Kinetics of Isothermal Nicotine Adsorption from the Aqueous Solution

Jun 21, 2010 - Military Technical Institute, Ratka Resanovica 1, 11132 Belgrade, Serbia; Faculty of Physical Chemistry, University of Belgrade, Studen...
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Ind. Eng. Chem. Res. 2010, 49, 6302–6308

Kinetics of Isothermal Nicotine Adsorption from the Aqueous Solution onto the USY Zeolite Natasa Lazarevic,† Jelena Jovanovic,‡ Milutin Jevremovic,† Miroslava Nikolic,§ and Borivoj Adnadjevic*,‡ Military Technical Institute, Ratka ResanoVica 1, 11132 Belgrade, Serbia; Faculty of Physical Chemistry, UniVersity of Belgrade, Studentski trg 12-16, 11001 Belgrade, Serbia; and Faculty of Agriculture, UniVersity of Belgrade, Nemanjina 6, 11080 Belgrade-Zemun, Serbia

The kinetics of isothermal adsorption of nicotine from an aqueous solution onto hydrophobic zeolite type USY was investigated. The adsorption isotherms of nicotine on zeolite type USY were measured in the temperature range from 298 to 322 K. Specific shape parameters of the adsorption degree curves were determined. The changes in the specific shape parameters of the adsorption degree curves with temperature and the kinetic parameters of nicotine adsorption (Ea, ln A) were determined. The kinetic model of nicotine adsorption, [1 - (1 - R)1/2] ) kt, was established by using the model-fitting method. It was found that the nicotine adsorption was a kinetically controlled process which was determined by the rate of bidimensional movement of the boundary layer of the adsorption phase. Nicotine is most probably chemisorbed onto the acidic centers of zeolite type USY. 1. Introduction Large quantities of tobacco waste from the production of cigarettes and significant solubility of nicotine in water lead to the increase in nicotine concentration both in the outer surface and in the underground water flows.1 Nicotine in small quantities acts as a stimulant and causes adrenaline secretion, but in larger quantities it becomes a highly toxic substance which is harmful for human health because it acts as a nerve toxin that causes general paralysis. Numerous methods of elimination of nicotine from the aqueous solution have been developed: catalytic oxidation,2 microbiological degradation,3 and adsorption on solid adsorbents. A. Lukas et al.4 investigated nicotine adsorption from an aqueous solution to the H+- form strong-acid ion exchanger. The isothermal adsorption of nicotine from the aqueous solution for different types of hydrophilic zeolite (NaA, NaX, NaY), hydrophobic zeolite (HZSM-5; silicate), and amorphous materials was investigated by Adnadjevic´ et al.5 The presented results showed that the obtained adsorption isotherms can be described by Freundlich’s model and the highest specific adsorption capacity was found for precipitated amorphous SiO2. The effects of the crystallinity degree, specific surface area, and pore volume in different cellulose powders on the nicotine adsorption were examined by Mihranyama et al.6 Stosˇic´ et al. investigated the adsorption of nicotine from an aqueous solution onto different types of zeolite: clinoptilolite, HZSM-5, FeZSM-5, CuZSM-5, β-zeolite, and activated carbon7 and the highest specific adsorption capacity was observed for β-zeolite (∼1 mmol/g) and active carbon. The effect of nicotine concentration in the solution, pH value of the nicotine solution, flocculate size, and zeta potential on the adsorption of nicotine to the magnesium aluminum silicate was investigated by Suksri et al. It was found that the isothermal adsorption of nicotine at different pH can be described by Langmuir and Freundlich models.8 * To whom correspondence should be addressed. Tel: +381113336871. Fax: +38111-2187133. E-mail: [email protected]. † Military Technical Institute. ‡ Faculty of Physical Chemistry, University of Belgrade. § Faculty of Agriculture, University of Belgrade.

