black cumin (nigella sativa) as low cost biosorbent for the ...black cumin (nigella sativa) was used...
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International Journal of Engineering & Technology IJET-IJENS Vol:15 No:02 46
I J E N S IJENS © April 2015 IJENS -IJET-6767-021584
Black cumin (Nigella sativa) as Low Cost Biosorbent
For the Removal of Toxic Cu (II) and Pb (II) From
Aqueous Solutions Hanaa H. Abdel Rahman, Amira H.E. Moustafa
, *, Mohamed G. Kassem
Chemistry Department, Faculty of Science, Alexandria University, Alexandria, Egypt
[email protected] (A.H.E. Moustafa), ([email protected]) H.H. Abdel Rahman, M.G. Kassem ([email protected]).
Tel.: +2035917883, Fax.: +2035932488 University P.O. Box: 426 Ibrahimia, Alexandria 21321, Egypt
Abstract-- Black cumin, an annual plant was used as effective biosorbent for the removal of toxic Cu (II) and Pb (II) from
aqueous solution. Continuous techniques due to large size of
black cumin were carried out under different pH, contact time,
initial metal ion concentration, dosage and temperature. The
biosorption capacity was found to be increased in the order of Pb
(II) >Cu (II). Kinetic studies demonstrate that, the data fit a
pseudo second- order mechanism. Different isotherm models
were applied to describe the biosorption in all analyzed cases.
SEM, FT-IR and EDX analysis were employed for the evaluation
of biosorption process.
Index Term-- Biosorption, Black cumin, Copper ions, Lead ions, Biosorption capacity, Kinetics, Isotherm models, SEM, FT-
IR, EDX.
1. INTRODUCTION Environmental problems have become more frequent
and complex in recent decades, as a result of human
population growth and increasing industrialization. Industrial
activities such as mining and metal processing can lead to
heavy metal contamination in surface water, groundwater, or
the sea. The presence of these heavy metals in the aquatic
environment has been of great concern because of their
toxicity and non-biodegradability. Agro industrial biomass has
been known to readily adsorb metal ion. The ability of metal
uptake by these biomasses (known as biosorption) has caught
great attention due to its potential to provide an effective and
economic means for the remediation of heavy-metal-polluted
wastewater. Biosorption can be defined as the ability of
inactive and dead biomasses to remove heavy metals from
aqueous solutions through physical and chemical pathways of
uptake, since is a sustainable method able to replace the most
widely applied industrial materials such as activated carbon
and ion-exchange resins [1-6]. Also it is passive, non-
metabolic process involving complexation, chelation, ion
exchange, adsorption and microprecipitation. Using
inexpensive sorbents, biosorption can achieve high purity in
treated wastewater.
The major advantages of biosorption over
conventional treatment methods include [2, 7]:
Low cost;
High efficiency of metal removal from dilute solutions;
Minimization of chemical and/ or biological sludge;
No additional nutrient requirements;
Regeneration of biosorbent; and
Possibility of metal recovery.
Among the heavy metals, copper and lead are the
ones that have most severely affected the environment.
Copper is an essential trace element in living systems, where it
serves as a cofactor in enzymes that function in energy
generation, oxygen transport and many other processes. If the
copper content exceeds the permissible limits, procedures
must be applied for metal removal. Also lead and lead
compounds are generally toxic pollutants. Lead (II) salts and
organic lead compounds are ecotoxicologically very harmful.
Lead has the most damaging effects on human health by
accumulating in organisms, sediments and sludge. In recent
years, greater attention has been gained by biomaterials
obtained from plant or plant wastes have been reported to
remove or recover heavy metals from aqueous solutions.
The seeds of the Nigella sativa plant, frequently
called kalajira or black cumin has been considered as a new
biosorbent. Black cumin is an annual herbaceous plant
belonging to the Ranuculacea family, small, black and
possesses an aromatic odor and taste. Mature seeds are
consumed for edible and medical purposes. However, there
has been very few study of the absorptive effect of black
cumin [7-9].
The primary objective of this study was to explore
the potential of using low-coast biosorbent (black cumin) to
eliminate Cu (II) and Pb (II) from aqueous solutions. The
effect of some parameters on black cumin sorption capacity
for copper and lead ions has been examined as a comparative
study and confirmed by some characterization techniques for
understand better the biosorption process.
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II. MATERIAL AND METHODS
A. Preparation of biosorbent
Black cumin (Nigella sativa) was used as a
biosorbent. A commercial pack of black cumin (particle size
in a range of length 784.21 μm and width 1352.63 μm) was
purchased from a local market in Alexandria, Egypt. Black
cumin was immersed in distilled water for 24 h to remove dirt,
and dried in an air oven at 65oC for 48 h for the removal of
moisture. Then, it was stored in desiccators. The analysis of black cumin mature seeds cultivated in Egypt has been
reported by M.B. Atta [9].
B. Synthetic wastewater preparation
All chemicals used were of analytical grade and were
purchased from LOBA Chemie. In order to obtain a synthetic
wastewater solutions (1000 mg/L); you have to dissolving
analytical grade CuSO4.5H2O or Pb (NO3)2 in distilled water.
Desired test solutions of copper (II) and lead (II) ions were
prepared using appropriate subsequent dilutions of stock
solution. The range in concentrations of copper (II) and lead
(II) ions prepared from standard solution varied between 50 to
200 mg/L. Before mixing the biosorbent, the pH of each test
solution was adjusted to the required value with nitric acid for
acidic solution or sodium acetate for basic solution.
C. Analysis
The concentration of copper (II) and lead (II) ions in
the solutions before and after equilibrium were determined by
Perkin-Elmer 2380 atomic absorption spectrophotometer (Cu
wavelength, λ = 324.8 nm, Pb wavelength, λ = 283.3 nm).
Crison GLP 21 pH-meter was used to adjust pH of solutions.
