Chilean Chemical Society
versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.55 n.2 Concepción jun. 2010
doi: 10.4067/S0717-97072010000200030
J. Chil. Chem. Soc, 55, N° 2 (2010), págs.: 278-282
BANANA TRUNK FIBERS AS AN EFFICIENT BIOSORBENT FOR THE REMOVAL OF Cd(II), Cu(II), Fe(II) AND Zn(II) FROM AQUEOUS SOLUTIONS
KATHIRESAN SATHASIVAMa,b AND MAS ROSEMAL HAKIM MAS HARISa*
a School of Chemical Sciences, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia.
b Department of Materials Science, Faculty of Applied Sciences, AIMST University, 08100 Bedong, Kedah, Malaysia
b Department of Materials Science, Faculty of Applied Sciences, AIMST University, 08100 Bedong, Kedah, Malaysia
ABSTRACT
The biosorption of Cd(II), Cu(II), Fe(II) and Zn(II) from aqueous solutions by an agrowaste, namely banana trunk fibers (BTF), was investigated. The effect of pH, contact time, metal ions concentration, adsorbent dose and change in [M2+]/biomass were studied at ambient temperature (25 °C). The equihbrium process was described well by the Freundlich isotherm model with adsorption capacity, Kf of 8.49, 2.68, 6.58 and 1.74 mg/g for Cd(II), Cu(II), Fe(II) and Zn(II), respectively. Kinetic studies showed good correlation coefficients for a pseudo-second-order kinetic model. The BTF were subjected to different chemical modification methods (mercerization, acetylation, formaldehyde treatment, peroxide treatment, stearic acid treatment and sulphuric acid treatment) and the adsorption capacity (qe) of each modified BTF for the metal ions was obtained. Our findings hitherto reveal that the qe values are practically similar to that of the unmodified BTF confirming that the latter by its nature, that is its chemical composition, is already an efficient biosorbent for the removal of the heavy metal ions.
Keywords: Banana Trunk Fibers; Agrowaste; Biosorption; Heavy Metal Ions; Isotherm
INTRODUCTION
Industrialization has enhanced the degradation of our environment through the discharge of wastewaters. This hasoresulted in significant amounts of heavy metal ions such as Cd(II), Cu(II), Fe(II) and Zn(II) being deposited into our ecosystems1. These metals are not biodegradable and known to cause severe dysfunction of the kidney, reproductive system, liver, brain and central nervous system2. Several methods for removing heavy metal ions such as chemical precipitation, electrodeposition, ion exchange, reverse osmosis and adsorption have been used to treat wastewater. Of these methods, chemical precipitation is the most economic but is inefficient for dilute solution. Electrodepostion, ion exchange and reverse osmosis are generally effective, but have rather high maintenance and operation costs and subjectto fouling. Biosorption, a process that utilizes biomass3 for the decontamination of metal-containing effluents is a promising alternative. Low-cost natural sorbents such as cork and yohimbe bark4, spent grain5, peanut hull pellets6, rice milling by-products7, grape stalk waste8, pectin rich fruit wastes9 and biowaste from fruit juice industry10.
Banana plants are of the family Musacease and cultivated primarily fortheir fruit. As such, after harvesting the fruit, the matured pseudostems are generally disposed at a landfill or left to decompose slowly in a plantation field. The composition of a typical BTF obtained by elemental analysis, as determined by Bilba et al,11 is as follows: Cellulose (31.27 ± 3.61 %), Hemicellulose (14.98 ±2.03%), Lignin (15.07 ± 0.66 %), Extractives (4.46 ± 0.11 %), (Moisture 9.74 ± 1.42 %) and Ashes (8.65 ± 0.10 %).
The aim of the study is to evaluate the efficiency of the removal of Cd(II), Cu(II), Fe(II) and Zn(II) from aqueous solutions by unmodified BTF. The effect of various operating parameters such as pH, contact time, metal ions concentration, adsorbent dose and change in [M2+]/biomass was studied. The adsorption isotherm study was also carried out on two isotherm models, namely Langmuir and Freundlich. The adsorption capacity were determined and compared by first and second order kinetic models.
