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Biosorption of lead(II) ions onto waste biomass of Phaseolus vulgaris L.: estimation of the equilibrium, kinetic and thermodynamic parameters
Desalination 244 (2009) 188–198
Biosorption of lead(II) ions onto waste biomass of Phaseolus vulgaris L.: estimation of the equilibrium, kinetic and thermodynamic parameters A. Safa Özcana, Sibel Tunalib, Tamer Akarb, Adnan Özcana* a
Department of Chemistry, Faculty of Science, Yunusemre Campus, Anadolu University 26470, Eskişehir, Turkey Tel. +90 (222) 335-0580/4815; Fax: +90 (222) 320-4910; email: [email protected] b Department of Chemistry, Faculty of Arts and Science, Eskişehir Osmangazi University, 26480, Eskişehir, Turkey Received 21 June 2006; Accepted 23 May 2008
Abstract Biosorption of lead(II) ions onto Phaseolus vulgaris L. waste was investigated with the variation in the parameters of pH, contact time, biosorbent and lead(II) concentrations and temperatures. The nature of the possible biosorbent and metal ion interactions was examined by the FTIR technique. The lead(II) biosorption equilibrium was attained within 20 min. Biosorption of lead(II) ions onto P. vulgaris L. waste followed by the Langmuir and Dubinin– Radushkevich isotherm models. Maximum biosorption capacity (qmax) of biosorbent for lead(II) ions was 2.064×10!4 mol g!1 or 42.77 mg g!1 at 20EC. Thermodynamic parameters such as the changes of free energy, enthalpy and entropy were also evaluated for the biosorption of lead(II) ions onto P. vulgaris L. waste. It was indicated that the biosorption of lead(II) ions onto P. vulgaris L. waste is a spontaneous endothermic process. Keywords: Biosorption; Lead(II) ions; Isotherm; Kinetics; Thermodynamics
1. Introduction Environmental pollution from one of the toxic heavy metals, which is lead(II), arises as a result of many activities, mostly industrial such as the metallurgical industry, electroplating and metal finishing industries, tannery operations, chemical *Corresponding author.
manufacturing, and it uses matches, explosives, photographic materials, fuels and printing processes. Lead is known as an environmental pollutant that acts as accumulative poison. Inorganic lead(II) ions affect the nervous system, metallic lead and its salts and oxides are used in paints and pigments, battery industries, lead smelter and it can enter and be adsorbed into the human body through inhalation, diet or skin contact, and
0011-9164/09/$– See front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.desal.2008.05.023
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produce adverse effects on virtually every system in the human body. It causes severe dysfunction of kidneys, liver, nervous system, reproductive system and causes high blood pressure. In addition, lead is harmful to developing brains of fetuses and children and may affect children’s mental and physical health. In other words, it affects children’s learning abilities and can cause behavioral problems and mental retardation [1–5]. Various suitable methods including chemical precipitation, evaporation, reverse osmosis, electroplating, ion-exchange, membrane separation, etc. can be used for the removal of such contaminants from liquid wastes when they are present in high concentration levels. In contrast, it is much more difficult to remove very low concentrations of these contaminants. The permissible level for lead in drinking water is 0.05 mg L!1 according to the Environmental Protection Agency (EPA). Therefore, a very low concentration of lead in water is very toxic [6]. For this reason, there is a need to develop economic and eco-friendly methods for waste minimization and fine tuning of wastewater [7]. The most respective alternative method for the removal of heavy metal ions is adsorption. Even though commercially activated carbon with a high surface area, microporous character and high adsorption capacity has proven its potential as an adsorbent for the removal of heavy metals from industrial wastewater, it is expensive with relatively high operating costs. Hence, there is a growing demand to find low-cost and efficient, locally available adsorbents for the sorption of lead such as the Nile rose plant [8], chaff [9], rice husk [10,11], coir fibre waste [12], banana stems [13], wheat bran [14], a clay-polymer composite [15], coffee grounds [16], tree ferns [17–19], palm kernel fibres [20,21], crop milling waste [22], spent grain [23], seaweed [24], pomegranate peels [25], modified silica [26], peanut skins [27] and rice hulls [28].