Due to the physicochemical properties of zeolite type USY (high specific area and volume and significant hydrophobicity), it was assumed that it would be suitable as a novel adsorbent for nicotine removal from aqueous solutions. With the intention of developing a new adsorption system for nicotine removal from waste waters, in this work the kinetics of nicotine adsorption (kinetic model and kinetic parameters) onto zeolite type USY was investigated. 2. Materials and Methods Nicotine (p.a.) was purchased from Merck, KGaA, Darmstadt Germany. Zeolite type USY was synthesized at the Faculty of Physical Chemistry, Belgrade (Adnadjevic´, 1997). Basic physicochemical properties of the zeolite are presented in Table 1. Distilled water was used in all of the experiments. 2.1. Determination of the Specific Adsorption Capacity of Zeolite to Nicotine. Specific adsorption capacity of zeolite to nicotine was measured by the batch method. Zeolite powder (m ∼ 1 g) was added to 1 wt % aqueous nicotine solution (ms ∼ 100 g), and the adsorption vessel was placed in the thermostat at a predetermined temperature (298, 311, and 322 K). The accuracy of temperature measurements was (0.2 K. The vessel was equipped with a reflux condenser in order to prevent evaporation of water. During the adsorption process, the adsorption system was homogenized by stirring at 400 rpm. Samples were taken from this adsorption system at regular time intervals. After centrifugation, concentration of nicotine remaining in supernatant was determined by measuring the absorbance Table 1. Basic Physicochemical Properties of Zeolite Type USY physicochemical properties

value

degree of crystallinity (%) SiO2 (wt %) anhydrous basic Al2O3 (wt %) anhydrous basic SiO2/Al2O3, molar ratio degree of hydrophobicity (%) unit constant cells (nm) crystal size SEM (µm) surface area BET (m2/g) specific volume (cm3/g)

100 77.1 21.2 6.18 98 2.42 1.2 600 0.38

10.1021/ie901351h  2010 American Chemical Society Published on Web 06/21/2010

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Table 2. Set of the Kinetic Reaction Models Used To Determine the Model of Nicotine Adsorption Kinetics symbol P1 P2 P3 P4 R1 R2 R3 F1 F2 F3 A2 A3 A4 D1 D2 D3 D4

kinetics models

f(R)

power law power law power law power law zero-order (Polany-Winger equation) phase-boundary-controlled reaction (contracting area, i.e., bidimensional shape) phase-boundary-controlled reaction (contracting area, i.e., tridimensional shape) first-order (Mampel) second-order third-order Avrami-Erofe´ev Avrami-Erofe´ev Avrami-Erofe´ev one-dimensional diffusion two-dimensional diffusion (bidimensional particle shape) three-dimensional diffusion (tridimensional particle shape), Jander equation three-dimensional diffusion (tridimensional particle shape), Ginstling-Brounshtein

of nicotine solution at 259 nm. For that measurement a UV-visible spectrometer (Cintra 10e, GBC Scientific Equipment) was used. The nicotine concentration in water solution was determined by the ISO 2881 method.9 2.1.1. Specific Adsorption Capacity. The specific adsorption capacity of zeolite USY for nicotine (xs) at a given temperature after certain adsorption period can be calculated from the following equation: xs )

(c0 - ci)ms 100(%) m

(1)

where c0 is the initial concentration of the nicotine solution before adsorption (wt %), ci is the concentration of the nicotine solution after a certain adsorption time (wt %), and m is the mass of zeolite (g). 2.1.2. Adsorption Degree of Nicotine. The degree of adsorption of nicotine (R) is calculated from the following equation: R)

xs xmax

(2)

where xmax is the maximum specific adsorption capacity of zeolite for nicotine at a given temperature and is determined from experimental kinetic curves. 2.2. Kinetics Model of Nicotine Adsorption at Zeolite Type USY. The kinetics model of nicotine adsorption onto zeolite type USY was examined by the so-called “modelfitting procedure”. The model-fitting procedure is widely used to determine the suitability of various kinetic reaction models for solid-state reactions.10 According to the model-fitting method, the kinetic reaction models for solid-state reaction are classified into five groups depending on the reaction mechanism: (1) a power law reaction; (2) a phase-controlled reaction; (3) reaction order reaction; (4) a reaction described by the Avrami equation; and (5) diffusion-controlled reactions. The model-fitting method is based on the following. The experimentally determined conversion curve Rexp ) f(t)T has to be transformed into the so-called reduced conversion curve R ) f(tR)T, where tR is the so-called reduced time. The reduced time, tR, introduced to normalize the time interval of the

g(R)

4R 3R2/3 2R1/2 2/3R-1/2 1 2(1 - R)1/2

R R1/3 R1/2 R3/2 R [1 - (1 - R)1/2]

