Kinetics of adsorption and desorption of Pb(II) in aqueous solution on activated carbon by two-site adsorption model

Colloids and Surfaces A: Physicochem. Eng. Aspects 240 (2004) 179–186

Kinetics of adsorption and desorption of Pb(II) in aqueous solution on activated carbon by two-site adsorption model M. Machida a,∗ , Y. Kikuchi b,1 , M. Aikawa c,2 , H. Tatsumoto a,3 a Faculty of Engineering, Chiba University, Yayoi-cho 1-33, Inage-ku, Chiba 263-8522, Japan Graduate School of Science and Technology, Chiba University, Yayoicho 1-33, Inage-ku, Chiba 263-8522, Japan Faculty of Science, Kisarazu National College of Technology, Kiyomidai-higashi 2-11-1, Kisarazu-city, Chiba 292-0041, Japan b

c

Received 14 October 2003; accepted 14 April 2004

Abstract The adsorption and desorption equilibrium and kinetics of lead ions from aqueous solutions on a granular activated carbon (GAC) were examined. Rapid increase followed by slow increase in Pb(II) amount on the GAC was observed as a function of time for the adsorption, while rapid decrease and consecutive very slow decrease was observed in desorption. Based on the experimental results, a two-site adsorption model was proposed for the adsorption and the desorption of Pb(II) under the study conditions. The Pb(II) adsorption on the GAC was estimated to have simultaneously occurred on the strong and the weak adsorption sites. Conventional Langmuir-type kinetic equations were introduced to quantitatively predict the adsorption and desorption with the two-site model by optimizing the parameters to fit the equilibrium and the kinetic experimental results. The equilibrium and kinetic experimental results could be represented by the equations by using one set of the common Langmuir parameters. Resultant kinetic parameters revealed that the adsorption equilibrium constant was two orders of magnitude greater for strong adsorption site than for weak adsorption site, though the maximum number of weak adsorption site was 1.5 times as great as that of strong adsorption site. The strong adsorption equilibrium constant resulted from a small desorption rate constant for the site. The equations were demonstrated to be applicable for predicting other desorption performances as well. © 2004 Elsevier B.V. All rights reserved. Keywords: Adsorption; Activated carbon; Kinetics; Two-site model; Langmuir

1. Introduction Heavy metals are generally recognized to be a threat toward humans and ecosystems because of their high potential toxicity. They could not be biologically decomposed into harmless materials and, to matters worse, were accumulated in the organisms [1,2]. Lead, as well as mercury, cadmium, chromium and arsenic, is in the group of serious hazardous heavy metals. It is widely found in contaminated soil, acidic leachate water from land fill or industrial sites, and many other places [3]. The lead in aqueous solution could be removed using precipitation by pH control or ion exchange. Adsorption was also an attractive option for removing lead ∗

Corresponding author. Tel.: +81 43 290 3129; fax: +81 43 290 3129. E-mail address: [email protected] (M. Machida). 1 Tel.: +81 43 290 3129; fax: +81 43 290 3129. 2 Tel.: +81 438 30 4069; fax: +81 438 98 5717. 3 Tel.: +81 43 290 3559; fax: +81 43 290 3559.

0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.04.046

and other hazardous materials [4]. Activated carbons [5–13], polymers [14], silica [15], clay [16], metal oxides [17,18] and many other materials [19–22] were examined as adsorbents for the lead removal from aqueous solution. For utilizing the adsorbents conveniently, kinetic models were proposed to express the adsorption performance for a lot of combinations of aqueous solutions and adsorbents as well as models for the equilibrium isotherms. The kinetic study is useful not only for the optimum adsorption operation, but also for accessing adsorption active sites though it is indirect manner. A pseudo-first-order model was most widely applied to the adsorption due to its simplicity. Manju et al. [23] studied the adsorption of lead, mercury and copper onto the modified iron oxide. They concluded that the adsorption would obey the first-order reversible kinetics in which the intraparticle diffusion of metal ions in the adsorbent was a rate-determining step. Evans et al. [24] examined the adsorption of cadmium onto crab shells and observed a high initial rate of cadmium uptake followed by a slow uptake

