Adsorption of Pb(II) Ions from Aqueous Solution Using Activated Carbon Prepared from Areca Catechu Shell: Kinetic, Isotherm and Thermodynamic Studies

The ACS waste was collected from the areca catechu processing in the plantations of Simeulue district, Aceh Province. The 2 kg ACS was rinsed thoroughly using distilled water, and then it was dried at temperature of 30 °C (± 2 °C) under the sun for 3 days with relative humidity being 12% on average. The stock Pb(II) aqueous solution of 517.914 mg/L was prepared using Pb(NO₃)₂ (99% pure from 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 general dilution formula for batch studies of Pb(II) ions adsorption.

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 800 °C (±2 °C) for 3 h. The pyrolisis and physical 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 decreasing 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 activation. It was stirred at 75-rpm stirring speed (IKA Hotplate Stirrer C-MAG HS 7) at room temperature (27 °C, ± 2 °C) 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 105 °C (± 2 °C) 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 Spectrophotometer (Shimadzu Co. Kyoto, Japan) groups on the ACS powder and the ACS AC.³² To investigate surface micrographs of the ACS powder and the ACS AC, Scanning Electron Microscopy (SEM) (Hitachi-TM3000, made in Japan) with 500VA (1 phase 50/60Hz) of accelerating voltage was used.³²

Pb(II) ions adsorption experiments were carried out in batch mode by contacting 1 g of the ACS AC with 100 mL of Pb(II) solutions in 200-mL erlenmeyer flask at 100-rpm stirring speed, 1 atm and neutral pH (6.5 ± 0.1). The effect of independent variables on the adsorption capacity of Pb(II) ions by the ACS AC was investigated by manipulating 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 (27–45 °C). Wide range of adsorbate 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 independent 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 filters. 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.

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

Figure1.

The FT-IR spectra of the ACS powder and the ACS AC.

Overall, the FT-IR analysis confirmed that pyrolysis followed by NaOH chemical activation can release the volatile componeds of hydrogen functional groups, aldehyde, aromatic rings and carboxyl acids, which would result in more potential pores and surface leading to higher capacity of the ACS AC adsorbent. Moreover, the presence of hydroxyl functional group left on the ACS AC might promote adsorption of Pb(II) ions, as addressed in the previous studies.^38,39,40

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 followed by NaOH activation produced completely irregular and uneven pores. As expected in the FT-IR analysis, volatile matter could be released from the ACS powder creating cavities.

Figure2.

The SEM micrographs of surface of (a) the ACS powder, (b) the 5 g NaOH ACS AC.

The adsorption trend of Pb(II) ions on adsorbent over contact time could be varied with different adsorbent. The equilibrium time was attained at about 120-min contact time for tartaric acid modified rice husk,¹⁹ 3-h and 20-min for activated carbon utilised from Militia ferruginea plant leaves⁴¹ and heartwood powder of Areca catechu,⁴² respectively. As can be seen in Fig. 3, the Pb(II) adsorption on the 15 g NaOH ACS AC increased exponentially over contact time reaching the equilibrium at 120-min contact time with the Pb(II) adsorption capacity being 49.48 mg/g. The same trend was also reported for Pb(II) adsorption on activated carbon from Militia ferruginea plant leaves.⁴¹ Based on the equilibrium time, Pb(II) adsorption kinetic of the ACS AC was investigated.

Figure3.

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 concentration being 517,914 mg/L, 1 g of the 15 g NaOH ACS AC, 75 rpm magnetic stirring, 1 atm and room temperature of 27 °C (± 2 °C).

As clearly shown in Fig. 4, the Pb(II) adsorption capacity 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 efficiency increased gradually from 0.72 to 15.32%, respectively. 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) adsorption efficiency was 95.56% at the initial Pb(II) ions concentration being 517.91 mg/L, and the available active site on the ACS AC was very limited leading to the saturation of active sites on the ACS AC.

Figure4.

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 initial 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 temperature of 27 °C (± 2 °C).

The linearised forms of pseudo first-order kinetic model (PFOKM) by Lagergren⁴³ and pseudo second-order kinetic model⁴⁴ (PSOKM) resulted in equations (1) and (2), respectively.^30,32,45

where q_t (mg/g) presents the Pb(II) adsorption capacity at the time of t (min), q_e (mg/g) denoates as the equilibrium adsorption capacity, k_L (/min) is the pseudo first-order adsorption rate constant, and k_H (g/mg.min) is known as the PSOKM adsorption rate constant.

The PSOKM gave the best fit for the three runs experimental 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.

Figure5.

The PFOKM (a) and PSOKM (b) for Pb(II) adsorption 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 temperature of 27 and 45 °C (± 2 °C).