3(1 - R)2/3

[1 - (1 - R)1/3]

(1 - R) (1 - R)2 (1 - R)3 2(1 - R)[-ln(1 - R)]1/2 3(1 - R)[-ln(1 - R)]2/3 4(1 - R)[-ln(1 - R)]3/4 1/2R 1/[-ln(1 - R)] 3(1 - R)2/3/2[1 - (1 - R)1/3]

-ln(1 - R) (1 - R)-1 - 1 0.5 [(1 - R)-2 - 1] [-ln(1 - R)]1/2 [-ln(1 - R)]1/3 [-ln(1 - R)]1/4 R2 (1 - R) ln(1 - R) + R [1 - (1 - R)1/3]2

3/2 [(1 - R)-1/3 - 1]

(1 - 2R/3) - (1 - R)2/3

3/4

1/4

monitored process in accordance with Brown,10 is defined by the following equation: tR )

t t0.9

(3)

where t0.9 is the adsorption time at which R ) 0.9. By applying the reduced time, it was possible to calculate the reduced conversional curves for different kinetics models.11 The kinetics model of the investigated process was determined by analytically comparing the reduced experimentally curves with the models’ reduced curves. The criterion for comparing was the sum of squares of the deviation from the models’ reduced curves. As a kinetics model, the model for which the sum of squares of the deviation from the experimental curve gives a minimal value is chosen. A set of the reaction models used to determine the model which best describes the kinetic of nicotine adsorption onto zeolite is given in Table 2, where f(R) is the analytical expression describing the kinetic model and g(R) is the integral form of the kinetics model. 2.3. Differential Isoconversion Method. The activation energy of investigated nicotine adsorption process for various degrees of nicotine adsorbed was established by the Friedman differential method.12 The rate of a process in the condensed state is generally a function of temperature and conversion degree: dR ) f(T, R) dt

(4)

dR ) k(T)f(R) dt

(5)

i.e.

where dR/dt is the reaction rate, R the conversion degree, k(T) the rate constant, t the time, T the temperature, and f(R) the reaction model associated with a certain theoretical reaction mechanism. The dependence of the rate constant on temperature is usually described by the Arrhenius law:

( )

k(T) ) A exp -

Ea RT

(6)

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Figure 1. Dependence of the isothermal specific adsorption capacity on time for nicotine adsorption onto zeolite type USY at 298 K (9), 311 K (b), and 322 K (2).

where Ea is the activation energy, A is the pre-exponential factor, and R is the gas constant. Then, on substitution of eq 6 into eq 5, the following equation is obtained:

( dRdt ) ) A exp(- RT )f(R) Ea,R

(7)

According to the isoconversional principle, f(R) does not change with R, and then eq 7 can easily be transformed to

( dRdt )

ln

R

) ln[Af(R)] -

Ea,R RT

(8)

Based on eq 8, from the slope and intercept of the dependence of ln(dR/dt)R vs 1/T, the values of the kinetics parameters, the activation energy for a particular degree of conversion Ea,R and ln AR are obtained. 3. Results and Discussion The isothermal dependence of specific adsorption capacity of the zeolite for nicotine vs adsorption time (kinetic curves) is shown in Figure 1. Three distinct regions of shape of change of adsorption capacity with time can be clearly seen from the adsorption curves, i.e., linear, nonlinear, and saturation regions. In order to determine the temperature influence on the shape of adsorption curves, the so-called shape parameters of the adsorption curves are defined: period of linearity tin and saturation time ts13 (inset, Figure 1). The period of linearity is the time interval within which the degree of adsorption increases linearly with the adsorption time. The period of linearity is determined graphically. Actually, by physical meaning the period of linearity is inversely proportional to the initial adsorption rate of nicotine. The saturation time represents the adsorption time required to achieve the maximum specific adsorption capacity of zeolite for nicotine at a given temperature. It is also determined graphically and its reciprocal value corresponds to the adsorption rate at the end of adsorption process by physical meaning. The adsorption kinetic curve parameters (tin and ts) at different temperatures are presented in Table 3. Based on the results given in Table 3, it can be seen that the values of tin and ts decrease with the increase in temperature of the adsorption system; meanwhile, the values of xmax increase.