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According to the above literature survey, little information exists on the use of agricultural waste biomass materials as adsorbents for the removal of lead(II) ions and this needs further research. One of these materials, Phaseolus vulgaris L., which is an agricultural waste material, can be used as a biosorbent source for the biosorption of lead(II) ions. The study of biosorption equilibrium isotherms is essential in supplying the basic information required for the design and operation of biosorption equipment for wastewater treatment. Various isotherm models including the Langmuir, Freundlich and Dubinin–Radushkevich (D–R) have been put forward to describe or predict the biosorption isotherms. The aim of this research is to investigate the possible use of Phaseolus vulgaris L. waste as an alternative biosorbent material for the removal of lead(II) ions from aqueous solutions. It is an abundantly available agricultural material and it was chosen as a biosorbent material due to a lack of information on its biosorption abilities. The effects of temperature, pH, contact time and concentration were examined for the biosorption of lead(II) ions onto P. vulgaris L. The biosorption isotherm and thermodynamic parameters were deduced from biosorption measurements. 1.1. Equilibrium parameters of biosorption Equilibrium data that are related to the biosorption isotherms are a basic requirement to understand the mechanism of biosorption. Classical biosorption isotherm models—Langmuir, Freundlich and D–R—are used to describe the equilibrium between biosorbed lead(II) ions on P. vulgaris L. waste (qe) and lead(II) ions in solution (Ce) at a constant temperature. The Langmuir biosorption isotherm assumes that biosorption occurs at specific homogeneous sites within the biosorbent and has found successful application in many monolayer biosorption
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processes. The linear form of the Langmuir isotherm equation [29] is:
1 1 1 1 qe qmax qmax K L Ce
(1)
where qe is the equilibrium lead(II) concentration on the biosorbent (mol g!1), Ce is the equilibrium lead(II) concentration in the solution (mol dm!3), qmax is the monolayer biosorption capacity of the biosorbent (mol g!1), and KL is the Langmuir biosorption constant (dm3 mol!1) related to the free energy of biosorption. A plot of 1/qe versus 1/Ce for the biosorption gives a straight line of slope 1/(qmax KL) and intercept 1/qmax. The effect of isotherm shape has been discussed by Weber and Chakravorti [30] with a view to predict whether a biosorption system is favorable or unfavorable. The essential feature of the Langmuir isotherm can be expressed by means of RL, a dimensionless constant referred to a separation factor or equilibrium parameter. RL is calculated using the following equation:
1 RL 1 K LC0
ln qe ln qm 2
(4)
where β is a constant related to the mean free energy of biosorption per mole of the biosorbate (mol2 kJ!2), qm is the theoretical saturation capacity, and ε is the Polanyi potential, which is equal to RT ln[1+(1/Ce)] where R (J mol!1 K!1) is the gas constant, and T (K) is the absolute temperature. Hence by plotting ln qe against ε2, it is possible to generate the value of qm (mol g!1) from the intercept and the value of β from the slope. 1.2. Thermodynamic parameters of biosorption
(2)
where C0 is the initial lead(II) concentration (mol dm!3). The Freundlich isotherm is an empirical equation employed to describe heterogeneous systems. A linear form of the Freundlich equation [31] is:
1 ln qe ln K F ln Ce n
lnqe versus lnCe for the biosorption was employed to generate KF and n from the intercept and the slope values, respectively. The D–R isotherm is more general than the Langmuir isotherm since it does not assume a homogeneous surface or constant biosorption potential. It was applied to distinguish between the physical and chemical biosorption of lead(II) ions. The linear form of the D–R isotherm equation [32] is
Since KL is the equilibrium constant, its dependence with temperature can be used to predict thermodynamic parameters including changes in the free energy (ΔG0), enthalpy (ΔH0) and entropy (ΔS0) associated to the biosorption process and which were determined by using the following equations:
G 0 RT ln K L (3)
where KF (dm3 g!1) and n (dimensionless) are Freundlich biosorption isotherm constants, being indicative of the extent of the biosorption and the degree of nonlinearity between solution concentration and biosorption, respectively. The plot of
ln K L
ΔG 0 ΔH 0 ΔS 0 RT RT R
(5) (6)
The plot of ln KL as a function of 1/T yields a straight line from which ΔH0 and ΔS0 can be calculated from the slope and intercept, respectively.
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2. Experimental 2.1. Preparation of the biosorbent material The P. vulgaris L. used in this study was obtained as a residual biomass of a canned food factory in Bartin, Turkey. It was washed with deionized water three times and dried at 80EC in an oven. Dried biosorbent was crushed, ground and sieved to select a particle size of 300 µm by using an ASTM standard sieve and then stored for further use. 2.2. Lead(II) solutions A stock solution of lead(II) was prepared by dissolving an accurate quantity of Pb(NO3)2 in deionized water. Other concentrations prepared from stock solution by dilution varied between 100 and 250 mg dm!3 and the pH of the working solutions was adjusted to desired values with 0.1 M HNO3 or 0.1 M NaOH. Fresh dilutions were used for each experiment. All the chemicals used were of analytical grade. 2.3. Batch biosorption studies All batch experiments were conducted with biosorbent samples in a beaker on a magnetic stirrer at 200 rpm to elucidate the optimum conditions of pH, biosorbent and lead(II) ion concentrations. The effect of pH on the biosorption of lead(II) onto P. vulgaris L. waste was determined by equilibrating the biosorption mixture with dried biosorbent and 50 cm3 of 100 mg dm!3 lead(II) solution at different pH values between 1 and 6. The effect of biosorbent concentration was studied by using biosorbent sample ranging from 0.4 to 6.0 g dm!3. The optimum pH and biosorbent concentration were then determined as 5 and 4.0 g dm!3, respectively and used throughout all biosorption experiments, which were conducted at various time intervals between 10 and 75 min and tempera-
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tures of 20, 30, 40 and 50EC. When the biosorption procedure was completed, the solutions were centrifuged at 4500 rpm for 2 min and the supernatants were then analyzed for residual lead(II) ion concentrations. The biosorption of lead(II) onto P. vulgaris L. waste was evaluated at constant temperatures of 20, 30, 40 and 50EC for the biosorption isotherms. 2.4. Analytical methods The final lead(II) concentrations of the solutions were determined by using an atomic absorption spectrophotometer (Hitachi 180-70, Japan) with an air-acetylene flame. Deuterium background correction was used. The spectral slit width, the working current and wavelength were 1.3 nm, 7.5 mA and 283.3 nm, respectively. The instrument calibration was periodically checked by using standard metal solutions for every 10 readings. Infrared spectra of unloaded and lead(II) loaded P. vulgaris L. waste prepared as KBr discs were recorded in a Bruker Tensor 27 infrared spectrophotometer.
