Artigo de Adsorção

1. Accepted Manuscript
Understanding the adsorption behaviour of acid yellow 99 on
Aspergillus niger biomass
Animesh Naskar, Rajib Majumder
PII: S0167-7322(17)31265-5
DOI: doi: 10.1016/j.molliq.2017.05.155
Reference: MOLLIQ 7665
To appear in: Journal of Molecular Liquids
Received date: 23 March 2017
Revised date: 3 May 2017
Accepted date: 15 May 2017
Please cite this article as: Animesh Naskar, Rajib Majumder , Understanding the
adsorption behaviour of acid yellow 99 on Aspergillus niger biomass, Journal of
Molecular Liquids (2017), doi: 10.1016/j.molliq.2017.05.155
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2. ACCEPTED
MANUSCRIPT
1
Understanding the adsorption behaviour of Acid yellow 99 on
Aspergillus niger biomass
Animesh Naskar1
*, Rajib Majumder2
1
Department of Food Technology and Biochemical Engineering, Jadavpur University,
Kolkata – 700 032, India.
2
Structural Biology and Bioinformatics Division, CSIR-IICB, Kolkata – 700 032, India.
*Address correspondence to: Animesh Naskar
Email:animesh.ftbe@gmail.com
Department of Food Technology and Biochemical
Engineering, Jadavpur University, Kolkata – 700 032,
India
Tel: +91 9051527550
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Abstract
The adsorption behaviour of the textile dye Acid yellow 99 onto Aspergillus niger biomass in
aqueous medium has been investigated to explore the biomass efficacy in combating
pollution and to understand the mechanisms of the phenomenon. The maximum adsorption
capacity is found to be 544.30 mg g-1
of the biomass. The adsorption process depends on pH
of the solution, temperature and biomass concentration also evident from response surface
methodology (RSM) analysis with optimum being 3.0, 30 °C and 2 g L-1
, respectively.
Adsorption process is very fast initially and attains equilibrium within 245 min following
pseudo-second order rate model. The isotherm data can best be explained by Redlich-
Peterson model suggesting a possible physical and chemical adsorption of the dye molecules
on the surface of the biomass. FTIR results suggest that amino and carboxyl groups play vital
roles in binding through electrostatic and complexation reaction. SEM analysis shows that on
dye adsorption biomass surface becomes more irregular and rough compared to the regular
and smooth surface of pristine biomass. XRD study indicates amorphous characteristics of
the biomass even after incorporation of dye.
Keywords: Adsorption; Acid dye; Fungal biomass; Binding mechanism; RSM.
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1. Introduction
Textile, paper, paint, printing and cosmetics industries discharge a huge amount of
wastewater containing synthetic dyes [1]. Most of the dyes are toxic, carcinogenic,
teratogenic and also causes allergic dermatitis and mutations. Besides, they impart colour to
wastewater streams restricting sunlight penetration ultimately resulting in reduced
photosynthesis of aquatic plants thereby endangering aquatic life [2, 3]. It is reported that
over one million tons of dyes are produced annually all over the world [4]. Because of
different types of dyestuffs used in dyeing industries, wastewater characteristics vary with
respect to salt, colour and chemical oxygen demand [5]. Most of the dyes are of synthetic
origin having complex aromatic structure and non biodegradable. They are resistant to
biodegradation, oxidation as well as fading by chemicals [6]. Effluent of these industries
must be treated to reduce the dye concentration to permissible limit as determined by the
environmental protection agency of the concerned country before discharging into water-
bodies [7]. Various conventional processes for the removal of dyes from aqueous solution
comprise chemical coagulation, oxidation or ozonation, reverse osmosis, flocculation,
membrane separation etc. which are either inadequate or expensive besides generating toxic
sludge difficult to dispose of [6]. Hence, there is a continuous surge for developing new
technology in the field of environmental pollution. Recently most of the researches are
focused on adsorption technology to remove dye from wastewater. Removal of dye using
activated charcoal as adsorbent is very effective but finds restricted application in many
countries due to its high cost [8]. Therefore, considerable research activity is going on for
low-cost adsorbent mainly of biological origin like peat, fly ash, orange peel, banana pith,
rice husk, activated sludge, microbial biomass etc. for the removal of pollutants from
wastewater. In this context, Fungal cells used as biosorbent have been proved their worth
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offering large surface area and a high percentage of active functional groups, which become
to superior entity for dye binding [9].