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rate for which the intraparticle diffusion was proposed as a rate-limiting step. In several experiments conducting adsorption of heavy metals onto adsorbents, two- or three-site adsorption model was proposed to represent the experimental results. Chiron et al. [15] carried out kinetic studies on lead and copper adsorption onto a grafted silica by using the pseudo-first- and second-order models, the Langmuir model and the double exponential model. The metal uptake was proposed to proceed by a diffusion-controlled mechanism or a two-site adsorption process from their results. Lazaridis and Asouhidou [25] investigated chromium(VI) adsorption on a hydrotalcite, and kinetically observed three type of nature of adsorption by plotting changes in cadmium adsorption onto the adsorbent against square root of time. A similar two-site model was also proposed to fit the adsorption equilibrium isotherms. Bi-Langmuir equations were successfully applied to represent equilibrium adsorption data of lead and zinc onto goethite [26], and cadmium onto bone char [27]. Furthermore, most of the kinetic and equilibrium analysis found in the literature was based on the adsorption experiments [7,15]. Those based on both adsorption and desorption experiments, however, were limited in the literature [11,28]. This study, investigated the adsorption and desorption equilibrium and kinetics of Pb(II) on an activated carbon. Particularly, the desorption was examined in detail by changing the initial amount of Pb(II) on the activated carbon. The influence of pH on adsorption was also examined. A simple Langmuir-type two-site kinetic adsorption model was proposed from the observation of the adsorption and desorption rates of Pb(II) on the activated carbon. The parameters in the kinetic equations were determined to predict the adsorption and desorption equilibrium and kinetics data. A brief discussion was made for the resultant parameters and the validity of the kinetic equations.

2. Materials and method 2.1. Adsorbent and Pb(II) solutions A commercially available granular activated carbon (GAC) made from coconut shell (Calgon Mitsubishi Chemical Corporation, Diasorb W10-30) was used for the experiments. The GAC was dried at 110 ◦ C overnight followed by 350 ◦ C for 2 h prior to use and was then allowed to cool in a desiccator. The BET surface area and the pore size distribution were measured by a surface analyzer (Beckman Coulter Model SA3100, USA). All chemicals were of reagent grade. The lead solutions were prepared by dissolving lead chloride in de-ionized water in the concentration range of 0.05–0.50 mmol L−1 . 2.2. Adsorption and desorption procedures Adsorption and desorption were carried out by batch processes. For adsorption experiments, the mixture of 100 mg

of GAC and 50 mL of Pb(II) solution at a desired concentration in a 100 mL conical flask was agitated at 100 rpm to measure adsorption rate and equilibrium isotherm at 25 ◦ C. The 100 mg GAC on which Pb(II) was pre-adsorbed in a desired amount and 50 mL de-ionized water mixture was used for desorption experiments at 100 rpm and 25 ◦ C. The pre-adsorbed GAC was prepared by removing Pb(II) solution by decantation technique after adsorbing Pb(II) onto the GAC for 2 h or 1 week. The agitation time was 0.5–70 h for rate measurement, and 1 week for equilibrium measurement. After the agitation was completed, the Pb(II) solution was separated from the GAC by decantation technique. The amount of Pb(II) on the GAC was determined by the difference between Pb(II) amount contained in the initial solution and that remained in the agitated solution. The pH was adjusted by adding hydrochloric acid to the Pb(II) solutions. The solution equilibrium pH was used to examine the influence of pH on the removal of Pb(II) from the aqueous solution. The pH was measured by a pH meter (HORIBA Model D-21).

3. Results and discussion 3.1. Properties of adsorbent and influence of pH As tabulated in Table 1, the surface area of the GAC was 1035 m2 /g, and mesopore and macropore defined by IUPAC were dominant for pore size distribution. It is generally known that pH strongly affects adsorption of heavy metals on activated carbons [29,30]. Fig. 1 shows the changes in percent Pb(II) removal as a function of pH. A sharp increase in the removal rate was observed between pH 5.5 and 6.0. However, the Pb(II) removal was changed within only 10% (29–39%) when the pH was changed between 5.4 and 6.0 without pH adjustment. The Pb(II) removal, therefore, would not exhibit remarkable change by drifting solution equilibrium pH for the GAC used as long as hydrochloric acid was not added to the solution. The solution equilibrium pH was within 5.4–6.0 for all the experiments in the study. Fig. 2 shows the calculated speciation diagram of Pb(II) as a function of pH for Pb(II) concentration of 0.05 mmol L−1 at 25 ◦ C. Including the other Pb(II) concentrations used in the study, more than 98% of Pb(II) species was Pb2+ even at the pH of 6.0. Similar distribution diagrams for Pb(II) species in aqueous solution were also given by Faur-Brasquet et al. [6] and Herrera-Urbina and Fuerstenau [31]. Table 1 Physical properties of activated carbon Surface area (m2 g−1 ) pore volume

