Adsorption of Cd(II) ions from aqueous solutions using activated carbon prepared from olive stone by ZnCl2 activation

Available online at www.sciencedirect.com

Bioresource Technology 99 (2008) 492–501

Adsorption of Cd(II) ions from aqueous solutions using activated carbon prepared from olive stone by ZnCl2 activation Ibrahim Kula a, Mehmet Ug˘urlu a

b,*

, Hamdi Karaog˘lu b, Ali C ¸ elik

c

Department of Chemistry, Faculty of Science and Arts, Middle East Technical University, Ankara 06561, Turkey b Department of Chemistry, Faculty of Science and Arts, Mug˘la University, Mug˘la 48000, Turkey c Department of Chemistry, Faculty of Science and Arts, Celal Bayar University, Manisa 45000, Turkey Received 13 July 2006; received in revised form 16 January 2007; accepted 16 January 2007 Available online 12 March 2007

Abstract This study is aimed to remove Cd(II) ions from aqueous solutions by adsorption. As adsorbent, activated carbon prepared from olive stone, an agricultural solid by-product was used. Different activating agent (ZnCl2) amounts and adsorbent particle size were studied to optimize adsorbent surface area. The adsorption experiments were conducted at different parameters such as, adsorbent dose, temperature, equilibrium time and pH. According to the experiments results, the equilibrium time, optimum pH, adsorbent dosage were found 60 min, pH > 6 and 1.0 g/50 ml respectively. The kinetic data supports pseudo-second order model and intra-particle model but shows very poor fit for pseudo-first order model. Adsorption isotherms were obtained from three different temperatures. These adsorption data were fitted with the Langmuir and Freundlich isotherms. In addition, the thermodynamic parameters, standard free energy (DG0), standard enthalpy (DH0), standard entropy (DS0) of the adsorption process were calculated. To reveal the adsorptive characteristics of the produced active carbon, BET surface area measurements were made. Structural analysis was performed using SEM-EDS. The resulting activated carbons with 20% ZnCl2 solution was the best sample of the produced activated carbons from olive stone with the specific surface area of 790.25 m2 g1. The results show that the produced activated carbon from olive stone is an alternative low-cost adsorbent for removing Cd(II).  2007 Elsevier Ltd. All rights reserved. Keywords: Adsorption; Cadmium(II) ions; Chemically prepared activated carbons; Olive stone; Kinetic equations

1. Introduction It is well known that heavy metals, such as Cd2, Cu2, Pb , Hg2 and Zn2 are toxic to human beings and other living organisms, if their concentration exceed the tolerance limit. These heavy metals introduced into natural water resources by waste water discharged from industries such as smelting, metal plating, Cd–Ni batteries, phosphate fertilizer, mining pigments, stabilizer and alloy manufacturing. The serious incident of itai itai-disease, which was caused by cadmium poisoning owing to mining in Japan, 2

*

Corresponding author. Fax: +90 252 2238656. E-mail address: [email protected] (M. Ug˘urlu).

0960-8524/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2007.01.015

was related to cadmium ions. The harmful effects of Cd(II) ions are renal damage, hypertension, proteinuria, kidney stone formation and testicular atrophy. Cd(II) ions may replace Zn(II) ions in some enzymes thereby affecting the enzyme activity. This necessitates the removal of Cd(II) ions from wastewater and water (Naseem and Tahir, 2001; Nagarethinam and Gurusamy, 2005; Erosa et al., 2001). Physical and chemical processes have been extensively studied to remove the heavy metal pollutants from wastewaters at high concentrations. Some of these processes are adsorption, coagulation, flotation, biosorption, chemical precipitation, ultra filtration and electrochemical methods. In this process, adsorption can be seen as an efficient and economic method to remove the heavy metal