3. Results and discussion 3.1. FTIR analysis The FTIR spectra of dried unloaded and lead(II)-loaded P. vulgaris L. waste in the range of 400–4000 cm!1 were recorded and compared with each other to obtain information on the nature of the possible biosorbent–metal ion interactions. They are presented in Fig. 1. The broad stretching absorption bands at 3290–3345 cm!1 represent –NH and bonded –OH groups. These band intensities decreased only in the FTIR spectrum of lead(II)-loaded P. vulgaris L. waste. The change in the intensities decrease in the amino and hydroxyl groups in the FTIR spectrum indicated that these two groups are possibly involved
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Fig. 1. FTIR spectra of (a) unloaded and (b) lead(II) loaded P. vulgaris L. waste.
in lead(II) biosorption. The bands observed at 2865 and 2936 cm!1 are assigned to the symmetric and asymmetric stretching vibrations of the –CH3 and –CH2 groups and their bending vibrations are 1376 and 1419 cm!1 for unloaded and lead(II)-loaded P. vulgaris L. waste. The bands at 1643 cm!1 and 1544 cm!1 correspond to carbonyl stretching vibration of amide considered to be due to the combined effect of double-bond stretching vibrations [33] and –NH deformation band for unloaded and lead(II)-loaded P. vulgaris L. waste, respectively. Their intensities in the spectrum of unloaded biomass decrease, and separately the –NH deformation band shifts to 1541 cm!1 in the lead(II)-loaded biomass. This behavior reflects the interaction between the amino groups and metal ions. Therefore, P.
vulgaris L. waste provides more biosorption sites for lead(II) ions. The 1039 cm!1 band is due to C-O stretching of carbonyl groups and the bending vibration of hydroxyl groups for unloaded P. vulgaris L. waste but the disappearance of this band after lead(II)-loaded P. vulgaris L. waste suggests that this type of functional groups is likely to participate in metal binding. 3.2. Effect of pH on metal removal The pH of the solution has been identified as the most important variable governing by the metal biosorption on the agricultural waste. This is partly because hydrogen ions themselves are strongly competing with biosorbates. Fig. 2
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Fig. 2. Effect of pH for lead(II) biosorption onto P. vulgaris L. waste at 20EC (m: 4.0 g dm!3; C0: 100 mg dm!3).
Fig. 3. Effect of biosorbent concentration for lead(II) biosorption onto P. vulgaris L. waste at 20EC (pH: 5.0, C0: 100 mg dm!3).
indicates the effect of pH on the removal of lead(II) onto P. vulgaris L. waste from aqueous solutions. It can be seen from Fig. 2 that biosorption capacity is very low at strong acidic medium (pH = 1–3). After pH 3, uptakes increase sharply up to pH 5 because more metal binding sites could be exposed and carried negative charges, with subsequent attraction of the positively charged metal ions with the biosorbent surface. A decrease in biosorption of lead(II) was noticed above pH 5. Experiments were carried out with the pH values of up to 5 due to the fact that metal precipitation appeared at higher pH values and interfered with the accumulation or biosorbent deterioration [34–36].
P. vulgaris L. waste for further biosorption experiments was selected as 4.0 g dm!3.
3.3. Effect of biosorbent concentration on metal removal The results of the experiments with varying biosorbent concentrations are presented in Fig. 3. With an increase in biosorbent concentration from 0.4 to 6.0 g dm!3, the percentage of biosorbed lead(II) removal increases from 4.73% to 92.11% as the number of possible binding sites are increased. The biosorbent concentration of
3.4. Effect of equilibrium contact time The biosorption capacity of lead(II) removed by P. vulgaris L. waste versus contact time is illustrated in Fig. 4. It can be seen that the biosorbed amount of lead(II) ions increased with contact time up to 20 min; after that maximum removal was attained. Therefore, 20 min was selected as the optimum contact time for all further experiments. 3.5. Effect of temperature on metal uptake The equilibrium biosorption capacity of lead(II) onto P. vulgaris L. waste was favored at higher temperatures. An increase in the temperature from 20 to 50EC leads to an increase in the biosorption capacity from 21.85±0.06 to 23.85± 0.04 mg g!1 at an equilibrium time of 20 min. After equilibrium was attained, the uptake increases with increasing temperature. This effect may be explained by availability of more active sites of biosorbent at higher temperatures.
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Fig. 4. Effect of contact time for lead(II) biosorption onto P. vulgaris L. waste at various temperatures (pH: 5.0, m: 4.0 g dm!3; C0: 100 mg dm!3).
Fig. 5. General isotherm plots for lead(II) biosorption onto P. vulgaris L. waste at various temperatures (pH: 5.0; m: 4.0 g dm!3).
