Effect of acid activation on the structural characteristics and methylene blue adsorption performance of pan-derived carbon nanofibers

Abstract

Activated carbon nanofibers (ACNFs) were successfully fabricated from electrospun polyacrylonitrile (PAN) precursors via stabilization, carbonization, and subsequent chemical activation using a H₂SO₄/HNO₃ mixture. This study systematically investigates the impact of acid activation on the structural characteristics and adsorption performance of PAN-derived carbon nanofibers for methylene blue (MB) removal from aqueous environments. Morphological and chemical characterizations using SEM, FTIR, and EDX revealed that while the interconnected fibrous network was well-preserved, the acid treatment effectively etched the fiber surface, increasing its roughness. Furthermore, activation successfully introduced abundant oxygen-containing functional groups and increased the overall surface oxygen content, thereby significantly enhancing the availability of active adsorption sites. Batch adsorption experiments demonstrated that the ACNFs exhibited a substantially improved adsorption performance compared to their pristine counterparts. Adsorption equilibrium data were best described by the Langmuir isotherm model (R2 = 0.9566 to 0.9924). Kinetic studies revealed that the adsorption process closely followed the pseudo-second-order model, confirming that chemisorption was the rate-limiting step. The remarkable enhancement in MB removal efficiency is attributed to the synergistic effects of increased surface roughness, the elimination of surface impurities, and the functionalization with oxygenated groups. These findings highlight the immense potential of acid-activated PAN-derived carbon nanofibers as efficient and sustainable adsorbents for the remediation of dye-contaminated wastewater.

Keywords: Carbon nanofibers, Electrospinning, Surface activation, Methylene blue, Adsorption, Langmuir isotherm, Polyacrylonitrile.

JEL Classification: Q51, Q52, Q53, Q55.

1. Introduction

The discharge of dye-containing wastewater from textile, printing, paper, and pharmaceutical industries has become a serious environmental issue due to its persistence, toxicity, and resistance to biodegradation [1-4]. Among various synthetic dyes, methylene blue (MB) is widely used in industrial and medical applications and is frequently detected in wastewater streams. Excessive release of MB can negatively affect aquatic ecosystems and may cause health problems in humans, highlighting the need for effective treatment technologies [3, 5].

Various methods, including coagulation, membrane filtration, biological treatment, photocatalysis, and advanced oxidation processes, have been developed for dye removal [2, 4, 6]. However, many of these techniques suffer from limitations such as high operational costs, complicated procedures, or incomplete pollutant removal [6, 7]. Adsorption has emerged as one of the most promising approaches because of its simplicity, high efficiency, and economicfeasibility [5-8]. Carbon-based materials are widely used as  adsorbents owing to their large surface area, chemical stability, and tunable surface properties [9, 10]. In particular, carbon nanofibers (CNFs) have attracted increasing attention due to their interconnected fibrous structure, high porosity, and excellent mechanical properties [12-14]. Electrospinning is a versatile technique for producing continuous nanofibers with controllable morphology, while polyacrylonitrile (PAN) is the most commonly used precursor because of its high carbon yield and good spinnability [18, 19]. After stabilization and carbonization, PAN-derived nanofibers can be converted into carbon nanofibers suitable for adsorption applications.

Despite their advantages, pristine carbon nanofibers generally possess limited adsorption capacity because of the relatively low number of surface functional groups and active adsorption sites [21-24]. Chemical activation has therefore been widely employed to improve the adsorption performance of carbon materials. Among different activation methods, acid treatment using sulfuric acid and nitric acid is particularly effective in increasing surface roughness, removing impurities, and introducing oxygen-containing functional groups such as hydroxyl, carbonyl, and carboxyl groups [21-25]. These functionalities can enhance interactions between carbon surfaces and dye molecules through electrostatic attraction, hydrogen bonding, and π–π interactions [26, 27].

Adsorption isotherm and kinetic models are commonly applied to investigate adsorption mechanisms and evaluate adsorbent performance. The Langmuir and Freundlich isotherm models are frequently used to describe adsorption equilibrium, while pseudo-first-order and pseudo-second-order models provide insight into adsorption kinetics [31-33]. Although PAN-derived carbon nanofibers have been extensively studied, reports directly comparing the structural characteristics and adsorption performance of pristine and acid-activated carbon nanofibers remain limited.

Therefore, this study aims to investigate the effect of H2SO4/HNO3 activation on the morphology, surface chemistry, and methylene blue adsorption behavior of PAN-derived carbon nanofibers. The prepared materials were characterized using SEM, FTIR, and EDX analyses, while adsorption performance was evaluated through equilibrium and kinetic studies. The findings provide valuable insight into the role of acid activation in enhancing the adsorption capability of carbon nanofibers for wastewater treatment applications.

2. Materials and Methods

2.1. Materials

Polyacrylonitrile (PAN, average molecular weight ≈ 150,000 g mol⁻¹) was used as the precursor polymer for carbon nanofiber fabrication. N,N-Dimethylacetamide (DMAc) was employed as the solvent for preparing the electrospinning solution. Sulfuric acid (H₂SO₄, 98%) and nitric acid (HNO₃, 65%) were used during the activation process. Methylene blue (MB) was selected as the model adsorbate for adsorption experiments. All chemicals were of analytical grade and used without further purification. Deionized water was used throughout the study.

2.2. Preparation of Carbon Nanofibers

Adsorption isotherm and kinetic models are commonly applied to investigate adsorption mechanisms and evaluate adsorbent performance. The Langmuir and Freundlich isotherm models are frequently used to describe adsorption equilibrium, while pseudo-first-order and pseudo-second-order models provide insight into adsorption kinetics [31-35].