Acid yellow 99 (AY 99), an anionic azo dye is extensively used in industries to colour
different fibers made of protein, silk, nylon etc. Only a few studies have been reported
dealing with the removal of AY 99 by adsorption technology using
polyacrylonitrile/activated carbon, clay material and coir pith [10-12]. The efficiency of a
biosorbent depends on several factors such as availability, cost effectiveness and
microenvironment of the adsorbate. Therefore, there is continuously a need to explore for
new effective biosorbent. Further, it is known that even with low degree of understanding of
the mechanism, biosorption technology can be applied, but better understanding will
definitely improve the efficiency of the adsorbent. In recent times, response surface
methodology (RSM) based on polynomial equation has gained huge interest for analyzing the
effects of different independent variables [7, 13].
Present manuscript describes the adsorption behaviour of AY 99 on Aspergillus niger
biomass (ANB) with special reference to RSM approach and binding mechanism on the basis
of experimental results obtained from Fourier transform infrared spectrophotometer (FTIR),
X-ray diffractometer (XRD), Scanning electron microscope (SEM) and elemental analyzer.
Since the wastewaters of dyeing industries usually contain sodium chloride as well as other
anions, effect of these substances on the adsorption process has also been investigated.
2. Experimental
2.1 Materials
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Acid yellow 99 (C.I. 13900) used in this study were obtained from Sigma Aldrich
(U.S). Microbiological media were supplied from Hi-media, India. All other chemicals and
biochemicals were purchased from E-Merck, Germany.
2.2. Methods
2.2.1. Dye solution
Stock solution (1000 mg L–1
) of dyes have been prepared in double distilled water and
diluted to desired concentration. In the solution, concentration of AY 99 is determined from
the calibration curve obtained by measuring the absorbance of different concentration of dye
solution at 446 nm using UV-visible spectrophotometer (Perkin-Elmer, Singapore).
2.2.2. Preparation of biosorbent
Aspergillus niger (MTCC 1344) was obtained from the Microbial Type Culture
Centre, Institute of Microbial Technology, Chandigarh, India. Biomass of Aspergillus niger
was prepared by growing the organism in potato dextrose broth (pH 5.0) at 27 °C for 72 h
under shaking condition (120 rpm). At the end of incubation period, biomass was separated
from the fermented broth by filtration, dried by lyophilization and kept in desiccator at room
temperature after grinding to fine powder until use.
2.2.3. Batch biosorption experiment
Biosorption experiments were carried out with 0.1 g of Aspergillus niger biomass
(ANB) and 50 mL of AY 99 (100 mg L–1
) taken in 250 mL Erlenmeyer flasks. This was
incubated at 30 °C for 24 h with shaking (120 rpm) until stated otherwise. At the end of
incubation, ANB was separated by centrifugation at 5500 rpm for 10 min and residual
concentration of AY 99 in the supernatant was determined spectrophotometrically as
mentioned above.
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The amount of dye adsorbed by the biomass was calculated from the following mass balance
equation (Eq. 1).
W
VCC
q e
e
1000
)( 0 

[1]
where, C0 and Ce is the initial and dye concentration after adsorption, respectively in mg L–1
.
V is the volume (mL) of the dye solution and W is the amount (g) of biosorbent.
