Optimized Adsorptive Desulfurization Using Waste Tire-Derived Carbon

Abstract

The inclusion of adsorption thermodynamic analysis and performance benchmarking with existing adsorbents reinforces both the theoretical significance and practical applicability of this study. The modified rubber-derived carbon demonstrated a remarkably high DBT adsorption capacity of 254.45 mg/g. These results establish it as a promising alternative to conventional materials such as commercial activated carbon, zeolites, and even metal–organic framework materials. In addition to confirming the superior performance of the adsorbent, the findings provide a deeper understanding of the DBT adsorption mechanism and offer a solid scientific basis for large-scale fuel desulfurization applications. This research highlights the potential of transforming end-of-life tire waste into value-added functional materials and contributes to the advancement of sustainable and efficient desulfurization technologies. Future work should focus on optimizing surface functionalization and regeneration strategies to further improve long-term adsorption stability and practical deployment.

1. Introduction

Sulfur has long been an unavoidable impurity in fossil fuels. When burned, sulfur-containing compounds generate sulfur oxides (SOₓ), which contribute to acid rain, smog, and respiratory illnesses [1]. To mitigate these environmental and health hazards, governments worldwide have imposed increasingly stringent regulations on fuel sulfur content. The International Maritime Organization (IMO), for example, has lowered the sulfur cap in marine fuels to 0.5% since 2020, while the European Union and the United States have set even stricter standards for road transportation fuels [2,3]. However, achieving ultra-low sulfur fuels is not a straightforward process, particularly given the increasing reliance on heavier crude oils that contain higher sulfur levels [4].

Traditionally, hydrodesulfurization (HDS) has been the backbone of fuel desulfurization, utilizing high temperatures (300–400 °C) and high-pressure hydrogen (10 atm) to break sulfur-carbon bonds. While effective for removing simple sulfur compounds, HDS struggles with dibenzothiophene (DBT) and its derivatives, which are deeply embedded in the hydrocarbon matrix and resistant to catalytic hydrogenation [2,5]. Deep desulfurization requires more severe reaction conditions and costly catalysts, such as Co-Mo and Ni-Mo, making the process both economically and environmentally burdensome [6]. As a result, researchers have explored alternative methods, including oxidative desulfurization, biodesulfurization, extractive desulfurization, and most notably, adsorptive desulfurization, which offers a promising low-energy, high-selectivity approach [7,8,9,10].

The efficiency of adsorptive desulfurization hinges on a crucial factor: the choice of adsorbent. To effectively capture DBT from fuel, an ideal adsorbent must possess a high surface area, well-developed porosity, and strong chemical affinity for sulfur molecules [11]. Various materials have been studied, including metal–organic frameworks (MOFs) [12], zeolites [13], and metal oxide composites [14]. Among them, activated carbon stands out for its tunable surface chemistry, stability, and cost-effectiveness. However, unmodified activated carbon has limited affinity for DBT due to the lack of oxygen-containing functional groups, which are essential for strong adsorption interactions [12,15]. To enhance adsorption performance, researchers have modified activated carbon using different functionalization strategies. For instance, Sentorun-Shalaby et al. improved DBT adsorption by impregnating mesoporous molecular sieve MCM-48 with nickel oxide, which introduced strong metal–sulfur interactions and increased DBT removal efficiency [16]. Similarly, Nejad et al. demonstrated that ordered mesoporous carbon CMK-5 exhibits superior sulfur removal compared to conventional activated carbon due to its well-defined pore structure and tailored surface chemistry [17].

Beyond traditional carbon sources, waste tires have emerged as a promising precursor for activated carbon due to their high carbon content and inherent porosity. Every year, millions of discarded tires accumulate in landfills, creating an environmental crisis. Recent studies have shown that tire-derived activated carbon possesses a highly porous structure, making it suitable for adsorption applications. However, its adsorption efficiency for thiophenic sulfur compounds remains suboptimal, necessitating further chemical modifications to improve performance [18,19,20]. The ability to repurpose waste tires into highly effective desulfurization adsorbents not only addresses the persistent issue of sulfur emissions but also contributes to sustainable waste management.