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Nomenclature ACOS qt qe qe,exp qe,calc t k1 k2 ki t1/2 h

activated carbon from olive stone adsorbed amount at t time (mg/g) adsorbed amount at equilibrium (mg/g) experimental adsorbed amount at equilibrium (mg/g) calculated adsorbed amount at equilibrium (mg/g) time (min) rate constant for first order model (min1/2) rate constant for second order model (g/mg min) rate constant for intra-particle model (mg/g min1/2) half-time for adsorption (min) initial adsorption rate (mg/g min)

pollutants at low concentrations. Activated carbons were used as adsorbent materials because of their extended surface area, microporous structure, high adsorption capacity and high degree of surface reactivity. Furthermore, the presence of different surface functional groups on activated carbon, especially oxygen groups, leads to the adsorption of ions of heavy metals (Al-Asheh et al., 2003; Naseem and Tahir, 2001). In addition, many of the researchers (Viraghavan and Alfaro, 1998; Bailey et al., 1999; Amjad et al., 2003) have focused on preparing low-cost activated carbons. There are a quite large number of studies regarding the preparation of activated carbons from agricultural wastes (Kadirvelu et al., 2003 and Nag et al., 1999), nutshells (Mohanty et al., 2005), fruit stones (Lussier et al., 1994), bagasse (Mohan and Singh, 2002), oil palm waste (Lua and Guo, 1998) and agricultural residues from sugarcane (Blanco Castro et al., 2000), rice (Srinivasan et al., 1998) and peanut (Ricordel et al., 2001), sawdust (MarquezMontesinos et al., 2001), Rosa canina sp. seeds (Gu¨rses et al., 2006) and canes from some easy-growing wood species (Basso et al., 2001). Basically, there are two different processes for the preparation of activated carbon: physical activation and chemical activation. In comparison with physical activation, there are two important advantages of chemical activation. One is the lower temperature in which the process is accomplished. The other is that the global yield of the chemical activation tends to be greater since burn-off char is not required. Among the numerous dehydrating agents, zinc chloride in particular is the widely used chemical agent in the preparation of activated carbon. Knowledge of different variables during the activation process is very important in developing porosity of carbon which is sought for a given applications. For example, chemical activation done by ZnCl2, H3PO4, H2O2, etc., can improve the pore distribution and increase the surface area of adsorbents in the structure because of using differ-

C0 Ce ym, y K r2 SEM G0ads R T K c AAS AC

initial concentration (mg/L) C Equilibrium concentration (mg/L) the adsorbed amount in monolayer formation in Langmuir model equilibrium constant in Langmuir model statistical correlation coefficient scanning electron microscopy the free energy of adsorption (kJ/mol) universal gas constant (8,314 J/mol K) absolute temperature (K) the adsorption equilibrium constant intra-particle diffusion constant flame atomic absorption spectrometer activate carbon

ent chemicals (Ahmadpour and Do, 1997; Gu¨rses et al., 2006 and Mohanty et al., 2005). Turkey is one of the Mediterranean countries which are in first range of olive and olive oil producing. The annual production of olive oil in Turkey is 100.000–250.000 tons. This generates 100.000–250.000 tons of solid wastes (Oktay et al., 2002). In this study, olive stone (pirina in Turkish and Greece) from olive oil industry in Mug˘la Region in Turkey has been used for preparing activated carbon. Optimization of conditions and characterization of prepared activated carbon were investigated. The aim was to investigate the optimum conditions for removing of cadmium ions and to calculate the adsorption capacity and some thermodynamic constants. In addition, the adsorption mechanism through various adsorption kinetics models was investigated such as; pseudo-first-order model, pseudo-second-order model and intra-particle diffusion model. 1.1. Adsorption theory 1.1.1. Pseudo-first-order model In order to investigate the mechanism of adsorption, the pseudo-first-order adsorption and the pseudo-second order adsorption model were used to test dynamical experimental data. The pseudo-first order rate expression of Lagergren (1998) is generally described by the following equation (Ho and McKay, 1998, 1999; Chiou and Li, 2003; Jain and Sharma, 2002; Tseng et al., 2003) dqt ¼ k 1 ðqe  qt Þ; dt

