Adsorption of copper ion from its aqueous solution by a novel biosorbent Uncaria gambir: Equilibrium, kinetics, and thermodynamic studies

Chemical Engineering Journal 170 (2011) 145–153

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Adsorption of copper ion from its aqueous solution by a novel biosorbent Uncaria gambir: Equilibrium, kinetics, and thermodynamic studies K.S. Tong ∗ , M. Jain Kassim, A. Azraa Material and Corrosion Chemistry Laboratory, School of Chemical Sciences, Universiti Sains Malaysia, 11800 Penang, Malaysia

a r t i c l e

i n f o

Article history: Received 1 December 2010 Received in revised form 14 March 2011 Accepted 14 March 2011 Keywords: Biosorption Copper Pseudo-second order Langmuir isotherm Thermodynamic Gibbs free energy

a b s t r a c t In this paper, gambir was chemically modified into insoluble adsorbent (GA). The GA was characterized by FTIR, pHzpc , SEM, and BET. The effects of pH, adsorbent dosage, initial concentration of Cu2+ solution and contact time were studied. Batch experiments were performed with aqueous copper solutions of concentration 10 mg/L, at pH 5.0 and 0.30 g adsorbent. The experimental data was analyzed using pseudofirst order, pseudo-second order and Elovich kinetic models. The correlation coefficient calculated from pseudo-second order equation was higher than pseudo-first order and Elovich kinetic equations, indicating that equilibrium data fitted well with pseudo-second order model where adsorption process was chemisorptions. The adsorption equilibrium was well described by Langmuir isotherm model. The maximum adsorption capacity was found to be 9.950 mg/g at 333 K. The Gibbs free energy change, G◦ , values were negative indicating that the process of Cu2+ ions adsorbed onto GA was spontaneous. The positive values of H◦ and S◦ suggested that the process of adsorption was endothermic at high temperature. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The presence of toxic heavy metals in environment through industrial waste disposal is currently an important environmental concern. The removal of heavy metal ions from drinking water is an essential process because they are able to accumulate in living tissues causing various diseases, not biodegradable and present harmful effect to ecosystem. Copper is one of the most toxic heavy metal to living organisms and one of the most widespread heavy metal in the environment [1,2]. Copper is a very common substance that widely used in many industries. The potential sources of copper ion in industrial effluents include metal cleaning, plating bath, paper board, mining, anti-fouling for paint and pigment, fertilizer, wood pulp, etc. [3,4]. The effluents usually contained high concentration of copper ion, which has been reported that excessive intake of copper ions in human body may leads to several mucosal irritation, hepatic and renal damage, liver and brain damages, widespread capillary damages, central nervous problems followed by depression, gastrointestinal irritation and possible necrotic changes in the liver and kidney [4–6]. The World Health Organization recommended a maximum acceptable concentration of Cu2+ in drinking water is about 1.5 mg/L [7].

∗ Corresponding author. Tel.: +60 4 653 4023; fax: +60 4 6574854. E-mail address: [email protected] (K.S. Tong). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.03.044

There are several physicochemical methods available for removal of heavy metal ions concentration in industrial wastewater, including chemical precipitation, ion exchange, reverse osmosis, filtration, solvent extraction and electro deposition. However, some of these methods have disadvantage such as ineffective removal of low concentration of metal ions (10 mg/dm3 ), disposal of toxic sludge and less applicable to a wide range of pollutants. Adsorption process by using activated carbon is considerable the most effective technology for removal of heavy metal ions from aqueous solutions. However, activated carbon is still considered an expensive adsorbent due to the high cost of regeneration and is not eco-friendly [8–11]. This situation has led many researchers to extensively investigates for the use of cheap, easily available and eco-friendly adsorbent which might capable to remove significant quantities of heavy metal ions from industrial wastewater. Agricultural wastes and biomass such as grape stalks [2], peanut hull [4], sawdust [5,6], areca nut [8], algae [9], palm kernel fiber [12], banana stem [13], carrot residues [14], cassava waste [15], potato peels [16], papaya wood [17], sugar beet pulp [18], cocoa shells [19], groundnut shells [20], chestnut shells [21,22], mushroom biomass [23], camphor leaves [24], teak leaves [25], rubber leaves [26], leaves of saltbush [27] and dried sunflower leaves [28] have been widely studied for copper adsorption from aqueous solution. Uncaria gambir, a native of Southeast Asia herbal plant, can be found mostly in Malaysia, Singapore and Indonesia. It had been widely used as an astringent medicine, for treatment of spongy gums, tooth care, diarrheas and sore throat. Besides for chewing it

