Iranian Journal of Environmental Health Science & Engineering, Vol. 2, No. 3, 2005, pp. 159-168
KINETIC AND EQUILIBRIUM STUDIES OF LEAD AND CADMIUM BIOSORPTION FROM AQUEOUS SOLUTIONS BY SARGASSUM SPP. BIOMASS
*1R. Nabizadeh, 1K. Naddafi, 1R. Saeedi, 1A. H. Mahvi, 1F. Vaezi, 2K. Yaghmaeian and 1S. Nazmara
1Department of Environmental Health Engineering,
School of Public Health, Tehran University of Medical Sciences, Tehran, Iran
Received 26 February 2005; revised 12 March 2005; accepted 23April 2005
Code Number: se05023
Contamination of the aqueous environment by heavy metals is a worldwide environmental problem. Biosorption of lead (II) and cadmium (II) from aqueous solutions by brown algae Sargassum spp. biomass was studied in a batch system. The heavy metals uptake was found to be rapid and reached to 88-96% of equilibrium capacity of biosorption in 15min. The pseudo second-order and saturation rate equations were found in the best fitness with the kinetic data (R2 > 0.99). The data obtained from experiments of single-component biosorption isotherm were analyzed using the Freundlich, Langmuir, Freundlich-Langmuir and Redlich-Peterson isotherm models. The Redlich-Peterson equation described the biosorption isotherm of Pb2+ and Cd2+ with high correlation coefficient (R2 > 0.99) and better than the other equations. The effect of Na+, K+, Mg2+ and Ca2+ on the biosorption of Pb2+ was not significant, but the metal ions affected the biosorption of Cd2+ considerably. According to the Langmuir model, the maximum uptake capacities (qm) of Sargassum spp. for Pb2+ and Cd2+ were obtained as 1.70 and 1.02mmol/g, respectively. Although the Sargassum spp. used in this study can be classified as an efficient biosorbent.
Key words: Biosorption, sargassum, lead (II); cadmium (II), kinetic, isotherm
Aqueous heavy metals pollution represents an important environmental problem due to their toxic effects and accumulation throughout the food chain. Among heavy metals, lead and cadmium have high priority for removal from aqueous environments (Kapoor et al., 1999; Volesky, 2001; Rama et al., 2002 and Anonymous, 2002). The conventional technologies for the removal of heavy metals from wastewater mainly include: chemical precipitation, ion exchange, adsorption, membrane processes and evaporation that require high capital investment and running costs (Anonymous, 2000; Gupta et al., 2001 and Aksu, 2002). Therefore, there is an urgent need for development of innovative but low cost processes, where metal ions can be removed economically. The search for new treatment technologies has focused on biosorption (Dönmez et al., 1999; Figueira et al., 2000 and Loukidou et al., 2003). Biosorption is a term that describes the removal of heavy metals by the passive binding to nonliving microorganisms (bacteria, fungi and algae) and other biomass (such as peat, rice hull, fruit peel, leave and bark of tree etc.) from an aqueous solution (Davis et al., 2003 and Ma and Tobin, 2003). A number of different metal binding mechanisms have been postulated to be active in biosorption such as ion exchange, complexation, coordination, chelation, physical adsorption and microprecipitation (Volesky, 2001). Biosorption has many advantages including low capital and operational costs, the selective removal of metals, biosorbent regeneration and metal recovery potentiality, rapid kinetics of adsorption and desorption and no sludge generation. Biosorption technology has been shown to be a feasible alternative for removing heavy metals from wastewater. This technology can utilize naturally abundant biomass such as seaweeds, and of these Sargassum has been identifie for its high sorption capacity (Volesky, 2001; Davis et al., 2003 and Diniz and Volesky, 2003). Biosorption of lead (II) and cadmium (II) from aqueous solutions using various biomasses has been studied. Cadmium (II) biosorption on Asppergillus oryzae reached equilibrium in 1h with 90% of biosorption taking place in the initial 10 min (Kiff and Little, 1986). Kinetic data of cadmium (II) biosorption by chitin presented high correlation with the pseudo second order rate equation (Benguella and Benaissa, 2002). Matheickal and Yu (1999) observed that the maximum uptake capacities of Durvillaea potatorum and Ecklonia radiata for Pb2+ were 1.6 and 1.3 mmol/g, respectively.
The objective of this research was to study the biosorption of lead (II) and cadmium (II) from aqueous environments by brown algae Sargassum spp. biomass. Kinetic and isotherm of lead (II) and cadmium (II) biosorption and the effects of pH and light metal ions (Na+, K+, Mg2+ and Ca2+) on lead (II) and cadmium (II) biosorption were investigated.