D. Biosorption experiments
The continuous techniques used due to large size of
black cumin. Therefore, it has not affected the continuous
sample taking with time intervals. These experiments were
performed by stirring black cumin and 100 ml of copper
sulphate and lead nitrate solution using Dragon digital
(hotplate) magnetic stirrer MS-H-Pro with temperature sensor
PT 1000. Experiments were carried out at different variables
of temperature 301, 308, 313 and 318 K, 700 rpm, initial
copper (II) and lead (II) ions concentration 50, 75, 100, 150,
and 200 mg/l and black cumin dosage 0.2, 0.4, 0.7 and 0.9
g/L. Samples (0.25ml) were withdrawn in the storage tank for
analysis at regular time intervals. Then, they were analyzed by
using atomic absorption spectrophotometer. The data was used
to calculate the equilibrium biosorption capacity qe (mg/g) as
the difference between the initial and equilibrium metal
concentrations, and qt (mg/g) as the difference between the
initial and time changes (t) metal concentrations:
m
VCCq ee
)( (1)
m
VCCq tt
)( (2)
Also the change in % Removal with time was determined
from this equation:
100)(
Re%
o
t
C
CCmoval (3)
where, Co (mg/L) is the initial metal ions concentration in
solution, Ce (mg/L) is the equilibrium concentration of metal
ions in the solution, Ct (mg/L) is the concentration of metal
ions in the solution after time (t), m is the mass of black cumin
used (g) and V is the volume of the solution (L).
a. Biosorption Kinetics modeling
Kinetic measurements were conducted for a series of
solutions containing different initial concentrations (50, 75,
100, 150 and 200 mg/L) of copper (II) and lead (II) ions. Also,
the continuous adsorption studies were carried out at different
parameters of dose, pH and temperature. The applicability of
pseudo first-order, pseudo second-order and intra-particle
diffusion kinetic models was checked under the specified
conditions.
b. Biosorption isotherms modeling
A series of solutions containing different initial
concentrations of copper(II) and lead (II) ions was prepared
and continuous adsorption studies were carried out at, 700
rpm, 0.9 g/L black cumin dose and 301 K. The applicability of
the Langmuir, Freundlich and Dubinin–Radushkevich (D-R)
adsorption isotherms was checked under the specified
conditions.
E. Characterization
For characterization of black cumin before and after
biosorption process; the reaction was chosen of 700 rpm, 200
mg/L [Cu2+
or Pb2+
], and 0.9 g/L of black cumin at 301 K for
studying by different techniques [SEM, EDX and FTIR]. The
reaction was terminated after 60 min, and black cumin were
dried at room temperature to avoid any unexpected effects of
the high temperature then the solution was filtered to separate
it and left to dry at oven at 65oC for 8 h.
a. SEM and EDX Analysis
Scanning electron microscopy (SEM) and energy
dispersive X-Ray fluorescence (EDX) technique (Jeol-JSM-
5300) were used in studying the morphology and chemical
composition of black cumin surface.
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b. FTIR Analysis
An IR analysis was performed with a Fourier
Transform Infrared Spectrometer (Perkin-Elmer Spectrum BX
FTIR) to identify the chemical groups present in the black
cumin before and after metal binding with copper and lead.
The samples were crushed well in order to synthesize KBr
pellets under hydraulic pressure of 400 kg/cm2. Spectra were
recorded in the range of 400-4000 cm-1
. For all samples scan,
the amount of the samples and KBr were kept constant to
investigate the changes in the intensities and shifting of the
characteristic peaks as a result of the structural changes.
III. RESULTS AND DISCUSSION
A. Effect of pH
In order to establish the effect of pH on the
biosorption of copper and lead ions, the equilibrium studies at
different pH values have been carried and in the range of 2 to
5.3 for copper and from 2 to 6 for lead. Experiments could not
be performed at higher pH value due to hydrolysis and
precipitation of copper (II) and lead (II) ions [10-12]. The
effect of initial pH on the biosorption process is presented in
Fig.1. It is shown that the adsorbed amount of copper and lead
increases with increasing pH and maximum biosorption are
obtained at pH 5.3 for Cu2+
and pH 4.9 for Pb2+
due to the
negative surface charge of black cumin at high pH values. The
acidity of the medium can affect the metal ions uptake amount
of the black cumin adsorbent because, at very low pH values,
copper and lead biosorption was found to be very low due to
the competition between H+ and Cu (II) ions or Pb (II) ions for
the active sites on black cumin surface. With increasing the
pH value, the deprotonation of the functional groups provided
the chance to coordinate with copper (II) or lead (II) resulting
in higher biosorption capacity.
B. Effect of contact time
Fig. 2 shows that the removal increases with time
and reaches a maximum at 60 min. This indicates that the
concentration of copper in the solution decreased rapidly
within the first 30 min and the removal was virtually
completed within 60 min. It is clear that the removal of metal
ions can be derived into two stages: one in which the removal
rate is very high. The second is very important to determine
the equilibrium time that is; the contact time characterized by
unchanging Cu2+
or Pb2+
concentration in the solution was
achieved after 30 min for all used concentrations of
solutions [13].
High biosorption rate at the beginning of the
adsorption process is due to the numerous readily available
active adsorbing sites on the biosorbent surface that is the
large uncovered surface area of black cumin which was
provided by high amount of black cumin. Additionally, the
driving force for the biosorption is the difference between
concentration of copper and lead ions in the solution and
solid/liquid interface which has the highest value at the
beginning of the process, resulting in fast biosorption. The low
rate of biosorption after the first 30 minutes may be attributed
to the onset of intra particle diffusion which is slow compared
to liquid phase diffusion. In addition, the decrease in the active
centers on the biosorbent and the decrease of biosorbent
concentration contribute to slowing down the rate of
biosorption.
C. Effect of biosorbent dose
Fig.3 shows that the removal percentage of Cu (II)
and Pb (II) ions increase as the biosorbent amount increases.