3.6. Biosorption isotherms The equilibrium biosorption isotherms are most important to understand the mechanism of biosorption. Several isotherm equations are available and three important ones (Langmuir, Freundlich and D–R) were chosen for this study. The general biosorption isotherm plots at the temperatures of 20, 30, 40 and 50EC are illustrated in Fig. 5. All of the isotherm model para-
meters for the biosorption of lead(II) onto P. vulgaris L. waste are listed in Table 1. It is evident from these data that the surface of P. vulgaris L. waste is made up of homogeneous biosorption patches. In other words, the Langmuir and D–R isotherm models fit better than the Freundlich isotherm model when the r2 values are compared in Table 1. It concluded that the biosorption of lead(II) ions onto P. vulgaris L. waste
Table 1 Isotherm constants for the biosorption of lead(II) ions onto P. vulgaris L. waste at various temperatures t, EC
2.064×10!4 2.258×10!4 2.314×10!4 2.424×10!4
Langmuir
Freundlich
Dubinin–Radushkevich
KL, dm3 mol!1
r2L
RL
KF, dm3 g!1
n
r2F
qmax, mol g!1
β, mol2 kJ!2
r2D–R
1.727×104 1.906×104 2.923×104 4.087×104
0.996 0.982 0.988 0.993
4.580×10!2 4.167×10!2 2.757×10!2 1.987×10!2
1.621×10!3 2.397×10!3 2.088×10!3 2.091×10!3
3.617 3.251 3.605 3.788
0.980 0.987 0.980 0.970
4.932×10!4 6.154×10!4 5.961×10!4 6.109×10!4
2.687×10!3 2.721×10!3 2.238×10!3 1.945×10!3
0.988 0.991 0.988 0.981
Table 2 Biosorption results of lead(II) ions from the literature by various agricultural biosorbents and operating conditions Biosorbent material
Chaff Rice husk Banana stem Wheat bran Tree fern Palm kernel fibre Pinus sylvestris P. vulgaris L.
Biosorption capacity (mg g!1)
12.50 8.60 91.74 87.00 40.00 49.90 11.38 42.68
Operating conditions pH
t (EC)
Initial concentration range (mg dm!3)
Biomass (g dm!3)
Reference
5.5 5.0 6.0 4.0–7.0 — 5.0 4.0 5.0
20 30–60 30 60 20 65 25 20
8–96 50 10–400 50–400 74.1–344 20–200 10–100 100–250
2–12 10 2 10 4 2.5 4.0 4.0
[9] [10] [13] [14] [18] [21] [37] This study
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20 30 40 50
qmax, mol g!1
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is thus complex, involving more than one mechanism. The maximum biosorption capacity (qmax) of biosorbent calculated from Langmuir isotherm equation defines the total capacity of the biosorbent for lead(II). The biosorption capacity of biosorbent increased with an increase in the temperature. The highest value of qmax obtained at 50EC is 2.424×10!4 mol g!1. It appears to be the highest in comparison with the uptake obtained at the other temperatures (Table 1). The results obtained from this study were also found to be higher than that of many corresponding biosorbents reported in the literature (Table 2) [9,10,13, 14,18,21,37]. The value of RL calculated from Eq. (2) is incorporated in Table 1. As the RL values lie between 0 and 1, the biosorption process is favorable [30,38]. Further, the RL values for this study at all temperatures studied are between 1.987×10!2 and 4.580×10!2. Therefore, the biosorption is spontaneous.
The Freundlich constant KF indicates the biosorption capacity of the biosorbent and the values of KF at equilibrium at all temperatures lie at a range of 1.621×10!3–2.397×10!3 dm3 g!1. The other Freundlich constant n is a measure of the deviation from linearity of the biosorption and the numerical values of n at all temperatures lie between 3.251 and 3.788 and are greater than unity, indicating that lead(II) ions are favorably adsorbed by P. vulgaris L. waste at all the temperatures studied. 3.7. Thermodynamic parameters of biosorption The thermodynamic parameters of ΔH0 and ΔS0 were obtained from the ln KL versus 1/T plot (Fig. 6). The Gibbs free energies (ΔG0) were calculated from Eq. (5), and the results are given in Table 3. The negative values of ΔG0 at all temperatures studied are due to the fact that the biosorption process is spontaneous. The positive value of ΔH0 suggests the endothermic nature of
Fig. 6. Plot of KL versus 1/T for estimation of thermodynamic parameters for lead(II) biosorption onto P. vulgaris L. waste.
A. Safa Ózcan et al. / Desalination 244 (2009) 188–198 Table 3 Thermodynamic parameters calculated from Langmuir isotherm constants (KL) for the biosorption of lead(II) ions onto P. vulgaris L. waste at different temperatures t (EC)
ΔG0 (kJ mol!1)
ΔH0 (kJ mol!1)
ΔS0 (J K!1 mol!1)
20 30 40 50
!23.78 !24.84 !26.77 !28.53
23.63
160.98
biosorption. The positive value of ΔS0 suggests increased randomness at the solid/solution interface during the biosorption of lead(II) onto P. vulgaris L. waste.
4. Conclusions This study has shown that an agricultural byproduct, P. vulgaris L., can be used to remove lead(II) ions from aqueous solution as a function of pH, contact time and temperature. The maximum biosorption capacity of biosorbent for the removal of lead(II) was obtained at pH 5. The biosorption equilibrium data fit well to the Langmuir and D–R isotherm models. The negative values of ΔG0 confirm a favorable biosorption of lead(II) onto P. vulgaris L. waste and the positive value of ΔH0 leads to an endothermic nature of biosorption. The equilibrium was attained in 20 min. It may be concluded from above results that P. vulgaris L. waste can be used for elimination of heavy metal pollution from wastewater because it is a low-cost, abundant waste product and is a locally available biosorbent.
