Batch Study of Liquid-Phase Adsorption of Lead Ions Using Lalang (Imperata cylindrica) Leaf Powder

A batch adsorption method was applied to study the adsorption of Pb²⁺ ions from aqueous solution by lalang or Imperata cylindrica (IC) leaf powder. Effect of initial lead concentration, contact time, pH of solution, temperature and adsorbent dose were also investigated. A strong dependence of adsorption capacity on pH was observed as the capacity increased with increasing pH. Adsorption equilibrium was established in about 120 min. Adsorption of lead was found to increase with increasing temperature indicating endothermic nature of adsorption. The kinetic data were analyzed using various kinetic models particularly pseudo-first-order, pseudo-second-order, Ritchie`s-second-order, Elovich and intraparticle diffusion equations. Results show that the pseudo-second order kinetic model was found to agree well with the experimental data. Equilibrium isotherm data were tested using three models-Langmuir, Freundlich and Temkin. Equilibrium data agreed well with Langmuir model. The maximum adsorption capacity of lead by IC leaf powder determined from Langmuir model was 5.89 mg g^-1 at pH 4.

INTRODUCTION

The presence of heavy metals in the environment poses a serious and complex pollution problem that has become a major attention by scientists all over the world. Lead is one of the toxic heavy metals widely discharged into the environment as industrial waste in many developing countries, causing serious soil and water pollution. Lead has been known to pollute the environment by waste streams, which originates from metallurgical industry, electroplating and metal finishing industries, paint manufacture, storage battery manufacture, petroleum refining and drainage from ore mines. Excessive exposure to lead may lead to anemia, mental retardation, coma, seizures and bizarre behavior (Botkin and Keller, 2000). Numerous methods have been introduced for removing lead from wastewaters mainly chemical precipitation, chemical oxidation or reduction, ultrafiltration, electrochemical treatment, reverse osmosis, phytoextraction, electrodialysis, application of membrane technology, evaporation recovery, solvent extraction and ion exchange processes. However, the aforementioned techniques are very expensive, may produce large volume of wastes and not economically feasible for small and medium industries (Volesky, 1990; Aksu, 2005). Adsorption processes using natural adsorbents or agricultural waste products are becoming the new alternative for wastewater treatment because they are cheap, simple, sludge free and involve small initial cost and land investment. Some of the examples of adsorbent materials that have been applied for the removal of lead from aqueous solutions are given in Table 1. In this research, the effectiveness of adsorption of lead ions by IC leaf powder was studied. IC has been ranked as one of the ten worst weeds of the world (Holm et al., 1977). It is widely distributed in Africa, Asia, Australia and America.

Table 1:	Categories of adsorbent material successfully tested for the removal of lead from wastewaters

This species is considered a pernicious pest plant due its ability to successfully colonize, spread and subsequently compete and displace desirable vegetation and disrupt ecosystems (Holm et al., 1977). This study determines the parameters that influence adsorption of lead such as pH of the solution, temperature, initial lead concentration, contact time and adsorbent dose. The kinetic or rate of adsorption based on pseudo-first-order, pseudo-second-order, Ritchie’s-second-order, Elovich and intraparticle diffusion equations were also studied.

MATERIALS AND METHODS

Preparation of adsorbent: IC leaves were collected from the surrounding area of Universiti Teknologi MARA, Pahang campus, Malaysia. The leaves were extensively washed with distilled water to remove dirt, dried in an oven at 105°C for a period of 24 h, then ground and screened to obtain the average particle size of 355 μm. The powder was preserved in glass bottles for use as adsorbent.

