Kinetic and equilibrium studies on the biosorption
of reactive black 5 dye by Aspergillus foetidus
Rachna Patel 1, Sumathi Suresh
*
Centre for Environmental Science and Engineering, Indian Institute of Technology-Bombay, Powai, Mumbai 400 076, Maharashtra, India
Abstract
An isolated fungus, Aspergillus foetidus had the ability to decolourize growth unsupportive medium containing 100 mg L1 of reactive
black 5 (RB5) dye with >99% efficiency at acidic pH (2–3). Pre-treatment of fungal biomass by autoclaving or exposure to 0.1 M sodium
hydroxide facilitated more efficient uptake of dye as compared to untreated fungal biomass. Pre-equilibrium biosorption of RB5 dye
onto fungus under different temperatures followed pseudo-second-order kinetic model with high degree of correlation coefficients
(R2 > 0.99). Biosorption isotherm data fitted better into Freundlich model for lower concentrations of dye probably suggesting the heterogeneous nature of sorption process. Based on the Langmuir isotherm plots the maximum biosorption capacity (Q0) value was calculated to be 106 mg g1 at 50 C for fungal biomass pre-treated with 0.1 M NaOH. Thermodynamic studies revealed that the biosorption
process was favourable, spontaneous and endothermic in nature. Recovery of both adsorbate (dye) and adsorbent (fungal biomass) was
possible using sodium hydroxide. Recovered fungal biomass could be recycled number of times following desorption of dye using 0.1 M
NaOH. Fungal biomass pre-treated with NaOH was efficient in decolourizing solution containing mixture of dyes as well as composite
raw industrial effluent generated from leather, pharmaceutical and dye manufacturing company.
Keywords: Azo dyes; Biosorption; Decolourization; Fungus; Reactive black 5
1. Introduction
Azo dyes are characterized by the presence of one or
more –N@N–(azo) bonds and used in products such as textile, leather and foodstuffs. The textile industry ranks first in
the usage of azo dyes for colouration of the fiber. Slokar
and Marechal (1998) reported that the worldwide annual
consumption of dyes is around 7 · 104 tonnes, out of which
50% is lost in manufacturing and processing units. In India,
an average mill producing 60 · 104 m of fabric discharges
approximately 1.5 million litres of effluent per day (Sandhya
et al., 2005). Disposal of untreated effluent to the surroundings often leads to the following consequences (a) makes the
water bodies coloured and creates aesthetic problem (b)
limits the reoxygenation capacity of the receiving water
and cuts-off sunlight which in turn disturbs the photosynthetic activities in the aquatic system and (c) causes chronic
and acute toxicity (Arami et al., 2006; Kadirvelu et al.,
2005). Thus it is mandatory to treat dyebath effluents prior
to discharge into the surrounding aquatic systems.
Physico-chemical processes such as electro-coagulation,
ozonation, photocatalysis, membrane filtration and
adsorption have been employed for the treatment of dye
containing wastewater (Capar et al., 2006; Senthilkumaar
et al., 2006; Shu, 2006; Silva et al., 2006; Alinsafi et al.,
2005). Among these technologies, adsorption process is
considered to be a promising technology which involves
phase transfer of dye molecules onto adsorbent leaving
behind the clear effluent. Research groups have explored
the possibility of using a wide variety of sorbents derived
from carbonaceous materials (such as wood, peat, rice
52
husk and coconut shell), raw agricultural solid wastes (such
as sawdust, wheat straw, orange peels and bagasse), industrial solid wastes (such as fly ash, red mud and metal
hydroxide sludge), natural materials (such as clays, zeolites,
siliceous materials) and biological materials (Crini, 2006).
