Bacterial Degradation of Azo Dye Containing Wastes - Microbial Degradation of Synthetic Dyes in Wastewaters - page 68

4.9 Redox Mediators

Redox mediators are the compounds that speed up the reaction rate by shuttling
electrons from the biological oxidation of primary electron donors to the electron
accepting compounds, as

rst described by Bourbonnais and Paice ( 1990 ). Flavin-

based compounds, such as avin adenide

mononucleotide (FMN), and quinone-based compounds, such as anthraquinone-
2,6-disulfonate (AQDS), anthraquinone-2-sulfonate (AQS), riboavin (vitamin
B2), cyanocobalamin (vitamin B12) and lawsone (2-hydroxy-1,4-naphthoquinone),
have been reported as redox mediators. Redox mediators are characterized by a
redox potential ranging from

avin adenide dinucleotide (FAD) and

350 mV. The performance of redox
mediators is dependent on the redox potential. The dye removal rate is the highest,
when the redox potential of the system is at its most negative, and it decreases when
the redox potential of the system increases. The transfer of reducing equivalents
from a primary electron donor (co-substrate) to a terminal electron acceptor (azo
dye) lowers the electron density in the azo link which enhances the color removal
rate. The supplementation of redox mediators accelerates the transfer of reducing
equivalents to the terminal electron acceptor (i.e. azo dye), and also minimizes the
steric hindrance of the dye molecule. The effect of redox mediators on the decol-
orization of azo dyes and textile wastewaters was investigated by Saratale et al.
( 2011 ). However, the effect of redox mediators in enhancing the decolorization of
textile wastewater is still unclear due to a wide range of redox potentials among azo
dyes (

− 200 to −

430 mV) present in wastewater, high dye COD concentration
(about 1.4 g l − 1 ) and the different properties of the dyes (dos Santos et al. 2004 ).

− 180 to −

5 Enzyme System Involved in Azo Dye Degradation

The bacterial decolorization of azo dyes is mainly mediated by the various oxi-
doreductive enzymes, such as lignin peroxidase, manganese peroxidase, laccase,
veratryl alcohol oxidase, tyrosinase, amino pyrine N-demethylase, DCIP reductase,
azo reductase. According to reaction type, these oxidoreductase enzymes transfer
electrons from a substrate to an acceptor (azo dye). As per earlier reports, it has
been suggested that initial reduction of azo group takes place by reductive enzymes,
such as azo reductase, NADH-DCIP reductase and resulted intermediates are fur-
ther mineralized by oxidative enzymes. In general, azo dyes are electron de

cient

compounds due to presence of azo group ( N=N ) and other electron withdrawing

- -

groups, such as sulphonic (SO 3 ). Electron de cient azo dyes are not easily

degraded by bacteria, as they need supplementation. Enzymatic treatment is most
effective, when there is a high concentration of target contaminants and low con-
centration of interference.