Effects of Phosphate and Thermal Treatments on the Characteristics of Activated Carbon Manufactured from Durian (Durio zibethinus) Peel

Abstract

The availability of fossil energy is dwindling, so renewable fuels are the alternative choices, one of which is bioethanol. To increase the purity of the ethanol produced via the fermentation process, activated carbon (AC) was made from durian (Durio zibethinus) peel. The steps for making AC consist of carbonization (300 °C and 400 °C), chemical activation using phosphoric acid (10–40%), pyrolysis (700 °C and 800 °C), and neutralization. The results showed that the maximum surface area (326.72 m²/g) was obtained from 400 °C carbonization, 800 °C pyrolysis, and activation using a 40% phosphoric acid solution. Other characteristics are the surface area of 326.72 m²/g, pore radius of 1.04 nm, and total pore volume of 0.17 cc/g with phosphate residue in the form a P₂O₅ molecule of 3.47% by weight, with COOH, OH, CO, C=C, C=O, P-OC, and Fe-O groups with wavenumbers (cm⁻¹), respectively, of 3836, 3225, 2103, 1555, 1143, and 494. The AC also demonstrated the highest number of carbon (86.41%) upon detection using EDX, while XRF analysis verified an average carbon content of 94.45 wt%. The highest ethanol adsorption efficiency (%) and the lowest yield (%) of AC (%) were 90.01 ± 0.00 and 23.26 ± 0.01. This study shows that durian peel has great potential as the raw material for the activated carbon manufacture of ethanol adsorbents.

1. Introduction

Significant global climate change due to greenhouse gasses generated via fossil fuel combustion has led to much research on alternative fuels that are efficient and environmentally friendly, including microbial bioethanol [1,2]. Unfortunately, the ethanol content in the broth obtained from the fermentation process is very low [3]. Meanwhile, the purity of ethanol as a biofuel is very high, which is about 99 wt% [4]. Therefore, an appropriate and energy-efficient purification technology is required, and the adsorption process using activated carbon (AC) can be a good option [5]. The research results of Downarowicz and Aleksandrzak (2017) [5] show that AC has a strong affinity for organic polar compounds, including bioethanol. Functionally, AC can also be used as a catalyst carrier [6], drinking water disinfection, energy storage, and gas storage [7].

Durian (Durio zibethinus) peel is a fruit waste that is known as an attractive AC precursor for its high carbon (C) content and affinity to polar compounds [8]. It is the fifth largest fruit commodity in Indonesia with around 60–75% waste of its peel [9]. The presence of lignocellulosic content in biomass requires a heating process at low and controlled temperatures in order to develop porosity in the charcoal, which is called pyrolysis/hydrothermal carbonization [10]. The hydrothermal carbonization temperature for cellulose is in the range of 300–400 °C [11] to allow complete removal of moisture and volatile compounds [10].

The production of AC via carbonization and then activation (chemical and physical) produces large amounts of carbon and expels most of the non-carbon element in gas form, which is mainly oxygen. The use of chemical activation has the advantage of transforming the pore structure, high conversion of raw materials to carbon, and resisting the formation of tar [12]. Meanwhile, physical activation by heating at high temperatures will open carbon pores [13]. Phosphoric acid is a chemical activation agent that is widely used for large-scale synthesis of activated carbon from lignocellulosic materials because it produces high pore diameter and total volume and is relatively environmentally friendly [7].

Several studies on the carbonization temperature and activators for the manufacture of AC as adsorbents from durian peel at 400 °C with KOH and H₂SO₄ [12] and at 500 °C with KOH [14]. Unfortunately, both of them used KOH activation, which is hygroscopic [15] and leads to increased hydrophilicity of the resulting AC [16]. In addition, durian peel AC has also been studied using 10% v/v H₃PO₄ without going through carbonization, and the activation temperatures are 400 and 500 °C [17]. We know that ethanol production is still a mixture of ethanol–water, and the hydrophobicity of AC aims to allow ethanol to be adsorbed while water is retained outside of it. AC characteristics strongly depend on its pore structure which can be analyzed via surface area, volume, and pore size evaluation [18]. Other analyzes include the presence of functional groups with FTIR instruments, morphology and atomic content in AC with SEM-EDX, and chemical constituents with XRF.

