Jurnal Rekayasa Kimia dan Lingkungan Vol. 7, No. 4, hal. 149-155, 2010 ISSN 1412-5064
Sorption Kinetics for the Removal of Cadmium and Zinc onto
Palm Kernel Shell Based Activated Carbon
Muhammad, Thomas S. Y. Choong*, Chuah T. G., Robiah Yunus, Suaraya A. R. Department of Chemical and Environmental Engineering, Faculty of Engineering,
Universiti Putra Malaysia, UPM Serdang, Selangor D. E., Malaysia 43300 *E-mail: tsyc2@eng.upm.edu.my
Abstract
The kinetics and mechanism of cadmium and zinc adsorption on palm kernel shell based activated carbons (PKSAC) have been studied. A series of batch laboratory studies were conducted in order to investigate the suitability of palm kernel shell based activated carbon (PKSAC) for the removal of cadmium (cadmium ions) and zinc (zinc ions) from their aqueous solutions. All batch experiments were carried out at pH 7.0 and a constant temperature of 30+1°C using an incubator shaker that operated at 150 rpm. The kinetics investigated includes the pseudo first order, the pseudo-second order and the intraparticle diffusion models. The pseudo-second order model correlate excellently the experimental data, suggesting that chemisorption processes could be the rate-limiting step.
Keywords: adsorption, cadmium, kinetics, palm kernel shell, zinc
1. Introduction
Heavy metal contamination exists in aqueous waste streams of many industries, such as metal plating, mining, as well as agricultural fields. Zinc and cadmium are among the harmful heavy metal waste produced by the-se industries, and pothe-se a risk of conta-minated groundwater and other water re-sources. Since heavy metals are not biode-gradable and tend accumulate in living orga-nisms, causing various disease and disorders. The mechanism of sorption often involves chemical reaction between functional groups on the biosorbent and the metal ions, thus forming metal-organic complexes, or a cation exchange reaction due to the high cation-exchange capacity of the biosorbent. In addition, other mechanisms may be involved as well as mass-transport processes, such as transport in the bulk of the liquid phase, diffusion across the liquid film surrounding the solid particles and diffusion in macropores or micropores. Sorption kinetics have been extensively studied and it has been com-monly observed that the sorption rate is very rapid at the beginning of the process, then becomes slower as equilibrium is approached. Such kinetics are often described by a pseudo-second-order kinetics reaction (Taty et al., 2003; Rakhshaee et al., 2005; Horsfall et al., 2006).
Treatment processes for metals contaminated waste streams include chemical precipitation, membrane, filtration, ion exchange, carbon adsorption, and coprecipitation/adsorption. These processes are usually need an expensive facility and high maintenance cost. Adsorption using low cost sorbents such as palm kernel shell may be an economical alternative technology for the treatment of metals contaminated waste streams.
Natural materials that are available in large quantities, or certain waste products from or agricultural operations such as rice husk, strew and oil palmwastes, may have potential as inexpensive sorbents. These abundance and availability of agricultural byproducts make them good sources of raw materials for activated carbons. The possible use of palm shell as activated carbon precursor was highlighted by several investigators (Hussien et al., 1996; Daud et al., 2000) and they concluded that a good quality product can be obtained.
2. Methodology 2.1 Materials
In this study, analytical grade Cd(No3)2.4H2O
and Zn (NO3)2.6H2O used in the experiments
Stock solutions of metal ions were prepared using deionized water. All solutions were adjusted to pH 7.0 using dilute NaOH and HNO3. The palm kernel shell activated carbon
(PKSAC) was donated by K.D. Technology Sdn. Bhd., in granular form with particle sizes of 0.25 – 0.60 mm.