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with betel, it also used for skin tanning, calico printing and textile dying purposes [29]. The aqueous extract of leaves and young twigs of Uncaria gambir or known as gambir showed that it consists of flavanol monomers (+)-catechin and (+)-epicatechin, several other dimeric compounds related to (+)-catechin, as well as alkaloid [30]. Since the polyphenolic compounds are capable of absorbing significant quantities of metal ions, thus it might be possible that gambit to be used as adsorbent for removal of metal ions from aqueous solution. In this paper, gambir was chemically treated to produce gambir adsorbent (GA) and investigated its adsorption kinetics and thermodynamic for removal of copper ions from aqueous solutions. SEM and FTIR analysis were carried out to understand the surface and functional group of GA. The effects of pH, adsorbent dosage mass, and initial copper concentration were studied. Four kinetic models including pseudo-first and second order, Elvoich and intraparticle diffusion were used to describe the adsorption process of copper ion. The Langmuir, Freundlich and Temkin isotherms models were applied in order to evaluate the discrepancy between the experimental data with the theoretical equilibrium capacity which calculated from the kinetic equation. Adsorption thermodynamic parameters were also calculated and discussed. 2. Experimental 2.1. Materials and chemicals Gambir used in this study was obtained from Medan, Indonesia. The gambir was ground into powder form and extracted with 70% aqueous acetone under maceration condition for 1 h [31]. The 70% aqueous acetone was concentrated and dried in oven for 48 h. The content of polyphenolic compounds in extract gambir was determined by Folin–Cioculteau assay [32], the total flavanoid compounds was determined by Prussian blue assay [33]. The percentage of polyphenolic compounds and total flavanoid was determined from the standard calibration curve of catechin. The content of catechin was analyzed by High Performance Liquid Chromatography (HPLC). All chemicals used in this study were analytical grade. 2.2. Preparation of gambir adsorbent (GA) Ten grams of gambir extract was dissolved in 20 mL of 0.225 M NaOH solution at room temperature. To the solution, 20 mL 37% (w/w) formaldehyde was added as the cross linking agent and stirred to ensure uniform mixing. The polymerization or gelation process was took place in 80 ◦ C water bath for 4 h. The gel that obtained was ground into powder form. After that, 100 mL of 0.10 M HNO3 was added and stirred for 60 min to recover the untreated NaOH [34]. The mixture was then filtered with Buchner funnel and filter paper (Advantec No. 1) and rinsed with distilled water until the pH of filtrate was neutral. Finally, the gel was dried in oven to form insoluble gambir adsorbent (GA). The GA was sieved to obtain a size fraction of 250 ␮m using ASTM Standard Sieve No. 60. The morphology and surface structure of GA was analyzed by Leo Supra 50VP Field Emission Scanning Electron Microscope. The Brunauer–Emmett–Teller (BET) method [21] was applied for the determination of the surface area values of GA. The BET isotherm plot which is used to determine the total surface area of GA was obtained by using the adsorbed gas versus gas pressure data. The specific surface area was calculated by dividing the total surface are to GA weight. 2.3. Point zero charge The point zero charge (pHpzc ) of the GA was determined by solid addition method [35]. 50 mL of known concentration NaCl

solution was transferred into a series of 100 mL conical flasks. The pH initial (pHi ) value of these solutions were recorded and adjusted from pH 2 to 10 by adding either 0.1 M HCl or 0.1 M NaOH. After that, 0.15 g of GA was added into each flask and then securely capped immediately. These suspensions were shaken at 150 rpm for 24 h. These suspensions were filtered and the pH of filtrates was recorded. The difference between initial pH value and final pH value was plotted against the initial pH. The point of intersection from the curve at which pH = zero gave the pHpzc value. 2.4. Adsorption equilibrium studies The stock solution of Cu2+ was prepared by dissolving CuSO4 ·5H2 O (Merck Co.) in deionized water to the concentration of 1 g/L. The experiment solutions were prepared by diluting the copper stock solution in accurate proportions to needed initial concentrations. The adsorption experiments were conducted in 250 mL borosil conical flask with 50 mL of standard solution. The solution was shaken at 150 rpm for 180 min. After that, the filtrate was analyzed by atomic absorption spectrophotometer (Perkin Elmer 3100 Model) at wavelength of 325 nm. Each experiment was duplicated under identical conditions. The effect of pH on the Cu2+ ions adsorption was studied over the pH range from pH 2.0 to pH 8.0 [5]. The adsorbent dosage mass added was 0.30 g and the initial concentration of Cu2+ was 10 mg/L. The pH of Cu2+ solution was adjusted by using either 0.1 M HCl or 0.1 M NaOH. The effect of adsorbent dosage mass was conducted by different adsorbent dosage mass ranging from 0.05 to 1.50 g with Cu2+ solution (10 mg/L, pH 5.0) at 150 rpm for 180 min. The adsorption capacity of the adsorbent at equilibrium was calculated using the equation: qe =

