Printed in the Republic of Korea https://doi.org/10.5012/jkcs.2017.61.3.89
Adsorption of Pb(II) Ions from Aqueous Solution Using Activated Carbon Prepared from Areca Catechu Shell: Kinetic, Isotherm
and Thermodynamic Studies
A. Muslim*, S. Aprilia, T. A. Suha, and Z. Fitri
Process Technology Laboratory, Department of Chemical Engineering, Faculty of Engineering, Syiah Kuala University, Banda Aceh, Indonesia.
*E-mail: abrar.muslim@che.unsyiah.ac.id (Received January 22, 2017; Accepted April 14, 2017)
ABSTRACT. This study proposed adsorption of Pb(II) ions from aqueous solution using activated carbon prepared from areca catechu shell (ACS AC) using Timphan Method. The effects of independent variables on adsorption kinetic and iso- therm have been investigated by conducting experiments in batch mode at neutral pH. The structural characterization of adsor- bent was done by FT-IR and SEM analysis. The Pb(II) adsorption was correlated very well with the pseudo second-order kinetic (PSOKM) and Langmuir isotherm models (LIM). Increasing NaOH mass for activation and adsorption temperature increased weakly all the parameters of adsorption kinetic and isotherm. The Pb(II) ions adsorption capacity of the ACS AC at 27 and 45oC was 50.51 and 55.25 mg/g, respectively. Thermodynamic parameters were determined, and the results con- firmed the Pb(II) ions adsorption should be endothermic and spontaneous process, and both physical and chemical adsorption should be taken place.
Key words: Activated carbon, Areca catechu shell, Isotherm, Kinetic, Thermodynamic
INTRODUCTION
Most of non-biodegradable toxic pollutants which can be bio-accumulated in the humans and animals are waste products of heavy metal from uncontrollable mining, petro- leum refining, metallurgical processes,1,2 agricultural indus- tries3,4,5 and educational industries.6 Among others, lead is one of the most hazardous heavy metals for human, which can cause blood cancer, brain disorder,7,8 miscarriage in preg- nant women, damage the kidneys and death.9,10
Various adsorbent with low cost material had been investi- gated for the adsorption of Pb(II) ions. Adsorbent prepared from non-renewable adsorbents of zeolite,11 clay12 and phos- phate13,14 had been applied to remove Pb(II) ions from aque- ous solutions. Biological wastes based adsorbents using marine alga,15 oedogonium species, nostoc species,16 ulva lactuca and bacillus17 have been also proposed for the adsorp- tion of Pb(II) ions. Moreover, modified agriculture wastes such as rice husk,18,19 husk and maize cob,20 Garciana mangos- tana fruit shell21 have been investigated for the adsorption of heavy metals including Pb(II) ions.
The global consumption of activated carbon has been increasing to answer the global industrial and environmental protection. It is forecasted to reach approximately 1.7 million tons by the next two years. Therefore, effective and low
cost materials for activated carbon to remove heavy metal has been investigated but there are still limited studies on Pb(II) ions adsorption using activated carbon in the past 15 years. Activated carbon prepared from agriculture waste i.e. lignocellulosic residues,22 European black pine cone,23 sugar cane husk and sawdust,24,25 apricot stone,26 Bois carré seeds,27 sludge and sugarcane bagasse,28 and olive stones,29 have been proposed for Pb(II) adsorption. However, acti- vated carbon prepared from areca catechu shell has not been proposed for adsorption of Pb(II) ions in aqueous solu- tions, and it is necessary to study since areca catechu shell has potential pores and surface30 to increase by pyrolysis.
Meanwhile, according to the FAO, world production of areca catechu was approximately 1,023,050 tons in 2010 with the annual world production growth was expected following India, about 5%.31
The objective of this study is to prepare activated car- bon from areca catechu shell using Timphan Method, and to investigate the adsorption of Pb(II) ions from aqueous solutions by areca catechu shell activated carbon (ACS AC).