Adsorption experiments: All the chemicals used in the experiments were of analytical grade reagents. Pb (NO₃)₂ (Merck) was used as the source of Pb²⁺ and all of the solutions were made in distilled water. The solutions of Pb²⁺ were prepared from a stock solution containing 1000 mg L^-1 of Pb²⁺. In general, batch adsorption experiments were carried out in a series of Erlenmeyer flasks of 100 mL capacity by agitating 0.1 g (unless otherwise stated) of the adsorbent with 25 mL of the aqueous Pb²⁺ solution in a constant temperature (30°C), water bath shaker (Memmert, Germany) for a pre-determined time interval at a constant speed of 150 rpm. The initial pH of the solution was fixed at 4 by adding drops of 0.1 M HCl or NaOH solutions. For isotherm study, 0.1 g of adsorbent was mixed with 25 mL lead solutions at various concentrations (5-100 mg L^-1) and the mixtures were shaken for 120 min (equilibrium time). The effect of pH on lead adsorption was performed at pH range 2-5, using a lead concentration of 20 mg L^-1. The study on the effect of temperature was carried out at two different temperatures, 30 and 40°C. After adsorption, the mixture was filtered through Whatman filter paper (No. 40). The concentrations of lead in the solutions before and after equilibrium were determined by atomic absorption spectrophotometer (Perkin Elmer, Aanalyst 400) using air-acetylene flame. All experiments were performed in duplicates and results are reported as average.

Metal uptake: The amount of Pb²⁺ adsorbed, q_t (mg g^-1) was computed by using the following expression:

(1)

where C_o and C_t are Pb²⁺ concentrations in mg L^-1 before and after adsorption at time t, V is the volume of adsorbate in liter (L) and m is the weight of the adsorbent in gram (g).

FTIR analysis: The FTIR spectrum of IC leaf powder was recorded with JASCO (Japan) FTIR Spectrophotometer. Samples of 200 mg KBr and 2 mg IC leaf powder were prepared and subjected to analysis.

RESULTS AND DISCUSSION

FTIR spectrum: FTIR analysis was carried out in order to identify the functional groups in the IC leaf powder that might be involved in the adsorption process. The FTIR spectrum in the range of 400-4000 cm^-1 is shown in Fig. 1. As shown in the figure, the spectrum displays a number of absorption peaks, which indicates the complex nature of the material examined. The broad and strong band ranging from 3000 to 3600 cm^-1 may be due to the overlapping of OH and NH stretching, which is consistent with the peak at 1047 and 1162 cm^-1 assigned to alcoholic C-O and C-N stretching vibration. The absorption band wave numbers of 2920 and 2851 cm^-1 can be assigned to-CH and-CH₂ stretching, respectively.

Fig. 1:	FTIR spectrum of IC leaf powder

Fig. 2:	Effect of initial lead concentration and contact time on lead adsorption onto IC leaf powder

Table 2:	FTIR spectrum of IC leaf powder

The absorption band wave number of the amide group is about 1630 cm^-1. Table 2 shows the infrared absorption frequencies of each peak and the corresponding functional groups of IC leaf powder.

Adsorption kinetic: Adsorption kinetic, which describes the adsorbate adsorption rate, is an important characteristic in evaluating the efficiency of adsorption. The adsorption data at different initial Pb²⁺ concentrations is shown in Fig. 2. The plots show that kinetic of adsorption of Pb²⁺ consisted of two phases; an initial rapid phase where adsorption was fast and a second slower phase where equilibrium uptake was achieved. The first phase is related to external surface adsorption of lead, which occurs instantaneously. The second phase is the gradual adsorption stage before the lead uptake reaches equilibrium. The time to reach equilibrium is 120 min and the maximum amounts of Pb²⁺ adsorbed are 0.979 and 3.594 mg g^-1 for lead concentrations of 5 and 20 mg L^-1, respectively. According to Bhattacharyya and Gupta (2006), the initial high rate of metal uptake may be attributed to the existence of the bare surface. The number of available adsorption sites decreased as the number of Pb²⁺ ions adsorbed increases. The enhanced adsorption of metal ion with increase in agitation time may also be due to the decrease in boundary layer resistance to mass transfer in the bulk solution and an increase in the kinetic energy of hydrated ions (Horsfall and Abia, 2003). By increasing the agitation time, the boundary layer resistance will be reduced, but the mobility of ions in the solution will increase. For the following experiments, the contact time was maintained for 120 min to ensure that equilibrium was really achieved.

Identifying the slowest step or rate-determining step is also crucial in any kinetic study. In order to investigate the controlling mechanisms of adsorption processes, the pseudo-first-order (Ho and McKay, 1998), pseudo-second-order (Ho, 1995), Elovich (Chien and Clayton, 1980), Ritchie’s-second-order (Rithcie, 1977) and intraparticle diffusion (Weber and Morris, 1963) rate equations were used to test the experimental data. The pseudo-first-order kinetic model is given as:

(2)

where q_t and q_e are the amount adsorbed (mg g^-1) at time t and at equilibrium and k₁ is the rate constant of the pseudo-first-order adsorption process (min^-1). Straight-line plots of log (q_e-q_t) against t were used to determine the rate constants, k₁ and correlation coefficients, R² for different lead concentrations.