(C0 − Ce )V m

(1)

where C0 and Ce (mg/L) are the initial and equilibrium concentration of metal ion solution, respectively. V (L) is the volume of adsorbate in liter and m (g) is the amount of adsorbent in grams. The percentage adsorption was determined using the equation: % adsorption =

C0 − Ce × 100 C0

(2)

where C0 (mg/L) is the initial concentration of the metal ion solution and Ce (mg/L) is the final equilibrium concentration of the metal ion solution. Adsorption equilibrium experiments were conducted under the optimum conditions of pH 5.0, and 0.30 g of adsorbent. For kinetic studied, 50 mL of different initial concentration Cu2+ solution (10, 30, 50, 100, 150 mg/L) with 0.30 g adsorbent were shaken at 150 rpm for a varied contact time in a range of 5–180 min at room temperature. For thermodynamic studied, 0.30 g of adsorbent with 50 mL of different initial concentration of Cu2+ solutions were shaken for 180 min in a range of temperature from 303 K to 343 K. 2.5. FTIR analysis The surface functional groups of GA before and after adsorption were analyzed by Fourier Transform Infrared Spectrophotometer, Shimazdu 2000. Potassium bromide disks were prepared by mixing 1 mg of the samples with 200 mg KBr, and the spectra were recorded from 4000 cm−1 to 400 cm−1 .

K.S. Tong et al. / Chemical Engineering Journal 170 (2011) 145–153

a

147

100

Adsorption %

80

60

40

20

0 0

2

4

6

8

10

pH

b Fig. 1. Typical SEM micrograph of GA.

6 5 4

ΔpH

3. Results and discussion

3

3.1. Gambir extraction and modification 2

The percentage yield of gambir extraction was 44.90%. The gambir extract contained 72.80% of polyphenolic compounds, 76.20% of total flavanoid compounds and 68.00% of catechin. The percentage yield of GA produced by chemically modified gambir extract was 99.6% which indicated that all polyphenolic components in gambir are converted into water insoluble GA.

1 0 0 -1

2

4

6

8

10

12

14

pHi

Fig. 2. (a) Effect of pH on sorption of Cu2+ onto GA (initial concentration of solution = 10 mg/L, adsorbent dosage = 0.20 g, temperature = 30 ◦ C, contact time = 180 min, agitation rate = 150 rpm), (b) point zero charge of GA.

3.2. Physical characteristic of GA Fig. 1 shows the SEM micrographs of GA sample. It is clear that GA has considerable numbers of heterogeneous layers of rounded and pores where there is a good possibility for copper ions to be adsorbed. The multipoint BET surface area analysis of GA was performed with Quantachrome Nova 2200e and the result is given in Table 1. The surface area of GA was found to be 0.97 m2 /g. The mesopore area for GA was 0.97 m2 /g, implying that the surface area of GA was dominated by mesopore structures. 3.3. Effect of pH The effect of pH solution is an important variable in the adsorption process. The influence of pH in the initial solution of Cu2+ was examined at a different pH ranging from 2.0 to 8.0. Results are shown in Fig. 2a. As seen in Fig. 2a, the Cu2+ adsorption was constant at pH 2–3. It can be seen that Cu2+ uptake increased significantly from 48.30% to 81.70% at pH ranging 3.5–5.0. A similar observation was reported for adsorption of Cu2+ on grape stalks wastes [2], chitosan–cellulose hydrogel beads [3] and dried sunflower leaves [28]. Low pH value of Cu2+ solution contains high concentration of H+ ions that may compete effectively with Cu2+ ions for exchange-

Table 1 The BET characterization of GA. Characterization

GA

Surface area (BET), m2 /g Micropore area, m2 /g Mesopore area, m2 /g Total pore volume, cm3 /g Average pore diameter, Å

0.97 0 0.97 4.05E−03 1.67E+02

able cations on the surface active sites of GA [24]. This could be also due to the protonation of the functional groups on adsorbent surface, inducing an electrostatic repulsion of Cu2+ ions and the surface active sites of GA [36]. At pH 5.0, Cu2+ solution consists of free Cu2+ ions that are mainly involved in adsorption process causing an increase in the amount of Cu2+ adsorbed. From Fig. 2a, it was observed that the adsorption percentage of Cu2+ increased significantly from pH 6.0 to pH 8.0. This phenomenon is mainly caused by the presence of three ion species in the solution as suggested by Larous et al. [5] that the Cu2+ ions were presence in a very small quantity whereas Cu(OH)+ and Cu(OH)2 in a large quantities. Therefore, at higher pH (>pH 6.0), Cu2+ are attached by the hydroxide ions to form Cu(OH)2 and the phenolic hydroxyl groups of GA would more readily to be oxidized, making it impractical to apply on adsorption process [36]. The pH influences not only on the speciation of metal ions, but also the charges on the active sites of GA. By changing the pH of metal ions solution, the functional group of adsorbent such as hydroxyl groups also will be changed. To verify the effect of pH on adsorbent, it is essential to determine the pH point of zero charge (pHpzc ) of GA. The value of pHpzc was found to be 3.9 (Fig. 2b). When the pH of Cu2+ solution lower than pHpzc , the hydroxyl groups of adsorbent will be protonated, thus GA surface behaves as a positively charged polymatrix. It will attract the negatively charged ions that presence in the solution. Otherwise, the hydroxyl groups will be deprotonated and act as negative species; it binds with positive metal ions [37,38]. Based on effect of the Cu(OH)2 precipitation and pHpzc of GA, pH 5.0 was selected as the optimal pH condition.