MATERIALS AND METHODS
Preparation of biosorbent
The biosorbent used in experiments was brown algae Sargassum spp. biomass. The biomass was harvested from Oman Sea on the coast of Chabahar, Iran. The biomass was washed with tap water and deionized water to remove sand and other impurities. The biomass was sundried and then dried in an oven at 70 °C. Dried biomass was ground in a laboratory blender. After this, the biomass was sieved to select particle between 0.2 -0.3 mm for use. The biomass was subsequently loaded with H+ in a solution of 0.1M HCl (biomass concentration of 50 g/L) for 30 min under slow stirring. Later the biomass was washed with deionized water to remove excess hydrogen ions. Finally the biosorbent again dried at 70 °C for 24 hr.
Synthetic solutions were prepared using de-ionized water and salts of Pb(NO3)2, Cd(NO3)2.4H2O, NaCl, KCl, MgCl2.6H2O and CaCl2.2H2O (Merck supplied). Initial pH of solutions was adjusted with a pH meter (CAMLAB Ltd, Model CG842) to the desired values by using 0.1-1M HCl and 0.11M NaOH.
In all batch biosorption experiments, solution volume was 1L and the mixture of solution and biosorbent was agitated in 200 rpm. The experiments were conducted at room temperature (20±1 °C). Initial pH of the solutions was adjusted to desired values. The reaction mixture pH was not regulated after the initiation of experiments and final pH was measured.
Kinetic experiments were done in three initial concentrations of Pb2+ and Cd2+ and fixed initial ratio of adsorbate to biosorbent inside a single component system. Initial metal concentrations were 0.5, 1 and 5 mM and initial ratio of adsorbate to biosorbent was 2 mmol/g, therefore 2.5, 0.5 and 0.25g of the biosorbent were added to experiment vessels with initial metal concentrations of 5, 1 and 0.5 mM, resppectively. Initial pH of solutions was adjusted to 5 and pH of the solutions was monitored continuously. The experiments were continued for 5 hr and samples were drawn from the mixture at predetermined time intervals for analysis.
Kinetic experiments presented that maximum time required to reach equilibrium was 2 h; therefore, the equilibrium time for equilibrium experiments was chosen 3 hr.
Biosorption isotherm experiments were conducted in a single component system. The initial Pb2+ and Cd2+ concentrations were varied from 0.05 to 5 mM. Initial pH of the solutions was adjusted to 5 and then 500 mg of Sargasssum spp. biomass was added to experiment vessels.
Effect of pH on biosorption
The effect of pH on equilibrium capacities of Pb2+ and Cd2+ biosorption was studied in a single component system. Initial heavy metal ions concentration was 1 mM and initial pH of solutions was varied from 2 to 5.5. After pH adjustment, 500 mg of Sargasssum spp. biomass was added to experiment vessels.
Effect of light metal ions on biosorption
The effect of Na+, K+, Mg2+ and Ca2+ on equilibrium uptake of Pb2+ and Cd2+ was studied in a binary system (one heavy metal and one light metal). Initial heavy metal ion concentration was 1mM and initial light metal ion concentration was varied from 0 to 6 mM. Initial pH of the solutions was adjusted to 5 and then 500 mg of Sargasssum spp. biomass was added to experiment vessels.
Metal analysis: The biomass was removed by filtration through 0.45µm membrane filters (mixed cellulose ester) and filtrates were analyzed for residual heavy metal (Pb2+ or Cd2+) concentration by a flame atomic absorption sppectrophotometer (FAAS, Chem. Tech Analytical, Model ALPHA4).
Kinetic modeling: Kinetic of Pb2+ and Cd2+ biosorption was modeled by the pseudo first-order (Langergren), pseudo second-order, saturation (mixed-order) and second-order rate equations presented below as Eqs. (1)-(4), resppectively:
where qe and qt are the amounts of metal ion sorbed (mmol/g-) at equilibrium and at any time, resppectively; k1 is the pseudo first-order rate constant of adsorption (min-1); k2 is the pseudo second-order rate constant of adsorption (gm/mol/ min); C0 and Ct are the concentrations of metal ion (mM) at t=0 and at any time, resppectively; k0 (mM/min) and K (mM) are saturation rate constants of adsorption and k is the second-order rate constant of adsorption (gm/mol/min) (Benguella and Benaissa, 2002; Metcalf & Eddy, 2003;Azizian, 2004).
The isotherm of Pb2+ and Cd2+ biosorption was analyzed using the Freundlich, Langmuir, Freundlich-Langmuir and Redlich-Peterson models.
