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Removal of Cu(II) and Pb(II) ions from aqueous solutions by adsorption on sawdust of Meranti wood
DESALINATION Desalination 247 (2009) 636–646
www.elsevier.com/locate/desal
Removal of Cu(II) and Pb(II) ions from aqueous solutions by adsorption on sawdust of Meranti wood Anees Ahmada*, Mohd. Rafatullahb, Othman Sulaimanb, Mahamad Hakimi Ibrahima, Yap Yee Chiia, Bazlul Mobin Siddiquea a
Environmental Technology Division, School of Industrial Technology, Universiti Sains Malaysia, 11800 Penang, Malaysia b Bioresource Paper and Coatings Technology Division, School of Industrial Technology, Universiti Sains Malaysia, 11800 Penang, Malaysia Tel.: +604-653-2214, Fax: +604-657-3678; email: [email protected], [email protected] Received 6 August 2007; accepted 19 January 2009
Abstract Meranti tree sawdust, an inexpensive material, is currently being investigated as an adsorbent for the removal of Cu(II) and Pb(II) ions from aqueous solutions. In this work, adsorption of Cu(II) and Pb(II) ions on Meranti tree sawdust has been studied by using batch techniques. The equilibrium adsorption level was determined to be a function of the solution of pH, contact time, and adsorbent dosage. Adsorption isotherms of Cu(II) and Pb(II) ions on adsorbents were determined and correlated with common isotherm equations such as Langmuir and Freundlich models. The thermodynamic parameters like free energy, enthalpy, and entropy changes for the adsorption of Cu(II) and Pb(II) ions have also been computed and discussed. The heat of adsorption [DH = 31.47 kJ/mol for Cu(II) and DH = 20.07 kJ/mol for Pb(II)] implied that the adsorption was endothermic in nature. Keywords: Heavy metals; Adsorption; Endothermic; Meranti wood; Sawdust
1. Introduction The presence of heavy metal in the aquatic environment has been of great concern to scientists and engineers because of their increased discharge, toxic nature, and other adverse effects on receiving waters. Unlike most organic pollutants, *Corresponding author.
heavy metals are generally refractory and nondegradable or readily detoxified biologically. Copper is essential to human life and health, but like all heavy metals, it is potentially toxic, especially at high concentrations. Copper and its compounds are ubiquitous in the environment and are thus found frequently in surface waters. Potential sources of copper bearing waste include plating baths, fertilizer industry, paints
0011-9164/09/$– See front matter Ó 2009 Published by Elsevier B.V. doi:10.1016/j.desal.2009.01.007
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and pigments, municipal and storm water run-offs. Intake of excessively large doses of copper by man leads to severe mucosal irritation and corrosion, widespread capillary damage, hepatic and renal damage, and central nervous system irritation followed by depression. Severe gastrointestinal irritation and possible necrotic changes in the liver and kidney could occur. The maximum recommended concentration for drinking water that is regulated in Environmental Quality Act 1974 is 0.2 mg/L [1–3]. Another heavy metal lead has been introduced into natural waters from a variety of sources such as storage batteries, lead smelting, tetraethyl lead manufacturing, mining, plating, ammunition, and the ceramic glass industries. The permissible limit of lead in drinking water by Environmental Quality Act is 0.10 mg/L. The presence of excess lead in drinking water causes diseases such as anemia, encephalopathy, and hepatitis. Lead ions have an affinity for ligands containing thiol and phosphatic groups and they inhibit the biosynthesis of heme, causing damage to both the kidney and liver. However, lead can remain immobilized for years, and hence it is difficult to detect the metabolic disorders it causes [4–6]. In developed countries, removal of heavy metals in wastewater is normally achieved by advanced technologies such as ion exchange, chemical precipitation, ultra filtration, or electrochemical deposition [7–9]. But these technologies do not seem to be economically feasible because of their relatively high costs and that developing countries may not afford such technologies. Therefore, there is a need to look into alternatives to investigate a low-cost method, which is effective and economical. To overcome this difficulty there is a strong need to develop cheap adsorbents which can be used in developing countries. Adsorption by natural materials is another alternative method to solve this kind of problems. The natural materials form complexes with metal ions using their ligand or functional groups. Process for metal removal like adsorption
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has been suggested as being cheaper and more effective than the other technologies [10]. Adsorption was first observed by Lowitz in 1785 [11] and was soon applied as a process for removal of color from sugar during refining. In the second half of the nineteenth century, American water treatment plants used inactivated charcoal filters for water purification. Currently, adsorption on activated carbon is a recognized method for the removal of heavy metals from wastewater [12, 13], while the high cost of activated carbon limits its use in adsorption. A search for a low-cost and easily available adsorbent has led to the investigation of materials of agricultural and biological origin as potential metal sorbents [14]. Agricultural by-products, such as sugar beet pulp and sawdust [15, 16], coconut shell [17], clay [18], red mud [19], tea leaves [20], and so on, have received attention in these types of application. The sorption of metals by these kinds of materials might be attributed to their proteins, carbohydrates, and phenolic compounds that have carboxyl, hydroxyl, sulfate, phosphate, and amino groups that can bind metal ions. Most cases have confirmed that the use of large quantities of wastes from agricultural products for the treatment of polluted water is an attractive and promising option with a double benefit for the environment [21]:
• •
It reduces the residues whose disposal becomes a major, costly problem and It converts the wastes into useful and inexpensive sorbents for water purification
The aim of this article is to assess the potential of Meranti tree sawdust to adsorb Cu(II) and Pb(II) ions from aqueous solutions. The effect of the solution pH, temperature, contact time, and adsorbent doses on the removal of Cu(II) and Pb(II) ions was studied. The adsorption isotherm and probable mechanism are explained. The thermodynamic parameters for