148

K.S. Tong et al. / Chemical Engineering Journal 170 (2011) 145–153 25

5

100

4.5

3.5

80

3 2.5

70 Adsorption %

qe

2

60

1.5 1

50

10 mg/L

30 mg/L

50 mg/L

100 mg/L

150 mg/L

20

qt, mg/g

4

Adosrption capacity qe, mg/g

Adsorption %

90

15

10

5

0.5 0

0

40 0

0.25

0.5

0.75

1

1.25

1.5

1.75

0

50

100

t, min

Adsorbent dosage, g Fig. 3. Effect of adsorbent dosage on the adsorption capacity and percentage removal of Cu2+ ions (initial pH of solution = 5.0, initial concentration of solution = 10 mg/L, temperature = 30 ◦ C, contact time = 180 min, agitation rate = 150 rpm).

3.4. Effect of adsorbent dosage The effect of GA dosage on Cu2+ ions adsorption was studied and the results are shown in Fig. 3. Based on Fig. 3, it was observed that the removal of Cu2+ increased rapidly from 45.20% to 83.78% with an increase of adsorbent dosage from 0.05 to 0.30 g. This is because by increasing the adsorbent dosage, the total number of adsorption sites available for Cu2+ ions interaction increased as well. Further increase in GA dosage mass from 0.50 to 1.50 g seemed not affect the sorption in any wise. However, the adsorption capacity of Cu2+ is observed to be decreased with an increase of adsorbent dosage. When the adsorbent dosage mass increased from 0.05 g to 0.30 g, the loading capacity of GA decreased from 4.52 mg/g to 0.30 mg/g. This phenomenon may be due to two reasons, first is an increasing of adsorbent amount at constant Cu2+ ions concentration and volume, may cause unsaturation of the adsorption sites and secondly due to the particulate interaction such as aggregation resulting from high adsorbent dosage. Moreover, the adsorbent surface and metal ions have been reached to an equilibrium point which no further Cu2+ ion will be adsorbed [10,39]. The adsorbent dosage mass was fixed at 0.30 g for further equilibrium and kinetic studies. 3.5. The effect of Cu2+ initial concentration and contact time The influence of contact time on the adsorption of Cu2+ onto GA was investigated at various initial concentrations of Cu2+ , 10–150 mg/L is shown in Fig. 4. It can be seen that by increasing the initial concentration of Cu2+ led to an increase of adsorption capacity, qt at various contact time. This may be attributed by the initial concentration which provides an important driving force to overcome all mass transfer resistances of Cu2+ ions between the aqueous and solid phase, hence a higher initial concentration of Cu2+ ions will enhance the adsorption process [40]. The equilibrium sorption capacity of the GA increased with an increase initial Cu2+ concentration, while the percentage removal of Cu2+ showed the opposite trend. When the initial concentration increased from 10 to 150 mg/L, the loading capacity of GA increased from 1.44 to 19.99 mg/g and the percentage removal decreased from 86.40% to 79.90%. A similar observation was reported for the adsorption of copper on hazelnut shell activated carbon [38]. At low initial concentrations (10–50 mg/L), the Cu2+ uptake reached equilibrium faster at 50 min. This is because the ratio of Cu2+ ions to the number of available adsorption sites is small and consequently, less competition among the Cu2+ ions and binding sites [41]. Meanwhile, the adsorption capacity increased rapidly

150

200

Fig. 4. Effect of contact time for the adsorption of Cu2+ ions onto GA at various initial concentrations (initial pH of solution = 5.0, adsorbent dosage = 0.30 g, temperature = 30 ◦ C, agitation rate = 150 rpm).

