Microbe-Mediated Degradation of Synthetic Dyes in Wastewater - Microbial Degradation of Synthetic Dyes in Wastewaters - page 214

Fig. 6 E0 ' values for both quinone-based and non quinone-based redox mediators (Rau et al.

2002a )

ef ' 0 values for both

quinone-based and non-quinone-based redox mediators. The standard redox
potential value (E

ciently reduce the azo bond at high rates. Figure 6 shows the E

′ 0 ) is a good indication of a compound capacity to function as a
redox mediator. However, other factors are also of importance as well, since dif-
ferent decolorization rates in the presence of mediators with similar E

′ 0 values have
been reported, and similar decolorization rates with mediators with different E

′ 0
values (Dos Santos et al. 2004a ). For instance, Brown ( 1981 ) tested the polymeric
nitro dye Poly Y-607 and found that methyl viologen and benzyl viologen increased
the decolorization rates by 4.5-fold, even though the E ′ 0 of methyl viologen
(

0.360 V (Fig. 6 ).
Walker and Ryan ( 1971 ) postulated that decolorization rates were related to the
electron density in the azo bond region. They suggested that color removal rates
would increase by lowering the electron density in the azo linkage. Therefore, the
use of redox mediators would not only tend to accelerate the transfer of reducing
equivalents to the terminal electron acceptor, i.e. the azo dye, but also to minimize
the steric hindrance of the dye molecule (Bragger et al. 1997 ; Moir et al. 2001 ) and
to decrease the activation energy of the chemical reaction (Dos Santos 2005 ). Thus,
in evaluation of theoretical decolorization rates by using speci

0.440 V) is much lower than that of benzyl viologen, i.e.

− −

c redox mediators,
differences in electrochemical factors between mediator and azo dye should also be
considered.