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the adsorption of Cu(II) and Pb(II) ions have also been computed and discussed. 2. Experimental 2.1. Apparatus and instrumentation The concentrations of Cu(II) and Pb(II) in aqueous solutions were determined using atomic absorption spectrophotometer (Analyst 100 Perkin Elmer) operating with an air-acetylene flame. The pH measurements were done with a pH-meter (HACH Model SensION-3). The pH-meter was standardized using buffer solutions with pH values of 4.0, 7.0, and 10.0. The BET surface area was determined through N2 gas adsorption by a Micromeritics PulseChemiSorb 2705 instrument. Temperature-controlled shaker (Rillins Sains Sdn Bhd Model IKA KS 260) was used for equilibrium studies. All filtrations during this work were carried out using Whatman No. 1 filter paper. 2.2. Adsorbent Meranti tree sawdust was collected from Bio-resource, paper and coating (PBC) division of School of Industrial Technology of USM. The sawdust was washed with distilled water and then dried in a dryer at 708C until all the moisture had evaporated. The material was ground to a fine powder in a still mill. The resulting material was sieved in the size range of 100–150 mm particle size. To immobilize the color and water-soluble substances, the ground powder was treated with 2% formaldehyde in the ratio of 1:4 (sawdust: formaldehyde, w/v) at room temperature for 4 h. After that, the sawdust was filtered out, washed with distilled water to remove free formaldehyde, and activated at 708C in a dryer for 24 h. The material was placed in an airtight container for further use. 2.3. Adsorbate solution The aqueous solutions (1000 mg/L) of Cu(II) as well as Pb(II) were prepared in distilled water
using Cu(II) sulfate pentahydrate, CuSO4.5H2O and Pb(II) nitrate, Pb(NO3) respectively. The solutions of different dilutions (25–250 mg/L) required for the adsorption studies were prepared by dilution of the stock solutions. 2.4. Batch adsorption studies Batch adsorption studies were carried out by shaking 0.25 g of the sawdust with 25 ml of the aqueous solutions of Cu(II) and Pb(II) ions in different conical flask at 150 rpm, respectively. The mixtures were filtered out and analyzed for its metal ion concentrations using Atomic Adsorption Spectrometer (AAS) (Analyst 100 Perkin Elmer). All metal solutions were used at neutral pH and performed at room temperature unless otherwise stated. The effect of pH of the initial solution on the equilibrium uptake of Cu(II) and Pb(II) ions was analyzed over a pH range from 2.0 to 7.0 and 2.0 to 7.5, respectively. The pH was adjusted using 0.1 M NaOH and 0.1 M HCI solutions. The adsorption experiments were also conducted to determine the equilibrium time, the optimum pH, and dosage of the adsorbent for maximum adsorption. 2.5. Adsorption model To quantify the adsorption capacity of Meranti tree sawdust for the removal of Cu(II) and Pb(II) ions from aqueous solution, the Langmuir and Freundlich models were used. Langmuir mode: Langmuir proposed the following model: Ce =Am ¼ ð1=KÞð1=bÞ þ ð1=bÞðCe Þ
ð1Þ
where Ce is the equilibrium concentration (mg/L) and Am is the amount adsorbed per specified amount of adsorbent (mg/g), b is the equilibrium constant, and K is the amount of adsorbate required to form a monolayer. Hence, a plot of Ce/Am versus Ce should be a straight line with a slope (1/b) and an intercept as 1/Kb.
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Freundlich model: According to Freundlich model: Am ¼ ðKÞðCe1=n Þ log Am ¼ log K þ 1=n log Ce
ð2Þ
where n is an empirical constant. Thus, a plot of log Am versus log Ce should be a straight line with a slope 1/n and an intercept of log K. This model deals with the multilayer adsorption of the substance on the adsorbent. A computer modeling technique has been applied to fit the Freundlich and Langmuir equations for the adsorption data. The coefficients of determination of least square fitting to a straight line (r2) were computed for these two models.
Fig. 1. SEM image (Mag:500) of sawdust of meranti tree.
3. Results and discussion 3.1. Characterization of Meranti tree sawdust
2.6. Thermodynamic parameters of sorption The Gibbs free energy (DG) is the fundamental criterion of spontaneity of a process and can be determined using equilibrium constant as below: DG ¼ RT ln K
ð3Þ
where R is the universal gas constant (8.314 Jmol1K1) and T is the absolute temperature (K). Similarly, the enthalpy was computed from the following equation ln K ¼ DS=R DH=RT
ð4Þ
DH8 can be obtained from the slope of plot of ln K versus 1/T. The entropy was calculated from the equation, DG ¼ DH TDS
ð5Þ
A SEM micrograph of Meranti tree sawdust is shown in Fig. 1. Meranti tree sawdust is a heterogeneous material consisting largely of small spheres, irregular, porous, coke like particles of cell wall of plant cells. The surface seems to be rough, and protrusions can be seen throughout the micrograph. Pores can be seen however, not extending into the matrix. The surface roughness is indicative of high surface area. 3.2. Physical–chemical characteristics Characteristics of the adsorbent such as surface area, bulk density, moisture content, ash content, solubility in water (inorganic and organic matter) were determined. The results are summarized in Table 1. 3.3. The pH of adsorbent surface A 25 ml of metal solution of 100 mg/L concentration was agitated with 0.25 g of adsorbent at room temperature for 60 min at 150 rpm to
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Table 1 Various physical parameters for the adsorbent (Meranti tree sawdust) Parameters
Values
pH (aqueous solution) Particle size (mm) Surface area (m2/g) Bulk density (g/cm3) Moisture contents (%) Water soluble components (inorganic matter) (%) Insoluble components (organic matter) (%)
7.10 100–150 630.00 0.27 6.25 21.73 76.22
reach equilibrium. Then, the pH of the slurry was measured. The results are shown in Table 1. 3.4. Effect of contact time The dependence of adsorption of Cu(II) and Pb(II) ions with time is presented in (Fig. 2). The data obtained from the adsorption of Cu(II) and Pb(II) ions on the Meranti tree sawdust showed that the adsorption increases with an increasing contact time. The plot reveals that the rate of 80 70
% Adsorption
60 50
percentage removal of Cu(II) and Pb(II) ions is initially high which is probably due to the availability of larger surface area of the sawdust for the adsorption of these ions. As the surface adsorption sites become exhausted, the rate of uptake is controlled by the rate of transport from the exterior to the interior sites of the adsorbent particles. Further, the maximum percentage removal of Cu(II) and Pb(II) ions was attained after 60 min of stirring time at different concentrations. Therefore, the contact time of 60 min was sufficient to achieve equilibrium for both these ions. The adsorption did not change much with further increase in contact time. Therefore, in each experiment, the shaking time was set to be 60 min. Figure 2 shows that the percentage adsorption of Pb(II) ions is higher than Cu(II) ions. The adsorption of Pb(II) ions only slightly increases with the increase in contact time compared to the Cu(II) ions. A similar result has been found by Unlu and Ersoz [22] in adsorption characteristic of heavy metal ions onto a lowcost biopolymeric sorbent from aqueous solutions. It is necessary to point out that these tests were done under a condition without any pH adjustment. However, it is supposed that the pH of the metal ions solution may affect metal affinity for adsorption on the Meranti tree sawdust and that the process of metal adsorption may change the pH of an unbuffered solution. Anyway, pH may play an important role in the metals adsorption, which is an important aspect of our further studies.
