Abstract: Studies on a batch sorption process using Triplochiton scleroxylon sawdust as a low cost sorbent produced by wood industry was investigated to remove lead ions from aqueous solution. The sorption process follows a pseudo second-order rate kinetics at different dosage of sawdust and at different Pb(II) initial concentration. The Freundlich and Langmuir sorption models were used for the mathematical description of the equilibrium and isotherm constants were also evaluated, the maximum sorption capacity of Pb(II) in single component system was determined to be 26.38 mg g-1 at 30±2°C and at an initial pH of 4. The sorption process showed that copper, cadmium or both metals in aqueous solutions suppressed the removal of lead ions by sawdust. These results indicated that ion exchange is one of the major sorption mechanisms for binding Pb(II) ions into sawdust.
INTRODUCTION
The increase in the industrial activities has lead to more environmental problems leading to pollution and deterioration of several ecosystems due to the accumulation heavy metals. Heavy metal ions are extremely toxic and harmful even at low concentrations; consequently, their drainage from water and wastewater is significant to meet legislative standards and to protect the public health (Kumar et al., 2006).
Lead, a heavy metal, is widely used in many important manufacturing processes such as storage battery, manufacturing, printing, pigments, fuels, photographic materials and explosive manufacturing. It is meanwhile one of the most toxic heavy metals (Martins et al., 2006).
Many physicochemical techniques for removing lead from wastewaters include chemical precipitation, carbon adsorption, ion exchange, evaporation and membrane processes (Bulut and Tez, 2007; Abia et al., 2006). The selection of a particular technique depends notably on a number of factors, such as metal type ant its concentration, other constituents, the level of clean-up required and the cost of its use. Precipitation methods are particularly reliable but require large settling tanks for the precipitation of voluminous alkaline sludges and a subsequent treatment is needed (Dang et al., 2009). Adsorption is an important means for controlling the extent of pollution due to heavy metal ions. Activated carbons are widely used because of their high adsorption abilities for a large number of heavy metal ions. However, the price of activated carbons is relatively high, which limits their use (Oliveira et al., 2005). This situation led many researchers to seek cheaper materials such as, the agricultural by-products.
A number of agricultural waste and by-products have been cited in the literature for their capacity to remove lead from aqueous solutions. These comprise bagasse sugar (Gupta and Ali, 2004), sawdust (Adouby et al., 2007; Li et al., 2007), rice husk (Wong et al., 2003), bark (Nehrenheim and Gustafsson, 2008; Al-Asheh and Duvnjak, 1997), peat (Ho et al., 2002), wheat bran (Nouri et al., 2007), tree fern (Ho et al., 2004), modified corn cobs (Khan and Wahab, 2007) oil palm tree (Zuhairi et al., 2007).
Among low-cost adsorbents, sawdust has been considered promising, due to abundance and availability mostly in wood-based industries (Shukla et al., 2002). Sawdust is a solid waste produced in large quantities at saw mills. It basically contains lignin and cellulose and has a good mechanical stability as well as some other advantages.
The objective of this study was to obtain the basic information for the design of the process of lead sorption on sawdust, such as kinetic data and equilibrium in batch system. In order to describe the isotherm mathematically, the experimental data of the sorption equilibrium were correlated with Langmuir and Freundlich equations. The kinetics of the process was determined, especially in relation to the effects of lead concentration and sawdust dosage on the sorption. The competitive sorption of lead with copper, cadmium or both metals on sawdust were also investigated. The regenerative capacity and reusability of the sawdust were also assessed in the present study.
MATERIALS AND METHODS
Sorbent: This study was conducted from April 2007 to December 2008. Sawdust was obtained from locally used wood, Triplochiton scléroxylon. It was collected from a local sawmill and dried in sunlight for 15 days until almost all the moisture evaporated. It was ground to a fine powder and sieved to 0, 5 mm size then directly used without any pretreatment, as a sorbent for the metal removal from aqueous solutions.