ð1Þ

where qe and qt are the amounts of carotene and acidity, (mg/g) adsorbed on sorbents at equilibrium, and at time t, respectively and k1 is the rate constant (min1), respectively. Integrating and applying the boundary condition, t = 0 and qt = 0 to t = t and qe = qt Eq. (1) takes the form:

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logðqe  qt Þ ¼ log qe 

k1 t: 2:303

ð2Þ

The rate k1 was obtained form slope of the linear plots of (qe  qt) against t. 1.1.2. Pseudo-second-order model The pseudo-second order model is based on the assumption that the rate-limiting step may be chemical sorption or chemical sorption involving valence forces through sharing or exchange of electrons between sorbent and sorbate. The surface site-sorbate reaction may be represented as follows: Active surface site þ sorbate ! surface site-sorbate surface complex: It is assumed that the sorption capacity is proportional to the number of active sites occupied on the sorbent, then the kinetic rate law can be written as follows: (Ho and McKay, 1998, 2000; Ho, 2003; Li et al., 1999; Tseng et al., 2003; Gu¨ndog˘an et al., 2004) dqt 2 ¼ k 2 ðqe  qt Þ dt

qt ¼ k i t0:5 þ C;

ð9Þ

where ki and c is intra-particle diffusion rate constant (mg/ g min1/2) and a constant, respectively. The ki is the slope of straight-line portions of plot of qt versus t0.5. These plots generally have a dual nature, i.e., an initial curved portion and a final linear portion. This is explained by the fact that the initial curved portion represents boundary layer diffusion effects. The final linear portions are a result of intraparticle diffusion effects. Extrapolating the linear portion of the plot to the ordinate yields the intercept (c) which is proportional to the extent of boundary layer thickness (Jain and Sharma, 2002). 2. Methods

ð3Þ 2.1. Preparation and characterization of activated carbon

where k2 the rate constant of pseudo-second order sorption (g/mg min). Integrating and applying boundary conditions t = 0 and qt = 0 to t = t and qe = qt Eq. (3) becomes t qt ¼ 1 ; ð4Þ þ qt k q2 e

2 e

Morris (1963) is used. The rate constants, for intra-particle diffusion (ki) are determined using equation given by Weber and Morris (1963). This equation can be described by as Sivaraj et al. (2001), Jain and Sharma (2002), and Ug˘urlu et al. (2005)

Olive stone samples used for production of activated carbon were taken from olive oil factories in Mug˘la region in Turkey. The general procedure of the activation process in this study is described by Kim et al. (2001) and Mohanty et al. (2005) with a few modifications as below:

which has linear form of t 1 1 ¼ þ t: qt k 2 q2e qe

ð5Þ

If initial adsorption rate is h ¼ k 2 q2e ;

ð6Þ

then Eqs. (5) and (6) become t qt ¼ 1 t þq h

ð7Þ

e

and t 1 1 ¼ þ t: qt h qe

ð8Þ

If second-order kinetics is applicable, the plot of t/qt against t of Eq. (8) should give a linear relationship from which the constants qe, h and k2 can be determined. 1.1.3. Intra-particle diffusion model It has also been demonstrated that AC has a highly porous structure. This type of adsorbent structure often results in intra-particle diffusion being the rate-limiting step. The nature of the rate-limiting step in a batch system can be determined from the properties of the solute and sorbent. In adsorption systems where there is the possibility of intra-particle diffusion being the rate-limiting step, the intra-particle diffusion approach described by Weber and

1. Impregnation was carried out about 70 C in a water bath until the excess water was evaporated. The samples were taken to oven-dried at 120 C for 24 h. 2. Olive stone samples were mixed with 200 ml of 10%, 20% and 30% ZnCl2. 3. The samples of impregnated olive stone were placed at the centre of a stainless steel tubular reactor. This reactor was placed horizontally in side the muffle furnace. The activation temperature was set at 650 C under an inert flow N2 gas (150 cm3 min1 STP) for 2 h carbonization. 4. The products were washed sequentially with 0.5 N HCl, hot water and finally cold distilled water to remove residual organic and mineral matters. 5. The samples were then vacuum-dried at 102 C. A final product yield or loss due to washing process was recorded. The final product was stored in a desiccators filled with N2 gas to prevent oxidation. In all experiments, carbonization temperature and N2 flow were kept constant. The activated carbons were weighed to determine activation burn-off or mass loss due to activation and calculated the following equation (Fan et al., 2004): Activation burn-off ð%Þ ¼ 100  f½mass after activationðgÞ =original massðgÞ  100g:

ð10Þ

I. Kula et al. / Bioresource Technology 99 (2008) 492–501

The BET surface areas of prepared activated carbons were calculated from N2 adsorption by using the Brunauer–Emmett–Teller (BET) method with Micrometritics model, Flowsorb II-2300 adsorption meter. Chemical compositions of produced activated carbons were analyzed with EDX detector. Scanning electron micrographs (SEM) were recorded without sample coating by Philips XL-30S FEG model scanning electron microscope with 1000· magnification. SEMicrographs of raw material (olive stone) and ACOS2 are shown in Fig. 1. SEMicrograph of Cd(II) ions adsorbed activated carbon (ACOS2) is shown in the figure as well. The various physico-chemi-

495

cal characteristics of the activated carbon prepared from olive stone is given in Table 1. It can be concluded that surface area of the activated carbons highly depend on the amount of the activation agent and particle size. The surface area of the resulting activated carbons are maximum with % 20 ZnCl2 solution. The BET surface area of ACOS2 is 790.25 m2/g. The BET surface area of Cd(II) ions adsorbed ACOS2 was found 701.50 m2/g. The reason is the sorption of Cd(II) ions in the cavities of material. In addition, the chemical composition of Cd(II) ions adsorbed ACOS2are; C: % 88.47, O: % 9.65, S: % 0.60 and Cd: % 1.29.

Fig. 1. SEMs of (a) ACOS2, activated carbon (b), raw olive stone (c) and Cd(II) adsorbed ACOS2.

Table 1 Physico-chemical and other properties of activated carbon from olive stone ACOS Sample nos.

Impegranation solution by ZnCl2 (w/w)

Particle size (lm)

Weight loss (%)

BET surface area (m2/g)

Chemical comp. (%) C

O

S

ACOS ACOS1 ACOS2 ACOS3 ACOS4 ACOS5 ACOS6 ACOS7

0.0% 10% 20% 30% 20% 20% 25% 15%

350> 350P 350> 350< 350< 150< 350< 150<

40.02 47,24 45.21 51.45 48.13 47.41 47.89 48.12

3,00 58,89 790.25 166.58 107.73 505.77 638.41 552.77

64.41 86.11 90.08 87.27 89.11 88.13 89.92 89.71

34.01 13.10 9.49 11.73 10.24 10.88 9.10 9.33

0.50 0.35 0.43 0.10 0.65 0.99 0.98 0.96

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2.2. SEM analysis of the activated carbons

3. Results and discussion

Scanning electron microscopy (SEM) technique was employed to observe the surface physical morphology of the olive stone derived activated carbon. Fig. 1 shows the SEM photographs of the olive stone before and after the carbonization at the optimum operating condition with 1000· magnification. Pores of different size and different shape could be observed. It can be seen from the micrographs that the external surface of the chemically activated carbon is full of cavities. The reason for the formation of the cavities on the ZnCl2-activated carbon is not clear. According to this micrograph, it seems that the cavities resulted from the evaporation of ZnCl2 during carbonization, leaving the space previously occupied by the ZnCl2. The carbonization temperature for chemical activation was too low to cause the agglomeration of the char structure. Since the carbonization temperature for physical activation is high (900 C), caking and agglomeration occurred on the char structure and thus resulted in the formation of chars with an intact external surface. It can also be seen that some salt particles are scattered on the surface of the activated carbon, probably due to the presence of remaining zinc chloride or other metal compounds on the activated carbon. Some particles were even trapped into the pores and could possibly block the entry of pores to some extent.