MATERIALS AND METHODS
Biosorbent material
BTF from the family of Musa acuminate x balbisiana Colla (ABB Group) cv Pisang Awak' were obtained locally, Penang, Malaysia. The pseudostems were chopped into cubes of average size of 2 cm x 2 cm. The cubes were submerged in boiling water for 1 hr and then dried in an oven at 70 °C until a constant weight was obtained. The resulting material was ground using a Waring Commercials high speed blender and sieved to isolate fibers of the size 212 - 350 micron. The native metal content the BTF were observed by using a scanning electrón microscopy (SEM-EDX) machine Model Leica Cambridge AS-360 at an accelerating voltage 15kV. Prior to examination, a surface of specimen was coated with a thin layer of gold approximately 30 nm using Sputter Coater Polaron SC 515.
Metal Solution
Cd(II), Cu(II), Fe(II) and Zn(II) standard solutions (1000 ± 2 mg/L) from Merck were diluted to desired concentrations for Atomic Absorption Spectroscopy (AAS) analysis. The experiment was conducted at pH value range 2 to 6. The pH was adjusted by adding appropriate amount of either 0.1 M NaOH or 0.1 M HC1 solution before each experiment
Equilihrium studies
Adsorption experiments were carried out by adding 0.2 g of sorbent into 250-mL Erlenmeyer flasks containing 50 mL solutions of different concentrations (1, 10 and 100 mg/L) of metal ions. The temperature was controlled at 25 °C. Agitation was provided at 150 rpm for 180 min. The initial and equihbrium metal concentrations were determined by absorbance measurement using the Atomic absorption Spectroscopy (AAnalyst 700, Perkin-Elmer, Waltham, MA, USA). When the equihbrium was established, the supernatant was carefully filtered through Whatman filter paper (No. 1) which was pre-saturated with distilled water. It is worthwhile to note that no adsorption of the metals occurred on the filter paper: a comparative study was done by measuring the concentration of the metal solution before and after filtering, and the result showed insignificant variation of concentration of the metal solution meaning that the amount of the metals adsorbed on the filter paper, if any, was negligible. It was then computed to metal concentration using standard calibration curve. The adsorption at equihbrium, q (mg/g), was calculated using equation (1).
where Co and Ce (mg/L) are the liquid-phase concentrations of metals at initial and equihbrium, respectively. V is the volume (L) of the solution and W is the weight (g) of dry sorbent. For the determination of rate of sorption and the sorption equihbrium time, the residual metal in the supernatant was determined by allowing metal-BTF contact for different periods between 5 and 180 min. The metal-BTF sorption suspension was equilibrated at different pH values of 2 — 6. For the adsorption isotherms studies, metal concentrations used for sorption ranged between 1 and 500 mg/L. The quantity of biomass was varied between 0.1 and 1.0 g to determine the BTF required for optimum level of sorption.
Treatments of Banana Trunk Fibers
The BTF were modified according to the methodsoreported in literature: (i) mercerization, the fibers were immersed in 5% NaOH solution for 48 hr at 25 °C12, (ii) acetylation, the mercerized fibers were soaked in glacial acetic acid for 1 hr, separated by decantation and then soaked in acetic anhydride containing 2 drops of concentrated H2SO4 for 2 min13, (iii) formaldehyde treatment, using 1% formaldehyde in the weight to volume ratio of 1:5 at 50 °C for 4 hr14, (iv) peroxide treatment , the mercerized fibers (30 g) were immersed in 1 L of a 6% solution of benzoyl peroxide in acetone for 30 min13, (v) stearic acid treatment, a mixture containing 1.0 g of the fibers, 0.2 g of stearic acid, 2 drops of concentrated H2SO4 in 100 mL of n-hexane wasorefluxed in a Dean-Stark apparatus at 65 °C for 6 hr15, and (vi) sulphuric acid treatment, 1 : 1 weight ratio of the fibers : concentrated H2SO4 was heated in a muirle furnace for 24 h at 150 °C14. All resulting fibers were washed ampie amount of water till a pH close to neutral was obtained.
RESULTS AND DISCUSSION
Elemental and Surface Area Analysis
The results obtained by energy dispersive X-ray (EDX) analysis of BTF are presented in Figure 1. It shows that BTF contained several elements such as carbon (40.25 %), oxygen (55.85%), potassium (2.41%), calcium (1.49%) and the presence of the metals understudy were not observed. Surface área of BTF was characterized according to method described by Horsfall and Spiff6 and was foundto be 28.75 ± 1.57 m2/g.