Biosorption increases from 47 to 81 % with increase in
biosorbent dose from 0.2 to 0.9 g/L in case of copper ions and
from 60 to 100 % in the case of lead ions for 50mg/L and after
60 min. The increase in removal percentage with an increase
in biosorbent dosage is due to the availability of larger surface
area and more biosorption sites.
D. Effect of initial concentration on copper and lead removal
At the initial stage of biosorption of metal ions from
aqueous solution, the surface of the biosorbent is free of these
metal ions and large amounts of copper (II) or lead (II) ions
species move across from the solution to the black cumin
surface.
It is necessary to highlight that the biosorption
capacity depends on the concentration of metal ions [11].
Biosorption of Cu (II) or Pb (II) ions was carried out at
different initial concentrations ranging from 50 to 200 mg/L at
pH 5.3, 4.9 for copper and lead respectively, 60 min of contact
time, 301 K, 700 rpm and 0.9 g/L of black cumin. The amount
of metal ions, qe (mg/g), increased with increasing initial
concentration as shown in Fig. 4 for copper and lead
respectively. Furthermore, the results presented in Fig. 5,
showed that the amount of the percent removal of Cu2+
or Pb2+
ions decrease with the increase of the initial concentration.
This decrease in copper or lead removal percentage could be
due to lack of sufficient active sites on black cumin to absorb
more metal ions available in the solution (i.e. saturation of
black cumin sites at higher concentrations of copper or lead
ions) [2, 13]. So, the percentage removal depended upon the
initial metal ions concentration. This indicates the possible
mono layer formation of metal ions on the outer surface of
black cumin.
E. Effect of temperature and thermodynamic parameters
To study the thermodynamic properties of
adsorption, the studies were carried out at 301, 308, 313 and
318 K.
The biosorption of Cu and Pb onto black cumin as a
function of temperature is illustrated in Fig. 6 shows that the
degrees of Cu biosorption at equilibrium increases with
increasing temperature and maximum biosorption of Cu ions
are obtained at 318 K. An increase in temperature involves
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increasing the mobility of Cu ions and decreasing in the
retarding forces acting on the diffusing ions; these result in the
enhancement in the sorptive capacity of the adsorbent,
increasing the chemical interaction between adsorbate-
adsorbent and creation of active surface centers or by an
enhanced rate of intra-particle diffusion of Cu (II) ions into the
pores of the biosorbent at higher temperature. This increase
indicates that the adsorption is an endothermic process [12-
16]. But for Pb, at high temperatures, the sorbed Pb amount
decreased with the increase of temperature. The increase of
biosorption capacity with decreasing temperature from 301 to
318 K favors the sorbate transport within the pores of the
sorbent Fig. 6(i.e. decrease in surface activity suggesting that
biosorption between Pb (II) ions and black cumin is an
exothermic process [7]).
The biosorption equilibrium data obtained for
different temperatures were used to calculate the important
biosorption thermodynamic properties such as standard Gibbs
free energy (ΔGo), standard enthalpy change (ΔH
o) and
standard entropy change (ΔSo). These biosorption parameters
were estimated using the following equations:
e
e
eC
qK (4)
eKRTG ln
(5)
STHG (6)
RT
H
R
SKe
ln (7)
where qe (mg/g) is the amount of Cu (II) or Pb (II) ions
adsorbed onto the black cumin from the solution at
equilibrium, Ce (mg/L) the equilibrium concentration of Cu
(II) or Pb (II) ions the solution, R (J/mol.K) the gas constant
8.314, T (K) the absolute temperature, and Ke (L/g) the
biosorption equilibrium constant. ΔHo
and ΔSo were obtained
from the slope and intercept of the Van’t Hoff’s plot of ln (Ke)
versus 1/T as shown in Fig. 7 and the values of ΔGo, ΔH
o, and
ΔSo were collected in Table 1.
From the recorded values in this work, it could be
observe that, in case of copper, although the Gibbs free energy
of Cu (II) biosorption onto black cumin decreased with
increasing temperatures, its values were positive at all of the
temperature tested. These results suggest that the biosorption
process not occur spontaneously [14, 15]; in other words, the
degree of non-spontaneity decreases with increasing
temperature. The positive value of ΔHo indicates that the
biosorption process is endothermic in nature. The decrease in
ΔGo
with increasing temperature shows that the biosorption
reaction is more favorable at higher temperatures. At high
temperature, the metal ions are readily adsorbed due to the
high biosorption rate and capacity in equilibrium time [17].
The positive ΔSo suggests the increased randomness at solid /
liquid interface during the biosorption of Cu (II). Also, during
the adsorption of copper, the adsorbed solvent molecules,
which are displaced by the copper ions, gain more
translational entropy than that is lost by the adsorbate ions,
thus allowing for the prevalence of randomness in the system
[14, 16]. These results contrast with those in case of lead. The
Gibbs free energy ΔGo
of Pb (II) onto black cumin increased
with increasing temperature; it was negative at 301-308 K,
which suggests the biosorption might not occur spontaneously
at other temperatures. The negative ΔHo values confirm the
exothermic nature of biosorption and the negative values of
ΔSo indicate the stability of sorption process with no structural
change at solid-liquid interface [11, 12]. The exothermic
nature of the reaction explains why the value ΔGo
becomes
more positive with the rise in temperature indicating a
decrease in the feasibility of the biosorption process [18]. By
other words, from equation (6) the entropic term TΔSo is the
dominant factor that determines the sign of ΔGo. By
increasing temperature, this term is increased and its
contribution dominant over ΔHo. Thus, a conclusion could be
drawn that the overall biosorption process of Pb (II) is
entropically driven [19].
