Journal of environmental chemical engineering volume issue 2014 doi 10 1016 j jece 2014 06 015 pandey, ruchi prasad, ram lakhan ansari, nasreen ghazi murthy utilization of NaOH modified desmostachya bi

Accepted Manuscript
Title: Utilization of NaOH modified Desmostachya bipinnata
(Kush Grass) Leaves & Bambusa arundinacea (Bamboo)
Leaves for Cd(II) removal from aqueous solution
Author: Ruchi Pandey Ram Lakhan Prasad Nasreen Ghazi
Ansari Ramesh Chandra Murthy
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DOI:
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S2213-3437(14)00131-6
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Please cite this article as: Ruchi Pandey, Ram Lakhan Prasad, Nasreen Ghazi
Ansari, Ramesh Chandra Murthy, Utilization of NaOH modified Desmostachya
bipinnata (Kush Grass) Leaves & Bambusa arundinacea (Bamboo) Leaves for Cd(II)
removal from aqueous solution, Journal of Environmental Chemical Engineering
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Utilization of NaOH modified Desmostachya bipinnata
(Kush Grass) Leaves & Bambusa arundinacea

ip

t

(Bamboo) Leaves for Cd(II) removal from aqueous

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solution

Ruchi Pandeya, b, Ram Lakhan Prasadb, Nasreen Ghazi Ansaria and

a

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Ramesh Chandra Murthy*, a

CSIR, Indian Institute of Toxicology Research, Analytical Chemistry Section,

Department of Chemistry, Faculty of Science, Banaras Hindu University,

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b

an

Post Box-80, M.G. Marg, Lucknow-226001, India

Corresponding Author.

Abstract

te

*

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Varanasi, 221005, India

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ce
p

A fundamental investigation for the Cd(II) ions removal from aqueous solutions by NaOH
modified Desmostachya bipinnata (Kush Grass) Leaves (MDBL) and Bambusa arundinacea
(Bamboo) Leaves (MBAL) was conducted in batch experiments. The influence of different
experimental parameters such as pH, contact time, initial Cd(II) ion concentration, adsorbent
dosage, on the Cd(II) adsorption was studied. The Cd(II) uptake by MDBL and MBAL was
quantitatively evaluated using sorption isotherms. Freundlich and Langmuir isotherm models
were used to fit the equilibria data, of which Langmuir model is considered better in correlation
and the maximum adsorption capacity was found to be 15.22 mg g−1 for MDBL and 19.70 mgg1

for MBAL at room temperature. The kinetic data were found to follow closely the pseudo

second order kinetic model by both adsorbents. FTIR and SEM were recorded, before and after
adsorption, to explore number and position of the functional groups available for Cd(II) binding
on to studied adsorbents and changes in surface morphology of adsorbent. Desorption studies
show 94.18% and 92.08% recovery for adsorbed Cd(II) ions from MDBL and MBAL,
respectively using 0.1 N HNO3. Thermodynamic studies indicated that the adsorption reaction

was a spontaneous and exothermic process. It can be concluded that MDBL and MBAL are

Page 1 of 32


low-cost biosorbent alternatives for wastewater treatment, since both have a considerable high
adsorption capacity.
Highlights
Highlights

• A Novel and efficient biosorbent is developed from NaOH modification of

ip
t

•

Desmostachya bipinnata (Kush Grass) Leaves (MDBL) & Bambusa arundinacea

•

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(Bamboo) Leaves (MBAL) for removal of Cd(II) ions from aqueous solution.

• Apparent high adsorption capacity of 19.84 and 19.71 mg g−1 was shown by

• 94.18 percent and 92.08% desorption of adsorbed Cd(II) ions from MDBL and

an

MBAL, respectively was observed using 0.1 N HNO3.

Keywords

d

M

Graphical Abstract

Isotherm, Kinetics

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Desmostachya bipinnata leaves, Kush grass, Bambusa arundinacea leaves, Bamboo leaves,

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ce
p

•

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MDBL and MBAL at pH = 6.5, respectively with a fast adsorption rate.

