Adsorption of Cu(II) Ions onto Myristica Fragrans Shell-based Activated Carbon: Isotherm, Kinetic and Thermodynamic Studies

The MFS waste was collected from a Myristica Fragrans oil refinery in Tapaktuan, in the district of South Aceh, Aceh Province. The South Aceh Myristica Fragrans plantations covers an area of approximately 14,000 ha with the dried MF production being approximately 4,600. Materials preparation in the previous studies^28,29 was taken into account. The 1.5 kg MFS was rinsed using tap water, and then it was dried in an oven (Memmert: NN-ST342M, Germany) at 110 °C for 24 h. The Cu(II) ions aqueous solution was made by dissolving 4 g (0.001) of CuSO4·5H₂O (99% pure, from Aldrich) in 2000 mL distilled water in 2000-mL Erlenmeyer flask. The sample of 2 mL was taken, it was diluted with 10 mL distilled water in 15-mL vial for Atomic Absorption Spectrophotometer (AAS) analysis. The concentration of Cu(II) ions in the stock solution was calculated based on the dilution factor of 6. The predetermined Cu(II) ions concentration for all the adsorption experiments was prepared using the stock solution and general dilution formula.

The dried MFS was milled to powder using a rice mill,²⁹ and then it was sieved to get the size in the range of 80-100 mesh. A tube furnace (TF-80/120/160, HumanLab Inc. 300-1500 °C, Korea) was used for pyrolysis of the dried MFS powder. The tube furnace heating rate was at 45 °C/min, and it took 12.75 min to get the TF final temperature of 600 °C (1 °C) for pyrolysis with nitrogen gas pumped at 5 ml/min with the initial TF temperature of 30 °C. The physical activation took place at 600 °C (1 °C) for 1 h. After the TF was turned off, the MFS carbon was left in the TF over night to gradually decrease the MFS carbon temperature. Chemical activation of the MFS carbon was conducted for 6 h in 250-mL beaker glass consisting of 200 ml NaOH solution at 0.1 M (prepared using 97% NaOH, Merk). It was stirred at 75-rpm using a hotplate stirrer (IKA, type C-MAG HS 7) at 30 °C. The MFS activated carbon (MFS AC) was washed using distilled water and filterred using vacuum filter. Washing and filterring took many times until the waste water reaching neutral pH of 7.¹⁸ The chemical activation was separately repeated to prepare the MFS AC with 0.3 M NaOH and the MFS AC with 0.5 M NaOH. The two types of MFS AC were separately dried in porcelain cups using an oven drier (Memmert, type NN-ST342M, Western Germany) at 110 °C for 2 h to remove the remain water. The MFS AC were stored in different sealed bottles in a desiccator for adsorption experiments within 2 weeks. The FTIR analysis was conducted using a spectrophotometer (Shimadzu IR Prestige 21, Kyoto, Japan). The SEM analysis was done using JOEL (type JSM-6360LV, Tokyo, Japan) with 240VA (single-phase, 60 Hz) of accelerating voltage was used. The BET surface area and total pore volume of the MFS AC were measured using Nova Station A (Quantachrome Instruments version 11.0, USA) with 0.0633 g of the MFS AC. Nitrogen was degassed with the pressure tolerance of 0.100/0.100 (adsorption/desorption) at 200 °C for 4.0 h.

Adsorption experiments of Cu(II) ions were conducted in batch mode. The system of the MFS AC-solution consisted of 0.5 g of the MFS AC and 100 mL of Cu(II) ions solution in 200-mL beaker glass, and it was stirred at 75-rpm and 1 atm. The Cu(II) ions adsorption capacity of the MFS AC over independent variables was investigated by varying the contact time (0–80 min), NaOH concentration for chemical activation (0.1–0.5 M), adsorbate concentration (6.465–645.540 mg/L based on the AAS reading), initial pH of Cu(II) ions solution (3–6) and adsorption temperature (30–60 °C). The optimum condition with the highest Cu(II) ions adsorption capacity of the MFS AC was obtained.