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Biosorption of lead(II) ions onto waste biomass of Phaseolus vulgaris L.: estimation of the equilibrium, kinetic and thermodynamic parameters A. Safa Özcana, Sibel Tunalib, Tamer Akarb, Adnan Özcana* a
Department of Chemistry, Faculty of Science, Yunusemre Campus, Anadolu University 26470, Eskişehir, Turkey Tel. +90 (222) 335-0580/4815; Fax: +90 (222) 320-4910; email: [email protected] b Department of Chemistry, Faculty of Arts and Science, Eskişehir Osmangazi University, 26480, Eskişehir, Turkey Received 21 June 2006; Accepted 23 May 2008
Abstract Biosorption of lead(II) ions onto Phaseolus vulgaris L. waste was investigated with the variation in the parameters of pH, contact time, biosorbent and lead(II) concentrations and temperatures. The nature of the possible biosorbent and metal ion interactions was examined by the FTIR technique. The lead(II) biosorption equilibrium was attained within 20 min. Biosorption of lead(II) ions onto P. vulgaris L. waste followed by the Langmuir and Dubinin– Radushkevich isotherm models. Maximum biosorption capacity (qmax) of biosorbent for lead(II) ions was 2.064×10!4 mol g!1 or 42.77 mg g!1 at 20EC. Thermodynamic parameters such as the changes of free energy, enthalpy and entropy were also evaluated for the biosorption of lead(II) ions onto P. vulgaris L. waste. It was indicated that the biosorption of lead(II) ions onto P. vulgaris L. waste is a spontaneous endothermic process. Keywords: Biosorption; Lead(II) ions; Isotherm; Kinetics; Thermodynamics
1. Introduction Environmental pollution from one of the toxic heavy metals, which is lead(II), arises as a result of many activities, mostly industrial such as the metallurgical industry, electroplating and metal finishing industries, tannery operations, chemical *Corresponding author.
manufacturing, and it uses matches, explosives, photographic materials, fuels and printing processes. Lead is known as an environmental pollutant that acts as accumulative poison. Inorganic lead(II) ions affect the nervous system, metallic lead and its salts and oxides are used in paints and pigments, battery industries, lead smelter and it can enter and be adsorbed into the human body through inhalation, diet or skin contact, and
0011-9164/09/$– See front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.desal.2008.05.023
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produce adverse effects on virtually every system in the human body. It causes severe dysfunction of kidneys, liver, nervous system, reproductive system and causes high blood pressure. In addition, lead is harmful to developing brains of fetuses and children and may affect children’s mental and physical health. In other words, it affects children’s learning abilities and can cause behavioral problems and mental retardation [1–5]. Various suitable methods including chemical precipitation, evaporation, reverse osmosis, electroplating, ion-exchange, membrane separation, etc. can be used for the removal of such contaminants from liquid wastes when they are present in high concentration levels. In contrast, it is much more difficult to remove very low concentrations of these contaminants. The permissible level for lead in drinking water is 0.05 mg L!1 according to the Environmental Protection Agency (EPA). Therefore, a very low concentration of lead in water is very toxic [6]. For this reason, there is a need to develop economic and eco-friendly methods for waste minimization and fine tuning of wastewater [7]. The most respective alternative method for the removal of heavy metal ions is adsorption. Even though commercially activated carbon with a high surface area, microporous character and high adsorption capacity has proven its potential as an adsorbent for the removal of heavy metals from industrial wastewater, it is expensive with relatively high operating costs. Hence, there is a growing demand to find low-cost and efficient, locally available adsorbents for the sorption of lead such as the Nile rose plant [8], chaff [9], rice husk [10,11], coir fibre waste [12], banana stems [13], wheat bran [14], a clay-polymer composite [15], coffee grounds [16], tree ferns [17–19], palm kernel fibres [20,21], crop milling waste [22], spent grain [23], seaweed [24], pomegranate peels [25], modified silica [26], peanut skins [27] and rice hulls [28].