Effect of pH on metal ion sorption
pH of solution has been identified as the most important variable governing metal ions uptake because it influences the ionization of functional groups at the surface of a sorbent and hydrogen ions themselves may compete strongly with the adsorbates17,18. Henee, comparative studies on metal ions uptake must be carried out in solutions of similar pH value as any variation in the pH can drastically change the adsorption capacity of a sorbent19. With this in view, the uptake of Cd(II), Cu(II), Fe(II) and Zn(II) by BTF was investigated at various pH values (Figure 2). At the pH 2, it was observed that the sorptions were poor. This was attributed to the active sites being widely protonated which limits the adsorbing sites for the metal ions20. In the pH range of 3—5, a prominent increase in the sorptions was observed. The increase in metal removal as the pH increases can be explained on the basis of decrease in competition between protons and metal cations for the same binding sites21. Furthermore, the decrease in the positive surface chargesoresulting in a lower electrostatic repulsion between the surface and metal ions22. Further increase in sorptions was insignificant as the optimum biosorption for all the four metals wasoreached at about pH 5.
Effect of contact time and metal concentration on metal adsorption
Figure 3 illustrates the time-course studies on sorption of Cd(II), Cu(II), Fe(II) and Zn(II) that were performed by contacting 10 mg/L of the solutions containing metal ions at pH 5 with 0.2 g of BTF in 50 mL of the solutions. All the four metal ions showed a fast rate sorption during the initial 15 min (first stage) and the equihbrium (second stage) wasoreached at about 60 mins. The two-stage sorption, the first stage which is quantitatively predominant and the second slower stage which is quantitatively insignificant, has been extensively reported in literature23. The rapid stage is attributed to the abundant availability of active sites onthebiomass and with the gradual oceupaney ofthese sites, the sorption becomes less efficient in the slower stage24. The adsorption capacity (qe) for Cd(II), Cu(II), Fe(II) and Zn(II) of 2.43, 2.39, 2.42 and 2.18 mg/g, respectively. These results are in keeping with conclusion by Romera et al.25 Pejic et al.26 and Nasernejad et al27. The uptake level of the metal ions can be explained, similar to that provided by Okieimen et al.28 and Ricordel et al.29 in terms of (i) the difference in the ionic size of metals, (ii) the nature and distribution of active groups on the biosorbent, (iii) the mode of interaction between the metal ions and the biosorbent (iv) hydration energy, (v) ionic mobility and (vi) diffusion coefficient.
Optimization of amount of adsorbent
This adsorption study was carried out with 10 mg/L metal solution at pH 5 in an orbital shaker agitated at 150 rpm with 50 mL of the metal ions solution in a 250 mL Erlenmeyer flask. Results shown in Figure 4 indicate that the adsorbent dosage apparently increases proportionately to the availability of the adsorbent sites. The increase of adsorbent sites and surface área of contact with the metals increases the amount of metal uptakes and consequently leads to a better adsorption. This observed trend is mainly due to the increase in sorptive surface área and availability of more adsorption sites.
Concentration dependent study
The experimental results of the sorption of the metals on BTF at various concentrations are shown in Figure 5. The uptake rate ofthe metal ions will increase along with increasing concentration if the amount of biomass is kept unchanged. At low concentrations metals adsorb by available sites and with increasing metal concentrations the sites become saturated. The adsorption experimentsorelated to the concentration of metals were carried out over a range of 1 — 500 mg/L at pH 5 using 0.2 g of the fibers in 50 mL of metal ion solutions with an agitation speed of 150 rpm. The adsorption capacity of Cd(II), Cu(II), Fe(II) and Zn(II) increased from 0.24 to 20.82, 0.22 to 23.18, 0.22 to 22.67 and 0.19 to 22.23 mg/g, respectively. The metal concentration provides the necessary driving force to overcome the resistances to the mass transfer of metal between the aqueous and the solid phases. The increase in concentration also enhances the interaction between metal and the sorbent. Therefore, an increase in concentration of the metals enhances the adsorption uptake of metal ions. This indicates that the metal ions concentration plays an important role in the adsorption capacity of metal onto the sorbent. It was also observed that the q value increases as the C value increased16. It is also worthwhile to note that the qc of all the four metal ions were steep at lower equilibrium concentration which is desirable for a sorbent to possess a high affinity for the sorbate species30.
