Production of High Specific Surface Area Activated Carbon from Tangerine Peels and Utilization of Its By-Products

Abstract

Biomass waste, generated globally in vast quantities, represents an underutilized yet highly valuable resource for advanced material production. This study highlights a novel valorization pathway for waste tangerine peels, sourced from Jeju Island, South Korea, by converting them into high-performance activated carbon (T-AC) with exceptional pore characteristics, specifically designed for volatile organic compound (VOC) removal. Utilizing a unique combination of hydrothermal carbonization (HTC) and dry carbonization (DC) processes, the structural properties of the biomass were optimized, significantly enhancing the fixed carbon content. Subsequent chemical activation with an alkaline agent yielded T-AC with an outstanding specific surface area (1530–3375 m²/g) and total pore volume (0.73–2.00 cm³/g), with a tailored pore distribution favoring the sub-mesopore range (2.0–4.0 nm). The T-AC demonstrated remarkable performance in removing methylene chloride (MC), a hazardous VOC, with methylene chloride activity (MA) increasing from 44.7% to 76.3% as the activation agent ratio increased, while methylene chloride working capacity (MWC) improved significantly from 17.1% to 55.9%. These results underscore the transformative potential of tangerine peel-derived AC as a sustainable solution for VOC remediation, combining environmental waste management with advanced adsorption technology. The findings not only advance the field of biomass utilization but also offer a scalable approach for tackling pressing environmental and industrial challenges.

1. Introduction

Tangerines are one of the most widely produced and consumed fruits, grown in many countries with tropical and subtropical climates. Major producers include China, Spain, and Turkey, contributing significantly to global tangerine production [1]. Approximately 18% of the production is used in juice factories to produce juice, bioactive essential oils, jams, and other food or cosmetic products [2,3]. In this process, more than 400,000 tons of tangerine peel waste, which accounts for 40–50% of the tangerine, are generated annually [4]. Managing this substantial residue is an ongoing challenge, as improper disposal not only incurs significant economic costs but also leads to severe environmental consequences [5,6].

To mitigate these issues, various recycling and reuse methods, such as biochemical recovery, vermicomposting, and composting, have been explored.

A notable example of successful waste valorization can be found in China, the world’s largest tangerine producer, where most tangerine peels are repurposed into Chenpi, a traditional medicine.

By contrast, South Korea, despite being a significant tangerine producer with over 200,000 tons produced annually on Jeju Island alone in 2023, struggles with managing tangerine peel waste. While most of the fruit is consumed domestically, large quantities of peels and waste tangerines are treated as disposable waste. Common disposal methods, such as landfilling and incineration, not only incur substantial costs but also exacerbate environmental pollution [7]. Although some are used as feed or fertilizer, this, too, is difficult to manage, leading to disposal through ocean dumping.

Recent studies underscore the importance of utilizing agricultural waste as a resource to reduce management costs and mitigate environmental risks. For instance, 13% of the waste has been reported to be usable products such as animal feed [8], soil amendment [9], biogas [10], and bioethanol [11].

Thus, tangerine peels, which are applicable in various uses, contain a high content of organic carbon components such as cellulose, hemicellulose, and pectin, which have high calorific value, making them usable as fuel briquettes [12] or in the form of biochar [13]. Such characteristics highlight their potential as a renewable resource in addressing both waste management and energy challenges.

In particular, tangerine peels can serve as precursors for producing activated carbon (AC), a versatile material widely used in adsorption technologies. Among carbonaceous materials, activated carbon (AC) is a porous carbon-based material widely used as an adsorbent in various fields due to its high surface area and well-developed pore distribution. The inherent properties of AC vary depending on the activation method and inherent precursor properties. The activation method is generally classified into two types: physical and chemical activation. Physical activation is conducted in an oxidizing atmosphere (such as steam and CO₂) at relatively high temperatures, whereas chemical activation involves the use of various chemical reagents (KOH, NaOH, ZnCl₂, H₃PO₄) to produce AC [14], allowing for the production of AC with high yield and excellent pore characteristics at relatively lower temperatures compared to physical activation. This makes chemical activation particularly attractive for producing high-performance AC from waste biomass.