Biosorption process employing biopolymers (such as sawdust, wood chips, chitin/chitosan, starch, cyclodextrin
and cross linked chitosan/cyclodextrin) and non-viable
microbial (fungi, algae and bacteria) biomass has emerged
as one of the powerful and attractive option since it is inexpensive, effective and simple to operate (Batzias and Sidiras, 2007; Crini, 2006; Kumari and Abraham, 2006;
Maurya et al., 2006; Renganathan et al., 2006). Biosorption is a metabolism independent mechanism that involves
binding of pollutants to the surface of cell membranes and/
or cell walls through physical adsorption, electrostatic
interaction, ion exchange, chelation and chemical precipitation (Aksu, 2005). In some cases the pollutants may be
internalized within the cells. The materials to be used as
biosorbents for the removal of dyes should meet the following criteria: (a) low cost and abundant supply (b) high rates
and capacity for sorption (c) applicable to effluents containing wide variety of dyes and (d) ability to tolerate the
presence of co-pollutants such as salts, heavy metals and
other conditions of wastewater (Crini, 2006).
The ability of a fungus, Aspergillus foetidus to uptake azo
reactive dye(s) such as drimarene red under active growth
and growth unsupportive conditions was reported in earlier
publications (Sumathi and Manju, 2000, 2001; Sumathi and
Phatak, 1999). Subsequent experiments revealed that the
pre-treatment of fungal biomass with agents such as sodium
hydroxide enhanced its biosorption capacity to a great
extent. Therefore the possibility of extending the application
of pre-treated fungus for uptake of very recalcitrant azo dye
such as reactive black, decolourization of raw industrial
effluent and solutions containing mixture of commercial textile dyes was explored. Specific objectives of the present
investigations were to: (a) characterize pre-equilibrium
kinetics of uptake of reactive black 5 (RB5) dye as well as
evaluation of equilibrium isotherm constants (b) examine
the thermodynamics of biosorption of RB5 dye (c) explore
the possibility of reuse of fungal biomass for decolourization
of dye containing effluent in order to make the process economical and (d) assess the applicability of fungal biomass for
the decolourization of solution containing mixture of dyes
and raw industrial effluent containing textile dyes. These
results are relevant from the point of view of the design
and operation of biosorption system, recovery of dyes, regeneration of fungal biomass and recycling of treated water.
2. Methods
colorant, mol. wt. 534.37) and sunset yellow FCF (food
colorant, mol. wt. 452.37) were purchased from Sigma
Aldrich Chemical Company. The purity of dyes was 90%.
A stock solution of 1000 mg L1 of each dye were prepared
in double distilled water and used for further studies by
diluting as required.
2.2. Adsorbent (fungus)
2.2.1. Maintenance and growth of the fungus
A previously isolated fungal culture of A. foetidus was
used for all our experiments. The mineral salts medium
(initial pH 7.5) used for the cultivation of the fungus contained the following constituents: 200 mg KH2PO4; 600 mg
K2HPO4; 500 mg (NH4)2SO4; 100 mg MgSO4; 10 mg
CaCl2; 5 mg FeCl3; 1.0 mg ZnSO4; 0.25 mg NaMoO4;
0.1 mg MnCl2 and 10 g of glucose dissolved in 1 l of deionized water (Sumathi and Phatak, 1999). For routine growth
of the fungal culture, flasks containing autoclaved mineral
salts medium were inoculated with fungal spores (1 ml of
spore suspension in 100 ml of medium) and placed on a
rotary platform incubator shaker at 30 C and 200 rpm.
2.3. Biosorption studies using RB5 dye
All biosorption experiments were conducted in Erlenmeyer flasks containing desired concentrations of RB5
dye dissolved in 100 ml of growth unsupportive medium
at pH 2.3 (Bidisha et al., 2006). Following 48 h of growth
in 100 ml of mineral salts medium, the fungal pellets
(0.2 g ± 0.05) were harvested, washed, introduced into the
above solution and incubated further at 30 C (unless
otherwise specified) and 200 rpm on a temperature controlled rotary platform shaker. Following the completion
of kinetic or equilibrium biosorption experiments the fungal biomass was collected by filtration and oven dried at
105 C for 24 h to calculate its dry weight. The residual
dye concentration (or the extent of uptake of dye or colour
by the fungal biomass) in the treated medium was analyzed
by UV–Visible absorption spectroscopy.
