi, demon-
strated a high ability in color removal in the presence of up to 100 g l − 1 NaCl or
Na 2 SO 4 (Meng et al. 2012 ; Liu et al. 2013b ). These salt-tolerant microbial strains
would facilitate treatment of saline dye wastewater.

3.5 Effects of Carbon/Nitrogen Sources

Ef cient decolorization of azo dye by bacteria generally requires the supplement of

carbon and nitrogen sources. Glucose has been frequently demonstrated to improve
the decolorization ef

cacy (Khan et al. 2013 ). However, higher concentration of
glucose could also decrease the decolorization performance, probably due to
acidication of the medium. Other organic substances, such as acetate, glycerin,
sucrose, salicylate and citrate etc. could also act as carbon source for azo dye
decolorization. However, the most effective carbon source is strain-speci c and

needs detailed investigation for different bacterial strains. The addition of organic
nitrogen sources including peptone, beef extract, yeast extract etc. could help the
regeneration of NAD(P)H, which donates electrons for bacterial azo dye reduction
(Saratale et al. 2011 ). There have been some reports indicating that certain bacterial
species could utilize azo dye itself or its reduction products as carbon/nitrogen
sources, but such microorganisms were very speci c to their substrates (Pandey

et al. 2007 ; Ali 2010 ). However, their effectiveness for practical azo dye wastewater
treatment needs further investigation.

3.6 Effects of Dye Structure/Concentration

Besides the common chromophore azo bond, azo dyes have diverse structures,
which signi cantly affects the decolorization performance of bacteria. Dyes with

simple structures and low molecular weights are generally easier to be decolorized.
Substitution of electron withdrawing groups (e.g. SO 2 NH 2 ) in the para-

position of phenyl ring relative to the azo bond makes dyes dif

- SO 3 H, -

cult to be reduced. It
is well known that the decolorization rates of monoazo dyes are generally higher
than those of diazo and triazo dyes (Saratale et al. 2011 ). However, sometimes the
above-mentioned rules do not work. Besides molecular structure and molecular
weight, the decolorization of an azo dye is also dependent on its redox potential.