In this study, AC from durian peel was manufactured via carbonization and chemical activation with phosphoric acid. The AC was characterized, its yield was optimized, and adsorption efficiency was investigated for its ability to adsorb ethanol.

2. Materials and Methods

2.1. Material Pretreatment

Durian peel was collected in Gunungpati area (Semarang City) in Central Java Province, Indonesia, and then they were cut into 6 to 8 cm. Furthermore, the raw material was dried at 70 °C until a constant weight was reached. Dried durian peel were stored in an airtight container filled with silica gel to prevent fungal growth until they were ready to be processed when needed. Commercial activated carbon (CAC) (local market) was made from coconut shells in Indonesia.

2.2. AC Synthesis

The AC was produced via carbonization (CN) process at 300 °C and 400 °C by carbonizing the durian peel in the furnace for 1 h accompanied by flowing N₂ gas at a rate of 1 L/min. Next, chemical activation was performed by soaking the carbon in various concentrations of H₃PO₄, the activator, namely 10%, 20%, 30%, and 40 wt.%, at room temperature with constant stirring at 100 rpm for 24 h. Then, the carbon was physically activated by heating it in the furnace at 700 °C and 800 °C for 1 h, accompanied by flowing N₂ gas at a rate of 1 L/min.

2.3. AC Neutralization

The AC produced was then neutralized with distilled water for 30 min at 85 °C until no longer detectable phosphor ions or other contaminants were found in the wash water, then filtered, and the filtrate was tested for its pH using a pH meter (Mettler Toledo). Washing using distilled water and 0.5 N KOH was repeated until a pH of 7 was reached. After the neutralization stage was completed, the AC was heated in an oven at 110 °C until the achievement of constant weight. Then, the AC was sieved at 8–10 mesh.

2.4. Ethanol Adsorption Test

One gram of AC was introduced into an Erlenmeyer flask containing 20 mL of 9.91% ethanol. The adsorption was carried out at room temperature with the shaker speed was 100 rpm. Ethanol samples were drawn using the same technique at 15, 30, 45, 60, and 75 min.

2.5. Determination of Ethanol Concentration

The Conway method [19] was used to determine the remaining ethanol content after being adsorbed with AC. The Conway cup containing 3 mL of dichromic acid, 0.5 mL of ethanol after adsorption, and 1 mL of 20% Na₂CO₃ was placed in the oven at 90 °C for 20 min before cooling back to room temperature. The dichromic acid in the Conway cup was diluted to 25 mL, and the absorbance was measured using a UV Vis Spectrophotometer (Genesys 10) with a maximum wavelength of 444 nm. Then, the absorbance value obtained was substituted for the linear equation resulting from the ethanol calibration curve to determine the ethanol concentration resulting from adsorption. AC adsorption of durian peel compared to CAC.

2.6. Determination of Ethanol Adsorption Efficiency via AC

The adsorption efficiency via AC in this study was calculated using Equation (1) [20].

$$\% adsorption efficiency=\frac{{C_0-C}_{\mathrm{t}}}{C_0}\times 100\%$$

(1)

where $C_0$ and $C_t$ refer to concentration of ethanol solution (%) before and after the adsorption process

2.7. Determination of Yield of AC

The yield of AC in this study was calculated using Equation (2) [21].

$$\mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d} \mathrm{o}\mathrm{f} \mathrm{A}\mathrm{C} \left(\mathrm{\%}\right)=\frac{\mathrm{AC}}{\mathrm{C}}\times 100\mathrm{\%}$$

(2)

where AC and C refer to total mass (g) of activated carbon and carbon

2.8. Characterizations of AC and CAC before and after Activation

The surface area, pore volume, and pore diameter were quantified using nitrogen adsorption–desorption (Brunauer–Emmett–Teller, BET) model Nova 1200 e, Quantachrome). The pore size distribution is obtained from the desorption branch of the hysteretic adsorption isotherm. Functional groups were analyzed using Fourier Transform Infrared Spectroscopy (FTIR) model Perkin Elmer Spectrum IR 10.6.1. (PerkinElmer, Waltham, MA, USA) Scanning Electron Microscope–Energy Dispersive X-ray (SEM-EDX) model SEM-EDX JEOL JSM-6510LA (JEOL, Showima City, Tokyo, Japan) was used to study surface chemistry and pore structure (SEM) and atomic content (EDX, Thermo Fisher Scientific, Waltham, MA, USA). Prior to SEM analysis, the sample was coated with thin layer of gold and then mounted to a puncture made of copper with the help of carbon double-sided tape. Meanwhile, the elemental composition of the AC was analyzed via EDX. Determination of the chemical constituents of AC was evaluated using X-ray Fluorescence (XRF) Spectrometry (Wavelength Dispersive X-ray Fluorescence (WDXRF) model Rigaku Supermini200, Quantachrome TouchWin ver. 1.11, Rigaku, The Woodlands, TX, USA).