The effects of activator (NaOH) concentration and tem- perature were examined at neutral pH. The structural charac- terization of the raw ACS and the ACS AC was performed by Fourier Transform Infrared Spectroscopy and Scanning Electron Microscopy analyses, respectively. Adsorption
Aldrich). The Pb(II) concentration was obtained using Atomic Absorption Spectrophotometer (AAS, Shimadzu AA 6300, made in Japan). The predetermined concentration of Pb(II) was prepared using a variable volume pipette and gen- eral dilution formula for batch studies of Pb(II) ions adsorp- tion.
Activated Carbon Preparation
The dried ACS was milled to powder using a ball mill, and then sieved (80-100 mesh). The procedure of activated carbon preparation in the previous study32 was adopted with physical activation of in a furnace (Nabertherm, made in Germany) at 800oC (±2oC) for 3 h. The pyrolisis and phys- ical activation by coating the dried ACS using at least four layers of aluminum foil to limit oxygen diffusion is now presented as Timphan Method. The higher temperature of pyrolysis was taken into account to get better performance of carbon as reported in the literatures.33,34 After gradual decreas- ing to room temperature, the ACS carbon of 40 g was put in 500-mL beaker glass consisting of 250 mL distilled water and 5 g NaOH (97% pure, Merk) for chemical acti- vation. It was stirred at 75-rpm stirring speed (IKA Hot- plate Stirrer C-MAG HS 7) at room temperature (27oC,
± 2oC) for 3 h. The ACS activated carbon (ACS AC) was washed using distilled water many times until reaching neutral pH of 7 (± 0.2). The ACS AC was finally filtered using vacuum filter. The procedure of chemical activation was repeated separately for 10 and 15 g NaOH. The ACS AC with different NaOH concentration was dried separately in an oven drier (Memmert, NN-ST342M, made in Western Germany) at 105oC (± 2oC) for 3 h to remove the excess water. Each the ACS AC was stored in a sealed bottle to use within 3 weeks of adsorption experiments. The same procedure of Fourier Transform Infrared Spectroscopy (FT- IR) analysis using a Shimadzu IR Prestige 21 Spectro- photometer (Shimadzu Co. Kyoto, Japan) groups on the
ulating the contact time (0–120 min), NaOH mass of adsorbent activator (5–15 g), initial concentration of Pb(II) ions in aqueous solution (3.729-517,914 mg/L), and adsorption temperature (27–45oC). Wide range of adsor- bate initial concentration is possibly applied to obtain maximun adsorption capacity of activated carbon, and neutral pH of adsorption had been also applied by many authors due to environmental concern as reviewed in the previous study.35 The maximum adsorption capacity of Pb(II) ions by the ACS AC was obtained over the inde- pendent variables.
For each experiment of Pb(II) adsorption, 1-mL samples of Pb(II) solutions were taken using volume macropipette at 0, 10, 20, 40, 60, 90 and 120 min of contact time in series.
The stirring was stopped for 1 min prior to sampling. Each sample was diluted 10 times with distilled water in 20-mL vial to limit adsorbate lost when filtered using syringe fil- ters. The Pb(II) ions concentration was obtained using the AAS, calculated based on dilution factor, and an average value of the AAS reading was taken from experiments conducted in duplicate. The Pb(II) adsorption isotherm, kinetic and thermodynamic studies were conducted using the experimental data, and the parameters value were obtained based on adsorption kinetic, isotherm and thermodynamic equations.
RESULTS AND DISCUSSION
The ACS Powder and ACS AC Functional Groups The FT-IR transmission spectra ranging from 400 to 4000 cm−1 were obtained to characterize the the chemical functional groups on the ACS powder, and the ACS AC.
As shown in Fig. 1, there could be five major absorption bands can be observed from the three samples in overall.
