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Heavy metal adsorption characteristics of a submerged aquatic plant (Myriophyllum spicatum)
Process Biochemistry 39 (2003) 179 /183 www.elsevier.com/locate/procbio
Heavy metal adsorption characteristics of a submerged aquatic plant (Myriophyllum spicatum) O. Keskinkan a, M.Z.L. Goksu b, A. Yuceer a, M. Basibuyuk a, C.F. Forster c,* a
Department of Environmental Engineering, Faculty of Engineering and Architecture, Cukurova University, 01330 Balcali-Adana, Turkey b Fisheries Faculty, Cukurova University, 01330 Balcali-Adana, Turkey c Department of Civil Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK Received 9 December 2002; accepted 13 January 2003
Abstract Submerged aquatic plants can be used for the removal of heavy metals. In this paper, the adsorption properties of Myriophyllum spicatum (Eurasian watermilfoil) for lead, zinc, and copper were investigated and the results were compared with other aquatic submerged plants. Data obtained from the initial batch adsorption studies have indicated that M. spicatum is capable of removing lead, zinc, and copper from solution. Metal biosorption was fast and equilibrium was attained within 20 min. Data obtained from further batch studies fitted the Langmuir model. The maximum adsorption capacities (qmax) were 10.37 mg/g for Cu(II), 15.59 mg/g for Zn(II) and 46.49 mg/g for Pb(II). The kinetics of adsorption of zinc, lead and copper were also analysed and rate constants were derived for each metal. It was found that the overall adsorption process was best described by the pseudo second order kinetics. The results showed that this submerged aquatic plant M. spicatum can be successfully used for heavy metal removal. # 2003 Elsevier Ltd. All rights reserved. Keywords: Aquatic plants; Biosorption; Heavy metals; Langmuir model; Pseudo second order
1. Introduction Heavy metals may come from various industrial sources such as electroplating, metal finishing, textile, storage batteries, lead smelting, mining, plating, ceramic and glass industries. Zinc, copper and lead are common contaminants of industrial wastewaters. Because they pose serious environmental problems and are dangerous to human health, considerable attention has been paid to methods for their removal from industrial wastewaters. The methods for removal of many heavy metals include precipitation, oxidation, reduction, ion exchange, filtration, electrochemical treatment, membrane technologies, reverse osmosis and solvent extraction [1,2]. Adsorption is a well established technique for heavy metal removal and activated carbon is the most widely used adsorbent. However, the use of activated carbon can be expensive and there has been considerable
* Corresponding author. Tel.: /44-121-414-5049; fax: /44-121414-3675. E-mail address: [email protected] (C.F. Forster). 0032-9592/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0032-9592(03)00045-1
interest in the use of other adsorbent materials, particularly biosorbents [3,4]. This technique is now recognised as an alternative method for the treatment of wastewaters containing heavy metals [5 /7]. It has been long known that aquatic plants, both living and dead, are heavy metal accumulators and, therefore, the use of aquatic plants for the removal of heavy metals from wastewater gained high interest [8 / 10]. Some freshwater macrophytes including Potamogeton lucens , Salvinia herzogoi , Eichhornia crassipes , Myriophyllum brasillensis , Myriophyllum spicatum , Cabomba sp., Ceratophyllum demersum have been investigated for the removal of heavy metals [11 /13]. There are various mechanisms for biosorption and, in some cases, they are not understood very well. The removal of metals by biosorption and the mechanisms of biosorption has been discussed by Veglio and Beolchini [14] who reported that there were many ways for the metal to be captured by cells. For example, biosorption may be classified as being: extracellular accumulation/precipitation, cell surface sorption/precipitation, and intracellular accumulation [14] and can
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O. Keskinkan et al. / Process Biochemistry 39 (2003) 179 /183
occur by complexation, co-ordination, chelation of metals, ion exchange, adsorption and micro precipitation. [11]. The bioremoval process using aquatic plants contains two uptake processes: . An initial fast, reversible, metal-binding processes (biosorption). . A slow, irreversible, ion-sequestration step (bioaccumulation). Schneider et al. [15] have studied the biosorption of metals onto plant biomass extensively in order to determine whether it was exchange adsorption or surface precipitation. It was found that sorption of heavy metals onto plant surface was a function of pH, and was greatest at a pH value which was slightly more acidic than the pH at which there was bulk precipitation of the metal hydroxide [15]. These workers have also studied the fit, or approximate fit, of the sorption data to the Langmuir isotherm. The fit was considered to be evidence that sorption stopped at one monolayer, consistent with specific and strong sorption onto specific sites [15]. Some submerged aquatic plants are invasive and can be seen generally in aquatic environments like streams, littoral zones of the lakes, drainage systems and wetlands. In Cukurova (Turkey) agricultural areas are very large and drainage channel systems are common around the fields for irrigation. M. spicatum is very common in these drainage systems and can be found throughout the year. M. spicatum (Eurasian watermilfoil) is a submerged aquatic perennial plant, which reproduces primarily by vegetative fragmentation. These fragments are produced during much of the year with the roots often developing on a fragment before it is released by the plant [16]. Plants may grow in water from 0.5 to 10 m deep. However, most plants appear to grow in water 0.5 /3.5 m deep. It is rooted to the bottom and grows to the surface. When the surface is reached, the plant branches profusely to form a dense canopy. Flowering and seed production are common. However, the seeds exhibit prolonged dormancy and their germination is erratic [17]. From one point of view, therefore, it can be thought of as a waste product. The main purpose of this study, therefore, is to investigate the adsorption characteristics of zinc, copper and lead onto watermilfoil (M. spicatum ).