2.2 Procedure
Batch sorption studies were performed at different concentration (20, 30, and 40 mg/L) to obtain the equilibrium kinetics. The metals chosen for the investigation studies were cadmium (II), and zinc (II). A series of 1000 ml conical flask were employed. Each conical flask was filled with 500 ml of metal ion solution of varying concentrations and adjusted to pH 7.0. The conical flask with heavy metal ions solution was then shaken using an orbital incubator shaker, which operated at 150 rpm and 30+10C. When the
desired temperature was reached, a known amount of adsorbent was added into each conical flask and mechanical shaking agitated the solutions. Generally equilibrium was reached after six hours (from contact time studies). After this period the solution was filtered using Whatman Cat. No. 1002 110 filter paper and analyzed for the concen- tration of the metal ions remaining in the solution using AAS (HITACHI, Ltd. Z-8100, Japan.) at the maximum wavelength (λ) of 228.8 nm for cadmium and 213.9 nm for zinc (Martin et al., 2004), respectively. The metal ions concentration at equilibrium, qe (mg/g),
can be calculated from (Lao et al., 2005):
m
C
C
V
q
e
o
e (1)where C0 (mg/L) is the initial metal ions
concentration in liquid phase, Ce (mg/L) is
metal ions concentration in liquid phase at equilibrium, V (L) is total volume of metal ions solution and m (g) is mass of adsorbent 3. Theory
3.1 Pseudo First-Order
The Lagergren pseudo first-order rate equation is the simplest rate equation for the sorption in liquid/solid system based on solid capacity (Ho and McKay, 1998a):
e t
tq
q
k
dt
dq
1 (2) where k is the equilibrium rate constant of pseudo first-order sorption (1/min),q
e isthe amount of cadmium or zinc ions sorbed
at equilibrium (mg/g) and qt is the amount of
cadmium or zinc ions sorbed on the surface of the activated carbon at any time t (mg/g). Integrating Eq. (2) for the boundary conditions
t
0
to t andq
t
0
toq
t, yields:
q
eq
t
q
ek
t
303
.
2
)
log(
log
1 (3) If the pseudo first order kinetics are applicable to the system, then the plot of log(qe-qt) versust
of equation (3) will give alinear relationship, with k1/2.303 and log(qe)
as slope and intercept, respectively. 3.2 Pseudo Second-Order
The pseudo second-order kinetics rate equations can be written as follows (Ho and Ofomaja, 2005):
2 2 e t tq
q
k
dt
dq
(4) where k2 is the equilibrium rate constant ofpseudo second-order sorption (g/mg min), e
q
is the amount of cadmium or zinc ions adsorbed at equilibrium (mg/g) and qt is theamount of cadmium or zinc ions adsorbed on the surface of the activated carbon at any time t (mg/g). Separating the variables in Eq. (4) gives:
q
q
k
dt
dq
t e t 2 2
(5)Integrating equation (5) for the boundary conditions
t
0
to t andq
t
0
toq
t , gives:
k
t
q
q
q
e t e 21
1
(6)A linear form of Eq. (6) can be obtained:
t q q k q t e e t 1 1 2 2 (7) If the pseudo second-order kinetics are applicable to the system, then the plot of
t
q
tversus t of equation (7) will give a linear
relationship, with
1
q
e and 1k2qe2 as a slopeand intercept, respectively. The values of
q
eand
k
2 are determined from the slope andintercept. The pseudo second-order kinetics model has been successfully applied to several biosorp- tion systems as reported by Ho and McKay (1999a).
Muhammad et al. / Jurnal Rekayasa Kimia dan Lingkungan Vol. 7 No. 4 151
3.3 Intraparticle Diffusion
For a solid–liquid sorption process, the mass transfer mechanism is usually characterized by external mass transfer, or intraparticle diffusion, or both. The three steps that describe the mechanisms of adsorption are: (i) mass transfer of solute from the bulk of the solution to the adsorbent’s particle surface (film diffusion), (ii) uptake of the adsorbate at the binding sites (solid diffusion), (iii) intraparticle diffusion of solute via either pore diffusion or solid diffusion (pore diffusion). A comprehensive three resistance model has recently been developed by Choong et al. (2006). This model requires rather involved computer programming and therefore will not be used in this work. The most widely used intraparticle diffusion equation is given by Weber and Morris (1963):
5 . 0 t k qt id (8)
where kid is the intra-particle diffusion rate
constant (mg/g min0.5). The k
id values were
calculated from the slope of the straight line plot qt versus t0.5. Equation (8) has been
used by a number of researchers such as Ho and McKay (1999), Atun and Hisarli (2003); Koumanova et al. (2003), and Mall et al. (2006).