Cu(II) solution samples of 2-mL at the contact time 0, 10, 20, 40, 60, 90 and 120 min (in time series) were analyzed using the AAS based on the procedure in the previous study.¹⁸ The Cu(II) ions adsorption isotherm, kinetic and thermodynamic studies were preformed using the experimental data.

To characterize the samples’ chemical functional groups, the FTIR transmission spectra of 400 to 4000 cm⁻¹ were obtained. Seven major bands^18,29,31 are shown in Fig. 1. A band with the peak at approximately 3649.32 cm⁻¹ refers to hydroxyl functional groups (O–H stretch) at 3500-3700 cm⁻¹ of wavenumber. A band with 2 peaks at 2833.43 and 2922.15 cm⁻¹ is assigned to aldehyde and alkanes (C–H stretch), respectively at 2695-3000 cm⁻¹. A wide band has 2 peaks at 1708.93 and 1741.72 cm⁻¹ refers to carbonyls (C=O stretch) at 1655-1760 cm⁻¹. A weak band with multiple peaks at 1367.53, 1427.32 and 1452.41 cm⁻¹ is attributed to alkanes (C–H bend and C–H rock) at 1350-1470 cm⁻¹. There is also a weak band with multiple peaks at 1039.63, 1122.57, 1238.31 and 1267.23 cm⁻¹ is assigned to alcohols, carboxylic acids, ester and ethers at 1000-1320 cm⁻¹ (C-O stretch). A band with multiple peaks at 700.16, 823.6 and 896.9 cm⁻¹ refers to alkenes (=C–H bend) at 650-1000 cm⁻¹. The last band with 2 peaks at 516.92 and 580.57 cm⁻¹ is for C–C stretch^18,32 at 400-700 cm⁻¹.

Overall, pyrolysis followed by chemical activation especially at 0.5 M of NaOH released all the volatile matters of the MFS AC except alcohols, carboxylic acids, ester and ethers (C-O stretch) and C–C stretch, as shown by the bottom curve in Fig. 1. More volatile matters being released would produce more potential pores and adsorption surface area leading to higher capacity of the MFS AC.

Figure1.

The FTIR spectra of the MFS powder and the MFS AC.

The transmittance of C–C stretch of the MFS AC with 0.5 M of NaOH was higher than the one with 0.1 M NaOH. The transmittance of other chemical functional groups of the MFS AC with 0.5 M of NaOH was lower than the one with 0.1 M NaOH. These conditions would confirm that physical adsorption was dominant rather than chemical adsorption.

Fig. 2(a), (b) and (c) show the surface morphology of the MFS powder and MFS AC by the SEM analysis. As can be seen in Fig. 2, the MFS AC with 0.1 M NaOH and the MFS AC with 0.5 M NaOH almost had the same surface morphology with the irreglar pores and unclear pores mouth. In other words, the cross section area of pores could not be clearly seen. On the other hand, the cross section area of pores could be clearly seen by the MFS AC with 0.5 M NaOH. It can be observed in Fig. 2 that the MFS AC with 0.5 M NaOH had deeper the pores, compared to the others. It could result in the larger adsorption surface area of the MFS AC with 0.5 M NaOH compared to other. The result of SEM analysis was in line with the result of FTIR analysis where the more volatile matters being released would produce the more potential pores and surface area. This prediction was also supported by the result of BET analysis in which the total surface area of the MFS AC for 0.1, 0.3 and 0.5 M NaOH was approximately 13.26, 71.11 and 99.85 m²/g, respectively. The total pore volume was 0.045, 0.084 and 0.086 cc/g, respectively with the average pore radius being 28.91, 31.55 and 31.61 Å.

Figure2.

The SEM micrographs of surface of (a) 0.1 M NaOH MFS AC (b) 0.3 M NaOH MFS AC (c) 0.5 M NaOH MFS AC.