as the metal sources and stock solutions of these metal ions were prepared in deionised water. Sorption tests were conducted at 25 8C in conical flasks (250 ml) using an orbital shaker in a constant room temperature. The initial pH values were between 5 and 6 during the batch experiments and no pH adjustment was made. Therefore, all the sorption experiments were carried out at pH values of B/6 M. spicatum biomass (about 2 g wet weight) was added to each flask and placed on the orbital shaker. The initial metal concentrations for the contact time experiments was 10 mg/l and the incubation times ranged from 5 to 160 min. The data used to derive the Langmuir constants were obtained using M. spicatum biomass (about 2 g wet weight) and metals concentrations of 2, 4, 8, 16, 32 and 64 mg/l. The contact time was 120 min. After contacting, the contents of the flask were filtered to separate the biomass from the solution. The filtrates were then analysed with an atomic absorption spectrophotometer (Perkin Elmer model 3100) to determine the metal concentrations in the samples. Control experiments were performed for each metal to measure any adsorption onto the glassware. Neither precipitation nor adsorption onto the wall of the flasks were observed. The results of metal analysis were used to calculate specific adsorption (mg metal adsorbed per g of biomass, dry weight). All the experiments were duplicated.
3. Results and discussion 3.1. Basic adsorption Initial adsorption tests for zinc, lead and copper showed that M. spicatum would adsorb all three metals tested (Fig. 1). The data also showed that a contact time of 20 min was sufficient to achieve equilibrium. After these initial experiments, further adsorption tests were carried out and it was seen that the data did not fit the Freundlich model but conformed well to the Langmuir
2. Materials and methods Plant biomass, which had been harvested locally, was washed with diluted HCl solution (3%) and distilled water before being used. Analytical grade copper(II) sulphate, zinc(II) sulphate and lead(II) nitrate were used
Fig. 1. Initial adsorption isotherm for zinc (") lead (') and copper (m) on M. spicatum .
O. Keskinkan et al. / Process Biochemistry 39 (2003) 179 /183
equation which is considered to be evidence that sorption was as a monolayer. The Langmuir equation can be linearalised as; Ce 1 a L Ce qe KL KL where Ce is the equilibrium concentration of adsorbate in solution after adsorption (mg/l), qe the equilibrium solid phase concentration (mg/g), KL and aL is the Langmuir constants. When the data were plotted as Ce/qe against Ce, good straight lines were obtained and the values of the regression analyses are shown in Table 1. The results indicate the applicability of the Langmuir equation for M. spicatum and the metals examined. The qmax values were found to be 46.49, 15.59 and 10.37 mg/g for lead, zinc and copper, respectively. As can be seen from the results, lead has the highest adsorption and copper has the lowest adsorption. The adsorption and the qmax results are consistent with the earlier work reported by Wang et al. for zinc, lead and copper adsorption by M. spicatum [11]. Table 2 shows a comparison of the qmax values from the current research with those obtained for other aquatic plants by various researchers. Wang et al. have also investigated the effect of pH of heavy metal adsorption by M. spicatum . According to their findings, pH is one of the main factor affecting the adsorption with the highest removal rates being obtained at pH values between 5 and 7.4 [11]. As discussed earlier, the effect of pH on heavy metal biosorption by algae, bacteria and aquatic plants has also been examined by Schneider et al. [15]. In this present work with pH values B/6, the concentrations of metal hydroxide would be negligible and, therefore, metal removal would, essentially, be only by adsorption. Monolayer coverage of the surface by the metal ions can be used for the calculation of the specific surface area S according to following equation [18]; S
qmax NA M
S is the specific surface area, m2/g adsorbent; qmax the monolayer sorption capacity, g metal/g M. spicatum ; N is the Avagadro number, 6.02 /1023; A is the cross sectional area of the metal ion, m2; M is molecular weight of the metal.