T (K)

tin (min)

ts (min)

xmax (%)

298 311 322

22 ( 2 15 ( 2 6(1

90 ( 8 40 ( 4 30 ( 3

3.65 ( 0.05 3.90 ( 0.05 4.05 ( 0.05

Bearing in mind that the shape of adsorption kinetic curve, the increase in xmax with increase in temperature, the fact that nicotine in aqueous solution acting like weak base14 and surface of zeolite type USY has strong acid centers,15 with great degree of certainty it may be assumed that nicotine is chemisorbed onto zeolite type USY. In this process, the pyridine ring of nicotine is most probably protonated and so PyH+ complex is formed, which means that nicotine is chemically bonded to the acidic centers of zeolite. In order to determine the kinetic adsorption model, the possibility of preliminarily applying the most common kinetics models of adsorption was examined. It was also assumed that the adsorption kinetics could be determined by the rate of diffusion of the dissolved nicotine to the adsorption centers of zeolite or by the kinetic of interaction of nicotine with adsorption centers. In the case when the adsorption kinetics of nicotine is determined by the rate of diffusion of nicotine molecules to the adsorption centers of zeolite, the dependence R2 ) f(t) would be the straight line whose slope corresponds to the constant of adsorption rate. Figure 2 shows the isothermal dependence R2 ) f(t) at different temperatures. According to the results presented in Figure 2, it is easy to recognize that the plots of R2 vs time give straight lines only in limited ranges of R, in the range from 0.5 to 0.8. That proves that the kinetics of nicotine adsorption on the zeolite was not determined by the rate of diffusion of the nicotine molecules. When the adsorption kinetics is determined by the kinetics of interaction nicotine- adsorption center, i.e., by the concentration of the nicotine remained in the aqueous solution, the dependence of -ln(1 - R) vs time would give a straight line. The plot of the isothermal dependence -ln(1 - R) ) f(t) for different temperatures is shown in Figure 3. From Figure 3, it can be clearly seen that the isothermal dependence of -ln(1 - R) on the adsorption time gives straight lines in the limited range of R, in the range from 0 to 0.40. These results imply that the kinetics of nicotine adsorption is not determined by the kinetics of interaction nicotine molecules’ adsorption center of zeolite.

Figure 2. Isothermal dependence of R2 on adsorption time at 298 K (9), 311 K (b), and 322 K (2).

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Figure 3. Isothermal dependence of -ln(1 - R) vs adsorption time at 298 K (9), 311 K (b), and 322 K (2).

Figure 4. Plot of R vs tR at 298 K (9), 311 K (b), and 322 K (2).

Bearing in mind the previously obtained results, in order to determine the kinetic model of adsorption of nicotine onto zeolite the model-fitting method was applied. Figure 4 shows the isothermal dependence of the degree of adsorption on the reduced time at all the investigated temperatures. Experimentally determined reduced isothermal adsorption degree curves, at all of the investigated temperatures, are identical in shape and all of them conform to a single model. Figure 5, a and b, shows the isothermal dependence of adsorption degree on reduced time for the used theoretical reactions models (solid curves) and experimental determined reduced curves (denoted with symbols). From Figure 5a,b, it can be clearly seen that the model R2 appears to be a best match with experimental reduced time plot, while the experimental curves significantly deviate from any other theoretical reaction model. Based on the experimentally determined isothermal dependence R ) f(tR), by the use of the analytical method, with great degree of certainty (sum of squares of residues is from 0.058 to 0.060) it could be stated that the kinetics of nicotine adsorption onto zeolite USY type at all the investigated temperatures can be best described by the kinetic model R2, which is given by the following equation: [1 - (1 - R)1/2] ) kMt

(9)

where kM is the model constant of adsorption rate. This model is characteristic for the physicochemical processes whose kinetics is determined by the rate of movement of the boundary layer of the adsorption phase during the process (phase-

Figure 5. (a) Plots of R vs reduced time for the reaction models (solid curves) and isothermal experimental data for nicotine adsorption at 298 K (9), 311 K (b), and 322 K (2). (b) Plots of R vs reduced time for the theoretical reaction models R1 ( · · · ), R2 (s), and R3 (--) and isothermal experimental data for nicotine adsorption at 298 K (9), 311 K (b), and 322 K (2).

