Chemicals

All chemicals employed in this study, including Na₂CO₃, RR 120, NaOH, and HCl, were Merck and of analytical grade with purity exceeding 99%.

RR 120 was with 99 % purity.

Making Activated Carbon from Durian Shell

Durian shells, upon procurement, underwent a rigorous cleansing procedure involving successive rinsing with tap water and distilled water to effectively eliminate any particulate matter. Subsequently, these cleaned durian shells were subjected to thermal treatment of 600ºC for 3 h, hereby referred to as DS for brevity. Then DS continued with chemical activation by Na₂CO₃ with mass ratio DS : Na₂CO₃ equal to 1:1, 2:1, and 3:2. It was then calcined at 600ºC for 3 h. After chemical activation, the resulting materials underwent a neutralization process using a hydrochloric acid (HCl) solution until the filtrate attained a state of neutrality. Subsequently, they were subjected to a drying procedure at a temperature of 110ºC for 24 h. The resulting product is designated as ACDSB. Finally, ACDSB was crushed and stored in glass vials for subsequent experiments.

Selection of Material Ratio DS : Na₂CO₃

Take 50 mL of RR 120 solution with a concentration of 47.85 mg/L (the concentration has been determined) and place it in a 100 mL conical flask containing 0.05 g of ACDSB material for the DS mass ratios : Na₂CO₃ equals 1:1, 2:1, 3:2, pH = 3. Shake in a shaker at 300 rpm at room temperature (25±10ºC) for 90 min. Then, centrifuge and determine the concentration of the active red 120 solution after adsorption. The results are shown in Table 1.

From Table 1, the adsorption efficiency of RR 120 clearly changes between the ratios of materials DS : Na₂CO₃. At a mass ratio of DS:Na₂CO₃ by 2:1, the adsorption efficiency of Active Red Food 120 reached the highest value of 90.18%. Therefore, the material DS : Na₂CO₃ with the ratio 2:1 (in terms of volume) was chosen to study and investigate the surface characteristics, structure, composition, specific surface area of the material, and adsorption capacity of RR 120 in aqueous medium.

Analysis of Physicochemical Properties of Activated Carbons DS and ACDSB

The fundamental characteristics of DS and ACDSB were investigated using a range of advanced equipment. SEM (transmission electron microscope) measurements were performed using the equipment JEM-2001F (Jeol, Tokyo, Japan). The EDX method, which determines sample composition, was performed on an MS-7001F device (Jeol, Tokyo, Japan). The FT-IR infrared spectrum was measured on a Shimadzu device made in Japan. The sample surface area was measured on a Tristar 3000 machine (Micromesitics). Crystallographic structures in the activated carbon sample were determined through X-ray diffraction (XRD) analysis utilizing a computer-controlled X-ray diffractometer (D2 Phraser; Brucker) fitted with a stepping motor and a graphite crystal monochromator.

The quantification of RR 120 concentrations both prior to and after adsorption was conducted utilizing a dual-beam UV-Vis 1700 (Shimadzu, Japan).

Experiment-Evaluated Factors Affecting the Adsorption Process

Solution pH, duration of exposure, mass of ACDSB, initial concentration of RR 120, and ambient temperature were systematically examined to assess the efficacy of RR 120 removal. The experimental procedure was replicated 3 times.

Study on the influence of pH: Add to each flask 0.05 g of ACDSB and 20 mL of RR 120 solution with the average initial concentration ranging from 60 mg/L to 70 mg/L (concentration has been determined exactly), whose pH varies from 3 to 10, stabilized by HNO₃ and NaOH solutions. Shake in a shaker for 90 min at 300 rpmat a temperature of 303 K. Then the solutions were centrifuged with a centrifuge at a speed of 4000 rpm for 30 min, using a micropipette to aspirate the solution after centrifugation and determine the concentration of RR 120 after adsorption.

The effect of time: Weigh 0.05 g of ACDSB into a 100 mL conical flask, then add the solution with an average concentration of 50-110 mg/L (concentration has been determined) to a 20 mL conical flask. The above solutions are kept stable at pH = 3. Shake for 5, 15, 30, 45, 60, and 90 min at 300 rpm at a temperature of 303 K.

Study on the influence of material weight: Weigh ACDSB into a 100 mL conical flask with a mass varying from 0.01 to 0.125g ACDSB, then add 20 mL of RR 120 solution with concentration in the flask. 54.26 mg/L (concentration has been determined). The above solutions are kept stable at pH = 3. Shake for 30 min at 300 rpm at 303 K.

