removal of lead ion from aqueous solution by bamboo activated

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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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



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



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



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,



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).



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).



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)



- 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



(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



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



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



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


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