F. Kinetics of biosorption
Various kinetics models [20], namely pseudo first-
order, pseudo second-order, and intra-particle diffusion, have
been used for their validity with the experimental biosorption
data for copper (II) and lead (II) onto black cumin.
a. The pseudo first-order kinetic model
The pseudo first-order reaction equation of Lagergren
[21] was widely used for the biosorption of liquid/solid system
on the basis of solid capacity. Its linear form is generally
expressed as the following [20]:
tkqqq ete 1) lnln( (8)
where qe (mg/g) and qt (mg/g) are the values of amount
adsorbed per unit mass at equilibrium and at any time
respectively, t (min) is time, k1 (min−1
) is the pseudo first order
biosorption rate coefficient. The values of k1 and qe can be
obtained from the slope and intercept of the linear plot of
ln(qe−qt) vs. t, and listed in Fig. 8 and Table 2.
It is necessary to know the value of qe for fitting the
experimental data to the equation (8). The real test of the
validity of equation (8) arises from a comparison of the
experimentally determined qe values and those obtained from
the plots of ln(qe−qt) vs. t. The correlation coefficients for the
pseudo first-order kinetic model are low and a difference of
equilibrium biosorption capacity qe between the experimental
and the calculated data was observed, indicating a poor pseudo
first-order fit to the experimental data.
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b. The pseudo second-order kinetic model
The kinetic data were further analyzed using Ho and
Mckay [22, 23], is based on the assumption that the adsorption
follows second-order. The linear form can be written as
follows:
tqqkq
t
eet
112
2
(9)
where, k2 (g/mg.min) is the rate constant of biosorption. By
plotting a curve of t/qt against t, qe and k2 can be evaluated.
The dependence of t/qt vs t, gives an excellent straight line
relation for all the experimental concentrations Fig. 9. The
values of qe, k2 and R2 are listed in Table 2, and all the R
2
values are very high in this study and close to 1.
It was also noted that the values of the calculated
pseudo second-order capacities, qe were close to the
experimentally determined capacities signifying the ability for
the model to predict the experimental data. It can also be seen
in Table 2 that, with an increase in initial metal concentration,
the rate constant of biosorption k2 decreases. A similar
observation was also reported by the earlier researchers [1,
12]. The reason for this behavior can be attributed to the lower
competition for the sorption surface sites at lower
concentrations. At high concentrations, the competition for the
surface active sites will be high and consequently lower
sorption rates are obtained.
The half life time of the adsorption process
(i.e: t = t 0.5), we have:
e
oqk
t2
5.
1 (10)
The values for pseudo second-order index k2qe were
determined for all samples and displayed in Table 2. The
results show that the values of the index k2qe, increased with
reducing initial concentration of copper (II) and lead (II). The
half-life of the biosorption process is shown in Table 2. The
results shown revealed that the half-life reduces with
decreasing initial concentrations of copper (II) and lead (II) in
solution. This means that, as the initial concentration of
copper (II) and lead (II) in solution reduces, less time is
needed to reduce the initial concentration by half its original
value [24].
c. Intra-particle diffusion Intra-particle diffusion process involves the
migration of ions into the internal surface of the biosorbent
particles through pores of different size. To understand the
mechanism and rate controlling steps influencing biosorption
kinetics, the intra-particle diffusion model was also applied.
The intra-particle diffusion model was plotted in
order to verify the influence of mass transfer resistance on the
binding of copper (II) or lead (II) to the black cumin. The
kinetic results were analyzed by the Weber and Morris intra-
particle diffusion model to elucidate the diffusion mechanism,
which is expressed as:
Ctkq bt 5.0
(11)
where, C is a constant that gives idea about the thickness of
the boundary layer (mg/g) and kp is the intra-particle diffusion
rate constant (mg/g.min0.5
), which can be evaluated from the
slope of the linear plot of qt vs. t0.5
as shown in Fig. 10. Larger
the intercept, the greater is the contribution of the surface
sorption in the rate controlling step. The calculated intra-
particle diffusion coefficient kp values are listed in Table 2.
The deviation from the origin is perhaps due to the difference
in the rate of mass transfer in the initial and final stages of the
biosorption. This is indication of some degree of boundary
layer control and this further showed that the intra-particle
diffusion was not the only rate-limiting step, but also be
controlling the rate of sorption or all may be operating
simultaneously. Thus, based on the high correlation
coefficient values Table 2 for high concentrations of copper
(II), the increase in kp with initial concentration can be
attributed to the increase in concentration gradient as initial
concentration of copper (II) and lead (II) in solution is
increased, forcing more copper (II) and lead (II) ions to
migrate to the biosorbent surface [24-26].
Thus, based on the high correlation coefficient values
Table 2, it can be inferred that the adsorption of copper (II)
onto black cumin followed pseudo-second order model than
that of pseudo-first order model and intra-particle diffusion
model.
G. Equilibrium biosorption isothermal
To estimate the potential application of black cumin,
Langmuir, Freundlich and Dubinin–Radushkevich (D-R)
isotherm models [27, 28] were employed to evaluate the
biosorption properties of that biosorbent. They indicate how
the adsorbate molecules distribute between the liquid phase
and the solid phase at equilibrium.
a. Langmuir model
The Langmuir model is obtained under the ideal
assumption of a totally homogenous adsorption, which each
molecule possess constant enthalpies and sorption activation
energy (all sites possess equal affinity for the adsorbate) and
represented as following [27]:
maxmax .
1
q
C
bqq
C e
e
e (12)
where, qmax the maximum capacity biosorption at high
equilibrium concentrations (attaining biosorbent monolayer)
and b is the Langmuir constant. Fig. 11 shows linear plots of
Ce/qe vs. Ce was used to calculate the parameters of the
Langmuir isotherm, by means of linear regression equation.
From this regression equation and the linear plot, the value of
the Langmuir constant were calculated and were tabulated in
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Table 3. The qmax and b were obtained from the slope and
intercept of the plots respectively. High R2 values for metal
ions reveal the extremely good application of Langmuir model
to these biosorption.
