JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
187, 338–343 (1997)
CS964537
Removal of Pb(II), Cd(II), Cu(II), and Zn(II) from Aqueous Solutions by Adsorption on Bentonite GO¨ZEN BEREKET,* AYS¨ E ZEHRA AROGˇUZ,† ,1
AND
¨ ZEL * MUSTAFA ZAFER O
*Faculty of Art and Sciences, Chemistry Department, University of Osmangazi, Eskis¨ ehir, Turkey; and †Faculty of Engineering, Department of Chemistry, University of Ig stanbul, 34850 Avcilar, Ig stanbul, Turkey Received May 10, 1996; accepted July 30, 1996
Removal of Pb(II), Cd(II), Zn(II), and Cu(II) from aqueous solutions using the adsorption process on bentonite has been investigated. In order to find out the effect of temperature on adsorption, the experiments were conducted at 20, 35, and 507C. For all the metals, maximum adsorption was observed at 207C. The rate of attaining equilibrium of adsorption of metal ions follows the order Zn(II) ú Cu(II) ú Cd(II) ú Pb(II). Equilibrium modeling of the adsorption showed that adsorption of Pb(II), Cd(II), and Cu(II) were fitted to a Langmuir isotherm, while the adsorption of Zn(II) was fitted to a Freundlich isotherm. Dynamic modeling of the adsorption showed that the first order reversible kinetic model was held for the adsorption process. The overall rate constant k*, the adsorption rate constant k1 , the desorption rate constant k2 , and the equilibrium constant Ke for the adsorption process were calculated. From the results of the thermodynamic analysis, standard free energy DG 0 , standard enthalpy DH 0 , and standard entropy DS 0 of the adsorption process were calculated. q 1997 Academic Press.
INTRODUCTION
The presence of metals in aquatic environments has been known to cause several health problems to animals and human beings (1). The heavy metal levels in waste water, drinking water, and water used for agriculture must be reduced to the maximum permissible concentration. Precipitation, ion exchange, solvent extraction, and adsorption on activated carbon are the conventional methods for the removal of heavy metal ions from aqueous solutions (2–5), but due to high maintenance cost these methods do not suit the needs of developing countries (6). The adsorption process is used especially in the water treatment field and the investigation has been made to determine inexpensive and good adsorbents. For this purpose bentonite is used as the adsorbent in the present investiga1
tion. It is an inexpensive clay mineral readily available in Turkey. It has been extensively used in drilling processes, but few scientists have used clay minerals in water and waste water pollution control (7–9). In the present study, removal of Pb(II), Cd(II), Cu(II), and Zn(II) from aqueous solution by adsorption was investigated. Adsorption isotherms and thermodynamic parameters of the adsorption are also presented. EXPERIMENTAL
Bentonite, used as an adsorbent in this study, was obtained from MTA, Ankara, Turkey. It was dried at 1107C for 2 h and used as received after sieving through a 200 mm sieve. The chemical composition of bentonite was done according to the methods described in literature (10). The results of the experiments are given in Table 1. Stock solutions of Pb(II), Cd(II), Cu(II), and Zn(II) were prepared by dissolving Pb(NO3 )2 , Cd(NO3 )2 , Cu(NO3 )2r3H2O, and Zn(NO3 )2r6H2O in distilled, deionized water and pH values of the solutions were adjusted to pH 5 by addition of 0.1 N HCl or 0.1 N NaOH. Batch adsorption experiments were conducted using 0.5 g of adsorbent with 50 ml of solutions containing heavy metal ions of desired concentration at different temperatures in 250 ml glasses which were immersed in a temperature controlled water bath. After the desired contact time, suspensions were filtered TABLE 1 The Chemical Composition of Bentonite Constituents
Percentage by weight
Si O2 Al2 O3 Fe2 O3 MgO CaO
56.28 26.79 4.00 3.67 1.34
To whom correspondence should be addressed.
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0021-9797/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved.
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FIG. 1. atures.
Percentage uptake of Pb ( II ) on bentonite at different temper-
339
FIG. 3. Percentage uptake of Cu ( II ) on bentonite at different temperatures.
Effect of contact time and temperature on the adsorption of the Pb(II), Cd(II), Cu(II), and Zn(II) ions on bentonite are illustrated in Figs. 1–4, respectively. Removal of these cations increased with time and then became constant. Higher removals for all heavy metal cations studied was
observed in the lower temperature range. Furthermore, the maximum removals (82.2%) for Pb(II), (71.1%) for Cd(II), (55.5%) for Cu(II), and (33.9%) for Zn(II) were observed at 207C. A decrease in uptake of heavy metal cations with the rise in temperature was observed. This was due to the increasing tendency of adsorbate ions to desorb from the interface to the solution with increasing temperature. Kinetic studies. Adsorption of heavy metal cations from the liquid phase to the solid phase can be considered as a reversible reaction with an equilibrium being established between the two phases. Therefore, a simple first order ki-
FIG. 2. Percentage uptake of Cd ( II ) on bentonite at different temperatures.