Effect of contact time on Cu(II) ions adsorption capacity of the MFS AC is shown in Fig. 3. The three kinds of the MFS AC has 2 steps of exponential increase. As revealed by the MFS AC with 0.1 and 0.3 M NaOH, it sharply increased in the first 10 min, slowly increased from 10 to 40 min, and gradually increased from 40 to 60 min. It was almost steady until 80-min contact time. The same trend was also shown by the curve of the MFS AC with 0.5 M NaOH, but the second step increase of the MFS AC with 0.5 M NaOH was shapper compared to the one with 0.1 and 0.3 M NaOH. The equilibrium time chosen was at 80-min contact time, and Cu(II) ions adsorption capacity of the MFS AC was approximately 42.81, 47.26 and 64.58 mg/g for 0.1, 0.3 and 0.5 M NaOH, respectively.

Figure3.

Effect of contact time on Cu(II) ions adsorption capacity on the MFS AC. The MFS AC-solution system: 100 mL Cu(II) ions aqueous solution at pH 4.5, initial Cu(II) ions concentration at 645.540 mg/L, 1 g of MFS AC (0.1, 0.3 and 0.5 M NaOH), 75-rpm magnetic stirring, 1 atm and 30 °C.

Effect of initial Cu(II) ions concentration on Cu(II) ions adsorption capacity of the MFS AC is clearly shown in Fig. 4. It increased a bit sharp from 0.93 to 5.12 mg/g with the increase of initial Cu(II) ions concentration from 6.455 to 32.277 mg/L respectively. It also gradually increased to 9.27, 16.96, 36.27 and 64.58 mg/g when the initial Cu(II) ions concentration increased to 64.554, 129.108, 322.771 and 645.540 mg/L respectively at pH 4 and 30 °C of the MFS AC-solution system. The same trend of increase of Cu(II) ions adsorption capacity over initial Cu(II) ions concentration also occured at 45 and 60 °C. The trend was almost linear because the active sites on the MFS AC migh not been saturation at 645.540 mg/L of the initial Cu(II) ions concentration. The linear trend was also reported in the previous study.³³ The result showed that the MFS AC could be a promising activted carbon for Cu(II) ions adsorption.

Figure4.

Effect of initial Cu(II) ions concentration on Cu(II) ions adsorption capacity of the MFS AC. The MFS AC-solution system: 100 mL Cu(II) ions aqueous solution at pH 4.5, initial Cu(II) ions concentration at 6.465–645.540 mg/L, 1 g of MFS AC (0.5 M NaOH), 75-rpm magnetic stirring, 1 atm and 30–60 °C.

In general, the maximum Cu(II) ions adsorption capacity of adsorbents was obtained in acid condition of adsorbate solution.^16,23,25,34 Therefore, the initial pH range of 3–6 was chosen for the MFS AC-solution system. The initial pH of system was adjusted by dropping 0.01 M HCl and 0.01 M NaOH. As clearly shown in Fig. 5, the Cu(II) ions adsorption capacity of the MFS AC at 3, 4.5 and 6 was 24.06, 64.58 and 5.42 mg/g, respectively, and the maximum one was at pH 4.5.

Figure5.

Effect of initial pH on Cu(II) ions adsorption capacity of the MFS AC. The MFS AC-solution system: 100 mL Cu(II) ions aqueous solution at pH 3–6, initial Cu(II) ions concentration at 645.540 mg/L, 1 g of MFS AC (0.5 M NaOH), 75-rpm magnetic stirring, 1 atm and 30 °C.

