33 International Journal of Water Research 2015; 5(2): 33-46
ISSN 2348 – 2710
Original Article
Removal of Lead ion from aqueous solution by Bamboo activated Carbon
Masood Akhtar Khan*, Amanual Alemayehu, Ramesh Duraisamy* and Abiyu Kerebo Berekete
Department of Chemistry, College of Natural Sciences
Arba Minch University, Arba Minch, Ethiopia (East Africa)
Corresponding Author’s:
e-mail: [email protected], [email protected]; [email protected]
Mobile: + 251-938607570; + 251-910171048; + 91-9042725600
Received 11 June 2015; accepted 09 July 2015 Abstract
The current study focusing the removal of Pb2+ from it aqueous solution using activated carbon obtained from bamboo
stem has been investigated by batch adsorption method. The results were obtained and indicate that the maximum sorption
for lead ion was found at pH 5. The bamboo activated carbon (BAC) dosage reveals better results even at lower metal ion
concentrations. Greater adsorption occurs at smaller particle size of adsorbent and at high solution temperature. The results
were also confirmed that the adsorption process follows Freundlich isotherm model with a better sorption fit and supported
for the multilayer adsorption of Pb2+ ions on BAC. The kinetic model of this study shows a pseudo-second order kinetic
model with good correlation coefficient. Thermodynamic parameters such as change in Gibbs free energy, enthalpy and
entropy were also evaluated. Thus, these results were reveals the negative free energy changes (ΔG) and positive entropy
(ΔS) and enthalpy changes (ΔH) that were recognized the spontaneous and endothermic nature of the adsorption process.
© 2015 Universal Research Publications. All rights reserved
Key words: Bamboo activated carbon (BAC), Pb2+ions, Adsorption isotherm, Kinetics, Thermodynamics.
1. Introduction
Heavy metals are natural components of the Earth's
crust, and their concentrations in an aquatic environment
have increased due to mining and industrial activities and
geochemical processes. They are toxic or poisonous, if
avail as more than the recommended enough amounts in
water bodies. Heavy metals are common in industrial
applications such as the manufacture of pesticides,
batteries, mining operations, alloys, plating facilities, textile
dyes, tanneries, etc (1). Living organisms require trace
amounts of some heavy metals, e.g., iron, copper, and zinc,
etc., as they are essential to maintain the metabolism of the
human body. But, at higher concentrations of heavy metals
can promote poisoning and other hazardous nature because
they cannot be degraded or destroyed, and tend to bio
accumulate. They pose risks not only to humans but also to
other animals and plants because of their extremely toxic
effects and have been the main reason behind the number
of affliction (2).
Heavy metals become toxic when they are not
metabolized by the body and accumulate in the soft tissues
they can enter the bodies of humans via the food chain,
drinking water, air or absorption through the skin.
Commonly encountered metals include Fe3+, Pb2+, Cu2+,
Zn2+, Co2+ and Ni2+ etc. These metals are toxic in both their
chemically combined forms as well as the elemental form.
These are resulted in heavy metal pollution problems in the
eco-system. Toxic metallic compounds not only
contaminate the water bodies like seas, lake reservoirs and
ponds also enter the underground water in traceable
amounts. Unlike the organic pollutants which are
biodegradable and the heavy metals are not biodegradable
thus making a source of great concern. Exposure of these
contaminants present even in low concentrations in the
environment can prove to be harmful to the human health.
Agricultural development, human health and the eco-
systems are all at risk unless water and land systems are
effectively managed the availability of heavy metals (3).
Nowadays, the important toxic metals with the exponential
increase in population, measures for controlling heavy
metal emissions into the environment are essential. Lead
like heavy metals cause many serious disorders like
anemia, kidney disease, nervous disorders, and even death
(4). At present lead pollution is considered a worldwide
problem because this metal is commonly detected in
several industrial wastewaters.
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International Journal of Water Research
Universal Research Publications. All rights reserved
34 International Journal of Water Research 2015; 5(2): 33-46
In order to solve heavy metal pollution in the
environment, it is important to bring applicable solutions.
Several treatment technologies such as chemical
precipitation, ion exchange, coagulation, bioremediation
and sorption/adsorption are available for the removal of
heavy metal ions from its aqueous solutions (5). The most
commonly known biological method is biosorption using
microorganisms and microbial products. Biosorption is a
passive non-metabolically mediated process of metal
binding by biosorbent. Bacteria, yeasts, fungi, algae and
some higher plants are used as biosorbents for the removal
of heavy metals (6). Among all these techniques adsorption
of heavy metals on solid substrate is preferred because of
its high efficiency, easy handling and cost effectiveness as
well as availability of different adsorbents (7). Activated
carbon (AC) is still the main notable adsorbent for the
removal of pollutants from polluted gaseous and liquid
streams. The challenge in utilizing activated carbon is,
however, to cater to the demands with reasonable costs for
end-users. Activated carbon production costs can be
reduced by either choosing a cheap raw material for
instance using agricultural waste and/or by applying a
proper production method (8). Agriculture waste materials
are inexpensive and available in large quantities, thus they
can be disposed without concerning expensive regeneration
process (9).