The essential features of the Langmuir isotherm can
be expressed in term of a dimensionless constant separation
factor (RL) which is defined as:
RL = )bC(1
1
o (13)
The values of RL indicate the type of isotherm to be
either unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL
< 1) or irreversible nature of biosorption (RL = 0). The values
of RL are given in Fig. 12. The separation factor RL for the
Langmuir model equations at pH 5.3 for Cu2+
and pH 4.9 for
Pb2+
was greater than zero and less than one indicating
Langmuir isotherm was favorable for describing the
biosorption of copper and lead by black cumin. Fig. 12 also
indicates that the biosorption is more favorable for the higher
initial Cu (II) and Pb (II) ion concentrations than for the lower
ones. It is thus apparent that the biosorption of Cu (II) and Pb
(II) ions on black cumin is favorable within the experimental
conditions studied [2, 28].
b. Freundlich model
The Freundlich isotherm is the earliest known
relationship describing the non-ideal and reversible
biosorption not restricted to the formation of monolayer. This
empirical model can be applied to multilayer biosorption, with
non-uniform distribution of adsorption heat and affinities over
the heterogeneous surface and expressed by the following
equation [1]:
efe Cn
Kq log1
loglog (14)
where KF and 1/n are the Freundlich constant denoting the
biosorption capacity and intensity, respectively. Fig. 13 shows
the linear plot of log qe vs. log Ce, and the constants 1/n and
KF were calculated from the slope and intercept respectively,
and then collected in Table 3. The value of 1/n less than 1
represent of favorable biosorption and confirmed the
heterogeneity of the biosorbent. Also, it indicates that the
bond between heavy metal ions and black cumin are strong
[11, 18].
c. Dubinin–Radushkevich (D-R) isotherm model Langmuir and Freundlich isotherm don’t give any
idea about sorption mechanism. The D-R isotherm is an
analogue of Langmuir type, but it is more general because it
does not assume a homogenous surface or constant sorption
potential. It was applied to distinguish between the physical
and chemical biosorption of Cu2+
and Pb2+
ions.
The D-R isotherm equation is [1]:
2lnln adse Kqq (15)
where, qs is the theoretical isotherm saturation capacity
(mg/g), Kad is the D-R isotherm constant ( mol2/J
2) and ε is the
Polanyi potential which is equal to RT ln ( 1 + 1
𝐶𝑒), where R
(J/mol.K) is the gas constant and T (K) is the absolute
temperature.
Fig. 14 shows a linear relation between ln qe vs. ε2.
The slope of the plot gives Kad (mol2/J
2) and the intercept
yields the sorption capacity qs (mg/g) [Table 3]. The constant
Kad gives an idea about the mean free energy E (kJ/mol) for
biosorption per molecule of adsorbate when it is transferred to
the surface of the solid from infinity in the solution and can be
calculated using the relationship [27, 28]:
E = 1 / (2 kad)1/2
(16)
This parameter gives information about biosorption
mechanism either chemical or physical. The magnitude of E is
between 8 and 16 kJ/mol, the biosorption process follows
chemical; while for values of E < 8 kJ/mol, the biosorption
process is of a physical nature. The numerical values of the
biosorption of the mean free energy are 0.41 kJ/mol for copper
and 4.08 kJ/mol in case of lead which is correspond to a
physical mechanism.
Finally in other words, all of the isotherm models fit
very well when the R2 values are compared in Table 3.
H. Mechanism of biosorption process and SEM analysis
Sorption is a general term that includes sorption
process that occurs at the solid solution interface, as well as
those in which a solute (molecule or ion) penetrated the bulk
of the sorbent phase. Biosorption of metal ions on this type of
material (black cumin) is generally attributed to weak
interactions between biosorbent and adsorbate. Surface
charges on substrates as well as softness or hardness of the
solute are mostly responsible for the intensity of the
interaction. Columbic interaction can be observed for the ionic
inter-exchange of cationic species with anionic sites in the
black cumin and is determined by their surface areas. In the
present work, it is found that the percentage removal of the
softer Pb2+
ions using black cumin is higher than the harder
Cu2+
ions since the black cumin has nitrogen and oxygen
atoms as soft and hard centers of biosorption respectively. So,
it could be predicted that the biosorption process occurs
mainly through the N sites of the black cumin.
In this work, SEM analysis was conducted to observe
any changes in the surface structure of black cumin after the
biosorption experiments. Biosorption of both Cu (II) ions and
Pb (II) ions made a clear change in the surface structure of
black cumin Fig. 15. These finding indicate that the
biosorption of metal ions onto the surface of black cumin is
may be complexation reaction [11, 29].
I. EDX spectroscopy study of the black cumin
Table 4 and Fig. 16 show the SEM-EDX analysis of
black cumin, black cumin-Cu and black cumin-Pb. After
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biosorption of copper onto black cumin an increase in copper
percentage (10.8%) was observed, appearance of lead (14%)
could be noticed in case of Pb biosorption and decrease of
some wt % values of other elements which verified of metal
ion biosorption on the black cumin surface by ion exchange
with other elements especially Ca [17].
J. Infrared Spectra of black cumin
FTIR spectra have been a usuful tool in identifying
the existence of certain functional groups in a molecule as
each specific chemical bond often shows a unique energy
absorption band. FTIR spectra of black cumin were measured
before and after biosorption of both copper and lead in order
to determine the frequency changes in the functional groups in
the adsorbent. The spectra were measured within the range of
500-4000 cm-1
and given in Fig. 17.