Chemicals: All chemicals used in this study were of analytical grade. Metal solutions were prepared by dissolving Pb(NO3)2, (Cu(NO3)2, 3H2O); (Cd(NO3)2, 4H2O) (99.5% MERCK), in deionized water to desired initial concentrations. Nitrate salt was selected as a possibly inhibiting anion because of its low tendency for complex formation with most metals.
Sorption kinetics: Batch kinetic experiments were carried out at room temperature (30±2°C) using 250 mL Erlenmeyer flasks. The experiments were conducted at the initial pH 4; the mixtures were agitated at 300 rpm.
At appropriate time intervals, Sawdust from the samples was separated by vacuum filtration using a 45 μm membrane filter and the metal content of the filtrate was analyzed using a Varian Atomic Absorption Spectrophotometer (AAS) Varian Model AA 20 in an air-acetylene flame.
The effects of sorbent dosage (5-20 g L–1) and of the initial concentrations of lead (100-400 mg L–1) on the sorption rate were studied. The procedure was the same as described above. Each batch experiment was carried out in duplicate.
The concentrations of Pb(II) adsorbed were obtained from the difference between the initial and the final concentration of the metal in the filtrate.
The amount of Pb(II) adsorbed in mg g–1 at a given time was computed by using the following equation:
| (1) |
where, Co and Ct are the metal concentrations (in mg L–1) initially and at a given time t, respectively and V is the volume of the metal solutions in mL and m is the weight of sawdust in g.
The lead percentage of removal was calculated using the following equation:
| (2) |
where, C0 and Ct are the initial and final metal concentrations in solution, respectively.
Equilibrium studies for single-metal systems: Batch sorption experiments were performed at room temperature (30°C±1) in 250 mL Erlenmeyer flasks. Accurately weighed amounts (0.2-3 g) of sorbent (sawdust) were agitated at 300 rpm with 100 mL of Pb(II) solution (100 mg L–1) for 45 min. At the end of the equilibrium period, sawdust was separated by vacuum filtration using a 45 μm membrane filter. The metal content of the filtrate was analyzed using an Atomic Absorption Spectrophotometer as indicated earlier.
Equilibrium studies for multiple-metal systems: The competitive sorption experiments were developed from the method described by Han et al. (2006). In these experiments, competitive sorption of Pb(II), Cd(II) and Cu(II) ions in binary and ternary solutions was investigated by following a similar procedure as described above. These studies were performed at initial pH of 4.0 and at 30°C. The experiments of competitive sorption of Pb(II), Cd(II) and Cu(II) included two parts: (1) The competitive sorption of Pb(II) with Cd and Cu(II) at binary and ternary system in the total metal concentration was fixed (100mg L–1); (2) In another binary system, the initial concentration of Pb(II) was constant in 100 mg L–1 and the concentration of Cu(II) or Cd(II) were varied from 0 to 100 mg L–1.
Desorption/reuse experiments: For the desorption study, 2.0 g of sawdust
was put in contact with 100 mL Pb(II) solution (200 mg L–1). After
45 min, the solution was filtrated on 0.45 μm Millipore filter. Sawdust
was collected, washed three times with 500 mL distilled water in order to remove
residual Pb (II) on the sorbent surface. The sawdust was dried at 60°C was
transferred into 100 mL of desorbing solutions containing: deionised water,
0.2 M HCl, HNO3, H2SO4, CaCl2, NaCl
and EDTA. The mixtures were shaken for 45 min, filtrated and the filtrates were
analyzed for the determination of Pb(II) after desorption. These sorption/desorption
studies were repeated four times using the same sawdust.
Desorption ratio was given as:
| (3) |
RESULTS AND DISCUSSION
Kinetic studies: The rate at which sorption takes place is an important factor to consider when designing batch sorption systems. It is therefore important to establish the time dependence of such systems under various process conditions. Many authors have studied the sorption kinetics in bivalent metal with several sorbent materials by the kinetics of pseudo second order (Ofomaja and Ho, 2007; Khormaei et al., 2007; Wang et al., 2006). The rate equation for the reaction may be represented by the following expression:
| (4) |
where, k is the sorption rate constant (g mg–1 min), qe the amount of metal ion sorbed at equilibrium (mg g–1) and qt is the amount of metal ion on the sorbent surface at any time t (mg g–1).
