black cumin (nigella sativa) as low cost biosorbent for the ...black cumin (nigella sativa) was used...

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.

mailto:[email protected]:[email protected]



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.



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



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.



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



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



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



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



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



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



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



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



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



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



.

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)



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)



(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.



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).



Fig. 17. FTIR spectra of blank black cumin and after removal of lead and copper.

black cumin (nigella sativa) as low cost biosorbent for the ...black cumin (nigella sativa) was used...

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