The black cumin is a composite mixture of organic
compounds such as carbohydrates, fats, protiens and surface
adsorbed water. These molecules are characterized by large
number of functional groups which could act as active sites for
biosorpting metal ion on the black cumin surface. The
presence of these functional groups as a main component of
black cumin is confirmed using the FTIR analysis. The IR
spectrum of black cumin showed a broad band in the range of
3300-3400 cm-1
which confirm the prsence of functional
groups such as OH, NH and chemisorbed water on the surface
of black cumin. The peak near 3010 cm-1
is assigned to the
unsaturated =C-H streching while the bands in the range of
2850- 3000 cm-1
could be assigned to the aliphatic C-H
streching. The bands at 1650 and 1750 cm-1
are characteristic
frequencies for the conjugated and uncongugated carbonyl
group respectively. The peak around 1540 cm-1
is due to the
C=C streches. The peak around 1458 cm-1
was attributed to the
C-O-H bending of the carboxylate group. The bands in the
range of 1000-1250 cm-1
were attributed to the streching
vibrations of the C-O and C-N groups [14, 15, 30].
On the other hand, changes in FTIR spectra were
observed after copper and lead biosorption onto black cumin.
The intensity clearly decreased after Cu2+
and Pb 2+
biosorption. It suggests that there may be an ion exchange
process, and metal, more voluminous, somehow prevents the
vibration of the bands. The decrease in the IR intensities is
higher in case of lead compared to copper ions. Also the band
at 3300-3400 cm-1
is shifted to higher frequency in case of the
former compared to the latter. These result indicate the
stronger interactions between the lead ion and black cumin
compared to the copper ions [ 2, 3, 7].
According to the theory of hard and soft acid and
bases (HSAB), lead and copper ions are considered in the
borderline between hard and soft acid, but lead is
intermediate-to-soft metal, and copper is intermediate-to-hard
metal [31]. HSAB theory supposes that lead (II) is a softer
cation which is able of forming strong complexes with the
softer N-sites [32]. On the other hand, copper is a harder
cation which is able of forming strong complexes with harder
O-sites [33]. Since the selectivity of the black cumin to lead
ions was very high so a conclusion can be drawn that most of
the functional groups of black cumin responsible for the
biosorption of the metal ions are the N-sites rather than O-
sites. Therefore, removal of lead ions with black cumin
occurred to a higher extent than copper ions. Also, the high
value of the ionic radius of lead, small hydrated radius and its
higher atomic weight are factors which increase the softness
of lead and its selectivity [29].
IV. CONCLUSIONS
Black cumin was effectively potential biosorbent for Pb
2+ and Cu
2+removal. The qe was found to be increased in the
order Pb2+
> Cu2+
. The difference of qe between Pb2+
and Cu2+
was explained according to HSAB theory. The biosorption
data fit well to the second order kinetic model. Langmuir
adsorption model was the best in data explanation. The
thermodynamics biosorption data reveal non-spontaneous,
ordered and exothermic biosorption of Pb2+
and non-
spontaneous, random and endothermic biosorption of Cu2+
.
FTIR was used to study the participated function group of
black cumin. SEM and EDX studied changes of black cumin
surface before and after reaction.
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LIST OF TABLES
Table 1 Thermodynamic biosorption parameters of copper and lead onto black cumin
at constant initial concentration of 100 mg/L.
Heavy metal
Thermodynamic biosorption parameters
T ΔGo ΔH
o ΔS
o
(K) (kJ/mol) (kJ/mol) (J/mol.K)
Copper 301 7.75
55.10
157.32
308 6.65
313 5.86
318 5.07
Lead 301 -3.68
-106.08
-340.18
308 -1.30
313 0.40
318 2.10
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Table 2 Kinetic models for biosorption of a) copper and b) lead onto black cumin
at different initial concentrations of metal ions, 0.9 g/L dose, 700 rpm and 301 K.
(a)
Kinetic models Parameters Concentration of copper(II) solution (mg/L)
50 75 100 150 200
qe(Exp) (mg/g) 0.51 0.72 0.89 1.01 1.15
Pseudo-first order
equation qe (Calc.) (mg/g)
0.53 0.75 0.77 0.75 0.90
k1(min-1
) 0.13 0.11 0.08 0.04 0.04
R2 0.9435 0.9018 0.974 0.9891 0.9983
Pseudo-second
order equation
qe (Calc.) (mg/g) 0.55 0.78 0.98 1.15 1.32
k2 (g/mg. min) 0.48 0.27 0.18 0.07 0.06
k2qe2 (mg/g.min) 0.14 0.16 0.17 0.10 0.10
k2qe 0.26 0.21 0.17 0.08 0.08
t0.5 3.78 4.75 5.77 11.97 13.31
R2 0.999 0.9987 0.9996 0.9829 0.9826
Intra-particle
diffusion equation
kp (mg/g. min-0.5
) 0.04 0.06 0.09 0.11 0.13
C 0.25 0.30 0.31 0.15 0.13
R2 0.8876 0.9096 0.8679 0.9882 0.9935
(b)
Kinetic models Parameters Concentration of lead (II) solution (mg/L)
50 75 100 150 200
qe(Exp) (mg/g) 0.56 0.82 1.10 1.66 2.21
Pseudo-first order
equation
qe (Calc.) (mg/g) 0.19 0.33 0.45 0.80 1.31
k1(min-1
) 0.12 0.11 0.09 0.07 0.06
R2 0.9616 0.9732 0.9585 0.9524 0.9785
Pseudo-second
order equation
qe (Calc.) (mg/g) 0.57 0.85 1.15 1.77 2.40
k2 (g/mg. min) 1.56 0.72 0.37 0.14 0.08
k2qe2 (mg/g.min) 0.51 0.52 0.49 0.45 0.44
k2qe 0.89 0.61 0.43 0.26 0.18
t0.5 1.13 1.64 2.34 3.91 5.50
R2 0.9998 0.9998 0.9996 0.9994 0.9998
Intra-particle
diffusion equation
kp (mg/g. min-0.5
) 0.02 0.04 0.07 0.14 0.21
C 0.41 0.54 0.64 0.74 0.78
R2 0.7137 0.6672 0.6966 0.7801 0.8597
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Table 3 Biosorption isotherm constants for the biosorption of copper (II) and lead (II)
onto black cumin at 301 K.