The pseudo-second-order equation is expressed as:

(3)

where h = kq_e² can be regarded as the initial sorption rate as t → 0 and k is the rate constant of second-order adsorption (g mg^-1 min^-1). The plot t/q_t versus t should give a straight line if second-order kinetic is applicable and q_e, k and h can be determined from the slope and intercept of the plot, respectively. The plots of the linearized form of the pseudo-first-order and pseudo- second-order model at different Pb²⁺ concentrations by IC leaf powder are shown in Fig. 3 and 4, respectively.

Table 3:	Comparison between adsorption rate constants, qe estimated and coefficients of correlation associated to the pseudo-first-order and to the pseudo-second-order kinetic models

Fig. 3:	Pseudo-first-order plots for lead adsorption onto IC leaf powder

Fig. 4:	Pseudo-second-order plots for lead adsorption onto IC leaf powder

The pseudo-first-order rate constants (k₁) and q_e determined from Eq. 2 are presented in Table 3 along with the corresponding correlation coefficients. The rate constants, calculated from the slopes of the best-fit lines were in the range of 3.36x10^-2 to 3.38x10^-2 min^-2 for pseudo-first order model. The dependence of the first-order rate constant with concentration and the coefficients of correlation were lower than those found for the pseudo-second-order model. It can be observed in this study that q_e values computed from the experiment did not agree with q_e values obtained from pseudo-first-order plots. This indicates that pseudo-first-order kinetic might not be sufficient to describe the mechanism of Pb²⁺:IC interactions.

The plot of t/q_t versus t for pseudo-second-order model (Fig. 4) yields very good straight lines (correlation coefficient, R²>0.99).

Fig. 5:	Ritchie’s second-order plots for lead adsorption onto IC leaf powder

The pseudo-second-order rate constants were in the range of 1.77x10^-1 to 3.38x10^-1 g mg^-1 min^-1, as shown in Table 3. The theoretical values of q_e also agree very well with the experimental ones. Both facts suggest that the adsorption of Pb²⁺ ions by IC leaf powder follows the pseudo-second-order kinetic model. This finding was similar to other studies on the adsorption of lead by low-cost adsorbents. For instance, pseudo-second-order kinetic was also observed in the adsorption of cadmium and lead by spent grain (Low et al., 2000), adsorption of cadmium and lead onto Hevea brasiliensis leaf powder (Hanafiah et al., 2006a,b) and adsorption of lead onto activated carbon prepared from coconut shell (Sekar et al., 2004).

Ritchie’s second-order equation based on adsorption equilibrium capacity is expressed in the form:

(4)

where k₂ is the rate constant of the second-order adsorption. From Eq. 4, a plot of 1/q_t versus 1/t should give a straight line (Fig. 5) and the rate constants (k₂) and adsorption at equilibrium (q_e) can be determined from the slope and intercept, respectively. The theoretical values of q_e obtained from Ritchie second-order plots were found to be lower than those of the experimental values. The summary of q_e and k₂ values determined from Ritchie second-order are given in Table 4.

The Elovich equation has been applied satisfactorily to some chemisorption processes and the equation is often valid for systems in which the adsorbing surface is heterogeneous.

Table 4:	Parameters for Ritchie second-order and Elovich equations

Fig. 6:	Elovich plots for lead adsorption onto IC leaf powder

The Elovich equation is generally express as:

(5)

where α is the initial adsorption rate (mg g^-1 min^-1), β is the desorption constant (g mg^-1). To simplify the Elovich equation, Chien and Clayton (1980) assumed αβt>>1 and by applying the boundary conditions q_t = 0 at t = 0 and q_t = q_t at t = t, Eq. 5 becomes:

(6)

If lead adsorption fits the Elovich model, a plot of q_t versus ln t should give a linear relationship with a slope of 1/β and an intercept of 1/β ln αβ. The Elovich plots for lead adsorption at two different concentrations are shown in Fig. 6. Table 4 lists the kinetic constants obtained from Elovich equation for two different lead concentrations. It can be seen that on increasing the lead concentration, the value of α increases from 0.121 to 3763.1 mg g^-1 min^-1.

