Equilibrium Study on the Adsorption of Zn(II) and Pb(II) Ions from Aqueous Solution onto Vitex...

Copyright © 2012 by Modern Scientific Press Company, Florida, USA

International Journal of Modern Chemistry, 2012, 3(2): 82-97

International Journal of Modern Chemistry

Journal homepage: www.ModernScientificPress.com/Journals/IJMChem.aspx

ISSN: 2165-0128

Florida, USA

Article

Equilibrium Study on the Adsorption of Zn(II) and Pb(II) Ions

from Aqueous Solution onto Vitex doniana Nut

Paul Ocheje Ameh 1,

*, Raphael Odoh 2, Adedirin Oluwaseye

3

1 Department of Chemistry, Ahmadu Bello University, Zaria Kaduna State, Nigeria

2 Department of Chemistry, Federal University, Wukari Taraba State, Nigeria

3 Sheda Science and Technology Complex, Abuja, Nigeria

* Author to whom correspondence should be addressed; E-Mail: [email protected].

Article history: Received 21 October 2012, Received in revised form 22 November 2012, Accepted 26

November 2012, Published 28 November 2012.

Abstract: The adsorption of Zn(II) and Pb(II) ions from aqueous solution by carbon

prepared from Vitex doniana nut was studied at varying metal ion concentrations,

adsorbent dose, contact time, temperature and pH. Batch adsorption studies were used. It

was found that the capacity of adsorption depends on pH value. The uptake of zinc and

lead increased from 0.4869 to 0.5994 mg/g and 0.5361 to 0.6450 mg/g, respectively, when

the solution’s pH value was increased from 2 to 8 and thereafter decreases. The highest

percentage of metal removal was achieved in the adsorbent dosage of 0.4 g and at an initial

concentration of 100 ppm metal ion. The removal percentage was found to be higher for

Pb(II) when compared with Zn(II). The three most common adsorption equations, Temkin,

Freundlich and Langmuir adsorption isotherms, were used in the study to verify the

adsorption performance. From interpretation of the equations, the Langmuir adsorption

isotherm was found to fit the experimental data better than the other two. This suggests the

formation of monolayer of Zn(II) and Pb(II) ions onto the outer surface of the adsorbents.

Vitex doniana nut can be considered as potential adsorbents for Zn(II) and Pb(II) ions from

dilute aqueous solutions.

Keywords: activated carbon; heavy metal; adsorption; adsorption isotherm; Vitex doniana.

1. Introduction

The problems of the ecosystem are increasing with developing technology. Heavy metal

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pollution is one of the main problems. This pollution by heavy metals is mainly caused by industrial

and agricultural processes. Removal of heavy metals from wastewater is usually achieved by physical

and chemical processes which include precipitation, coagulation, reduction, membrane processes, ion

exchange and adsorption [1]. Activated carbon (AC) is widely used as an adsorbent for the removal of

these heavy metals due to its high adsorption capacity. This capacity is related to the pore structure and

chemical nature of the carbon surface in connection with preparation conditions [2].

Literature survey generally indicated that there have been many attempt to obtain low cost AC

or adsorbent from agro-waste. These, according to Zabaniotou and Ioannidou [3] include the

comprehensive studies carried out on wheat, corn straw, olive stones, bagasse, sunflower shell,

pinecone, and rapeseed. Adsorption of Zn(II) ions from aqueous solutions was studied by Eddy and

Odoemelam in a batch system using tiger nut shells [4]. The optimum condition for the adsorption of

Zn(II) ions from aqueous solution by these shells was investigated by considering the extent of

adsorption with respect to contact time, initial metal ion concentration, particle size of the adsorbent

and modification of the adsorbent with H2SO4. They found out that the extent of metal ions removed

decreased with increasing contact time but increased with increase in the initial metal ion

concentration and the adsorption equilibrium data were best described by Langmuir adsorption

isotherm.

Gimba researched on the preparation and adsorption of activated carbon from coconut shell [5].