during the first 15 min at high initial concentrations (100 mg/L and 150 mg/L). After 15 min, the adsorption capacity increased slowly and the adsorption equilibrium was established at 90 min. This strongly suggests that adsorption of Cu2+ onto GA was fast and significantly practical important, as it will facilitate the use of small adsorbent volumes in order to ensure efficiency and economy [23]. 3.6. Adsorption kinetics Four kinetic models: pseudo-first order, pseudo-second order, Elovich and intraparticle diffusion were applied to the experimental date and thus elucidated the kinetic adsorption process. The pseudo-first order model of Lagergren [42] is based on solid capacity and can be expressed in a linear form as follows: log(qe − qt ) = log qe −

k1 t 2.303

(3)

where k1 (1/min) is the rate constant of pseudo-first order adsorption, qe and qt (mg/g) are the amount of the metal ion adsorbed per gram of GA at equilibrium and at any time t, respectively. The values of k1 and qe for Cu2+ adsorption by GA were determined from the plot of log (qe − qt ) against t (Fig. 5a). The pseudo-second order kinetic model is based on the assumption that the limiting rate of metal ion adsorption may due to chemisorption involving valence forces through sharing electrons or exchange of electrons between the hydroxyl groups and metal ions [43,44]. The pseudo-second order kinetics can be expressed in a linear form as [45,46]: t 1 1 = + t qt qe k2 q2e

(4)

where k2 (g/mg min) is the rate constant of pseudo-second order adsorption. The plot of t/qt versus t is shown in Fig. 5b. The values of k2 and qe can be calculated from the slope and intercept of the plot in Fig. 5b. The adsorption experiments data were analyzed by using the Elovich equation [47–49] which is in linear form: qe =

1 1 ln(˛ˇ) + ln t ˇ ˇ

(5)

where ˛ (mg/g min) is the initial rate constant and the parameter ˇ (g/mg) is related to the extent of surface coverage and activation energy of chemisorptions. Fig. 5c shows a linear plot of Elovich equation for the same experimental data. The values of correlation coefficients calculated from pseudo-first order, pseudo second order and Elovich equation are given in Table 2.

K.S. Tong et al. / Chemical Engineering Journal 170 (2011) 145–153

149

Table 2 A comparison of pseudo-first order, pseudo-second order, and Elovich kinetic models rate constants and calculated equilibrium from experimental data. C0 (mg/L)

Kinetic models

10 qt exp (mg/g) Pseudo-first order qe cal (mg/g) k1 (1/min) R2 Pseudo-second order qe cal (mg/g) k2 (g/mg min) R2 Elovich equation ˇ (g/mmol) ˛ (mmol/g min) R2

100

150

1.441

30 4.300

6.180

14.750

19.989

0.728 0.041 0.908

3.095 0.039 0.923

3.211 0.022 0.931

9.576 0.031 0.953

15.650 0.032 0.950

1.508 0.102 0.999

4.543 0.0227 0.999

6.519 0.0136 0.999

15.576 0.0059 0.996

21.786 0.0030 0.999

28.91 6.94 × 1018 0.979

17.48 4.95 × 1010 0.910

268.80 1.40 × 1040 0.869

50

84.84 4.26 × 1024 0.951

As seen in Table 2, the correlation coefficients of pseudo-first order kinetic were calculated to be in a range of 0.908–0.953 and it shows the applicability of pseudo-first order kinetic model on the experimental data. However, the values of qe for pseudo-first order are not in an agreement with the experimental qe values. This indicates that pseudo-first order equation might not be sufficient to describe the interaction between Cu2+ ion-adsorbent. From Table 2, the lower correlation coefficients (R2 < 0.979) show poor agreement of Elovich kinetic model with the experimental data. Thus, pseudo-first and Elovich kinetic model are not suggested to use for describing the kinetic process of Cu2+ adsorption. As seen in Table 2, the correlation coefficient R2 for pseudo-second order kinetic is greater than 0.99 and its calculated qe values agree with the experimental values, qe . This confirms that the adsorption data are well represented by the pseudo-second order kinetic model. From Table 2, it was observed that the pseudo-second order kinetic rate constants decreased with the increase of Cu2+ initial concentrations. This is because of less competition of sorption surface active sites at lower concentration. At higher concentration, the competition for the adsorption active sites will be increased and consequently the adsorption rate will become slower. A similar result was reported for the adsorption of Cu2+ on chitosan-coated sand [1], sugar beet pulp [18], Cinnamomum camphora leaves [24] and hazelnut shell activated carbon [38]. The kinetics results were analyzed by the intraparticle diffusion model in order to study the steps of diffusion mechanisms. The adsorption process on a porous adsorbent will generally have multi-step process. These steps involve the transport of the adsorbate from the bulk solution, film diffusion, intraparticle diffusion in the pores and in the solid phase and finally adsorption on the sites [50]. The intraparticle diffusion equation can be written by the following equation [38,39]: qt = kint t 1/2

(6)

where kint is the intraparticle diffusion rate constant (mg/g min1/2 ) that can determine from the straight line plots of qt against t1/2 . According to Eq. (6), if the plots give a straight line, then the adsorp-