Each year, approximately 1.5 billion tires reach end-of-life globally, leading to significant waste management concerns [21]. Simultaneously, conventional adsorbents used for fuel purification, such as commercial activated carbons, remain expensive and energy-intensive to produce. Compared with these materials, waste tire-derived carbon offers a sustainable and low-cost alternative. This study not only repurposes tire waste but also optimizes the activation and modification process to achieve high DBT removal performance. These advantages highlight the environmental and economic viability of this method for future fuel desulfurization technologies. Although the valorization of waste tire rubber into carbon-based adsorbents has been previously reported, most studies have focused either on surface area enhancement or elemental sulfur removal under idealized conditions. This study advances the field by systematically optimizing both activation (H₃PO₄) and functionalization (HNO₃) conditions to tailor pore structure and surface chemistry for selective DBT adsorption.

This study explores the transformation of waste tires into high-performance DBT adsorbents through a two-step chemical modification process. First, phosphoric acid activation is used to increase porosity and surface area. Then, nitric acid oxidation introduces oxygen-containing functional groups, such as hydroxyl and phenol, which strengthen DBT adsorption through π–π interactions and hydrogen bonding. The results reveal that the adsorption process follows the Langmuir–Freundlich isotherm model, indicating monolayer adsorption, while kinetic studies confirm a PFO mechanism, suggesting that physisorption plays a dominant role.

2. Results

In the pursuit of an efficient and sustainable adsorbent for fuel desulfurization, the structural and chemical transformation of rubber-derived carbon plays a crucial role. Through a carefully designed process involving sulfuric acid treatment, phosphoric acid activation, and nitric acid modification, we examine how pore structure, surface chemistry, and crystalline properties evolve to enhance DBT adsorption performance.

The untreated rubber-derived carbon, despite possessing some porous characteristics, contained a significant amount of Si, S, and Zn, which were introduced during tire manufacturing. These impurities hinder adsorption performance by blocking active sites and introducing unwanted competing interactions. Upon sulfuric acid treatment, a significant purification effect was observed. EDS (

Table 1. EDS results showing elemental composition before and after sulfuric acid treatment.

Sample	Carbon Content (%)	Major Impurities (Si, Zn, S) (%)
Untreated	85.32	14.68
Sulfuric Acid-Treated	95.68	4.32

) revealed a marked reduction in impurities after acid treatment, with the carbon content increasing to 95.68%. This suggests that sulfuric acid effectively removed unwanted elements while retaining the essential carbon framework.

Additionally, XRD analysis (Figure 1) further confirmed the structural transformation induced by acid treatment. The diffraction pattern of the untreated carbon sample displayed several sharp peaks in the 2θ range of 25–60°, which can be attributed to the presence of inorganic impurities such as SiO₂, ZnO, and CaCO₃, commonly found in tire-derived materials [22]. After sulfuric acid treatment, these crystalline peaks largely disappeared, and the XRD pattern exhibited two broad humps centered around 2θ = 20–30° and 40–50°. This pattern is indicative of an amorphous carbon structure with a turbostratic, graphitic-like arrangement, lacking long-range order [19]. The absence of well-defined peaks implies enhanced structural disorder, which is favorable for adsorption due to the increase in surface heterogeneity and accessible active.

As shown in Figure 2, the nitrogen adsorption–desorption isotherms of the activated carbon samples exhibit typical Type IV behavior, which is indicative of mesoporous structures. The adsorbed nitrogen quantity increases progressively with activation temperature, reaching a maximum at 650 °C. This trend reflects enhanced surface area and pore development resulting from effective phosphoric acid activation. However, at 750 °C, a significant decrease in nitrogen uptake is observed, suggesting structural degradation and partial pore collapse due to excessive thermal treatment [23].

As shown in

Table 2. Effect of activation temperature on adsorbent pore structure.