He reported that coconut shell carbon can be used for both laboratory and industrial purpose,

depending on their method of preparation. Turoti et al. studied the effect of different activation

methods on the adsorption characteristics of AC from Khaya senegalensis and Delonix regia pods [6].

They discovered that the order of effectiveness of the methods follows the sequence; two-step >

microwave > impregnation > one step. Others include research on almond shell by Aygun et al. [7],

peach stone by Tsai et al. [8], peanut hulls by Girgis et al. [9] and nut shells by Yang and Lua [10].

Vitex doniana called black plum, and is widely spread in tropical West Africa and extending

eastward to Uganda, Kenya and Tanzania. It is also grown throughout the world as ornamental and as

sources of wood [11]. Vitex doniana nut is a common waste in Nigeria and other countries. The aim of

this paper is to assess the ability of Vitex doniana nut to adsorb Zn(II) and Pb(II) ions from aqueous

solutions. This aim was achieved by determining some physicochemical properties of the adsorbent

and examining the effect of the solution pH, temperature, contact time, particle size and adsorbent

dosage on the removal of Zn(II) and Pb(II) ions.

2. Materials and Methods

2.1. Sample Collection and Treatment

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Vitex doniana fruit was collected from Yargaya in Warawa Local Government Area of Kano

State, Nigeria. The fruit was then de-fleshed. The method of sample treatment by Itodo [12] was

adopted. The sample was separately washed with plenty of water to removes surface impurities and

sun dried. After this, they were oven dried at 105 °C for 96 h, and the dried Vitex doniana nut was

milled and sieved to different particle sizes.

2.2. Preparation of Activated Vitex doniana nut (AVDN)

About 3 g of the pretreated adsorbent was introduced into clean and pre-weighed 3 crucibles.

They were introduced into a furnace at 500 oC for 5 min after which they were poured from the

crucible into a bath of ice block. The excess water was drained and the samples were sun dried. This

process was repeated until a substantial amount of carbonized samples were obtained [13]. The

carbonized sample was washed using 10% HCl to remove surface ash, followed by hot water wash and

rinsing with distilled water to remove residual acid [14]. The solids were sun dried, then, dried in the

oven at 105 oC for one hour.

2.3. Characterization of the Adsorbent

2.3.1. Moisture content

The 1 g of the fresh precursor was weighed in clean, dried and pre-weighed Petri dish. This

sample was thinly spread in the dish. It was dried in air- circulated oven at 105 oC for 24 h. The dried

sample was cooled in a desiccator for 30 min. To ascertain constant weight, the process was repeated

in one hour interval. The percentage moisture content and dry matter (%) was calculated using

equations 1 and 2 below [15]:

Moisture (%) =

100 (1)

Dry Matter (%) =

100 (2)

The analysis was carried out in triplicate and the average was recorded.

2.3.2. Ash content determination

Copper crucible was heated in a furnace at 500 oC, cooled in a dessicator and weighed. Oven

dried sample from the moisture content determination was used. The crucible containing the dry

sample was placed in a muffle furnace and temperature was allowed to rise to 500 oC. After 3 h, it was

removed and allowed to cool in a dessicator and weighed. The percentage ash content was calculated

using the equation 3 below [16]:

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Ash (%) =

(3)

2.3.3. pH measurement

The 1% solution of the sample was made using deionized water. The pH of the supernatant was

obtained after 1 h using a pre-calibrated pH meter (Oaklon pH meter, Model 1100).

2.3.4. Bulk density

The bulk density was estimated by placing the product into a graduated cylinder and compacted

by tapping on the bench top until an expected volume (cm3) was occupied by mass (g). The cylinder

was tapped on the bench top until the volume of the sample stop decreasing. The mass and volume

were recorded and density calculated as equation 4 [17].