3. Results and Discussion

3.1. Surface Area of AC of Durian Peel

The results of BET analysis with thermal CN 300 and 400 °C at various concentrations of phosphoric acid at pyrolysis temperatures of 700 °C and 800 °C are presented in

Table 1. BET result of AC pyrolized at 700 °C.

Sample	Surface Area (m2/g)	Pore Size (Å)	Pore Volume (cc/g)
AC700-300-10%	51.1	19.5	0.05
AC 700-300-20%	37.5	15.6	0.03
AC 700-300-30%	81.0	15.6	0.04
AC 700-300-40%	259	16.7	0.04
AC 700-400-10%	16.3	15.7	0.02
AC 700-400-20%	35.8	15.7	0.04
AC 700-400-30%	51.5	15.7	0.04
AC 700-400-40%	102	15.6	0.04

and

Table 2. BET result of AC pyrolized at 800 °C.

Sample	Surface Area (m2/g)	Pore Size (Å)	Pore Volume (cc/g)
AC 800-300-10%	2.81	15.2	0.002
AC 800-300-20%	67.2	13.2	0.044
AC 800-300-30%	258	11.1	0.14
AC 800-300-40%	292	11.6	0.16
AC 800-400-10%	2.09	16.4	0.003
AC 800-400-20%	216	10.2	0.11
AC 800-400-30%	254	10.3	0.13
AC 800-400-40%	327	10.4	0.17

. Accordingly, the surface area comparison of the ACs obtained from this study and CAC is shown in

Table 3. Surface area comparison of this study and CAC.

Sample		Surface Area (m2/g)
Before Activation	CN300	51.738
After Activation	AC 700-300-40%	259.243
	AC 800-300-40%	291.64
Before Activation	K400	49.460
After Activation	AC 700-400-40%	101.897
	AC 800-400-40%	326.72
CAC		50.374

.

Table 1 shows that the CA samples activated with 40% H₃PO₄ exhibit a larger surface area and is most compared to other concentrations in the same CN and pyrolysis temperature group. The results of the BET analysis in this study agreed well with those reported by [22,23], where CAs activated with H₃PO₄ possessed a surface area of 16, 56.78, and 257.50 m²/g, respectively.

Table 2 shows that the CN temperature of 400 °C produces a higher surface area (326.72 m²/g) than 300 °C (291.64 m²/g). Indeed, raising the activator concentration while also raising the surface area and pore volume can be seen in Table 1 and Table 2. It is due to the intensive interaction between the phosphoric acid molecules and the carbon structure. So, the carbon decomposes into cellulose to form phosphates oxide (P₂O₅), which forms pores [24]. The pore size decreased sharply as the activator concentration increased from 10% to 20%. However, the reduction in pore size was not significant at activator concentrations exceeding 20%. It is due to the lower the volatile matter content, the smaller the pore diameter [25]. The increase in pore volume is to have a good effect on adsorption because it can provide optimal adsorption space in AC [26].

The increase in surface area (Table 3) occurs as activation temperature, pyrolysis temperature, and H₃PO₄ concentration increase. The best results were when the pyrolysis, activation, and H₃PO₄ concentration conditions were AC800-400-40%, which reached about 6.6 times the increase in surface area after activation. It is also the same as the results of the SEM-EDX test, where AC 800-400-40% has the most wrinkles, so it has a large surface area.

In contrast, the AC obtained from AC700-400-10% has a small surface area, consists of fragments, and is less porous. It is presumably due to variations in temperature conditions during pyrolysis that aims for thermal decomposition [27]. So, the higher the pyrolysis temperature, the more open pores there are, but the yield will decrease [28]. Several studies on the effect of using phosphoric acid activation on yield and surface area in the manufacture of AC with biomass precursors are presented in

Table 4. Several studies on the effects of using phosphoric acid activation on yield and surface area in the manufacture of AC with biomass precursors.