1 Introduction

An increased flux of heavy metals in the aquatic environment due to their swift use [1] in

industries, has led severe threat to human being. Beyond permitted concentration, they
can cause grave health disorders; therefore, considerable attention has been paid to
wastewater treatment prior to its discharge in the environment. Among these metallic
pollutants, Cd(II), an extremely toxic heavy metal causes a potential risk to
environmental and human health because it is incorporated into the food chain, mainly
by plant uptake [2]. The main anthropogenic pathway through which Cd(II) enters the
water bodies is via wastes from industrial processes such as electroplating, plastic
manufacturing, metallurgical processes, and Cd/Ni batteries. Over exposures may befall
even in conditions where a little amount of Cd(II) found because of its low permissible
limit (0.005 mg L−1) in drinking water [3]. So many surface chemistry practices for

Page 2 of 32


wastewater treatment such as precipitation, adsorption, membrane processes, ionic
exchange, floatation, and others [4,5] have been studied. However, because of inherent
limitation of such techniques as less competent, perceptive operating settings, and
production of sludge, they further require costly disposal [6], whereas, adsorption is by

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far the most versatile and widely used method, and activated carbon is the furthermost
commonly used adsorbent [7]. Conversely, the use of activated carbon is expensive, so
considerable interest has been shown towards the use of other efficient sorbent

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materials, particularly biosorbents [8]. In recent years, agricultural by-products have
been widely considered for metal sorption studies including peat, banana pith, pine bark,

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peanut, shells, hazelnut shell, rice husk, wood, sawdust, wool, soybean and cottonseed
hulls, orange peel, leaves and compost [9-13]. In a previous study we have carried out

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Cd(II) removal using Cucumber peel and obtained a maximum adsorption capacity of
7.142 mg g−1[14]. In the adsorption process, various metal-binding mechanisms are
thought to be involved, including ion exchange, surface adsorption, chemisorption,

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complexation, and adsorption–complexation [15-18].

In the present study, Cd(II) sorption using Desmostachya bipinnata (Kush Grass)

d

Leaves (DBL) and Bambusa arundinacea (Bamboo) Leaves (BAL), members of true

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grass family: Poaceae, had been studied and as both adsorbents correspond to same
family, their major constituents must be same. These materials are the major organic

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ce
p

components of the solid waste, comprising about 14.6% of total municipal solid waste
(MSW) and about 50% of the organic fraction of the MSW [19]. However, in the entire
world, India has the huge rate of biomass production, including organic wastes, such as
grass, leaves and flowers. Therefore, it is essential to search for a better use of these
abundant agricultural wastes such as, remediation of heavy metal from contaminated
aqueous solutions. Both materials found abundantly throughout the year, and these
kinds of materials exhibit strong potential due to their high content of lignin and cellulose
[20] that abide numerous polar functional groups, including phenolic and carboxylic acid
groups, which may be involved in metal binding [21,22]. Due to the low cost, DBL and
BAL are an attractive and inexpensive option for the adsorption of Cd(II) ion from
aqueous solution. Further NaOH was used in the modification process because it can
enhance surface characteristics of DBL and BAL with increased adsorption capacity
[23]. The adsorption capacity of modified DBL and BAL (MDBL & MBAL, respectively)
was investigated by batch experiments. The influences of parameters such as pH,

Page 3 of 32


adsorbent dosage, contact time, initial ion concentration were investigated and the
experimental data obtained were evaluated and fitted using adsorption equilibrium and
kinetic models.

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t

2 Materials and methods
2.1 Adsorbent & chemicals

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DBL and BAL were obtained from the Indian Institute of Toxicology Research, Gheru
Campus (Lucknow, India). Both were dried under the sunlight for 2 days then, ground,

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washed several times with double distilled water (DDW) and afterwards screened to
obtain 80 µm sized particles. These samples were modified using 0.5 M NaOH solution
with 1:20 for 30 min (solid–liquid ratio) [24]. The MDBL and MBAL were again dried at
MDBL & MBAL are presented in Table 1.

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100 °C for 24 h and stored in an airtight container. The physical characteristics of the

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Table 1 Physical characteristic of MDBL & MBAL.
Parameter

MBAL

≤ 80

0.144

0.196

0.127

0.166

Porosity (%)

0.113

0.15

Moisture content (%)

10.93

8.30

Ash content (%)

2.57

1.42

Phzpc

5.50

5

Particle density

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ce

p

Bulk density (g ml−1)

d

≤ 80

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Particle size (µm)

MDBL

The stock solution of Cd(II) (1000 mg L−1) was prepared in DDW using Cd
(NO3)2.4H2O salt (Merck); all working solutions were prepared by diluting the stock
solution with DDW.