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According to the above literature survey, little information exists on the use of agricultural waste biomass materials as adsorbents for the removal of lead(II) ions and this needs further research. One of these materials, Phaseolus vulgaris L., which is an agricultural waste material, can be used as a biosorbent source for the biosorption of lead(II) ions. The study of biosorption equilibrium isotherms is essential in supplying the basic information required for the design and operation of biosorption equipment for wastewater treatment. Various isotherm models including the Langmuir, Freundlich and Dubinin–Radushkevich (D–R) have been put forward to describe or predict the biosorption isotherms. The aim of this research is to investigate the possible use of Phaseolus vulgaris L. waste as an alternative biosorbent material for the removal of lead(II) ions from aqueous solutions. It is an abundantly available agricultural material and it was chosen as a biosorbent material due to a lack of information on its biosorption abilities. The effects of temperature, pH, contact time and concentration were examined for the biosorption of lead(II) ions onto P. vulgaris L. The biosorption isotherm and thermodynamic parameters were deduced from biosorption measurements. 1.1. Equilibrium parameters of biosorption Equilibrium data that are related to the biosorption isotherms are a basic requirement to understand the mechanism of biosorption. Classical biosorption isotherm models—Langmuir, Freundlich and D–R—are used to describe the equilibrium between biosorbed lead(II) ions on P. vulgaris L. waste (qe) and lead(II) ions in solution (Ce) at a constant temperature. The Langmuir biosorption isotherm assumes that biosorption occurs at specific homogeneous sites within the biosorbent and has found successful application in many monolayer biosorption
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processes. The linear form of the Langmuir isotherm equation [29] is:
1 1 1 1 qe qmax qmax K L Ce
(1)
where qe is the equilibrium lead(II) concentration on the biosorbent (mol g!1), Ce is the equilibrium lead(II) concentration in the solution (mol dm!3), qmax is the monolayer biosorption capacity of the biosorbent (mol g!1), and KL is the Langmuir biosorption constant (dm3 mol!1) related to the free energy of biosorption. A plot of 1/qe versus 1/Ce for the biosorption gives a straight line of slope 1/(qmax KL) and intercept 1/qmax. The effect of isotherm shape has been discussed by Weber and Chakravorti [30] with a view to predict whether a biosorption system is favorable or unfavorable. The essential feature of the Langmuir isotherm can be expressed by means of RL, a dimensionless constant referred to a separation factor or equilibrium parameter. RL is calculated using the following equation:
1 RL 1 K LC0
ln qe ln qm 2
(4)
where β is a constant related to the mean free energy of biosorption per mole of the biosorbate (mol2 kJ!2), qm is the theoretical saturation capacity, and ε is the Polanyi potential, which is equal to RT ln[1+(1/Ce)] where R (J mol!1 K!1) is the gas constant, and T (K) is the absolute temperature. Hence by plotting ln qe against ε2, it is possible to generate the value of qm (mol g!1) from the intercept and the value of β from the slope. 1.2. Thermodynamic parameters of biosorption
(2)
where C0 is the initial lead(II) concentration (mol dm!3). The Freundlich isotherm is an empirical equation employed to describe heterogeneous systems. A linear form of the Freundlich equation [31] is:
1 ln qe ln K F ln Ce n
lnqe versus lnCe for the biosorption was employed to generate KF and n from the intercept and the slope values, respectively. The D–R isotherm is more general than the Langmuir isotherm since it does not assume a homogeneous surface or constant biosorption potential. It was applied to distinguish between the physical and chemical biosorption of lead(II) ions. The linear form of the D–R isotherm equation [32] is
Since KL is the equilibrium constant, its dependence with temperature can be used to predict thermodynamic parameters including changes in the free energy (ΔG0), enthalpy (ΔH0) and entropy (ΔS0) associated to the biosorption process and which were determined by using the following equations:
G 0 RT ln K L (3)
where KF (dm3 g!1) and n (dimensionless) are Freundlich biosorption isotherm constants, being indicative of the extent of the biosorption and the degree of nonlinearity between solution concentration and biosorption, respectively. The plot of
ln K L
ΔG 0 ΔH 0 ΔS 0 RT RT R
(5) (6)
The plot of ln KL as a function of 1/T yields a straight line from which ΔH0 and ΔS0 can be calculated from the slope and intercept, respectively.
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2. Experimental 2.1. Preparation of the biosorbent material The P. vulgaris L. used in this study was obtained as a residual biomass of a canned food factory in Bartin, Turkey. It was washed with deionized water three times and dried at 80EC in an oven. Dried biosorbent was crushed, ground and sieved to select a particle size of 300 µm by using an ASTM standard sieve and then stored for further use. 2.2. Lead(II) solutions A stock solution of lead(II) was prepared by dissolving an accurate quantity of Pb(NO3)2 in deionized water. Other concentrations prepared from stock solution by dilution varied between 100 and 250 mg dm!3 and the pH of the working solutions was adjusted to desired values with 0.1 M HNO3 or 0.1 M NaOH. Fresh dilutions were used for each experiment. All the chemicals used were of analytical grade. 2.3. Batch biosorption studies All batch experiments were conducted with biosorbent samples in a beaker on a magnetic stirrer at 200 rpm to elucidate the optimum conditions of pH, biosorbent and lead(II) ion concentrations. The effect of pH on the biosorption of lead(II) onto P. vulgaris L. waste was determined by equilibrating the biosorption mixture with dried biosorbent and 50 cm3 of 100 mg dm!3 lead(II) solution at different pH values between 1 and 6. The effect of biosorbent concentration was studied by using biosorbent sample ranging from 0.4 to 6.0 g dm!3. The optimum pH and biosorbent concentration were then determined as 5 and 4.0 g dm!3, respectively and used throughout all biosorption experiments, which were conducted at various time intervals between 10 and 75 min and tempera-
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tures of 20, 30, 40 and 50EC. When the biosorption procedure was completed, the solutions were centrifuged at 4500 rpm for 2 min and the supernatants were then analyzed for residual lead(II) ion concentrations. The biosorption of lead(II) onto P. vulgaris L. waste was evaluated at constant temperatures of 20, 30, 40 and 50EC for the biosorption isotherms. 2.4. Analytical methods The final lead(II) concentrations of the solutions were determined by using an atomic absorption spectrophotometer (Hitachi 180-70, Japan) with an air-acetylene flame. Deuterium background correction was used. The spectral slit width, the working current and wavelength were 1.3 nm, 7.5 mA and 283.3 nm, respectively. The instrument calibration was periodically checked by using standard metal solutions for every 10 readings. Infrared spectra of unloaded and lead(II) loaded P. vulgaris L. waste prepared as KBr discs were recorded in a Bruker Tensor 27 infrared spectrophotometer.