40
3.5. Effect of pH
30
The pH is one of the most important environmental factors influencing not only the site dissociation, but also the solution chemistry of the heavy metals: hydrolysis, complexation by organic and/ or inorganic ligands, redox reactions, and precipitation are strongly influenced by pH and, on the other hand, strongly influence the speciation and adsorption availability of heavy metals. The effect of pH on the adsorption of Cu(II) and Pb(II) ions on Meranti tree sawdust has
20 copper lead
10 0 0
30
60
90 120 Time (min)
150
180
Fig. 2. Effect of time in the removal of Cu(II) and Pb(II) by sawdust.
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been studied by varying it in the ranges of 2.0–7.0 for Cu(II) ions and 2.0–7.5 for Pb(II) ions as shown in (Fig. 3). As shown in Fig. 3, the uptake of Cu(II) and Pb(II) ions depends on pH, it increases with the increase in pH from 2.0 to 6.6 and 2.0 to 7.0, respectively. Above this pH range, a decrease in the uptake of both the metal ions was observed. The peak percentage adsorptions of Cu(II) as well as Pb(II) ions were attained at pH 6.6 and 7.0, respectively. Based on the behavior of heavy metal adsorption on sawdust, it is speculated that the ion exchange and hydrogen bonding may be the principal mechanism for the removal of heavy metals [15]. There are a number of parameters to support this speculation, including the components and complexing properties of the sawdust, the properties of heavy metals and the adsorption behavior, such as the effect of pH of the aqueous media leading to change in speciation. Based on the electron-donating nature of the O-, S-, N-, and P-containing groups in sawdust and the electron-accepting nature of heavy metal ions, the ion exchange mechanism could 100
641
be preferentially considered. For instance, a divalent heavy metal ion may attach itself to two adjacent hydroxyl groups and oxyl groups which can donate two pairs of electrons to the metal ion, forming four coordination number compounds and releasing two hydrogen ions into solution. It is then readily understood that the equilibrium is quite dependent on pH of the aqueous solution. At lower pH, the H+ ions compete with metal cations for the exchange sites on the sawdust, thereby partially releasing the latter. The heavy metal cations are completely released under circumstances of extreme acidic conditions [23, 24]. At pH value lower than 3, the adsorption capacities were found to be low due to the competitive adsorption of HO3+ ions and metal ions for the same active adsorption site. As the pH increases, the adsorption surface becomes less positive and therefore electrostatic attraction between the metal ions and sawdust surface is likely to be increased. The maximum sorption efficiency in the range of 2.0–6.6 for Cu(II) and 2.0–7.0 for Pb(II) ions may be due to the interaction of M+, M(OH)+, and M(OH)2 with surface functional groups present in the sawdust. A decrease in adsorption at high pH is due to the formation of soluble hydroxyl complexes.
90 80
3.6. Effect of adsorbent dosage
% Adsorption
70 60 40 50 30 20
copper lead
10 0
0
1
2
3
4 pH
5
6
7
8
Fig. 3. Effect of pH in the removal of Cu(II) and Pb(II) by sawdust.
Adsorbent dosage is an important parameter because it determines the capacity of an adsorbent for a given concentration of the adsorbate. The adsorption studies of Cu(II) and Pb(II) ions on Meranti tree sawdust were done at room temperature by varying the quantity of adsorbent from 0.25 to 2.00 g while keeping the volume of the metal solutions constant at different pH. The influence of adsorbent dosage in the removal of Cu(II) and Pb(II) ions is shown in Fig. 4. The increase in the adsorbent dosage from 0.25 to 2.0 g at pH 6.6 for Cu(II) resulted in an increase of adsorption from 90% to 97.5%. On the other
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3.7. Adsorption behavior of sawdust (isotherm studies)
98 97
The related parameters for the fitting of Freundlich and Langmuir equations at different temperatures are summarized in Table 2 and isotherm plots are shown in Figs 5–8 for Cu(II) and Pb(II) ions, respectively. The adsorption of Cu(II) and Pb(II) ions on sawdust at pH 6.6 and 7.0, respectively, follow both Freundlich- and Langmuir-type adsorption isotherms. However, the Freundlich equation is better obeyed by the system than the Langmuir one as is evident from the values of regression coefficient shown in Table 2. The Freundlich-type adsorption isotherm is an indication of surface heterogeneity of the adsorbent while Langmuir-type isotherm hints toward surface homogeneity of the adsorbent. This leads to the conclusion that the surface of sawdust is made up of small heterogeneous adsorption patches which are very much similar to each other with respect to adsorption phenomenon. With the increase in concentration at elevated temperature, the activation of adsorption sites takes place leading to increased adsorption probably via a surface exchange reaction. Actually at high temperatures, the aggregation of Cu(II) ions at the surface of sawdust increases which results in an exchange reaction with the already-adsorbed species along with the normal physio-sorption.
% Adsorption
96 95 94 93 92 91 copper lead
90 89 0
0.5 1 1.5 2 Amount of adsorbent (g)
2.5
Fig. 4. Effect of adsorbent dosage in the removal of Cu(II) and Pb(II) by sawdust.
hand, the percentage for Pb(II) ions adsorption at pH 7.0 is 90.4% to 93.9% with the adsorbent dosage ranging from 0.25 to 2.00 g. The results show that the adsorption increases with the increase in the dose of sawdust. This is because of the availability of more binding sites on the surface at higher concentration of the adsorbent for complexation of metal ions. From this point of view, it is easily understandable that the initial concentration of Cu(II) and Pb(II) ions gave some effect on its percentage removal (Fig. 4).
Table 2 The related parameters for the Cu(II) and Pb(II) adsorption on sawdust at different temperatures Temp (8C)
30 40 50 60
Freundlich constants Cu(II) K 1/n r2 (mg/g)
Pb(II) r2 K (mg/g)
1/n
0.99 0.99 0.99 0.99
0.99 0.99 0.99 0.99
0.76 0.74 0.73 0.68
2.74 3.20 3.83 5.43
0.70 0.68 0.67 0.59
1.01 1.17 1.28 1.76
Langmuir constants Cu(II) r2 b K (mg/g) (L/mg)
Pb(II) r2 b (mg/g)
K (L/mg)
0.95 0.96 0.97 0.95
0.97 0.99 0.99 0.97
0.02 0.02 0.03 0.04
37.17 34.84 34.13 30.67
0.07 0.09 0.12 0.22
37.04 34.84 33.90 31.95
A. Ahmad et al. / Desalination 247 (2009) 636–646 1.6
1.6
1.4
1.4
1.2
1.2
log Am
1
1 log Am
0.8 0.6 30°C 40°C 50°C 60°C
0.4
0
⫺0.5
0.8 0.6 30°C 40°C 50°C 60°C
0.4
0.2 ⫺1
643
0.2 0
0.5
1
1.5
log Ce
0 0
1 log Ce
0.5
Fig. 5. Freundlich plots for the adsorption of Cu(II) by sawdust.