The presence of an intense band at approximately 3647.68 cm−1 is assigned to O–H stretching of hydroxyl functional
groups at 3500−3700 cm−1 of wavenumber.36 A wide band with mutiple peaks at 2987.88, 2885.51 and 2821.86 cm−1 is attributed to C–H stretching at 2700-2980 cm−1. A strong band with two intense band at 1695.43 and 1519.91 cm−1 is ascribed to the C=C stretching of aromatic rings at 1500−1700 cm−1. A weak band of C–H asymmetrical and symmetrical stretching at 1317-1450 cm−1 has an intense band at 1429.25 cm−1. The last band with the band of 400- 700 cm−1 and the peak at 578.64 cm−1 refers to C–C.37
Overall, the FT-IR analysis confirmed that pyrolysis followed by NaOH chemical activation can release the volatile componeds of hydrogen functional groups, alde- hyde, aromatic rings and carboxyl acids, which would result in more potential pores and surface leading to higher capac- ity of the ACS AC adsorbent. Moreover, the presence of hydroxyl functional group left on the ACS AC might pro- mote adsorption of Pb(II) ions, as addressed in the pre- vious studies.38,39,40
The ACS Powder and ACS AC Surface Morphology Fig. 2(a) and (b) show the SEM images of the ACS powder and 15 g NaOH ACS AC, respectively. As can be observed in Fig. 2(a), all the irreglar pores are almost closed by round substances which could be the chemical functional groups of the ACS powder. It was also found on the surface of ACS raw material, but regular and smooth pores were found on the ACS raw material in the previous studies.30,39 It is clear to see that the effect of pyrolysis fol- lowed by NaOH activation produced completely irregu- lar and uneven pores. As expected in the FT-IR analysis, volatile matter could be released from the ACS powder creating cavities.
Effect of Contact Time on Adsorption Capacity The adsorption trend of Pb(II) ions on adsorbent over contact time could be varied with different adsorbent. The Figure 1. The FT-IR spectra of the ACS powder and the ACS AC.
Figure 2. The SEM micrographs of surface of (a) the ACS pow- der, (b) the 5 g NaOH ACS AC.
Figure 3. Pb(II) ions adsorption capacity over contact time.
Experimental condition: 100 mL Pb(II) ions aqueous solution at neutral pH with the predetermined initial Pb(II) ions concentra- tion being 517,914 mg/L, 1 g of the 15 g NaOH ACS AC, 75 rpm magnetic stirring, 1 atm and room temperature of 27oC (± 2oC).
Effect of Initial Pb(II) Ions Concentration
As clearly shown in Fig. 4, the Pb(II) adsorption capac- ity and efficiency were really affected by the initial Pb(II) ions concentration in solution. The Pb(II) adsorption capacity stepped up gradually from 0.37 to 7.93 mg/g for the increase in initial Pb(II) concentration from 3.72 to 79.54 mg/L, respectively, and the Pb(II) adsorption effi- ciency increased gradually from 0.72 to 15.32%, respec- tively. A very sharp increase in the Pb(II) adsorption capacity from 15.99 to 49.48 mg/g occurred when lifting up the initial Pb(II) concentration from 160.29 to 517.91 mg/L.
As seen in Fig. 4, the increase in Pb(II) adsorption capacity and efficiency could follow an exponential trend whereas it should increase slowly until reaching equilibrium when the initial Pb(II) concentration was increased to more than 517.91 mg/L. It could be reasonable because the Pb(II)
(2) where qt (mg/g) presents the Pb(II) adsorption capacity at the time of t (min), qe (mg/g) denoates as the equilibrium adsorption capacity, kL (/min) is the pseudo first-order adsorp-
qt ---- k---Hqe2
qe +----
=
Figure 4. Pb(II) ions adsorption capacity over initial Pb(II) ions concentration in solution. Experimental condition: 100 mL Pb(II) ions aqueous solution at neutral pH with the predetermined ini- tial Pb(II) ions concentration being 517,914 mg/L, 1 g of the 15 g NaOH ACS AC, 75 rpm magnetic stirring, 1 atm and room tem- perature of 27 oC (± 2oC).
Figure 5. The PFOKM (a) and PSOKM (b) for Pb(II) adsorp- tion by the ACS AC. Experimental condition: 100 mL Pb(II) aqueous solution at neutral pH, the predetermined initial Pb(II) ions concentration of 517.914 mg/L, 1 g of the 5 and 15 g NaOH ACS AC, 75 rpm magnetic stirring, 1 atm at different tempera- ture of 27 and 45oC (± 2oC).
tion rate constant, and kH (g/mg.min) is known as the PSOKM adsorption rate constant.
