India is one of the main producer and consumer
of synthetic organic chemicals including synthetic
dyes. Synthetic dyes are used extensively in
textile dyeing, paper printing and colour photography
and also as additives in petroleum products.
A wide variety of synthetic dyes namely azo,
polymeric, anthraquinone, triphenylmethane and
heterocyclic dyes is used in textile dyeing
processes. Worldwide more than 10,000 dyes and
pigments are used in dyeing and printing industries.
The total world colourant production is estimated
to be 8,00,000 tonnes per year and at least
10% of the used dyestuff enters the environment
through wastes (Levin et al., 2004;
Palmieri et al., 2005). The textile
industry accounts for two-thirds of the total
dyestuff market (Riu et al., 1998)
and consumes large volumes of water and chemicals
for wet processing of textiles. An estimated,
10-15% of dye is discharged or lost into the
effluents during different dyeing processes
(Zollinger, 2002).
Wastewaters from textile industries are a complex
mixture of many polluting substances like acids,
salts, organochlorine-based pesticides, heavy
metals, pigments, dyes etc., Due to complex
nature and hard-to-treat by conventional methods,
textile dyeing industries are facing problems
to safe discharge of wastewater. There have
been several successful methods developed based
on physical and chemical processes for colour
removal of textile dyeing effluents. They include
coagulation/flocculation, membrane filtration
and activated carbon adsorption. Unfortunately,
these methods of effluent treatment have high
operating costs and limited applicability. Further,
these treatment methods produce large quantities
of sludge, which again create a problem in waste
disposal (Moreira et al., 2000). In
recent years, biological decolourization using
potential microorganisms capable of decolourizing
and detoxifying the synthetic dyes has been
considered as a promising and eco-friendly method
(Couto et al., 2005; Camarero et
al., 2005).
Over the past few decades, numerous microorganisms
have been isolated and characterized for decolourization
of various groups of synthetic dyes. In general,
azo dyes are resistant to bacterial degradation.
However, certain bacteria can degrade dyestuff
by azoreductase activity (Chung and Stevens,
1993). White rot fungi (WRF), a group of basidiomycetous
are the potential organisms capable of mineralizing
the complex wood polymer and a wide variety
of recalcitrant compounds like xenobiotics,
lignin and dyestuff by their extracellular lignolytic
enzyme system. WRF offer significant advantages
over bacterial system since their extracellular
lignolytic enzyme system consisting of lignin
peroxidases, manganese dependent peroxidases,
manganese independent versatile peroxidases,
and laccases and they degrade a wide variety
of complex aromatic dyestuffs (Boer et al.,
2004; Kamistsuji et al., 2005). White-rot
fungi do not require preconditioning to particular
pollutants, because enzyme secretion depends
on nutrient limitation, nitrogen or carbon,
rather than presence of pollutant. The extracellular
enzyme system also enables white-rot fungus
(WRF) to tolerate high concentration of pollutants
(Knapp et al., 1997).
However, the fungi in waste treatment and bioremediation
do not always enable the culture conditions
for lignolytic to be fulfilled. Other white
rot fungi namely Bjerkandera adusta, Irpex
lacteus, Plebioa radiata, Pleurotus ostreatus,
P.sajor-caju, Ganodema lucidum, Pycnoporus cinnabarinus
and Trametes versicolor have been demonstrated
for the decomposition of several recalcitrant
dyes (Novotny et al., 2001; Murugesan
et al., 2006; 2007). The enzymatic
treatment of industrial waste has
exhibited several advantages over other physical
methods because it can be applied even to compounds,
which are biorefractory and it can be operated
at varied temperatures, pH and salinities.
Moreover, the enzymatic treatment of wastes
does not leave much sludge at the treatment
site. Much attention has been focused on the
development of processes to treat the wastewaters,
solid wastes, hazardous wastes and ameliorate
contaminated soils realizing the potential application
of enzyme treatments.
Immobilization
of enzymes
Biodegradation appears to be a promising technology,
particularly the use of oxidative enzymes as
biocatalyst included with a microorganism or
free enzyme. Laccase has received particular
attention because of its ability to catalyze
the oxidation of a wide spectrum of molecules
containing an aromatic ring substituted with
electron withdrawing groups (D’Annibale
et al., 1999). Enzyme immobilization
usually allows a good preservation of enzyme
activity over a long period (D’Annibale
et al., 1999). The efficiency of enzyme
extract is enhanced by selective adsorption
when immobilized, as reported by Tatsumi et
al. (1996), in the removal of chlorophenols
from wastewaters by peroxidase immobilized on
magnetite. In most cases, laccases are immobilized
on porous beads. Xenobiotics are degraded in
bed-packed column reactors. However, immobilization
of enzymes on a membrane and the use of filtration
offer several advantages. First it allows the
simultaneous downstream separation of the transformation
products, when they are insoluble and secondly
flow rates can be higher than with packed beads,
because all the substrate flows through the
support instead of diffusing in the bead pores.
Some of the intended applications e.g. kraft
pulp bleaching, dye effluent using laccase involve
high pH. Among the 40-50 known fungal laccases,
a few are active at alkaline pH (Schneider et
al., 1999). Being added to alkaline detergents,
the laccases are able to oxidize various textile
dyes to bleach the undesirable colour in washing
solution.
Effluent treatment
by immobilized mycelium
The efficiency of immobilized Pleurotus
sp. MAK-II for the decolourizing of the textile
dye effluent was assessed. Figure 1 shows the
steps involved for the textile dyeing effluent
treatment with immobilized mycelium of Pleurotus
sp. MAK-II. The SEM micrograph of immobilized
fungus alginate beads was completely different
from that of the beads without fungus. Table
1 shows the physicochemical properties of the
untreated and immobilized fungal treated effluents.
The initial and final pH of untreated effluent
was 9.5-9.8, whereas, the treated effluent pH
after 15 days decreased to 7.0-7.2. The values
of BOD and COD found high in the untreated effluent,
whereas the immobilized mycelium of the test
fungus removed up to 75% and 80% of BOD and
COD, respectively.
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Agar Plate culture |
After 1 day |
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Liquid plate
culture |
After 5 days |
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Immobilization |
After 10 days |
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Growth
of Immobilized
culture at 120 rpm 2days. |
After 15 days
Note the decolourization of effluent at
different days
of treatment in Bioreactor. |
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Bioreactor |
Without cell Immobilization
With cell
Immobilization |
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Fig. 1 Effluent treatment
with immobilized
mycelium of Pleurotus sp. MAK-II
Fig. 2 Declolourization
of the dye effluent and
laccase activity by immobilized mucelium
of Pleurotus sp. MAK - II
The fungus removed 55% of the
colour on 15th day and the maximum
laccase activity of 27.81 U/mL observed on 12th
day(Fig. 2). Reduction of peak height in the
UV spectrum clearly indicate decolourization
of effluent (Fig. 3).
Fig. 3 UV-visible spectram
of decolourization of effluent
by immobilized myceliam. Spectra after (1)1
day
(2) 5 days (3) 10 days (4) 15 days treatment.
Table 1. Physico-chemical
properties of untreated and treated effluents.
Parameter |
Effluent
and Medium (1:1) |
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|
Untreated |
Treated |
Colour |
Dark blue |
Light blue |
Odour |
Offensive |
No Odour |
pH |
9.5-9.8 |
7.0-7.2 |
BOD (mg/L) |
4500 |
1120 |
COD (mg/L) |
14000 |
2800 |
References
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White Rot Fungi for their ability to produce
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169-176.
Palmieri, G., Cennamo, G. and
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