Dye adsorption by ANB with a varying hydrogen ion concentration was examined
under a pH range of 2.0 to 7.5. Biomass was conditioned in phosphate buffer of desired pH
for 2 h with shaking (120 rpm), collected by centrifugation, washed and used for adsorption
experiments. Influence of temperature on dye adsorption process was studied over a
temperature range 20 – 40 °C at optimum pH 3.0. Experiments regarding equilibrium
isotherm were accomplished using AY 99 concentration varying from 10 – 2000 mg L–1
. In
addition, the sorption of AY 99 on ANB with respect to contact time was studied at regular
intervals of time upto 24 h using dye concentration 100 mg L–1
. Each data point was collected
from individual flask and therefore no correction was necessary for withdrawal of sampling
volume.
The effect of NaCl on dye adsorption by the biomass was studied using salt
concentration 0.05 to 0.5 M. The influence of different anions like SO4
–2
and HPO4
–2
on the
adsorption process was studied using Na2SO4 and Na2HPO4 separately at a concentration
varying from 0.05 to 0.2 M.
2.2.4. Batch biosorption experiments using Response surface methodology based on Box-
Behnken design
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The Box-Behnken design for three independent variables (pH [A], biomass dose [B]
and contact time [C]), each at three levels (Supplementary Table S1), was taken into account
to develop a second-order polynomial model for determining optimum levels. The predicted
responses with respect to the interactions between three process variables was obtained by the
following second-order polynomial equation (Eq. 2) [13]:
  
2
iiijiijiio xxxxY  [Eq. 2]
where, Y is the predicted response, βo, βi, βii, βij are the offset term, linear effect, squared
effect and interaction effect, respectively. xi is the ith
independent variable. xixj is the
interaction term and xi
2
represents the quadratic terms followed by the quadratic model
verification.
The RSM providing the mathematical model was established by conducting the
experiments on given optimal conditions. In each case, the differences in AY 99
concentration before and after biosorption are used to find out the efficiency of dye removal
by the biomass as equation 3.
100(%)Re
0
0



C
CC
moval e
[Eq. 3]
2.2.5. Characterization of ANB
The pristine and dye adsorbed ANB consist of carbon, hydrogen, nitrogen and sulphur
were determined by Elemental analyser (2400 series II, CHNS Analyzer, Perkin-Elmer). The
data relating to FTIR spectra of both pristine and dye laden ANB were recorded by Shimadzu
IR- Prestige-21 (Japan) FTIR spectrophotometer. The pressed pellets were prepared by
grinding powder sample with IR grade KBr in an agate mortar. The spectral data were
recorded in the range of 4000 – 400 cm–1
with a resolution of 4 cm–1
using KBr disc
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technique. Crystalline behaviour of the pristine and dye adsorbed ANB were determined by
X-ray difractometer (RIGAKU, Model: ULTIMA−III, Japan) attached with Cu Kα radiation
(λ = 0.15406 nm). This was operated at 40 kV with a scanning speed of 5° min–1
. Scanning
electron micrographs of pristine and dye adsorbed ANB were recorded on SEM (JEOL,
Model: JSM 6360, UK). Samples were coated with platinum ultrathin film.
2.2.6. Desorption study
Biomass collected after equilibrium adsorption experiment with 2000 mg L–1
of AY99
was transferred into 50 mL of eluant (0.01 M NaOH) in a 250 mL Erlenmeyer flask and
incubated with shaking (120 rpm) for 2 h. At the end of incubation, biomass was separated by
centrifugation and dye concentration in the supernatant was determined
spectrophotometrically to understand the desorption efficiency of eluant.
3. Results and discussion
3.1. Biosorption mechanism
3.1.1. CHNS analysis
The resultant data (Table S2, Supplementary data) demonstrate that sulphur content of
dye laden ANB increases to 1.7 % from that of 0.42 % of pristine biomass indicating the
adsorption of AY 99 on the biomass.