The common adsorption isotherm equations of Langmuir³⁵ and Freundlich³⁶ were used to obtain overall maximum Cu(II) ions adsorption capacity of the MFS AC and related parameters. The linearized Langmuir isotherm is expressed as:^{17,18,28,29,37,38}

where C_e (mg/L) denotes as Cu(II) ions concentration in aqueous solution at equilibrium time, q_e (mg/g) is the C_e based adsorption capacity, q_m (mg/g) represents the Langmuir based adsorption capacity, and K_L (L/mg) refers to the Langmuir constant. The values of K_L and q_m are calculated using the slope and intercept of equation (1) (C_e / q_e versus C_e). The adsorption nature and adsorption isotherm type are obtained by R_L value whereas R_L (1+ K_LC_o) = 1 with C_o (mg/L) being the higest initial Cu(II) ions concentration in aqueous solution. R_L = 0, 0 < R_L <1, R_L = 1, R_L > 1, or between, represents Cu(II) ions adsorption of the MFS AC being irreversible, favorable, linear, or unfavorable, respectively.⁴⁶ The linearized Freundlich isotherm is:^{17,18,28,29,37,38}

where K_F (L/g) represents the adsorption capacity based on Freundlich, and 1/n is the adsorption intensity. The K_F and 1/n values are worked out using the slope and intercept of equation (2) (log q_e versus log C_e).

As shown by the R² values in Fig. 6, Freundlich adsorption isotherm (FAI) fitted vey well compared to the Langmuir adsorption isotherm. Physical adsorption might fully control Cu(II) ions adsorption onto the MFSAC at the temperature being more than 30 °C. This view was also support by the FAI K_F values which decreased from 0.845 to 0.663 and 0.209 L/g for the temperature increase from 30 to 45 and 60 °C, respectively. Adsoprtion capacity should increase as the result of temperature increase if chemical adsorption occured. In fact, adsoprtion capacity decreased with the increase in temperature, as shown in Fig. 4.

Figure6.

Plots of linearized adsorption isotherm based on (a) Langmuir and (b) Freundlich for Cu(II) ions adsorption of the MFS AC. Experimental condition: 100 mL Cu(II) ions aqueous solution at pH 4.5, initial Cu(II) ions concentration at 6.465–645.540 mg/L, 1 g of MFS AC (0.5 M NaOH), 75-rpm magnetic stirring, 1 atm and 30–60 °C.

The decline in R_L values from 0.133 to 0.269 and 0.524 with the rising temperature from 30 to 45 and 60 °C, respectively, clearly denoted that the Cu(II) ions adsorption process was unfavourable at higher temperatures. However, chemical adsorption might partially occur at 30 °C, since the R² value (0.913) of Langmuir plot was still high and almost the same as the Freundlich one (0.946). The Langmuir based adsorption capacity, q_m obtained was 93.47 mg/g which 44.71% higher than the q_m value (64.58 mg/g) shown in Fig. 4 which was calculated using the AAS reading for initial Cu(II) ions concentration being at 645.540 mg/L. The Langmuir based q_m value at 45 and 60 °C which was 72.99 and 88.49 mg/g, respectively, cannot be presented because it did not fit well (R² = 0.591 and 0.139, respectively). Therefore, the q_m value at 45 and 60 °C was 43.33 and 28.71 mg/g, respectively based on the AAS reading viewed in Fig. 4. The Cu(II) ions adsorption capacity by the MFS AC at 30 °C was higher than by activated carbon prepared from watermelon (31.03 mg/g),¹⁶ biomass gasification (27.03 mg/g),²⁵ Tunisian date stones (31.25 mg/g),²³ hazelnut husks (6.65 mg/g), hazelnut shell (6.65 mg/g),¹⁹ Australian pine cone (26.71 mg/g),²⁹ apricot stone (22.80 mg/g)³⁹ and rubber wood sawdust (5.72 mg/g).⁴⁰

The linearised pseudo first order of adsorption kinetic (PFOAK) by Lagergren⁴³ and the linearised pseudo second order of adsorption kinetic by Ho et al.⁴⁴ (PSOAK) are expressed as follows: ^{17,18,28,29,37,38}

where q_t (mg/g) is the Cu(II) ions adsorption capacity (q) at the adsorption time, t (min), q_e (mg/g) represents the q value at the equilibrium time. The k_L (/min) denotes as the PFOAK rate constant, and k_H (g/mg·min) is the PSOAK rate constant.