2.2 Biosorption experiments
Batch experiments were executed for adsorption studies. A Pre-weighted sample of the
adsorbents (MDBL & MBAL) with a measured volume of Cd(II) solution were taken in
100 mL Erlenmeyer flask and stirred in an incubator shaker (250 rpm) at a steady

Page 4 of 32


temperature (25 ± 2 °C), for 240 min to ensure equilibrium. After shaking the flasks for
regular intervals, samples were withdrawn, filtered and the filtrates were analyzed by
Atomic absorption Spectrophotometer (PerkinElmer AAnalyst 300, USA) for the
concentration of Cd(II). A first series of sorption experiments was carried out with an

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t

initial concentration of 20 mg L−1. In these experiments the most favorable pH of
biosorption was determined. Subsequently, the influence of adsorbent dosage, contact
time, initial ion concentration was also evaluated. Percentage metal removal was

us

cr

calculated using the following formula:

(1)

where C0 is initial and Ct is the final concentration of Cd(II). The morphological
characteristics of adsorbents were evaluated by using a scanning electron microscope

an

(SEM) and disposition of the functional group present on the adsorbent surface were
spectrophotometer.

2.3 Adsorption isotherms studies

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studied before and after biosorption using Fourier Transform Infrared (FTIR)

d

Isotherm studies were recorded by varying the initial concentration of Cd(II) solutions

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from 10–150 mg L−1 with MDBL and MBAL separately. A known amount of adsorbents
was then added into solutions in different flasks followed by agitating them at 250 rpm till

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ce
p

equilibrium. The metal ion concentrations, retained in the adsorbent phase qe (mgg−1)
which is defined as adsorption capacity, was calculated by using the following mass
balance equation for the process at equilibrium condition:

(2)

where V is the volume of solution (L) and W is the mass of adsorbate (g).

2.4 Desorption study

Desorption experiments were performed to consider the practical usefulness of the
biosorbents. After the biosorption studies, 0.2 g of metal loaded sorbent were agitated in
100 mL of 0.1 M HCl and 0.1 M HNO3 same as described by Witek-Krowiak [25]. After
60 min of contact time the metal concentration in the solution was determined. To check
the applicability as the best eluent the sorption desorption steps were repeated five
times.

Page 5 of 32


3 Results and discussion
3.1 Characteristic of MDBL & MBAL before and after adsorption
Presence of functional groups on MDBL & MBAL powder were analyzed using FTIR, as

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t

shown in Fig. 1(A) and (B). Occurrences of diverse type of functional groups are
confirmed by the peaks, and their detailed illustration is shown in Table 2. Mainly, metal
ions were bonded by functional groups such as carboxylic groups (pectin, hemicellulose

cr

and lignin), phenolic groups (lignin and extractives) and a little amount may also
adsorbed by hydroxyl (cellulose, lignin, extractives, and pectin) and carbonyl groups

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(lignin) [26]. After sorption, several functional groups which were initially present
disappear, while some other had their position altered and thus confirming the active

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participation of bonded OH groups, secondary amine group, carboxyl groups, C−O
stretching of ether groups and −C−C− group [3,27] as shown in Fig. 1(A) and (B) and
Table 2. Hence, the good sorption properties of both MDBL and MBAL towards Cd(II)

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ions can be ascribed to the presence of these functional groups on their surfaces.
Fig. 2(A) and (B) shows SEM images for MDBL and MBAL before and after the
adsorption process, respectively. From Fig. 2(A & B), it is clearly visible that before

d

adsorption, both the adsorbents have rough heterogeneous porous surface and a large

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number of steps and kinks on the adsorbent surface, with wrecked edges [28]. The
change in the morphology of the adsorbent after adsorption indicates that there is a

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good possibility for Cd(II) ions to be trapped and adsorbed onto the surface.

Fig. 1FTIR spectra of MDBL (A) and MBAL (B)

before and after Cd(II) adsorption.

Table 2 Functional groups and mode of vibration from the FTIR spectrum of
MDBL and MBAL before and after adsorption.

Functional group

Stretching vibration of bonded −OH

Cd (II)

Cd (II)

loaded

loaded

MDBL

MDBL

MBAL

MDBL

(cm )

(cm )

(cm )

(cm−1)

3418.35

3415.14

3402.93

3431.08

−1

−1

−1

Page 6 of 32


group on surface
Asymmetrical stretching vibration of

2922.73

2920.52

2922.59

2923.30

1631.46

1639.24

1634.58

1644.39

−

1516.20

−

Strong stretching vibration of C−O from
carboxylic acid in presence of

Generation of this peak after adsorption

cr

intermolecular H bonding

bending vibration of N−H and stretching
vibration of C−N

Bending vibration oh OH and stretching
vibration of C−O−C in lignin structure

Bending vibration of OH and stretching

−

1376.61

1514.93

1059.02

1103.39

1246.16

1252.85

−

−

1049.61

1073.96

−

−

664.60

563.42

602.96

466.37

469.10

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C−O stretching of carboxylic acid

1377.07

an

1380.35

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is outcome of an overlapped band of

C−C stretching of aromatic ring

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t

−CH3

d

vibration of C−O−C in lignin structure

te

Fingerprint region: adsorption cannot be
clearly assigned to any particular

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p

vibration because they correspond to

605.21

complex interacting vibration systems

Fig. 2Scanning electron micrograph showing morphology of MDBL (A) and MBAL

(B) before and after Cd(II) adsorption.