1035 (␮L g−1 )

(vol.%)

<2 nm 2–50 nm 50 nm<

n.d. 45.7 4.6

0 91 9

Total

50.3

M. Machida et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 240 (2004) 179–186

181

-2.0

50

adsorption -2.5 desorption -3.0

30

ln Q e

Pb(II) removal, %

40

-3.5

20 -4.0

10

0 3.0

-4.5

3.5

4.0 4.5 5.0 5.5 6.0 Equilibrium solution pH

6.5

-5.0 -6.0

-4.0

-2.0

0.0

ln Ce

Fig. 1. Influence of solution equilibrium pH on percent Pb(II) removal by GAC at 25 ◦ C: (䊐, 䊏), no pH adjustment and pH adjustment by hydrochloric acid, respectively.

Fig. 3. Freundlich plot of Pb(II) adsorption and desorption on GAC at 25 ◦ C.

3.2. Equilibrium isotherm Fig. 3 shows the plot of the experimental results by the well-known empirical Freundlich isotherm for the adsorption and desorption. A fairly good linearity was obtained by the least square method. Freundlich parameters of KF and 1/n calculated from the slope and the intercept of the linear plot were shown in Table 2. The 1/n value of 0.38 indicates that the Pb(II) would preferentially adsorb onto the GAC under the conditions. A Langmuir plot was also shown in Fig. 4 by using the same experimental data with the Freundlich plot. Assuming a single adsorption site in the Langmuir isotherm, a linear relationship could be obtained for the adsorption and desorption as well as the Freundlich isotherm (not shown in the figure). The Langmuir parameters of the maximum number of adsorption sites (Xm ) and the

Pb(II) species distribution

1.0

Fig. 4. Langmuir plots of Pb(II) adsorption and desorption on GAC at 25 ◦ C: dotted lines, isotherms for strong (site-1) and weak (site-2) adsorption sites; solid line, sum of the isotherms of strong and weak sites.

adsorption equilibrium constant (Ke ) were 0.066 mmol g−1 and 25 L mmol−1 , respectively, as presented in Table 2. As described in detail in the latter section, two-site model also fitted the experimental data as drown in Fig. 4 in which the Xm and Ke were 0.047 mmol g−1 and 50 L mmol−1 for one site, and 0.075 mmol g−1 and 0.5 L mmol−1 for the other

0.8

0.6

0.4

Table 2 Freundlich and Langmuir constants for Pb(II) adsorption on activated carbon

0.2

Adsorption site 0.0 4

6

8

10

12

14

pH

Fig. 2. Speciation diagram of Pb(II) of 0.05 mmol L−1 in aqueous solution at 25 ◦ C: (䊉, 䊊, 䊏, 䉫, , 䊐, 䉱), Pb2+ , [Pb(OH)]+ , [Pb3 (OH)4 ]2+ , [Pb(OH)2 ], [Pb4 (OH)4 ]4+ , [Pb6 (OH)8 ]4+ , [Pb(OH)3 ]− , respectively.

Single-site model Two-site model Site-1 Site-2

Freundlich parameters

Langmuir parameters

KF

1/n

Xm (mmol g−1 )

Ke (L mmol−1 )

11

0.38

0.066

25

0.047 0.075

50 0.5

M. Machida et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 240 (2004) 179–186 0.06

0.07

0.05

0.06 0.05

0.04 Q, mmol/g

Q, mmol/g

182

Site-1 0.03 Site-2

0.04 0.03

0.02 0.02

0.01 0.00

0.01

0

20

40

60

0.00

80

0

15

time, h

Fig. 5. Changes in Pb(II) amount on GAC as a function of time in adsorption: dotted lines; drawn by Langmuir equations for strong (site-1) and weak (site-2) sites, solid lines; sum of adsorption amounts on strong and weak sites (site-1 and -2).