As as the result, the PFOAK fitted better and more favourable for Cu(II) ions adsorption of the MFS AC, as can be seen in Fig. 7. The PFOAK equilibrium adsorption capacity of the MFS AC for 0.1, 0.3 and 0.5 M NaOH was approximately 48.98, 56.88 and 107.65 mg/g, respectively. The equilibrium adsorption capacity which increased with the increase in activator concentration was reasonable as highlighted in the previous discussion of the FTIR and SEM. The more volatile matters being released at 0.5 M NaOH showed by decreasing the transmittance of chemical functional groups could result in the more potential pores and adsorption surface area leading to higher capacity of the MFS AC. Meanwhile, the MFS AC rate constant was 0.046, 0.052 and 0.053 /min for the MFS AC with 0.1, 0.3 and 0.5 M NaOH, respectively.

Figure7.

The PFOAK (a) and PSOAK (b) for Cu(II) adsorption by the MFS AC. Experimental condition: 100 mL Cu(II) aqueous solution at neutral pH, the predetermined initial Cu(II) ions concentration of 517.914 mg/L, 1 g of the 5 and 15 g NaOH MFS AC, 75 rpm magnetic stirring, 1 atm at different temperature of 30 and 45 °C.

Thermodynamic expression of equation (5) of the van’t Hoff linear form (6)^18,38,41 and (7)^18,41,42 were taken into account to work out the change of enthalphy (∆H⁰, J/mol), free energy (∆G⁰, J/mol), and entropy (∆S⁰, J/mol.K) for Cu(II) ions adsorption onto the MFS AC:

where the distribution coefficient, K_d (L/mg) equals q_e/C_e, and R represents the gas constant which is 8.314 J/mol K. The thermodynamic parameters of T₁, T₂, T₃ and C_e1,C_e2, C_e3, q_e1, q_e2, q_e3, K_d1 K_d2, and K_d3 which were taken from Langmuir adsorption isotherm plot (Fig. 6(a)) were 300.15 K (30 °C), 318.15 K (45 °C), 333.15 K (60 °C), 322.62 mg/L, 428.89 mg/L, 501.98 mg/L, 64.58 mg/g, 43.33 mg/g, 28.71 mg/g, 0.200 L/g, 0.101 L/g and 0.057 L/g, respectively.

As shown in Fig. 8, the trendline equation was ln k_d = 4220.315/T-15.539, and ∆H⁰ was approximately -35.087 kJ/mol. The negative sign of ∆H⁰ confirmed exothermic nature of the Cu(II) ions adsorption onto the MFS AC. The increasing the temperature from 30 to 60 °C resulted in the decreasing Cu(II) ions adsorption capacity from 64.58 to 28.71 mg/g was not relevant to Le Chatelier’s theory for chemical adsorption.⁴³ It means that physical adsorption might fully control Cu(II) ions adsorption onto the MFSAC, as discussed in the previous result of adsorption isotherm. The ∆G⁰ values worked out was approximately 4014.102 and 6063.537 kJ/mol at 300.15 and 318.15 K, respectively. The positive sign of ∆G⁰ value indicated the Cu(II) ions adsorption onto the MFS AC non-spontaneous nature of process.⁴² The ∆S⁰ value was -129.345 kJ/mol, and the negative sign of ∆S⁰ value corresponded to a decrease in the freedom degree of the Cu(II) ions adsorption.^44,45,46

Figure8.

The relationship between 1/T and ln K_d for Cu(II) ions adsorption capacity of the MFS AC. The MFS AC-solution system: 100 mL Cu(II) ions aqueous solution at pH 4.5, initial Cu(II) ions concentration at 6.465–645.540 mg/L, 1 g of MFS AC (0.5 M NaOH), 75-rpm magnetic stirring, 1 atm and 30–60 °C.