Activation Temperature °C	Specific Surface Area m2/g	Pore Volume cm3/g	Average Pore Diameter (nm)
350	661.47	0.40	3.33
450	728.44	0.42	2.64
550	813.43	0.44	2.62
650	926.44	0.51	2.21
750	641.89	0.39	3.30

, the activation temperature significantly influenced the pore structure of the resulting carbon adsorbents. The specific surface area increased steadily with a temperature from 350 °C (661.47 m²/g) to a maximum of 926.44 m²/g at 650 °C, accompanied by a corresponding rise in pore volume to 0.51 cm³/g. Simultaneously, the average pore diameter decreased from 3.33 nm to 2.21 nm, indicating the development of a more finely structured microporous network. These structural improvements at 650 °C facilitated the formation of high-density active sites and improved molecular diffusion pathways, which are crucial for effective DBT adsorption. As shown in Figure 2, the nitrogen adsorption desorption isotherms exhibit combined features of Type I and Type IV profiles, suggesting the coexistence of microporous and mesoporous structures. The pore size distribution confirms that, in addition to mesopores ranging from 2 to 5 nanometers, a substantial proportion of micropores between 0.8 and 2.0 nanometers is present. These structural features are most pronounced in the sample activated at 650 degrees Celsius, which also exhibits the highest surface area and pore volume, as detailed in Table 2. Given that the kinetic diameter of DBT is approximately 0.74 nanometers, the presence of large micropores allows effective physisorption through size-compatible interactions. These micropores provide access to internal surface sites and enhance van der Waals forces between DBT molecules and the carbon framework.

The impact of activation temperature on DBT removal efficiency is further illustrated in Figure 3. The adsorbent prepared at 650 °C exhibited the highest removal performance, confirming that the optimized pore architecture directly contributes to enhanced adsorption capacity. However, a sharp decline in performance was observed at 750 °C, despite the relatively high temperature. This reduction is attributed to excessive thermal degradation and the volatilization of phosphoric acid, which led to the collapse of micropores and a subsequent decrease in surface area and pore accessibility. As a result, both the physical structure and chemical functionality of the adsorbent were compromised, diminishing its effectiveness.

To visualize these morphological changes shown in Figure 4, at lower activation temperatures (350–450 °C), the carbon material exhibited irregular, non-porous structures. At 650 °C, a well-developed, interconnected porous network emerged, maximizing surface area and adsorption capacity. However, at 750 °C, structural collapse was evident, confirming the BET results. Thus, 650 °C was selected as the optimal activation temperature, balancing pore development and structural stability.

Although a high surface area enhances adsorption, the presence of oxygen-containing functional groups plays an equally important role in DBT removal by introducing π–π interactions and hydrogen bonding sites.

Table 3. Boehm titrates the preparation of sorbents at different activation temperatures.

Activation Temperature (°C)	Acid Functional Group, mmol/g
	Lipid	Hydroxy	Phenol	Total Acid
350	0.14	0.25	0.75	1.14
450	0.14	0.25	0.72	1.11
550	0.13	0.21	0.61	0.95
650	0.13	0.21	0.60	0.94
750	0.12	0.17	0.55	0.84

indicates that at lower activation temperatures, phenol and hydroxyl groups are more abundant, while higher temperatures favor pure carbonaceous structure. Given that DBT adsorption relies on both pore structure and chemical interactions, the balance between surface area and functional groups was found at 650 °C.

While phosphoric acid activation created a highly porous structure, DBT adsorption still relied on chemical interactions with surface functional groups. To enhance these interactions, the adsorbent underwent nitric acid oxidation, introducing oxygen-containing groups that facilitate π–π interactions and hydrogen bonding with DBT molecules. To further improve DBT adsorption, nitric acid oxidation was employed to introduce additional functional groups.

Table 4. Boehm titration of adsorbents prepared at different nitric acid modification temperatures. nitric acid concentration: 15 M, modifier time: 3 h.