3.1. Effect of adsorbents dosage on adsorption

2.3. Adsorption experiments Experiments were conducted with 15 mg/L dye concentration and samples having different ACOS2 and raw olive stone dosage ranging from 0.125 to 1.5 g/50 ml in order to determine the effect of solid/liquid ratio on adsorption. Batch adsorption experiments were carried out in which aliquots of 50 ml of Cd(II) solutions of known concentrations were poured into beaker glasses containing accurately weighed amounts of the adsorbents. The beaker glasses were shaken at 200 min1 using an electric shaker for a prescribed length of time to attain equilibrium at 293 K, 303 K and 313 K separately. The solution was filtered by using a 110 mm membrane filter and analyzed for residual metal content by FAAS. In all of the experiments, contact time, initial solution concentration, initial pH, adsorbent dose and temperature were selected as experimental parameters. A GBC Avanta model Flame Atomic Absorption Spectrometer was used for determination of cadmium(II) ions. Atomic absorption measurements were carried out using air/acetylene flame. The operating parameters for working element were set of as recommended by the manufacturer. WTW model pH-meter equipped with a combination pH electrode was used to measure the pH of all solutions. All the chemicals were of analytical grade. Reference solutions were prepared as required by further dilution with distilled water.

For Investigating of effect of ACOS2 and raw olive stone dosage on adsorption, the experiments were conducted with constant dye concentration (15 mg/L) and samples having different adsorbent dosage ranging from 0.125 to 1.50 g/50 ml (solid/liquid). The results of adsorption obtained for both adsorbents are given in Fig. 2. As seen from this figure, the adsorptive behaviour of Cd(II) ions on ACOS2 and raw olive stone are similar. However, it is seen in Fig. 1 that the adsorption of ACOS2 is four times more than that of raw olive stone (80–20%). This difference could be explained by the significantly larger surface area of ACOS2 compared to raw olive stone (see Table 1). In addition, as seen from Fig. 2, the removal percentage and adsorption of Cd(II) ions for both adsorbents increases with increasing adsorbed amount and then becomes constant indicating that 1.0 g/50 ml of adsorbent is sufficient for the optimum removal of Cd(II). Therefore, it was used only ACOS2 as adsorbents in all the experiments and dosage of this adsorbent was selected as 1.0 g/50 ml for the all experimental studies. 3.2. Effect of initial concentration at different times Effects of initial concentration were investigated for different times. The solid–liquid ratio and pH was selected to be constant (1.0 g/50 ml and 6.15). The results obtained from experiments are given in Fig. 3. As seen from this figure, the adsorption of Cd(II) ions onto ACOS2 increases when increasing treatment time from t: 0 to about 60 min and, there after, becomes constant 90 min. The adsorption processes reach equilibrium 90 min for all the initial concentrations studied. With changing the concentration of the solution from 15 mg/L to 45 mg/L, the absolute amount of Cd(II) ions per unit of adsorbent increases from 0.68 mg/g to 1.654 mg/g at 303.15 K.

Fig. 2. Effect of AC dosage and raw olive stone on adsorption of Cd(II) (303 K, pH: 6.15 and time: 1.5 h).

I. Kula et al. / Bioresource Technology 99 (2008) 492–501 1.8 1.6

q, (mg/g)

1.4 1.2 1 0.8 0.6 0.4 0.2

15 mg/L

30mg/L

45mg/L

0 0

30

60

90

120

Time (min.) Fig. 3. The effect of initial concentration on the adsorption of Cd(II) (1.0 g/50 mL solution, 303 K, pH = 6.15).

