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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88
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
qe (mg/g) %Rem qe (mg/g) %Rem
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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89
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.
Table 4. Amount of metal ion adsorbed and removal efficiency at equilibrium by Vitex doniana nut at
various solution pH
pH Zinc Lead
qe (mg/g) %Rem qe (mg/g) %Rem
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+
by Vitex doniana nut.
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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].
Table 5. Amount of metal ion adsorbed and removal efficiency at equilibrium by Vitex doniana nut at
various temperatures
Temp. (K) Zinc Lead
qe (mg/g) %Rem qe (mg/g) %Rem
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
by Vitex doniana nut.
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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