4. Results and Discussion
The experiments were carried out at three different initial concentrations i.e 20, 30 and 40 mg/L, at two different dosages of adsorbent, 2.0 and 3.0 g/L.
The relationship between contact time and heavy metal uptake by PKSAC was studied through batch kinetics experiments and the results were shown in Figures 1 and 2. Figure 1(a) showed that cadmium equilibrium was reached after 90 minutes, 100 minutes and 120 minutes for initial heavy metal concentrations of 20, 30 and 40 mg/L, respectively. Figure 1(b) showed that for zinc, the equilibrium time required for initial zinc concentrations of 20 mg/L, 30 mg/L and 40 mg/L are 90 minutes, 120 minutes and 180 minutes, respectively. For all cases under investigation, the metal uptake profile was a single, smooth, and continuous curve leading to saturation, suggesting the possible monolayer coverage of heavy metal on the surface of the
adsorbent. As shown in Figures 1 and 2, the amount of cadmium and zinc ions adsorbed at equili- brium, qe, increased with
increasing initial concentration. A longer time was needed to reach equilibrium for higher initial heavy me- tal concentrations. The adsorption process could be divided into three regimes. The first regime was fast as a result of the rapid attachment of heavy metal particles to the surface of the adsorbent. The second regime was slower possibly because adsorption took place in the pores (intraparticle diffusion) and in the third regime the adsorption process decreased, indicating the attainment of equi- brium. As initial heavy metal concentrations increased, a longer time was needed to reach equilibrium and the adsorption capacity was also found to be increased. 4.1 Pseudo First-Order
The Lagergren pseudo first-order kinetics model is the simplest rate equation proposed for the adsorption of liquid/adsorbent system based on adsorbent capacity. The linearised form of the pseudo first-order kinetics model is given in equation (3). The plots of log (qe -
qt) versus t are presented in Figures 3 and 4
to test the applicability of the experimental data to the pseudo first-order kinetics model. For plotting Figures 3 and 4, qe in log (qe - qt)
needed to be obtained from equilibrium experiments.
The pseudo first order rate constant, k1 and qe (qe,calc) were obtained from the slope and
intercept of the plot and the results are tabulated in Tables 1 and 2. For the applicability of the pseudo first-order kinetics model, the correlation coefficient R2 need to
be sufficiently high and qe,exp must be
sufficiently close to qe,calc. In all cases, it was
found that this model was only applicable over the initial stage of the adsorption processes and did not fit well to the whole range of contact time. This was because the sorption of cadmium and zinc ions was not the only ion-exchange mechanism but other sorption mechanisms were also occurring. In most cases from the literature, the pseudo-first order equation does not fit well over the range of contact times (Choy et al., 2004). This pattern was also observed by Tutem et al. (2000) for adsorption of chlorophenols from water by bituminous shale and also by Ho and McKay (1998b) for pollutant removal on various sorbents.
Muhammad et al./Jurnal Rekayasa Kimia dan Lingkungan vol.6 (2) 152
Table 1. Rate constants (k1) and R2 for adsorption of cadmium and zinc onto PKSAC for different initial concentrations by using the pseudo first-order kinetics model: adsorbent dosage = 2.0 g/L. Initial concentration mg/L Cadmium Zinc k1
min-1 mg/g qe exp mg/g qe calc R2 mink1 -1 mg/g qe exp mg/g qe calc R2 20 0.0076 6.03 1.33 0.795 0.0090 2.97 1.75 0.921 30 0.0081 7.05 1.67 0.761 0.0095 5.40 2.96 0.984 40 0.0090 9.07 2.28 0.952 0.0097 7.73 3.38 0.891
Table 2. Rate constants (k1) and R2 for adsorption of cadmium and zinc onto PKSAC for different initial concentrations by using the pseudo first-order kinetics model: adsorbent dosage = 3.0g/L. Initial concentration mg/L Cadmium Zinc k1
min-1 mg/g qe exp mg/g qe calc R2 mink1 -1 mg/g qe exp mg/g qe calc R2 20 0.0064 4.07 1.18 0.977 0.0060 2.00 1.07 0.964 30 0.0092 5.43 1.74 0.925 0.0085 3.87 2.18 0.951 40 0.0094 7.42 2.14 0.967 0.0108 6.62 3.90 0.914
Figure 1. Contact time study of the removal of (a) cadmium and (b) zinc from aqueous solutions using
PKSAC: Effect of initial heavy metal concentrations at 2.0 g/L dosage of PKSAC.