In view of the efficiency and the ease processing
of biosorbent from agricultural waste, with which it can be
apply for the treatment of waste water containing heavy
metals. Therefore, the present study had chosen bamboo
stem which is a self-regenerating agricultural products and
application of these bamboo activated carbon adsorbent
offers highly effective technological means in dealing with
the pollution of heavy metals with the requirement of
minimum investment. Bamboo is an agricultural product
and especially highland bamboo species is botanically
known as Yushania alpine was chosen for this study. These
highland bamboos grow naturally in ecological zones of the
country between 2200-3500 meters above sea level. The
coverage of this species in Ethiopia was roughly estimated
about 130,000 hectares in 2005. The present study is
confirmed that locally activated carbon produced from
bamboo stem is a good in adsorbing the Pb2+ ions from its
synthetically prepared aqueous solution. Thus, the optimum
removal condition was determined by using the suitable
adsorption isotherms and by its related constants.
2. Materials and Methods
2.1 Chemicals, reagents and standard solutions
Reagents were used in the present study are
analytical grade (AR) such as zinc chloride
(97%), phosphoric acid (85%) and sodium hydroxide
(97.5%) were obtained from THOMAS BAKER Chemicals
Company. Hydrochloric acid (37%) was obtained from
Scharlab.S.L Company.
2.1.1 Preparation of adsorbent from bamboo stem
Bamboo stem was selected as raw material to
prepare the activated carbon to be use as adsorbent for the
removal of Pb2+ from its aqueous solution. The bamboo
stem was collected from Semen Ari which is located in the
southern Nations, Nationalities and peoples’ Region
(SNNPR), South Omo Zone and which is 589 km south
from Addis Ababa (capital city, Ethiopia, East Africa) and
334 km from Arbaminch location with longitude N-06 10
36.1, E- 036 39 47.3 and altitude of 2678 meters above
sea level. The collected stem was cut into small pieces,
washed thoroughly under running tap water and followed
by washing with double distilled water to remove all the
dust and any adhering impure particles present on it, and
dried under sunlight about three days.
The small pieces of dried bamboo stems were kept
in muffle furnace and carried out carbonization at 500°C on
about two hours to set complete carbonized carbon and
allowed to cool into room temperature. The carbonized
material was crushed and finally sieved by using automatic
sieve shaker D406 with a desired particle size (10) and
stored in desiccators for further use.
2.1.2 Chemical Activation of Carbon produced from
bamboo
The carbonized adsorbent material was weighed
separately and poured in to different beakers containing
orthophosphoric acid. The content of the beakers was
thoroughly mixed until a paste was formed. The pasted
sample was transferred into the crucibles and were placed
in a carbolite furnace and heated at 800°C about two hours.
The activated sample from bamboo stem (BAC) was cooled
at room temperature and supernatant acidic solution was
decanted. It was repeatedly washed with distilled water
until the washing was free from acid (pH is 6-7). Activated
carbon was filtered, dried and again activated under
thermally in a hot air oven at 105°C upon three hours
according to Gimba et.al.,(11). The final product is
grounded well and sieved by using automatic sieve shaker
D406 with different desired particle size and stored in a
glass bottles and kept inside the desiccators for further use
as an adsorbent to remove Pb2+ ions from it aqueous
solution.
2.1.3 Preparation of synthetic feed Pb2+ solution
A stock solution (1000 mg/L) of Pb2+ was
prepared by dissolving 1.3557g of PbCl2 in 1000 ml
volumetric flask using double distilled water and it is
diluted as required for batch adsorption and other
experimental studies. It is used as synthetic effluent called
adsorbate; a fresh solution of heavy metallic effluent was
prepared for every trial, and utilized completely for the
entire set of experiments.
2.2 Analytical methods and Instrument were used
2.2.1 Instruments
Studies were undergone about the removal
efficiency of an adsorbent involves by determination of
amount of Pb2+ ions in the effluent solution before and after
adsorption takes place. This was done by using Atomic
Absorption Spectroscopy (AAS) BUCK SCIENTIFIC
MODEL 210 VGP East Norwalk, USA. It is equipped with
deuterium arc background, nebulizer and hallow cathode
lamp corresponding to metal of interest, and air-acetylene
flame was used.
The pH of different solutions was measured using
pH meter (JENWAY PH meter 3310). Magnetic stirrer with
hot plate (Model 04803-02, Cole-Parmer Instrument,
U.S.A.) was used for stirring the mixture of adsorbent and
35 International Journal of Water Research 2015; 5(2): 33-46
metal ion solution at known time intervals. Then, sample
solutions were aspirated in to the AAS instrument and
direct readings of total metal ion concentrations were
recorded by triplicate measurements on each sample. The
amount of metal ion adsorbed was calculated from the
difference between the amount of adsorption before and
after a certain period of time.
2.2.2 Methods
2.2.2.1 Characterization of adsorbent, BAC
The physico-chemical characterization such as pH,
surface area, bulk density, ash content, moisture content,
volatile content and fixed carbon of the adsorbent was
evaluated according to the literatures (12-14). The pH of
RHAC was determined taking about 0.5 g of adsorbent into
20 ml of distilled water and the resulting suspension
mixture was stirred at 300 rpm for 24 hrs. Thus, the
solution was filtered and pH of the filtrate was measured
using JENWAY pH meter 3310. Proximate analysis was
carried out using thermogravimetric analyzer (Perkin Elmer
TGA7, USA) and elemental analysis was performed using
Elemental Analyzer (Perkin Elmer series II 2400).
2.2.2.2 Batch adsorption experiments
Batch experiments for the removal of Pb2+ was
conducted in 250 mL Erlenmeyer flasks by taking 50 mL of
three different (30, 60 and 90 mg/L) Pb2+ solutions. The
experiments were carried out at room temperature by
shaking a mixture of 1g BAC powder introduced into the
metal ion solution containing flask with agitation rate of
200 rpm about 2 hours until equilibrium was reached. After
agitation, the residual adsorbent was removed by filtration
using filter paper. The experiment was conducted with
duplicate under the same conditions and the average results
were taken and recorded.