Precursors	Aqueous Solution with Concentration	Impregnation Ratio H3PO4 to Sample	CN and Activation Temperature (°C)	Adsorbate	Yield (%)	BET Surface Area (m2/g)	References
Durian peel	85 wt.%	3:1–6:1 ± α	Activation = 400–600 ± α	NH3-N	21.02	906.48 at 400 °C	[29]
Durian peel	85 wt.%	3:1–6:1 ± α	Activation = 400–600 ± α	NH3-N and COD	N.A	906.48	[30]
Durian shell	1 M	25:1	CN 250–350 Activation = 400–700	N.A	N.A	2004	[31]
Durian shell	5,10,20,30, 50%	0.7, 1.4, 2.8, 4.2, 8.4	CN = 500	toluene	N.A	1404 at 30%	[32]
Durian seed	85 wt.%	1:1–6:1	Activation = 600 and 900	N2	46.82	2123 at 600 °C	[33]
Durian seed (50%) and rambutan peels (50%)		each as much as 1 g to 2 mL H3PO4	Activation = 100	N2	N.A	104.2	[34]
Durian peel	10% v/v overnight	1:1.4	Activation = 400 and 500	N.A	63	1024 at 500 °C	[17]
Coconut shell	20, 30, 40, 50 wt.%	3.5:1	CN 400–600	methylene blue	36.9	891	[35]
Rice husk	40–85 wt.%	3:1–6:1	Activation = 300–600	N.A	44.6	1820	[36]
Rice husk	40–85 wt.%	3:1–6:1	Activation = 300–600	N.A	44.6	1820	[36]
Palm shell	65 wt.%	1.75:1	Activation = 375, 425, 475, and 525	N.A	55	1109	[37]
Durian Peel (This study)	10–40 wt.%	6:1	CN 300–400 Activation = 700–800	ethanol	49.67% for 700-300-40%	326.72 for 800-400-40%

.

3.2. FTIR Analysis

The results of the FTIR analysis of the AC’s surface with concentrations of 10%, 20%, 30%, and 40% are shown in Figure 1.

The wavenumber of 2500–3500 cm⁻¹ characterizes the steepest and dominant number of −OH compared to other functional groups in both the same and different treatments (Figure 1) so that the AC has a hydrophilic site [38].

The detected vibrations 3839–3567 cm⁻¹ were obtained from the -COOH functional group in the variables AC 800-400-20%, AC 800-400-30%, AC 800-400-40%, AC 700–300, and AC 700-400-20% (Figure 1). Meanwhile, the wavenumber of 3262–3225 cm⁻¹ indicates the presence of the −OH functional group of the hydroxyl group [39]. The wavenumber 2263–2261 cm⁻¹ is the C≡C vibration of the alkyne group [40] and was only found in the variables AC 800-300-10%, AC 800-400-10%, AC 800-300-20%, and AC 800-300-30%. Conversely, the C=C stretching disappeared at a concentrated activator concentration of 40%, indicating that many weak bonds disappeared in the activation process [41]. Based on Thue et al. 2017 [42], the discovery of carbonyl (C=O) and hydroxyl (−OH) on AC will reduce hydrophobicity because these functional groups easily bind water.

An aromatic bond C=C is found at a peak between 1532 and 1623 cm⁻¹, which is more stable at a higher pyrolysis temperature [40]. The stretching vibrations of the P-OR functional group of esters with the weak band had been found at peaks of 876 cm⁻¹ at AC 800-300-40%, 932 cm⁻¹ at AC 800-400-30%, and 933 cm⁻¹ at AC 800-400-20% [43]. The accumulation of Fe atoms on activated carbon was indicated by the Fe-O stretching vibrations found in all variables AC800-400-(10–40%) between 488 and 536 cm⁻¹ [44]. A strong band at approximately 1058–1177 cm⁻¹ as a result of the activation process with H₃PO₄ produces char oxidation, introducing oxygenated functionalities in the AC, was probably the stretching of the O–C bond in the P–O–C linkage (aromatic bond) or hydrogen-bonded P=O in a phosphate ester [37].

3.3. XRF Analysis

The chemical compositions of CN and AC were analyzed using XRF [41], with the results presented in

Table 5. Chemical components of CN temperature.