Figure 2. Contact time study of the removal of (a) cadmium and (b) zinc from aqueous solutions using
PKSAC: Effect of initial heavy metal concentrations at 3.0 g/L dosage of PKSAC. 0 2 4 6 8 10 0 50 100 150 200 250 300 350 400 450 q t (m g /g ) t (min) (a) 20 mg/L 30 mg/L 40 mg/L (b) 0 2 4 6 8 10 0 50 100 150 200 250 300 350 400 450 t (min) qt ( m g/ g) 20 mg/L 30 mg/L 40 mg/L 0 2 4 6 8 10 0 50 100 150 200 250 300 350 400 450 q t (m g /g ) t (min) (a) 20 mg/L 30 mg/L 40 mg/L (b) 0 2 4 6 8 10 0 50 100 150 200 250 300 350 400 450 t (min) qt (m g/ g) 20 mg/L 30 mg/L 40 mg/L
The values of correlation coefficient R2 were
considerably high. However, as shown in Tables 1 and 2, the values of qe,exp and qe,calc
differ a lot. Therefore, it can be concluded that the pseudo first-order kinetics model do not describe well the adsorption kinetics of cadmium and zinc onto palm kernel shell based activated carbon (PKSAC).
4.2 Pseudo Second-Order
As shown in equation (7), the pseudo second-order rate constant k2 and qe (qe,calc) can be
obtained from the slope of the plot of t/qt
versus t as shown in Figures 5 and 6, and the parameters of pseudo second-order were as listed in Tables 3 and 4. For the applicability of the pseudo second-order kinetics model, the correlation coefficient R2 need to be
sufficiently high and qe,exp must be sufficiently
close to qe,calc.
The adsorption kinetics of both cadmium and zinc were found to obey the pseudo second-order kinetics, with high values of the correlation coefficient, R2 (>0.99) and the
values of qe,exp and qe,calc are sufficiently
close. This indicated that the experimental data can be explained using this model. The rate of cadmium and zinc adsorption process appears to be controlled by chemisorption. The same phenomenon was reported by Jia and Aik (3003) on adsorption of sulphur dioxide onto activated carbon prepared from oil-palm shells and by Jia et al. (2005) on adsorption of NH3 onto activated carbon
prepared from palm shells.
Table 3. Rate constants (k2) for the adsorption of cadmium and zinc onto palm kernel shell based activated carbon by using the pseudo second-order kinetics model: adsorbent dosage = 2.0g/L. Initial concentration mg/L Cadmium Zinc k2
g mg-1min-1 mg/g qe exp mg/g qe calc R2 g mgk-12 min-1 mg/g qe exp mg/g qe calc R2 20 0.024 6.03 6.09 0.999 0.014 2.97 3.04 0.992 30 0.020 7.05 7.07 0.999 0.009 5.40 5.61 0.998 40 0.014 9.07 9.12 0.999 0.008 7.73 7.89 0.999
Table 4. Rate constants (k2) for the adsorption of cadmium and zinc onto palm kernel shell based activated carbon by using the pseudo second-order kinetics model: adsorbent dosage = 3.0g/L. Initial concentration mg/L Cadmium Zinc k2 g mg-1min -1 qe exp mg/g mg/g qe calc R2 k2 g mg-1min -1 qe exp mg/g mg/g qe calc R2 20 0.024 4.07 4.06 0.998 0.020 2.00 1.99 0.991 30 0.018 5.43 5.48 0.998 0.010 3.87 4.01 0.998 40 0.015 7.42 7.49 0.999 0.007 6.62 6.88 0.998
Figure 3. Pseudo first-order kinetics model for the removal of (a) cadmium and (b) zinc from aqueous
solutions using PKSAC: Effect of initial heavy metal concentrations at 2.0 g/L dosage of PKSAC. -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 0 50 100 150 200 250 300 350 400 lo g ( q e-q t) t (min) (b) 20 mg/L 30 mg/L 40 mg/L -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 0 50 100 150 200 250 300 350 400 lo g (q e-q t) t (min) (a ) 20 mg/L 30 mg/L 40 mg/L
Muhammad et al. / Jurnal Rekayasa Kimia dan Lingkungan Vol. 7 No. 4 154
Figure 4. Pseudo first-order kinetics model for the removal of (a) cadmium and (b) zinc from aqueous
solutions using PKSAC: Effect of initial heavy metal concentrations at 3.0 g/L dosage of PKSAC.