The concentration of metal ion in the filtrates as
well as in the control samples were determined by using
atomic absorption spectroscopy (AAS) spectrometer. The
effect of adsorbent dose (4 - 40 g/L), contact time (5 - 240
minutes), feed solution pH (2 - 9), initial concentration of
the metal ion (20 – 100 mg/L) and the particle size (150 –
425 m and 1.18 mm) of BAC were investigated by
varying any one of the parameters and keeping the other
parameters as constant. In addition to this, thermodynamic
study was also conducted by varying the temperatures
(about 298, 308 and 313 K). The solution pH was adjusted
to the desired value by drop wise addition of hydrochloric
acid (HCl) or sodium hydroxide (NaOH) solution and the
filtrates were analyzed for the influence of pH on metal ion
adsorption.
The experiments were performed in duplicate and
the average result is reported. The uptake of
metal ion was calculated using the equation:
Uptake (%) = x 100
Where = Initial concentration of metal ion (mg/l)
= Concentration of metal ion at equilibrium state
(mg/l)
2.2.2.3 Kinetic study of sorption
Kinetics of adsorption was determined by
analyzing sorptive uptake of the Pb2+ ion from an aqueous
solution at different time intervals. For the determination of
sorption isotherms, metal ion solution of different
concentrations was agitated with known amount of sorbents
till the equilibrium was achieved at room temperature.
Adsorption kinetic experiments (true and pseudo
order kinetics) were performed at pH 5 for Pb2+ with initial
concentrations of 30 - 90 mg/L solutions with their
respective optimum adsorbent doses. Then the residual
metal concentrations were measured at different time
intervals by taking samples periodically.
2.2.2.4 Study of sorption isotherm
The study of adsorption isotherm has been a
greater importance in water and wastewater treatment by
the batch absorption technique, as they provide an
approximate estimate of the monolayer adsorption capacity
of adsorbent. The equilibrium isotherm was determined by
using different amount of adsorbent ranged 0.2 - 2.0 g of
mixed with 50 ml of 30-90 mg/L concentration of Pb2+
solution. This mixture is agitated with the 200 rpm speed
for 4 hours, which was sufficient to reach equilibrium. The
amount of metal ion adsorbed at equilibrium (qe) was
calculated as:
Where, V = volume of solution (L)
m = mass of adsorbent (g)
The equilibrium data for the removal of Pb2+ ions
on the adsorbent at room temperature were estimated
through testing the Langmuir, Freundlich and Temkin
isotherms.
2.2.2.5 Thermodynamic Study
The effect of temperature on the sorption
characteristics was investigated by taking 30 mg/L and 60
mg/l initial concentrations of Pb2+ solution and 2 g/L of
adsorbent dose, at temperatures were ranged from 298 up to
318 K. Increase in temperature does affect the solubility
and chemical potential of the sorbate, which can be a
controlling factor for sorption. The dependence on
temperature of sorption of Pb2+ ions on BAC were
evaluated using the equation:
Kc =
ln = -
= - T S
Where, , Δ , and T are the enthalpy, entropy,
Gibbs free energy change and temperature, respectively, R
is the gas constant (8.314 J.mol-1.k -1) and is the
adsorption coefficient obtained from Langmuir equation. It
is equal to the ratio of the amount adsorbed (x/m in mg/g)
to the adsorptive concentration in (mg/l). These parameters
can be obtained from experiments at various temperatures
using the above equations. The values of H and S are
determined from the slope and intercept of the linear plots
of lnKc versus 1/T (15). In general these parameters
indicate that the adsorption process is spontaneous or not
and exothermic or endothermic.
3. Results and discussion
The present study deals with the removal of lead ion by
adsorption on a low cost adsorbent as activated carbon
36 International Journal of Water Research 2015; 5(2): 33-46
prepared from bamboo stem agricultural waste material.
This is a well-known non-conventional material, which
could be employed as an alternative to commercial
activated carbon for water and wastewater treatment. This
is an endeavor to present data for the design of
economical wastewater treatment plant for the removal of
metal/metal ions were discharged as effluent from the metal
finishing industries.
The experimental parameters, which affect the
extent of adsorption of pb2+ ion, are reported. The effect of
the initial concentration of dye, contact time, dose of
adsorbent and pH of the aqueous solution on the removal of
Pb2+ by adsorption onto BAC was studied in the present
investigation. The various experimental conditions and the
related results for the adsorption studies are reported for the
discussion about the study.
3.1 Characterization of Adsorbent, BAC
The chemical composition, ultimate and proximate
analysis of BAC used in the present study was carried out
and the data were presented in table.1. Results show good
agreement with the earlier reported literatures to work with
the precursors of BAC for removal of Pb2+ from its own
aqueous solution. The pH of BAC solution was found to be
6.4 (shown in table.1), which is higher than 6.0 and 4.6
obtained from RH with H3PO4 and FeCl3-H3PO4 activated
carbons (16, 17), respectively. The BAC of this present
investigation was found to have lower volatile content and
higher carbon content, indicating the suitability of BAC for
the precursor for the treatment of metallic effluents. The
results were shown in good agreement with the reported
literatures (16 - 19).