FIG. 4. Percentage uptake of Zn ( II ) on bentonite at different temperatures.
through Whatman No. 42 filter paper and solutions were analyzed for heavy metal ions by the atomic adsorption method (Hitachi 180-70 AA Spectrometer). RESULTS AND DISCUSSION
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FIG. 5. Typical first order reversible kinetic fit of Pb(II) adsorption data on bentonite.
FIG. 7. Typical first order reversible kinetic fit of Cu(II) adsorption data on bentonite.
netic model is used to establish the rate of reaction (11). The first order kinetic equation is
U(t) Å
ln[1 0 U(t)] Å 0k *t
C0 0 Ct C0 0 Ce
where Kc is the equilibrium constant and k1 and k2 are the first order forward and reverse rate constants, respectively.
C0 , Ct , and Ce (all in mg liter 01 ) are concentration of the metal cation in solution initially, at any time t, and at equilibrium, respectively. The typical plots of ln((C0 0 Ct )/(C0 0 Ce )) vs time are given for Pb(II), Cd(II), Cu(II), and Zn(II) in Figs. 5–8, respectively. A near straight line was generally observed for all concentrations indicating that the adsorption reaction can be approximated to first order reversible kinetics.
FIG. 6. Typical first order reversible kinetic fit of Cd(II) adsorption data on bentonite.
FIG. 8. Typical first order reversible kinetic fit of Zn(II) adsorption data on bentonite.
in which k * is the overall rate constant. Further,
S
k * Å k1 1 /
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FIG. 9. Langmuir adsorption isotherm of Pb(II) at 207C (r Å 0.998).
Adsorption isotherm. Typical adsorption isotherms for Pb(II), Cd(II), Cu(II), and Zn(II) are presented in Figs. 9–12, respectively. As seen from the figures, the experimental data of the adsorption of Pb(II), Cd(II), and Cu(II) fit much better to the Langmiur isotherm. In bentonite these metals seem to reach saturation which means that the metal had filled possible available sites and further adsorption could take place only at new surfaces.
FIG. 11. 0.920 ) .
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Langmuir adsorption isotherm of Cu ( II ) at 207 C ( r Å
x/m is the amount of metal ion adsorbed per unit weight of adsorbent, a and b are the Langmiur constants, and C is the equilibrium concentration of Pb(II), Cd(II), and Cu(II). On the other hand, adsorption data of Zn(II) fit well to the Freundlich isotherm. The shape of the isotherm strongly suggests that a description of the adsorption by the Freundlich equation is in the form
C 1 b Å / C x/m a a
log
x Å log k / n log C, m
FIG. 10. Langmuir adsorption isotherm of Cd(II) at 207C (r Å 0.991).
FIG. 12. Freundlich adsorption isotherm of Zn(II) at 207C (r Å 0.961).
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TABLE 2 Langmuir Constants of Pb(II), Cd(II), and Cu(II) Ions Temperature (7C)
a (g01)
b (g01rl)
20 35 50
16.66 15.38 12.50
10.80 10.10 8.75
20 35 50
11.10 7.40 4.65
6.77 5.00 2.25
Cd(II)
20 35 50
2.32 1.81 1.47
1.18 1.12 1.01
Cu(II)
Cations
DG 0 Å 0RT ln K
DS 0 Å
S
T 1rT 2 k2 ln T 1 0 T 2 k1 DH 0 0 DG 0 T
D
.
Here, R is the gas constant and K, k1 , and k2 are equilibrium constants at the temperature T, T 1 , and T 2 , respectively. TABLE 3 Freundlich Constants of the Zn(II) Ion
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k (g01)
n
20 35 50
0.46 0.33 0.22
0.72 0.82 1.22
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Temperature (7C)
CBe (grl01)
CAe (grl01)
Kc
k * 1 103 (min01)
k1 1 103 (min01)
k2 1 103 (min01)
20 35 50
0.822 0.749 0.715
0.178 0.251 0.285
4.62 2.98 2.50
11.7 10.4 9.97
9.62 7.78 7.12
2.08 2.42 2.85
Pb(II)
20 35 50
0.711 0.576 0.556
0.289 0.424 0.444
2.46 1.36 1.25
11.71 8.59 8.24
8.36 4.95 4.57
3.35 3.64 3.67
Cd(II)
20 35 50
0.555 0.476 0.463
0.445 0.524 0.537
1.25 0.90 0.86
11.65 10.10 9.97
6.47 4.78 4.60
5.18 5.32 5.37
Cu(II)
20 35 50
0.339 0.302 0.261
0.661 0.698 0.739
0.51 0.43 0.35
10.20 10.00 9.88
3.44 3.00 2.56
6.76 7.00 7.32
Zn(II)
Cations
Pb(II)