1.5
2
1.2
3
1
2.5
0.8
2 Ce /Am (mg/L)
Ce /Am (mg/L)
Fig. 7. Freundlich plots for the adsorption of Pb(II) by sawdust.
0.6
0.4
30°C 40°C 50°C 60°C
1 30°C 40°C 50°C 60°C
0.2
0
1.5
0
5
10
15
20
0.5
25
Ce (mg/L)
0 0
10
20
30
40
50
60
Ce (mg/L)
Fig. 6. Langmuir plots for the adsorption of Cu(II) by sawdust.
Fig. 8. Langmuir plots for the adsorption of Pb(II) by sawdust.
It was reported in our previous study [15] that the efficiency for the removal of copper from river water using Mango sawdust was 63%, and it was thus concluded that the sawdust is
an excellent adsorbent for copper removal from aqueous solution. In 2001, Yu et al. [25] on the adsorption behavior of maple sawdust for the removal of heavy metals, such as Cu(II)
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and Pb(II), presented some guidelines for the application of sawdust adsorption. In these studies, it was investigated that the adsorption behavior, capacity, and other factors, such as pH and sawdust dose, using untreated maple sawdust. Under optimized conditions, the percentage of metal removal by maple sawdust adsorption was over 90%; indicating sawdust is an appealing adsorbent for the removal of heavy metals. Raji and Anirudhan reported studies on the removal of Pb(II) using polyacrylamide grafted sawdust [26] and polymerized sawdust [27]. In those studies, adsorption and desorption experiments were conducted, and various affecting factors tested. Research data showed that the treated sawdusts might be effectively used as sorbents for the removal of Pb(II) from aqueous media. The maximum removal of over 98% was reported. By comparison of the results obtained from this study to the previously reported work on percentage removal of various low-cost adsorbents in aqueous solution for Cu(II) and Pb(II) ions, it can be stated that our findings are well. 3.8. Thermodynamic parameter of sorption The various thermodynamic parameters, that is free energy (DG), enthalpy (DH), and entropy (DS), and its values associated with the sorption
of Cu(II) and Pb(II) ions onto the Meranti tree sawdust are listed in Table 3 which show that the overall processes for both ions are endothermic. The DH for Cu(II) is 31.47 kJ/mol and the DH for Pb(II) is 20.07 kJ/mol. Besides that, the K value from Table 2 indicates the affinity toward the binding of metal ions [28]. The highest value of K was found at 608C for each metal ion. The free energy of adsorption (DG) can be related with the equilibrium constant K, corresponding to the reciprocal of the Langmuir constant from Eqn (3). The free energy of the process at all temperature is negative for both ions and it decreases with an increase in temperature which indicates that the process is spontaneous in nature and the spontaneity increases with the rise in temperature. Also enthalpy (DH) and entropy (DS) changes can be estimated by Eqns (4) and (5). The values of DS were found to be positive due to the exchange of the metal ions with more mobile ions present on the exchanger, which would cause increase in the entropy, during the adsorption process. Besides that, the entropy also can be increased in the case of physisorption which may also contribute to the total adsorption process. This is due to the water molecules released from the hydrated ions or water molecules present on the surface during the adsorption process [29].
Table 3 Thermodynamic parameters for the Cu(II) and Pb(II) adsorption on sawdust at different temperatures Temp (8C)
30 40 50 60
For Langmuir isotherms Cu(II) ln K DG DS 8.37 8.64 8.92 9.53
21.09 22.51 23.98 26.40
0.17 0.17 0.17 0.17
DH (30–608C) 31.47
DG = kJmole1, DS = kJmole1K1, and DH = kJmole1
Pb(II) ln K
DG
DS
DH (30–608C)
8.33 8.55 8.69 9.08
20.99 22.26 23.34 25.16
0.13 0.13 0.13 0.14
120.07
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4. Conclusions Based on the present study, it is clearly shown that sawdust obtained from Meranti wood is an effective adsorbent for the removal of Cu(II) and Pb(II) ions from aqueous solution. The adsorption process is strongly affected by parameters such as time, pH, adsorbent dosage, and temperature. The equilibrium time for Cu(II) and Pb(II) ions is determined as 60 min. The percentage adsorption for Pb(II) seems to be greater than adsorption of Cu(II) in this equilibrium time. The plot of pH versus percentage adsorption shows significant adsorption at pH 6.60 for the Cu(II) ions and at pH 7.00 for the Pb(II) ions. The removal is almost 100% for Cu(II) at this pH value being 99.39%. On the other hand, percentage removal for Pb(II) at this pH is 94.61%. The percentage adsorption of Cu(II) and Pb(II) ions was increased with increasing the adsorbent dosage. The adsorption of Cu(II) and Pb(II) ions on sawdust at pH 6.60 and 7.00, respectively, follow both Freundlichand Langmuir-type adsorption isotherms. Thermodynamic constants were also evaluated using equilibrium constants changing with temperature. The negative value of DG for Cu(II) and Pb(II) ions indicated the spontaneity of the process. The positive value of DH showed the endothermic nature of Cu(II) and Pb(II) sorption, respectively. Acknowledgment The study was funded through USM shortterm grant number 304/ PTEKIND/ 637044. The authors acknowledge the USM for providing research facilities.