The PSOKM gave the best fit for the three runs exper- imental condition, as can be seen in Fig. 5. The R2 value of the pseudo second-order kinetic for each run was the same which approximately 0.99 showing that the PSOKM was more favourable for the Pb(II) adsorption kinetic by the ACS AC.
The PSOKM based equilibrium adsorption capacity for the 5 g NaOH ACS AC and the 15 g NaOH ACS AC at 27 oC calculated was approximately 49.26 and 49.50 mg/g, respec- tively. The increase in NaOH concentration by 10 g resulted in only 0.49% increase of equilibrium adsorption capac- ity. Meanwhile, it was approximately 50.51 mg/g for the Pb(II) adsorption using the 15 g NaOH ACS AC at 45oC meaning that temperature also influenced weakly equi- librium adsorption capacity. It was only by approximately 2.04% increase for increasing the temperature of adsorp- tion from 27 to 45oC. In contrast, the PSOKM rate constant improved by 55.24% from 0.111 to 0.173 g/mg.min indi- cating faster Pb(II) adsorption on the ACS AC occurred for the higher mass of NaOH. It was decreased by 18.76%
from 0.111 to 0.173 g/mg.min for the higher adsorption temperature.
Pb (II) Adsorption Isotherm of the ACS AC
The maximum Pb(II) adsorption capacity by the ACS AC was obtained using the Langmuir equation,48 and the linear form30,32,39,45 is given as:
(3) where Ce (mg/L) is the equilibrium Pb(II) concentration in aqueous solution, qe (mg/g) represents the equilibrium adsorp- tion capacity, qm (mg/g) is the Langmuir mono-layer adsorp- tion capacity, and KL (L/mg) is the Langmuir constant.
The values of KL and qm were obtained using the slope, the intercept, 1/(qm KL) and 1/qm of the straight line, Ce /qe versus Ce. The nature of Pb(II) adsorption onto the ACS AC and the type of adsorption isotherm can be evaluted using the expression of RL = 1/(1 + KLCo) wherein Co (mg/L) is the higest initial Pb(II) ions concentration in solution. The RL value being equal to 0, more than 0, equal to 1, or between 0 and 1, presents the Pb(II) adsorption on the ACS AC being irre- versible, unfavorable, linear, or favorable, respectively.46 Meanwhile, the Freundlich equation,47 and the linear form30,32,39,45 is expressed as:
(4)
where KF (L/g) is the Freundlich constant presenting adsorption capacity, and 1/n denotes as the adsorption inten- sity. The values of KF and 1/n are obtained using with the intercept, log KF and the slope, 1/n of the straight line of log qe versus log Ce.
Investigation on isotherm adsorption of Pb(II) concluded that Langmuir model of Pb(II) adsorption isotherm pro- vided the best fit with the R2 value being approximately 0.96 on the average, can be obtained in Fig. 6. As expected in the previous discussion of adsorption kinetic, NaOH mass and adsorption temperature influenced weakly adsorption capacity. The Langmuir adsorption capacity increased from approximately 54.35 to 54.95 mg/g for the increase in NaOH mass by 10 g with the Langmuir constant being 0.37 and 0.40 L/mg, respectively. It was 55.25 mg/g with the Lang- muir constant being 0.58 L/mg when the adsorption tem- perature increased to 45oC. Meanwhile, adsorption temperature Ce
qe --- 1
qmKL --- 1
qm ---Ce +
=
logqe 1
n---logCe+logKF
=
Figure 6. Langmuir (a) and Freundlich (b) adsorption isotherm models for Pb(II) adsorption by the ACS AC. Experimental con- dition: 100 mL Pb(II) aqueous solution at neutral pH, the prede- termined initial Pb(II) ions concentration being 3.729 to 517.914 mg/L, 1 g of the 5 and 15 g NaOH ACS AC, 75 rpm magnetic stir- ring, 1 atm at the temperatur of 27 and 45 oC (± 2oC).
obtained was in the range of 0.003-0.005 which also con- firmed that Langmuir isotherm was realiable model to present the Pb(II) adsorption onto the ACS AC, and it was favour- able adsorption of Pb(II).