The PSOKM based equilibrium adsorption capacity for the 5 g NaOH ACS AC and the 15 g NaOH ACS AC at 27 °C calculated was approximately 49.26 and 49.50 mg/g, respectively. The increase in NaOH concentration by 10 g resulted in only 0.49% increase of equilibrium adsorption capacity. Meanwhile, it was approximately 50.51 mg/g for the Pb(II) adsorption using the 15 g NaOH ACS AC at 45 °C meaning that temperature also influenced weakly equilibrium adsorption capacity. It was only by approximately 2.04% increase for increasing the temperature of adsorption from 27 to 45 °C. In contrast, the PSOKM rate constant improved by 55.24% from 0.111 to 0.173 g/mg.min indicating 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.

The maximum Pb(II) adsorption capacity by the ACS AC was obtained using the Langmuir equation,⁴⁸ and the linear form^30,32,39,45 is given as:

where C_e (mg/L) is the equilibrium Pb(II) concentration in aqueous solution, q_e (mg/g) represents the equilibrium adsorption capacity, q_m (mg/g) is the Langmuir mono-layer adsorption capacity, and K_L (L/mg) is the Langmuir constant. The values of K_L and q_m were obtained using the slope, the intercept, 1/(q_m K_L) and 1/q_m of the straight line, C_e /q_e versus C_e. The nature of Pb(II) adsorption onto the ACS AC and the type of adsorption isotherm can be evaluted using the expression of R_L = 1/(1 +K_LC_o) wherein C_o (mg/L) is the higest initial Pb(II) ions concentration in solution. The R_L 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 irreversible, unfavorable, linear, or favorable, respectively.⁴⁶ Meanwhile, the Freundlich equation,⁴⁷ and the linear form^30,32,39,45 is expressed as:

where K_F (L/g) is the Freundlich constant presenting adsorption capacity, and 1/n denotes as the adsorption intensity. The values of K_F and 1/n are obtained using with the intercept, log K_F and the slope, 1/n of the straight line of log q_e versus log C_e.

Investigation on isotherm adsorption of Pb(II) concluded that Langmuir model of Pb(II) adsorption isotherm provided 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 Langmuir constant being 0.58 L/mg when the adsorption temperature increased to 45 °C. Meanwhile, adsorption temperature obviously affected the Pb(II) adsorption capacity, and it increased by 44.34% from 0.40 and 0.58 L/mg. The R_L values obtained was in the range of 0.003-0.005 which also confirmed that Langmuir isotherm was realiable model to present the Pb(II) adsorption onto the ACS AC, and it was favourable adsorption of Pb(II).

Figure6.

Langmuir (a) and Freundlich (b) adsorption isotherm models for Pb(II) adsorption by the ACS AC. Experimental condition: 100 mL Pb(II) aqueous solution at neutral pH, the predetermined 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 stirring, 1 atm at the temperatur of 27 and 45 °C (± 2 °C).

In addition, the AC and TS listed in Table 1 stand for activated 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.

Thermodynamic equations were used to calculate enthalphy, 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

where ∆H⁰ (J/mol) is enthalphy change in the Pb(II) adsorption process at the temperature T (K) with the distribution coefficient K_d (L/mg) defined as q_e/C_e) where q_e and C_e are the adsorbate equilibrium adsorption capacity and concentration in solution, respectively at the maximum initial Pb(II) ions concentration of Langmuir plot; ∆G⁰ (J/mol) is free energy change at the T and K_d; 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, q_e and C_e1, C_e2, q_e1, q_e2, K_d1 and K_d2 were taken from Langmuir adsorption isotherm in the previous discussion, which was 300.15 K (27 °C), 318.15 K (45 °C), 26.043 mg/L, 16.944 mg/L, 49.187 mg/g, 50.097 mg/g, 1.888 L/g and 2.956 L/g, respectively. As the result, the trendline form obtained was ln k_d = 8.56–2377.59/T, and ∆H⁰ obtained was about 19.77 kJ/mol. The positive value of ∆H⁰ indicated endothermic nature of the Pb(II) adsorption due to the entrophy increase as the adsoption driving force.^55,56 The increase in the temperature from 27 to 45 °C causing the increase in the Langmuir-based Pb(II) adsorption capacity from 54.95 to 55.25 mg/g was also relevant to Le Chatelier’s principle for chemical adsorption,⁵⁷ and the activation energy should be in positive sign⁵⁸ and can be calculated using the slope of linear plot 1/T versus ln q_m as given:

where q_mi (mg/g) represent the temperature independent Langmuir-based Pb(II) adsorption capacity. The value of q_m1 and q_m2 was was 54.95 to 55.25 mg/g at the temperature T₁ dan T₂ (K) being 300.15 and 318.15 K, respectively. As the result, the trendline form obtained was ln q_m = 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