ρ = Mass/Vol. occupied (4)

2.3.5. Determination of porosity based on swellings procedure

The 0.5 g sorbent was dispersed in 20 mL water (V1) in a graduated tube with the aid of a

shaker. This was further centrifuged for 10 min at 2000 rpm using centrifuge (Baird and Tatlock Auto

Bench). The resulting volume was read at V2 and recorded. The equation 5 was used to calculate the

porosity [15,17]:

Porosity = V1/V2 (5)

2.3.6. Determination of iodine adsorption number of adsorbent

The method used for the determination of iodine value was as that used by Itodo [18]. The

iodine adsorption number (IAN) was calculated from the relationship as equation 6:

IAN = Ms(Vb - Vs)/2Ma (6)

where Ms is the molarity of thiosulphate solution (mol/dm3), Vs is the volume of thiosulphate (cm

3)

used for titration of the AC aliquot, Vb is the volume of thiosulphate (cm3) used for blank titration and

Ma is the mass of AC (g).

2.4. Preparation of Solutions

The stock solutions of Zn(II) and Pb(II) ion were prepared by dissolving ZnSO4.7H2O and

Pb(NO3)2 in distilled water. The resulting solutions were diluted to obtained serial concentrations of

Zn(II) and Pb(II) ion solutions.

2.5. Batch Adsorption Studies

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Batch adsorption experiments were conducted using AVDN as adsorbent in 120 mL polythene

bottle or 300 mL reagent bottles containing 50 mL of various concentrations of Zn(II) and Pb(II)

solutions. The bottles were agitated at 100 rpm in a thermostatic shaking incubator to reach the

equilibrium. The effect of pH on the adsorption of the studied metal ions by AVDNs was studied in the

pH range 2.0 - 12.0 at 303 K. The solution pH was adjusted to the desired value with dilute HCl or

NaOH solution using a pH meter (Oaklon pH meter, Model 1100). The effect of temperature on

adsorption capacity of AVDNs was studied at 303, 313, 323 and 333 K with the adsorbent dosage of

0.4 g/L at pH 6. The effect of dosage on the adsorption capacity of AVDNs was determined at

different dosages in the range 0.2 - 1.0 g/L of the studied metal ions at initial concentration of 50

mg/L. The effect of contact time on the adsorption capacity of AVDNs was studied in the range 20 -

100 min at an initial concentration of 50 mg/L. Adsorption isotherms were studied at various initial

concentrations of the studied metal ions in the range of 0.1 - 100 g/L.

In each set of the experiment, the concentrations of Zn(II) and Pb(VI) ions were determined

using Perkin Elmer atomic absorption spectrophotometer. From the measured concentration of Zn(II)

and Pb(II) ions, amount of the metal adsorbed (qe) and the percentage metal ion removal (% Rem) was

calculated using equations 7 and 8, respectively [19].

V(Co – Ce)

qe = (7) 100 M

(Co – Ce) % Rem = 100 (8)

Co

where qe is the amount of adsorbate ion adsorbed in milligram per gram of the adsorbent, Co is the

initial concentration of the metal ion before the adsorption process, Co is the equilibrium concentration

of the metal ion in the filtrate after adsorption process, M is the mass in gram of the adsorbent and V is

the volume of the solution in mL.

3. Results and Discussion

3.1. Physicochemical Analysis of Vitex doniana Nut

The results obtained from the physicochemical parameters of the Vitex doniana nut are

presented in Table 1. The percent dry matter and moisture of Vitex doniana nut are 90.70% and 7.00%,

respectively. The percent moisture was higher than 6.48% reported by Erhan et al. [20] for cornelian

cherry and 5.75% reported by Shakirullah et al. [21] for sawdust. This high value may be as a result of

its hydrophilic nature.