Simultaneously, the rapid growth of the electric vehicle market has driven increased use of methylene chloride (MC), a volatile organic compound (VOC) widely employed as a cleaning agent in industrial processes [15,16]. As environmental regulations continue to tighten, awareness of MC emissions is changing, and various treatment technologies are being proposed to effectively remove them. Among these, adsorption treatment using AC has the advantage of effectively treating MC without generating reaction by-products and at a relatively low cost compared to other treatment processes [17,18].

Despite these promising developments, research on utilizing waste tangerine peels as a precursor for AC production especially for MC removal remains limited. In this study, waste tangerine-based AC was produced using a chemical activation (alkaline treatment) method, and the pore characteristics of AC were systematically investigated according to activation treatment conditions. By leveraging the unique properties of tangerine peel waste, this study provides a sustainable and economically viable solution for VOC management. The overall schematic of the manufacturing process is outlined in Figure 1. As shown in Figure 1, discarded tangerine peels are converted into tangerine-based biochar through the hydrothermal carbonization (HTC) process. This process is conducted at a temperature range of 140–220 °C for approximately 0.5 h. The resulting biochar is further transformed into tangerine peel-derived carbon through a dry carbonization (D-C) process. The carbonized product, with an enhanced fixed carbon content, can then be applied as activated carbon for MC removal through chemical activation.

2. Materials and Methods

2.1. Materials

The waste tangerine peel (W-T) used as a raw material in this study was supplied by Jeju Techno Park (TP, Jeju, Republic of Korea).

2.2. Carbonization

A two-step carbonization process was conducted in this study to obtain structurally stable tangerine-based char (T-C). Initially, a hydrothermal carbonization (HTC) process was performed to remove the extract from the slurry state of the W-T. The HTC reaction conditions involved heat treatment of 1 kg of W-T in slurry form within a temperature range of 140–220 °C, with a reaction time maintained for 0.5 h. The HTC reaction was conducted using the equipment shown in Figure 2. The tangerine-based biochar samples with HTC at 140–220 °C treated were dried overnight at 105 °C. The produced HTC-based samples were named T-HTC-140, 160, 180, 200, and 220 based on the HTC treatment temperature [19].

Subsequently, a secondary dry carbonization (DC) process was performed to maximize the fixed carbon content of the HTC-based samples. Approximately 30 g of T-HTC-treated samples were placed in a boat-shaped crucible alumina loaded into the center of a tubular furnace. The samples were then purged with nitrogen at a flow rate of 200 mL/min for 30 min to block the influx of external air. Heat treatment was maintained for 3 h within a temperature range of 400–600 °C (at a heating rate of 3 °C/min), followed by natural cooling to room temperature. The produced DC samples were named T-HTC-DC-400, 500, and 600 based on DC treatment temperature [20].

2.3. Chemical Activation

For chemical activation, KOH (95%, SAMCHUN Chemical Co., Ltd., Pyeongtaek, Republic of Korea), NaOH (98%, SAMCHUN Chemical Co., Ltd., Pyeongtaek, Republic of Korea), and K₂CO₃ (99%, SAMCHUN Chemical Co., Ltd., Pyeongtaek, Republic of Korea) were selected as agents, and the weight ratio of precursor to agent was prepared in ratios ranging from 1.0 to 4.0. The activation process involved placing the mixture in a boat-shaped alumina crucible, which was then positioned in the center of an activation tubular furnace. The activation atmosphere was maintained under a nitrogen environment (99.9%, 200 mL/min) from the start of the heating process until the end of the activation holding time, which lasted for 3 h. The produced AC was named T-AC-activation agent–agent ratio (carbon source) based on the activation conditions. The activation yield was obtained as shown in the following Equation (1).

$$Activationyield\left(\%\right)={\displaystyle \frac{{Weight}_{afterchemicalactivation}}{{Weight}_{aftercarbonization}}}$$

(1)

2.4. Methylene Chloride (MC) Ads-Desorption Behaviors

The adsorption–desorption behavior of MC on the produced T-AC was examined according to the modified ASTM D5228 Standard Test Method for Determination of Butane Working Capacity of Activated Carbon [21]. Prior to measurement, all AC samples were dried at 105 °C for 24 h in a vacuum oven. A 0.3 g sample of T-AC was packed into a U-shaped measurement cell, which was then placed in a 25 °C constant-temperature water bath. MC gas (20,000 ppm (2%)), the adsorbate, was introduced at a flow rate of 250 mL/min for about 15 min. The adsorption process was repeated every 10 min until no further weight change was observed. After complete adsorption (maximum saturation, MC activity (MA)), desorption was carried out by introducing N₂ (99.9%) at a flow rate of 300 mL/min for 40 min, and the weight was recorded to evaluate MC retentivity (MR). Considering MA and MR, the MC working capacity (MWC), which indicates the actual performance of the adsorbent, was evaluated, and MA, MR, and MWC were calculated as follows [22]. The MC adsorption/desorption experiment was conducted in triplicate, and the average values for MA, MR, and MWC were reported.