The intraparticle diffusion equation is given as:

(7)

where k_id (mg g^-1 min^-½) is the intraparticle rate constant and can be obtained from the slope of the second portion of the straight-line of plot q_t versus t^1/2 (Fig. 7). As can be seen from Fig. 7, the adsorption was controlled by three different stages: the first sharper portion being a rapid external surface adsorption, the second linear portion being a gradual adsorption where intraparticle diffusion is the rate limiting factor and the final portion being final equilibrium stage due to low concentration of lead in solution phase as well as less number of available adsorption sites.

Fig. 7:	Intraparticle diffusion plots for lead adsorption onto IC leaf powder

Fig. 8:	Effect of pH on lead adsorption onto IC leaf powder

It is obvious that intraparticle diffusion is not the sole rate-determining step as the second portions of plots of q_t versus t^1/2 did not have zero intercept (Ho and McKay, 2003). The values of k_id were 4.83x10^-2 and 1.36x10^-1 mg g^-1 min^-½ for lead concentrations of 5 and 20 mg g^-1, respectively.

Effect of pH: pH is an important parameter for adsorption process due to its influence on the ionization state of the functional groups on the surface of adsorbent (carboxylate, hydroxy, etc) and different ionic forms of lead. According to Sekar et al. (2004), above pH 5.5, lead

starts to precipitate as Pb (OH)₂. Therefore experiments were performed in the pH range 2-5. As can be seen in Fig. 8, the adsorption of Pb²⁺ ions from aqueous solution is mainly influenced by solution pH. It is noticed that adsorption increased continuously with the decrease in acidity until it reached maximum adsorption capacity at pH 5.

Fig. 9:	Effect of adsorbent dose on lead adsorption onto IC leaf powder

Table 5:	Amount of lead removed (%) and adsorbed (mg g-1) at three different adsorbent doses

The increase in lead adsorbed as pH increases can be explained on the basis of a decrease in competition between proton (H⁺) and lead ions on the surface of adsorbent. As pH increased, more adsorbent surface would be exposed and carried negative charges, with subsequent attraction of lead ions (Brady and Duncan, 1994).

Effect of adsorbent dose: The dependence of adsorption of lead on the amount of IC was studied at room temperature (30°C) at pH 4.0 by varying the adsorbent doses from 0.10 to 1.0 g while keeping the volume (25 mL) and concentration of the metal solution constant. The result is shown in Fig. 9 and Table 5. The percent lead removal from the solution is seen to increase with increasing adsorbent dose. The percent removal of Pb(II) ions increased from 71.9 to 86.0 for adsorbent dose of 0.10 and 1.0 g, respectively. According to Shukla et al. (2002), the decrease in adsorption density (mg g^-1) with increase in adsorbent dose is due to the high number of unsaturated adsorption sites.

Effect of temperature: The effect of temperature on lead adsorption for lead concentration of 20 mg L^-1 is shown in Fig. 10 and Table 6. It is found that the amount of lead adsorbed increased with increasing temperature, indicating endothermic nature of adsorption. A similar finding was also observed by Dursun (2006) for the biosorption of Cu(II) and Pb(II) ions onto pretreated Aspergillus niger. At equilibrium, the amount of lead adsorbed were 3.59 and 4.07 mg g^-1 at temperatures 30 and 40°C, respectively. At higher temperature (40°C), the time taken for adsorption to reach equilibrium was much shorter (20 min).

Fig. 10:	Effect of temperature on lead adsorption onto IC leaf powder

Fig. 11:	Adsorption isotherm plot for lead adsorption onto IC leaf powder

The Gibbs free energy (ΔG) for lead adsorption was calculated using the following equations:

(8)

(9)

where K_c is the equilibrium constant (unitless), C_e is the equilibrium concentration in solution (mg L^-1) and C_Ae is the solid-phase concentration at equilibrium (mg L^-1). Spontaneity of the adsorption process was demonstrated by the increase in Gibbs energy, ΔG (values varied from-2.363 kJ mol^-1 at 30°C to-3.851 kJ mol^-1 at 40°C) as shown in Table 6. The negative values of ΔG indicate that the equilibrium:

(10)

shifts to the forward direction in a spontaneous manner leading to binding of Pb²⁺ ions on to surface constituents of IC leaf powder. The increase in lead adsorbed with temperature could be due to changes in the pore size, desolvation of the adsorbate and the increase in the rate of intraparticle diffusion (Sekar et al., 2004).