site, respectively. The equation for two-site model could be described as Eq. (1) [26,27], Qe =

Ke,S-1 Ce Ke,S-2 Ce Xm,S-1 + Xm,S-2 , 1 + Ke,S-1 Ce 1 + Ke,S-2 Ce

30

45

60

time, h

(1)

where Ce is the equilibrium concentration of Pb(II), Qe is the equilibrium amount of Pb(II) on the GAC, Ke,S-1 and Ke,S-2 are adsorption equilibrium constants, and Xm,S-1 and Xm,S-2 are the maximum numbers of adsorption sites for the two different sites, respectively. 3.3. Adsorption and desorption rates Fig. 5 shows changes in Pb(II) amount adsorbed on the GAC as a function of time. A sharp increase in Pb(II) amount on the GAC was observed at the first step of adsorption followed by slow increase toward the equilibrium amount. As shown in Fig. 6, when the Pb(II) was pre-adsorbed on the GAC by 0.064 mmol g−1 of GAC under the equilibrium state, there was a sharp decrease in Pb(II) amount on the GAC at the first step of desorption followed by very slow decrease toward the equilibrium amount as well. These results indicated that there were more than two adsorption sites. Fig. 7 shows the results of repeated adsorption and desorption cycle of Pb(II) on the GAC. For the second adsorption through fourth desorption, the amounts of Pb(II) at the equilibrium were constant both for adsorption and desorption. The difference in the equilibrium amounts of Pb(II) between adsorption and desorption was considered to be corresponding to the Pb(II) desorbed from the weak adsorption site, whereas the equilibrium amount of Pb(II) in the desorption would be attributed to the Pb(II) mostly remained on the strong adsorption site. In this study, therefore, two-site model was employed to the kinetic analysis. Based on the above experimental results, two contrastive two-site models

Fig. 6. Changes in Pb(II) amount on GAC as a function of time in desorption: (䊊, 䊐), Pb(II) pre-adsorbed on GAC by equilibrium amounts of 0.064 and 0.020 mmol g−1 in advance, respectively; solid lines, sum of adsorption amounts on strong and weak sites drawn by Langmuir equations.

could be proposed as follows: (a) the first rapid adsorption would have mainly occurred on the relatively weak adsorption site on the external and/or in the macro pore of the GAC, and the slow adsorption would be attributed to a diffusion-controlled step in which the adsorption took place on the site in the intraparticle of the GAC, (b) the adsorption would proceed on strong and weak adsorption sites on the GAC simultaneously, with a large molecular diffusivity for accessing both of the sites. For the above mentioned two models, it was assumed that the migration of Pb(II) on the GAC could not have occurred in the adsorption and desorption. If the first two-site

Fig. 7. Changes in equilibrium amount of Pb(II) on GAC in repetition of adsorption and desorption cycle, Pb(II) concentration of 0.50 mmol L−1 for adsorption.

M. Machida et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 240 (2004) 179–186

70

Pb(II) adsorption, %

60 50 40 30 20 10 0 0.0

0.1

(A)