3. Results and discussion 3.1. FTIR analysis The FTIR spectra of dried unloaded and lead(II)-loaded P. vulgaris L. waste in the range of 400–4000 cm!1 were recorded and compared with each other to obtain information on the nature of the possible biosorbent–metal ion interactions. They are presented in Fig. 1. The broad stretching absorption bands at 3290–3345 cm!1 represent –NH and bonded –OH groups. These band intensities decreased only in the FTIR spectrum of lead(II)-loaded P. vulgaris L. waste. The change in the intensities decrease in the amino and hydroxyl groups in the FTIR spectrum indicated that these two groups are possibly involved
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Fig. 1. FTIR spectra of (a) unloaded and (b) lead(II) loaded P. vulgaris L. waste.
in lead(II) biosorption. The bands observed at 2865 and 2936 cm!1 are assigned to the symmetric and asymmetric stretching vibrations of the –CH3 and –CH2 groups and their bending vibrations are 1376 and 1419 cm!1 for unloaded and lead(II)-loaded P. vulgaris L. waste. The bands at 1643 cm!1 and 1544 cm!1 correspond to carbonyl stretching vibration of amide considered to be due to the combined effect of double-bond stretching vibrations [33] and –NH deformation band for unloaded and lead(II)-loaded P. vulgaris L. waste, respectively. Their intensities in the spectrum of unloaded biomass decrease, and separately the –NH deformation band shifts to 1541 cm!1 in the lead(II)-loaded biomass. This behavior reflects the interaction between the amino groups and metal ions. Therefore, P.
vulgaris L. waste provides more biosorption sites for lead(II) ions. The 1039 cm!1 band is due to C-O stretching of carbonyl groups and the bending vibration of hydroxyl groups for unloaded P. vulgaris L. waste but the disappearance of this band after lead(II)-loaded P. vulgaris L. waste suggests that this type of functional groups is likely to participate in metal binding. 3.2. Effect of pH on metal removal The pH of the solution has been identified as the most important variable governing by the metal biosorption on the agricultural waste. This is partly because hydrogen ions themselves are strongly competing with biosorbates. Fig. 2
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Fig. 2. Effect of pH for lead(II) biosorption onto P. vulgaris L. waste at 20EC (m: 4.0 g dm!3; C0: 100 mg dm!3).
Fig. 3. Effect of biosorbent concentration for lead(II) biosorption onto P. vulgaris L. waste at 20EC (pH: 5.0, C0: 100 mg dm!3).
indicates the effect of pH on the removal of lead(II) onto P. vulgaris L. waste from aqueous solutions. It can be seen from Fig. 2 that biosorption capacity is very low at strong acidic medium (pH = 1–3). After pH 3, uptakes increase sharply up to pH 5 because more metal binding sites could be exposed and carried negative charges, with subsequent attraction of the positively charged metal ions with the biosorbent surface. A decrease in biosorption of lead(II) was noticed above pH 5. Experiments were carried out with the pH values of up to 5 due to the fact that metal precipitation appeared at higher pH values and interfered with the accumulation or biosorbent deterioration [34–36].
P. vulgaris L. waste for further biosorption experiments was selected as 4.0 g dm!3.
3.3. Effect of biosorbent concentration on metal removal The results of the experiments with varying biosorbent concentrations are presented in Fig. 3. With an increase in biosorbent concentration from 0.4 to 6.0 g dm!3, the percentage of biosorbed lead(II) removal increases from 4.73% to 92.11% as the number of possible binding sites are increased. The biosorbent concentration of
3.4. Effect of equilibrium contact time The biosorption capacity of lead(II) removed by P. vulgaris L. waste versus contact time is illustrated in Fig. 4. It can be seen that the biosorbed amount of lead(II) ions increased with contact time up to 20 min; after that maximum removal was attained. Therefore, 20 min was selected as the optimum contact time for all further experiments. 3.5. Effect of temperature on metal uptake The equilibrium biosorption capacity of lead(II) onto P. vulgaris L. waste was favored at higher temperatures. An increase in the temperature from 20 to 50EC leads to an increase in the biosorption capacity from 21.85±0.06 to 23.85± 0.04 mg g!1 at an equilibrium time of 20 min. After equilibrium was attained, the uptake increases with increasing temperature. This effect may be explained by availability of more active sites of biosorbent at higher temperatures.
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Fig. 4. Effect of contact time for lead(II) biosorption onto P. vulgaris L. waste at various temperatures (pH: 5.0, m: 4.0 g dm!3; C0: 100 mg dm!3).
Fig. 5. General isotherm plots for lead(II) biosorption onto P. vulgaris L. waste at various temperatures (pH: 5.0; m: 4.0 g dm!3).