Figure 6. Isothermal dependences of [1 - (1 - R)1/2] vs adsorption time at 298 K (9), 311 K (b), and 322 K (2).

boundary-controlled reaction, i.e., contracting area). If eq 9 describes the kinetics of the isothermal adsorption of nicotine onto zeolite, then the dependence [1 - (1 - R)1/2] on the adsorption time should be a straight line. The isothermal dependences of [1 - (1 - R)1/2] vs adsorption time are shown in Figure 6 at the investigated temperatures. Over the whole range of the adsorption degrees, at all of the investigated temperatures, the dependence [1 - (1 - R)1/2] on

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Table 4. Changes of the Model Constants of Nicotine Adsorption Rate with Temperature and the Kinetic Parameters (Ea,M and ln AM) T (K)

kM × 10-4 (min-1)

R

kinetic parameters

298 311 322

170 ( 1 280 ( 1 370 ( 1

0.9999 0.9999 0.9999

Ea,M (kJ/mol) ) 27 ( 2 ln AM (min-1) ) 7.3 ( 0.5

the adsorption time gives a straight line. This confirms that the correct kinetic model was selected for describing the adsorption of nicotine onto zeolite USY type. The change of the model constant of nicotine adsorption rate with temperatures is given in Table 4. Since the increase of the model constant of nicotine adsorption rate with temperature is exponential, the nicotine adsorption kinetic parameters (Ea,M and ln AM) were determined by applying the Arrhenius equation and are presented in Table 4. Because nicotine is probably chemisorbed on zeolite and adsorption of nicotine is an elementary kinetics process whose kinetics is limited by the rate of movement of the boundary layer, it can be concluded that Ea is predetermined with the enthalpy of the formation of transition complex (∆H*) and can be expressed with eq 10: Ea ) ∆H* + 2RT

(10)

where ∆H* is the enthalpy of transition complex. Since the adsorption entropy of activated complex (∆S*) is in functional dependence with ln A: ln A )

kBT ∆S* exp h R

( )

(11)

where kB is Boltzmann constant and h is Planck constant. Based on eq 11, it can be easily calculated that the entropy of the activated complex ∆S* ) -218 J/(mol K). The activation energy and pre-exponential factor values obtained confirm the assumption of nicotine chemisorption on acid centers of zeolite type USY. In order to investigate the complexity of the adsorption process,16 the dependence Ea ) f(R) was examined by the differential isoconversion method. The dependences ln(dR/dt)R ) f(1/T) for different degrees of nicotine adsorbed are presented in Figure 7. As can be seen from the results presented in Figure 7, there was a linear relationship between ln(dR/dt)R and the inverse temperature (1/T) for all degrees of nicotine adsorbed. From the slopes and intercepts of these straight lines, the values

Figure 8. Dependence of Ea,R vs degree of nicotine adsorbed.

of the kinetics parameters (Ea,R and ln AR) for each value of the degree of nicotine adsorbed were obtained. The dependence of Ea,R vs R is shown in Figure 8. As can be seen from the results presented in Figure 8, values of the activation energy are practically independent of the degree of nicotine adsorbed, which implies that nicotine adsorption onto USY zeolite is elementary (overall single-stage kinetics) process. The determined kinetic model of the nicotine adsorption onto zeolite enables us to propose a realistic physical model of adsorption onto zeolite. On the basis of the previously presented data, it can be stated with a high degree of assurance that the adsorption system of zeolite can be modeled by the system which consists of the spherical pores with equal effective diameter. During the adsorption, the pores of zeolite is filled simultaneously and layer by layer by adsorbed nicotine. That leads to the decrease in the surface of the boundary phase of zeolite-nicotine interaction with the increasing amounts of the nicotine adsorbed. The main stages of the nicotine adsorption mechanism onto zeolite are the following: fast nicotine adsorption on zeolite; formation of continual, spherical, layered, boundary phase of adsorbed nicotine in porous zeolite structure; controlled decrease of the areas of boundary phase; and termination of the adsorption interaction due to the complete filled-up state of the adsorption system of zeolite. Based on the established model for the zeolite adsorption system, the specific surface of the adsorption interaction before nicotine adsorption S0 is given by the expression S0 ) n4πR02

(12)

where n is the number of the spherical holes per unit mass of zeolite and R0 is the radius of the zeolite’s specific pore before adsorption. Bearing in mind the previously suggested model of nicotine adsorption, the radius of the spherical pore of zeolite at the adsorption time, Rt, is equal to Rt ) R0 - kct

(13)

where kc is the rate constant of the decrease of the radius of the boundary layer. Based on that, it follows that the specific surface of the adsorption interaction at time, St, is given by the expression St ) n4πRt2 Figure 7. Dependences of ln(dR/dt)R on inverse temperature (1/T) for different degrees of nicotine adsorbed.

