Use of Fungal Laccases and Peroxidases for Enzymatic Treatment of Wastewater Containing Synthetic Dyes - Green Chemistry for Dyes Removal from Wastewater - page 235

redox mediator. They found that violuric acid was a better redox media-
tor than HBT for the L. edodes laccase. Crude and purified laccase from a
white rot fungus Ganoderma lucidum was investigated for the decoloriza-
tion of Reactive Black 5 and an anthraquinone dye, Reactive Blue 19 [165].
The crude enzyme was able to decolorize Reactive Blue 19 without a redox
mediator, while Reactive Black 5 required a mediator HBT. With 25 U/
mL of crude G. lucidum laccase, 50 mg/L of Reactive Black 5 and Reactive
Blue 19 could be decolorized by 77% and 92% in two hours, respectively.
Similarly, purified laccases from Daedalea quercina [166] and Perenniporia
tephropora [167] were evaluated for the decolorization of various syn-
thetic dyes. Lu et al. [168] isolated and purified laccase from the culture of
P. sanguineus and evaluated its use in decolorization of Reactive Blue 19.
Very rapid decolorization of this anthraquinone dye (50 to 200 mg/L) was
observed using 5 U/mL of P. sanguineus laccase without a redox mediator.
Soares et al. [169] performed a study on Reactive Blue 19 decoloriza-
tion by a commercial laccase from genetically modified A. niger . hey
found that in the presence of HBT as a redox mediator, the oxidation of
Reactive Blue 19 could be accelerated significantly. Within 60 min, 100%
of 20 mg/L of Reactive Blue 19 could be decolorized by 10 U/mL of lac-
case and 0.15% HBT. They also found that certain surfactants (e.g., Triton
X-100 and Tween 20) showed an inhibitory effect on the dye decoloriza-
tion. In another report, they found that another kind of redox mediator,
violuric acid (Figure 6.14), showed improved efficiency [170].

6.6.3.2 Kinetics and Reaction Mechanisms

Chivukula and Renganathan [171] studied the mechanisms of mono-azo
compound oxidation by laccase from Pyricularia oryzae by using synthe-
sized 4-(4'-sulfophenylazo)-phenol derivatives as model azo compounds.
They found that the dimethyl derivatives of the model compound were
the best substrate of P. o r y z a e laccase. Corresponding benzoquinones
and sulfophenylhydroperoxides were identified as degradation products
(Figure 6.15), suggesting the one electron oxidation of phenolic hydroxyl
group, followed by the liberation of benzoquinone and dinitrogen, and the
oxidation of benzensulfonate to peroxide. Similarly, Soares et al. [172] syn-
thesized four diazo-model dyes and used them to investigate the impact of
substituents on one of the aromatic rings in the laccase-catalyzed decolor-
ization. They used a commercial laccase formulation produced by geneti-
cally modified A. niger . Interestingly, without a sulfonate group (R-SO 3 - )
this laccase could not decolorize the synthesized dyes with or without a
redox mediator HBT. Those dyes were water insoluble. Violuric acid was