3.1.2. FT-IR analysis
To evaluate the surface functional groups of concerned biomass involved in effective
dye sorption from aqueous solution, FTIR spectroscopic study was done. The recorded
spectra of pristine and dye laden ANB has been depicted in Fig. 1. The pristine data related to
the broad and strong band (3200 – 3600 cm−1
) being centred on 3390 cm–1
is mainly due to
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the combining stretching vibration of hydroxyl and amine groups. However, the strong broad
band at the wavenumber region of 3300–3500 cm−1
is characteristic of the –NH stretching
vibration [14]. The peaks are significantly shifted in the bands at 3427.30 cm−1
after dye
sorption. Moreover, the signals at 1458.09 cm−1
and 856.34 cm−1
corresponding to N–H
bending of amine group [15], which appreciably shifts towards lower wavenumber region at
1452.31 cm–1
and 817.76 cm−1
, respectively demonstrating the contribution of this group for
rapid dye sorption. Likewise, the alteration of signal location from 1242 cm−1
to 1218.93
cm−1
with concomitant appearance of new peak (1298.01 cm−1
) was found on dye sorption,
which suggests the strong interaction between anionic dye molecules and amine groups. The
sharp peaks of amide I (1647.11 cm–1
), amide II (1544.89 cm–1
) and amide III (1311.51 cm–1
)
[16] reveal no possible involvement of these groups, as no changes in peak positions for dye
sorption are observed. The characteristic symmetric stretching peak of carboxyl groups
makes a signal at around 1409.88 cm−1
[17]. The obvious changes in peak position to higher
intensity (1415.66 cm–1
) and appearance of a new band (1371.30 cm−1
) may result due to the
involvement of such groups. The characteristics peak observed at 1149.50 cm−1
corresponds
to the −SO3 stretching vibration [18]. The sharp and strong bands at 1082 cm−1
and 1035.71
cm−1
may be responsible for stretching vibrations of P−O linkage and PO4 group, respectively
[19]. No significant changes of these peak positions occur on dye sorption. Interestingly, the
new peaks occurring at 1249.79 cm−1
and 559.32 cm−1
after the biosorption process may be
ascribed to characteristic SO3 group and aromatic ring in dye structure. This may serve as
another evidence for incorporation of dye molecules to the biomass surface [20]. New peaks
arising at 642.35 cm−1
and 530.39 cm−1
correspond to aromatic structure, which further prove
the sorption of AY 99 onto the biomass. Moreover, a weak band localized at 1560.31 cm–1
relating to azo bond (–N = N–) provides direct testimony to the incorporation of AY 99
molecules onto biomass surface [21]. Thus, the information obtained from FTIR study
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suggests that the carboxyl and amine groups play a crucial role for effective dye sorption by
ANB through hydrogen bonding as well as electrostatic interactions [22].
3.1.3. XRD study
XRD study was undertaken to evaluate the constituent phase types of biomass and
possible reaction mechanisms regarding dye sorption process. As illustrated in Fig. 2g,
pristine ANB shows a small hallo at 2θ value of 10.05° and a broad peak ranging from 15° –
29°. No perfect crystal peak is observed, indicating the amorphous characteristics of ANB
[23]. Hence, more sorption sites due to presence of various biomolecules were available for
dye binding. No additional peaks corresponding to the crystalline phases is obtained even
after incorporation of AY 99 on ANB (Fig. 2h) and the recorded data revealed the increase in
relative intensity, suggesting that the dye molecules could easily be permeated on amorphous
biomass surface. Thus the findings may account for electrostatic interactions, hydrogen
bonding etc. involving AY 99 sorption onto ANB [24].
3.1.4. SEM observation
In order to gain insight on the surface morphology as well as the fundamental physical
properties of the biosorbent surface, scanning electron microscopic study was performed.
Prior to dye adsorption, the micrographs of fungal biomass exhibits regular and smoother
surface structure (Fig. 2a, b and c). Conspicuous morphological alteration is observed on
treating with dye, which becomes more irregular and rough (Fig. 2d, e and f), thus implying
that the significant adsorption of dye onto ANB changed its surface topography.
3.2. Effect of pH and temperature
The pH of the solution affects the chemical nature of the surface of the adsorbent [17].