Table.1: Physico-chemical Characteristics of activated
carbon derived from bamboo stem
BAC/H3PO4
Parameter Value
pH 6.4
Surface area, (m2/g) 807
Bulk density (g/cm3 ) 0.65
Ash content (%) 5.55
Moisture content (%) 7.6
Volatile content (%) 24.4
Fixed carbon (%) 62.45
3.2 Batch Adsorption Studies
3.2.1. Effect of pH on the sorption of Pb2+on BAC
The effect of pH on the removal of lead was
studied, and it is revealed that the solution pH does affect
the amount of lead adsorption. The lead uptake was found
to be increase with increasing pH, and also shows the
removal efficiency does increasing rapidly upon pH starts
from 2 to 3 (shown in fig.1). Three different solutions (30 -
90 mg/L) of Pb2+ were studied, the maximum removal of
lead appeared at pH 5 in all three solutions. Therefore,
pH 5 was selected as optimum pH for further studies for the
removal of Pb2+ from it aqueous solution.
The increase in metal removal was observed as pH
increases. This may be due to decrease in competition
between hydronium ions and metal ions for the surface
sites. This is also by the decrease in positive surface charge
on the adsorbent, which resulted in a lower electrostatic
repulsion between the surface and the metal ions and hence
uptake of metal ions get increased. A similar theory was
proposed earlier (20) for metal adsorption on different
adsorbent. It is also supported in an alkaline medium lead
ions tend to hydrolyze and precipitate instead of adsorption
on adsorbent. It was deteriorated with accumulation of
metal ions, and making impossible true adsorption.
Fig.1 Effect of pH on sorption of Pb2+ onto BAC at 298 K;
[BAC]-10g/L, Time-120 min.
3.2.2. Effects of contact time of Pb2+ adsorption on BAC
Effect of contact time on the removal of lead is
illustrated in figure 2. It shows that the removal of lead
increased with contact time and it was rapid at initial up to
30 minutes, and then it proceeds at slower rate of increases
and finally attained saturation. This behavior suggests that
at the initial stage adsorption was takes place rapidly on the
external surface of the adsorbent followed by a slower
internal diffusion process, which may be the rate-
determining step. The trend in adsorption of Pb2+suggests
that the binding may be through the interactions with the
functional groups located on the surface of the carbon.
Equilibrium adsorption was established at 60 minutes (for
30 mg/L) and 120 minutes for other studied concentrations
of Pb2+ ion solutions respectively. It is clearly shows that
the maximum contact time is required for greater uptake of
metal ions by BAC is depends on the initial concentrations
of metal ion. It is evident that the contact time was fixed at
120 min for the batch experiments to make sure that
equilibrium was attained. Thus, the % removal for 30, 60
and 90 mg/L of lead ions upon contact time 120 min. were
96.07 %, 90.2 % and 85.16 %, respectively.
The results were demonstrated that at a fixed
adsorbent dosage, the amount of adsorbate increased with
increasing concentration of Pb2+solution, but the percentage
of adsorption was decreased. This is due to at lower
concentrations, the ratio of number of metal ions to the
available adsorption sites is almost fulfilled and
subsequently the adsorption becomes greater. But, at higher
concentrations of metal ions, however, the available sites
on BAC for adsorption become fewer and subsequently the
37 International Journal of Water Research 2015; 5(2): 33-46
Fig.2 Effect of contact time on sorption of Pb2+ onto BAC at 298 K; [BAC]-10 g/L, pH - 5
removal of lead depends on the concentrations of Pb2+ and
decreases with increase in initial Pb2+ concentration was
good agreement with reported (21).
3.2.3. Effect of adsorbent dosage onPb2+
As it can be seen from Figure 3, adsorption of Pb2+
increased from 32.93 % to 97.33 % with increasing
adsorbent dose from 4 g/L to 40 g/L, respectively. This is
because for a fixed initial metal concentration, while
increasing the adsorbent dose provides a greater adsorption
sites. On the other hand, the plot of capacity (metal uptake
per adsorbent unit) versus adsorbent dose revealed that the
capacity was high at low doses and low at greater dose of
adsorbent, which shows increase in adsorption with the
growth of adsorbent. Similar results were reported
(22) in adsorption of Cu2+ and Pb2+ using sawdust and clay
as adsorbent respectively.
This result can be attributed to the fact that some
of the adsorption sites remain unsaturated after the
adsorption process. It might be because of formation of
particle aggregation, resulting in a decrease in the total
surface area and an increase in diffusion path length, which
contribute to decrease in amount adsorbed per unit mass.
Studies were indicating that the efficiency of (hydroxide)
oxides to adsorb heavy metal ions is due to their high
surface/mass ratio (23). Even if the up-take of the metal
increased by increasing the adsorbent dose, beyond a dose
of 20 g/L of BAC, and the rise of the adsorption efficiency
is insignificant and the capacity of adsorbent is very low.
Therefore, further increase in the dose results the much
production of sludge and wastage of material. Thus, 20 g/L
of adsorbent dose was taken as an optimum dose for further
experiments.
Fig.3 Capacity and removal efficiency of Pb2+ at different
adsorbent dose
3.2.4. Effect of initial concentration of Pb2+ adsorption
on BAC
The initial metal ion concentration provides an
important driving force to overcome all mass transfer
resistances of metal ion between aqueous and solid phases.
The removal efficiency at a fixed adsorbent dose on the
effect of initial concentration of Pb2+ is depicted in figure 4.
The capacity of the adsorbent increased significantly even
though there is slight decrease in the adsorption efficiency
with the increment of initial concentration. The increase of
capacity can be due to increment of driving force that is
concentration gradient, which causes an increase in the
number of metal ions coming in contact with the adsorbent.