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Table 2 A comparison of the heavy metal uptake capacities (qmax, mg/g) of various macrophytes Adsorbent
Lead
Zinc
Copper
M. spicatum (this study) M. spicatum [11] P. lucens [12] S. herzegoi [12] E. crassipes [12]
46.69 55.6 141 / /
15.59 13.5 32.4 18.1 19.2
10.37 12.9 40.8 19.7 23.1
For Zn2, Cu2 and Pb2ions, the molecular weights are 65.4, 63.5 and 207 and the cross sectional areas of Zn2, Cu2 and Pb2 are 1.72, 1.58 and 5.56 ˚ 2 (Zn2, Cu2 and Pb2 radii are 0.74, 0.71 and 1.33 A ˚ , in a close packed monolayer) [18]. The maximum A specific surface area for M. spicatum for binding Zn2 is 2.46 m2/g, for Cu2 it is 1.56 m2/g and for Pb2 it is 7.51 m2/g. Table 3 shows a comparison of specific surface areas for M. spicatum and tree fern for each metal calculated on the same basis.
3.2. Kinetics of adsorption If the movement of the solute from the bulk liquid film surrounding the adsorbent is ignored, the adsorption process for porous solids can be separated into three stages; 1) Mass transfer (Boundary layer diffusion). 2) Sorption of ions onto sites. 3) Intraparticle diffusion. External mass transfer is characterised by the initial rate of solute adsorption. This can be calculated from the initial slope of the Ct/C0-time curves (Fig. 2). These slopes can be derived by assuming that the relationship was linear over the first 5 /10 min [19]. This gave initial rates of 0.097, 0.080 and 0.077 per min for lead, zinc and copper, respectively. In many cases there is the possibility that intraparticle diffusion will be the rate limiting step and this normally determined by using the equation described by Weber and Morris [20]; kp
q t1=2
Table 1 Regression results for Langmuir isotherm for M. spicatum for zinc, lead and copper Metal (mg/l)
Regression equation
KL (l/g)
aL (l/mg)
qmax (mg/g)
Correlation coefficient, R
Copper Lead Zinc
y/0.965x/0.1753 y/0.0215x/0.2118 y/0.0641x/0.3154
5.71 4.72 3.17
0.550 0.101 0.203
10.37 46.49 15.59
0.993 0.990 0.971
O. Keskinkan et al. / Process Biochemistry 39 (2003) 179 /183
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Table 3 A comparison of specific surface area for Zn2 , Cu2 and Pb2 for M. spicatum and tree fern Metal
qmax (mg metal/g M. spicatum )
S (m2/g M. spicatum )
qmax (mg metal/g tree fern)
S * (m2/g tree fern)a
Zinc Copper Lead
15.59 10.37 46.49
2.46 1.56 7.51
7.58 10.6 39.8
1.20 1.59 6.43
a
Data from Ho et al. [18].
in Table 4. These rate parameters have units of mg/g min0.5 and, as such, are not a direct quantification of the rates. Nevertheless, they can be interpreted in relative terms. According to the theoretical equations for diffusion, when intraparticle diffusion is the only rate determining step, the rate parameter is directly related to the square root of the initial concentration [23]. That is to say; j (C0 )n
Fig. 2. The variation of Ct/C0 with time for zinc (") lead (') and copper (m).