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[23] U. Forstner and G.T.W. Wittmann, Metal pollution in the aquatic environment, 2nd edition, Springer, New York, 1983. [24] A. Shukla, Y. Zhang, P. Dubey, J.L. Margrave and S.S. Shukla, The role of sawdust in the removal of unwanted materials from water. Journal of Hazardous Materials, B95 (2002) 137–152. [25] B. Yu, Y. Zhang, A. Shukla, S.S. Shukla and K.L. Dorris, The removal of heavy metals from aqueous solutions by sawdust adsorption — removal of lead and comparison of its adsorption with copper. Journal of Hazardous Material, B84 (2001) 83–94. [26] C. Raji and T.S. Anirudhan, Kinetics of Pb(II) adsorption by polyacrylamide grafted sawdust. Indian Journal of Chemical Technology, 4 (1997) 157. [27] C. Raji and T.S. Anirudhan, Use of chemically modified sawdust in the removal of Pb(II) ions from aqueous media. Indian Journal of Environment Health, 39 (1997) 230. [28] A.Y Dursun, A Comparative on determination of the equilibrium, kinetic and thermodynamic parameter of biosorption o copper(II) an lead(II) ions onto pretreated aspergillus niger. Biochemical Engineering Journal, 28 (2006) 187–195. [29] G. Uslu and M. Tanyol, Equilibrium and thermodynamic parameters of single and binary mixture biosorption of lead(II) and copper(II) ions onto pseudomonas putida: effect of temperature. Journal of Hazardous Materials, 135 (2006) 87–93.
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Removal of Cu(II) and Pb(II) ions from aqueous solutions by adsorption on sawdust of Meranti wood Anees Ahmada*, Mohd. Rafatullahb, Othman Sulaimanb, Mahamad Hakimi Ibrahima, Yap Yee Chiia, Bazlul Mobin Siddiquea a
Environmental Technology Division, School of Industrial Technology, Universiti Sains Malaysia, 11800 Penang, Malaysia b Bioresource Paper and Coatings Technology Division, School of Industrial Technology, Universiti Sains Malaysia, 11800 Penang, Malaysia Tel.: +604-653-2214, Fax: +604-657-3678; email: [email protected], [email protected] Received 6 August 2007; accepted 19 January 2009
Abstract Meranti tree sawdust, an inexpensive material, is currently being investigated as an adsorbent for the removal of Cu(II) and Pb(II) ions from aqueous solutions. In this work, adsorption of Cu(II) and Pb(II) ions on Meranti tree sawdust has been studied by using batch techniques. The equilibrium adsorption level was determined to be a function of the solution of pH, contact time, and adsorbent dosage. Adsorption isotherms of Cu(II) and Pb(II) ions on adsorbents were determined and correlated with common isotherm equations such as Langmuir and Freundlich models. The thermodynamic parameters like free energy, enthalpy, and entropy changes for the adsorption of Cu(II) and Pb(II) ions have also been computed and discussed. The heat of adsorption [DH = 31.47 kJ/mol for Cu(II) and DH = 20.07 kJ/mol for Pb(II)] implied that the adsorption was endothermic in nature. Keywords: Heavy metals; Adsorption; Endothermic; Meranti wood; Sawdust
1. Introduction The presence of heavy metal in the aquatic environment has been of great concern to scientists and engineers because of their increased discharge, toxic nature, and other adverse effects on receiving waters. Unlike most organic pollutants, *Corresponding author.
heavy metals are generally refractory and nondegradable or readily detoxified biologically. Copper is essential to human life and health, but like all heavy metals, it is potentially toxic, especially at high concentrations. Copper and its compounds are ubiquitous in the environment and are thus found frequently in surface waters. Potential sources of copper bearing waste include plating baths, fertilizer industry, paints
0011-9164/09/$– See front matter Ó 2009 Published by Elsevier B.V. doi:10.1016/j.desal.2009.01.007
A. Ahmad et al. / Desalination 247 (2009) 636–646
and pigments, municipal and storm water run-offs. Intake of excessively large doses of copper by man leads to severe mucosal irritation and corrosion, widespread capillary damage, hepatic and renal damage, and central nervous system irritation followed by depression. Severe gastrointestinal irritation and possible necrotic changes in the liver and kidney could occur. The maximum recommended concentration for drinking water that is regulated in Environmental Quality Act 1974 is 0.2 mg/L [1–3]. Another heavy metal lead has been introduced into natural waters from a variety of sources such as storage batteries, lead smelting, tetraethyl lead manufacturing, mining, plating, ammunition, and the ceramic glass industries. The permissible limit of lead in drinking water by Environmental Quality Act is 0.10 mg/L. The presence of excess lead in drinking water causes diseases such as anemia, encephalopathy, and hepatitis. Lead ions have an affinity for ligands containing thiol and phosphatic groups and they inhibit the biosynthesis of heme, causing damage to both the kidney and liver. However, lead can remain immobilized for years, and hence it is difficult to detect the metabolic disorders it causes [4–6]. In developed countries, removal of heavy metals in wastewater is normally achieved by advanced technologies such as ion exchange, chemical precipitation, ultra filtration, or electrochemical deposition [7–9]. But these technologies do not seem to be economically feasible because of their relatively high costs and that developing countries may not afford such technologies. Therefore, there is a need to look into alternatives to investigate a low-cost method, which is effective and economical. To overcome this difficulty there is a strong need to develop cheap adsorbents which can be used in developing countries. Adsorption by natural materials is another alternative method to solve this kind of problems. The natural materials form complexes with metal ions using their ligand or functional groups. Process for metal removal like adsorption
637
has been suggested as being cheaper and more effective than the other technologies [10]. Adsorption was first observed by Lowitz in 1785 [11] and was soon applied as a process for removal of color from sugar during refining. In the second half of the nineteenth century, American water treatment plants used inactivated charcoal filters for water purification. Currently, adsorption on activated carbon is a recognized method for the removal of heavy metals from wastewater [12, 13], while the high cost of activated carbon limits its use in adsorption. A search for a low-cost and easily available adsorbent has led to the investigation of materials of agricultural and biological origin as potential metal sorbents [14]. Agricultural by-products, such as sugar beet pulp and sawdust [15, 16], coconut shell [17], clay [18], red mud [19], tea leaves [20], and so on, have received attention in these types of application. The sorption of metals by these kinds of materials might be attributed to their proteins, carbohydrates, and phenolic compounds that have carboxyl, hydroxyl, sulfate, phosphate, and amino groups that can bind metal ions. Most cases have confirmed that the use of large quantities of wastes from agricultural products for the treatment of polluted water is an attractive and promising option with a double benefit for the environment [21]:
• •
It reduces the residues whose disposal becomes a major, costly problem and It converts the wastes into useful and inexpensive sorbents for water purification
The aim of this article is to assess the potential of Meranti tree sawdust to adsorb Cu(II) and Pb(II) ions from aqueous solutions. The effect of the solution pH, temperature, contact time, and adsorbent doses on the removal of Cu(II) and Pb(II) ions was studied. The adsorption isotherm and probable mechanism are explained. The thermodynamic parameters for