Effect of change in [M2+] /biomass
Various [M2+]/biomassoratio were obtained by increasing the mass of BTF while the concentration and volume of the metal ions solution are kept constan!The quantity of BTF was varied between 0.5 and 20 g l-1 to determine the optimum quantity of biomass needed for máximum sorption. From 10 mg/L metal ion solution at pH 5, the qe of Cd(II), Cu(II), Fe(II) and Zn(II) of 1.21, 1.19, 1.26 and 1.05 mg/g, respectively achieved using 10 g 1-1 BTF. Further increase in the BTF weight to 20 g 1-1increased the adsorption capacity. Table 1 indicates the change in [M2+]/biomass based on the concentration ofthe metal solution. The results obtained indicate a decrease in [M2+]/biomass. This is attributed to an increase in the adsorption of these metal ions due to the increase of BTF biomass. The increase in BTF biomass increases the adsorption surface área ofthe biomass31. The obtained results are in keeping with the conclusion by Saeed et al32.
Adsorption Isotherms
Langmuir isotherm model assumes the uniform energies of adsorption onto the surface and no transmigration of adsorbate in the plañe ofthe surface33.The linear form of Langmuir isotherm equation is given in equation (2).
where Ce is the equilibrium concentration of the adsorbate (mg/L), qe is the amount of adsorbate adsorbed per unit mass of adsorbent (mg/g), Q0 and b are Langmuir constantsorelated to adsorption capacity and rate of adsorption, respectively. When Ce/qe was plotted against Ce, a straight line with slope of 1/ Q0 was obtained. The value of Q0 was determined from the Langmuir plot at the concentration range 1 to 500 mg/L and then the b value was calculated and tabulated in Table 2. The essential characteristics ofthe Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor Rt that is given in equation (3).
The values of RL were found to be 0.385, 0.769, 0.417 and 0.935 for Cd(II), Cu(II), Zn(II) and Fe(II), respectively suggesting the isotherm to be favorable at the concentrations studied. The Freundlich isotherm model34 considers a heterogeneous adsorption surface that has unequal available sites with different energies of adsorption and can be represented by equation (4).
where Ce is the equilibrium concentration of the adsorbate (mg/L), q is the amount of adsorbate adsorbed per unit mass of adsorbent (mg/g), Kf and n are Freundlich constants. The Freundlich constants were derived from the slopes and intercepts of log qe versus log Ce and are presented in Table 2. Kt can be defined as the adsorption capacity that represents the quantity of metal ions adsorbed onto the fibers for a unit equilibrium concentration and value of n > 1 giving an indication of favorability ofthe adsorption process35. In this work, it is found that Kf increased in the order of Zn(II) (1.74 mg/g) < Cu(II) (2.68 mg/g) < Fe(II) (6.85 mg/g) < Cd(II) (8.49 mg/g) and the value of n to be 1.016, 1.036, 1.138 and 1.149 for Zn(II), Cu(II), Fe(II) and Cd(II), respectively. As seen from the Table 2, a high regression correlation coefficient, R2, was shown by the Freundlich model for all the metal ions. This indicates that the Freundlich model is very suitable for describing the sorption equilibrium of the metal ions by the BTF. When the linearity ofthe plots ofthe Freundlich and Langmuir models was compared, it is found that the former has a better fit. Thus it isoreasonable to conclude that the adsorption of the metal ions on the fibers that consist of heterogeneous adsorption sites that are very similar to each other in respect of adsorption phenomenon.
Adsorption Kinetics
Two simplified kinetic models were adopted to examine the mechanism ofthe adsorption process. First, the kinetics of adsorption was analyzed by the Langergren pseudo-first-order equation36 as depicted in equation (5).
where qe and qt are the amounts of the metals adsorbed (mg/g) at equilibrium and at time t (min), respectively, and k1 (min-1) is the rate constant adsorption. Values of k1 at ambient temperature were calculated from the plots of log (qe — qt) versus t for an concentration of 10 mg/L for the metals. The set of R2 values obtained were poor and the experimental q values did not agree with the calculated values obtained from the linear plots (Table 3). On the other hand, the pseudo-second-order equation based on equilibrium adsorption37 is expressed as equation (6):
where k (g/mg min) is the rate constant of second-order adsorption. The linear plot of tlq versus t at ambient temperature yielded set of iR2 values that are greater than 0.999 for all the metal at 10 mg/L. It also showed a good agreement between the experimental and the calculated qe values (Table 3). indicating the applicability of this model to describe the adsorption process of the metals onto the fibers.