Hence, its effect on the adsorption of anionic dye AY 99 by ANB was investigated. However,
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contradictory reports are available regarding the influence of solution pH on the adsorption of
a dye on biomass. Annadurai et al. [25] reported maximum adsorption of Rhodamine B on
orange peel at pH 7.0. On the other hand, Namasivayam et al. [26] concluded no significant
effect of solution pH using banana pith as adsorbent. Only minor influence of pH on the
adsorption of Congo red by neem leaf powder was reported by Bhattacharyye and Sharma
[27]. In the present investigation, a decrease in adsorption of AY 99 on ANB is noted with
increase in pH upto 7.0 (Fig. S1(a), Supplementary data). AY 99 being an anionic dye, its
chromophore structure is electrically negative. Thus, biosorption of dye anions will be
enhanced or decreased depending on the surface charge of the biomass. With increase in pH
of the solution, surface charge of the biomass becomes more negative due to decreased
availability of protons and negative surface charge will repulse the negatively charged AY 99
for binding and this is reflected in less binding at higher pH. The maximum sorption of AY
99 on ANB is noted at pH 3.0 and then decreases with increase in pH values, similar to that
reported earlier by Khan et al. and Ozcan and Ozcan [11, 28] for coir pith and DEDMA-
sepiolite, respectively.
Effect of temperature on dye sorption has been depicted in Fig. S1b (Supplementary
data).
3.3. Effect of biomass dosage
The results and discussions regarding the effect of biomass dosage on dye sorption
have been described in Fig. S2 (Supplementary data).
3.4. Sorption kinetics
The sorption kinetics provides important characteristics for describing the dye uptake
rate in accordance with time. As revealed in Fig. 3a, the instantaneous dye adsorption onto
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ANB is noted within first 90 min, which may be attributed to fast external diffusion and
surface adsorption due to abundant active sites on the exterior surface of biomass. Subsequent
milder and gradual ascend (90 – 245 min) is found to be observed because of progressively
decreasing active sites resulting in lesser interaction with dye molecules, followed by
saturation plateau from 245 to 300 min [22].
To obtain an idea of the feasible mechanism and rate-controlling step in the present
dye removal process, the experimental data are correlated to pseudo-first order (Eq. 4) and
pseudo-second order kinetic model (Eq. 5) with respective kinetics equations [29] mentioned
as follows:
tkqqq ete 1ln)ln(  [4]
eet q
t
qkq
t
 2
2
1
[5]
where, qe and qt are the amount of the dye adsorbed per gram of biomass at equilibrium and
time t (mg g−1
), respectively. k1 (min−1
) is the pseudo-first order rate constant and k2 (min g
mg−1
) represents the pseudo-second order rate constant.
The results acquired from the fitted kinetic models are summarized in Table 1. It is
found that pseudo-second order could better explain the adsorption process through the
analysis of correlation coefficients (R2
= 0.992) along with more consistent value of the
experimental equilibrium adsorption capacity (qe,exp) and the calculated ones (qe,cal).
Meanwhile, the prevailing mechanism for AY 99 sorption onto ANB suggests chemisorption,
since valence forces via sharing or exchange of electrons between sorbate and sorbent are
active in present dye removal process [30].
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To probe the steps involved during dye adsorption, intraparticle diffusion is also used
to analyze the kinetic data according to the model equation proposed by Weber and Morris
[15] can be expressed as follows (Eq. 6):
Ctkq pt  2/1
[6]
where, kp is the rate constant (mg g−1
min−1/2
) and C is the intercept (mg g−1
).
Results (Fig. S6, Supplementary data) exhibit that the line is non-linear and do not pass
through the origin, suggesting the intraparticle diffusion is not the only factor on the rate
limiting step deeming the entire dye removal process.