The effect of temperature: 0.05 g of ACDSB was mixed with 20 mL of RR 120 solution with an average initial concentration of 104.45 mg/L in 100 mL Erlenmeyer flasks. Use KOH and HNO₃ to adjust the solution’s pH to pH 3. Heat the solutions on a heated magnetic stirrer for 5-45 min at a shaking speed of 300 rpm at the respective temperature values of 303 K, 313 K, and 323 K.

The effect of concentration: Add 0.05 g of ACDSB and 20 mL of RR 120 solution to each conical flask with an average initial concentration of 50-500 mg/L (concentration has been determined). The above solutions were kept stable at pH 3 with HNO₃ solution. Shake for 30 min at 300 rpm at 303 K.

The adsorption effectiveness of RR 120 on activated carbon (ACDSB) was computed using the ensuing formula (1):

Where H is adsorption efficiency, C₀ is the initial solution concentration (mg/L), and C_e is the solution concentration when reaching adsorption equilibrium (mg/L).

ACDSB material then determined some physical parameters, the isoelectric point, and studied isotherm adsorption models, including the Langmuir, Freundlich, Dubinin-Radushkevich (D-R), Tempkin, Elovich, adsorption kinetics, activation energy, and thermodynamics, according to Quoc Toan Tran et al. (2022) [30].

Study of the Physical Attributes and Surface Properties of Activated Carbons DS and ACDSB

SEM images depicting DS and ACDSB materials are illustrated in Fig. 1 and Fig. 2.

Fig. 2 shows that after treatment with Na₂CO₃, the surface morphology of the durian peel underwent significant alteration compared to its untreated state. Prior to activation, the DS material displayed a surface characterized by varying degrees of smoothness, roughness, and reduced porosity (Fig. 1). After being activated with Na₂CO₃, the ACDSB material has formed tubular capillaries, creating additional voids and enhancing surface porosity, leading to potential applications as better adsorbents (Fig. 2). ACDSB-activated carbon incorporating Na₂CO₃ exhibited superior porosity compared to ACDSB-activated carbon derived from durian peels and activated using H₂SO₄ [20], which the authors have previously researched and published.

The results of the EDS energy scattering spectroscopy analysis of ACDSB materials are shown in Table 3. From the results, the carbon content of ACDSB was notably elevated in terms of mass (87.86%) and low oxygen content by mass (8.41%). Compared with and relative to activated carbon synthesized using H₂SO₄-activated durian shell (ACDS) (Table 3), ACDSB material activated with Na₂CO₃ had a higher carbon content than ACDS material [30].

FT-IR spectroscopy, as depicted in Fig. 3, was employed to investigate the surface functional groups of ACDSB. The findings demonstrated a significant peak at 3732.26 cm^–1, which signifies the -OH group. The spectral pattern at about 2980.02 and 2897.08 cm⁻¹ was assigned to the C–H stretching vibration of the methylene group (–CH₂), the spectral pattern at 2133.27 cm⁻¹ was assigned to the C–C vibration of the alkyne group (–C=C–), and the range from 1791.87 to 1994. 40 cm⁻¹ was assigned the C=O oscillations of saturated fatty esters and carboxylic acids, with strong peaks at 1680.00 and 1556.55 cm⁻¹, possibly due to prolonged oscillations of C=C and C=O in organic matter. The absorption peak observed between 1300-900 cm⁻¹ was assigned to the C–N and C–O stretching oscillation. This is consistent with the EDX analysis results, which show that the main components of ACDSB and ACDS materials are C and O [20].

The results of XRD spectrometry of ACDSB coal are shown in Fig. 4. Fig. 4 results show that the C (002) diffraction peak is wide (2θ = 22-30º), and the C (101) diffraction peak is weak and wide (2θ = 40-50º) and can be attributed to the amorphous carbon structures. From this structural feature, activated carbon has an amorphous carbon structure [19].

The ACDSB material has a specific surface area of 497.00 m²/g, which shows that its surface area is large. Compared with ACDS material, ACDSB material has a larger specific surface area than ACDS (348.00 m²/g) [30].

The analysis results show that ACDSB has higher porosity, carbon content, and specific surface area than ACDS.

Determining the Isoelectric Point of ACDSB

Fig. 5 presents the outcomes of the isoelectric point determination for ACDSB. The analysis results indicate that ACDSB exhibits greater porosity, carbon content, and specific surface area than ACDS.

The results of Fig. 6 determined the isoelectric point (pH_pzc) of ACDSB pH_pzc = 6.9. These findings indicate that ACDSB exhibited a positive surface charge at pH levels below pH_pzc and shifted to a negative charge as pH surpassed the pH_pzc threshold.