3.2 Effect of pH

The pH plays a very significant role in the sorption of heavy metals by affecting the
surface charge of adsorbent, the degree of ionization, and speciation of adsorbate.
Thus, the effects of initial pH of the solution on the Cd(II) removal efficiency were studied
at different pH ranging from 3.5 to 8.5. A sharp increase in the Cd(II) removal was
observed from 64.4% to 77.6% and 60.88%–75.2% at pH 6.5 (Fig. 3) for MDBL and
MBAL, respectively and after that with a slight decrease, the value became constant

Page 7 of 32


because of saturation of active sites on the adsorbents surface. So the pH 6.5 was
selected as the best pH to study the overall adsorption process. Precipitation of Cd(II)
ions was observed at pH 8 [29]. At low pH, the little removal efficiency is due to

occurrence of higher concentration of protons in the solution which compete with the

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t

Cd(II) ions for the adsorption sites of the adsorbents. As the pH increases, the H+
concentration decreases, leading to enhanced Cd(II) uptake. The effect of pH can be
explained in terms of pHzpc of the adsorbent. The pH at, which the charge of the whole

cr

surface is zero is referred as the zero point of charge (pHzpc) and above which the
surface become negatively charged. The obtained pHzpc of MDBL and MBAL is 5 and 5.5

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respectively by using the batch equilibration technique [30]. Positively charged Cd(II)
species are soft acids and as a rule the interaction of Cd2+ and Cd (OH)+ with the

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negatively charged adsorbent surface containing carboxyl and hydroxyl groups are
responsible for the sorption of Cd(II) ions and also supported by FTIR studies. At low pH,
particularly below pHzpc the Cd2+ and Cd (OH)

+

species present in the solution may

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exchange with H+ from peripheral. Apparently, at very low pH (≤ 3), the presence of
higher concentrations of H+ ions in the mixture, owes electrostatic repulsion between
both positively charged adsorbent surface and metal ion. A decreasing trend in

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adsorption was also observed at very high pH also, and this may be due to the formation

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of soluble hydroxy complexes [31]. Dissociation of the –COOH groups (pKa = 3. 8-5.0) is
the plausible reason for becoming the surface of MDBL and MBAL negatively charged at

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optimum pH 6.5 and thus, favorable to the adsorption of Cd(II) at this pH [32]. Cd(II) may
most likely be bound on the MDBL and MBAL surface via an ion exchange mechanism
as following equation:

(3)

Fig. 3Effect of pH on Cd(II) adsorption by MDBL and MBAL at 25 °C

(condition: 20 mg L−1 of Cd(II) solution, 250 rpm, 1 g L−1 adsorbent dosage,
60 min).

(where -R represents the matrix of the adsorbents).

3.3 Effect of biosorbent dosage

Page 8 of 32


One of another important parameter that strongly affects the sorption capacity is the
biosorbent dosage. As shown in Fig. 4 with the increasing adsorbent dosage from 2 to
12 g L−1, it can easily be inferred that the percent removal of metal ions boosts from
63.95% to 85.50% and 65.50%–82.05% for MDBL and MBAL, respectively, whereas the

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amount adsorbed per unit mass decreases. It is apparent that the percent removal of
heavy metals increases rapidly with an increase in the dosage of the adsorbents due to
the greater availability of the exchangeable sites or surface area [3], whereas the

cr

decrease in Cd(II) uptake with increasing adsorbent dosage is mainly due to
unsaturation of adsorption sites through the adsorption reaction and the similar results

an

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were obtained in a study performed by Chen et al. [33].

Fig. 4Effect of adsorbent dosage for Cd(II) adsorption by MDBL and MBAL at

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25 °C (condition: 20 mg L−1 of Cd(II) solution, 250 rpm, 60 min).

3.4 Effect of initial ion concentration

d

The Cd(II) ion uptake is particularly reliant on the initial Cd(II) concentration. At the lower
range, Cd(II) is adsorbed by specific active sites, while at higher sides; decreased

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adsorption is due to the saturation of adsorption sites and also because of lack of
sufficient surface area to accumulate further available ions. This is due to the

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ce
p

competition for the available active sites on the surface. The influence of the initial Cd(II)
concentration on its removal with MDBL and MBAL shown in Fig. 5, where a decrease in
removal percent from 74.2–66.60% and 77.90-62.61% (for Co = 10 − 150 mg L−1) could
be observed respectively.