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the adsorption of Cu(II) and Pb(II) ions have also been computed and discussed. 2. Experimental 2.1. Apparatus and instrumentation The concentrations of Cu(II) and Pb(II) in aqueous solutions were determined using atomic absorption spectrophotometer (Analyst 100 Perkin Elmer) operating with an air-acetylene flame. The pH measurements were done with a pH-meter (HACH Model SensION-3). The pH-meter was standardized using buffer solutions with pH values of 4.0, 7.0, and 10.0. The BET surface area was determined through N2 gas adsorption by a Micromeritics PulseChemiSorb 2705 instrument. Temperature-controlled shaker (Rillins Sains Sdn Bhd Model IKA KS 260) was used for equilibrium studies. All filtrations during this work were carried out using Whatman No. 1 filter paper. 2.2. Adsorbent Meranti tree sawdust was collected from Bio-resource, paper and coating (PBC) division of School of Industrial Technology of USM. The sawdust was washed with distilled water and then dried in a dryer at 708C until all the moisture had evaporated. The material was ground to a fine powder in a still mill. The resulting material was sieved in the size range of 100–150 mm particle size. To immobilize the color and water-soluble substances, the ground powder was treated with 2% formaldehyde in the ratio of 1:4 (sawdust: formaldehyde, w/v) at room temperature for 4 h. After that, the sawdust was filtered out, washed with distilled water to remove free formaldehyde, and activated at 708C in a dryer for 24 h. The material was placed in an airtight container for further use. 2.3. Adsorbate solution The aqueous solutions (1000 mg/L) of Cu(II) as well as Pb(II) were prepared in distilled water
using Cu(II) sulfate pentahydrate, CuSO4.5H2O and Pb(II) nitrate, Pb(NO3) respectively. The solutions of different dilutions (25–250 mg/L) required for the adsorption studies were prepared by dilution of the stock solutions. 2.4. Batch adsorption studies Batch adsorption studies were carried out by shaking 0.25 g of the sawdust with 25 ml of the aqueous solutions of Cu(II) and Pb(II) ions in different conical flask at 150 rpm, respectively. The mixtures were filtered out and analyzed for its metal ion concentrations using Atomic Adsorption Spectrometer (AAS) (Analyst 100 Perkin Elmer). All metal solutions were used at neutral pH and performed at room temperature unless otherwise stated. The effect of pH of the initial solution on the equilibrium uptake of Cu(II) and Pb(II) ions was analyzed over a pH range from 2.0 to 7.0 and 2.0 to 7.5, respectively. The pH was adjusted using 0.1 M NaOH and 0.1 M HCI solutions. The adsorption experiments were also conducted to determine the equilibrium time, the optimum pH, and dosage of the adsorbent for maximum adsorption. 2.5. Adsorption model To quantify the adsorption capacity of Meranti tree sawdust for the removal of Cu(II) and Pb(II) ions from aqueous solution, the Langmuir and Freundlich models were used. Langmuir mode: Langmuir proposed the following model: Ce =Am ¼ ð1=KÞð1=bÞ þ ð1=bÞðCe Þ
ð1Þ
where Ce is the equilibrium concentration (mg/L) and Am is the amount adsorbed per specified amount of adsorbent (mg/g), b is the equilibrium constant, and K is the amount of adsorbate required to form a monolayer. Hence, a plot of Ce/Am versus Ce should be a straight line with a slope (1/b) and an intercept as 1/Kb.
A. Ahmad et al. / Desalination 247 (2009) 636–646
639
Freundlich model: According to Freundlich model: Am ¼ ðKÞðCe1=n Þ log Am ¼ log K þ 1=n log Ce
ð2Þ
where n is an empirical constant. Thus, a plot of log Am versus log Ce should be a straight line with a slope 1/n and an intercept of log K. This model deals with the multilayer adsorption of the substance on the adsorbent. A computer modeling technique has been applied to fit the Freundlich and Langmuir equations for the adsorption data. The coefficients of determination of least square fitting to a straight line (r2) were computed for these two models.
Fig. 1. SEM image (Mag:500) of sawdust of meranti tree.
3. Results and discussion 3.1. Characterization of Meranti tree sawdust
2.6. Thermodynamic parameters of sorption The Gibbs free energy (DG) is the fundamental criterion of spontaneity of a process and can be determined using equilibrium constant as below: DG ¼ RT ln K
ð3Þ
where R is the universal gas constant (8.314 Jmol1K1) and T is the absolute temperature (K). Similarly, the enthalpy was computed from the following equation ln K ¼ DS=R DH=RT
ð4Þ
DH8 can be obtained from the slope of plot of ln K versus 1/T. The entropy was calculated from the equation, DG ¼ DH TDS
ð5Þ
A SEM micrograph of Meranti tree sawdust is shown in Fig. 1. Meranti tree sawdust is a heterogeneous material consisting largely of small spheres, irregular, porous, coke like particles of cell wall of plant cells. The surface seems to be rough, and protrusions can be seen throughout the micrograph. Pores can be seen however, not extending into the matrix. The surface roughness is indicative of high surface area. 3.2. Physical–chemical characteristics Characteristics of the adsorbent such as surface area, bulk density, moisture content, ash content, solubility in water (inorganic and organic matter) were determined. The results are summarized in Table 1. 3.3. The pH of adsorbent surface A 25 ml of metal solution of 100 mg/L concentration was agitated with 0.25 g of adsorbent at room temperature for 60 min at 150 rpm to
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Table 1 Various physical parameters for the adsorbent (Meranti tree sawdust) Parameters
Values
pH (aqueous solution) Particle size (mm) Surface area (m2/g) Bulk density (g/cm3) Moisture contents (%) Water soluble components (inorganic matter) (%) Insoluble components (organic matter) (%)
7.10 100–150 630.00 0.27 6.25 21.73 76.22
reach equilibrium. Then, the pH of the slurry was measured. The results are shown in Table 1. 3.4. Effect of contact time The dependence of adsorption of Cu(II) and Pb(II) ions with time is presented in (Fig. 2). The data obtained from the adsorption of Cu(II) and Pb(II) ions on the Meranti tree sawdust showed that the adsorption increases with an increasing contact time. The plot reveals that the rate of 80 70
% Adsorption
60 50
percentage removal of Cu(II) and Pb(II) ions is initially high which is probably due to the availability of larger surface area of the sawdust for the adsorption of these ions. As the surface adsorption sites become exhausted, the rate of uptake is controlled by the rate of transport from the exterior to the interior sites of the adsorbent particles. Further, the maximum percentage removal of Cu(II) and Pb(II) ions was attained after 60 min of stirring time at different concentrations. Therefore, the contact time of 60 min was sufficient to achieve equilibrium for both these ions. The adsorption did not change much with further increase in contact time. Therefore, in each experiment, the shaking time was set to be 60 min. Figure 2 shows that the percentage adsorption of Pb(II) ions is higher than Cu(II) ions. The adsorption of Pb(II) ions only slightly increases with the increase in contact time compared to the Cu(II) ions. A similar result has been found by Unlu and Ersoz [22] in adsorption characteristic of heavy metal ions onto a lowcost biopolymeric sorbent from aqueous solutions. It is necessary to point out that these tests were done under a condition without any pH adjustment. However, it is supposed that the pH of the metal ions solution may affect metal affinity for adsorption on the Meranti tree sawdust and that the process of metal adsorption may change the pH of an unbuffered solution. Anyway, pH may play an important role in the metals adsorption, which is an important aspect of our further studies.