In addition, the AC and TS listed in Table 1 stand for acti- vated carbon and this study, respectively; and the ACS, AS, CS, DP, EBPC, PH and PS stands for areca catechu shell, apricot stone, coconut shell, date pits, European black pine cones, peanut husks and palm shell, respectively. As listed in Table 1, the Pb(II) adsorption capacity by the ACS AC was higher than the one by the AS AC, DP AC, and EBPC AC, and it was a bit less than the one by the PH AC, CS AC and PS AC.
Pb (II) Adsorption Thermodynamics of the ACS AC Thermodynamic equations were used to calculate enthal- phy, free energy and entropy changes of Pb(II) adsorption onto the ACS AC are given as equations (5) of the van’t Hoff linear form,41,45 (6) and (7):45,54
(5) (6) (7) where ∆H0 (J/mol) is enthalphy change in the Pb(II) adsorption process at the temperature T (K) with the dis- tribution coefficient Kd (L/mg) defined as qe/Ce) where qe and Ce are the adsorbate equilibrium adsorption capacity and concentration in solution, respectively at the maxi- mum initial Pb(II) ions concentration of Langmuir plot;
∆G0 (J/mol) is free energy change at the T and Kd; and ∆S0 (J/mol K) denotes as entropy change at the temperature T (K); and R is the gas constant (8.314 J/mol K).
The value of thermodynamic parameters T1, T2, qe and
be in positive sign and can be calculated using the slope of linear plot 1/T versus ln qm as given:
(8) where qmi (mg/g) represent the temperature independent Langmuir-based Pb(II) adsorption capacity. The value of qm1 and qm2 was was 54.95 to 55.25 mg/g at the tempera- ture T1 dan T2 (K) being 300.15 and 318.15 K, respectively.
As the result, the trendline form obtained was ln qm= 4.10–
28.88/T, and the activation energy obtained from the slope was approximately 0.24 kJ/mol. Chemical adsorption mostly refers to monolayer adsorption of Langmuir. However, since the ∆H0 value was stil small which smaller than 50 kJ/mol, and the activation energy obtained being very small, chemical adsorption should not fully control Pb(II) ions adsorption, and physical adsorption should also control Pb(II) ions adsorption on the ACSAC. The ∆G0 values obtained using equation (6) was approximately -1.586 and -2.867 kJ/mol for the adsorption temparature of 300.15 and 318.15 K, respectively, and the negative sign of ∆G0 value confirms the Pb(II) adsorption onto the ACS AC spontaneous nature of process.54 Meanwhile, the ∆S0 value worked out using equations (5) and (7) was approximately 0.071 kJ/mol, and the positive sign of ∆S0 value corresponds to a increase in the degree of freedom of the Pb(II) adsorption.55,56
CONCLUSION
The adsorption of Pb(II) ions by areca catechu shell activated carbon (ACS AC) was studied by conduct experiments in batch mode at neutral pH. It followed the pseudo second- order kinetic (PSOKM) and Langmuir isotherm models (LIM). Increasing NaOH mass increase weakly all the PSOKM and LIM parameters, and the improvement was expected in the FT-IR and SEM analyses. Thermodynamic study Kd
ln ΔS
--- ΔR H ---RT –
=
ΔG0=–RT ln Kd ΔS ΔH0–ΔG0
---T
=
qm
ln ln qmi E R--- 1
T---
⎝ ⎠⎛ ⎞ –
=
confirmed that the Pb(II) adsorption by the ACS AC should be endothermic and spontaneous process, it should be controlled by both physical and chemical adsorption.
Acknowledgments. The authors would like to appre- ciate the Chemical Engineering Department and Mechan- ical Engineering Department at Syiah Kuala University for technical support in the experimental work. We wish to thank to Mathematics and Science Faculty at Syiah Kuala Uni- versity for sample analysis using an Atomic Absorption Spec- trometer Shimadzu AA 6300. Publication cost of this paper was supported by the Korean Chemical Society.
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