Fig. 5Effect of initial Cd(II) ion concentration for adsorption process by MDBL and

MBAL at 25 °C (condition: 1 g L−1 adsorbent dosage, 250 rpm, 60 min).

3.5 Effect of contact time
The effect of contact time on adsorption was studied up to 240 min. It appeared from
Fig. 6 that the metal uptake is very rapid up to 90 and 180 min of equilibrium for MBAL
and MDBL respectively, after that Cd(II) uptake does not significantly change with time.

Page 9 of 32


Therefore, 240 min of contact time is chosen to achieve the equilibrium [34]. For 20 mg
L−1 of Cd(II) concentration with an increase in contact time from 10–180 min for MDBL
and 10–150 min for MBAL, the percentage Cd(II) sorption increased from 36.80–89.55%
and 29.15–90.05%, respectively and after which a plateau is obtained, which showed

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saturation of the active points (Fig. 6). The subsequent increase in contact time had no

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effect on Cd(II) adsorption.

Fig. 6Effect of contact time on Cd(II) adsorption by MDBL and MBAL, at 25 °C,

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(condition: 20 mg L−1 of Cd(II) solution, 250 rpm, 1 g L−1 adsorbent dosage).

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3.6 Adsorption isotherms

The equilibrium isotherm study is very essential for designing adsorption systems and
also facilitate the comprehensive of the interaction involved between adsorbate and

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adsorbent. The adsorption data were analyzed Freundlich (1906) and Langmuir (1918)
isotherm models. The linearized forms of the Freundlich isotherm can be expressed as

(4)
(5)

Ac
ce
p

te

d

in Eq. 4[35] and of Langmuir isotherm as in Eq. 5[35] below:

where Ce (mg L−1) is the equilibrium Cd(II) concentration in solution, qe and qmax
(mgg−1) are the equilibrium and maximum adsorption capacities (mgg−1), 1/n is the
heterogeneity factor, kF (Lg−1) and kL(Lg−1) are Freundlich and Langmuir constant
respectively. The values of Freundlich and Langmuir parameters were obtained based
on the linear correlation between the values of (i) log qe vs. log Ce and (ii) (Ce/qe) vs. Ce,
respectively.

Fig. 7(a) and (b) shows the adsorption isotherms of Cd(II) using MDBL and MBAL,
respectively. Freundlich isotherm is the most basic relationship describing non-ideal and
reversible adsorption, valid for heterogeneous surface having non uniform energy
distribution and is not restricted to monolayer formation [36]. In the present study
(Table 3) at increasing temperature, higher kF values and n > 1 indicated, favorable
process [37] and are classified as L type isotherm reflecting a high affinity between

Page 10 of 32


adsorbent and adsorbate thus shows beneficial adsorption for MDBL and MBAL, and is
indicative of chemisorption [38].

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Fig. 7Langmuir isotherm for MDBL
and MBAL at 25 °C.

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Table 3 Adsorption isotherm parameters at different temperatures.
Freundlich
Freundlich

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n

L )

R

2

(mg g )
−1

kL

R2

(min−1)

298

1.251 ± 0.057

1.505 ± 0.002

0.977

15.222 ± 0.162

0.077 ± 0.006

0.997

308

1.460 ± 0.067

1.522 ± 0.045

0.979

19.843 ± 0.570

0.090 ± 0.006

0.992

318

1.552 ± 0.028

1.501 ± 0.006

0.955

15.540 ± 0.085

0.104 ± 0.003

0.995

298

1.424 ± 0.025

1.472 ± 0.016

0.977

19.705 ± 0.097

0.064 ± 0.001

0.987

308

1.961 ± 0.020

0.949

18.483 ± 0.151

0.107 ± 0.001

0.986

318

2.024 ± 0.061

0.933

17.331 ± 0.425

0.135 ± 0.008

0.993

d

MBAL

qmax

1/n

1.596 ± 0.011

te

MDBL

(mg1−1/n g−1

M

(K)

kF

an

Temp

Langmuir

1.602 ± 0.018

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ce
p

The Langmuir model is based on the hypothesis that all binding sites are, likewise,
active on the energetically homogeneous surface, and monolayer surface formation
takes place without any interaction between the adsorbed molecules [12]. As it can be
seen from Table 3, the coefficients of determination (R2 > 0.980) at varying temperatures
proved that the linear fit of the Langmuir model agrees well for both the adsorbents.
Initially, on increasing temperature from 298 to 308K adsorption capacity increases
for MDBL and decreasing values were noticed from 308–318 K. On the other side
continuous decrease in adsorption capacity is observed for MBAL up to 318 K, and it
may be due to either the damage of adsorbent’s active binding sites [39] or increasing
propensity of desorption of Cd(II) ions from the interface to the solution [40]. This is
because at the higher temperature the thickness of the boundary layer decreases and
thus tendency of the metal ion to escape from the biomass surface to the solution phase
increases [41]. This trend in the result indicates the exothermic nature of Cd(II)