3.3. Effect of pH on adsorption process The removal of metal ions from aqueous solution by adsorption is highly depend on the pH of the solution which affects the surface charge of the adsorbent and the degree of ionization and speciation of the adsorbate (Gu¨ndog˘an et al., 2004; Ho and McKay, 1999). Most research has been conducted on heavy metal sorption indicated that the decrease in ionsorption at acidic pH may be due to the increase in competition with protons for active sites. At the alkaline pH values, however, other effects may arise from the some processes, such as the predominant presence of hydrated species of heavy metals, changes in surface charge and the precipitation of the appropriate salt (Blazquez et al., 2005; Sarin and Pant, 2006; Kumar and Bandyopadhyay, 2006). To verify the effect of pH on Cd(II) adsorption using raw olive stone and ACOS2 as adsorbent, experiments were conducted modifying from 1 to 11. The results obtained for both adsorbents are shown in Fig. 4. As can be seen, since pH rises, the percentage of cadmium removal increases considerably for both adsorbent. Moreover, removal amounts obtained by using ACOS2 are higher than raw olive stones. Initially, as the pH of solution was increased from 1.0 to 7 for raw olive stone and ACOS2, percent removal of Cd(II) increase from 10 to 25 and from 40 to

497

80, respectively. The fact that the amount of Cd(II) removal at low pH is considerably lower may be accounted for by the competition between Cd2+ and H+ ions the active sites on both sorbents surface. Similar results have been reported by other researchers (Mohan and Singh, 2002; Ajmal et al., 2003; Blazquez et al., 2005). After pH 7 value, for both adsorbents, the adsorption increased highly up to pH 9.0. Optimum uptake of 95.0% and 43.0 by ACOS2 and raw stone was observed at this pH, respectively. Then, decreasing trend in uptake was observed above pH 9 due to formation of soluble hydroxyl complexes. It is assumed that OH ions in the alkaline medium affects firstly hydrolysis products of Cd(OH)+, then affects Cd(OH)2 hydrolysis complexes. Also, these effects decrease the adsorption. This result is consistent with finding by Blazquez et al. (2005). Moreover, the yield which was gained from ACOS2 at the all pH values is much higher than the yield from raw olive stone. This difference can be explained considering the nature of the adsorbent. The surface area of the activated carbon is higher than raw stone and contains a large number of functional groups. Furthermore, the adsorption of metal ions can largely be related to the type and ionic state of these functional groups. (Mohan and Singh, 2002; Ajmal et al., 2003).

3.4. Effect of temperature To investigate the effect of the temperature (293, 303 and 313 K) on the Cd(II) adsorption, the experiments were conducted constant concentrations of Cd(II) (15 mg/L) and different times. The results are given in Fig. 5. As can be seen from these figures, the adsorption of Cd(II) onto the surface of ACOS2 taken place quickly for three temperature until first 60 min. It is seen that adsorption rate is constant in 90 min by increasing times at all of studied temperatures. The absorbed amount of Cd(II) ions slightly decreases when increasing temperature from 293.15 K to 313.15 K. The observed decrease in the adsorption capacity with an increase of temperature from 293 to 313 K indicated that low temperatures is in favour Cd(II) ions removal by adsorption onto ACOS2. This may be due to 0. 8 0. 7

q, (mg/g)

0. 6 0. 5 0. 4 0. 3 0. 2

293 K

303 K

313 K

0. 1 0 0

20

406

08

0

100

120

Time (min.) Fig. 4. Effect of pH on adsorption Cd(II) onto activated carbon and raw olive stone (g/50 mL solution, 303 K, time: 1.5 h).

Fig. 5. The effect of the different temperatures on the adsorption of Cd(II) ions (pH: 6.15 and 15 mg/L).

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a tendency for the Cd(II) ions to escape from the solid phase to the bulk phase with an increase in the temperature of the solutions. This effects suggested that an explanation of the adsorption mechanism associated with the removal of Cd(II) ions onto ACOS2 involves a physical process in this situation, in which adsorption arises from the electrostatic interaction, which is usually associated with low adsorption heat (Karaca et al., 2006). This means that the adsorption process has an exothermic character. Additionally, it was seen that the equilibrium time was observed to be independent from the temperature.