3.6. Biosorption isotherms The equilibrium biosorption isotherms are most important to understand the mechanism of biosorption. Several isotherm equations are available and three important ones (Langmuir, Freundlich and D–R) were chosen for this study. The general biosorption isotherm plots at the temperatures of 20, 30, 40 and 50EC are illustrated in Fig. 5. All of the isotherm model para-
meters for the biosorption of lead(II) onto P. vulgaris L. waste are listed in Table 1. It is evident from these data that the surface of P. vulgaris L. waste is made up of homogeneous biosorption patches. In other words, the Langmuir and D–R isotherm models fit better than the Freundlich isotherm model when the r2 values are compared in Table 1. It concluded that the biosorption of lead(II) ions onto P. vulgaris L. waste
Table 1 Isotherm constants for the biosorption of lead(II) ions onto P. vulgaris L. waste at various temperatures t, EC
2.064×10!4 2.258×10!4 2.314×10!4 2.424×10!4
Langmuir
Freundlich
Dubinin–Radushkevich
KL, dm3 mol!1
r2L
RL
KF, dm3 g!1
n
r2F
qmax, mol g!1
β, mol2 kJ!2
r2D–R
1.727×104 1.906×104 2.923×104 4.087×104
0.996 0.982 0.988 0.993
4.580×10!2 4.167×10!2 2.757×10!2 1.987×10!2
1.621×10!3 2.397×10!3 2.088×10!3 2.091×10!3
3.617 3.251 3.605 3.788
0.980 0.987 0.980 0.970
4.932×10!4 6.154×10!4 5.961×10!4 6.109×10!4
2.687×10!3 2.721×10!3 2.238×10!3 1.945×10!3
0.988 0.991 0.988 0.981
Table 2 Biosorption results of lead(II) ions from the literature by various agricultural biosorbents and operating conditions Biosorbent material
Chaff Rice husk Banana stem Wheat bran Tree fern Palm kernel fibre Pinus sylvestris P. vulgaris L.
Biosorption capacity (mg g!1)
12.50 8.60 91.74 87.00 40.00 49.90 11.38 42.68
Operating conditions pH
t (EC)
Initial concentration range (mg dm!3)
Biomass (g dm!3)
Reference
5.5 5.0 6.0 4.0–7.0 — 5.0 4.0 5.0
20 30–60 30 60 20 65 25 20
8–96 50 10–400 50–400 74.1–344 20–200 10–100 100–250
2–12 10 2 10 4 2.5 4.0 4.0
[9] [10] [13] [14] [18] [21] [37] This study
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20 30 40 50
qmax, mol g!1
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is thus complex, involving more than one mechanism. The maximum biosorption capacity (qmax) of biosorbent calculated from Langmuir isotherm equation defines the total capacity of the biosorbent for lead(II). The biosorption capacity of biosorbent increased with an increase in the temperature. The highest value of qmax obtained at 50EC is 2.424×10!4 mol g!1. It appears to be the highest in comparison with the uptake obtained at the other temperatures (Table 1). The results obtained from this study were also found to be higher than that of many corresponding biosorbents reported in the literature (Table 2) [9,10,13, 14,18,21,37]. The value of RL calculated from Eq. (2) is incorporated in Table 1. As the RL values lie between 0 and 1, the biosorption process is favorable [30,38]. Further, the RL values for this study at all temperatures studied are between 1.987×10!2 and 4.580×10!2. Therefore, the biosorption is spontaneous.
The Freundlich constant KF indicates the biosorption capacity of the biosorbent and the values of KF at equilibrium at all temperatures lie at a range of 1.621×10!3–2.397×10!3 dm3 g!1. The other Freundlich constant n is a measure of the deviation from linearity of the biosorption and the numerical values of n at all temperatures lie between 3.251 and 3.788 and are greater than unity, indicating that lead(II) ions are favorably adsorbed by P. vulgaris L. waste at all the temperatures studied. 3.7. Thermodynamic parameters of biosorption The thermodynamic parameters of ΔH0 and ΔS0 were obtained from the ln KL versus 1/T plot (Fig. 6). The Gibbs free energies (ΔG0) were calculated from Eq. (5), and the results are given in Table 3. The negative values of ΔG0 at all temperatures studied are due to the fact that the biosorption process is spontaneous. The positive value of ΔH0 suggests the endothermic nature of
Fig. 6. Plot of KL versus 1/T for estimation of thermodynamic parameters for lead(II) biosorption onto P. vulgaris L. waste.
A. Safa Ózcan et al. / Desalination 244 (2009) 188–198 Table 3 Thermodynamic parameters calculated from Langmuir isotherm constants (KL) for the biosorption of lead(II) ions onto P. vulgaris L. waste at different temperatures t (EC)
ΔG0 (kJ mol!1)
ΔH0 (kJ mol!1)
ΔS0 (J K!1 mol!1)
20 30 40 50
!23.78 !24.84 !26.77 !28.53
23.63
160.98
biosorption. The positive value of ΔS0 suggests increased randomness at the solid/solution interface during the biosorption of lead(II) onto P. vulgaris L. waste.
4. Conclusions This study has shown that an agricultural byproduct, P. vulgaris L., can be used to remove lead(II) ions from aqueous solution as a function of pH, contact time and temperature. The maximum biosorption capacity of biosorbent for the removal of lead(II) was obtained at pH 5. The biosorption equilibrium data fit well to the Langmuir and D–R isotherm models. The negative values of ΔG0 confirm a favorable biosorption of lead(II) onto P. vulgaris L. waste and the positive value of ΔH0 leads to an endothermic nature of biosorption. The equilibrium was attained in 20 min. It may be concluded from above results that P. vulgaris L. waste can be used for elimination of heavy metal pollution from wastewater because it is a low-cost, abundant waste product and is a locally available biosorbent.
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