Page 11 of 32


adsorption. The admirable qmax data as shown in the Table 3, it is obvious that MDBL
and MBAL exhibited a favorable combination of a high number of accessible adsorption
sites, and evidenced excellent promising adsorbents for Cd(II) ions from aqueous
solution.

ip
t

Freundlich and Langmuir equations have their own limitations in describing the
equilibrium data adequately. Both are based on utterly unlike principles, and the fact that
the experimental fallouts fit one or another equation, imitates a purely mathematical apt

cr

[13]. So here it may be concluded from Table 3, that the Cd(II) adsorption isotherm
exhibit Langmuir behavior with comparative high coefficients of determination

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(R2 > 0.980) than Freundlich isotherm.

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3.7 Thermodynamic study

Langmuir isotherm constant kL for varying temperatures, i.e. 298, 308 and 318 K has
been used to evaluate the thermodynamic parameters, the change in Gibbs free energy

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(ΔGo), enthalpy (ΔHo) and entropy (ΔSo) for the adsorption process [38], and the

te

d

following equations are used for the calculations for these parameter:

(6)
(7)

Ac
ce
p

where R is the universal gas constant (1.987 cal mol−1) and T is the temperature (K).
The entropy (ΔSo) and enthalpy (ΔHo) change were obtained from the slope and
intercept of the plot of log kL against 1 T−1 (Fig. 8), respectively. All the thermodynamic
parameters of the adsorption process are shown in Table 4. As expected, the negative
ΔG° value indicates feasibility and spontaneity of the adsorption process. The change of
the standard free energy decreases with increasing temperature regardless the nature of
adsorbent, indicate that a better adsorption is actually obtained at the higher
temperature [42]. Moreover, the standard free energy change for multilayer adsorption
was more than −20 kJ mol−1 and less than zero. It should be noted that the magnitude of
ΔG◦ values is in the range of multilayer adsorption [43]. Each metal ion has to displace
more than one ion of the solvent. The net result corresponds to the endothermic process
[44]. Moreover, the positive ΔS° value corresponds to the increased randomness at the
solid/liquid interface during the adsorption process, which suggests that Cd(II) ions

Page 12 of 32


replace some water molecules from the solution previously adsorbed on the surface of
adsorbent. Positive value of ΔH◦ shows the endothermic nature of reaction [45]. The
values of adsorption (ΔH°) obtained in this study (< 20 kJ mol−1) are consistent with

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t

hydrogen bond and dipole bond forces for both adsorbents [46].

Fig. 8Plot of ln KL versus 1/T for Cd(II)

cr

sorption onto MDBL and MBAL.

Table 4 Thermodynamic parameters at different temperatures for Cd(II)

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adsorption using MDBL and MBAL.

ΔGo

Temp (K)

(kJ mol−1)

298

–

ΔHo (kJ mol−1)

ΔSo (J mol−1
K−1)

M

MDBL

an

Thermodynamic parameters

0.389 ± 0.143
–

d

308

5.50 ± 0.916

1.58 ± 0.089

9.283 ± 6.564

2.673 ± 1.890

Ac
ce
p

318

te

1.506 ± 0.174

MBAL

298

308

318

–

1.112 ± 0.053
–
0.661 ± 0.064
–
1.731 ± 0.031
–
2.051 ± 0.085

3.8 Kinetic study
Adsorption kinetic studies are important as they provide valuable information about the
mechanism of the adsorption process [47] such as mass transfer and chemical reaction.
In the present study, applicability of the pseudo-first-order [Eq. 8] [48] and pseudo-

Page 13 of 32

second-order kinetics [Eq. 9] [37] based on solid capacity and solid phase sorption has
been evaluated, respectively.

(8)

ip
t

(9)

cr

qe and qt are adsorption capacities (mgg−1) at equilibrium and at time t respectively.
k1ad is the first order rate constant (Lmin.−1) and k2ad is the rate equilibrium constant (g mg−1

(10)

an

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min−1) for pseudo second order kinetics.

where h is the initial sorption rate (mg g−1 min−1) [49].

In the present study, at different initial ion Cd(II) concentration all the kinetic data

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were considered up to 240 min for pseudo-second-order rate model, but for pseudo-firstorder rate model, only initial kinetic data up to 150 and 120 min for MDBL and MBAL
respectively have been used, because using a whole range of contact time, the

d

calculated values of qe (mgg−1) from pseudo-first-order rate model is physically

te

unacceptable. The pseudo-first-order rate model does not fit well to the whole range of
contact time in several other cases also and is generally applicable to the initial phase of

Ac
ce
p

the adsorption processes [49,50].