Separating the variables in Eq. 4 gives:
| (5) |
Integrating this for the boundary conditions t = 0 to t = t and qt = 0 to qt = qt; gives:
| (6) |
The plot of t/qt versus t should give a straight line if second-order kinetics is applicable and qe and k can be determined from the slope and intercept of the plot, respectively.
The initial sorption rate h (mg g–1. min) is given by the following equation:
| (7) |
Effect of initial concentration: Figure 1 show the plot of the experimental data points for the sorption of lead ions on sawdust for 20 g L–1 of biomass concentration and pH 4 as a function of time. The sorption increased rapidly at the beginning, then became slow until it reached the equilibrium.
| Fig. 1: | Effect of initial concentration of metal on the sorption of
Pb(II) by Triplochiton scleroxylon sawdust for 20 g L–1
of biomass concentration and pH 4 |
| Fig. 2: | Pseudo-second-order sorption kinetics of Pb(II)) by sawdust
at different initial concentration of metal for 20 g L–1 of biomass
concentration and pH 4 |
We observed that the metal uptake varied with varying initial lead concentration. The equilibrium uptake of Pb(II) was found to be 10.837, 17.593, 25.601 and 31.046 mg g–1 at 100, 200, 300 and 400 mg L–1 initial Pb(II) concentrations respectively. Equilibrium time for sorption of Pb(II) at different concentrations was found to be 45 min showing that equilibrium time is not dependent of initial concentration of Pb(II).
By plotting t/qt against t, a straight line was obtained in all cases. The values of the second-order rate constants k and qe were determined from the plots of the Eq. 6 (Fig. 2). Theses constants, the initial rate h of sorption at various initial concentrations and the regression coefficients for the linear plots are summarized in Table 1. The equilibrium sorption capacity (10.83 to 31.04 mg g–1) and the initial sorption rate (8.46 to 27.95 mg g–1 min) increased with an increase in the initial lead ion concentration.
| Table 1: | Pseudo-second order parameters for the sorption of Pb(II) by Triplochiton scleroxylon sawdust |
| Fig. 3: | Effect of sawdust dosage on the sorption of Pb(II) for 100 mg L–1 of metal solution and pH 4 |
Effect of the quantity of sawdust: The effect of sawdust dosage on the kinetics of Pb(II) sorption is shown by Fig. 3. The results indicated that the amount of lead sorbed per unit mass of sorbent decreased with increase in sorbent dosage. The maximum uptake of Pb(II) was found to be 12.27, 17.60, 13.20 and 10.84 mg g–1 for 5, 10, 15 and 20 g L–1 of sawdust dosage (II) .The decrease in the amount of lead sorbed with increasing sorbent dosage is due to the split in the flux or the concentration gradient between solute concentration in the solution and the solute concentration in the surface of the sorbent (Ghodbane et al., 2008). Thus with increasing sorbent mass, the amount of lead sorbed onto unit weight of sorbent is reduced leading to a decrease in sorption capacity with increasing sorbent dose.
By plotting t/qt against t, a straight line was obtained in all cases. The values of the second-order rate constants k and qe were determined from the plots of the Eq. 6 (Fig. 4).
The coefficients of determination, R2 (varying between 0.9995 and 0.9999, Table 1) are all higher than 0.9995 indicating that the sorption of Pb(II) by sawdust was well represented by the pseudo-second-order kinetic equation.
| Fig. 4: | Pseudo-second-order sorption kinetics of Pb(II)) by sawdust at different dosage for 100 mg L–1 of metal solution and pH 4 |
The equilibrium sorption capacity qe decreased from 22.27 to 10.84 mg g–1, when the sorbent dose of the system is changed from 5 to 20 g L–1.