55.35 1.12 × 1013 0.957

tion is controlled solely by the intraparticle diffusion but if the plots show multi-linear plots, such plots indicate that two or more steps take place. The first sharper portion is the external surface adsorption or the boundary layer diffusion of solute molecule or ions. The second portion is gradual adsorption stage, where intraparticle diffusion is rate-controlled. The final portion is the equilibrium adsorption stage where intraparticle diffusion starts to slow down due to low adsorbate concentrations [11,24,50]. The rate of uptake might be limited by the size of adsorbate molecule or ions, concentration of the adsorbate, diffusion coefficient of the adsorbate in the bulk phase, the pore size distribution of the adsorbent, and degree of mixing [51,52]. Fig. 5d shows the present of external surface adsorption (stage 1). For the first sharper portion, from 3 min to 10 min, it postulated that Cu2+ ion was transported to the external surface of the adsorbent through film diffusion. Table 3 shows the rate constant, kint,1 , was found to be in ranging 0.136–3.601 mg/g min, which means the adsorption process of Cu2+ onto GA is very fast. The second stage of intraparticle diffusion rate controlling is attained after 10 min until 50 min, kint,2 (0.160–0.782 mg/g min). Finally, equilibrium adsorption stage exists after 50 min which is the final equilibrium stage where intraparticle diffusion starts to slow down due to the extremely low solute concentration in solutions. As seen from Fig. 5d, the plots are not linear over whole time range, implying that more than one process affected the adsorption. 3.7. Adsorption isotherms Adsorption isotherm is important for the description of how molecules or ions of adsorbate interact with the adsorbent surface active sites and also, is critical in optimizing the use of adsorbent. Three isotherm models, Langmuir, Freundlich, and Temkin were applied to the experimental data in the present study. The Langmuir isotherm is based on the assumption that all adsorption sites are equivalent and adsorption in an active sites is independent of whether the adjacent active sites is occupied or not. The energy of the adsorption is constant and there is no transmigration of adsorbate in the plane of surface [16]. The linear form

Table 3 Intraparticle diffusion kinetic rate constants for adsorption of Cu2+ onto GA with various initial concentrations. C0 (mg/L)

10 30 50 100 150

Intraparticle diffusion equation kint,1 (mg/g min1/2 )

R2

kint,2 (mg/g min1/2 )

R2

kint,3 (mg/g min1/2 )

R2

0.136 0.572 1.034 2.944 3.601

0.997 0.985 0.916 0.888 0.832

0.160 0.419 0.528 0.587 0.782

0.787 0.900 0.979 0.845 0.983

0.010 0.095 0.140 0.452 0.453

0.882 0.838 0.938 0.838 0.855

150

K.S. Tong et al. / Chemical Engineering Journal 170 (2011) 145–153

1.5

10 mg/L

1

30 mg/L

50 mg/L

100 mg/L

30

10 mg/L

(a) 25

0 - 0.5 0

20

40

60

80

100

120

140

- 1

Ce/qt, L/mg

log(qe-qt)

0.5

- 1.5

(a)

-2

20 303 K

15

313 K 10

323 K

5

333 K 343 K

0

- 2.5

0

50

100 Ce, mg/L

t, min 140

100 mg/L

60

150 mg/L

40

0.8 log qe

t/qt , g min/mg

50 mg/L

80

(b)

1

30 mg/L

100

200

1.2

10 mg/L

120

150

(b)

0.6

303 K 313 K

0.4

323 K

20

0.2

333 K

0 0

50

100 t, min

150

343 K

0

200

0

0.5

1

10 mg/L 20

30 mg/L 50 mg/L

15

100 mg/L

qe, mg/g

qt, mg/g

1.5

2

2.5

log Ce

25

150 mg/L

10 5

(c)

0 0

1

2

3 ln t

4

5

6

10 9 8 7 6 5 4 3 2 1 0

(c)

303 K 313 K 323 K 333 K 343 K 0

1

2

25 10 mg/L

4

5

6

Fig. 6. Isotherm models plot: (a) Langmuir, (b) Freundlich, and (c) Temkin for the adsorption of Cu2+ onto GA at various temperature.