$$Methylene chloride activity={\displaystyle \frac{B-A}{A}}\times 100$$

$$Methylene chloride retentivity={\displaystyle \frac{C-A}{A}}\times 100$$

$$Methylene chloride working capacity={\displaystyle \frac{B-C}{A}}\times 100$$

Here, A is the weight of the adsorbate (T-AC), B is the weight of the adsorbate after MC adsorption, and C is the weight of the adsorbate after N₂ desorption.

3. Results

3.1. Hydrothermal Effect

3.1.1. Element Analysis of HTC Samples

According to Hoekman et al., hydrothermal carbonization (HTC) of cellulose-based precursors can be classified into three stages [23]; (1) Hydrolysis of cellulose, around 180 °C; (2) Hydrolysis of hemicellulose, around 150–230 °C; (3) Decomposition of lignin, above 600 °C. Therefore, in this study, the HTC process was conducted within the specified range (140–220 °C) to induce the hydrolysis of cellulose (OH and C-O compounds) and hemicellulose (C=O compounds), which have low thermal stability.

The proximate and elemental analysis data for the biochar obtained from the HTC process are listed in

Table 1. Element contents of tangerine-derived carbons at various hydrothermal treatment conditions.

Samples	Proximate Analysis (wt.%)				Element Analysis (wt.%)
	HTC Temp.	Volatile	Fixed Carbon	Ash	Carbon	Hydrogen	Nitrogen	Oxygen	O/C	H/C	Yield (%)
Raw	-	75.7	20.5	3.80	50.4	5.84	2.25	37.5	0.74	0.12	-
HTC	140 °C	79.1	18.0	2.90	51.5	5.95	2.76	36.8	0.71	0.12	4.25
	160 °C	76.8	19.2	3.20	52.6	5.98	2.24	35.9	0.68	0.11	6.35
	180 °C	74.4	22.3	3.90	53.9	5.84	2.68	34.3	0.64	0.11	6.65
	200 °C	72.7	24.1	3.27	54.8	5.68	2.59	33.6	0.61	0.10	5.09
	220 °C	68.3	29.0	2.64	59.0	5.59	2.59	30.2	0.51	0.09	4.20

. For the raw tangerine sample, the contents of volatile matter, fixed carbon, and ash were found to be 75.7%, 20.5%, and 3.80%, respectively. As the HTC process temperature increased from 140 °C to 220 °C, the volatile matter content decreased by about 10% (79.1% → 68.3%), while the fixed carbon content increased from 18.0% to 29.0%. As intended by the HTC process, which aims to induce hydrolysis and dehydration of W-T, the elemental analysis showed a decreasing trend in the O/C and H/C ratios. This is attributed to the removal of hydroxyl and carboxyl groups among the oxygen-containing functional groups in the precursor. As the HTC temperature increased, the yield initially rose from 4.25% to a maximum of 6.65%, before decreasing again to 4.20%. In this study, T-HTC-180 °C, which exhibited the highest yield, was determined to be the optimal sample.

3.1.2. Microstructural Properties of HTC Samples

To investigate the crystallinity of biochar under the influence of residence temperature, Figure 3 shows the XRD patterns of biochar produced at various temperature conditions for 0.5 h.

In the case of tangerine peel raw and T-HTC-140, 16° (100) and 22° (220) are clearly indistinguishable XRD patterns, which means relatively low crystallinity. On the other hand, from the HTC temperature of 160 °C, the two peaks were clearly differentiated, and an additional 34.5° (004) peak was observed, indicating that the HTC has changed to the cellulose-A type.

Notably, a peak corresponding to T-carbon was observed in biochar treated at temperatures above 160 °C. T-carbon is observed at 26°, which is known as a unique type of carbon with a structural order between amorphous carbon and crystalline carbon [24]. Through Raman spectroscopy analysis, the crystallite of HTC begins to change at a synthesis temperature above 220 °C (Supplementary Figure S2).