Ions exchanges: Ion exchange was proven the main mechanism involved in the sorption by biomass and there was a strong ionic balance between adsorbed (H+ and M2+) and released ions (Ca2+, Mg2+, K+ …) from the biomass.
The study of the removal of the cations on the sawdust in the metal ions solutions and of water at pH 4 is showed an ion-exchange phenomenon. The concentrations at different time of Ca2+ (6.34-11.29 mg L–1), Mg2+ (2.57- 4.09 mg L–1) and K+ (22.83 -24.23 mg L–1) leached in water (used as control) and in metal solution are shown in Table 2. In bidistilled water, the sawdust leaching is done according to the following order: (K + > Ca2+ > Mg2+). In the metal solutions, the sawdust leaching is higher than the leaching in bidistilled water; showing an additional leaching during the metal sorption by sawdust. The obtained order of leaching was: K+ > Ca2+ >> Mg2+.
The net release at different time of Ca2+ (varying from 9.98 to 14 mg L–1), Mg2+ (2.64 - 4.66 mg L–1) and K+ (0.92-1.84 mg L–1) due to the sorption process was calculated by subtracting the amount of cations released with water (control) to the amount of cations measured in the metal solutions after metal sorption process (Table 2). It appears that there was more Ca2+ cations released followed by Mg2+ and finally by K+ which takes tiny part in the ionic exchange (Ca2+ >> Mg2+ > K+).
We observed that the pH varies very little; from 4 to 4.5 in water and from 4 to 5.4 in lead solution. This variation of pH is due to a reduction of the metal ions in solution but especially to the leaching of ions (Ca2+, Mg2+, K+) in solution by the sawdust (Adouby et al., 2007).
| Table 2: | Cations release during sorption of Pb(II) into Triplochiton scleroxylon Sawdust pH = 4, Co = 100 g L–1 |
Sorption data for wide range of adsorbate concentration are most conveniently described by sorption isotherms, such as the Langmuir and Freundlich isotherms. Langmuir sorption isotherm is the most widely used model for the sorption process and is based on monolayer coverage of sorbate on the surface of adsorbents. According to Langmuir theory, it has been assumed that adsorption occurs at a specific homogenous site within adsorbent; each site is occupied only by an adsorbate molecule, all sites are equivalent and there are no interactions between adsorbate molecules (Langmuir, 1918).
The non-linear form of Langmuir isotherm model can be represented by Eq. 8:
| (8) |
where qe is the amount of metal sorbed per unit mass of sorbent, qo is the maximum metal uptake per unit mass of sorbent (mg g–1), Ce is the equilibrium concentration of sorbate in solution (mg L–1) and b is the Langmuir constant related to energy of sorption (L mg–1) which reflects quantitatively the affinity between the sorbent and the sorbate.
Linearization of Eq. 8 gives:
| (9) |
The values of the Langmuir constants b and maximum metal uptake qo were calculated from the slope and intercept of the plot of Ce/qe versus Ce.
| Fig. 5: | Isotherms for the sorption of Pb(II) using sawdust for 100 mg L–1 of metal solution, Triplochiton scleroxylon sawdust dosage (2-25 g L–1) and pH 4 |
Concerning the Freundlich equation, its empirical form is applicable to monolayer adsorption (Van der Waals adsorption) and multilayer adsorption (chemisorptions) (Fiol et al., 2006). In this model, lateral interaction between the adsorbed molecules and energetic surface heterogeneity are taken into account.
The non-linear form of Freundlich isotherm model can be represented by Eq. 10:
| (10) |
Linear form is:
| (11) |
where, Kf and 1/n are empirical constants relative to sorption capacity and sorption intensity, respectively. The values of Kf and n were calculated from the intercept and slope of the plot of Lnqe versus LnCe.