In summary, a conclusion could be made that the most suitable model to describe the kinetics of experiment was pseudo second order kinetics model with correlation coefficient, R2 > 0.99. The rate-limiting
process is chemisorption.
To further show that the process is not intraparticle diffusion controlled, empirical correlations of the pseudo second-order rate constant, k2, as a function of the initial
concentration can be expressed as: 719 . 0 0 2 0.21C k (9)
(for cadmium with adsorbent dosage 2.0 g/L) 631 . 0 0 2 0.16C k (10)
(for cadmium with - adsorbent dosage 3.0 g/L) 762 . 0 0 2 0.13C k (11)
(for zinc with adsorbent dosage 2.0 g/L) 547 . 0 0 2 2.02C k (12)
(for zinc with adsorbent dosage 3.0 g/L)
If the power of C0 is around the value 0.5,
then the intraparticle diffusion is a prominent factor in the adsorption process, all the values of power in equations (9 to 12) are negative and are different from 0.5, this confirms that the adsorption is not intraparticle diffusion controlled.
5. Conclusion
The analysis of contact time experiments suggests that the pseudo-second order kinetic model described well the dynamic behaviour for adsorption of both metal ions under different initial metal ions concen-
trations. This suggests that the chemisorption mechanism is the rate-limiting step in the adsorption uptake.
Nomenclature
Ce metal ions concentration in liquid phase at
equilibrium (mg/L)
C0 initial metal ions concentration in liquid phase (mg/L)
k1 rate constant of pseudo first-order sorption (L/min)
k2 rate constant of pseudo second-order sorption (g/mg min)
kid intra-particle diffusion rate constant (mg/g min0.5)
m mass of adsorbent (g)
qe amount of metal ions adsorbed by activated carbon at equilibrium (mg/g). qt amount of metal ions sorbed on the
surface of the activated carbon at any time (mg/g).
t time (min)
V total volume of metal ions solution (L) Acknowledgement
The authors would like to thank the Government of Malaysia for financial support of this research under Graduate Research fellowship (GRF).
References
Atun, G., Hisarli, G. (2003) Adsorption of carminic acid a dye onto glass powder. Chemical Engineering Journal, 95, 241-249.
Choong, T. S. Y., Wong, T. N., Chuah, T. G., Idris, A. (2006) Film-pore-concentration--2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 0 50 100 150 200 250 300 350 400 lo g (q e-q t) t (min) (a) 20 mg/L 30 mg/L 40 mg/L -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 0 50 100 150 200 250 300 350 400 lo g (qe -qt ) t (min) (b) 20 mg/L 30 mg/L 40 mg/L
Muhammad et al. / Jurnal Rekayasa Kimia dan Lingkungan Vol. 7 No. 4 155
dependent surface diffusion model for the adsorption of dye onto palm kernel shell activated carbon. Journal of Colloid Interface Science, 89, 301-309.
Choy, K. K. H., Porter, J. F., Mckay, G. (2004) Intraparticle diffusion in single and multicomponent acid dye adsorption from wastewater onto carbon, Chemical Engineering Journal, 103, 133–145. Chuah, T. G., Jumasiah, A., Azni, I., Katayon,
S., Choong, T. S. Y. (2005) Rice husk as a potentially low-cost biosorbent for heavy metal and dye removal: an overview. Desalination, 175, 305-316.