0.2 0.3 C0 , mmol/L

0.4

0.5

0.04

0.05

70 60 Pb(II) desorption, %

model (a) including diffusion-controlled rate-determining step would be operative, the rapid decrease in Pb(II) amount on the GAC observed in the desorption must be corresponding to the first rapid increase in Pb(II) amount in the adsorption. In contrast, when the second two-site model (b) consisting weak and strong adsorption sites would be working, the first rapid decrease in Pb(II) amount should be corresponding to the Pb(II) pre-adsorbed on the weak site. As shown in Fig. 6, when the pre-adsorbed Pb(II) on the GAC was decreased to 0.020 mmol Pb(II) per gram of GAC under the equilibrium state, only a slight decrease in Pb(II) amount on the GAC were observed, indicating that the first rapid adsorption for the low initial Pb(II) solution concentration would be mainly attributed to the strong adsorption site, not to the weak adsorption site. The experimental results, therefore, supported the two-site model (b) rather than (a) for the adsorption mechanism. Though the two-site model would be valid from the experimental results of adsorption and desorption, surface functional groups responsible for strong and weak adsorption sites for Pb(II) have not been clearly understood. In general, the carbon–oxygen surface groups were considered to be responsible for the adsorption of heavy metal ions [32–34]. Kadirvelu et al. [32] identified the carbon–oxygen functional groups on activated carbons as phenol-type hydroxyl, carboxyl, lactone and carbonyl groups by titration using the well-known Boehm method [35]. Goyal et al. [33] observed the significant decrease in Cu(II) uptake by activated carbon on vacuum evacuation at 650 ◦ C in which CO2 , CO and H2 O were evolved in the evacuation pre-treatment. Aggarwal et al. [34] also examined Cr(III) adsorption in aqueous solutions onto activated carbon oxidized at 350 ◦ C followed by vacuum evacuation in the temperature range of 400–950 ◦ C. The increase in H2 O, CO2 and CO in the pre-treatment and the decrease in Cr(III) adsorption onto activated carbon were observed with increasing evacuation temperature, and most of the adsorption sites for Cr(III) disappeared at 950 ◦ C in their study. On vacuum evacuation at 400 ◦ C, both CO2 , CO and H2 O were generated, while CO was dominantly detected in consecutive evacuations at 650 and 950 ◦ C. Consequently, phenol-type hydroxyl and carboxyl groups were supposed to be sequentially converted to lactone and carbonyl groups with releasing H2 O, CO2 and CO by heating under vacuum condition. Fig. 8(A) and (B), respectively, depict percent Pb(II) adsorption and desorption as functions of initial Pb(II) concentration in the solutions and initial Pb(II) amount on the GAC evacuated on vacuum at 560 ◦ C comparing to the GAC without vacuum evacuation. Whereas the Pb(II) adsorption was slightly increased, the Pb(II) desorption was significantly increased on vacuum evacuation suggesting that that the strong adsorption sites for Pb(II) on the GAC could be converted to weak adsorption sites by the evacuation. From the results in this study and the literature, phenol-type hydroxyl and/or carboxyl groups seem to be corresponding to the strong adsorption sites of Pb(II) on one hand, the lactone and/or carbony

183

50 40 30 20 10 0 0.00

(B)

0.01

0.02

0.03

Q0 , mmol/ g

Fig. 8. Changes in equilibrium Pb(II) adsorption (A) and desorption (B) rates as functions of the initial Pb(II) concentration in solution (C0 ) and the initial Pb(II) amount on GAC (Q0 ), respectively: (䊉, 䊏), adsorption on GAC and that evacuated at 560 ◦ C; (䊊, 䊐), desorption from GAC and that evacuated at 560 ◦ C, respectively.

groups could be the weak adsorption sites on the other hand. Further examination, however, would be required to quantitatively clarify the surface functional groups responsible for Pb(II) adsorption [32]. 3.4. Kinetic analysis (development of kinetic equations) As the two-site model including strong and weak adsorption sites could be proposed, Langmuir-type rate equations were introduced to quantitatively represent the adsorption and desorption as expressed in Eqs. (2a) and (2b) based on the experimental results, dQS-1 = kADS,S-1 C(Xm,S-1 − QS-1 ) − kDES,S-1 QS-1 dt

(2a)

dQS-2 = kADS,S-2 C(Xm,S-2 − QS-2 ) − kDES,S-2 QS-2 dt

(2b)

M. Machida et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 240 (2004) 179–186

where C is the Pb(II) concentration in the solution, QS-1 and QS-2 are the Pb(II) amounts per gram of GAC, and kADS,S-1 , kADS,S-2 , kDES,S-1 , kDES,S-2 are the adsorption and the desorption rate constants for the site-1 and -2, respectively. Application of the Langmuir-type kinetic equations as Eqs. (2a) and (2b) for heavy metal adsorption has been limited compared to the wide application of Langmuir-type isotherms. Chiron et al. [15], however, successfully predicted the results of copper and lead adsorption onto a grafted silica by using a Langmuir-type kinetic equation. As the equilibrium adsorption constant of Ke and the Pb(II) concentration of C could be expressed as Eqs. (3) and (4), Eqs. (2a) and (2b) could be transformed as Eqs. (5a) and (5b), Ke =

kADS , kDES

(3)

C = C0 − (QS-1 + QS-2 )

M , V

(4)

where M is the amount of the GAC, V is the volume of solution, and C0 is the initial Pb(II) concentration.
 dQS-1 M = kADS,S-1 C0 − (QS-1 + QS-2 ) dt V kADS,S-1 (5a) QS-1 × (Xm,S-1 − QS-1 ) − Ke,S-1
 M dQS-2 = kADS,S-2 C0 − (QS-1 + QS-2 ) dt V kADS,S-2 × (Xm,S-2 − QS-2 ) − (5b) QS-2 Ke,S-2 The total amount of Pb(II) adsorbed on the GAC (Q) was represented as Eq. (6), Q = QS-1 + QS-2 .