Langmuir
Adsorbate qmax
(mg/g)
b
(dm3/mg)
R2
Cu 1.21 0.14 0.9961
Pb 2.59 4.10 0.9986
Freundlish
Adsorbate 1/n KF R2
Cu 0.24 0.39 0.9402
Pb 0.38 2.05 0.9772
Dubinin–Radushkevich
Adsorbate Kads. E(kJ/mol) R2
Cu 3x10-6
0.41 0.8641
Pb 3x10-8
4.08 0.9677
Table 4 Element compositions of pure black cumin and black cumin after adsorption.
Element (wt %) Ca Al Si P S K Zn Mg Cu Pb
Black cumin 81.4 0.8 1.2 0.3 3.4 9.9 1.1 - 1.9 -
Black cumin-Cu 55.1 0.7 0.7 2.4 7.1 20.6 1.1 1.5 10.8 -
Black cumin- Pb 66.2 0.3 0.3 0.7 - 11.9 3.1 - 3.5 14
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LIST OF FIGURES
1) Effect of pH value for copper (II) and lead (II) adsorption onto black cumin (adsorbent concentration 100 mg/L, adsorbate dosage 0.9 g/L, stirring speed 700 rpm, temperature 301 K and contact time 60 min).
2) Effect of contact time for the adsorption of (a) copper (II), (b) lead (II) onto black cumin (adsorbent concentration 50 to 200 mg/L, adsorbate dosage 0.9 g/L, pH 5.3(Cu), 4.9 (Pb), 700 rpm, contact time 60 min and temperature 301 K).
3) Black cumin adsorbent dosage effect on copper (II) and lead (II) removal. (initial concentration 100 mg/L, temperature 301 K, 700 rpm, contact time 60 min and pH =5.3 for copper, pH = 4.9 for lead.
4) Relationship between qe and initial concentration for copper and lead (temperature 301 K, 700 rpm, contact time 60 min and pH =5.3 for copper, pH = 4.9 for lead).
5) Relationship between % Removal and Time at different initial concentrations a) copper b) lead (temperature 301 K, 700 rpm, contact time 60 min and pH =5.3 for copper, pH = 4.9 for lead).
6) Relationship between qe and temperature at constant initial concentration of 100 mg/L, 0.9 g/L dose of black cumin and 700 rpm.
7) Relationship between ln (Ke) and reciprocal of temperature, at constant initial concentration of 100 mg/L, 0.9 g/L dose of black cumin and 700 rpm for copper and lead.
8) Pseudo-first order kinetic fit for adsorption of metal ions onto black cumin at different initial concentrations, 0.9 g/L black cumin dose, 700 rpm and 301 K a) copper and b) lead.
9) Pseudo-second order kinetic fit for adsorption of metal ions onto black cumin at different initial concentrations, 0.9 g/L black cumin dose, 700 rpm and 301 K a) copper and b) lead.
10) Intra-particle diffusion kinetic model fit for adsorption of metal ions onto black cumin at different initial concentrations, 0.9 g/L black cumin dose, 700 rpm and 301 K a) copper and b) lead.
11) The linear Langmuir adsorption isotherm for a) copper (II) and b) lead (II) with black cumin at 301 K. 12) Equilibrium parameter, RL for the biosorption of Cu (II) and Pb (II) ion onto black cumin. 13) The linear Freundlich adsorption isotherm for a) copper (II) and b) lead (II) with black cumin at 301 K. 14) The Dubinin–Radushkevich (D-R) adsorption isotherm for a) copper (II) b) lead (II) with black cumin at 301 K. 15) SEM of black cumin: a) before adsorption, b) after copper removal of initial concentration 200 mg/L, c) after copper
removal of initial concentration 200 mg/L, d) after lead removal of initial concentration 200 mg/L and e) after lead
removal of initial concentration 200 mg/L.
16) EDX of a) black cumin, b) after copper adsorption and c) after lead adsorption (initial copper concentration = 200 mg/L, black cumin dose = 0.9 g/L, 60 min and 700 rpm).
FTIR spectra of blank black cumin and after removal of lead and copper.
Fig. 1. Effect of pH value for copper (II) and lead (II) adsorption onto black cumin (adsorbent concentration 100 mg/L, adsorbate dosage 0.9 g/L,
stirring speed 700 rpm, temperature 301 K and contact time 60 min).
50
60
70
80
90
100
110
1 2 3 4 5 6 7
% R
emo
va
l
pH
Copper Lead
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Fig. 2. Effect of contact time for the adsorption of (a) copper (II), (b) lead (II) onto black cumin (adsorbent concentration 50 to 200 mg/L, adsorbate dosage 0.9 g/L, pH 5.3(Cu), 4.9 (Pb), 700 rpm, contact time 60 min and temperature 301 K).
Fig. 3. Black cumin adsorbent dosage effect on copper (II) and lead (II) removal. (initial concentration 100 mg/L, temperature 301 K, 700 rpm, contact time 60
min and pH =5.3 for copper, pH = 4.9 for lead.
30
40
50
60
70
80
90
100
0 20 40 60
% R
emo
va
l
Time (min)
(b)
50 mg/l 75 mg/l 100 mg/l
150 mg/l 200 mg/l
0
20
40
60
80
100
0.2 0.4 0.7 0.9
% R
emo
va
l
Dose (g/L)
copper lead
0
20
40
60
80
100
0 20 40 60 80%
Rem
ov
al
Time (min)
(a)
50 mg/l 75 mg/l 100 mg/l
150 mg/l 200 mg/l
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Fig. 4. Relationship between qe and initial concentration for copper and lead (temperature 301 K, 700 rpm, contact time 60 min and pH =5.3 for copper, pH = 4.9
for lead).