Daud, W. M. A. W., Ali, W. S. W., Sulaiman, M. Z. (2000) The effects of carbonization temperature on pore development in palm shell based activated carbon. Carbon, 38, 1925-1932.
Ho, Y. S., Ofomaja, A. E. (2005) Kinetics and thermodynamics of lead ion sorption on palm kernel fibre from aqueous solution, Process Biochemistry, 40, 3455–3461. Ho, Y. S., McKay, G. (1998a) A comparison of
chemisorption kinetic models applied to pollutant removal on various sorbents.
Transaction Institute Chemical
Engineering, 76B, 1-10.
Ho, Y. S., McKay, G. (1998b) Kinetic model for lead (II) sorption onto peat, Adsorption Science Technology, 16, 243-255.
Ho, Y. S., McKay, G. (1999) A kinetic study of sorption by biosorbent waste product pith. Resource Conservation Recycling, 25, 171-193.
Ho, Y. S., Ng, J. Y. C., McKay, G. (2000) Kinetics of pollutant sorption by biosorbents: review. Separation and Purification Methods. 29, 189-232.
Horsfall, M. Jr., Abia, A. A., Spiff, A. I. (2006) Kinetic studies on the adsorption of Cd2+,
Cu2+ and Zn2+ ions from aqueous solutions
by cassava (Manihot sculenta Cranz) tuber bark waste, Bioresource Technology, 97, 283–291.
Hussien, M. Z., Tarmizi, R.S.H., Zainal, Z Ibrahim, R. (1996) Preparation and characterization of active carbons from oil palm shells, Carbon, 34, 1447-1454. Jia, G., Aik, C. L. (2003) Adsorption of
sulphur dioxide onto activated carbon prepared from oil-palm shells with and
without pre-impregnation. Separation and Purification Technolology, 30, 65-273. Jia, G., Xu, W. S., Chen, Y. L., Aik, C. L.
(2005) Adsorption of NH3 onto activated
carbon prepared from palm shells im-pregnated with H2SO4.Journal of Colloidal Interface and Science, 281, 285–290. Koumanova, B., Peeva, P., Allen, S. J. (2003)
Variation of intraparticle diffusion parameter during adsorption of p-chlo-rophenol onto activated carbon made from apricot stones. Journal of Chemical Technology and biotechnology, 78, 582-587.
Lao C., Zeledón, Z., Gamisans, X., Solé, M. (2005) Sorption of Cd(II) and Pb(II) from aqueous solutions by a low-rank coal (Leonardite), Separation and Purification Technology, 45, 79–85.
Mall, I. D., Srivastava, V. C., Agarwal, N. K. (2006) Removal of Orange-G and Methyl Violet dyes by adsorption onto bagasse fly ash kinetic study and equilibrium isotherm analyses, Dyes and Pigments, 69, 210-223.
Martins, R. J. E., Pardob, R., Boaventura, R. A. R. (2004) Cadmium (II) and zinc (II) adsorption by the aquatic moss Fontinalis antipyretica: effect of temperature, pH and water hardness, Water Research, 38, 693–699.
Rakhshaee, R., Khosravi, M. Masoud, T. G.
(2005) Kinetic modeling and
thermodynamic study to remove Pb(II), Cd(II), Ni(II) and Zn(II) from aqueous solution using dead and living Azolla filiculoides, Journal of Hazardous Materials, 134,120-129.
Taty-Costodes, V. C., Henri, F., Catherine, P. Alain, D. (2003) Removal of Cd(II) and Pb(II) ions, from aqueous solutions, by adsorption onto sawdust of Pinus sylvestris, Journal of Hazardous Materials, B105,121–142
Tutem, E., Apak, R, Unal, C. F. (1998) Adsorptive removal of chlorophenols from water by bituminous shale. Water Research, 32,2315-2324.
Weber, W. J., Morris, J. C. (1963) Kinetic of adsorption on carbon from solution. Journal of Sanitation Engineering Division American Society Civil Engineering 89, 31-60.