Component	CN300	CN400
C	97.5	96.2
P2O5	0.19	0.33
SiO2	0.25	0.27
Fe2O3	0.01	0.01
K2O	1.69	2.73
CaO	0.22	0.20

,

Table 6. Chemical components of AC 700–300.

Component	Variable Composition (wt.%)
	10%	20%	30%	40%
C	91.8	92.4	92.5	92.9
P2O5	5.60	5.32	4.86	4.86
SiO2	0.22	0.21	0.21	0.24
Fe2O3	0.02	0.02	0.03	0.17
K2O	2.17	1.69	2.04	1.54
CaO	0.16	0.29	0.17	0.14

,

Table 7. Chemical components of AC 700–400.

Component	Variable Composition (wt.%)
	10%	20%	30%	40%
C	91.7	94.1	91.7	92.1
P2O5	5.50	5.12	5.36	5.01
SiO2	0.32	0.22	0.28	0.24
Fe2O3	0.02	0.03	0.03	0.15
K2O	2.05	0.19	2.27	2.16
CaO	0.23	0.13	0.15	0.14

,

Table 8. Chemical components of AC 800–300.

Component	Variable Composition (wt.%)
	10%	20%	30%	40%
C	93.9	93.3	94.0	94.0
P2O5	5.03	5.12	4.62	4.71
SiO2	0.28	0.20	0.19	0.22
Fe2O3	0.01	0.01	0.01	0.02
K2O	0.66	1.27	1.03	0.87
CaO	0.13	0.11	0.09	0.11

and

Table 9. Chemical components of AC 800–400.

Component	Variable Composition (wt.%)
	10%	20%	30%	40%
C	96.8	97.1	93.6	94.9
P2O5	4.84	4.03	4.61	3.47
SiO2	0.26	0.200	0.21	0.16
Fe2O3	0.02	0.03	0.35	0.06
K2O	0.90	1.17	0.97	1.30
CaO	0.16	0.09	0.10	0.08

. Several inorganic species, such as K₂O, P₂O5, SiO₂, CaO, and Fe₂O₃, are present in durian peel and can be considered impurities. The concentration of these impurities increased with the CN temperature [45].

The mass percentage of component C in CN300 (97.5) is more than in CN400 (96.2) (Table 5) because the CN temperature of 400 C is higher than 300 C, so the release of volatile compounds is complete [45].

Table 6, Table 7, Table 8 and Table 9 have in common the components of AC from largest to smallest, namely C (more than 91%), P₂O₅, and K₂O. The carbon content after pyrolysis is lower than after CN. It may be due to the many carbon bonds removed at an average of 3–4 wt.%. However, the content of compound C in the variables AC800-400-10% and AC800-400-20% (Table 9) is higher than CN400 (Table 5). Increasing the pyrolysis temperature enriches the carbon content in the AC by removing non-carbon species. However, an increase in the concentration of H₃PO₄, especially 30 and 40%, reduced the carbon content, presumably due to the partial gasification of the carbon via P₂O₅ and volatilization of phosphorus compounds. Meanwhile, thermal pyrolysis ≥400 °C produces P₂O₅ due to the decomposition of H₃PO₄, and inorganic substances have active components despite low chemical reaction ability, such as silica (SiO₂)_, Fe₂O₃, K₂O, and CaO. The following is a sequence of amorphous SiO₂ (crystalline silica): AC700–400 (Table 7) > AC700–300 (Table 6) > AC800–400 (Table 9) > AC800–300 (Table 8). In contrast, as the phosphoric acid concentration increases, the SiO₂ content tends to decrease [46].

On the other hand, the chemical composition of AC carbonized at 400 °C is higher than at 300 °C because the deformation of complex chemical compounds becomes simpler than at pyrolysis temperatures of 700 °C and 800 °C, so many compounds with new bonds form [47].

3.4. SEM-EDX Analysis

The surface morphologies of C (Figure 2), AC (Figure 3) and CAC (Figure 4) were observed using SEM. Meanwhile, the atomic content of carbon, AC, and CAC are presented in Figure 5, Figure 6 and Figure 7, respectively.

The morphology of the durian peel carbonized at 300 °C showed a wrinkled carbon surface without the pores because the durian peel carbon has not been able to decompose the volatile content [48,49]. On the other hand, the durian peel C at 400 °C (Figure 2b) showed pores starting to form but not evenly distributed over the entire C surface due to good pore structure as temperature increases [48,50].