(6)

The Q was calculated by using Eqs. (5a) and (5b) with the fourth order Runge–Kutta routine, since the equations could not be integrated by numerical analysis [36]. The Langmuir rate equations parameters of kADS, s , Ke s and Xm s for the site-1 and -2 were iterated until the best fit to the experimental results was obtained. The experimental data used for the optimization were the adsorption and desorption equilibrium isotherm shown in Fig. 4, the changes in adsorption amount in Fig. 5, and the changes in desorption amount in case of pre-adsorbed amount of 0.064 mmol g−1 of GAC in Fig. 6. As drawn in Figs. 4–6, the equilibrium and kinetic experimental results could be represented simultaneously with one set of the common Langmuir parameters of kADS, s , Ke s and Xm s presented in Tables 2 and 3. Hereafter, the site-1 Table 3 Rate constants for Pb(II) adsorption and desorption on acitivated carbon Adsorption site

kADS (g L mmol−1 h−1 )

kDES (g h−1 )

Site-1 Site-2

0.25 0.30

0.005 0.6

and -2 were defined as the strong and weak desorption sites, respectively, in accordance with the decreasing order of the adsorption equilibrium constants of Ke,S-1  Ke,S-2 . The adsorption equilibrium constant for the site-1 (Ke,S-1 ) was two orders of magnitude higher than for the site-2 (Ke,S-2 ), though the maximum adsorption site (Xm ) was more than 1.5 times higher for the site-2 than for the site-1. The site-1 has, therefore, relatively a small adsorption area and a very strong adsorption character, while the site-2 has a large adsorption area and a weak adsorption character. The adsorption rate constants (kADS ,s) for the site-1 and -2 were similar though the desorption rate constant for the site-1 was two orders of magnitude smaller than that for the site-2 as shown in Table 3. Consequently, a very large adsorption equilibrium constant for the site-1 was ascribed to a very small desorption rate constant indicating that the Pb(II) was strongly held on the adsorption site-1 on the GAC once it had been adsorbed. In addition, since the difference between the adsorption rate constants for the two sites was small from the kinetic analysis, the adsorption under the study conditions are supposed not to be affected by a molecular diffusion control to such an extent as to be clearly appeared. 3.5. Prediction of desorption The kinetic equations derived from the equilibrium and kinetic results were applied to other desorption results to examine the validity of the equations. As drawn in Fig. 6, the desorption rate of the Pb(II) pre-adsorbed on the GAC by 0.020 mmol g−1 of the equilibrium amount could be predicted by the kinetic equations; most of the pre-adsorbed Pb(II) was remained on the strong adsorption site. Fig. 9 also shows the results of desorption of the Pb(II) pre-adsorbed for 2 h by 0.028 mmol g−1 with using a high initial solution

0.030 0.025 0.020 Q, mmol/g

184

0.015 Site-1 0.010 Site-2 0.005 0.000 0

15

30 time, h

45

60

Fig. 9. Changes in Pb(II) amount on GAC as a function of time in desorption: Pb(II) pre-adsorbed on GAC for 2 h by 0.028 mmol g−1 in advance; dotted lines, Pb(II) amounts on strong (site-1) and weak (site-2) sites on GAC predicted by Langmuir equations; solid line, sum of adsorption amounts on strong and weak sites (site-1 and -2).