40
3.5. Effect of pH
30
The pH is one of the most important environmental factors influencing not only the site dissociation, but also the solution chemistry of the heavy metals: hydrolysis, complexation by organic and/ or inorganic ligands, redox reactions, and precipitation are strongly influenced by pH and, on the other hand, strongly influence the speciation and adsorption availability of heavy metals. The effect of pH on the adsorption of Cu(II) and Pb(II) ions on Meranti tree sawdust has
20 copper lead
10 0 0
30
60
90 120 Time (min)
150
180
Fig. 2. Effect of time in the removal of Cu(II) and Pb(II) by sawdust.
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been studied by varying it in the ranges of 2.0–7.0 for Cu(II) ions and 2.0–7.5 for Pb(II) ions as shown in (Fig. 3). As shown in Fig. 3, the uptake of Cu(II) and Pb(II) ions depends on pH, it increases with the increase in pH from 2.0 to 6.6 and 2.0 to 7.0, respectively. Above this pH range, a decrease in the uptake of both the metal ions was observed. The peak percentage adsorptions of Cu(II) as well as Pb(II) ions were attained at pH 6.6 and 7.0, respectively. Based on the behavior of heavy metal adsorption on sawdust, it is speculated that the ion exchange and hydrogen bonding may be the principal mechanism for the removal of heavy metals [15]. There are a number of parameters to support this speculation, including the components and complexing properties of the sawdust, the properties of heavy metals and the adsorption behavior, such as the effect of pH of the aqueous media leading to change in speciation. Based on the electron-donating nature of the O-, S-, N-, and P-containing groups in sawdust and the electron-accepting nature of heavy metal ions, the ion exchange mechanism could 100
641
be preferentially considered. For instance, a divalent heavy metal ion may attach itself to two adjacent hydroxyl groups and oxyl groups which can donate two pairs of electrons to the metal ion, forming four coordination number compounds and releasing two hydrogen ions into solution. It is then readily understood that the equilibrium is quite dependent on pH of the aqueous solution. At lower pH, the H+ ions compete with metal cations for the exchange sites on the sawdust, thereby partially releasing the latter. The heavy metal cations are completely released under circumstances of extreme acidic conditions [23, 24]. At pH value lower than 3, the adsorption capacities were found to be low due to the competitive adsorption of HO3+ ions and metal ions for the same active adsorption site. As the pH increases, the adsorption surface becomes less positive and therefore electrostatic attraction between the metal ions and sawdust surface is likely to be increased. The maximum sorption efficiency in the range of 2.0–6.6 for Cu(II) and 2.0–7.0 for Pb(II) ions may be due to the interaction of M+, M(OH)+, and M(OH)2 with surface functional groups present in the sawdust. A decrease in adsorption at high pH is due to the formation of soluble hydroxyl complexes.
90 80
3.6. Effect of adsorbent dosage
% Adsorption
70 60 40 50 30 20
copper lead
10 0
0
1
2
3
4 pH
5
6
7
8
Fig. 3. Effect of pH in the removal of Cu(II) and Pb(II) by sawdust.
Adsorbent dosage is an important parameter because it determines the capacity of an adsorbent for a given concentration of the adsorbate. The adsorption studies of Cu(II) and Pb(II) ions on Meranti tree sawdust were done at room temperature by varying the quantity of adsorbent from 0.25 to 2.00 g while keeping the volume of the metal solutions constant at different pH. The influence of adsorbent dosage in the removal of Cu(II) and Pb(II) ions is shown in Fig. 4. The increase in the adsorbent dosage from 0.25 to 2.0 g at pH 6.6 for Cu(II) resulted in an increase of adsorption from 90% to 97.5%. On the other
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3.7. Adsorption behavior of sawdust (isotherm studies)
98 97
The related parameters for the fitting of Freundlich and Langmuir equations at different temperatures are summarized in Table 2 and isotherm plots are shown in Figs 5–8 for Cu(II) and Pb(II) ions, respectively. The adsorption of Cu(II) and Pb(II) ions on sawdust at pH 6.6 and 7.0, respectively, follow both Freundlich- and Langmuir-type adsorption isotherms. However, the Freundlich equation is better obeyed by the system than the Langmuir one as is evident from the values of regression coefficient shown in Table 2. The Freundlich-type adsorption isotherm is an indication of surface heterogeneity of the adsorbent while Langmuir-type isotherm hints toward surface homogeneity of the adsorbent. This leads to the conclusion that the surface of sawdust is made up of small heterogeneous adsorption patches which are very much similar to each other with respect to adsorption phenomenon. With the increase in concentration at elevated temperature, the activation of adsorption sites takes place leading to increased adsorption probably via a surface exchange reaction. Actually at high temperatures, the aggregation of Cu(II) ions at the surface of sawdust increases which results in an exchange reaction with the already-adsorbed species along with the normal physio-sorption.
% Adsorption
96 95 94 93 92 91 copper lead
90 89 0
0.5 1 1.5 2 Amount of adsorbent (g)
2.5
Fig. 4. Effect of adsorbent dosage in the removal of Cu(II) and Pb(II) by sawdust.
hand, the percentage for Pb(II) ions adsorption at pH 7.0 is 90.4% to 93.9% with the adsorbent dosage ranging from 0.25 to 2.00 g. The results show that the adsorption increases with the increase in the dose of sawdust. This is because of the availability of more binding sites on the surface at higher concentration of the adsorbent for complexation of metal ions. From this point of view, it is easily understandable that the initial concentration of Cu(II) and Pb(II) ions gave some effect on its percentage removal (Fig. 4).