Results of Physical Parameters and the Iodine Index of ACDSB

Table 4 displays the physical characteristics of ACDSB and ACDS.

ACDSB-activated carbon exhibits notably diminished moisture content and a correspondingly reduced density. In terms of its mechanical attributes, this feature is an advantageous characteristic of ACDSB-derived activated carbon. The elevated moisture content can significantly compromise the thermal engineering properties of activated carbon, and its structural integrity and overall durability are diminished. This result also shows that the physical parameters of ACDSB are better than ACDS. The iodine index of ACDSB material is 709 mg/g and higher than that of ACDS material (634 mg/g) [30].

Fig. 1.

SEM image of DS material.

Fig. 2.

SEM image of ACDSB materials.

Fig. 3.

FT-IR infrared spectrum of ACDSB.

The results of the investigation of factors affecting the adsorption capacity of RR 120 on ACDSB

Effect of pH

Fig. 6 shows that both adsorption efficiency and capacity increase with lower pH. This can be explained by the fact that, at low pH values, electrostatic attraction between the positive charge on the surface of ACDSB and the sulfate groups RR-120 (D-SO₃Na) has been dissociated into anions (D-SO₃-). When the pH value is high, a repulsive force appears between the surface of the negatively charged ACDSB material and the anion (D-SO₃). Therefore, the best adsorption pH for ACDSB’s RR 120 solution was selected as pH 3 and used for further experiments.

Fig. 4.

X-ray diffraction patterns of ACDSB material.

Fig. 5.

The isoelectric point of ACDSB.

Table 4.

Analysis of Activated Carbons ACDSB and ACDS Based on Physical Parameters.

Parameters	ACDSB	ACDS [30]
Ash content (%)	56.16±0.5	55.63±1
Moisture content (%)	2.1±0.05	4.74±0.03
Density (g/cm3)	0.65±0.05	0.83±0.05

Fig. 6.

The effect of pH on the adsorption efficiency of RR 120.

Model of Adsorption Isotherm

Langmuir Isotherm Adsorption Model

Fig. 10a) shows that the maximum adsorption capacity and constant were determined [30] as q_max = 46.51 mg/g and b = 0.056 (L/g), respectively. The calculated R_L parameter values shown in Fig. 10b) indicate that the R_L values ranging from 0.24 to 0.03 were all less than 1. Therefore, it is evident that the Langmuir isotherm model appropriately characterizes the adsorption process of RR 120 on ACDSB materials.

Fig. 7.

The effect of time on the adsorption efficiency of RR 120.

Fig. 8.

The effect of ACDSB mass on the adsorption efficiency of RR 120.

Fig. 9.

The effect of ACDSB time on the adsorption efficiency of RR 120 at different temperatures.

Examination of the adsorption capabilities of RR 120 across varied materials (Table 5) reveals that ACDSB-activated carbon exhibited a maximum adsorption capacity (q_max) for RR 120 that closely resembled that of synthetic materials of comparable nanometer scales [29, 33], higher than polyamide nylon 6 microplastic material.

Tempkin Isotherm Adsorption Model

According to the findings illustrated in Fig. 13, the coefficient of determination R² was determined to be 0.985. Therefore, the adsorption process of RR 120 on ACDSB was consistent with the Tempkin model.

Fig. 10.

a) The effect of C_cb/q on C_cb; b) the parameter value R_L with C_o.

Table 5.

Maximum absorption capacity RR 120 of various materials.

No.	Adsorption materials	Maximum adsorption capacity (mg/g)	Reference
1	Activated carbon made from herbal waste	47.88	[32]
2	Fe3O4 magnetic nanoparticles	49.26	[33]
3	Mg-hydrotalcites	45.94	[34]
4	Polyamide Nylon 6 Microplastics	3.96	[35]
5	Durian shell	46.51	This project

Elovich Isotherm Adsorption Model

The application of the Elovich adsorption isotherm model pertains to the process of multilayer adsorption. Fig. 14 showed that qmax was 7.67 mg/g and K_e was 6.39.

Overall, the Langmuir, Freundlich, Dubinin-Radushkevich, Tempkin, and Elovich models demonstrated R² values of 0.9911, 0.9733, 0.6953, 0.9895, and 0.9845, respectively.

The adsorption process of RR 120 on ACDSB-activated carbon adheres to the Langmuir, Freundlich, Tempkin, and Elovich adsorption isotherm models. Adsorption occurs via multilayer mechanisms amid the activated carbon’s non-uniform surface, where significant interactions between adsorbent and adsorbate are evident [21, 23, 27, 31].