M. Machida et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 240 (2004) 179–186

concentration of 0.46 mmol L−1 . The initial Pb(II) concentration was so high that the adsorption was estimated to take place not only on the strong adsorption site, but also on the weak adsorption site from the kinetic equations. As represented in Fig. 9, the desorption was started from the Pb(II) amounts per gram of the GAC estimated as much as 0.012 and 0.016 mmol g−1 for the site-1 and -2, respectively, by the kinetic equations. A rapid decline in Pb(II) amount on the GAC was observed for the first step, and then a slight slow increase in Pb(II) was observed for the desorption. The experimental results could also be predicted by the kinetic equations as drawn in Fig. 9; the amount of Pb(II) rapidly re-dissolved to the de-ionized water would have come from the weak adsorption site (site-2), and then the consecutive slight increase in Pb(II) amount on the GAC was interpreted in terms of the re-adsorption of Pb(II) from the solution to the strong adsorption site (site-1). The predicted results demonstrated that the introduced kinetic equations would be valid under the study conditions for desorption kinetics in a wide range of pre-adsorbed conditions as well as for adsorption kinetics and for equilibrium isotherms.

4. Conclusions By examining adsorption and desorption equilibrium and kinetics of Pb(II) on a granular activated carbon (GAC) from aqueous solutions, the following conclusions were deduced from the experiments and the kinetic analysis. (1) Based on the experimental results, a two-site model was suggested for the adsorption and desorption; the adsorption would simultaneously proceed on strong and weak adsorption sites on the GAC. (2) Langmuir-type kinetic equations could be introduced in which the parameters of the maximum number of adsorption sites (Xm s), adsorption rate constants (ke s) and adsorption equilibrium constants (KADS, s ) for the two sites were optimized to represent the kinetic and equilibrium results. (3) The resultant adsorption equilibrium constant was two orders of magnitude greater for the strong adsorption sites than for the weak adsorption site, while the maximum number of adsorption site was 1.5 times greater for the weak adsorption site than for the strong adsorption site. The strong adsorption equilibrium constant was estimated to have come from the small desorption rate constant of Pb(II) for the site from the kinetic analysis. (4) The adsorption and the desorption under the study conditions would not be affected by a molecular diffusion control, since the first rapid Pb(II) desorption was corresponding to the Pb(II) on the strong adsorption site, and the adsorption rate constants of Pb(II) were close for both of the sites from the kinetic analysis. (5) The two-site kinetic equations were demonstrated to be operative for desorption kinetics in a wide range of

185

pre-adsorbed conditions of Pb(II) as well as for adsorption kinetics and for equilibrium isotherms.

Acknowledgements The authors would like to thank Calgon Mitsubishi Chemical Corporation for providing the granular activated carbon for the study, and they would also like to appreciate Y. Yang, Senior System Specialist of Beckman Coulter Co., Lid. for supporting the experiments.

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[33] M. Goyal, V.K. Rattan, D. Aggarwal, R.C. Bansal, Colloids Surf. A 190 (2001) 229. [34] D. Aggarwal, M. Goyal, R.C. Bansal, Carbon 37 (1999) 1989. [35] H.-P. Boehm, High Temp.-High Press. 22 (1990) 275. [36] W.H. Press, S.A. Teukolsky, W.T. Vetterling, B.P. Flannery, Numerical Recipes in FORTRAN, second ed., Cambridge, 1992. Motoi Machida is employed by Chiba University, Department of Applied Chemistry and Biotechnology, as a faculty of Engineering, Yayoi-cho 1-33, Inage-ku, Chiba 263-8522, Japan. He holds the position of associate professor. He completed his Bachelors in Science from Hokkaido University in March 1983, and Masters in Science from Hokkaido University in March 1985. He got his Ph.D. in Engineering from Hokkaido University in September 1998. Yosuke Kikuchi is a master course student of Graduate School of Science and Technoloty, Chiba University, Yayoi-cho 1-33, Inage-ku, Chiba

263-8522, Japan. He completed his Bachelors in Engineering from Chiba University in March 2003. Masami Aikawa is employed by Kisarazu National College of Technology, Faculty of Science, Kiyomidai-higashi 2-11-1, Kisarazu city, Chiba 292-0041, Japan. He holds the position of professor. He completed his Bachelors in Science from Rikkyo University (Saint Paul’s University) in March 1974, and Masters in Science from the University of Tokyo in March 1976. He got his Ph.D. in Science from the University of Tokyo in March 1979. Hideki Tatsumoto is employed by Chiba University, Department of Applied Chemistry and Biotechnology, Faculty of Engineering, Yayoi-cho 1-33, Inage-ku, Chiba 263-8522, Japan. He holds the position of professor. He completed his Bachelors in Science from Chiba Institute of Technology in March 1966, and Ph.D. in Engineering from Kyoto University in March 1977.