where x / m is the amount of metal ion adsorbed per unit weight, k and n are the Freundlich constants, and C is the equilibrium concentration of Zn ( II ) . From the adsorption isotherm, the Langmuir constants for Pb ( II ) , Cd ( II ) , and Cu ( II ) and the Freundlich constants for Zn ( II ) and correlation coefficients were calculated. These correlation coefficients related with the equations are higher for both adsorption equations which reflect that these models fit. The results and the correlation coefficients are given in Tables 2 and 3 and in Figs. 9 – 12, respectively. Thermodynamic studies. In order to explain the effect of temperature on the adsorption thermodynamic parameters, standard free energy DG 0 , standard enthalpy DH 0 , and standard entropy DS 0 were determined. To calculate the values of the parameters the following equations were used:
DH 0 Å R
TABLE 4 Kinetic Parameters of the Adsorption of Pb(II), Cd(II), Cu(II), and Zn(II) Ions on Bentonite
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Numerical values of the equilibrium constants were calculated from KÅ
CBe , CAe
where CBe and CAe are the equilibrium concentrations of heavy metal cations on the sorbent and solute, respectively. Kinetic parameters of the adsorption are given in Table 4. It can be observed that k1 (sorption of solute from solution onto the sorbent) decreases as the temperature increases, while k2 (desorption of solute from sorbent) increases with increasing temperature. Calculated thermodynamic parameters for the adsorption of Pb(II), Cd(II), Cu(II), and Zn(II) on bentonite at different temperatures are given in Table 5. The negative values of these parameters are indicative of the spontaneous nature of the process. Entropy has been defined as the degree of chaos of the system and the negative value of this parameter found in our investigation reflects the adsorption of Pb(II), Cd(II), Cu(II), and Zn(II). During the adsorption process, the adsorbate ions become associated on the surface of the absorbent resulting in the loss of a degree of freedom and thus explaining the decrease in the value of this parameter (12). The negative values of DH 0 show the exothermic nature of Pb, Cd, Cu, and Zn adsorption. The results obtained in these experiments generally agree with the results previously found on the adsorption of heavy metals by pure clays (13). Compared with the treated clay, bentonite is a good adsorbent for Zn(II), Cd(II), Pb(II), and Cu(II) without the need for any chemical treatment (14). Good correlation can be observed with the calculated thermodynamic parameters and in the decreased percentage adsorption with increasing temperature.
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TABLE 5 The Thermodynamic Parameters for the Adsorption of Pb(II), Cd(II), Cu(II), and Zn(II) on Bentonite Temperature (7C)
Adsorption %
Kc
DG 7 (J. mol01)
DH 7 (J. mol01)
DS 7 (J. mol01)
20 35 50
82.2 74.9 71.5
4.62 2.98 2.50
03728 02796 02460
021932 09685 —
062.1 022.3 —
Pb(II)
20 35 50
71.1 57.6 55.6
2.46 1.36 1.25
02193 0787 0599
029645 04651 —
093.7 012.5 —
Cd(II)
20 35 50
55.5 47.6 46.3
1.25 0.90 0.86
0539 270 405
016311 02507 —
053.8 09.0 —
Cu(II)
20 35 50
33.9 30.2 26.1
0.51 0.43 0.35
1640 2161 2819
08535 011351 —
034.7 043.8 —
Zn(II)
REFERENCES
1. Irwing Sax, N., ‘‘Handbook of dangerous Materials,’’ pp. 218–222, 236. Reinhold, New York, 1951. 2. Gonzales-Davila, M., Santana-Casino, J. M., and Millero, F. J., J. Colloid Sci. 137(1) (1990). 3. Huang, C. D., and Blankenship, D. W., Water Res. 18, 37–46 (1984). 4. Naylor, L. M., and Daugue, R. R., J. Am. Water Works Assoc. 67, 560– 564 (1975). 5. Kahashi, Y. Y., and Imai, H., Soil Sci. Plant Nutr. 29(2), 111–122 (1983). 6. Battacharya, A. K., and Venkobachor, J. Environ. Eng. 110, 110 (1984).
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Cations
7. Arogˇuz, A. Z., and Arinel, N., VIII, Chem. Chem. Eng. Symp. 3, 207 (1992). 8. Sunderson, B. B., Bulusu, K. R., Kulkarni, D. N., and Pathack, B. N., Indian J. Environ. Health 20/21, 413 (1978). 9. Panday, K. K., Prasad, G., and Singh, V. N., Water, Air Soil Pollution 27, 287 (1986). 10. Solenay, E. ‘‘Seramik Bu¨nyeyi olus¨ turan hammadddelerin kimlik kartl¨ niversitesi,’’ Eskig s¨ ehir, 1991. arinin belirlenmesi Anadolu U 11. Lagergren, S. Big l, Svenska Watenskasal, Handl. 24, 111 (1898). 12. Panday, K. K., Prasad, G., and Singh, V. N., J. Chem. Tech. Biotechnol. A 34, 367 (1987). 13. Petruzzelli, G., Guidi, G., and Lubrano, L., Commun. In Soil Sci. Plant Anal. 16(9), 971–986 ( 1985). 14. Nld Filho, Gushikem, Y., and Polita, W. L., Anal. Chim. Acta 306(1), 167–172 (1995).
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