Figure 5 shows the Comparison of experimental equilibrium points with the theoretical equilibrium curves obtained from the non-linearized Langmuir and Freundlich sorption isotherms.
| Table 3: | Langmuir and Freundlich isotherms constants for the sorption of Pb(II) by Triplochiton scleroxylon sawdust in single component |
The average percentage errors (ε%) between the experimental and calculated values for the sorption isotherm models were evaluated using Eq. 12:
| (12) |
The constants related to linear and non-linear form were calculated and listed in Table 3.
The average percentage error (ε%) and the coefficient of correlation (R2) are the criteria used to select the most suitable isotherm model for a sorption process. According to Table 3, the correlation coefficients of the Langmuir isotherm (0.987) for linear model are high compared to those of non-linear models (0.951). It was also observed that the average percentage errors were smaller for linear model. These results indicated that Langmuir isotherm was more suitable for the experimental data compared to Freundlich isotherm because of the higher value of the coefficient of correlation and the smaller average percentage errors.
There is no remarkable difference between the values of qo calculated for the two Langmuir models; 26.37 mg g–1 for non-linear model and 26.38 mg g–1 for linear model. Similar results were found by two teams of researchers who studied Pb(II) uptake by sawdust. Li et al. (2007) under different conditions obtained maximum uptake of 21.05 mg g–1 for Pb(II) with poplar sawdust while Taty-Costodes et al. (2003) found Pb(II) maximum uptake of 15.77 and 22.22 mg g–1, respectively at pH 4 and 5 with Pine sawdust.
Effect of the presence of Cd(II) or Cu(II) in binary system: Wastewaters usually contain more than single metal. The presence of more than one metal in wastewater is expected to cause interactive effects depending on reasons such as the number of metals competing for binding sites; the metal concentrations; the nature and dosage of the sorbent (Abu-Al-Rub et al., 2006).
| Fig. 6: | The effect on sorption of lead when copper or cadmium in existence for 100 mg L–1 of initial lead solution and pH 4 |
These studies were carried out in order to investigate the competitive effects in binary systems ((Pb-Cu) and (Pb-Cd)). The concentration of lead was maintained constant (100 mg L–1) while varying the concentrations of other metal ions. The results (Fig. 6) showed that Pb(II) uptake decreases with the increasing of Cd(II) or Cu(II) initial concentration. Pb(II) uptake decreases from 22,43 to 5,1 mg g–1 when the initial concentration of Cu(II) ranged from 0 to 100 mg L–1. Pb(II) uptake decreases from 22,43 to 10,66 mg g–1 when the initial concentration of Cd(II) ranged from 0 to 100 mg L–1.
These results indicated that Pb(II) uptake is moderately affected by the presence of Cd(II) while the presence of Cu (II affected to a significant degree Pb(II) uptake , even at relatively low concentrations of Cu(II).
The simultaneous sorption of Pb(II) with Cd(II) or Cu(II) by the sawdust was studied using a solution containing 100 mg L–1 of each metal ion. T. The maximum uptake of Pb(II) (16.83 and 9.06 mg g–1) in the binary system were shown in Table 4.
In binary system (Pb-Cd), the presence of the Cd(II) lead to the inhibition of the Pb(II) maximum uptake which decreased from 26.38 to 16.83 mg g–1 (reduction of approximately 36%). This inhibition was more pronounced with (Pb-Cu) system; the capacity was reduced to 60%).
The observed reduction of Pb(II) maximum uptake in binary mixture can be due to the chemical interactions between these metals and also with the biomass resulting in the metal ions competition for the sorption sites on the surfaces of sawdust (Ziagova et al., 2007).