On the other hand, the number of available adsorption sites
in adsorbent is the same for all initial concentrations; thus,
38 International Journal of Water Research 2015; 5(2): 33-46
Fig.4 Percent removal of and Pb2+ as a function of initial concentration
the initial concentration increases with more number of
ions and the same change to be adsorbed and competes the
same adsorption sites. This may cause to left many ions
without being adsorbed and to decrease the efficiency of
the removal upon increases the concentration of Pb2+ ions.
3.2.5 Effect of particle size of BAC on the adsorption of
Pb2+
Particle size of adsorbent is an important factor
that affecting the adsorption capacity as it influences the
surface area of adsorbent. The effect of particle size on the
adsorption of Pb2+ ions was investigated in the range of 150
μm - 1.18 mm. Figure 5 shows that the variation of Pb2+
uptake with time of different particle size of adsorbent. The
results were indicated that increases the uptake of Pb2+ ion
with lower particle size. The higher uptake with in lower
particle size was attributed to the fact that smaller particles
had larger external surface area compared to larger
particles, hence more binding sites were exposed on the
surface and thus, leading to higher adsorption capacity
since adsorption is a surface process. Apart from that,
particles with smaller size also moved faster in the solution
compared to larger particles, consequently the adsorption
rate was faster.
Utilized activated carbon prepared from bamboo
waste for the removal of studied metal ion and investigated
the uptake of Pb2+ ions at different size of particles (150 μm
- 1.18 mm). The results were found that the % removal was
increased (shown in figure 5) as in the lower particle size
upon 30 mg/L metal ion solution. According to Sekar et.al.,
(24), larger particles that resist the diffusion to mass
transport and most of the internal surface of the particle
might not be utilized for adsorption, hence the smaller
amount of metal ions were adsorbed.
Fig.5 Effect of particle size on uptake of Pb2+ ions by BAC
3.3 Equilibrium sorption study
Sorption studies describe the interaction of
adsorbates with adsorbent, and established equilibrium
between adsorbed metal ions and the residual metal ions in
solution during the surface sorption. The interaction
between adsorbate and adsorbent is characterized using
adsorption isotherm models (25). Adsorption isotherms are
mathematical models that describe the distribution of
adsorbate species among liquid and adsorbent. Based on a
set of assumptions, that is mainly related to the
heterogeneity or homogeneity of adsorbents, type of
coverage and possibility of interaction between adsorbate
species (26).
39 International Journal of Water Research 2015; 5(2): 33-46
The adsorption equilibrium data is obtained at a
fixed initial concentration and varying adsorbent dose have
been fit into the linearized Langmuir, Freundlich and
Temkin adsorption isotherms.
3.3.1 Langmuir isotherm model (27)
The linear Langmuir isotherm model was expressed
mathematically as:
------------ (1)
= + . ------------- (2)
Where;
- Concentration of metal ions at equilibrium
(mg/L)
- Amount of metal ions adsorbed at equilibrium
(mg/g)
- Langmuir isotherm constant related to free
energy of adsorption (L/mg)
- Maximum adsorption capacity (mg/g)
In this current study, the plot of 1/qe against 1/Ce
gives straight line (seefig.6) with a slope of 1/qmKL and
intercept of 1/qm. Figure.6 shows the Langmuir plot of Pb2+
adsorption on BAC with a correlation coefficient of 0.8996
respectively, which is ≥ 0.828, indicates that the data are
fitted in Langmuir isotherm. Thus, values obtained by
linear regression correlation coefficient (R2) for Langmuir
suggests that monolayer sorption may exist under that
experimental condition as well. According to the Langmuir
equation the maximum uptake capacity (qm) of Pb2+ ions
have 3.30 mg/g (shown in table.2). The Langmuir
parameters were also used to predict the affinity of the
adsorbent (BAC) surfaces towards the metal ions by using
dimensionless constant called equilibrium parameter, RL,
which is expressed according to the literature (28). The
shape of isotherm is described in terms of RL is shown in
table below.
The RL value (0.055) is obtained in the range of 0 and 1,
which indicates a favorable isotherm shape for adsorption
of Pb2+ ions on BAC. The adsorption capacity (qm - 3.30
mg/g) obtained in this experiment is in agreement with the
results were reported (29) in the range of 2.00 - 16 mg/g.
Fig.6 Langmuir isotherm plot for the sorption of Pb2+ ions
4.3.2 Freundlich isotherm model
The Freundlich isotherm assumes a heterogeneous
surface with a non-uniform distribution of heat of
biosorption over the surface and a multilayer biosorption
can be expressed according to Freudlich.M. (30) model as:
= --------------- (4)
Where;
- Freundlich indicative of relative adsorption capacity
of adsorbent
n - Freundlich indicative of the intensity of adsorption
Equation 4 could be linearized by taking logarithms as
followed:
log = log + log ------------------ (5)
The plot of log qe against log Ce gives a straight
line with slope of 1/n and intercept of log KF. This
Freundlich type behavior is indicative of the surface
heterogeneity of the adsorbents, i.e. the adsorptive sites or
surface of the studied adsorbents are made up of small
heterogeneous adsorption patches that are homogeneous in
themselves (31).
40 International Journal of Water Research 2015; 5(2): 33-46
Figure.7 shows the Freundlich isotherm plot of Pb2+ ions
adsorption on BAC with a correlation coefficient of 0.935.
The greater values of R2 (over than Langmuir isotherm
model) indicate the adsorption is favorable for a Freundlich
isotherm. The value of Freundlich constant, KF and n
obtained for Pb2+ from the plot were 1.468 and 1.485
respectively. It is also noted that the value of 1/n (0.6732)
was between 0 and 1 indicating that the sorption of metal
ions into the studied adsorbents was favorable. Thus, the
results of KF values indicate that the BAC surface is
heterogeneous in the long range, but may have short range
of uniformity. Also, the ‘n’ values lying in the range of 1 to
10, reveals the favorability of sorption (n 1) of all Pb2+
ions according to Chen, et al., (32).