2.3.1. Efficiency of biosorption of dye onto various forms of
fungal biomass
Treatment of fungal biomass involved autoclaving or
incubation in 0.1 M HCl or 0.1 M NaOH or 1 M NaOH
for 1 h. Pre-treated fungal biomass was thoroughly washed
with distilled water, introduced into 100 ml of solution containing 100 mg L1 of RB5 dye and incubated for 3 h. The
supernatant was analyzed for residual concentration of the
dye. We also examined the efficiency of biosorption using
untreated fungal biomass using protocol similar to one
described above for pre-treated fungal biomass.
2.1. Adsorbate (dye)
Commercial textile dyes such as RB5 (reactive anionic
azo dye, mol. wt. 991), remazol brown and drimarene navy
dye were procured from a local company. Tartrazine (food
2.3.2. Pre-equilibrium kinetics of biosorption of dye onto
fungal biomass
Untreated or 0.1 M NaOH pre-treated fungal pellets
were introduced into 100 ml of solutions containing
53
50 mg L1 or 100 mg L1 of RB5 dye. Aliquots of the
supernatant were withdrawn at various time points (5–
60 min) and analyzed for residual dye concentration.
2.3.3. Equilibrium biosorption isotherm studies using
untreated and NaOH treated fungal biomass
Untreated or 0.1 M NaOH pre-treated fungal pellets
were introduced into 100 ml of solutions containing varying concentrations (10–400 mg L1) of RB5 dye. Following
24 h of incubation at 30 C or 50 C, the extent of uptake
of dye by the fungal biomass was noted. Equilibrium biosorption data were analyzed using Langmuir and Freundlich models to obtain the isotherm constants. Based on
the Langmuir and Freundlich constants theoretical isotherms were generated and compared with experimental
isotherm profiles.
2.3.4. Dye desorption studies
Set of eight flasks each containing 100 ml of RB5 dye
solution (100 mg L1) were incubated for 24 h with 0.1 M
NaOH pre-treated fungal biomass. Subsequently the dye
loaded fungal pellets were collected from each flask,
washed with distilled water and resuspended in 100 ml of
solutions maintained at pH values ranging from 3–10 (prepared by dosing appropriate concentration of sodium
hydroxide (NaOH) or acetic acid into distilled water).
The extent of desorption of dye from fungal biomass was
noted as a function of pH after 30 min of incubation time.
2.3.5. Reuse efficiency of fungal biomass for biosorption of
dye
Biosorption studies were conducted using 1 L of solution containing 100 mg of RB5 dye and 3.8 g of fungal biomass pre-treated with 0.1 M NaOH. The quantity of
biomass required was calculated based on the Langmuir
isotherm constants. The reuse efficiency was tested by subjecting the pre-treated fungal biomass to repeat biosorption
of dye at pH 2.3 followed by desorption using 0.1 M
NaOH. The contact time periods chosen for each biosorption cycle were 0.5 h, 1 h and 2 h.
2.4. Biosorption experiment using mixture of dyes
Biosorption study was done using 0.2 g ± 0.05 of 0.1 M
NaOH pre-treated fungal biomass and 100 ml of solution
(pH 2.3) containing 20 mg L1 each of RB5, tartrazine,
sunset yellow FCF, remazol brown and drimarene navy
dye. The extent of decolourization was noted after 1 h
and 24 h of incubation.
2.5. Biosorption experiment using composite industrial
effluent
Biosorption study was conducted using composite
industrial effluent generated from combining streams of
dye manufacturing, leather tanning and pharmaceutical
units of Clariant (India) Ltd. A portion (100 ml) of the
effluent collected from the equalization tank was acidified
to pH 3 and subjected to biosorption using 0.4 g ± 0.05
of 0.1 M NaOH pre-treated fungal biomass. The extent
of decolourization was noted after 1 h and 24 h of
incubation.