The researchers determined that an HTC temperature of 180 °C was the optimal condition, based on previous findings [19] and the highest HTC yield. Accordingly, samples performed at 180 °C were selected for the second stage (dry carbonization).

3.2. Dry Carbonization Effect

3.2.1. Element Analysis of D-C Samples

Table 2. Element contents of tangerine-derived carbons at various carbonization temperatures.

Samples		Proximate Analysis (wt.%)				Element Analysis (wt.%)					Yield (%)
	Temp.	Volatile	Fixed Carbon	Ash	Carbon	Hydrogen	Nitrogen	Oxygen	O/C	H/C
T-HTC-180	-	74.4	22.3	3.90	53.9	5.84	2.68	34.3	0.64	0.11	-
T-DC-400	400 °C	30.2	63.0	6.72	66.2	4.05	2.88	20.1	0.30	0.06	38.6
T-DC-500	500 °C	19.1	74.3	6.66	71.9	2.75	3.37	15.3	0.21	0.04	32.3
T-DC-600	600 °C	17.1	76.2	6.70	73.5	1.80	3.22	14.8	0.20	0.02	31.3

lists the elemental analysis results for the carbonized materials. The char material heat-treated at 600 °C had the highest carbon mass fraction (76.2%), which decreased due to the dissociation and decomposition of relatively weak oxygen functional groups during the heat treatment stage. As the heat treatment temperature increased, the H/C ratio decreased, indicating that the structure stabilized with increasing temperature.

3.2.2. Microstructural Properties of D-C Samples

Figure 4 presents the XRD patterns of carbon materials that secondary dry carbonization (DC) at 400, 500, and 600 °C after HTC treatment. It is seen that the pattern is significantly different from the T-HTC-180 sample that underwent only the HTC process, indicating an XRD pattern of amorphous carbon. As the carbonization temperature increased, the full width at half maximum (FWHM) of the relatively broad (002) peak decreased. From the results, the amorphous structure of biochar is inversely proportional to the carbonization temperature.

Additionally, a weakly developed 10ℓ peak, where the (100) and (101) peaks are mixed, was observed near 43° at T-DC-500 °C and T-DC-600 °C, respectively. This indicates that the carbon atomic layers within the structure are incompletely stacked, typically observed in turbostratic structures. Raman spectra revealed distinct D and G bands in the samples subjected to dry carbonization. At carbonization temperatures of 500 °C and above, the G’ band, indicative of carbon structural alignment and graphitization, also became apparent (Supplementary Figure S2). This suggests a reduction in defects and the development of a relative crystallite structure at these temperatures. Additionally, the narrow peak around 30 degrees may correspond to the CaCO₃ form added during the tangerine cultivation process.

3.3. Chemical Activation Effect

3.3.1. Textural Properties

FE-SEM analysis was conducted to examine the surface morphology of the produced char and activated carbon, with the results presented in Supplementary Figure S3. As the activation ratio increased, significant changes in surface roughness were observed. In the image (h), a slight distribution of snowflake-like crystals was identified, while image (i) has higher thermal stability than other activation agents, resulting in a relatively smooth surface despite the activation process.

Nitrogen adsorption–desorption isotherms are generally developed analytical techniques to investigate the specific surface area and porosity of porous materials. The N₂ adsorption results for tangerine-based AC measured at 77 K are shown in Figure 5a. As the ratio of the activation agent increased, the amount of nitrogen adsorption increased, indicating that the microporosity of the sample had developed. The shape of the adsorption isotherm curves slightly varied with increasing activation agent ratio. All samples with relatively low agent ratios (T-KOH-1.0 and 2.0) and T-K₂CO₃-3.0 were classified as IUPAC Type I [25]. This indicates monolayer adsorption due to strong interactions between the AC’s pores and N₂ molecules at relative pressures (P/P₀) below 0.05, signifying a well-developed microporous structure. The isotherm adsorption curves of T-KOH-3.0, 4.0, NaOH-3.0, and commercial AC were observed as IUPAC Type IV, showing adsorption phenomena and a slight hysteresis loop at relative pressures above 0.05, indicating that both micropore and mesopore are developed in these materials [26].