The equilibrium adsorption capacity and the second-order rate constant were
calculated from the slope and the intercept of the plot t/qt against t (Fig. 9a & b). A
competitively high adsorption capacity (qe) was obtained for MBAL than MDBL. The data
illustrated good compliance with the pseudo-second order rate law based on sorption
capacity because the coefficients of determination, R2 from Table 5 were higher than
0.990 for both MDBL and MBAL. It can also be perceived by values of Table 5 that, for
both the adsorbents, with an increase in initial metal concentration, the adsorption
capacity increases while the rate constant of adsorption (k2ad) decreases. A similar
observation was also reported by some earlier researchers [49]. The observed decrease
in the rate constants with an increase in initial metal ion concentration may be because,
at higher concentration, the average distance between the adsorbed ion is contracted to

a point where each affect the charge distribution of its adjacent ions. The second-order

Page 14 of 32


rate constant decreases with an increase in initial Cd(II) concentration, while the initial
sorption rate, h, generally increases with an increasing initial Cd(II) concentration at all
temperatures. This interaction can alter the ability of the ions to adsorb onto adsorbent,
hence higher concentration of metal ions may limit the ability of the biomass to adsorb

ip
t

metal ions and consequently sorption process may require several cycles in order to

cr

meet regulatory standards [31].

Fig. 9Kinetic parameters for the

us

adsorption of Cd(II) on MDBL and
MBAL at various initial Cd(II) ion

an

concentrations.

Table 5 Parameters of pseudo-first-order and pseudo-second-order kinetics for Cd(II) adsorption us

PseudoPseudo-first order

Co
(mg g−1)

d

L−1)

MBAL

PseudoPseudo-second order

k1ad

(min−1)

te

qe, cal

R2

qe, cal

k2ad

(mg g−1)

(g mg1 min−1)

h

20

1.097 ± 0.054

0.008 ± 0.001

0.924

1.882 ± 0.020

0.058 ± 0.004

0.194 ± 0.0

30

2.105 ± 0.023

0.012 ± 0.001

0.985

2.828 ± 0.006

0.030 ± 0.001

0.236 ± 0.0

50

3.546 ± 0.014

0.008 ± 0.001

0.978

4.785 ± 0.001

0.010 ± 0.000

0.233 ± 0.0

20

1.528 ± 0.014

0.007 ± 0.001

0.920

2.384 ± 0.007

0.028 ± 0.0002

0.157 ± 0.0

30

2.880 ± 0.018

0.013 ± 0.001

0.930

3.586 ± 0.025

0.016 ± 0.0004

0.213 ± 0.0

50

5.389 ± 0.092

0.014 ± 0.001

0.963

5.950 ± 0.007

0.011 ± 0.0004

0.396 ± 0.0

Ac

ce
p

MDBL

(mg

M

and MBAL at various initial Cd(II) ion concentration.

3.9 Sorption mechanism

The kinetic data were analyzed by intraparticle diffusion model [Eq. 11] to elucidate the
key steps involved during the adsorption process.

(11)

Page 15 of 32


ki is the intra-particle diffusion rate constant (mg g−1 min0.5).
In this direction, the kinetic data obtained by plotting qt vs. t0.5 plays a significant role
to understand the mechanism of adsorption: as migration of metal species from the
solution to adsorbent, diffusion, intraparticle diffusion through boundary layer and

ip
t

adsorption to the internal sorbent surface. Table 6 shows that with an increase in the

initial Cd(II) concentration, the intra-particle diffusion rate constant (ki) also increases

cr

because ki is directly related to qe and intra-particle diffusivity (I).

Table 6 Intra-particle diffusion parameters for Cd(II) adsorption using MDBL and MBAL at various i

Co
(mg

I

(mg g−1

L−1)

(mg g−1)

min−1/2)

20

–

0.308 ± 0.016

0.239 ± 0.061

R2

I

ki2

I

(mg g−1)

(mg g−1 min−1/2)