Fig. 11.

The effect of Lnq on LnC_cb for RR 120 adsorption.

Fig. 12.

The effect of Lnq ̅ on ℇ²

Fig. 13.

The effect of q ̅ on LnCcb ̅ according to the Tempkin model.

Fig. 14.

The impact of ln (q/Ccb) on q ̅.

Adsorption Kinetic Models

Tables 6 and 7 display the findings from the kinetic analysis of the RR 120 adsorption process on ACDSB-activated carbon. The values of the kinetic parameters from the first-order kinetic equation in Table 5 and Fig. 15a) indicate that the coefficient of determination (R²) falls within the range of 0.7453 to 0.8556. Furthermore, a noticeable disparity exists between the experimentally determined adsorption capacity and the calculated capacity derived from the kinetic equations. Hence, the first-order kinetic equation is not suitable for describing the adsorption process of RR 120 onto ACDSB.

The values in Table 6 and the graph in Fig. 15b) show that the coefficient of determination (R²) is consistently high, exceeding 0.99. Additionally, the experimentally determined adsorption capacities closely align with the results obtained by applying kinetic equations. Thus, it can be deduced that the pseudo-second-order kinetic equation is well-suited for elucidating the adsorption mechanism of RR 120 on ACDSB.

The Weber and Morris intraparticle diffusion model reveals that the plot of q_t as a function of t^0.5 can be divided into two regions (Fig. 16a)). Region 1, the initial portion, is attributed to the diffusion of the adsorbate from the bulk solution to the external surface of the adsorbent (external surface adsorption). Region 2 describes the adsorption stage, where intraparticle diffusion plays a decisive role in the adsorption rate to the solid phase surface of the ACDSB. Moreover, the linear plots observed in Region 2 do not cross the origin, suggesting that factors beyond intraparticle diffusion contribute to determining the adsorption rate [17, 20].

The liquid film diffusion model illustrates that Ln(1-F), as a function of time, exhibits a linear graph (Fig. 16b)). Hence, film diffusion emerges as a pivotal determinant of the adsorption rate.

Fig. 15.

a) The first-order and b) the second-order kinetic equation for RR 120.

Fig. 16.

a) q_t depends on time t^0,5 and b) Ln(1-F) on time.

The membrane diffusion model for the liquid indicates that the plot of Ln(1-F) over time exhibited a linear relationship, so membrane diffusion determines the rate in the adsorption process (Fig. 16b)). Consequently, the thick diffusion film enveloping the ACDSB impeded RR 120 migration from the liquid phase to the solid phase surface of the ACDSB. Hence, membrane diffusion predominated in the adsorption of ACDSB on ACDS-activated carbon [17, 18].

Activation Energy

Table 8 displays the findings concerning the RR 120 adsorption activation energy of ACDSB-activated carbon. Analysis revealed that E<25 kJ/mol, suggesting that external diffusion governed the RR 120 adsorption process on ACDS.

RR 120 Adsorption Thermodynamics of ACDSB Materials

The findings from the thermodynamic analysis of the RR 120 adsorption process using ACDSB-activated carbon are detailed in Table 9.

From Table 9, it can be seen that: The free energy change (∆G⁰) obtained has a negative value from -1.110 to -4.224 kJ/mol, and the entropy change (∆S⁰) has a positive value of 0.1607 kJ/mol, which proves the adsorption process ACDSB’s RR 120 is spontaneous. The measured enthalpy change (∆H⁰) was 47.68 kJ/mol positive, signifying the adsorption process as being endothermic.

Zhang et al. [25] have shown that: In the adsorption process, if ΔH^₀ < 25 kJ/mol, van der Waals forces were the main interactions leading to physical adsorption. For ΔH^₀ valued in the range of 40 to 200 kJ/mol, chemical bonding predominantly drives chemisorption in the material. Therefore, the results of thermodynamic calculations (Table 9) show that the adsorption process of RR 120 was chemisorption.

Conversely, the findings from the investigation into the adsorption dynamics of RR 120 onto ACDSB materials conform to the second-order apparent kinetics equation and align with the Tempkin and Elovich adsorption isotherm models. This observation suggests that the interaction between RR 120 and ACDSB is governed by chemisorptive forces, indicating a propensity for chemical adsorption processes. This trend notably parallels the outcomes of thermodynamic calculations and bears a resemblance to prior research on the adsorption of Methylene Blue (MB) onto ACDS-activated carbon treated with sulfuric acid (H₂SO₄) [18].