where q (mg/g) is the amount of the metal adsorbed at time t, kp is the intraparticle rate constant (mg/g min0.5). Fig. 3 shows that the relationships for the M. spicatum and copper, zinc, lead system are not linear over the entire time range, indicating that more than one process is affecting the adsorption. This type of nonlinearity has been reported previously by various authors [21 /25] and has been interpreted as showing that both boundary layer diffusion, the initial curved portion, and intraparticle diffusion, the final linear portion, are occuring. The slope of final linear portion can be used to derive a rate parameter, kp, for intraparticle diffusion. It is also possible to derive an initial rate parameter, j, by linear regression between t/0 and tlim which is the first breakpoint in the relationship [23]. Values for these parameters are given
However, values for n were found to be 0.100 for zinc, 0.077 for copper and 0.1909 for lead. This confirms that intraparticle diffusion is not the only operative mechanism and not the rate determining step for the lead, copper, zinc and M. spicatum system. In many cases, the kinetics of adsorption by any biological material have only been tested for the first order expression given by Lagergren. However, it has also been shown that a pseudo second order approach can sometimes provide a better description of the adsorption kinetics [1,23,26]. The first order Lagergren equation is [23]; ln(qe qt )ln(qe )Kt The pseudo second order equation is [26]; t 1 t 2 K?qe qt 2 qe where qe is the mass of metal adsorbed at equilibrium (mg/g), qt the mass of metal at time t (min), K the first order reaction rate constant of adsorption (per min), K ? is the pseudo second order rate constant of adsorption (mg/g min). It was found that although the first order equation was suitable for some of the data, it was not applicable to all the results. Therefore, no further consideration was given to it. The pseudo second order reaction Table 4 Initial rate parameters for each metal (mg/g min0.5) Metal
Fig. 3. Plots for the intraparticle diffusion for zinc (") lead (') and copper (m) onto M. spicatum .
Reaction rate constant kp (mg/g min0.5)
Zinc 1.2594 Copper 1.1967 Lead 1.5521
Correlation coefficient 0.984 0.992 0.976
O. Keskinkan et al. / Process Biochemistry 39 (2003) 179 /183 Table 5 Pseudo second order rate constants for each metal Metal
Reaction rate constant K (mg/g min) Correlation coefficient
Zinc 0.0338 Copper 1.508/10 3 Lead 3.695/10 3
0.999 0.999 1
model, however, was applicable to all the data and values of the reaction rate constants and correlation coefficients for each metal are listed in Table 5.
4. Conclusions From the work presented here, the aquatic submerged plant M. spicatum can be effective as a biosorbent for the removal of zinc, lead and copper. Batch adsorption studies showed that, based on the Langmuir coefficients, the maximum adsorption capacity was 15.59 mg/g for zinc, 46.49 mg/g for lead and 10.37 mg/g for copper. The initial part of the adsorption of each metal was governed by first degree film diffusion process. The data also confirm that intraparticle diffusion was not the rate determining step. The overall adsorption rate showed that the zinc, copper, lead and M. spicatum system was best described by the pseudo second order model.
Acknowledgements This project was supported by the Cukurova University Research Fund.
References [1] Cheung CW, Porter CF, McKay G. Sorption kinetics for the removal of copper and zinc from effluents using bone char. Sep Purif Technol 1997;19:55 /64. [2] Sternberg SPK, Dom RW. Cadmium removal using Cladophora in batch, semi-batch and flow reactors. Bioresour Technol 2002;81:249 /55. [3] Wase DAJ, Forster CF. Biosorbents for metal ions. London: Taylor and Francis, 1997. [4] Ajmal M, Rao AKR, Rais A, Jameel A. Adsorption studies on Citrus reticulata (fruit peel of orange): removal and recovery of Ni(II) from electroplating wastewater. J Hazardous Mater 2000;B79:117 /31. [5] Aksu Z, Akpnar D. Modelling of simultaneous adsorption of phenol and nickel II onto dried aerobic activated sludge. Sep Purif Technol 2000;21:87 /9.
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[6] Gupta VK, Shrivastava AK, Neeraj J. Biosorption of chromium (VI) from aqueous solutions by green algae Spirogyra species. Water Res 2001;35:4079 /85. [7] Kaewsarn P. Biosorption of copper(II) from aqueous solutions by pre-treated biomass of marine algae Padina sp. Chemosphere 2002;47:1081 /5. [8] Kuyucak N, Volesky B. Biosorbents for recovery of metals from industrial solutions. Biotechnol Lett 1989;10:137 /42. [9] Lee CL, Wang TC, Lin CK, Mok HK. Heavy metals removal by a promising locally available aquatic plant, Najas graminea del ., in Taiwan. Water Sci Technol 1999;39(10 /11):177 /81. [10] Lacher C, Smith RW. Sorption of Hg by Potamogeton natans dead biomass. Min Eng 2002;15:187 /91. [11] Wang TC, Weissman JC, Ramesh G, Varadarajan R, Benemann