2.6. Analytical methods
Uptake of dye(s) by the fungal culture was monitored
using an Agilent UV–Visible absorption spectrophotometer (model 8434) equipped with photo diode array detector.
The kmax value of the RB5 dye was 599 nm and the reduction in the absorbance value at this wavelength was taken
as an indication of colour removal from the medium. The
effect of pH on the optical density value and absorption
spectral profile of the dye was negligible. For composite
industrial effluent and solution containing mixture of dyes,
reduction in the absorbance values at 361 nm and 472 nm
were used to calculate the extent of colour removal,
respectively.
3. Results and discussion
3.1. Biosorption studies using RB5 dye
3.1.1. Efficiency of biosorption of dye onto various forms of
fungal biomass
Various research groups have shown that the biosorption efficiency of biomass can be significantly enhanced
by pre-treatment methods such as autoclaving, drying
and exposure to chemicals such as formaldehyde, acid,
NaOH, NaHCO3 and CaCl2 (Aksu, 2005). Therefore biosorption capacities of A. foetidus biomass subjected to various types of pre-treatments were compared. It was noted
that autoclaving or treatment of fungal biomass with
0.1 M NaOH improved its biosorption efficiency (97% colour removal). In comparison, the extent of decolourization
by untreated and acid treated biomass was 67% and 70%,
respectively. Lower efficiency (69%) of biosorption of dye
by 1 M NaOH pre-treated fungus could be a consequence
of reduction in the number of binding sites for dye molecules due to biomass agglomeration. Zhou and Banks
(1993) reported that pre-treatment of Rhizopus arrhizus
with 2 M NaOH enhanced its capacity to biosorb humic
acid. This phenomenon may be attributed to the removal
of protein by the alkali and exposure of chitin/chitosan.
Gallagher et al. (1997) observed that methods such as autoclaving, calcium saturation and exposure to NaOH
enhanced the biosorption capacity of Rhizopus oryzae from
7% to 15%. Uzun (2006) reported that chitosan is an excellent adsorber of RB5 dye.
3.1.2. Pre-equilibrium kinetics of biosorption of dye onto
untreated and NaOH pre-treated fungal biomass
Plots of Ct (residual dye concentration in the aqueous
phase) versus time during the course of biosorption of
RB5 dye (100 mg L1) onto untreated and pre-treated
54
120
t
1 1
¼ þ t
qt h qe
Untreated fungal biomass
100
ð2Þ
ct (mg L-1)
Pre-treated fungal biomass
80
60
40
20
0
0
100
50
150
Time (min)
Fig. 1. Comparison of time course profiles showing reduction in the
concentration of RB5 dye due to biosorption using untreated and pretreated (0.1 M NaOH) fungal biomass. Ct represents remaining concentration of dye in the aqueous phase at any time, t. Biosorption conditions:
dye concentration = 100 mg L1, weight of dry biomass = 0.2 g ± 0.05
per 100 ml, pH 2.3, speed of shaker = 200 rpm, temperature = 30 C.
(0.1 M NaOH) fungal biomass at 30 C are compared in
Fig. 1. The following salient points may be noted: (a)
NaOH pre-treated fungal biomass exhibited higher rate
of uptake of dye as compared to that of untreated biomass.
Also biosorption capacity (qt) values of the NaOH pretreated fungal biomass were much higher at all time points
as compared to those obtained using untreated biomass
(plot not shown) and (b) extent of decolourization of the
medium by pre-treated fungal biomass was significantly
higher. Thus pre-treatment of fungal biomass with NaOH
enhanced its biosorption capacity.
3.1.2.1. Kinetic models for the pre-equilibrium biosorption
studies. Pre-equilibrium kinetic profiles were characterized
in order to determine the rate limiting steps involved in
the process of biosorption of RB5 dye onto fungal biomass.
