Environmental Engineering Reference
In-Depth Information
2.1.2 Fungi
The capacity of fungi to reduce azo dyes is related to the formation of exoenzymes
such as peroxidases and phenol oxidases. Peroxidases are hemoproteins that cata-
lyze reactions in the presence of hydrogen peroxide (Duran et al. 2002 ). Lignin and
manganese peroxidases have a similar reaction mechanism which initiates with the
enzyme oxidation by H 2 O 2 to an oxidized state during their catalytic cycle.
Afterwards, in a mechanism involving two successive electron transfers, substrates
like azo dyes reduce the enzyme to its original form (Stolz 2001 ). Eighteen fungal
strains, which were able to degrade lignocellulosic material or lignin derivatives,
were tested with the azo dyes, Reactive Orange 96, Reactive Violet 5 and Reactive
Black 5. Only strains of Bjerkandera adusta, Trametes versicolor and Phanero-
chaete chrysosporium were able to decolorize all azo dyes (Hein
ing et al. 1997 ).
Although lignin peroxidases are able to oxidize both phenolic and non-phenolic
aromatic compounds, manganese peroxidases convert Mn 2+ to Mn 3+ in order to
oxidize phenolic compounds (Glenn et al. 1986 ). Phenoloxidases, which can be
divided into tyrosinases and laccases, are oxidoreductases that can catalyze the
oxidation of phenolic and other aromatic compounds without the use of co-factors
(Duran et al. 2002 ). Laccases are copper-containing enzymes which have a very
broad substrate speci
city with respect to electron donors, e.g. dyes (Abadulla et al.
2000 ). However, despite the fact that laccases from T. versicolor, Polyporus pin-
isitus and Myceliophthora thermophila were found to decolorize anthraquinone and
indigoid-based dyes at high rates, the azo dye Direct Red 29 (Congo Red) was a
very poor substrate for laccases (Claus et al. 2002 ). Chivukula and Renganathan
( 1995 ) stated that the azo dye must be electron-rich to be susceptible to oxidation
by laccase of Pyricularia oryzae. This situation is suitable for the generation of a
phenoxy radical, with consequent azo bond cleavage, and the release of molecular
nitrogen (Fig. 2 ). However, an addition of redox mediators has been shown to
further extend the substrate speci
city of laccases to several dye classes. Redox
mediators can also be formed from laccase oxidation of phenolic azo dyes (Li et al.
1999 ; Soares et al. 2001 ; Claus et al. 2002 ).
2.2 Biological Color Removal by Strict Anaerobes
or Facultative Microorganisms Under Anaerobic
Condition
Under anaerobic conditions, a low redox potential (< -50 mV) can be achieved,
which is necessary for the effective decolorization of dyes (Beydilli et al. 1998 ;
Bromley-Challenor et al. 2000 ). Color removal under anaerobic conditions is also
referred as dye reduction. The azo bond cleavage
involves a transfer of
four-electrons (reducing equivalents), which proceeds through two stages at the azo
linkage. At each stage, two electrons are transferred to the azo dye, which acts as a
N=N
-
-
nal electron acceptor:
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