Fig.7 Freundlich isotherm plot for the sorption of Pb2+ions
on BAC
4.3.3 Temkin Isotherm
This isotherm clearly takes into account the
interactions between adsorbing species and the adsorbate. It
assumes that (i) the heat of adsorption of all the molecules
in the layer decreases linearly with coverage due to
adsorbate–adsorbent interactions, and (ii) adsorption is
characterized by a uniform distribution of binding energies
up to some maximum binding energy (32). The Temkin
isotherm has been used in the form as follows:
= ln (KT ) ------------------------- (6)
The linearized form of the above equation has the
following form, which can be plotted as qe against ln Ce to
determine the isotherm constants BT, and KT from slops and
intercepts (fig. 8), respectively.
= ln KT + ln Ce --------------- (7)
BT = ----------------------------- (8)
= BTlnKT + BT ln ------------------- (9)
Where, BT and bT are Temkin constants
KT is Temkin adsorption potential (L/g)
Fig.8 Temkin isotherm plot for the sorption of Pb2+ ions on
BAC
The isotherm constants and correlation
coefficients for all three isotherms of studied metal ion
adsorption are presented in table 2:
Table 2 Isotherm model constants and correlation
coefficients for adsorption of Pb2+ ions on BAC
The results shown in table.2 revealed that the
Freundlich isotherm model achieved best fit with the
equilibrium adsorption data, which have highest correlation
coefficient value (R2) of Pb2+ is 0.9350. It indicates the
multilayer adsorption nature of this metal ion takes place
on BAC. The adsorption capacity (KF) of the adsorbent of
Pb2+ had a value of 0.68 mg/g respectively.
3.4 Adsorption kinetic study
The study of adsorption kinetics in wastewater
treatment is important as it not only provides valuable
insight into the reaction pathways and the mechanism of
sorption reactions, but also describes the solute uptake rate,
which in turn control the residual time of sorbate uptake at
the solid-solution interface (33).
The kinetic data was obtained from the adsorption
of Pb2+ ions on BAC. This was studied by includes the
common kinetics such as Zero, first, second, third order,
pseudo-first order, pseudo-second order and Intraparticle
diffusion models. The best fit model was selected based on
the linear regression correlation coefficient (R2). The R2
values of zero, first, second and third order kinetics does
not come under recommended (shown in table.3). So, the
corresponding kinetic adsorption plots are not shown.
3.4.1 Pseudo-first order kinetic model
The pseudo-first order kinetic model assumes that
the rate of occupation of sorption sites is proportional to the
number of unoccupied sites. The pseudo-first order
equation was expressed according to Lagergren (34).
= ( - ) --------- (10)
Where - amount of metal ions adsorbed at equilibrium
(mg/g)
- amount of metal ions adsorbed at time t
(mg/g)
41 International Journal of Water Research 2015; 5(2): 33-46
- pseudo first order rate constant ( )
The equation applicable to experimental results
are generally differs from a true first order equation in two
ways: the parameter k1(qe − qt) does not represent the
number of available sites, and the parameter log qe is an
adjustable parameter which is often not found in equal to
the intercept of a plot of log (qe − qt) against t, whereas in a
true first-order sorption reaction, log qe should be equal to
the intercept of log(qe − qt) against t. In order to fit equation
10 to the experimental data, the equilibrium sorption
capacity qe must be known.
The pseudo first order kinetics of Pb2+ was studied
for different concentrations of Pb2+, results were obtained
and presented in figure 9.
Fig.9 Pseudo first order plots of Pb2+ sorption on BAC
Figure 9 showed the linear plots of log (qe-qt) against t at initial
metal ion concentration of 30 mg/L, 60 mg/L and 90 mg/L.
The k1 and values were determined from the slope and
intercept of the linear plots respectively and given in table 3.
Table 3 reveals the values of , experimental
and calculated values of qe, as well as the R2 values for the
pseudo-first order kinetic plots. As can be seen, also the R2
values obtained from the plots were high. The calculated
values of qe were far lower than the corresponding
experimental data obtained. This suggested that a poor fit
between the kinetics data and the pseudo-first order model.
Theses results were confirmed that this adsorption system
is not follows a pseudo first order reaction.
3.4.2 Pseudo-second order kinetic model
The adsorption kinetics may also be described by
a pseudo second order. The pseudo second order is based
on the assumption that the rate limiting step may be
chemical sorption involving valence forces through sharing
or exchange of electrons between heavy metal ions and
adsorbent. The pseudo-second order kinetic rate equation
was used in this study according to Gupta, et al., (35).
= + t ------------------- (11)
Where h = k2qe2 (mg/g min) is the initial sorption rate.
The pseudo second order kinetic model was
studied with different concentrations and the results were
described in figure 10. The figure does give the linear plots
of t/qt against t at all studied concentrations of Pb2+
solutions. The values of qe and h were calculated from the
slope and intercept of the respective plots, and also
calculated the k2 for each plots are presented in table.3.
Fig.10 Pseudo second order plots of Pb2+ sorption on BAC
Table 3 reveals that all three linear plots with
different initial concentrations of Pb2+ (shown in fig.10)
have R2 values of 1. This indicates that the kinetics data
fitted perfectly well with the pseudo second order model. In
addition to the high values of R2, the calculated qe values
also almost agreed well with the experimental data obtained
from the pseudo second order kinetics.