Then, the degree of adsorption is given by the equation

(14)

Ind. Eng. Chem. Res., Vol. 49, No. 14, 2010

n4πR0 - n4πRt 2

R)

2

2

n4πR0

(15)

i.e.

( )

R) 1-

Rt2

(16)

R02

Replacing the Rt value in the eq 16 with R0 - kct, the following is given: kc t R0

(17)

[1 - (1 - R)1/2] ) kMt

(18)

kM ) kc /R0

(19)

[1 - (1 - R)1/2] ) i.e.

where

Since eq 18 is identical to the experimentally obtained kinetic model of nicotine adsorption onto the USY zeolite, it is possible to claim with a great degree of confidence that the suggested model of the nicotine adsorption is realistic and that the kinetics of adsorption was determined by the rate of movement of the boundary layer of the adsorption phase. 4. Concluding Remarks Nicotine is most probably chemisorbed onto zeolite type USY from water solution. Nicotine adsorption is an elementary kinetics process whose rate is limited by the rate of movement of the boundary layer. Nicotine adsorbs into the pores of zeolite according to model “layer by layer”. The activation energy of nicotine adsorption corresponds to the enthalpy of the transition complex formation, while the value of pre-exponential factor is in functional correlation with the entropy of the transition complex. Acknowledgment This investigation was supported by the Ministry of Science of Serbia, through project 142025G. Notation xs ) specific adsorption capacity of zeolite USY for nicotine at a given temperature after certain adsorption period of time (%), defined by eq 1 c0 ) initial concentration of the nicotine solution before adsorption (wt %) ci ) concentration of the nicotine solution after a certain adsorption time (wt %) m ) mass of zeolite (g) ms ) mass of solution (g) xmax ) maximum specific adsorption capacity of zeolite for nicotine at a given temperature and is determined from experimental kinetic curves (%) t ) time (min) tR ) so-called reduced time (min), defined by eq 3 t0.9 ) adsorption time at which R ) 0.9 tin ) period of linearity (min), the time interval within which the degree of adsorption increases linearly with the adsorption time at a given temperature (min)

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ts ) saturation time, the adsorption time required to achieve the maximum specific adsorption capacity of zeolite for nicotine at a given temperature (min) kM ) model constant of adsorption rate (min-1) k(T) ) reaction rate constant (min-1) Ea ) kinetic parameter, activation energy (kJ/mol) Ea,M ) activation energy for selected model (kJ/mol) ln A ) kinetic parameter in the Arrhenius equation ln AM ) kinetic parameter in the Arrhenius equation for selected model (kJ/mol) ∆S* ) adsorption entropy of transition complex (J/(mol K)) kB ) Boltzmann constant (1.38 × 10-23 J/K) h ) Planck constant (6.62 × 10-34 J · s) T ) temperature (K) R ) universal gas constant (8.314 J/(mol K)) S0 ) specific surface of the adsorption interaction before nicotine adsorption (cm2/g), defined by eq 12 n ) number of the spherical holes per unit mass of zeolite (g-1) R0 ) radius of the zeolite’s specific pore before adsorption (cm) Rt ) radius of the spherical zeolite pore at the adsorption time t (cm), defined by eq 13 kc ) rate constant of the decrease of the radius of the boundary layer (cm/min) ∆H* ) the enthalpy of transition complex (kJ/mol) Greek Letters R ) adsorption degree of nicotine, defined by eq 2 f(R) ) analytical expression describing the kinetic model g(R) ) integral form of the kinetics model; g(R) ) ∫R0 dR/f(R) ) kt Ea,R ) activation energy for a particular degree of conversion (kJ/ mol) dR/dt ) reaction rate (min-1)

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ReceiVed for reView August 28, 2009 ReVised manuscript receiVed May 6, 2010 Accepted June 4, 2010 IE901351H