JR. Parameters for removal of toxic heavy metals by water milfoil (Myriophyllum spicatum ). Bull Environ Contam Toxicol 1996;57:779 /86. [12] Schneider IAH, Rubio J. Sorption of heavy metal ions by the nonliving biomass of freshwater macrophytes. Environ Sci Technol 1999;33:2213 /7. [13] Schneider IAH, Rubio J, Smith RW. Effect of some mining chemicals on biosorption of Cu(II) by the nonliving biomass of the freshwater macrophyte Potamogeton lucens . Miner Eng 1999;12:255 /60. [14] Veglio F, Beolchini F. Removal of heavy metal ions by biosorption: a review. Hydrometallurgy 1997;44:301 /16. [15] Schneider IAH, Rubio J, Smith RW. Biosorption of heavy metals onto plant biomass: exchange adsorption or surface precipitation. Int J Miner Processing 2001;62:111 /20. [16] Aiken SG, Newroth PR, Wile I. The biology of Canadian weeds. 34. Myriophyllum spicatum L. Can J Plant Sci 1979;59:201 /21. [17] Aiken SG, Picard RR. The influence of substrate on the growth and morphology of Myriophyllum exalbescens and Myriophyllum spicatum . Can J Bot-Rev Can Bot 1980;58:1111 /8. [18] Ho YS, Huang CT, Huang HW. Equilibrium sorption isotherm for metal ions on tree fern. Process Biochem 2002;37:1421 /30. [19] Forster CF, Mehrotra I, Alibhai KRK. The multiple binding of heavy metals by digested sludge. J Chem Technol Biotechnol 1985;35B:145 /54. [20] Weber WJ, Morris JC. Kinetics of adsorption on carbon from solutions. J Sanit Eng Div Am Soc Civ Eng 1963;89:31 /60. [21] McKay G, Otterburn MS, Sweeney AG. The removal of colour from effluent using various adsorbents-III silica rate processes. Water Res 1980;14:15 /20. [22] Gupta GS, Shukla SP, Prasad G, Singh VN. China clay as an adsorbent for dye house wastewaters. Environ Technol 1992;13:925 /36. [23] Ho YS, Wase DAJ, Forster CF. Kinetic studies of competitive heavy metal adsorption by sphagnum moss peat. Environ Technol 1996;17:71 /7. [24] Fernandez N, Chacin C, Garcia C, Alastre N, Leal F, Forster CF. The use of seed pods from Albizia lebbek for the removal of alkyl benzene sulphonates from aqueous solution. Process Biochem 1996;32:383 /7. [25] Kandah M. Zinc adsorption from aqueous solutions using disposal sheep manure waste (SMW). Chem Eng J 2001;84:543 /9. [26] Benquella B, Benaissa H. Cadmium removal from aqueous solutions by chitin: kinetic and equilibrium studies. Water Res 2002;36:2463 /74.
Heavy metal adsorption characteristics of a submerged aquatic plant (Myriophyllum spicatum) O. Keskinkan a, M.Z.L. Goksu b, A. Yuceer a, M. Basibuyuk a, C.F. Forster c,* a
Department of Environmental Engineering, Faculty of Engineering and Architecture, Cukurova University, 01330 Balcali-Adana, Turkey b Fisheries Faculty, Cukurova University, 01330 Balcali-Adana, Turkey c Department of Civil Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK Received 9 December 2002; accepted 13 January 2003
Abstract Submerged aquatic plants can be used for the removal of heavy metals. In this paper, the adsorption properties of Myriophyllum spicatum (Eurasian watermilfoil) for lead, zinc, and copper were investigated and the results were compared with other aquatic submerged plants. Data obtained from the initial batch adsorption studies have indicated that M. spicatum is capable of removing lead, zinc, and copper from solution. Metal biosorption was fast and equilibrium was attained within 20 min. Data obtained from further batch studies fitted the Langmuir model. The maximum adsorption capacities (qmax) were 10.37 mg/g for Cu(II), 15.59 mg/g for Zn(II) and 46.49 mg/g for Pb(II). The kinetics of adsorption of zinc, lead and copper were also analysed and rate constants were derived for each metal. It was found that the overall adsorption process was best described by the pseudo second order kinetics. The results showed that this submerged aquatic plant M. spicatum can be successfully used for heavy metal removal. # 2003 Elsevier Ltd. All rights reserved. Keywords: Aquatic plants; Biosorption; Heavy metals; Langmuir model; Pseudo second order
1. Introduction Heavy metals may come from various industrial sources such as electroplating, metal finishing, textile, storage batteries, lead smelting, mining, plating, ceramic and glass industries. Zinc, copper and lead are common contaminants of industrial wastewaters. Because they pose serious environmental problems and are dangerous to human health, considerable attention has been paid to methods for their removal from industrial wastewaters. The methods for removal of many heavy metals include precipitation, oxidation, reduction, ion exchange, filtration, electrochemical treatment, membrane technologies, reverse osmosis and solvent extraction [1,2]. Adsorption is a well established technique for heavy metal removal and activated carbon is the most widely used adsorbent. However, the use of activated carbon can be expensive and there has been considerable