Lagergren pseudo first order (Eq. (1)) and pseudo second
order (Eq. (2)) kinetic models were applied (Wang et al.,
2006; Allen et al., 2005; Aksu, 2005; Hamadi et al., 2001;
Ho and McKay, 1999; Lagergren, 1898).
lnðqe qt Þ ¼ lnðqe Þ k 01 t
ð1Þ
where,
k 01
pseudo first order biosorption rate constant,
qe
amount of dye biosorbed on the fungal biomass at
equilibrium (mg g1),
qt
amount of dye biosorbed on the fungal biomass at
any time t (mg g1)
where, h ¼ k 02 q2e is the specific biosorption rate
(mg g1 min1) and k 02 is the pseudo second order biosorption rate constant (g mg1 min1).
As per Eq. (1) the slope and intercept of a linear plot of
ln (qe qt) versus t yield the values of k 01 and ln (qe), respectively. Table 1 lists the k 01 values and predicted qe (qcal) and
experimental qe (qexp) values obtained for the biosorption
of 50 mg L1 and 100 mg L1 RB5 dye onto untreated
and 0.1 M NaOH pre-treated fungal biomass. It may be
noted that in spite of the high R2 values the predicted qe
values did not match with the experimental qe values
thereby suggesting that the biosorption process did not follow first order kinetics (Allen et al., 2005). Therefore we
attempted to fit the biosorption data into pseudo second
order kinetic model. Based on Eq. (2) the values k 02 and
qe can be determined from the intercept and slope, respectively, from a linear plot of qt and t. Table 1 lists the h valt
ues, k 02 values, the predicted and experimental qe values for
1
the biosorption of 50 mg L and 100 mg L1 of RB5 dye
onto untreated and 0.1 M NaOH pre-treated fungal biomass. R2 values were >0.98 and the predicted values of qe
nearly matched the experimental values thereby suggesting
the goodness of the plot. The values of h and qe increased
with increasing concentration of dye presumably due to the
enhanced mass transfer of dye molecules to the surface of
fungal biomass. This observation suggested that the
boundary layer resistance was not the rate limiting step
(Allen et al., 2005). On the other hand, biosorption was
probably controlled by a chemical process (Gulnaz et al.,
2006; Ho and McKay, 1999). Higher qe, h and k 02 values
for the biosorption of RB5 dye onto 0.1 M NaOH pre-treated fungal pellets may be related to alkali induced denaturation and removal of proteins from the cell wall with
concomitant enrichment of chitin/chitosan in the biomass
fraction. Eren and Acar (2006) reported that the biosorption of RB5 dye on powdered activated carbon and fly
ash followed pseudo second order kinetics. The second
order rate constants values obtained in our investigation
was found to be relatively greater than the values reported
using powdered activated carbon but lower in comparison
to fly ash. It may be noted that sodium hydroxide pre-treated fungal biomass used in our investigation exhibited
higher initial sorption rates as compared to those obtained
Table 1
Comparison of the kinetic constants for the pre-equilibrium biosorption of RB5 dye onto untreated and NaOH pre-treated fungal biomass
Initial concentration of dye, mg L1
(treatment given to biomass: a or b)
50 (a)
100 (a)
50 (b)
100 (b)
Pseudo first order values
Pseudo second order values
k 01 ,
min1
R2
qexp mg,
g1
qcal mg,
g1
k 02 g, mg1,
min1
R2
h mg, g1,
min1
qexp mg,
g1
qcal mg,
g1
0.04
0.03
0.06
0.06
0.97
0.94
0.96
0.95
23
30
27
62.5
16.8
16.6
17.6
19
0.004
0.003
0.005
0.005
0.98
0.99
0.99
0.99
2.4
3
5
20
23
30
27
62.5
24
30
29
60
(a) and (b) represent untreated and 0.1 M NaOH pre-treated fungal biomass, respectively.