Table 6:	Comparison of ΔG values at two different temperatures for Pb(II) adsorption onto IC leaf powder

Table 7:	Langmuir, Freundlich and Temkin adsorption isotherms for removal of Pb2+ ions onto IC leaf powder

Fig. 12:	Langmuir plot for lead adsorption onto IC leaf powder

Fig. 13:	Freundlich plot for lead adsorption onto IC leaf powder

Adsorption isotherms: The maximum adsorption capacity of IC leaf powder for lead was investigated over a range of lead concentrations. Figure 11 shows the plot of the adsorption capacity, q_e (mg g^-1) versus the equilibrium concentration of lead ions in the solution, C_e (mg L^-1). The amount of lead adsorbed was found to increase with increasing lead concentration until the maximum adsorption capacity was achieved.

The distribution of metal ions between liquid and solid phases is generally described by using the Langmuir (1916), Freundlich (1906) and Temkin and Pyzheh (1940) adsorption isotherm models. The Langmuir model assumes uniform energies of adsorption onto the adsorbent surface and no transmigration of adsorbate in the plane of the surface. The Freundlich isotherm model however is applicable to highly heterogeneous surface. The Langmuir and Freundlich adsorption isotherms are given as:

Fig. 14:	Temkin plot for lead adsorption onto IC leaf powder

(11)

(12)

where, C_e is the equilibrium Pb²⁺ concentration (mg L^-1), q_e the amount of Pb²⁺ adsorbed at equilibrium (mg g^-1) and Q_max, b, K_F and n are the isotherm constants. Q_max and K_F are defined as maximum adsorption capacity (mg g^-1) for Langmuir and Freundlich isotherms, respectively. Figure 12 and 13 show the Langmuir and Freundlich adsorption isotherms obtained by plotting C_e/q_e versus C_e and log q_e versus log C_e, respectively. According to Mohanty et al. (2006), if the value of n is greater than 1, it indicates favorable adsorption of metal ions on the surface of adsorbent. The value of n determined from Freundlich isotherm was 2.669 as shown in Table 7, indicating that Pb(II) ions are favorably adsorbed by IC leaf powder.

Temkin isotherm model is given as;

(13)

where T is the temperature (^oK), R is the universal gas constant, 8.314 J mol^-1, K_T is the equilibrium constant (L mg^-1) and b_Te is related to the heat of adsorption (J mol^-1). According to Aharoni and Ungarish (1977), the derivation of Temkin isotherm assumes that the fall in the heat of adsorption is linear rather than logarithmic, as implied in the Freundlich equation. Temkin plot is shown in Fig. 14 and the values of K_T and b_Te are given in Table 7. Based on the R² values obtained from the three adsorption isotherm models, it is clear that the Langmuir has best fitted for the adsorption of lead onto IC leaf powder. The maximum adsorption capacity determined from the Langmuir model was found to be 5.89 mg g^-1, as shown in Table 7.

CONCLUSIONS

This study indicates that IC leaf powder has rapid adsorption rate and good adsorption capacity for lead. The Pb²⁺ adsorption was found to be dependent on initial lead concentration, contact time, pH, temperature and adsorbent dose. The adsorption of lead fitted the Langmuir isotherm model, which suggests monolayer coverage of adsorbent surface. Kinetic of lead adsorption however obeyed the pseudo-second order model, which suggests chemisorption as the rate-determining step in adsorption process. Maximum adsorption of lead occurred at pH 5 and a higher adsorption rate occurred at higher temperature, indicating endothermic nature of adsorption. Since IC leaves are highly abundant and can be easily synthesized at relatively low cost, the adsorbent could be applied for the removal of lead from wastewaters.

ACKNOWLEDGMENT

The authors thank the Institute of Research, Development and Commercialization (IRDC), Universiti Teknologi MARA for the financial support.