Fig. 5. Relationship between % Removal and Time at different initial concentrations a) copper b) lead (temperature 301 K, 700 rpm, contact time 60 min and
pH =5.3 for copper, pH = 4.9 for lead).
0.0
0.5
1.0
1.5
2.0
2.5
40 70 100 130 160 190 220
qe (
mg/g
)
C (mg/L)
copper lead
30
40
50
60
70
80
90
100
0 20 40 60 80
% R
emoval
Time (min)
(b) 50 mg/l 75 mg/l
100 mg/l 150 mg/l
200 mg/l
0
20
40
60
80
100
0 20 40 60 80
% R
emoval
Time (min)
(a)
50 mg/l 75 mg/l100 mg/l 150 mg/l200 mg/l
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Fig. 6. Relationship between qe and temperature at constant initial concentration of 100 mg/L, 0.9 g/L dose of black cumin and 700 rpm.
Fig. 7. Relationship between ln (Ke) and reciprocal of temperature, at constant initial concentration of 100 mg/L, 0.9 g/L dose of black cumin and 700 rpm for
copper and lead.
Fig. 8. Pseudo-first order kinetic fit for adsorption of metal ions onto black cumin at different initial concentrations, 0.9 g/L black cumin dose, 700 rpm and 301 K a) copper and b) lead.
0.85
0.90
0.95
1.00
1.05
1.10
1.15
300 305 310 315 320
qe (
mg/g
)
T (K)
copper lead
-4
-3
-2
-1
0
1
2
3.13 3.23 3.33
ln (
Ke)
1/T x 103
Copper Lead
-8
-7
-6
-5
-4
-3
-2
-1
0
0 20 40 60
ln(q
e-q
t)
Time (min)
(a)
50 mg/l 75 mg/l 100 mg/l
150 mg/l 200 mg/l
-8
-7
-6
-5
-4
-3
-2
-1
0
1
0 20 40 60
ln(q
e-q
t)
Time (min)
(b)
50 mg/l 75 mg/l 100 mg/l
150 mg/l 200 mg/l
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Fig. 9. Pseudo-second order kinetic fit for adsorption of metal ions onto black cumin at different initial concentrations, 0.9 g/L black cumin dose, 700 rpm and 301 K a) copper and b) lead.
Fig. 10. Intra-particle diffusion kinetic model fit for adsorption of metal ions onto black cumin at different initial concentrations, 0.9 g/L black cumin dose, 700
rpm and 301 K a) copper and b) lead.
0
30
60
90
120
0 20 40 60
t/q
t
Time (min)
(b)
50 mg/l 75 mg/l 100 mg/l
150 mg/l 200 mg/l
0.2
0.4
0.6
0.8
1.0
1.2
1 3 5 7 9
qt (m
g/g
)
t0.5
(a)
50 mg/l 75 mg/l 100 mg/l
150 mg/l 200 mg/l
0.3
0.8
1.3
1.8
2.3
2.8
1 3 5 7 9
qt (m
g/g
)
t0.5
(b)
50 mg/l 75 mg/l 100 mg/l
150 mg/l 200 mg/l
0
30
60
90
120
0 20 40 60
t/q
t
Time (min)
(a)
50 mg/l 75 mg/l 100 mg/l
150 mg/l 200 mg/l
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.
Fig.11. The linear Langmuir adsorption isotherm for a) copper (II) and b) lead (II) with black cumin at 301 K.
Fig. 12. Equilibrium parameter, RL for the biosorption of Cu (II) and Pb (II) ion onto black cumin.
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 0.5 1 1.5
Ce/q
e
Ce
(b)
0.02
0.06
0.10
0.14
40 70 100 130 160 190 220
RL
Co mg/L
(a)
0.000
0.002
0.004
40 70 100 130 160 190 220
RL
Co mg/L
(b)
5
25
45
65
85
105
0 20 40 60 80 100
Ce/
qe
Ce
(a)
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Fig. 13. The linear Freundlich adsorption isotherm for a) copper (II) and b) lead (II) with black cumin at 301 K.
-0.40
-0.30
-0.20
-0.10
0.00
0.10
0.20
0 0.02 0.04 0.06 0.08
ε2
x 10-6
ln q
e
(a)
Fig. 14. The Dubinin–Radushkevich (D-R) adsorption isotherm for a) copper (II) b) lead (II) with black cumin at 301 K.
-0.1
0.0
0.1
0.2
0.3
0.4
-1.1 -0.6 -0.1 0.4
log q
e
log Ce
(b)
-0.3
-0.1
0.1
0.3
0.5
0.7
0.9
0 10 20 30 40
ln q
e
ε2 x 10-6
(b)
-0.3
-0.2
-0.1
0.0
0.1
0.5 1 1.5 2
log q
e
log Ce
(a)
-
International Journal of Engineering & Technology IJET-IJENS Vol:15 No:02 64
I J E N S IJENS © April 2015 IJENS -IJET-6767-021584
(a)
(b) (c)
(d) (e)
Fig. 15. SEM of black cumin: a) before adsorption, b) after copper removal of initial concentration 200 mg/L, c) after copper removal of initial concentration 200
mg/L, d) after lead removal of initial concentration 200 mg/L and e) after lead removal of initial concentration 200 mg/L.
-
International Journal of Engineering & Technology IJET-IJENS Vol:15 No:02 65
I J E N S IJENS © April 2015 IJENS -IJET-6767-021584
Fig. 16. EDX of a) black cumin, b) after copper adsorption and c) after lead adsorption (initial copper concentration = 200 mg/L, black cumin dose = 0.9 g/L, 60
min and 700 rpm).
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International Journal of Engineering & Technology IJET-IJENS Vol:15 No:02 66
I J E N S IJENS © April 2015 IJENS -IJET-6767-021584
Fig. 17. FTIR spectra of blank black cumin and after removal of lead and copper.