From table 3 also observed that the values of ‘h’
increased from 7.64 to 18.52 when the initial concentration
of Pb2+ ions increased from 30 mg/L to 90 mg/L
respectively. This was because the higher the initial
concentration of Pb2+ ions, the greater chances of collision
with the binding sites of adsorbent and hence leads to a
higher initial sorption rate. The values of k2 was observed
as higher than the corresponding values of k1. So, the
pseudo second order kinetic model assumed as the best fit
for this adsorption studies and also the sorption rate is
proportional to the square of number of unoccupied sites
42 International Journal of Water Research 2015; 5(2): 33-46
(36). In addition, the values of k2 get decreases from
1.02g/mg min to 0.28 g/mg min with increasing the initial
concentration of Pb2+. This is occurred because at higher
concentration of metal ions, the competition for surface
active sites was high and consequently lower sorption rates
are obtained. The similar result was reported earlier (37).
The pseudo-second order kinetic model was also
reported to fit well with the kinetics data from studies of a
number of authors, including the adsorption of Cd2+ ions on
pomelo peel (37), adsorption of Cu2+ ions on Tectona
grandis leaves (38), adsorption of Pb2+ ions on pumpkin
seed shell activated carbon (39), adsorption of Ni2+ions on
potato peel, and adsorption of Cr6+ ions on cooked tea dust
(40).
Table 3 The adsorption rate constant and correlation coefficient (R2) of adsorption kinetics model in zero, 1st, 2nd, and 3rd
orders and pseudo first and second orders at different [Pb2+] at constant pH - 6, and and 40 g/l of adsorbent dose at 298K.
Initial
conc.
(mg/l)
Zero-order First-order Second-order Third-order
K
30 0.0048 0.4916 0.0015 0.5134 4.9130 0.5331 3.1902 0.5502
60 0.0086 0.4548 0.0012 0.5021 1.8477 0.5443 5.5542 0.5910
90 0.0013 0.5861 0.0011 0.6323 1.1078 0.6760 2.0839 0.7233
Pseudo first-order Pseudo second – order
K1
(min-1)
qe (mg/g) R2
K2
(g/mg)
qe (mg/g) h
(mg/mg)
R2
Exp. Calc. Exp. Calc.
30 0.0234 2.736 0.990 0.99 1.02 2.736 2.741 7.6394 1
60 0.016 5.418 0.828 0.8279 0.4665 5.418 5.426 13.7363 1
90 0.0106 8.055 0.971 0.9713 0.2843 8.055 8.071 18.5185 1
3.4.3 Intra-particle Diffusion
Adsorption is a surface phenomenon, but the
adsorbate may also diffuse into the interior pores of the
adsorbent, which may influence the rate of the reaction.
Thus the result also analyzed in terms of intraparticle
diffusion model to investigate whether the intraparticle
diffusion is the rate controlling step or not in adsorption of
lead ion on bamboo activated carbon. According to
Vadivelan and Kumar (41), sorption mechanisms
between solid-liquid solution systems follow certain stages:
movement of solutes to the exterior surface of adsorbent
which implies boundary surface diffusion (external mass
transfer or film diffusion) that the movement of solute from
external surface of the adsorbent which is intraparticle
diffusion.
The amount of metal ion sorbed per unit mass of
adsorbents, qt at any time t, was plotted as a function of
square root of time, t 1/2. The diffusion model can be
expressed by following equation.
= Ø + √t ----------- (12)
Where, qt is the amount adsorbed (mg/g) at time t and kip
(mg/g min1/2) is the intra-particle diffusion rate constant
which was obtained using the equation and Ø is
intraparticle diffusion constant, i.e. intercept of the line
(mg/g). If plot of qt versus t 1/2 gives a straight line that pass
through the origin, then it suggests that the intra-particle
diffusion contributes predominantly in the rate-determining
step (42).
Figure 11 depicts the linearity of plot between qt
(amount adsorbed) vs. time, t1/2 that does not pass through
the origin. The values obtained from the intercept (5.19 –
7.623) of the intraparticle diffusion kinetic model at
different adsorbate concentrations of Pb2+ are not the same
and did not pass through the origin which indicates the
intra-particle diffusion is not the rate controlling step. It
implied that the adsorption process of Pb2+ was controlled
by only a film diffusion. Also the value of the intercept at
the different concentrations gives an idea about the
thickness of the boundary layer. If the intercept become
larger confirms the thicker the boundary layer (41).
However, some factors have been attributed to be
responsible for the rate determining step of adsorption of
particular adsorbate. The factors that assign rate
determining step mechanism as film diffusion or external
transport mechanism have been reported to be poor mixing,
small particle size, and dilute concentration of the
adsorbate and high affinity of the adsorbate for the
adsorbent. The factors that include good mixing, large
particle size, high concentration of adsorbate and low
affinity of adsorbate for adsorbent assigned intraparticle
diffusion mechanism as the rate determining step (43).
Consequently the prepared bamboo activated carbon has
high affinity for the metal ion and as such followed film
diffusion mechanism.
Fig.11 Plot of Qt versus (time)1/2 for the adsorption of Pb2+
on BAC
43 International Journal of Water Research 2015; 5(2): 33-46
Table 4 Kinetics Parameter of intra-particle diffusion models for Pb2+ sorption on BAC.