3.5. Isotherm study
Fig. 3b exhibits the dye sorption capacity of the concerned biomass with respect to
increasing the concentration of dye in the system. However, the uptake capacity for AY 99
increases with the increase in initial dye concentration and maximum uptake capacity is
found to be 544.30 mg g−1
. The increased initial concentrations of dye facilitate the binding
of more dye molecules onto the biomass, leading to higher sorption yield [31].
The three popular isotherm models namely Langmuir (Eq. 7) and Freundlich (Eq. 8)
and Redlich - Peterson (Eq. 9) isotherms are used to describe the sorption behaviour of the
system [29]. The Langmuir model is based on the assumption of surface homogeneity with
monolayer coverage, while Freundlich isotherm model determines the adsorption intensity of
biosorbent towards adsorbate on heterogeneous surface:
eL
eL
CK
CKq
eq  1
max
[7]
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n
eFe CKq
/1
 [8]
The Redlich–Peterson is a mixed one and can be expressed as follows:
g
e
e
BC
AC
eq 
 1
[9]
where, qe is the equilibrium adsorption capacity (mg g−1
), qmax (mg g−1
) and KL (L mg−1
) are
the Langmuir adsorption constants related to maximum adsorption capacity and adsorption
rate, respectively. Ce is the concentration of dye at equilibrium (mg L−1
). KF (L mg−1
) and n
are the Freundlich adsorption constants, which represent the sorption capacity and intensity
of the dye sorption, respectively. A (L g−1
) and B (L mg−1
)g
are the Redlich–Peterson
constants and ‘g’ is the Redlich–Peterson isotherm exponent. The exponent value occurs on
the range of 0 to 1. When g = 1, this model assumes the form of Langmuir model.
The corresponding parameters evaluated from three non linear fitted models are
presented in Table 2. The experimental data were found to fit better to the Redlich - Peterson
model considering highest correlation coefficient (R2
= 0.998) and lowest chi square (χ2
=
73.01) value, suggesting a possible physical and chemical adsorption of AY99 molecules on
the surface of ANB [11].
3.6. Effect of NaCl concentration
Sodium chloride is used in the dye processing industries and often present in the
effluents. For this reason, its influence on sorption of AY 99 using ANB has been
investigated. Results show (Fig. S3, Supplementary data) that dye adsorption decreases from
44.10 mg g−1
to 41.15 mg g−1
with increasing concentration of NaCl upto 0.2 M. Further
increase in salt concentration, the sorption of dye molecules decreases to some extent,
probably due to saturation of Cl–
concentration. Negatively charged chloride ions possibly
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compete with the dye anions for inhabiting to the positively charged biomass surface and
consequently inhibit the dye adsorption. The findings suggest that electrostatic interaction is
one of the main driving forces for the binding of AY 99 to ANB. Similar observations were
also reported earlier [32].
3.7. Effect of coexisting anions
The anionic species commonly found in wastewater of dyeing industries may compete
with anionic dye for occupying the sites of biomass surface and their resultant effects on
target dye removal can eventually be deteriorated. In this regard, the influence of some
commonly encountered anionic species e.g., SO4
2−
and PO4
3−
in the dye solution was
investigated. As shown in Fig. 4, the occurrence of coexisting anions at various
concentrations dramatically influence on the adsorption efficiency of ANB towards AY 99. It
is obvious that the adsorption capacities of ANB for AY99 were decreased by 10.9 % and
26.01 % in the presence of SO4
2−
and PO4
3−
at lower concentration (0.05 M), respectively.
Such anions could utilize the adsorption sites through direct site competition with AY99,
indicating the inhibition of dye adsorption [33]. This adverse effect of ionic strength on dye
biosorption process may be attributed to electrostatic interactions, which strongly supported
the results of FTIR study. More interestingly, as increased ionic strength from 0.05 to 0.2 M
is shown to have a positive influence on dye sorption. The findings may be related increase
ionic strength of the solution and decreasing the degree of dye dissociation in their ionic
forms, reducing the solubility of dye molecules. As a result, non-ionic dye molecules can be
easily extracted from aqueous phase to solid phase [34].