A similar phenomenon had been observed in binary sorption of Pb(II) and Cu(II) on Aspergillus flavus (Akar and Tunali, 2006). This study revealed that Pb(II) and Cu(II) are in competition each other and that the capacities of biosorption of the metal ions of the binary mixture were lower than the capacities of these same cations under non competitive conditions.
| Table 4: | Langmuir isotherms constants for the sorption of Pb(II) by Triplochiton scleroxylon sawdust in single, binary and ternary component. |
| Fig. 7: | Effect of the presence of copper or cadmium on lead sorption
isotherm for 100 mg L–1 of metals solutions, Triplochiton
scleroxylon sawdust dosage (2-25 g L–1) and pH 4 |
Effect of the presence of other metals in ternary system: The results of the sorption of the ternary (Pb-Cd-Cu) system on sawdust are shown in Fig. 7 also the Langmuir constants and coefficient of correlation (R2) is summarized in Table 4. In ternary solution, the maximum uptake of Pb(II) was 8.33 mg g–1 whereas it was 26.38 mg L–1 in single system. This value is close to that obtained in binary solution with Cu(II) (9.06 mg g–1). This result shows once more that the presence of Cd(II) has little effect on the maximum uptake of Pb(II). Similar results were obtained with Al-Asheh and Duvnjak (1997), who compared the capacity of several metal cations (Cd, Cu, Pb and Ni) in binary, ternary and quaternary mixtures. This study showed that in binary mixture Cd(II) has little effect on the capacity of sorption of Pb(II) compared to Cu(II) and that in ternary mixture (Pb-Cd-Cu), the maximum uptake of Pb(II) was close to that found in binary mixture with Cu(II).
Desorption of Pb(II) and regeneration of the sawdust: The metal sorbed on the sawdust can be desorbed by a suitable solution of desorption, thus the sawdust can be employed in successive sorption/desorption cycles. Various solutions of desorption: double distilled water and 0.2 M of HCl, HNO3, H2SO4, CaCl2, NaCl and EDTA) were used for desorption of Pb(II) off the sawdust.
| Fig. 8: | Recovery of lead from Triplochiton scleroxylon sawdust using different desorbents |
| Fig. 9: | Adsorption/desorption cycles of lead for Triplochiton scleroxylon sawdust |
The effectiveness of desorption is expressed by the difference between Pb(II) quantity in solution after desorption and the quantity adsorbed by the sawdust.
Figure 8 expresses the percentage of Pb(II) (2.11, 9.87, 20.82, 31.28, 58.61 and 77,29% with H2O, HCl, NaCl, HNO3, H2SO4, CaCl2 and EDTA respectively) desorbed after treatment with the various solutions. It was observed that the EDTA (77.29% of recovery) was the most effective among all the studied desorbents while desorption with double distilled water (2.11%) was almost negligible (Vijayaraghavan et al., 2006).
The Pb(II) sorbed on the sawdust were desorbed with 0,2 M of EDTA, this process was repeated four times. The results (Fig. 9) show that the sawdust could be employed on several occasions for sorption process without significant losses of its initial capacity of adsorption. It was observed that the quantity of Pb(II) absorbed decreases with each reuse of the sawdust. This is due to the non leaching of previously sorbed ions that resisted to the desorption process.
CONCLUSION
This study showed the potentiality offers by the sawdust of Triplochiton scléroxylon for the sorption of Pb(II) in wastewater. The kinetic study of Pb(II) on the sawdust was fast and proved to obey to a pseudo-second kinetic order. The rate of Pb(II) sorption on sawdust was determined as a function of the initial metal concentrations and sawdust dose.
Ion exchange was proven to be the main mechanism involved in the sorption of Pb(II) by sawdust.
The obtained coefficient of correlation and the average percentage error indicated that the sorption of lead by the sawdust was described better by applying linear Langmuir model. The maximum uptake of sorption is 26.38 mg g–1 in single component. In binary system the presence of the Cd(II) and Cu(II) resulted in the inhibition of the Pb(II) maximum uptake which decreased from 26.38 mg g–1 to 16.83 mg g–1 with Cd(II) and to 9.06 mg g–1 in the presence of Cu.
This study shows that sawdust of Triplochiton scléroxylon is a suitable material for the removal of the lead from a liquid waste in successive cycles of sorption/desorption after the optimization of various physicochemical parameters of the effluent.