3.5. Kinetic studies The first-order, the pseudo-second order and intra-particle diffusion kinetic models are employed in this work. The obtained parameters for all of kinetic models are given in Table 2. As seen from this table, the first-order kinetic model did not adequately fit the experimental values (for all values, r2 < 89). In contrast, the pseudo-second order rate equation and intra-particle diffusion kinetic models for adsorption of Cd(II) ions onto ACOS2 agreed well with the data for r2 > 97 and r2 > 95, in turn. The correlation coefficients calculated for all models, i.e., r2, and the other parameters are shown in Table 2. Also, as seen from this table, it was observed that correlation coefficient, r2, for the pseudo-first-order model was much lower (<0.90) than for the pseudo-second order rate equation and intra-particle diffusion kinetic models. While the calculated equilibrium sorption capacity for the first order model at all of the initial concentration and temperature, qe,calc, values are not close to the experimental values (qe,exp), for the second-order model, qe,calc, values are close to qe,exp for all the initial concentration and temperatures. For this model, the linear regression analysis gave high values (r2 P 0.97). In addition, due to the porous nature of the ACOS2, diffusion of Cd2+ ions is also expected at the surface. Therefore, the plots of qt versus t1/2 according to intra-particle diffusion kinetics were obtained. The constants of intra-particle transport (ki) and the c values were calculated from the slopes and intercepts of the linear portions of the plots at various contact times (Table 2). The r2 values obtained

from intra-particle diffusion model and pseudo-secondorder model is higher than those of pseudo first-order. From these results, it is seen that the intra-particle diffusion model and pseudo-second-kinetic model are applicable for adsorption system. The applicability of both models showed that adsorption process is complex and involves more than one mechanism.

3.6. Adsorption isotherms Several models have been published in the literature to describe experimental data of adsorption isotherms. The Langmuir and Freundlich models are the most frequently employed models. In this work, both models were used to describe the relationship between the amount of Cd(II) ions adsorbed and its equilibrium concentration in solution at three temperature (293 K, 303 K, 313 K) for 1 h (see Figs. 6 and 7). The linear form of the Langmuir isotherm model can be represented by the using equation below: C 1 C ¼ þ : y ymK ym

ð11Þ

The linear form of the Freundlich isotherm model is given by the following equation:

Fig. 6. Freundlich adsorption isotherms of Cd(II) ions onto ACOS2 at different temperatures (time: 1.5 h, solid/liquid: 1 g/50 ml).

Table 2 First and pseudo-second order kinetics model parameters for the adsorption systems in the study Parameter

Pseudo-first-order model

Pseudo-second-order model

Intra-particle diffusion

qe,exp (mg/g)

qe,calc (mg/g)

k1 (g/mg min)

r2

qe,exp (mg/g)

qe,calc (mg/g)

k2 (g/mg min)

h (g/mg min)

r2

ki (mg/g min1/2)

ci

r2

Cint (mg/L) 15 30 45

0.680 1.184 1.654

0.985 3.735 5.500

0.0216 0.0907 0.0916

0.87 0.89 0.81

0.680 1.184 1.654

0.685 1.181 1.650

0.022 0.017 0.011

0.020 0.024 0.030

0.99 0.96 0.97

0.112 0.142 0.225

1.60 0.21 0.41

0.95 0.98 0.98

Temp, (K) 293 303 313

0.698 0.680 0.649

0.792 0.985 0.686

0.0256 0.0216 0.0149

0.87 0.87 0.82

0.698 0.680 0.649

0.670 0.682 0.650

0.049 0.022 0.032

0.024 0.020 0.133

0.99 0.99 0.98

0.182 0.112 0.081

0.60 1.60 0.14

0.95 0.95 0.97

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499

Table 4 The values of the thermodynamic of adsorption at various temperatures

Fig. 7. Langmuir adsorption isotherms of Cd(II) ions onto ACOS2 at different temperatures (time: 1.5 h, solid/liquid: 1.0 g/50 ml).