30 mg/L

20

3 ln Ce

qt, mg/g

50 mg/L 100 mg/L

15

150 mg/L 10

(d)

5 0 0

5

10

15

t 0.5, min

log qe = log KF +

Fig. 5. Kinetics plot: (a) pseudo-first order, (b) pseudo-second order, (c) Elovich, and (d) intraparticle for adsorption of Cu2+ onto GA at various initial concentrations.

of the Langmuir isotherm [53] equation is given by the following equation: 1 Ce Ce = + qe Q0 Q0 b

monolayer adsorption capacity and b (L/mg) is a constant related to free energy of adsorption. The linear plot of specific adsorption (Ce /qe ) against the equilibrium concentration (Ce ) is shown in Fig. 6a. The Langmuir constants Q0 and b were determined from the slope and intercept of the plot and are presented in Table 4. The Freundlich expression is an empirical model based on adsorption onto a heterogeneous surface. The linearized form of the Freundlich isotherm [54] is expressed as:

(7)

where qe (mg/g) and Ce (mg/L) are the amount of solute adsorbed per unit weight of adsorbent and the equilibrium concentration of solute in the bulk solution, respectively. Q0 (mg/g) is the maximum

1 n

log Ce

(8)

where KF (mg/g (L/mg)1/n ) is the constant indicative of the relative adsorption capacity of the bonding energy and n related to the adsorption capacity and heterogeneity of the adsorption surface sites. The magnitude of the exponent, 1/n, gives an indication of the favorability of adsorption. Values of n > 1 represent favorable adsorption condition. The constants KF and n were calculated from the plot of log qe versus log Ce (Fig. 6b). The equilibrium sorption data has been analyzed by Temkin isotherm model which is contained a factor that takes into consideration of the adsorbate–adsorbent interactions. Temkin isotherm

K.S. Tong et al. / Chemical Engineering Journal 170 (2011) 145–153 Table 5 Adsorption capacity of Cu2+ by various adsorbents.

Langmuir isotherm Q0 (mg/g) b (L/g) R2 Freundlich isotherm KF (mg/g) n R2 Temkin isotherm bT (kJ/mol) AT (dm3 /mol) R2

T (K) 303

313

323

333

343

6.650 0.044 0.978

7.090 0.045 0.985

9.390 0.041 0.981

9.950 0.064 0.996

8.950 0.053 0.983

1.080 2.970 0.981

1.040 2.750 0.983

1.060 2.350 0.982

1.350 2.400 0.954

1.240 2.540 0.976

2.354 1.106 0.914

2.176 0.926 0.930

1.607 0.695 0.935

1.501 0.950 0.986

1.808 0.935 0.954

equation is given by the following [55]: qe = BT

ln AT + BT

ln Ce

(9)

where BT =

RT bT

(10)

T is the absolute temperature in K and R is the universal gas constant (J/K mol). The constant bT is related to the heat of adsorption, AT (L/mol) is equilibrium binding constant corresponding to the maximum binding energy. Therefore, by plotting qe against ln Ce enables one to determine the constants AT and bT as shown in Fig. 6c. The constants AT and bT are listed in Table 4. As seen in Table 4, the adsorption of Cu2+ onto GA at different temperatures is well correlated (0.978–0.996) by Langmuir isotherm model compare with Freundlich and Temkin isotherms which give lower correlation coefficients. The values n for experimental data were found to be 2.970, 2.750, 2.350, 2.400, and 2.540, respectively. In average, the values of Freundlich constant n between 1 and 10 tend to have a favorable adsorption process (see Table 4). The fitness of the adsorption equilibrium data on Langmuir isotherm implying that all the adsorption active sites are equivalent and the surface is uniformed. Therefore, the adsorbed Cu2+ do not interact or compete with each other and the equilibrium is established where a monolayer is formed at the adsorbent. From Table 3, the maximum adsorption capacity (Q0 ) calculated from Langmuir isotherm equation defines the total capacity of the adsorbent. The adsorption capacity increased from 6.650 mg/g to 9.950 mg/g with an increasing of temperature from 303 K to 333 K. The Q0 values increased with the increasing of temperature reveals that the adsorption process was favorable in high temperature and endothermic in nature, while the opposite trend indicates the process release energy (exothermic). Several studies have been conducted using various types of adsorbents for Cu2+ adsorption. Table 5 presents a comparison of the adsorption capacity of the results. 3.8. Adsorption thermodynamics Thermodynamic parameters such as enthalpy change (H◦ ), entropy change (S◦ ), and Gibbs free energy change (G◦ ) must take into consideration in order to determine the spontaneous of a process. Standard Gibbs free energy is calculated by the following equation: ◦

G = −RT ln b

(11)

where b (L/mol) is the equilibrium constant obtained from Langmuir model, T (K) is the absolute temperature and R (8.314 J/mol K) is the gas universal constant. The relationship between Gibbs

Adsorbents

Q0 (mg/g)

Reference

Sugar beet pulp Dried sunflower leaves Chlorella vulgaris Grape seed activated carbon Carrot residues Peanut hull Papaya wood Tectona grandis L.F. leaves powder Hevea brasiliensis rubber leaves powder Atriplex canescens leaves of saltbush Chestnut shell Atriplex canescens flowers of saltbush Cotton boll Grape stalks wastes Uncaria gambir Dye loaded sawdust Dye loaded groundnut shells Atriplex canescens stems of saltbush Cassava waste Potato peel