It was also observed that the slope of the knee, the inflection point that appears in the relative pressure region below 0.05 P/P₀, decreased sharply as the agent ratio increased. Samples with a low agent ratio (T-KOH-1.0 and 2.0) showed relatively high curvature, and the knee curvature was found to decrease sharply as the ratio of the agent involved in the activation reaction increased [27]. This indicates an expansion of the micropore diameter.

The adsorption isotherm expressed in logarithmic pressure can be useful for identifying structural differences in the relative pressure region. Figure 5b shows the adsorption volume of T-AC in logarithmic pressure to investigate high-pressure behavior. At relatively low-pressure ranges, the difference in adsorption capacity was not significant, but as the relative pressure increased, the difference in adsorption capacity became more pronounced.

Table 3. Textural properties of tangerine-derived carbons at various activation agent ratio conditions.

Samples	SBET (m2/g)	VTotal (cm3/g)	VMeso (cm3/g)	VMicro (cm3/g)	FMicro (%)	Yield (%)
T-KOH-1.0	1530	0.73	0.11	0.62	84.9	64.2
T-KOH-2.0	2500	1.18	0.10	1.08	91.5	61.4
T-KOH-3.0	3158	1.65	0.17	1.48	89.7	52.6
T-KOH-4.0	3375	2.00	0.18	1.72	86.0	47.0
T-NaOH-3.0	1985	1.21	0.25	0.96	79.3	55.0
T-K2CO3-3.0	1258	0.57	0.08	0.43	84.3	63.2
Commercial AC	2242	1.43	1.31	0.12	8.39	-

shows the pore characteristics of T-AC depending on the ratio and type of activation agent. As the KOH agent ratio increased from 1.0 to 4.0, the activation yield decreased from 64.2% to 47.0%. This is attributed to the ongoing oxidation reaction between the KOH agent and the carbon material.

6KOH + 2C → 2K* + 3H₂ + 2K₂CO₃

(2)

K₂CO₃ + 2C → K₂O + 2CO

(3)

K₂CO₃ → K₂O + CO₂

(4)

2K* + CO₂ → K₂O + CO

(5)

The reaction indicates that when carbonaceous materials are heated with KOH in an inert atmosphere at temperatures above approximately 700 °C, metallic potassium (K*) is formed. Previous studies [6] have reported the formation of K₂CO₃ starting at around 400 °C, with nearly all of the KOH converting to K₂CO₃ by about 600 °C. Additionally, the presence of K₂O was detected at temperatures exceeding 700 °C, becoming the sole remaining compound by 800 °C. The formation of K₂O at temperatures above 700 °C can be attributed to several reactions with significantly negative free energy values at this temperature, such as the decomposition of K₂CO₃ into K₂O and CO₂, the reaction between K₂CO₃ and carbon, or the reaction of K* with CO₂. Furthermore, K₂O is expected to be reduced by carbon to potassium, a process that shows a negative free energy change starting at around 800 °C [28,29,30].

In contrast, at the same activation agent ratio (3.0), the activation yield was highest with K₂CO₃ (63.2%). This suggests that K₂CO₃, with its higher thermal stability (decomposition temperature = 891 °C), had little reactivity with the carbon material.

The specific surface area (S_BET) and total pore volume (V_Total) of T-AC increased with the KOH activation agent ratio, reaching 1530–3375 m²/g and 0.73–2.00 cm³/g, respectively. Additionally, the micropore volume continuously increased from 0.62 cm³/g to 1.72 cm³/g. In contrast, the specific surface area (S_BET) and total pore volume (V_Total) for NaOH and K₂CO₃ were observed to be 1985 m²/g and 1258 m²/g, respectively, which is attributed to the differences in thermal stability between KOH and K₂CO₃. The thermal decomposition temperature of K₂CO₃ is 891 °C, and for NaOH, it is 1388 °C. Due to their higher thermal stability compared to KOH (700 °C) at the activation temperature (850 °C), this results in a relatively larger difference, even at the same activation agent ratio.

The pore size distributions of all the ACs produced in this study were calculated based on non-localized density functional theory (NLDFT) and are shown in Figure 6. Also, the pore distribution calculated from NLDFT was obtained from the nitrogen adsorption isotherm. All ACs exhibited a sharp pore volume near 0.7 nm. In T-KOH-1.0 and 2.0, the pore volumes corresponding to 1.0 and 2.0 nm gradually increased, and particularly, as the KOH ratio increased from 3.0 to 4.0, an increase in pore volume in the specific pore region (2.0–4.0 nm) of T-AC was observed.