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The pH of AVDN was found to be 5.84. This value falls within the acceptable pH conditions

for adsorption [22]. Bulk density determines to a large extent the length of the filtration cycle of

activated carbon [23]. The bulk density for AVDN was found to be 0.491 g/cm3. This value according

to Erhan et al. [20] suggests that AVDN is a good adsorbent in terms of volume activity. The ash

content and porosity value of the sample were found to be within the range reported by other

researchers [24]. This is favourable because it is relatively low. The lower the ash content (5.9) of

AVDN suggests that it is a better material for adsorption since the ash serves as interferences during

the adsorption process [24].

Table 1. Physicochemical parameters of the Vitex doniana nut

Parameters Vitex doniana nut

pH of 1% solution 5.840

Bulk density (g/cm3) 0.491

Moisture content (%) 7.000

Dry matter (%) 90.800

Ash content (%) 5.900

Porosity 0.992

Iodine adsorption number (mM/g) 0.1542

The iodine adsorption number (IAN) measures the adsorption of iodine from an aqueous

solution. It is a measure of micropores and it is used as an indication of the total surface area.

Adsorbents with high iodine number perform better in removing small sized contaminants. It is the

most fundamental parameter used to characterize the performance of activated carbon. High value

indicates high degree of activation [17]. From the result presented in Table 1, the IAN for AVDN

estimated in mM iodine per gram of adsorbent is fairly high (0.1542), as compared to that reported by

Itodo [18] for shea nut shells (0.1338 - 0.1505) and groundnut shells (0.1115 - 0.1394). It thus implies

that the AVDN presents high degree of activation and high affinity for small sized contaminants.

3.2. Batch Studies

3.2.1. Effect of particle size

Table 2 shows the effect of particle size on the adsorption of lead(II) and zinc(II) by Vitex

doniana nut. It is seen that the removal of heavy metals increases as the particle size diameter

decreases. According to Kumar et al. [25], decrease in particle size increases the percentage removal

due to increase in surface area as well as micropore volume. Smaller particle size means more interior

surface and micropore volume and hence more will be the area of active sites for adsorption. Also for

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larger particles the diffusion resistance to mass transfer is higher and most of the internal surfaces of

the particle may not be utilized for adsorption and consequently the amount of the studied metal ions

adsorbed is small [26].

Table 2. Amount of metal ion adsorbed and removal efficiency at equilibrium by various sizes of Vitex

doniana nut

Size (mm) Zinc Lead

qe (mg/g) %Rem qe (mg/g) %Rem

0.250 0.5625 68.40 0.5710 69.49

0.775 0.5213 66.24 0.5285 67.16

1.125 0.5111 65.73 0.5183 66.66

2.00 0.5048 65.00 0.5128 66.03

3.2.2. Effect of initial metal ion concentration

The effect of initial metal ion concentration on percentage metal removal is shown in Table 3.

The concentrations in the range from 10 to 100 ppm for the metal ions have been studied. The removal

of metal ions by Vitex doniana nut was found to increase with increase in initial metal concentration.

This indicates that the adsorption process of Zn2+

and Pb2+

on Vitex doniana nut is dependent on

concentration of adsorbate up to some extent.

Table 3. Amount of metal ion adsorbed and removal efficiency at equilibrium by Vitex doniana nut at

various concentrations of Zn2+

and Pb2+

solution

Initial Conc. (ppm) Zinc Lead


10 0.9445 84.28 0.9644 89.10

20 1.2688 89.42 1.3330 94.20

30 1.4966 92.27 1.5682 95.58

40 1.7789 93.20 1.7984 95.76

60

100

1.9128

2.0812

93.64

93.98

1.9284

2.1021

95.85

95.93

3.2.3. The effect of initial solution pH

The effect of initial solution pH on Pb2+

and Zn2+

adsorption is presented in Table 4. It was

noticed that the ability of removing zinc and lead ions by the adsorbent depends on pH value and this

depends on the ion state and nature of material. The uptake of zinc and lead increased from 0.4869 to

0.5994 mg/g and 0.5361 to 0.6450 mg/g, respectively when the solution’s pH value was increased

from 2 to 8. There was decrease in the adsorption capacity of the adsorbent for lead when the pH value