The empirical Freundlich model based on sorption onto a heterogeneous surface is given below by Eq. (5):
where Ce is equilibrium concentration of metal ion (mM); KF and n are indicators of biosorption capacity and biosorption intensity, respectively (Loukidou et al., 2004; Selatnia et al., 2004b).
The Langmuir equation is based on the assumption that maximum adsorption correspponds to a saturated monolayer of solute on the adsorbent surface, that energy of adsorption is constant and that there is no transmigration of adsorbate in the plane of the surface. The Langmuir equation is given by Eq. (6):
where qm is the maximum capacity of biosorption (mmol/g) and b is a constant related to the affinity of the binding sites (mL/mol) (Yalçýnkaya, et al., 2002; Sheng, et al., 2004).
The three-parameter Freundlich-Langmuir model was developed to improve the fitness found by the Freundlich or Langmuir model. This model is given by Eq. (7):
where b, qm and n are the Freundlich-Langmuir parameters (Volesky, 2003).
The three-parameter Redlich-Peterson model is given below by Eq. (8):
where KRP (L/g), aRP (Lmmol-1)â and â (dimensionless) are the Redlich-Peterson constants. â lies between 0 and 1. For â = 1 the Redlich-Peterson model converts to the Langmuir model (Aksu, 2002 and Volesky, 2003).
The kinetic profiles of Pb2+ and Cd2+ biosorption by Sargassum spp. were shown in Fig. 1(a)-(b).
Fig. 2 (a)-(d) and Fig. 3 (a)-(d) show kinetic modeling of Pb2+ and Cd2+ biosorption by linear plots of the pseudo first-order, pseudo second-order, saturation and second-order rate equations (Eqs. (1)-(4)). Kinetic parameters of these equations for biosorption of Pb2+ and Cd2+ by Sargassum spp. biomass were shown in Tables 1 and 2.
Fig. 4(a)-(d) shows isotherm modeling of Pb2+ and Cd2+ biosorption by linear plots of the Freundlich, Langmuir, Freundlich-Langmuir and Redlich-Peterson models (Eqs. (5)-(8)).
The maximum biosorption capacities (qm) obtained from this research with those of other biosorbents reported in the literature were given in Table 5.
Effect of pH on biosorption
Fig. 5 shows the effect of pH on equilibrium uptake capacities of Sargassum spp. biomass for Pb2+ and Cd2+.
Effect of light metal ions on biosorption
The effect of Na+, K+, Mg2+ and Ca2+ on equilibrium capacities of Pb2+ and Cd2+ biosorption by Sargassum spp. biomass was shown in Fig. 6.
Kinetic study: According to Fig. 1, Pb2+ and Cd2+ uptake was relatively fast for all the concentrations studied. At the initial Pb2+ concentration of 5mM, the system reached to equilibrium within 30min. In general, the heavy metals uptake reached to 88-96% equilibrium capacity of biosorption in 15 min. This rapid kinetic has significant practical importance as it will facilitate smaller reactor volumes ensuring efficiency and economy. Similar rapid metal uptake has been reported for the biosorption of Pb2+ using Ecklonia radiata wherein the system reached over 50-60% of the equilibrium uptake capacity in 10 min (Matheickal and Yu, 1996). The kinetic of chromium (III) biosorption by Sargassum spp. biomass was fast, reaching 60% of the total uptake capacity in 10min (Cossich et al., 2002). The biosorption of lead (II) by Durvillaea potatorum was rather rapid and 90% of the total uptake occurred in 30min (Matheickal and Yu, 1999). The pseudo secondorder and saturation rate equations described the biosorption kinetic of Pb2+ and Cd2+ with high correlation coefficient (R2 > 0.99) and better than the other equations. Kinetic analysis of Pb2+, Cd2+, Ni2+ and Zn2+ biosorption by Mucor rouxii represented that the pseudo second-order rate equation described the biosorption kinetic better than the Langergren model (Yan and Viraraghavan, 2003).
The rate constants of the pseudo second-order rate equation for Pb2+ biosorption were obtained 1.86, 0.83 and 0.52gm/mol/min at initial Pb2+ concentrations of 5, 1 and 0.5mM, resppectively. In addition, the rate constants of saturation rate equation for Pb2+ biosorption were determined to be 0.0034, 0.0010 and 0.0007mM/min at initial Pb2+ concentrations of 5, 1 and 0.5mM, resppectively. The rate constants obtained from the pseudo second-order rate equation for Cd2+ biosorption were 1.07, 0.93 and 0.87gm/mol/min at initial Cd2+ concentrations of 5, 1 and 0.5 mM, resppectively. Also the rate constants of saturation rate equation for Cd2+ biosorption were determined to be 0.0011, 0.0004 and 0.0002mM/min at initial Cd2+ concentrations of 5, 1 and 0.5mM, resppectively. An increase in initial concentration of Pb2+ and Cd2+ led to an increase in the rate constant value, therefore there was a direct relationship between initial concentration of Pb2+ and Cd2+ and the rate of Pb2+ and Cd2+ biosorption by Sargassum spp. biomass. In other words, the biosorption of Pb2+ and Cd2+ by Sargassum spp. was faster in higher initial metal ion concentration.