Modification Temperature (°C)	Acid Functional Group, mmol/g
	Lipid	Hydroxy	Phenol	Total Acid
AC *	0.13	0.21	0.60	0.94
30	0.21	0.30	1.21	1.72
60	0.28	0.38	1.51	2.17
70	0.30	0.41	1.53	2.24
80	0.25	0.44	1.56	2.28
90	0.27	0.48	1.61	2.36

* AC refers to the unmodified activated carbon prepared at 650 °C without nitric acid treatment.

shows a significant increase in oxygen-containing functional groups, particularly at 70 °C, beyond which excessive oxidation caused structural degradation. Boehm titration results show that as modification temperature increased, the total surface acidity rose from 0.94 mmol/g to 2.24 mmol/g at 70 °C, beyond which the adsorption efficiency plateaued.

As shown in Figure 5, SEM images of the activated carbon samples treated with nitric acid at varying temperatures (30 °C to 90 °C) reveal significant morphological evolution. The sample treated at 30 °C (Figure 5a) exhibits a relatively smooth and compact surface, indicating limited pore development. With increasing treatment temperature to 60 °C and 70 °C (Figure 5b,c), the surface becomes progressively rougher and more porous, suggesting enhanced surface etching and pore exposure. Notably, the 70 °C sample shows the most well-developed and interconnected pore structure, which is favorable for mass transfer during adsorption. These morphological changes are further supported by the elemental composition analysis shown in the adjacent table in Figure 5. The oxygen content increases from 10.45 wt% at 30 °C to a maximum of 16.77 wt% at 70 °C, indicating the introduction of oxygen-containing functional groups such as carboxyl, hydroxyl, and lactonic groups due to nitric acid oxidation. Beyond 70 °C, the oxygen content slightly decreases or stabilizes, suggesting possible degradation or saturation of surface functionalization. This correlation between surface morphology and oxygen enrichment confirms that moderate nitric acid treatment at 70 °C optimizes both surface area and chemical functionality, which are critical factors governing DBT adsorption efficiency.

As shown in Figure 6, all nitric acid-modified samples exhibit Type IV nitrogen adsorption–desorption isotherms with H3-type hysteresis loops, confirming the presence of mesoporous structures. Among these, the sample treated at 70 °C shows the highest nitrogen uptake across the entire relative pressure range, indicating a larger accessible surface area. This result aligns with the enhanced DBT adsorption performance and suggests that moderate acid treatment effectively improves the porosity of the carbon framework. The observed increase in nitrogen adsorption may be attributed to the oxidative effect of nitric acid, which can selectively etch the carbon surface and open previously inaccessible pores. On the other hand, treatments at higher temperatures (80–90 °C) show slightly reduced nitrogen uptake, likely due to partial pore collapse or over-oxidation. These findings corroborate the role of surface area and pore accessibility, in addition to functional group chemistry, in determining adsorption performance after HNO₃ modification.

To further understand the role of surface functional groups in adsorption behavior, Boehm titration was conducted to quantify the concentration of oxygen-containing acidic sites on the carbon surface. As illustrated in Figure 7, the total content of carboxylic, lactonic, and phenolic groups significantly increased after HNO₃ modification. Notably, the HNO3-70 sample exhibited the highest concentration of all three functional groups, with carboxylic groups increasing from 0.35 to 0.55 mmol/g, and phenolic groups from 0.45 to 0.61 mmol/g compared to the unmodified AC-650. These oxygen-containing groups provide additional polar sites, which enhance hydrogen bonding and polar–π interactions with the sulfur-containing DBT molecule. This rise in surface acidity is consistent with the observed improvement in adsorption capacity, highlighting that surface chemistry—particularly the introduction of acid functional groups—is a critical factor in determining DBT uptake performance.