119.43 89.37 58.80 48.78 32.74 21.25 19.88 15.43 14.97 13.50 12.56 12.40 11.40 10.10 9.95 8.07 7.60 7.40 0.95 0.39

[18] [28] [9] [21] [14] [4] [17] [25] [26] [27] [22] [27] [40] [2] This study [20] [20] [27] [15] [16]

Table 6 Thermodynamic constants for adsorption of Cu2+ onto GA. T (K)

b (L/mol)

G◦ (kJ/mol)

H◦ (kJ/mol)

S◦ (kJ/mol)

303 313 323 333 343

2805.58 2844.78 2607.97 4066.94 3367.94

−20.00 −20.69 −21.12 −23.00 −23.16

6.28

0.086

free energy change, entropy change, and enthalpy change can be expressed as: G◦ = H ◦ − TS ◦

(12)

Fig. 7 shows the plot of Gibbs free energy change versus temperature and the thermodynamic parameters S◦ and H◦ were calculated from the slope and intercept of the plot and are summarized in Table 6. The Gibbs free energy change address the possibility and feasibility of a certain reaction which more negative values reflect a more energetically favorable adsorption process [56]. As seen in Table 6, the negative values of G◦ indicated that the adsorption process of Cu2+ onto GA was spontaneous in nature. By rising the temperature, the G◦ values become more negative led to a conclusion that the adsorption process was more favorable at high temperature. H◦ gave a positive value (6.28 kJ/mol), implying that endothermic process occurred during adsorption of Cu2+ . Positive -19.5 300 -20

310

320

330

340

350

-20.5

DG 0 (kJ/mol)

Table 4 Isotherm constants for adsorption of Cu2+ onto GA at various temperatures. Isotherm equation

151

-21 -21.5 -22 -22.5 -23 -23.5

T (K) ◦

Fig. 7. Gibbs free energy change, G , calculated from Langmuir constant, KL versus the reaction temperature.

152

K.S. Tong et al. / Chemical Engineering Journal 170 (2011) 145–153

GA

1257 687

3014 2858

604

873

1403 1104 1451

2925

1048 821

%T

1608

1492

GA--Cu

683

1257 1399

2858 3432

4000.

1492

2925

300

1451

604

873

1104

1048

821

1608

150

200

100

50

cm-1 Fig. 8. FTIR spectra of GA and Cu2+ ions loaded onto GA.

value of S◦ (0.086 kJ/mol) denoted the increase in randomness at the solid-solution interface during adsorption resulting in the irreversibility of the process. 3.9. FTIR studies Fourier transform infrared spectrum analysis is to identify the functional groups of GA which are responsible to the interaction of metal ions. Fig. 8 represented the comparison of FTIR spectrums of GA samples before and after Cu2+ adsorption. The broad peak in the region of 3500–3200 cm−1 is the characterization of –OH stretching in the phenolic and methylol groups. The peak at 2925 cm−1 in both spectrums is related to the aromatic C–H stretching vibrations [57]. Those peaks at 1608 cm−1 and 1451 cm−1 are related to the aromatic –C C– bonds. Also, the formation of –CH2 –O–CH2 – linkage is appeared at 1085–1104 cm−1 [11,57]. The peak at 1451 cm−1 is the formation of methylene bridges from the reaction of phenolic groups with formaldehyde [58]. As comparison between the GA and GA–Cu spectrums, the intensity of the –OH stretching vibration at the wavenumber 3432 cm−1 decreased due to the Cu2+ ions adsorption. Chen et al. [24] also summarized that decreased of intensity at peak 3413 cm−1 indicated that complexation process occurred is considered the formation of surface complexes. The intensity of –C–O benzene ring stretching at 1048 cm−1 was observed to be decreased due to the formation of complex between atom oxygen and Cu2+ . The intensity of catechol peak at 1492 cm−1 was significantly decreased which predicted the phenol degradation to other products [59]. 4. Conclusion This study is revealed that gambir extract can be used as an alternative adsorbent for heavy metal ions removal in industrial wastewater due to its efficiency of Cu2+ adsorption in aqueous solution. The adsorption of Cu2+ onto GA was affected by pH, adsorbent dosage, and temperature. The Cu2+ uptake percentage by GA was found to be 83.78% when 0.30 g of adsorbent was agitated with 50 mL of Cu2+ solution of 10 mg/L for 180 min at pH 5.0. The adsorption data was fitted well by pseudo-second order kinetic indicating that chemical reaction is involved in the adsorption process. The adsorption process was found to be controlled by three steps of diffusion mechanisms. The temperature equilibrium data fitted

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