* Corresponding author. Tel.: /44-121-414-5049; fax: /44-121414-3675. E-mail address: [email protected] (C.F. Forster). 0032-9592/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0032-9592(03)00045-1
interest in the use of other adsorbent materials, particularly biosorbents [3,4]. This technique is now recognised as an alternative method for the treatment of wastewaters containing heavy metals [5 /7]. It has been long known that aquatic plants, both living and dead, are heavy metal accumulators and, therefore, the use of aquatic plants for the removal of heavy metals from wastewater gained high interest [8 / 10]. Some freshwater macrophytes including Potamogeton lucens , Salvinia herzogoi , Eichhornia crassipes , Myriophyllum brasillensis , Myriophyllum spicatum , Cabomba sp., Ceratophyllum demersum have been investigated for the removal of heavy metals [11 /13]. There are various mechanisms for biosorption and, in some cases, they are not understood very well. The removal of metals by biosorption and the mechanisms of biosorption has been discussed by Veglio and Beolchini [14] who reported that there were many ways for the metal to be captured by cells. For example, biosorption may be classified as being: extracellular accumulation/precipitation, cell surface sorption/precipitation, and intracellular accumulation [14] and can
180
O. Keskinkan et al. / Process Biochemistry 39 (2003) 179 /183
occur by complexation, co-ordination, chelation of metals, ion exchange, adsorption and micro precipitation. [11]. The bioremoval process using aquatic plants contains two uptake processes: . An initial fast, reversible, metal-binding processes (biosorption). . A slow, irreversible, ion-sequestration step (bioaccumulation). Schneider et al. [15] have studied the biosorption of metals onto plant biomass extensively in order to determine whether it was exchange adsorption or surface precipitation. It was found that sorption of heavy metals onto plant surface was a function of pH, and was greatest at a pH value which was slightly more acidic than the pH at which there was bulk precipitation of the metal hydroxide [15]. These workers have also studied the fit, or approximate fit, of the sorption data to the Langmuir isotherm. The fit was considered to be evidence that sorption stopped at one monolayer, consistent with specific and strong sorption onto specific sites [15]. Some submerged aquatic plants are invasive and can be seen generally in aquatic environments like streams, littoral zones of the lakes, drainage systems and wetlands. In Cukurova (Turkey) agricultural areas are very large and drainage channel systems are common around the fields for irrigation. M. spicatum is very common in these drainage systems and can be found throughout the year. M. spicatum (Eurasian watermilfoil) is a submerged aquatic perennial plant, which reproduces primarily by vegetative fragmentation. These fragments are produced during much of the year with the roots often developing on a fragment before it is released by the plant [16]. Plants may grow in water from 0.5 to 10 m deep. However, most plants appear to grow in water 0.5 /3.5 m deep. It is rooted to the bottom and grows to the surface. When the surface is reached, the plant branches profusely to form a dense canopy. Flowering and seed production are common. However, the seeds exhibit prolonged dormancy and their germination is erratic [17]. From one point of view, therefore, it can be thought of as a waste product. The main purpose of this study, therefore, is to investigate the adsorption characteristics of zinc, copper and lead onto watermilfoil (M. spicatum ).
as the metal sources and stock solutions of these metal ions were prepared in deionised water. Sorption tests were conducted at 25 8C in conical flasks (250 ml) using an orbital shaker in a constant room temperature. The initial pH values were between 5 and 6 during the batch experiments and no pH adjustment was made. Therefore, all the sorption experiments were carried out at pH values of B/6 M. spicatum biomass (about 2 g wet weight) was added to each flask and placed on the orbital shaker. The initial metal concentrations for the contact time experiments was 10 mg/l and the incubation times ranged from 5 to 160 min. The data used to derive the Langmuir constants were obtained using M. spicatum biomass (about 2 g wet weight) and metals concentrations of 2, 4, 8, 16, 32 and 64 mg/l. The contact time was 120 min. After contacting, the contents of the flask were filtered to separate the biomass from the solution. The filtrates were then analysed with an atomic absorption spectrophotometer (Perkin Elmer model 3100) to determine the metal concentrations in the samples. Control experiments were performed for each metal to measure any adsorption onto the glassware. Neither precipitation nor adsorption onto the wall of the flasks were observed. The results of metal analysis were used to calculate specific adsorption (mg metal adsorbed per g of biomass, dry weight). All the experiments were duplicated.