As shown in Figure 8, the equilibrium data for DBT adsorption onto the HNO₃-modified activated carbon were fitted using the Langmuir, Freundlich, and Langmuir–Freundlich isotherm models [24,25]. The Langmuir model (Figure 8a), which assumes monolayer adsorption on a homogeneous surface with a finite number of identical active sites, provided a strong correlation with the experimental data (R² = 0.9933). The maximum adsorption capacity (q_max) predicted by the Langmuir model was 254.45 mg g⁻¹, indicating that DBT molecules form a single molecular layer over the adsorbent surface without significant lateral interaction. The Freundlich model (Figure 8b), which assumes multilayer adsorption on a heterogeneous surface with a non-uniform distribution of heat of adsorption, showed a comparatively lower R² value, suggesting that it is less suitable for this system. However, the fitting still reveals a moderate degree of surface heterogeneity. In contrast, the Langmuir–Freundlich model (Figure 8c), a hybrid that incorporates features of both monolayer and heterogeneous adsorption, yielded the highest correlation coefficient (R² = 0.9971). This suggests that while monolayer adsorption is the dominant mechanism, the surface of the modified carbon exhibits a degree of energetic and structural heterogeneity. This heterogeneity likely arises from variations in surface chemistry and pore structure introduced during the phosphoric acid activation and nitric acid functionalization processes. The superior fit of the Langmuir–Freundlich model highlights the dual contribution of structural porosity and chemical functionalization in determining DBT adsorption. The presence of micropores enhances molecular confinement and physisorption, while oxygen-containing surface groups provide additional affinity via π–π interactions and hydrogen bonding. Together, these features account for the high adsorption capacity and support the interpretation that the adsorbent surface is not perfectly uniform but includes a range of adsorption site energies.

As shown in Figure 9, the adsorption kinetics of DBT were analyzed using both the pseudo-first-order (PFO) and pseudo-second-order (PSO) models [25,26]. In Figure 9 a, the PFO model demonstrates a moderate linear relationship between ln (qₑ–qₜ) and time (R² = 0.946), and the calculated equilibrium adsorption capacity (qₑ = 1.89 mg/g) is nearly identical to the experimental value (1.88 mg/g), indicating a good physical fit. This suggests that the adsorption is primarily governed by physisorption mechanisms such as van der Waals interactions or pore filling. In contrast, the PSO model shown in Figure 9 b yields an excellent linear fit (R² = 0.999), implying a strong statistical correlation. However, the calculated qₑ is 10.00 mg/g substantially higher than the experimental value indicating a significant overestimation. This suggests that while the PSO model fits the data mathematically, it may misrepresent the actual mechanism or rate-controlling step in this system. Such discrepancies could be attributed to surface heterogeneity or non-ideal adsorption behavior.

The adsorption kinetics were further evaluated by comparing the calculated and experimental equilibrium adsorption capacities derived from the PFO and PSO models, as summarized in

Table 5. Comparison of kinetic model fitting results for DBT adsorption.

	Cal. qe (mg/g)	Exp. qe (mg/g)	K Value	R2
PFO	1.89	1.88	0.1	0.946
PSO	10	1.88	0.0052	0.999

. The PFO model yielded a calculated qe of 1.89 mg/g, which is in excellent agreement with the experimentally observed value of 1.88 mg/g, with a deviation of less than 0.5%. This close fit indicates that the adsorption process is likely governed by physisorption mechanisms such as van der Waals interactions or pore diffusion. In contrast, the PSO model produced a much higher calculated qe value of 10.00 mg/g, resulting in a deviation exceeding 400% despite the model’s high correlation coefficient (R² = 0.999). Such a large overestimation suggests potential overfitting or numerical error, and it may not accurately reflect the actual adsorption behavior. Therefore, while the PSO model appears statistically superior, the PFO model is more physically reasonable and consistent with the experimentally observed adsorption performance.

The exothermic nature of DBT adsorption was confirmed by adsorption temperature studies (Figure 10), which showed that increasing temperature resulted in a decrease in DBT removal efficiency. This behavior is characteristic of physisorption supplemented by physisorption, where higher temperatures disrupt van der Waals forces, π–π interactions, and hydrogen bonding, leading to lower adsorption capacity.

