Microbe-Mediated Degradation of Synthetic Dyes in Wastewater - Microbial Degradation of Synthetic Dyes in Wastewaters - page 210

et al. ( 2003 ) observed that different sulphate concentrations did not have an adverse
effect on the reduction of RR2 in either batch assays or reactor experiments. Hence,
they concluded that sulphate, even present at concentrations up to 60 mM, did not
obstruct the transfer of electron to the azo dye. However, any noticeable effect of
sulphate was veri

ed on the dye removal effciency in the lab experiment. In another
investigation on color removal, Albuquerque et al. ( 2005 ) used an anaerobic-aer-
obic sequencing batch reactor fed with sulphate (0.35 mM). The results indicated
that the decolorization capacity was not improved while testing the dye Acid
Orange 7, even though a sulphate reducing microbial population was used.
Therefore, for a real perspective application, sulphide generated by sulphate
reduction has no role to play and therefore, the color removal is mainly due to
biological processes. Color removal by anaerobic granular sludge under mesophilic
conditions has been re

cacy
of different method because of the differences in type and concentration of dyes,
sludge sources and concentrations, electron donor,

ected in Table 2 . It is very dif cult to compare the ef

the way of calculating the

decolorization rates etc.
As evident from Fig. 4 that the electron ow preference in the presence of

different redox couples involves biological processes. Thus, oxygen is a more
effective electron acceptor than azo dyes, which justi

es the low decolorization

rates (10 - 30 %) under aerobic conditions.

2.2.3 Reductive Decolorization of Azo Dyes in the Presence
of Redox Mediators

Redox mediators are compounds that accelerate the electron transfer from a primary
electron donor to a terminal electron acceptor, which may increase the reaction rates
by one to several orders of magnitude (Cervantes 2002 ; Dos Santos 2005 ). Redox
mediators have shown to be effective not only for reductive decolorization, but also
for the reductive transformation of many contaminants such as iron (Lovley et al.
1998 ), nitroaromatics (Dunnivant et al. 1992 ), polyhalogenated compounds
(O Loughlin et al. 1999 ) and radionuclides (Fredrickson et al. 2000 ). Recently, it

was found that during the aerobic degradation of naphthalene-2-sulfonate (2NS) by
Sphingomonas xenophaga strain BN6, quinoid redox mediators were produced,
which mediated the reduction of azo dye under anaerobic conditions (Keck et al.
2002 ). Flavin-based compounds, like FAD, FMN and ribo

'

avin, as well as qui-
none-based compounds, like AQS, AQDS and lawsone, have been extensively
reported as redox mediators during azo dye reduction (Semde et al. 1998 ; Cervantes
2002 ; Rau et al. 2002a ; Field and Brady 2003 ; Dos Santos et al. 2004a ; Dos Santos
2005 ; Encinas-Yocupicio et al. 2006 ). As re ected in Fig. 5 , reductive decolor-

ization of azo dyes in the presence of redox mediators occurs in two distinct steps,
the

c enzymatic mediator reduction, and the second step
being a chemical re-oxidation of the mediator by the azo dyes (Keck et al. 1997 ).
Theoretically, feasible redox mediators for biological azo dye reduction must
have redox potentials between the half reactions of the azo dye and the primary

rst step being a non-speci