Additionally, the pore distribution revealed differences in pore diameters developed depending on the type of agent, even at the same agent ratio. The developed diameter of T-K₂CO₃-3.0 showed characteristics very narrow range. K₂CO₃, with a melting point of 891 °C in an N₂ atmosphere, faced limitations in directly reacting with and penetrating the carbon material at the experimental condition of 850 °C, resulting in less developed porosity.

On the other hand, T-NaOH-3.0 showed a wider pore distribution ranging from 1.0 nm to 5.0 nm compared to T-KOH-3.0, likely due to the differences in ionic diameter between K+ (133 pm) and Na+ (95 pm) ions.

The specific pore region developed in T-KOH-AC produced in this study corresponds to the sub-mesopore (1.5–4.0 nm) region, which previous studies have reported to improve pore selectivity for specific gases (e.g., n-butane) [31].

3.3.2. Physicochemical Properties

The elemental content of the ACs observed through elemental analysis, and XPS is listed in

Table 4. Element contents of tangerine-derived ACs at various chemical activation conditions.

	Element Analysis (wt.%) via E.A						Element Analysis (at.%) via XPS
	Carbon	Hydrogen	Nitrogen	Oxygen	O/C	H/C	C1s	O1s
T-KOH-1.0	78.9	0.30	1.82	4.47	0.06	0.005	75.46	24.44
T-KOH-2.0	81.1	0.48	2.10	4.18	0.05	0.005	79.40	20.56
T-KOH-3.0	88.3	0.20	2.64	2.58	0.03	0.003	89.77	10.23
T-KOH-4.0	87.3	0.67	0.24	2.61	0.04	0.008	88.21	11.77
T-NaOH-3.0	83.8	0.33	1.03	3.31	0.04	0.004	80.26	19.46
T-K2CO3-3.0	84.1	2.58	0.37	3.44	0.04	0.031	86.16	13.81

. Both analyses showed that T-KOH-3.0 had the highest carbon content (88.3 wt.%) and the lowest oxygen content (2.58 wt.%) among the samples.

In general, oxygen functional groups on the surface of carbon materials are classified based on their acidity or basicity. During the activation process, the activation agent acts as an oxidizing agent that reacts with the carbon material, and the type and content of oxygen functional groups vary depending on the agent ratio, temperature, or type.

In this study, the composition and content of oxygen functional groups were investigated by deconvoluting the O_1s peak into four distinct peaks, and the results are shown in Figure 7: O-I (530.6 eV, C=O quinine-type groups), O-II (531.8 eV, C=O in anhydrides, esters, and C-OH in hydroxyl), O-III (533.2 eV, C-O-C in anhydrides and esters), O-IV (534.9 eV, COOH groups) [32].

The concentration of O_1s determined via XPS exhibits a declining trend as the proportion of the KOH agent increases. In contrast, NaOH and K₂CO₃ had relatively higher oxygen functional group content compared to the same agent ratio (T-KOH-3.0). This suggests that NaOH and K₂CO₃ were more effective in facilitating the introduction of oxygen functional groups compared to KOH.

Moreover, as the KOH agent ratio increased, the proportion of the O-II peak (C=O in anhydrides, esters, and C-OH in hydroxyl) also increased.

In all the produced ACs, the content order was observed as O-III > O-II > O-IV > O-I. Notably, the proportion of O-III increased from 31.5% to 35.2% as the agent ratio increased. The C_1s spectrum, deconvoluted into five peaks as shown in Supplementary Figure S4, showed a significant decrease in the peak corresponding to sp² carbon bonds at 283 eV during the activation process. Concurrently, the content of COOH groups increased with progressive KOH activation, a trend that aligns with the rise in the O-IV component observed in the O1s spectrum. This suggests that KOH activation oxidizes sp² carbon bonds, promoting the formation of oxygen-containing functional groups such as COOH, which results in a reduction in sp² carbon bonds and an increase in COOH groups.