Table 2 The related parameters for the Cu(II) and Pb(II) adsorption on sawdust at different temperatures Temp (8C)
30 40 50 60
Freundlich constants Cu(II) K 1/n r2 (mg/g)
Pb(II) r2 K (mg/g)
1/n
0.99 0.99 0.99 0.99
0.99 0.99 0.99 0.99
0.76 0.74 0.73 0.68
2.74 3.20 3.83 5.43
0.70 0.68 0.67 0.59
1.01 1.17 1.28 1.76
Langmuir constants Cu(II) r2 b K (mg/g) (L/mg)
Pb(II) r2 b (mg/g)
K (L/mg)
0.95 0.96 0.97 0.95
0.97 0.99 0.99 0.97
0.02 0.02 0.03 0.04
37.17 34.84 34.13 30.67
0.07 0.09 0.12 0.22
37.04 34.84 33.90 31.95
A. Ahmad et al. / Desalination 247 (2009) 636–646 1.6
1.6
1.4
1.4
1.2
1.2
log Am
1
1 log Am
0.8 0.6 30°C 40°C 50°C 60°C
0.4
0
⫺0.5
0.8 0.6 30°C 40°C 50°C 60°C
0.4
0.2 ⫺1
643
0.2 0
0.5
1
1.5
log Ce
0 0
1 log Ce
0.5
Fig. 5. Freundlich plots for the adsorption of Cu(II) by sawdust.
1.5
2
1.2
3
1
2.5
0.8
2 Ce /Am (mg/L)
Ce /Am (mg/L)
Fig. 7. Freundlich plots for the adsorption of Pb(II) by sawdust.
0.6
0.4
30°C 40°C 50°C 60°C
1 30°C 40°C 50°C 60°C
0.2
0
1.5
0
5
10
15
20
0.5
25
Ce (mg/L)
0 0
10
20
30
40
50
60
Ce (mg/L)
Fig. 6. Langmuir plots for the adsorption of Cu(II) by sawdust.
Fig. 8. Langmuir plots for the adsorption of Pb(II) by sawdust.
It was reported in our previous study [15] that the efficiency for the removal of copper from river water using Mango sawdust was 63%, and it was thus concluded that the sawdust is
an excellent adsorbent for copper removal from aqueous solution. In 2001, Yu et al. [25] on the adsorption behavior of maple sawdust for the removal of heavy metals, such as Cu(II)
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and Pb(II), presented some guidelines for the application of sawdust adsorption. In these studies, it was investigated that the adsorption behavior, capacity, and other factors, such as pH and sawdust dose, using untreated maple sawdust. Under optimized conditions, the percentage of metal removal by maple sawdust adsorption was over 90%; indicating sawdust is an appealing adsorbent for the removal of heavy metals. Raji and Anirudhan reported studies on the removal of Pb(II) using polyacrylamide grafted sawdust [26] and polymerized sawdust [27]. In those studies, adsorption and desorption experiments were conducted, and various affecting factors tested. Research data showed that the treated sawdusts might be effectively used as sorbents for the removal of Pb(II) from aqueous media. The maximum removal of over 98% was reported. By comparison of the results obtained from this study to the previously reported work on percentage removal of various low-cost adsorbents in aqueous solution for Cu(II) and Pb(II) ions, it can be stated that our findings are well. 3.8. Thermodynamic parameter of sorption The various thermodynamic parameters, that is free energy (DG), enthalpy (DH), and entropy (DS), and its values associated with the sorption
of Cu(II) and Pb(II) ions onto the Meranti tree sawdust are listed in Table 3 which show that the overall processes for both ions are endothermic. The DH for Cu(II) is 31.47 kJ/mol and the DH for Pb(II) is 20.07 kJ/mol. Besides that, the K value from Table 2 indicates the affinity toward the binding of metal ions [28]. The highest value of K was found at 608C for each metal ion. The free energy of adsorption (DG) can be related with the equilibrium constant K, corresponding to the reciprocal of the Langmuir constant from Eqn (3). The free energy of the process at all temperature is negative for both ions and it decreases with an increase in temperature which indicates that the process is spontaneous in nature and the spontaneity increases with the rise in temperature. Also enthalpy (DH) and entropy (DS) changes can be estimated by Eqns (4) and (5). The values of DS were found to be positive due to the exchange of the metal ions with more mobile ions present on the exchanger, which would cause increase in the entropy, during the adsorption process. Besides that, the entropy also can be increased in the case of physisorption which may also contribute to the total adsorption process. This is due to the water molecules released from the hydrated ions or water molecules present on the surface during the adsorption process [29].
Table 3 Thermodynamic parameters for the Cu(II) and Pb(II) adsorption on sawdust at different temperatures Temp (8C)
30 40 50 60
For Langmuir isotherms Cu(II) ln K DG DS 8.37 8.64 8.92 9.53
21.09 22.51 23.98 26.40
0.17 0.17 0.17 0.17
DH (30–608C) 31.47
DG = kJmole1, DS = kJmole1K1, and DH = kJmole1
Pb(II) ln K
DG
DS
DH (30–608C)
8.33 8.55 8.69 9.08
20.99 22.26 23.34 25.16
0.13 0.13 0.13 0.14
120.07
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4. Conclusions Based on the present study, it is clearly shown that sawdust obtained from Meranti wood is an effective adsorbent for the removal of Cu(II) and Pb(II) ions from aqueous solution. The adsorption process is strongly affected by parameters such as time, pH, adsorbent dosage, and temperature. The equilibrium time for Cu(II) and Pb(II) ions is determined as 60 min. The percentage adsorption for Pb(II) seems to be greater than adsorption of Cu(II) in this equilibrium time. The plot of pH versus percentage adsorption shows significant adsorption at pH 6.60 for the Cu(II) ions and at pH 7.00 for the Pb(II) ions. The removal is almost 100% for Cu(II) at this pH value being 99.39%. On the other hand, percentage removal for Pb(II) at this pH is 94.61%. The percentage adsorption of Cu(II) and Pb(II) ions was increased with increasing the adsorbent dosage. The adsorption of Cu(II) and Pb(II) ions on sawdust at pH 6.60 and 7.00, respectively, follow both Freundlichand Langmuir-type adsorption isotherms. Thermodynamic constants were also evaluated using equilibrium constants changing with temperature. The negative value of DG for Cu(II) and Pb(II) ions indicated the spontaneity of the process. The positive value of DH showed the endothermic nature of Cu(II) and Pb(II) sorption, respectively. Acknowledgment The study was funded through USM shortterm grant number 304/ PTEKIND/ 637044. The authors acknowledge the USM for providing research facilities.
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