All the variables shown in Figure 3 have a morphology in which many tunnel-like pores are formed on the surface of the AC. However, AC pyrolysis at 700 °C and 800 °C tends to produce the morphology of fragments and wrinkles. Figure 3 shows the pore volume of the SEM images, which is wider with higher CN and pyrolysis temperatures. Figure 3a shows a blockage in the pores in the form of C loss, which is thought to be due to the activation process. The appearance of a small spherical shape that sticks around the pores (Figure 3b) is thought to be residual KOH when the AC was neutralized.

The significant difference between the morphology of AC (Figure 3) and C (Figure 2) is the formation of porosity as a result of the activation of phosphoric acid on carbon to produce new products in the form of phosphate esters and polyphosphates [24].

Figure 4 shows forming many narrow fissure structures [35].

Figure 5 shows that the carbon content increases with increasing temperature. On the other hand, oxygen is decreasing. In addition, increasing the temperature also produces Zr.

After C was activated (Figure 6), new atoms appeared, namely Fe, P, Cu, and Mg. Figure 6 showed that C, O, and phosphorus (P) were the dominant compounds in the AC produced. The number of C atoms (%) produced from AC700-300-40%, AC700-400-40%, AC800-300-40%, and AC800-400-40%, respectively, are 85.14, 81.55, 86.88, and 86.14. The presence of P in AC is due to the availability of the phosphoric acid activator [51]. that helps the adsorbent to increase the hydrophobic properties of C [52]. AC. with a pyrolysis temperature of 800 °C, has a higher C content and a lower O content than 700 °C due to the organic matter’s contaminants, which prevented the N₂ from removing them during the carbon synthesis process and resulted in poor product qualities [48].

The low C content (~42%) in CAC (Figure 7). In addition, the levels of O, P, K, and Mg in CAC are similar to the results of previous studies [53]. The Mg, aluminum (Al), and ferrum (Fe) atoms in Figure 6 are due to be ashes in the activated carbon, which are considered impurities and unnecessary trace amounts of elements like Fe, Si, Mn, and Ca that are frequently found in coconut shells. Chemicals like P and K are also available for activation procedures [54].

3.5. Effect of Temperature on Various Pyrolysis, CN, and H₃PO₄ Concentrations on Adsorption Efficiency via AC

The relevant adsorption efficiency measurements taken at various pyrolysis and CN temperature ranges are shown in

Table 10. Effect of temperature on various pyrolysis, CN, and H₃PO₄ concentrations on AC adsorption efficiency.

Temperature (°C)		Adsorption Efficiency (%)
Pyrolysis	CN	10%	20%	30%	40%
700	300	86.17 ± 0.41	50.36 ± 0.20	53.61 ± 0.00	79.66 ± 0.20
	400	81.28 ± 0.00	41.00 ± 0.00	77.21 ± 0.00	56.87 ± 0.20
800	300	89.82 ± 0.20	89.71 ± 0.06	89.85 ± 0.02	89.83 ± 0.20
	400	89.74 ± 0.06	89.76 ± 0.21	89.82 ± 0.40	90.01 ± 0.00

. The concentration of phosphoric acid caused noticeable changes to be found.

Table 10 shows the adsorption efficiency calculated using Equation (3) that the highest efficiencies have AC800-400-40% (90.01%). It is suspected that nearly all of the ethanol can be adsorbed via the activated carbon. The AC adsorption efficiency increases with the pyrolysis temperature. It has proven that the adsorption efficiency of AC at AC800-300 and AC800-400 is higher than pyrolysis at 700 °C. On the other hand, the decrease in its efficiency is due to the adsorption sites on AC being covered by adsorbate particles, causing a reduction in the total surface area and inhibiting the adsorption process [55].

Table 10 shows the fluctuating adsorption efficiency at AC700-300°C, namely 86.17%, 50.36%, 53.61%, and 79.66% for H₃PO₄ concentrations of 10%, 20%, 30%, and 40%, respectively. It is due to the excessive cross-linking reaction between C-O-P during the impregnation process [52]. A similar phenomenon also occurs at AC700-400 °C, where the adsorption efficiency was the highest for 10% H₃PO₄, but a fluctuating adsorption efficiency is obtained for 20–40% H₃PO₄. It is due to the cross-linking of biopolymers from phosphate esters, and polyphosphates are a new product resulting from the activation of phosphoric acid, which expands porosity. Nevertheless, the excessive concentrations of phosphoric acid form an insulating layer on the surface of the AC, thereby inhibiting ethanol adsorption [24]. On the other hand, the higher the CN temperature treatment and activation, the more the insulating layer on the AC surface disappears, so the ethanol adsorption is also higher.