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was increase above 8. According to Forsner and Wittman [27], Esposito et al. [28]; in low pH value,

binding sites are generally protonated or positively charged (by the hydronium ions). Thus, repulsion

occurs between the metal cation and the adsorbent. At a higher pH value, binding sites start

deprotonating and makes different function groups available for metal binding. In general, cation

binding increases as pH increases. This can be used to explain the reason for the observed trend of zinc

and lead ions by the adsorbent at different pH.


various solution pH

pH Zinc Lead


2 0.4869 42.40 0.5361 46.95

4 0.5586 48.46 0.5872 51.38

6 0.6002 56.40 0.6322 60.28

8

10

0.5994

0.5990

56.02

56.00

0.6450

0.6450

61.90

61.90

3.2.4. Effect of adsorbent dosage

The results of variation of adsorption capacity with the dosage of Vitex doniana nut are shown

in Fig. 1. Adsorption capacity increases first with an increase in the adsorbent dosage up to 0.4 g and

decreases with further increase in the adsorbent dosage. The increase in the percentage removal with

increase in the adsorbent dosage is due to the increase in the number of adsorption sites [29]. The

lesser adsorption capacity observed at higher adsorbent doses may be attributed to overlapping or

aggregation of adsorption sites resulting in decrease in total adsorbent surface area available to metal

ions and an increase in diffusion path length.

3.2.5. Effect of contact time

Uptake of the zinc and lead ions with the effect of contact time by the Vitex doniana nut was

studied and the results are shown in Fig. 2. The efficiency increases with increase in time of contact,

due to the availability of more time for metal ions to make an attractive complex with the composite.

Initial removal occurs immediately as soon as the metal and composite came into contact and after

some extent further increase in contact time did not increase the uptake due to decrease of the easily

available active sites for the binding of metal ions, the equilibrium is reached. This result is important,

as equilibrium time is one of the important parameters for an economical waste water treatment

system.

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Figure 1. Effect of adsorbent dose on adsorption of Zn2+

and Pb2+

by Vitex doniana nut.

Figure 2. Effect of contact time on adsorption of Zn2+

and Pb2+


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3.2.6. Effects of temperature

Temperature is a highly significant parameter in the adsorption processes. For adsorption of

Zn(II) and Pb(II) ions onto Vitex doniana nut, adsorption experiments were run to study the effect of

temperature variation at 303, 313, 323 and 333 K at optimum pH value of 6 and adsorbent dose level

of 0.4 g/L. The equilibrium contact time for adsorption was maintained at 40 min. The results are

presented in Table 5. It was observed that the percentage of adsorption increased along with an

increase of temperature. This could be due to increase in average kinetic energy of the metal ions in

solutions containing the adsorbent which increases the number of metal ions interacting with the

adsorbent surface by increasing the rate at which the metal ions hit the binding sites at the surface of

the adsorbent thus increasing the adsorption capacities [30].


various temperatures

Temp. (K) Zinc Lead


303 0.4826 69.82 0.4723 69.13

313 0.4850 69.84 0.4723 69.13

323 0.4850 69.84 0.4800 69.26

333 0.5200 71.00 0.5184 70.06

3.3. Isotherm studies

The adsorption isotherms reveal the specific relation between the concentration of the

adsorbate and its adsorption degree onto adsorbent surface at a constant temperature. To quantify the

adsorption capacity of AVDN for the removal of Zn(II) and Pb(II) ions from aqueous solution, the

Langmuir, Freundlich and Temkin isotherm models were used.

3.3.1. Langmuir adsorption isotherm

The Langmuir adsorption isotherm model assumes that adsorption takes place at specific

homogeneous sites within the adsorbent [31]:

(9)

The linear form of the Langmuir isotherm model is:

1/qe = 1/(qmax bCe) + 1/qmax (10)

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where, qe is the amount of adsorbate adsorbed per gram of dried adsorbent at equilibrium (mg

adsorbate/g of dried adsorbent), qmax is the constant relating to the maximum amount of adsorbate ion

bound per g of adsorbent for a monolayer (mg/g), b is Langmuir constant or adsorption coefficient or

the adsorption affinity (L/mg) for binding of adsorbate on the adsorbent sites, and Ce is equilibrium

(residual) adsorbate concentration in solution after sorption (mg/L).