Table 6. Comparative adsorption performance of different adsorbents for DBT removal.

	DBT Adsorption (mg/g)	Surface Area (m2/g)	Ref.
Waste tire-derived activated carbon	254.45	926.44	This work
Magnetic activated carbon/γ-Fe2O3 composite	38.0	200–400	[27]
ZnCl2-activated mesoporous carbon nanospheres	70–90	3000	[28]
CMK-5 mesoporous carbon	130	1000	[29]
MOF-derived TiO2/carbon composite	190	1500–2000	[30]
Coconut shell activated carbon	80–120	750–800	[31]
Polystyrene-based activated carbon	153	2022	[32]

presents a comparative analysis of DBT adsorption performance among a range of reported adsorbents. Materials such as CMK-5 and MOF-derived TiO₂/carbon composites demonstrate high adsorption capacities; however, their synthesis often requires complex multi-step procedures, specialized templates, or expensive precursors, which may limit their practical scalability and cost-efficiency. In comparison, the waste tire-derived activated carbon developed in this study exhibits a DBT adsorption capacity of 254.45 mg/g. This performance not only surpasses that of conventional activated carbons and low-cost biowaste-derived materials such as coconut shell carbon but also approaches or exceeds the performance of highly engineered porous structures. Remarkably, this high adsorption capacity is achieved with a surface area of 926.44 m²/g, which is moderate relative to some mesoporous counterparts. This indicates that surface chemistry, particularly the presence of oxygen-containing functional groups introduced through nitric acid modification, plays a more dominant role than surface area alone in enhancing DBT affinity.

3. Discussion

The inclusion of adsorption thermodynamic analysis and performance benchmarking with existing adsorbents reinforces both the theoretical significance and practical applicability of this study. The modified rubber-derived carbon exhibited an outstanding DBT adsorption capacity of 254.45 mg/g, surpassing many conventional materials such as commercial activated carbon, zeolites, and metal–organic framework materials. In addition to confirming the superior adsorption performance, the results provide valuable insights into the adsorption mechanism of DBT and establish a solid scientific basis for the development of large-scale fuel desulfurization technologies. This research demonstrates the potential of converting waste tires into high-performance functional materials, supporting both environmental sustainability and industrial relevance. Future work may focus on enhancing surface functionalization strategies and improving long-term operational stability for practical deployment.

4. Materials and Methods

4.1. Materials

The recycled tire particle was supplied by You Hsien Trading Co., Ltd. (Tainan, Taiwan). All of the chemicals (e.g., nitric acid 85%; sodium bicarbonate, 99.5% SHOWA, sulfuric acid 95.9%, aqueous hydrochloric acid, 37%, J. T. Baker, sodium hydroxide, sodium carbonate, phosphoric acid, 69.5%, Hexane, 99%, dibenzothiophene, 98%, Echo Chem. Co., Ltd., Toufen, Taiwan, and pure nitrogen gas (99.99%, San Fu Chem. Co., Ltd., Taipei, Taiwan) used in this work were not further purified before usage.

4.2. Methods

4.2.1. Waste Tire Cleaning

Recycled tire granules were thoroughly washed with distilled water to remove iron filings and ash residues. The cleaned rubber was then dried in an oven at 70 °C for 12 h.

4.2.2. Sulfonation and Thermal Cracking

A total of 5 g of dried tire rubber was mixed with 40 mL of concentrated sulfuric acid and stirred at 60 °C for 4 h. The sulfonated rubber was filtered and rinsed with distilled water until neutral, then dried at 70 °C. The dried material was carbonized under nitrogen atmosphere by heating to 650 °C at a rate of 10 °C/min and maintained at this temperature for 30 min. After natural cooling to room temperature, the resulting carbon black was ground using a ball mill at 240 rpm for 6 h.