3.6. Effect of Temperature on Various Pyrolysis, CN, and H₃PO₄ Concentrations on Yield of AC

The effect of pyrolysis temperature, CN temperature, and H₃PO₄ concentrations on the yield of AC is shown in

Table 11. Effect of pyrolysis temperature, CN temperature, and H₃PO₄ concentrations on the yield of AC.

Temperature (°C)		Yield (%)
Pyrolysis	CN	10%	20%	30%	40%
700	300	32.46 ± 0.1	37.56 ± 0.08	42.06 ± 0.05	49.67 ± 0.03
	400	31.85 ± 0.01	36.33 ± 0.02	40.61 ± 0.05	46.80 ± 0.06
800	300	30.62 ± 0.05	32.33 ± 0.1	33.34 ± 0.05	35.07 ± 0.06
	400	21.10 ± 0.2	21.98 ± 0.03	22.51 ± 0.06	23.26 ± 0.01

.

Table 11 shows that as the concentration of H₃PO₄ increases, the yield of AC decreases. The mechanism of pore formation begins with a reaction in the form of dehydration, degradation, and coagulation changes between phosphoric acid and hemicellulose and lignin contained in carbon which causes the formation of aliphatic structures. The process is continued by heating the AC to high temperatures to form phosphate ester bonds and polyphosphate ester bonds. Then, when the temperature is above 600 °C, the compounds H₃PO₄, H₂PO₄⁻¹, and H₂P₂O₇⁻² can evaporate, causing the phosphate ester and polyphosphate bonds to break in the precursor matrix. As a result, many new pores are formed [56,57]. The results of this study are similar to previous studies [56,57,58].

A comparison of adsorption efficiency and yield of AC from biomass is presented in

Table 12. Comparison of adsorption efficiency and yield of AC from biomass precursors using phosphoric acid activation.

Precursor	Activating Agent	Carbonization and Activation Temperature (°C)	Yield (%)	Adsorbate	Highest Adsorption Efficiency (%)	References
Durian Peel	H3PO4 30%	Activation: 600	86.90	-	-	[51].
Banana Peel	H3PO4 25%	CN: 470	73.83	-	-	[47]
Durian Peel	-	CN: 250	-	COD Removal	68.65	[59]
Eucalyptus Residue	H3PO4 40%	Activation: 300–600	54.3 for 300 °C	Methylene Blue Solution	99.6	[51]
Mangrove Pile Leftover	H3PO4	Activation: 300–500	44.7 for 300 °C	Methylene Blue Solution	96.3 for 300 °C	[60]
Periwinkle Shell	H3PO4 (0.5M)	Activation: 700	-	Cyanide Removal	76.6	[61]
Durian Peel	H3PO4 10–40%	CN: 300–400 Activation: 700–800	49.67 for 700-300-40%	Ethanol	90.01 for 800-400-40%	This study

.

4. Conclusions

The sequence of AC manufacturing stages from durian peel for bioethanol purification consists of CN (300–400 °C), the activation of H₃PO₄ (10–40%), pyrolysis (700–800 °C), and neutralization. The best results were characterized via carbonization at 400 °C, activator concentration at 40% H₃PO₄, and pyrolysis at 800 °C as follows: surface area of 326.72 m²/g, pore radius of 1.04 nm, and total pore volume of 0.17 cc/g with phosphate residue in the form a P₂O₅ molecule of 3.47% by weight, with COOH, OH, CO, C=C, C=O, P-OC, and Fe-O groups with a wavenumber (cm⁻¹), respectively, are 3836, 3225, 2103, 1555, 1143, and 494 as well as XRF with phosphate residue (P₂O₅), which is 3.47% by weight. The AC also demonstrated the highest amount of carbon (86.41% by mass), while the adsorption efficiency (%) of bioethanol and yield (%) of AC were 90.01 ± 0.00 and 23.26 ± 0.01. The surface area of AC from durian peel was 6.5 times larger than CAC derived from coconut shells.