3.8. Thermodynamic study
The detail explanation of results obtained from thermodynamic study regarding the
present dye sorption process is described in Content S1 (Supplementary data).
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3.9. Response surface methodology (RSM)
To unravel the combined effect of the above mentioned factors, experiments were
performed in different combination of parameters using experimental design. Table 3 depicts
the results of three factors in Box Behnken design. The predicted responses are substantially
correlated to our actual response in terms of percentage removal of AY 99. The detail
explanation is described in Content S2 (Supplementary data).
3.10. Regeneration study
The desorption study is important to explicate the sorption mechanism as well as
highlighting the commercial feasibility for using such biosorbent. In this context, the dye
desorption processes involving acidic and alkaline treatments are proposed. The findings
report that 98 % of desorption efficiency is achieved using 0.1 M NaOH over the tested
eluants (Fig. not shown). However, the data described above suggests an opposite trend to the
biosorption process, thereby supporting the electrostatic interaction playing a key role in this
dye sorption experiment. Furthermore, the adsorption – desorption cycles were conducted
using 0.1 M NaOH to evaluate the stability of ANB and to examine a possible subsequent
application of such biosorbent. As illustrated in Fig. 6, a very high removal efficiency of AY
99 upto 44.01 mg g−1
(~89 %) is obtained in the first run and almost 98 % of loaded dye is
desorbed. After the first cycle, a significant decrease of removal efficiency is noted.
Afterwards, the removal efficiency does not change appreciably and is still very consistent
even next three cycles. The declined nature of adsorption seems to be postulated for structural
detoriation due to prolonged alkaline treatment and subsequent mass loss of biosorbent.
Although, the sorption capacity of ANB is found to be 31.75 mg g−1
even after four
successive cycles. Therefore, it may be concluded that biosorbent can be renewed easily with
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NaOH solution and used repeatedly as an efficient sorbent for real dyeing wastewater
treatment.
3.11. Removal of dye from simulated wastewater
Simulated wastewater was prepared by dissolving the ingredients in tap water. The
composition of simulated solution is summarized in Content S3 (Supplementary data). The
pH of the feed solution was adjusted to 3.0 and other conditions remained same as described
earlier. Results exhibit that dye removal efficiency of ANB from simulated effluent is almost
similar to dye bearing aqueous solution (Fig. not shown). The existence of auxiliary
contaminants has no significant impact on dye removal process.
4. Conclusions
In summary, ANB serves as an eco-friendly and low-cost adsorbent for efficient
removal of AY 99 from aqueous solutions. The FTIR studies suggest that the carboxyl and
amine groups are involved in intermolecular interactions for rapid and efficient dye
adsorption. RSM analysis shows that the adsorption efficiency of ANB towards AY 99 is
strongly pH dependent and the maximum adsorption (44.35 mg g−1
) occurs at pH 3.0. The
experimental data can be well described by the pseudo-second order kinetic as well as
Redlich-Peterson isotherm model. Presence of NaCl, SO4
2−
and PO4
3−
considerably influence
the sorption capacity of AY 99. Thermodynamic study suggests that the present adsorption
process is spontaneous, endothermic and entropy driven. Regeneration experiment reveals
that AY 99 sorbed onto ANB can be successfully recovered in 0.1 M NaOH solution and four
consecutive adsorption-desorption cycles can be carried out. The experimental results speak
in favour of the usefulness of such biosorbent in removal of AY 99 from textile wastewaters
and their safe disposal.
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Acknowledgement
We acknowledge to University Grants Commissions [Rajiv Gandhi National
Fellowship] for financial support to carry out this research work. Author gratefully
acknowledge to Dr. Akhil R. Das, Indian Association for the Cultivation of Science, India for
his kind support during the research work.
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Figure legends
Fig. 1: FT-IR spectra of pristine and dye loaded biomass.
Fig. 2: SEM micrographs of ANB: (a, b, c) before dye adsorption and (d, e, f) after dye
adsorption. XRD pattern of (g) pristine and (h) dye treated biomass.