3. Results and discussion 3.1. Basic adsorption Initial adsorption tests for zinc, lead and copper showed that M. spicatum would adsorb all three metals tested (Fig. 1). The data also showed that a contact time of 20 min was sufficient to achieve equilibrium. After these initial experiments, further adsorption tests were carried out and it was seen that the data did not fit the Freundlich model but conformed well to the Langmuir
2. Materials and methods Plant biomass, which had been harvested locally, was washed with diluted HCl solution (3%) and distilled water before being used. Analytical grade copper(II) sulphate, zinc(II) sulphate and lead(II) nitrate were used
Fig. 1. Initial adsorption isotherm for zinc (") lead (') and copper (m) on M. spicatum .
O. Keskinkan et al. / Process Biochemistry 39 (2003) 179 /183
equation which is considered to be evidence that sorption was as a monolayer. The Langmuir equation can be linearalised as; Ce 1 a L Ce qe KL KL where Ce is the equilibrium concentration of adsorbate in solution after adsorption (mg/l), qe the equilibrium solid phase concentration (mg/g), KL and aL is the Langmuir constants. When the data were plotted as Ce/qe against Ce, good straight lines were obtained and the values of the regression analyses are shown in Table 1. The results indicate the applicability of the Langmuir equation for M. spicatum and the metals examined. The qmax values were found to be 46.49, 15.59 and 10.37 mg/g for lead, zinc and copper, respectively. As can be seen from the results, lead has the highest adsorption and copper has the lowest adsorption. The adsorption and the qmax results are consistent with the earlier work reported by Wang et al. for zinc, lead and copper adsorption by M. spicatum [11]. Table 2 shows a comparison of the qmax values from the current research with those obtained for other aquatic plants by various researchers. Wang et al. have also investigated the effect of pH of heavy metal adsorption by M. spicatum . According to their findings, pH is one of the main factor affecting the adsorption with the highest removal rates being obtained at pH values between 5 and 7.4 [11]. As discussed earlier, the effect of pH on heavy metal biosorption by algae, bacteria and aquatic plants has also been examined by Schneider et al. [15]. In this present work with pH values B/6, the concentrations of metal hydroxide would be negligible and, therefore, metal removal would, essentially, be only by adsorption. Monolayer coverage of the surface by the metal ions can be used for the calculation of the specific surface area S according to following equation [18]; S
qmax NA M
S is the specific surface area, m2/g adsorbent; qmax the monolayer sorption capacity, g metal/g M. spicatum ; N is the Avagadro number, 6.02 /1023; A is the cross sectional area of the metal ion, m2; M is molecular weight of the metal.
181
Table 2 A comparison of the heavy metal uptake capacities (qmax, mg/g) of various macrophytes Adsorbent
Lead
Zinc
Copper
M. spicatum (this study) M. spicatum [11] P. lucens [12] S. herzegoi [12] E. crassipes [12]
46.69 55.6 141 / /
15.59 13.5 32.4 18.1 19.2
10.37 12.9 40.8 19.7 23.1
For Zn2, Cu2 and Pb2ions, the molecular weights are 65.4, 63.5 and 207 and the cross sectional areas of Zn2, Cu2 and Pb2 are 1.72, 1.58 and 5.56 ˚ 2 (Zn2, Cu2 and Pb2 radii are 0.74, 0.71 and 1.33 A ˚ , in a close packed monolayer) [18]. The maximum A specific surface area for M. spicatum for binding Zn2 is 2.46 m2/g, for Cu2 it is 1.56 m2/g and for Pb2 it is 7.51 m2/g. Table 3 shows a comparison of specific surface areas for M. spicatum and tree fern for each metal calculated on the same basis.