Isotherm study: Isotherm data are basic requirements for the design of biosorption reactors, moreover analysis of biosorption isotherm is important to develop an equation which accurately represents the results and which can be used for design purposes (Volesky, 2001; Aksu, 2002).
The Redlich-Peterson equation described the isotherm of Pb2+ and Cd2+ biosorption by Sargassum spp. biomass with high correlation coefficient (R2 > 0.99) and better than the other models. Also the other models were found in relatively good fitness with the experimental data (R2 > 0.93). According to Langmuir equation, the maximum capacities of Pb2+ and Cd2+ biosorption (qm) were obtained 1.70 and 1.02mmol/g, resppectively. The Langmuir parameter qm (maximum uptake capacity) is a suitable measure for comparing different sorbents for the same sorbate. Although due to the various experimental conditions employed in different studies, comparison of their results is difficult, but maximum uptake capacity of Sargassum spp. biomass for Pb2+ and Cd2+ far exceed those of most of the biosorbents (Table 5); consequently, the Sargassum spp. used in this study can be classified as a good biosorbent.
Effect of pH on biosorption
Other studies on heavy metal biosorption have presented that pH was an important parameter affecting the biosorption process (Yan and Viraraghavan, 2003; Selatnia et al., 2004a).
The effect of pH on Pb2+ and Cd2+ biosorption was studied in the initial pH range of 2 to 5.5. At higher pH values, the experiments were not conducted to avoid formation of insoluble Pb2+ and Cd2+ hydroxides. The optimum initial pH values for Pb2+ and Cd2+ biosorption were determined as 4.5 and 5, resppectively. The sharpest increase in Pb2+ uptake was obtained between pH 2 and 3. The sharpest increase in Cd2+ uptake was determined between pH values of 2 and 4. The dependence of Pb2+ and Cd2+ biosorption on pH could be largely related to ionic state of binding sites on the algal cell wall (Matheickal and Yu, 1996; Sheng et al., 2004). Measurement of final pH represented the simultaneous release of H+ with the uptake of heavy metal ions, because final pH of solutions was less than initial pH of solutions, therefore ion exchange confirmed to be one of the biosorption mechanisms. Other studies with seaweed and fungal biomass have indicated ion exchange as the dominant mechanism of biosorption (Fourest and Roux, 1992; Schiewer and Volesky, 1996; Ahuja et al., 1999; Volesky, 2001).
Effect of light metal ions on biosorption
Industrial effluents contaminated with heavy metals contain various kinds of impurities such as light metal ions (Na+, K+, Mg2+ and Ca2+) that affect the heavy metal removal processes. Matheickal and Yu (1999) investigated the effect of Na+, K+, Mg2+ and Ca2+ on biosorption of Pb2+ by Durvillaea potatorum and Ecklonia radiata. The results showed that the biosorbents had much higher relative affinities for Pb2+ than for the light metal ions. The presence of Na+, K+, Mg2+ and Ca2+ in solution did not affect the biosorption capacity of Cu2+ by Padina spp. significantly (Kaewsarn, 2002).
The effect of Na+ and K+ on Pb2+ uptake by Sargassum spp. biomass was insignificant even at 6 mM concentration of these ions, but Mg2+ and Ca2+ had influence on Pb2+ biosorption. The equilibrium capacity of Pb2+ biosorption was reduced at initial Mg2+ and Ca2+ concentration of 6 mM by 8% and 13%, resppectively. The presence of Na+, K+, Mg2+ and Ca2+ in solution affected the biosorption of Cd2+ conciderably, so that equilibrium uptake capacity of Cd2+ was reduced at initial concentration 0.5-6mM of Na+, K+, Mg2+ and Ca2+ by 1-10%, 6-17%, 7-35% and 12-56%, resppectively.
Altogether the Sargassum spp. used in this study can be classified as an efficient biosorbent because of rapid kinetic, remarkable biosorption capacity and selective removal of metals. Thus the biosorbent has a high potential for application in full-scale for removal of heavy metals from aqueous environments.
The authors are most grateful to the laboratory staff of the Department of Environmental Health Engineering, School of Public Health, Tehran University of Medical Sciences, Iran, for their collaboration in this research.
© 2005 Tehran University of Medical Sciences Publications
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