Microbe-Mediated Degradation of Synthetic Dyes in Wastewater - Microbial Degradation of Synthetic Dyes in Wastewaters - page 226

10 (Rajee and Patterson 2011 ). Temperature affects microbial growth and enzyme
production and, consequently, the percentage of decoloration. For example,
Micrococcus sp. decolorates 60 % of 300 ppm of Orange MR at 30

° C, 80 % at

35 C (Rajee and Patterson 2011 ). Pseudomonas aeruginosa

degrades 97 % of 50 ppm of Remazol Red at 40

° C and 42 % at 45 °

° C, 72 % at 10 ° C and 82 % at

30 C (Jadhav et al. 2011 ). Temperature also affects biosorption as evident from the

fact that T. versicolor absorbs 44 mg g − 1 of Sirius Blue K-FCN at 7

°

C, 58 mg g − 1 °

C and 48 mg g − 1 at 45

at 26 C, and this increase in biosorption could be due to the

increased surface activity and kinetic energy of the dye molecules (Erden et al.
2011 ). The presence of oxygen can either favour or inhibit the microbial degra-
dation of azo dyes. Shaking increases mass and oxygen transfer between cells and
the medium, and enzyme activity can depend on the presence of oxygen, if the
mechanism is aerobic. The time to decolorate Methyl Red using a Micrococcus
strain was reduced from 24 to 6 h under a supply of oxygen (Olukanni et al. 2009 ).
Shewanella oneidensis shows higher dye decoloration under static conditions
(97 %) than in aeration (8 %), although cell growth is comparatively faster under
shaking conditions (Wu et al. 2009 ). Orange II decoloration with A. niger improves
with the shaking (84 %) compared to static growth (61 %) (Ali et al. 2009 ).

° °

4 Involvement of Oxidoreductive Enzymes in Degradation
Process

Azo dyes are electron-decient xenobiotic compounds because of their azo linkage
( NN), and in many cases, they have sulphonic (SO 3 − ) or other electron-withdrawing

groups, which generate an electron de

-

ciency and make the dye less susceptible to
degradation by microorganisms (Hsueh et al. 2009 ; Enayatizamir et al. 2011 ; Kub-
eran et al. 2011 ; Kurade et al. 2011 ). The anaerobic mechanism of microbial deg-
radation of azo dyes to their corresponding amines is initiated by the cleavage of the
azo linkage with the aid of an anaerobic azoreductase and electron transfer by a redox
mediator that acts as an electron shuttle between the extracellular dye and the
intracellular reductase (Ramalho et al. 2002 ). The oxidative degradation of azo dyes
is catalyzed by peroxidases and phenoloxidases, such as manganese peroxidase
(MnP), lignin peroxidase (LiP), laccase (Lac), tyrosinase (Tyr), demethylase (Duran
et al. 2002 ; Jadhav et al. 2011 ; Oturkar et al. 2011 ; Martorell et al. 2012 ).

5 Toxicity of Decoloration Products and Evaluation
Methods

The metabolites produced from dye degradation are, in many cases, more toxic that
the parent dye. For example, the products of the oxidation of indigo blue via electro
incineration, coagulation with Al 2 (SO 4 ) 3 or the use of Lac are more toxic than the