Reductive Decolorization of Azo Dye by Bacteria - Microbial Degradation of Synthetic Dyes in Wastewaters - page 123

5.2 Indirect/Mediated Bacterial Decolorization

During the past decades, the use of redox mediator or electron shuttle to stimulate
bacterial decolorization of azo dye has attracted many interests. Redox mediator
itself can rapidly get transformed between oxidized and reduced forms which helps
the transfer of electron or reducing power from bacterial cells to extracellular azo
dyes. Keck et al. ( 1997 ) for the rst time reported that redox intermediates gen-

erated during aerobic degradation of a xenobiotic compound could cause the
enhancement of azo dye decolorization under anaerobic conditions. Chang et al.
( 2004 ) also showed that the addition of culture supernatants containing metabolites
of a dye-decolorization strain E. coli NO 3 enhanced azo dye decolorization rate.
Field and Brady ( 2003 ) found that ribo cantly

improved the decolorization performance of anaerobic granular sludge. Recent
studies indicated that S. oneidensis MR-1 could self-excrete

avin in catalytic amounts signi

avins to improve
extracellular electron transfer (von Canstein et al. 2008 ). The addition of synthetic
electron carriers, such as anthraquinone-2-sulfonate, could also greatly enhance azo
dye decolorization (van der Zee et al. 2001 ). More recent studies tested the effec-
tiveness of many different quinone compounds as redox mediators to stimulate
bacterial decolorization. Besides, various immobilization methods were developed
to prevent the loss and repeated dosage of quinone compounds. A detailed sum-
mary of this study and other investigations on redox mediator have been discussed
(van der Zee and Cervantes 2009 ).
The use of synthetic quinones as redox mediators could signi

cantly speed up
the decolorization process; however, the high cost of quinone addition limited its
practical use. Recently, we found that natural humic acids containing lots of qui-
none groups could also stimulate the decolorization of azo dyes by S. oneidensis
MR-1 (Liu et al. 2011 ). On the other hand, carbon material, such as activated
carbon, carbon black and carbon nano tube, were also found capable of acting as
redox mediators for dye wastewater treatment (Pereira et al. 2014 ). These studies
offered practical application to bacteria-mediated decolorization.

5.3 Azo Dye Decolorization by Biogenic Inorganic
Compounds

Inorganic compounds, such as sul de and ferrous ion generated during metabolic

reactions under anaerobic conditions, could also decolorize azo dyes via purely
chemical reactions. However, this way of azo dye decolorization is less important
compared to the above two ways (Pandey et al. 2007 ).