55
by Eren and Acar (2006) using powdered activated carbon
and fly ash.
consistent with each other and suggested that higher temperature and pre-treatment using 0.1 M NaOH had a positive impact on the fungal biosorption of dye molecules.
The usefulness of equilibrium biosorption models for
estimating the saturation sorption capacity of fungal biomass was confirmed by comparing the non-linear experimental isotherms with the corresponding theoretical
profiles (generated from Langmuir and Freundlich constants shown in Table 2). Analyses of the experimental
and theoretical data set (Fig. 2a–d) revealed that Freundlich model was more appropriate for the prediction of the
isothermal profiles for the lower range of dye concentrations. This observation suggested that the sorption process
was heterogeneous in nature. However at higher dye concentrations, Langmuir as well as Freundlich models were
equally good for prediction purposes.
3.1.3. Equilibrium biosorption isotherm studies using
untreated and NaOH pre-treated fungal biomass
The equilibrium data obtained for biosorption of RB5
dye onto untreated and 0.1 M NaOH pre-treated fungal
pellets were analyzed using linearized Langmuir (1918)
and Freundlich (1906) isotherm (Eqs. (3) and (4),
respectively).
Ce
1
1
¼
þ Ce
qe bQ0 Q0
1
ln qe ¼ ln K f þ ln C e
n
where
Ce
Q0
b
Kf
n
ð3Þ
ð4Þ
concentration of dye in aqueous phase at equilibrium (mg L1),
maximum capacity of adsorbent for adsorbate
(mg g1),
Langmuir constant related to the measure of affinity of the adsorbate for adsorbent (L mg1),
biosorption capacity (mg g1),
biosorption intensity i.e., affinity of the adsorbate
for adsorbent.
3.1.4. Thermodynamic analyses of biosorption isotherm data
The values of thermodynamic parameters are relevant
for the practical application of biosorption process (Ozcan
et al., 2006). Isotherm data related to biosorption of RB5
dye onto untreated and pre-treated (0.1 M NaOH) fungal
biomass at 30 C and at 50 C were analyzed to obtain
the values of thermodynamic parameters. Change in free
energy (DG), enthalpy (DH) and entropy (DS) for the biosorption process were calculated using the Eqs. (5)–(7)
(Rajgopal et al., 2006) and computed values for untreated
and NaOH pre-treated fungal biomass are listed in Table 3.
The values of Langmuir and Freundlich constants are
presented in Table 2. The Langmuir constant Q0 and Freundlich constant Kf values increased with increasing temperature from 30 to 50 C for untreated as well as NaOH
pre-treated fungal biomass thereby implying that the biosorption process was endothermic in nature (Gupta et al.,
2005). The value of Langmuir constant b also increased
with temperature thereby suggesting that the dye molecules
exhibited higher affinity for fungal biomass at higher temperature than at lower temperature. An n value greater
than unity indicated that the biosorption of RB5 dye onto
the fungal biomass was favourable (Wang et al., 2006).