4.2.3. Phosphoric Acid Activation

Tire-derived carbon black was mixed with concentrated H₃PO₄ at different weight ratios (50%, 100%, 150%, 200%, and 250%) and soaked for 6 h at room temperature. The samples were then heated under N₂ atmosphere to 650 °C at a rate of 10 °C/min and held for 30 min. For temperature-dependent studies, samples with a fixed H₃PO₄ ratio (2:1 by weight) were activated at 350, 450, 550, 650, and 750 °C under the same conditions. All activated samples were washed with 1 M NaOH and distilled water until neutral pH was reached, then dried at 100 °C for 12 h.

4.2.4. Chemical Modification via Nitric Acid on the Surface of Tire Particle

The chemical modification of activated tire-derived carbon was performed to enhance its surface functional groups. In this procedure, 3 g of activated carbon was added to 40 mL of nitric acid solution with a fixed concentration of 15 M. The suspension was stirred continuously at controlled temperatures of 30 °C, 60 °C, 70 °C, 80 °C, or 90 °C for a fixed modification time of 3 h. After the reaction, the nitric acid-modified carbon was thoroughly washed with distilled water until the filtrate reached neutral pH, and then dried in an oven at 100 °C for 12 h.

4.3. Characterization

The crystallinity and phase composition of the samples were analyzed by X-ray diffraction (XRD) using a diffractometer (Bruker D8 Advance, Bruker, Billerica, MA, USA) with Cu Kα radiation (λ = 1.5406 Å), operated at 40 kV and 30 mA. Data were recorded over a 2θ range of 10° to 80° with a step size of 0.02°. Surface morphology was observed using scanning electron microscopy (SEM, JEOL JSM-6510, JEOL Ltd., Tokyo, Japan), operated at an accelerating voltage of 5 kV with a working distance of approximately 10 mm and secondary electron detection. Energy dispersive X-ray spectroscopy (EDS) was used to determine the surface elemental composition. Textural properties were determined by nitrogen adsorption–desorption isotherms at 77 K using a Micromeritics ASAP 2020 analyzer, Micromeritics Instrument Corp., Norcross, GA, USA. Prior to measurement, samples were degassed at 200 °C for 8 h. The Brunauer–Emmett–Teller (BET) method was used to determine specific surface area, and the pore size distribution was derived using the Barrett–Joyner–Halenda (BJH) method.

4.4. Adsorption Performance Test

The adsorption capacity of dibenzothiophene (DBT) was used to evaluate the adsorptive performance of the synthesized carbons. In a typical test, 20 mg of adsorbent was dispersed into 20 mL of a DBT/n-hexane solution (500 ppm) and stirred at room temperature (25 °C) for 6 h. For temperature-dependent studies, experiments were conducted at 25 °C, 35 °C, 45 °C, and 55 °C under the same conditions. After adsorption, the suspensions were filtered, and the residual DBT concentration was analyzed using a UV–Vis spectrophotometer (Hitachi U-5100, Hitachi, Tokyo, Japan) at 325 nm. The adsorption capacity (Q, mg/g) was calculated by the equation

$$\mathrm{Q}={\displaystyle \frac{\left(C_0-C_e\right)\times V}{m}}$$

(1)

where Co and Ce are the initial and equilibrium DBT concentrations (mg/L), V is the solution volume (L), and m is the adsorbent mass (g). All experiments were conducted in triplicate and averaged to ensure reliability. The adsorption experiments were conducted using a single-solute DBT solution without coexisting sulfur compounds. Selectivity analysis was not included in this work.

5. Conclusions

The integration of adsorption thermodynamic analysis and performance benchmarking against existing adsorbents underscores both the theoretical significance and practical potential of this study. The modified rubber-derived carbon demonstrated an exceptional DBT adsorption capacity of 254.45 mg/g, outperforming conventional materials such as commercial activated carbon, zeolites, and MOFs. Beyond confirming the superior adsorption efficiency, the findings offer critical insights into the DBT adsorption mechanism, thereby establishing a robust scientific foundation for the advancement of large-scale fuel desulfurization technologies. This work highlights the viability of transforming waste tires into high-performance functional materials, aligning with goals of environmental sustainability and industrial applicability.