3.4. MC Adsorption Property

The MC adsorption–desorption performance of the T-AC produced in this study was determined according to the modified ASTM D5228. The ASTM D5228 method evaluates adsorption by measuring the saturated weight after MC is adsorbed for a certain period. As shown in Figure 8, the KOH agent ratio increased, methylene chloride activity (MA) increased from 44.7% to 76.3%, while methylene chloride retentivity (MR) decreased from 27.6% to 20.4%. Consequently, the methylene chloride working capacity (MWC) demonstrated a substantial increase, ranging from 17.1% to 55.9%.

Compared to KOH-based AC, NaOH and K₂CO₃-based AC demonstrated distinct properties in contrast trends. The MA of NaOH and K₂CO₃-based AC was 36.9% and 37.3%, respectively, and the MR was found to be 9.74% and 14.7%. The MWC, considering both MA and MR, was found to be 27.1% and 22.7%, respectively. Notably, the MAs and MRs of T-NaOH-3.0 and T-K₂CO₃-3.0 differed by about one and a half times. These observations are likely related to differences in pore characteristics, corroborating findings from previous research indicating a robust correlation between MA and micropore volume, and MR and mesopore volume. Applying these findings to the current study, the micropore volumes of T-NaOH-3.0 and T-K₂CO₃-3.0 were similar at 0.96 cm³/g and 1.08 cm³/g, respectively, while the mesopore volumes differed by about twofold at 0.25 and 0.10 cm³/g. T-NaOH-3.0, which had the highest mesopore volume, showed the lowest MR. Considering that the MWC performance evaluates both a high MA and a low MR, it is evident that among the activated carbons produced, the T-KOH-4.0 sample, which exhibits the highest MWC at 55.9%, is optimally suited for the adsorption of methylene chloride.

Additionally, the MC adsorption performance of commercial coal-based AC (S_BET: 2242 m²/g) was compared with that of tangerine-based AC. The MA and MR values of the commercial AC were 52.5% and 15.6%, respectively, resulting in an MWC value of 36.9%. When considering only the MWC, it was found to be comparable to that of T-KOH-3.0, which exhibited a similar value of 37.0%.

3.5. Correlation of MC Adsorption Property and Surface Properties

Several studies indicated that the adsorption selectivity of activated carbon is influenced by its surface chemical properties [33,34]. Given that MC is a polar gas, this study examined the relationship between MC and the oxygen functional groups present on the adsorbent surface, and the results are shown in Figure 9.

To investigate the relationship between the MA value, which indicates adsorption performance, and the content of oxygen functional groups, the fraction of each peak was considered. According to the results, for KOH-activated samples, the O-IV peak (COOH groups) showed the highest correlation (R² = 0.83). In contrast, NaOH and K₂CO₃-activated samples showed a low correlation with all oxygen functional group peaks.

The correlation was more distinct for MR, which indicates desorption performance. Generally, during the desorption process, the binding energy between the adsorbed gas and the oxygen functional groups on the surface of the adsorbent plays a more dominant role than in adsorption.

As with the MR results, KOH-activated samples showed the highest correlation with the O-IV peak (R² = 0.92), while NaOH and K₂CO₃-activated samples showed a high correlation with the O-III peak (R² = 0.96).

As shown in Figure 10, In the study by Jeong et al. [35], cellulose-based ACF was fabricated and tested for MC adsorption–desorption performance. In contrast, our research focuses on tangerine peel-based AC. From a material perspective, ACF and AC exhibit distinct properties, which explains the higher MR value observed in our study compared to previous research. Additionally, the enhanced pore characteristics of our sample resulted in a higher MA value, ultimately leading to an approximate 4.1% increase in the MWC value.

4. Conclusions

This study selected waste tangerine peels (W-T), which are generated as waste, as a precursor for AC to examine its potential for removal of MC from volatile organic compound gases. By applying alkaline agents, it was converted into AC with a high specific surface area, and as the KOH agent ratio increased, the specific surface area (S_BET) and total pore volume (V_Total) of the AC were obtained as 1530–3375 m²/g and 0.73–2.00 cm³/g, respectively, with a selective increase in pores in the sub-mesopore range of 2.0–4.0 nm. NaOH and K₂CO₃ activations were found to be less favorable for porosity development compared to KOH, but more advantageous in the introduction of oxygen functional groups. The MWC performance, conducted to apply the AC for MC removal, reached a maximum of 55.9%, indicating that KOH activation is more suitable compared to NaOH or K₂CO₃. The tangerine peel-based AC produced in this study could contribute to future society by repurposing difficult-to-manage waste materials into precursors for AC.