Adsorption capacity of the modified biomass
The adsorption experiment related to the treated banana fibers were carried outat 10mg/LatpH 5 using 1.0 g/L of the fibers with an agitation speed of 150 rpm. Mercerization and acetylation leads to the leaching out of the amorphous waxy cuticle layer. Peroxide treatment is found to fibrillate the fibers due to the leaching out of the waxes, gums and pectic substances12. Carvalho et al.,38 reported that formaldehyde treatment had reduced the fiber elasticity but increased the tensile strength. The stearic acid treatment reduced the moisture content of the fiber and improved the mechanical properties of the fibers39. Acid treatment is generally used for cleaning the cell wall and replacing the natural mix of ionic species bound on the cell wall with protons and other functional groups exposing new active sites for metal removal causing increase in sorption capacity40-42. It is observed in this study (Tab.4) that all modified BTF except peroxide treated BTF showed a further slight increase in sorption capacity. The order of the sorption capacity is sulphuric acid treated BTF > mercerized BTF, formaldehyde treated BTF > acetylated BTF, stearic acid treated BTF > untreated BTF > peroxide treated BTF. The slight drop in sorption capacity of the peroxide treated BTF is attributed to delignification of the fibers12since lignin plays an important role in heavy metal adsorption43. The slight variation in the adsorption capacity of the modified BTF as compared to that of the unmodified BTF suggests the latter is by its nature already an efficient biosorbent for the removal of Cd(II), Cu(II), Fe(II) and Zn(II).
CONCLUSION
The present work establishes that banana pseudostem fibers, an agro-waste is an efficient biosorbent in the removal of Cd(II), Cu(II), Fe(II) and Zn(II) ions from aqueous solutions at pH 5. The kinetics of sorption of the four metal ions on BTF follows a pseudo-second-order pattern. Moreover, sorption capacity is strongly dependent on the metal concentration and pH of solution. Banana trunk fibers are very cheap, easily available and renewable. This study revealed that this biosorbent could be used as a tool for the development of low-cost biomaterial-for the treatment of heavy metal waste. It is also worthwhile to note that by introducing new chemical sites on the biomass may not necessary aid in the adsorption capacity of BTF.
ACKNOWLEDGEMENTS
This study is partially funded by the Ministry of Science, Technology and Innovation (MOSTI) of Malaysia. The authors are also grateful to the School of Chemical Sciences, Universiti Sains Malaysia and the Faculty of Applied Sciences, AIMST University for providing the facilities to carry out this research.
REFERENCES
1. D. Sanyahumbi, J.R. Duncan, M. Zhao M, R. Hille, Biotechnol. Lett. 20, 745,(1998). [ Links ]
2. S.E. Manahan, Environmental Chemistry, CRC Press, Boca Raton, Florida 2004. [ Links ]
3. D. Kratochvil, B. Volesky, Trends Biotechnol., 16, 291, (1998). [ Links ]
4. I. Villaescusa, N. Martínez, N. Miralles, J. Chem. Technol. Biotechnol. 75, 1, (2000). [ Links ]
5. K.S. Low, C.K. Lee, S.C. Liew, Process Biochem. 36, 59, (2000). [ Links ]
6. P.D. Johnson, M.A. Watson, J. Brown, I. A. Jefcoat, Waste Manage. 22, 471, (2002). [ Links ]
7. C.R.T. Tarley, M.A.Z. Arruda, Chemosphere 54, 987, (2004). [ Links ]
8. M. Martínez, N. Miralles, S. Hidalgo, N. Fiol, J. Hazard. Mater. B133, 203, (2006). [ Links ]