Similar trend was reported by Uzun (2006) who studied
the adsorption of RB5 dye onto chitosan. Comparison of
Langmuir and Freundlich isotherm constants (Table 2)
clearly indicated that NaOH pre-treated fungal biomass
was more effective as an adsorbent as compared to the
untreated biomass. Thus results obtained from pre-equilibrium kinetic and equilibrium biosorption experiments were
DG ¼ RT ln b
b2 DH 1
1
ln ¼
R
T2 T1
b1
ð5Þ
DG ¼ DH T DS
ð7Þ
ð6Þ
where,
DG
change in Gibbs free energy (J mol1)
R
universal gas constant (8.314 J K1 mol1)
T, T1 and T2 temperatures (Kelvin)
DH
change in enthalpy (J mol1)
b1 and b2 are Langmuir constants at temperatures T1 and
T2, respectively
DS
Change in entropy (J mol1 K1)
Negative values of DG indicated that the biosorption
process was favorable and spontaneous in nature. It may
be noted that with the increase in temperature from 30 to
Table 2
Langmuir and Freundlich isotherm constants for the biosorption of RB5 dye onto untreated and 0.1 M NaOH pre-treated fungal biomass at varying
temperatures
Treatment given to fungal biomass
Biosorption temperature (C)
Langmuir constants
1
Q0 (mg g )
None
30
50
65
76
0.1 M NaOH
30
50
92
106.4
Freundlich constants
3
R2
Kf (mg g1)
125
169
0.99
0.99
17
30
3.9
5.6
0.91
0.81
195
262.5
0.99
0.99
59
76
13
16.7
0.82
0.94
b · 10
1
(L mg )
n
R2
80
60
b
100
80
40
Experimental
Langmuir
Freundlich
20
qe (mg g-1)
a
qe (mg g-1)
56
0
60
40
Experimental
Langmuir
Freundlich
20
0
0
50
0
100 150 200 250 300 350
50
-1
qe (mg g-1)
80
60
40
Experimental
Langmuir
Freundlich
20
d
140
120
100
80
60
40
20
50
100
150
200
250
300
350
250
Experimental
Langmuir
Freundlich
0
0
0
200
Ce (mg L )
qe (mg g-1)
100
150
-1
Ce (mg L )
c
100
0
300
50
100
150
200
250
300
Ce (mg L-1)
-1
Ce (mg L )
Fig. 2. Comparison of experimental and theoretical (based on Langmuir and Freundlich constants shown in Table 2) isotherms for biosorption of RB5
dye onto (a) untreated fungal biomass at 30 C (b) untreated fungal biomass at 50 C (c) 0.1 M NaOH pre-treated fungal biomass at 30 C and (d) 0.1 M
NaOH pre-treated fungal biomass at 50 C. The solid line in all the graphs represents the average curve drawn for experimental data set. Biosorption
conditions: pH 2.3, weight of dry biomass = 0.2 g ± 0.05 per 100 ml, agitation at 200 rpm and contact time = 24 h.
Table 3
Thermodynamic parameters for the biosorption of RB5 dye onto untreated and 0.1 M NaOH pre-treated fungal biomass as a function of temperature
Biosorption temperature (C)
Untreated fungal biomass
1
30
50
NaOH pre-treated fungal biomass
1
DG (kJ mol )
DS (J mol
12.2
13.8
80.4
80.4
K)
50 C, the value of DG decreased from 12.2 kJ mol1 to
13.8 kJ mol1 for untreated fungal biomass and from
13.3 kJ mol1 to 15 kJ mol1 for 0.1 M NaOH pre-treated fungal biomass. Thus biosorption of RB5 dye onto fungal biomass was enhanced at higher temperature. The
positive value of enthalpy change (DH) confirmed the
endothermic nature of the biosorption process. Positive
values of DS suggested good affinity of the dye towards
the adsorbent (both untreated and treated fungal biomass)
and that the change, either physical or chemical occurred
spontaneously (Gupta et al., 2005). Thermodynamic analyses (Table 3) clearly demonstrated that biosorption of
RB5 dye onto NaOH pre-treated fungal biomass was more
favourable in comparison to untreated biomass.
3.1.5. Dye desorption studies
Desorption studies as a function of pH were conducted
to explore the possibility of recovery of adsorbent and
adsorbate, which in turn is expected to make the biosorption process economical (Arami et al., 2005). The percentage desorption of RB5 dye increased with raise in pH from
3–10 (data not shown). It was further noted that the percentage desorption of RB5 from the dye loaded fungal biomass was 90% and 32% using 0.1 M and 1 M NaOH,
1
DH (kJ mol )
DG (kJ mol1)
DS (J mol1 K)
DH (kJ mol1)
12.2
13.3
15
83.7
83.7
12.1
respectively, thereby suggesting that higher strength of
NaOH was disadvantageous. Desorption at higher pH
may be explained on the basis of electrostatic repulsion
between the negatively charged sites (which increase at
higher pH values) on the fungal biomass and the anionic
dye molecules. The same trend was reported by Arami
et al. (2005) and Kadirvelu et al. (2005) during desorption
of dyes from orange peel and activated carbon,
respectively.