A plot of 1/qe vs 1/Ce (Fig. 3) should be a straight line with an intercept as 1/qmax and a slope as

1/(qmax b). The values of constants qmax and b can be calculated, and were reported in Table 6.

Figure 3. Langmuir adsorption isotherm for Zn2+

and Pb

2+ adsorption by Vitex doniana nut.

Table 6. Langmuir, Freundlich and Temkin isotherm constants for the adsorption of the selected metal

ions on Vitex doniana nut

Metals Langmuir Constant Freundlich Constant Temkin Constant

qmax (mg/g) b (L/mg) R2 n KF (mg/g) R2 A (L/g) B (J/mol) R2

Zinc 2.356 0.065 0.9860 0.3438 0.4675 0.9530 0.6263 0.5187 0.9787

Lead 2.404 0.066 0.9958 0.3555 0.4379 0.9597 0.7091 0.5087 0.9823

3.3.2. Freundlich isotherm

The isotherm model is defined as equation 11 below [32]:

= (11)

Its linearized form is given as:

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1 log qe = log KF + log Ce (12)

n In Eq. 12, qe is the amount of the metal ions adsorbed at equilibrium (mg/g), Ce is the

equilibrium concentration of the metal ion in solution (mg/L), KF and n are Freundlich constant and

intensity factors, respectively. The values of n and KF are calculated from slope and intercept of plots

of log qe versus log Ce (Fig. 4), which are presented in Table 6.

Figure 4. Freundlich adsorption isotherm for Zn2+

and Pb2+

adsorption by Vitex doniana nut.

3.3.3. Temkin isotherm

Temkin isotherm assumes that heat of adsorption decrease linearly with the adsorption onto the

surface at a particular temperature and the adsorption is characterized by a uniform distribution of

binding energies. Temkin isotherm is expressed in linear form by the following equation [33]:

qe = B lnA + B lnCe (13)

where, B is related to the heat of adsorption, T (K) is the absolute temperature, R is the universal gas

constant (8.3143 J/mol), b indicates the adsorption potential of the adsorbent (J/mol), A is the

equilibrium binding constant (L/mg). The parameters for the Temkin model are obtained from the plot

of qe versus lnCe (Fig. 5). The values for these parameters are given in Table 6.

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Figure 5. Temkin adsorption isotherm for Zn2+

and Pb

2+ adsorption


The straight line plots of Langmuir, Temkin and Freundlich adsorption isotherm models

indicate that the adsorption of Pb(II) and Zn(II) on AVDN follows both Langmuir, Temkin and

Freundlich isotherms. The higher value of correlation coefficient (R2) for Langmuir than for the other

two isotherms indicates that the experimental adsorption data provided better fit in Langmuir isotherm

model. This suggests the formation of monolayer of Zn(II) and Pb(II) ions onto the outer surface of the

adsorbents.

4. Conclusions

From the present study, the following conclusions are drawn: (1) Activated carbon prepared

from Vitex doniana nut was found to be a promising adsorbent for the removal of Pb(II) and Zn(II)

ions from aqueous solutions. (2) The sorption capacity of Vitex doniana nut was strongly dependent on

the adsorbent nature and dosage, initial metal ions concentration, temperature, and initial pH. (3) The

experimental data well fitted to the Langmuir equations with good correlation coefficients. This

suggests the formation of monolayer of Zn(II) and Pb(II) ions onto the outer surface of the adsorbents.

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Equilibrium Study on the Adsorption of Zn(II) and Pb(II) Ions from Aqueous Solution onto Vitex...

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