R2

(

0.996

1.049 ± 0.070

0.057 ± 0.007

0.927

2

0.050 ± 0.111

0.316 ± 0.026

0.9967

2.092 ± 0.154

0.046 ± 0.013

0.9841

2

50

0.244 ± 0.551

0.301 ± 0.238

0.962

2.830 ± 0.010

0.009 ± 0.0002

0.999

4

20

0.660 ± 0.127

0.566 ± 0.075

0.985

1.125 ± 0.010

0.160 ± 0.101

0.965

2

0.172 ± 0.029

0.989

1.432 ± 1.155

0.040 ± 0.008

0.735

3

0.485 ± 0.043

0.985

2.718 ± 1.143

0.247 ± 0.110

0.913

5

te

d

30

Ac
ce
p

MBAL

an

ki1

M

MDBL

us

Intra particle diffusion

30

–

1.286 ± 0.658

50

0.742 ± 0.198

The plots of qt vs. t0.5 at different initial Cd(II) ion concentrations (Fig. 10a & b) show
multi-linearity characterizations for both the adsorbents, indicates the occurrence of a
three step adsorption process [51]. For all three stages, the intra particle diffusion
constant (ki1, ki2, ki3) for MDBL and MBAL are shown in Table 6, that shows first sharp
section is the external surface adsorption or an instantaneous adsorption stage, then
intraparticle diffusion decelerated down and stabilized [52]. The initial kinetic data do not
pass through the origin for both the absorbents, so on the basis of these data it is
obvious that, intra-particle diffusion is not the sole rate controlling step [53]. So here the
overall mechanism involves adsorption and intra-particle diffusion.

Page 16 of 32


Fig. 10Intra
particle diffusion model for Cd(II) adsorption by MDBL and MBAL at
10
various initial Cd(II) ion concentrations.

ip
t

3.10 Desorption

Apart from adsorption, desorption also plays an important role to define the versatility of

cr

an adsorbent as it allows reuse of adsorbent and recovering of valuable heavy metals.
Dssorption studies were carried using 0.1 N HNO3 and 0.1 N HCl. Results showed that

us

MDBL achieved 94.18% and 93.65% desorption of adsorbed Cd(II) ions using HNO3 and
HCl, whereas for MBAL, attained desorption is 92.08% using HNO3 and 91.60% using
HCl, respectively. It indicates that HNO3 acts as a better eluting agent for Cd(II) ions

an

from the used biosorbent than HCl. After the desorption of Cd(II) ions, both sorbents
were washed thoroughly with DDW. Five cycles of successive sorption-desorption were

M

carried out to test the reusability of MDBL and MBAL using 0.1 N HNO3 and the outcome
indicated that the adsorption capacity decreases in the range of 93.75-80% and 91.13-

d

73.80% of HNO3 regenerated MDBL and MBAL, respectively.

te

3.11 Comparison of MDBL & MBAL with other adsorbents
The adsorption capacity of Cd(II) onto MDBL & MBAL was noticeable as compared with

Ac
ce
p

other NaOH treated adsorbents reported in literature as sawdust (73.62 mg g−1) [54],
Juniper fiber (29.54 mg g−1) [55], Rice husk (20.24 mg g−1) [56], Coconut bagasse (17.51
mg g−1) [57] and spent grains (17.3 mg g−1) [58]. It can be observed that a pH near about
6.0 was found to be an optimum in nearly all cases with the same modification method.
The adsorption capacity differences of metal uptake are due to the properties of each
adsorbent such as structure, functional groups and surface area. Thus the capacities
found in this work with MDBL (19.84 mg g−1) and MBAL (19.71 mg g−1) are significant
related to other.

4 Conclusion
In this detailed study, the NaOH modified DBL and BAL have been found to be very
effective biosorbents for Cd(II) ions removal from aqueous solution. The data of the
present study is comparable, as the both materials represent same family. All the
operational parameters such as the pH, adsorbent dosage, contact time and initial metal

Page 17 of 32


ion concentrations evidently affected the removal efficiency. The Langmuir adsorption
isotherm fits well to the equilibrium adsorption data and suggest that the monolayer and
homogeneous adsorption process take place. The adsorption process follows a pseudosecond-order kinetics rate model. The overall mechanism involves adsorption and intra-

ip
t

particle diffusion. Regeneration of used MDBL and MBAL can be performed efficiently
using 0.1 N HNO3 as an eluting agent. The adsorption process was spontaneous and
exothermic under natural conditions. These results demonstrate the remarkable potential

cr

of MDBL and MBAL as the low-cost substitutes with a considerable high adsorption

us

capacity of 15.22 mg g−1 and 19.70 mg g−1, respectively at room temperature.

Acknowledgments

an

The authors would like to acknowledge and extend their heartfelt gratitude to the
Director, CSIR-Indian Institute of Toxicology Research, Lucknow, and to, the HOD,

M

Department of Chemistry, Faculty of Science, BHU, Varanasi for taking interest in the
study. The authors thank the Council of Science and Technology, Uttar Pradesh, India

te

References

d

(Project Code, GAP-271) for providing their financial support for this research work.

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