3.1.6. Reuse efficiency of NaOH pre-treated fungal biomass
for the biosorption of RB5 dye
The total amount of 0.1 M NaOH pre-treated fungal
biomass required for achieving 95% decolourization of
1 L of medium containing 100 mg of RB5 dye was calculated to be 3.8 g by substituting the values of b (0.195)
and Q0 (92 mg g1) into Eq. (9). This equation was derived
by equating the mass balance Eq. (8) and Langmuir Eq.
(3).
qe M ¼ VC 0 VC e
M¼
V ð1 þ bC e ÞðC 0 C e Þ
Q0 bC e
ð8Þ
ð9Þ
57
Table 4
Reuse efficiency of 0.1 M NaOH pre-treated fungal biomass for the
biosorption of RB5 dye
Cycle no.
1
2
3
4
5
% Biosorption as a function of contact time
0.5 h
1h
2h
99
98
94
91
90
>99
>99
98
97
97
>99
>99
>99
>99
98
Experimental conditions: dye concentration = 100 mg L1; temperature = 30 C; pH = 3; dry weight of 0.1 M NaOH pre-treated biomass = 3.8 g L1; aqueous phase volume = 1 L; contact time periods as
indicated in the table.
where,
M
total mass of adsorbent (fungal biomass), g
V
total volume of dye solution in the reactor, L
initial concentration of dye in the liquid phase,
C0
mg L1
Results related to the reuse efficiency of pre-treated fungal biomass for the biosorption of RB5 dye (100 mg L1)
are presented in Table 4. It may be noted that percentage
colour removal decreased slightly with increase in the number of cycles. However near complete biosorption could be
achieved by increasing the contact time for later cycles. The
percentage desorption of dye ranged from 80–90% for each
cycle. Thus pre-treated fungal biomass could be reused for
number of cycles, which in turn is expected to reduce the
cost of biosorption process.
3.2. Biosorption experiments using raw industrial effluent and
solution containing mixture of dyes
The spectral profiles of raw industrial effluent and solution containing mixture of dyes were recorded after 1 h and
24 h incubation with 0.1 M NaOH pre-treated fungal biomass (profiles not shown). The extent of decolourization
of industrial effluent was 90% and 95% following 1h and
24 h of incubation, respectively. On the other hand, the efficiency of decolourization of solution containing mixture of
dyes was >98% following 1 h of incubation. Thus NaOH
pre-treated fungal biomass could be employed for decolourizing composite industrial effluent and solution containing
mixture of dyes.
4. Conclusions
The results obtained from this investigation demonstrate
that 0.1 M NaOH pre-treated A. foetidus biomass is very
efficient in decolourizing solutions containing RB5 dye or
mixture of dyes and composite industrial effluent. The biosorption capacity is strongly dependent on the temperature
and increases significantly with increase in temperature
from 30 to 50 C. Desorption studies shows that the recov-
ery of dye and regeneration of fungal biomass is possible at
alkaline pH (10) using sodium hydroxide as the leaching
agent. Sodium hydroxide has dual role to perform: (a)
helps in the recovery and regeneration of dye and fungal
biomass, respectively, and (b) activates the surface of fungal biomass for efficient biosorption. Thus sodium hydroxide pre-treated fungal biomass is a promising biosorbent
for treating textile mill wastewater.
Acknowledgements
The authors would like to acknowledge Department of
Biotechnology (DBT), Government of India for providing
financial support to implement this project. The authors
also thank Clariant (India) Ltd., Mumbai for allowing
them to collect the composite industrial effluent.
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