9. S. Schiewer, S.B. Patil, Bioresour. Technol. 99, 1896, (2008). [ Links ]
10. S. Senthilkumaar, S.Bharathi, D. Nithyanandhi, V. Subburam, Bioresour. Technol. 75, 163, (2000). [ Links ]
11. K. Bilba, M.A. Arsene, A. Ouensanga, Bioresour. Technol. 98, 58, (2007). [ Links ]
12. M.S. Sreekala, S.Thomas, Compos. Sci. Tech. 53, 861, (2003). [ Links ]
13. A. Paul, K. Joseph, S. Thomas, Compos. Sci. Tech. 57, 67, (1997). [ Links ]
14. S.S. Azhar, A. Ghaniey Liew, D.Suhardy, K. Farizul, M.D. Hatim, Am. J. App Sc. 2, 1499, (2005). [ Links ]
15. S.S. Banerjee, M.V. Joshi, R.V. Jayaram, Chemosphere 64, 1026, (2006). [ Links ]
16. M. Horsfall, A.I. Spiff, Electron. J. Biotechn. 7, 313, (2004). [ Links ]
17. F. Pagnanelli, A. Esposito, L. Toro, F. Veglio, Water Res. 37, 627, (2003). [ Links ]
18. E. Pehlivan, B.H. Yanik, G. Ahmetli, M. Pehlivan, Bioresour. Technol. 99, 3520, (2008). [ Links ]
19. S Schiewer, B. Volesky, Environmental Microbe-Metal Interactions (Ed: D.R Lovely), ASM Press, Washington DC, 2000. [ Links ]
20. J.M. Tobin, D.G. Cooper, R.J. Neufeld, App. Environ. Microbiol. 47, 821, (1984). [ Links ]
21. S. Schiewer, B. Volesky, Environ. Sci. Technol. 29, 3049, (1995). [ Links ]
22. Z. Reddad, C. Gerente, Y. Andrés, P. LeCloirec, Environ. Sci. Technol. 36, 2067, (2002). [ Links ]
23. Y. Sag, T. Kutsal, Process Biochem. 31, 561, (1996). [ Links ]
24. A.C.A. da Costa, S.G.F. Leite, Biotechnol. Lett. 13, 559, (1991). [ Links ]
25. E. Romera, F. Gonzales, A. Balleter, M.L. Blazquez, J.A. Muñoz, Bioresour. Technol. 98, 3344, (2007). [ Links ]
26. B. Pejic, M. Vukcevic, M. Kostic, P. Skundric, J. Hazard. Mater. 164, 146, (2009). [ Links ]
27. B. Nasernejad, T. Esslam Zadeh, B. Bonakdar Pour, M. Esmaail Bygi, A. Zamani, Process Biochem. 40, 1319, (2005). [ Links ]
28. F.E. Okieimen, D.E. Ogbeifeur, G.N. Nwala, C.A. Kumsah, Bull. Environ. Contam. Toxicol. 34, 866, (1985). [ Links ]
29. S. Ricordel, S. Taha, I. Cisse, G. Dorange, Sep. Purif. Technol. 24, 389, (2001). [ Links ]
30. B. Volesky, S. Schiewer, Encyclopedia of Bioprocess Technology: Fermentation, Biocatalysis, and Bioseparation (Eds: M.C. Flickinger, S.W. Drew), John Wiley & Sons, New York, 1999. [ Links ]
31. N. Goyal, S.C. Jain, U.C. Banerjee, Adv. Environ. Res. 7, 311, (2003). [ Links ]
32. A. Saeed, M.W. Akhter, M. Iqbal, Sep. Purif. Technol. 45, 25, (2005). [ Links ]
33. I. Langmuir, J. Am. Chem. Soc. 40, 1361, (1918). [ Links ]
34. H.M.F. Freundlich, J. Phys. Chem. 57, 385, (1906). [ Links ]
35. B.H. Hameed, H Hakimi, Biochem. Eng. J. 39, 338, (2008). [ Links ]
36. Y.S. Ho, Scientometrics 59, 171, (2004). [ Links ]
37. Y.S. Ho, G. McKay, Resour. Conserv. Recycl. 25, 171, (1999). [ Links ]
38. R.A. de Carvalho, C.R.F. Grosso, Food Hydrocolloid. 8, 717, (2004). [ Links ]
39. K. Nattaspol, K. Narumol, P. Thirawudh, P. Chantaraporn, J. Polym. Eng. 27,411,(2007). [ Links ]
40. Y. Y. Sang, J. Microbiol. Biotechnol. 14, 29, (2004). [ Links ]
41. Y.Y. Saug, D. Park, J.M. Park, B. Volesky, Environ. Sci. Technol. 35, 4535, (2001). [ Links ]
42. T.A. Davis, B. Volesky, A. Mucci, Water Res. 37, 4311, (2003). [ Links ]
43. M.N.M Ibrahim, W.S.W Ngah, M.S. Norliyana, W.R. Daud, Clean, 37, 80, (2009). [ Links ]
* Corresponding author: Mas Rosemal Hakim Mas Haris, School of Chemical Sciences, Universiti Sains Malaysia, Minden, 11800, Penang, Malaysia
e-mail address: masl@usm.my, rhmas@tm.net.my
(Received: November 25, 2009 - Accepted: May 3, 2010)
No comments:
Post a Comment