Fig. 3: Effect of (a) contact time (the inset photographs are the colour changes of the dye
solutions) and (b) initial dye concentration with fitted isotherm models of the AY 99
biosorption onto ANB: ±SD shown by the error bar.
Fig. 4: Effect of different anions on the sorption of AY 99 by ANB: ±SD shown by the error
bar.
Fig. 5: 3D surface plot of (a) pH vs. biomass dose, (b) pH vs. time, (c) biomass dose vs. time.
(d) Represent actual vs. predicted plot.
Fig. 6: Adsorption-desorption cycles of ANB for AY 99 sorption: ±SD shown by the error
bar.
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Table 1:
Kinetic parameters obtained from respective kinetic models for adsorption of AY 99 by ANB
First-order kinetic Second-order kinetic Intraparticle diffusion
____________________ ____________________ ____________________
k1 qe R2
k2 qe R2
kp C R2
0.0133 32.96 0.927 7.9×10-4
45.39 0.992 2.654 1.875 0.944
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Table 2:
Relevant constants of adsorption isotherms for the adsorption of AY 99 onto ANB
Langmuir Freundlich Redlich-Peterson
_____________________ _____________________ ______________________________
qmax KL R2
χ2
KF n R2
χ2 A B g R2
χ2
535.60 0.0031 0.990 283.94 14.70 1.76 0.994 149.08 9.357 0.472 0.78 0.998 73.01
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Table 3: Box-Behnken Design matrix for four factors along with observed and predicted
responses for AY 99 biosorption.
Run
Factor 1 Factor 2 Factor 3 Removal [%]
A:pH
B: biomass
dose
C: contact
time Actual Predicted
1 3 1 30 47.50 46.93
2 3 1.5 45 53.19 52.62
3 3 1.5 15 42.96 42.96
4 4 1 15 65.69 65.71
5 3 1 30 65.68 65.71
6 2 0.5 30 55.46 54.89
7 3 0.5 45 70.23 70.80
8 2 1 15 41.82 41.82
9 2 1 45 65.69 65.71
10 4 1.5 30 38.41 38.41
11 4 1 45 47.50 47.50
12 3 1 30 36.14 36.71
13 3 1 30 36.14 36.71
14 3 0.5 15 65.80 65.71
15 2 1.5 30 57.73 57.16
16 4 0.5 30 65.67 65.71
17 3 1 30 66.82 67.39
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Table 4: ANOVA analysis for response surface second order model in relation to AY 99
biosorption.
ANOVA for Response Surface Quadratic model
Source
Sum of
Squares
df
Mean
Square
F
Value
p-value
Prob > F
Remark
Model 2361.62 9 262.40 706.70 < 0.0001 significant
A-pH 52.33 1 52.33 140.93 < 0.0001
B-Dose 470.48 1 470.48 1267.09 < 0.0001
C-Time 0.64 1 0.64 1.73 0.2293
AB 26.16 1 26.16 70.46 < 0.0001
AC 15.80 1 15.80 42.55 0.0003
BC 8.07 1 8.07 21.72 0.0023
A2
1691.04 1 1691.04 4554.29 < 0.0001
B2
6.99 1 6.99 18.81 0.0034
C2
37.72 1 37.72 101.58 < 0.0001
Residual 2.60 7 0.37
Lack of Fit 2.59 3 0.86 304.81 < 0.0001 significant
Pure Error 0.011 4
2.830E-
003
Cor Total 2364.22 16
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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Figure 5
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Figure 6
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Highlights
 Aspergillus niger biomass (ANB) proved to be efficient in AY 99 removal.
 Physico-chemical parameters were optimized by statistical approach (RSM).
 Dye removal capacity of ANB was evaluated in presence of NaCl and other anions.
 Experimental data correlated to different isotherm and kinetic models.
 Dye binding mechanisms were explored by FTIR, XRD and SEM studies.
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