Initial con.(mg/l) Kip (g/mg/min1/2) Ø (mg/g)
30 0.0173 5.1931 0.6355
60 0.0173 5.1931 0.6355
90 0.0254 7.7226 0.7625
The relatively higher initial rates K2 (shown in table 3)
with the large intercepts (Ø in mg/g) of the linearized intra-
particle plots (shown in table 4) which are almost the same
with the (exp), suggest that the process was larger
surface adsorption (22). It is also possible to suggest that
from the parameters, Kip and Ø values. Moreover, it can be
observed that the linearity of intra-particle diffusion could
not be applicable for the whole time interval of the
adsorption process and also is higher than Kip, which
may indicate that the overall adsorption process can be
represented better by pseudo second order.
3.5. Thermodynamics study of adsorption of Pb (II)
onto BAC
Thermodynamic parameters G, H and S can
be obtained from the studies of Pb2+ adsorption from
aqueous solution of BAC at various temperatures (at 298 –
318K) using recommended empirical equations. The values
of H and S are determined (15) from the slope and
intercept of the linear plots of lnKc versus 1/T shown in
fig.12.
In general these parameters indicate that the
adsorption process is spontaneous or not and exothermic or
endothermic. The standard enthalpy change (Hº) for the
adsorption process is:
(i) Positive value indicates that the process is
endothermic in nature.
(ii) Negative value indicates that the process is
exothermic in nature and a given amount of heat is evolved
during the binding metal ion on the surface of adsorbent.
This could be obtained from the plot of percent of
adsorption (Efficiency %) vs. Temperature (T) present in
fig.13. It shows that the percent adsorption increase with
increase temperature; this indicates for the endothermic
processes and the opposite is correct (15). The positive
value of (Sº) indicate an increase in the degree of freedom
(or disorder) of the adsorbed species. This can be also seen
from the positive value of ΔH0 for metal ion adsorption,
that is the endothermic nature of the adsorption according
to the calculated data presented in table 6 for a given
temperature range. This result is in agreement with the
findings of other researchers for Cu2+ adsorption on
surfactant modified montmorillonite, and for lead on
kaolinite, montmorillonite and Celtek clay (44).
Fig.11 Vant’Hoff plot of lnKc versus 1/T at temperatures range of 298-318 K
44 International Journal of Water Research 2015; 5(2): 33-46
Fig. 13 Effect of temperature on the adsorption efficiency of Pb2+
Table.5 Thermodynamic parameters that are computed
from the linearized plot of ln kc versus 1/T at the
temperatures range of 298-318 K. Metal
ion
Temperature
(K) G
(KJ/mol )
H
(kJ/mol)
S
(J/k.mol )
Pb2+
298 -1.1322
91.963
306.37 308 -1.9358
318 -7.1146
From the entropy value of metal ion adsorption
(see table 5) could be observed that the metal ions in
aqueous solution were in more stable arrangement, since
stability is associated with an ordered distribution than
those in the adsorbed state. So the rise in temperature
might have positive contribution to enhance the
adsorption efficiency by causing the increased collision
between the metal ions and the surface sites (44).
The result shown table 5 is in an acceptable
range of H, indicates the favorability of physisorption. It
is very clear that from the results, physisorption is much
more possible for the adsorption of lead ion. The positive
values of H also indicate the endothermic nature of
adsorption. The negative value of ΔG indicates the
feasibility and spontaneous nature of the adsorption
process and more negative which indicates that the
adsorption process becomes more spontaneous with rise
in temperature, which favors the adsorption process. In
other words that the adsorption process is spontaneous
and the degree of spontaneity increases with increasing
the temperature (45). The value of ΔS can be used to
describe the randomness during adsorption process; the
positive value of ΔS reflected the affinity of the adsorbent
for particular heavy metal ions and confirms the increased
randomness at the solid–solution interface during
adsorption (45).
4. Conclusions
The isotherm, Kinetics and thermodynamics of
batch adsorption of Pb2+ ions from it aqueous solution
using activated carbon prepared from South Ethiopian
based bamboo has been investigated and drawn following
conclusions:
Adsorption capacity of adsorbate had seen to decrease
with increasing adsorbent dose while the efficiencies
increased. In addition, a decrease in efficiency of
adsorbent was observed with increasing initial metal ion
concentration.
The adsorption process follows Langmuir, Freundlich
and Temkin isotherms but a better sorption fit of Pb2+
ions by bamboo activated carbon (BAC) using
Freundlich isotherm model was obtained. It indicates a
multilayer formation over a surface of the material with
the correlation coefficient of (R2) of 0.935 and the
maximum adsorption capacity determined is 0.686 mg
of Pb2+ ions adsorbed per g of BAC was obtained.
Adsorption kinetics was modeled using true and pseudo
order kinetics and intra-particle diffusion models. The
kinetic data obtained from this study fitted well with the
pseudo-second order model. Also the sorption profiles
derived based on the pseudo second order kinetic model
showed a good agreement with the experimental curves
and the pseudo second order kinetic reaction is the rate
controlling step with some intra particle diffusion taking
place.
The determined negative free energy changes (ΔG) and
positive entropy change (ΔS) indicate the feasibility and
spontaneous nature of the adsorption process. The
positive value of enthalpy change (ΔH) suggests that the
adsorption process was an endothermic. Finally
activated carbon produced from bamboo demonstrated
that they are a promising adsorbent derived from
45 International Journal of Water Research 2015; 5(2): 33-46
agricultural waste material used for the removal of
heavy metal ions like Pb2+ from an aqueous solution.
Acknowledgment The author Ramesh Duraisamy express thanks to
his wife Mrs.V.Vidya Ramesh, for her constant
encouragement and heartfelt moral support for making
the research paper.
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Source of support: Nil; Conflict of interest: None declared