Environmental Engineering Reference
In-Depth Information
Substrate
Azo Dye
Product of
Substrate Oxidation
Aromatic Amines
Fig. 3 Schematic for direct enzymatic azo dye reduction
as an electron shuttle from NADPH-dependent avoproteins to azo dye as electron
acceptor (Gingell and Walker 1971 ). Intracellular azo dye reduction cannot be
responsible for the conversion of all types of azo dyes, especially for sulfonated azo
dyes, which have limited membrane permeability (Stolz 2001 ). Kudlich et al.
( 1997 ) demonstrated an increase on color removal rates of sulfonated azo dyes by
cell free-extracts, as well as after addition of toluene, i.e. a membrane-active
compound which increases cell lysis, thus showing the limited membrane perme-
ability of this type of dye. The current hypothesis is that azo dye reduction mostly
occurs by extracellular or membrane-bound enzymes (Stolz 2001 ). Reduced cyto-
plasmic co-factors, such as reduced
avins, do not contribute to the chemical dye
reduction due to their inability to cross living cell membranes (Russ et al. 2000 ).
However, cell fractionation experiments demonstrated that a quinone reductase
activity, located in the cell membranes, enhanced the reductive decolorization of a
sulfonated azo compound. In such case, no dye cross-membrane transport was
required (Kudlich et al. 1997 ). Recently, a NADH-dependent lawsone reductase
activity, located in the cytosolic fraction of Escherichia coli also showed the
capacity for azo dye reduction (Rau and Stolz 2003 ).
2.2.2 Biological and Chemical Reductive Decolorization
The reductive decolorization of azo dyes under anaerobic conditions is a combi-
nation of both biological and chemical mechanisms. The biological contribution can
be attributed to specialized enzymes, called azo reductases, which are present in
bacteria that are able to grow using only azo dye as a carbon and energy source.
However, there is no clear evidence till date for anaerobic azo reductase; or non-
speci
c enzymes that catalyze the reduction of a wide range of electron-with-
drawing contaminants, including azo dyes (Stolz 2001 ). Thus, a co-metabolic
reaction is probably the main mechanism of dye reduction (Fig. 3 ), in which the
reducing equivalents or reduced co-factors like NADH, NAD(P)H, FMNH 2 and
FADH 2 act as secondary electron donor and channel electrons to cleave the azo
bond (Gingell and Walker 1971 ). The chemical contribution to the reductive
decolorization of azo dyes under anaerobic conditions is linked to biogenic
reductants like sulphide, cysteine, ascorbate or Fe 2+ (Yoo 2002 ; Van der Zee et al.
2003 ). Among these sulphides can be formed by sulphate reduction in anaerobic
bioreactors. Therefore, there will be a competition between sulphate and dye to
become the terminal electron acceptor of the reducing equivalents. Van der Zee
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