log qe ¼ log K F þ

1 log C e : n

ð12Þ

The three equilibrium curves that were obtained at the three temperatures in this study are well represented by the Freundlich isotherm model (Fig. 6) When the Langmuir isotherm model was applied to these data, a very good fit was obtained at these temperatures (Fig. 7). In general, this model is also applicable to describe the experimental equilibrium data for all temperature values. The Langmuir constant ym and K and Freundlich constant KF and 1/n at various temperatures are displayed in Table 3. K is the Langmuir adsorption constant (L/mol) and related to the free energy of adsorption. It is obvious that the parameters K and KF, which are related to the sorption capacity, decrease with increasing the temperature values. This is consistent with the experimental observation. r2 values (>0.90) show that the Langmuir and Freundlich isotherm models can adequately describe the adsorption data. 3.7. Thermodynamic study The thermodynamic parameters including change in the Gibbs free energy (DG0), enthalpy (DH0), and entropy (DS0) were determined by using following equations and represented in Table 3: CA KC ¼ ; CS DG0 ¼ RT ln K C ;

ð13Þ ð14Þ

0

ln K C ¼

DS DH 0  ; R RT

ð15Þ

where R is the gas constant, KC is the equilibrium constant, CA the amount of Cd(II) adsorbed on the adsorbent from the solution at equilibrium (mg/L), and CS is the equilib-

Temperature (K)

KC

DG0 (kJ/mol)

DH0 (kJ/mol)

DS0 (J/mol K)

293 303 313

36.45 10.44 4.00

8.76 5.91 3.61

92.31 75.72 –

285.29 231.02 –

rium concentration of Cd(II) in the solution (mg/L). The qe,exp of the pseudo-second-order model in Table 2 was used to obtain CA and CS. It was given the plot of ln Kc versus 1/T to Eq. (15). DH0 and DS0 was calculated from this plot (Van’t Hoff plots). The results are given in Table 4. Generally, the change of free energy for physisorption is between 20 and 0 kJ/mol, however, chemisorption is a range of 80 to 400 kJ/mol (Atkins, 1990). The overall free energy change during the adsorption process was negative for the experimental range of temperatures (see Table 4), corresponding to a spontaneous physical process of Cd(II) adsorption and that the system does not gain energy from an external source. When the temperature decreases from 313 to 293 K, the magnitude of free energy change shifts to high negative value (from 3.61 to 8.76 kJ/ mol) suggested that the adsorption was more spontaneous at low temperature. The negative value of the enthalpy change (92.31 and 75.72 kJ/mol) shows the adsorption to be exothermic. This can be explained by the fact that Cd(II) ions are bound to the active areas of ACOS2 surface, such as hydroxyl, carboxyl, methoxyl and phenolic groups via Vander Waals binding, causing displacement of H+ (Yadava ¨ zcan et al., et al., 1991; Acemog˘lu and Alma, 2004 and O 2006). The negative entropy change (DS0) value (for 293 K, 285.29 J mol1 K1 and for 303 K, 231.02 J mol1 K1, respectively) corresponds to a decrease in the degree of freedom of the adsorbed species. 4. Conclusions This study has demonstrated that activated carbons containing high surface area can be prepared from the chemical activation of olive stone with ZnCl2 activating agent. Under the experimental conditions investigated, the best conditions for the production of high surface area activated carbon are; using 20% ZnCl2, smaller than 48 mesh particle size, carbonization time of 2 h and carbonization temperature of 650 C. At this optimal condition, the BET surface area was 790.25 m2/g (ACOS2). Scanning electron microscopy showed the development of pores after carbonization.

Table 3 Parameters of Langmuir and Freundlich isotherm models and the values of the thermodynamic of adsorption at various temperatures Temperature (K)

293 303 313

Langmuir constants

Freundlich constants

ym (mg/g)

K (1/mg)

r2

KF (mg/g) (1/g)1/n

n

r2

1.851 1.670 1.557

0.388 0.358 0.350

0.99 0.98 0.99

0.455 0.444 0.384

1.993 2.166 2.119

0.94 0.97 0.96

DG0 (kJ/mol)

DH0 (kJ/mol)

DS0 (J/mol K)

8.76 5.91 3.61

92.31 75.72 –

285.29 231.02 –

500

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