3.2. Kinetics of adsorption If the movement of the solute from the bulk liquid film surrounding the adsorbent is ignored, the adsorption process for porous solids can be separated into three stages; 1) Mass transfer (Boundary layer diffusion). 2) Sorption of ions onto sites. 3) Intraparticle diffusion. External mass transfer is characterised by the initial rate of solute adsorption. This can be calculated from the initial slope of the Ct/C0-time curves (Fig. 2). These slopes can be derived by assuming that the relationship was linear over the first 5 /10 min [19]. This gave initial rates of 0.097, 0.080 and 0.077 per min for lead, zinc and copper, respectively. In many cases there is the possibility that intraparticle diffusion will be the rate limiting step and this normally determined by using the equation described by Weber and Morris [20]; kp
q t1=2
Table 1 Regression results for Langmuir isotherm for M. spicatum for zinc, lead and copper Metal (mg/l)
Regression equation
KL (l/g)
aL (l/mg)
qmax (mg/g)
Correlation coefficient, R
Copper Lead Zinc
y/0.965x/0.1753 y/0.0215x/0.2118 y/0.0641x/0.3154
5.71 4.72 3.17
0.550 0.101 0.203
10.37 46.49 15.59
0.993 0.990 0.971
O. Keskinkan et al. / Process Biochemistry 39 (2003) 179 /183
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Table 3 A comparison of specific surface area for Zn2 , Cu2 and Pb2 for M. spicatum and tree fern Metal
qmax (mg metal/g M. spicatum )
S (m2/g M. spicatum )
qmax (mg metal/g tree fern)
S * (m2/g tree fern)a
Zinc Copper Lead
15.59 10.37 46.49
2.46 1.56 7.51
7.58 10.6 39.8
1.20 1.59 6.43
a
Data from Ho et al. [18].
in Table 4. These rate parameters have units of mg/g min0.5 and, as such, are not a direct quantification of the rates. Nevertheless, they can be interpreted in relative terms. According to the theoretical equations for diffusion, when intraparticle diffusion is the only rate determining step, the rate parameter is directly related to the square root of the initial concentration [23]. That is to say; j (C0 )n
Fig. 2. The variation of Ct/C0 with time for zinc (") lead (') and copper (m).
where q (mg/g) is the amount of the metal adsorbed at time t, kp is the intraparticle rate constant (mg/g min0.5). Fig. 3 shows that the relationships for the M. spicatum and copper, zinc, lead system are not linear over the entire time range, indicating that more than one process is affecting the adsorption. This type of nonlinearity has been reported previously by various authors [21 /25] and has been interpreted as showing that both boundary layer diffusion, the initial curved portion, and intraparticle diffusion, the final linear portion, are occuring. The slope of final linear portion can be used to derive a rate parameter, kp, for intraparticle diffusion. It is also possible to derive an initial rate parameter, j, by linear regression between t/0 and tlim which is the first breakpoint in the relationship [23]. Values for these parameters are given
However, values for n were found to be 0.100 for zinc, 0.077 for copper and 0.1909 for lead. This confirms that intraparticle diffusion is not the only operative mechanism and not the rate determining step for the lead, copper, zinc and M. spicatum system. In many cases, the kinetics of adsorption by any biological material have only been tested for the first order expression given by Lagergren. However, it has also been shown that a pseudo second order approach can sometimes provide a better description of the adsorption kinetics [1,23,26]. The first order Lagergren equation is [23]; ln(qe qt )ln(qe )Kt The pseudo second order equation is [26]; t 1 t 2 K?qe qt 2 qe where qe is the mass of metal adsorbed at equilibrium (mg/g), qt the mass of metal at time t (min), K the first order reaction rate constant of adsorption (per min), K ? is the pseudo second order rate constant of adsorption (mg/g min). It was found that although the first order equation was suitable for some of the data, it was not applicable to all the results. Therefore, no further consideration was given to it. The pseudo second order reaction Table 4 Initial rate parameters for each metal (mg/g min0.5) Metal
Fig. 3. Plots for the intraparticle diffusion for zinc (") lead (') and copper (m) onto M. spicatum .
Reaction rate constant kp (mg/g min0.5)
Zinc 1.2594 Copper 1.1967 Lead 1.5521
Correlation coefficient 0.984 0.992 0.976
O. Keskinkan et al. / Process Biochemistry 39 (2003) 179 /183 Table 5 Pseudo second order rate constants for each metal Metal
Reaction rate constant K (mg/g min) Correlation coefficient
Zinc 0.0338 Copper 1.508/10 3 Lead 3.695/10 3
0.999 0.999 1
model, however, was applicable to all the data and values of the reaction rate constants and correlation coefficients for each metal are listed in Table 5.
4. Conclusions From the work presented here, the aquatic submerged plant M. spicatum can be effective as a biosorbent for the removal of zinc, lead and copper. Batch adsorption studies showed that, based on the Langmuir coefficients, the maximum adsorption capacity was 15.59 mg/g for zinc, 46.49 mg/g for lead and 10.37 mg/g for copper. The initial part of the adsorption of each metal was governed by first degree film diffusion process. The data also confirm that intraparticle diffusion was not the rate determining step. The overall adsorption rate showed that the zinc, copper, lead and M. spicatum system was best described by the pseudo second order model.
Acknowledgements This project was supported by the Cukurova University Research Fund.
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