Although PAN-derived carbon nanofibers have been extensively studied, reports directly comparing the structural characteristics and adsorption performance of pristine and acid-activated carbon nanofibers remain limited. Therefore, this study aims to investigate the effect of H₂SO₄/HNO₃ activation on the morphology, surface chemistry, and methylene blue adsorption behavior of PAN-derived carbon nanofibers. The prepared materials were characterized using SEM, FTIR, and EDX analyses, while adsorption performance was evaluated through equilibrium and kinetic studies. The findings provide valuable insight into the role of acid activation in enhancing the adsorption capability of carbon nanofibers for wastewater treatment applications. 

2.3. Acid Activation of Carbon Nanofibers

The obtained pCNFs were chemically activated using a mixed-acid solution containing H₂SO₄ and HNO₃ under ultrasonic treatment. After activation, the samples were thoroughly washed with deionized water until neutral pH was reached and then dried at 80 °C. The activated samples were denoted as activated carbon nanofibers (ACNFs). Acid activation was performed to increase surface roughness, remove impurities, and introduce oxygen-containing functional groups onto the carbon surface [21-25].

2.4. Material Characterization

The morphology of the prepared materials was examined using scanning electron The morphology of the prepared nanofibers was examined using scanning electron microscopy (SEM). Surface functional groups were analyzed using Fourier-transform infrared spectroscopy (FTIR) in the range of 400 to 4000 cm⁻¹, while elemental composition was determined by energy-dispersive X-ray spectroscopy (EDX).

2.5. Adsorption Experiments

Batch adsorption experiments were conducted to evaluate the adsorption performance of the prepared carbon nanofibers toward methylene blue. The effects of contact time and adsorbent dosage on dye removal efficiency were investigated.

A predetermined amount of adsorbent was added to MB solutions of known concentration and stirred under ambient conditions. At selected time intervals, aliquots were withdrawn and analyzed using a UV-Visible spectrophotometer to determine the residual dye concentration.

The removal efficiency R (%) was calculated according to Equation (1):

where C0 (mg L-1) is the initial concentration of methylene blue and Ct (mg L-1) is the concentration at time (t).

The adsorption capacity at time t (Qt) was calculated using Equation (2):

where V (L) is the solution volume and m (g) is the adsorbent mass.

The equilibrium adsorption capacity (Qe) was determined using Equation (3):

 where Ce (mg L⁻¹) is the equilibrium concentration of methylene blue [31,37].

2.6. Adsorption Isotherm Models

The adsorption equilibrium data were analyzed using Langmuir and Freundlich isotherm models.

2.6.1. Langmuir Isotherm Model

The Langmuir model assumes monolayer adsorption on a homogeneous surface containing identical adsorption sites and no interaction between adsorbed molecules [34].

where Qmax (mg g-1) is the maximum adsorption capacity and KL (L mg-1) is the Langmuir adsorption constant.

2.6.2. Freundlich Isotherm Model

The Freundlich model describes adsorption on heterogeneous surfaces with non-uniform energy distributions [35].

where (KF) and (n) are Freundlich constants related to adsorption capacity and adsorption intensity, respectively.

2.7. Adsorption Kinetic Models

The adsorption kinetics were evaluated using pseudo-first-order and pseudo-second-order kinetic models.

2.7.1. Pseudo-First-Order Model

The pseudo-first-order kinetic model was originally proposed by Lagergren [36]:

where K1 (min⁻¹) is the pseudo-first-order rate constant.

2.7.2. Pseudo-Second-Order Model

The pseudo-second-order kinetic model developed by Ho and McKay [32] is expressed as:

where K2 (g mg-1 min-1) is the pseudo-second-order rate constant.

The suitability of each model was assessed based on the correlation coefficient (R2) and the agreement between calculated and experimental adsorption capacities.

2.8. Regeneration Study

The regeneration performance of the prepared adsorbents was evaluated through consecutive adsorption desorption cycles. After each adsorption experiment, the spent adsorbent was regenerated, washed, and reused under identical experimental conditions.

The adsorption efficiency retained after each cycle was used to assess the reusability, structural stability, and long-term applicability of the prepared carbon nanofibers for wastewater treatment applications [39,40].

3. Results and Discussion

3.1. Comparison of Morphological and Surface Characteristics

3.1.1. SEM Analysis

The surface morphology of carbon nanofibers before and after acid activation was investigated using SEM analysis. As shown in Figure 1, both samples maintained a continuous three-dimensional fibrous network after carbonization and activation, indicating that the overall structural integrity of the nanofibers was preserved throughout the preparation process.

The pristine carbon nanofibers (pCNFs) exhibited smooth and uniform surfaces with average fiber diameters ranging from approximately 200 to 250 nm. The fibers were randomly distributed and interconnected, forming a porous network favorable for mass transfer during adsorption. However, the surface of pCNFs appeared relatively compact with limited visible defects.

In contrast, the activated carbon nanofibers (ACNFs) displayed noticeably rougher surfaces with the presence of etched regions and surface irregularities. These morphological changes can be attributed to the oxidative action of the H₂SO₄/HNO₃ treatment, which partially removed amorphous carbon and generated additional surface defects. The increase in surface roughness is beneficial for adsorption because it increases the number of accessible adsorption sites and enhances dye diffusion toward the adsorbent surface. Similar observations have been reported for acid-treated carbon nanomaterials, where chemical activation significantly improved surface accessibility and adsorption performance [26,27].

Overall, SEM analysis demonstrated that acid activation effectively modified the surface morphology of carbon nanofibers without destroying their fibrous architecture, creating a more favorable structure for adsorption applications.

3.1.2. FTIR Analysis

The FTIR spectra of pristine carbon nanofibers (pCNFs) and activated carbon nanofibers (ACNFs) are presented in Figure 2. Noticeable differences were observed after acid activation, indicating significant changes in the surface chemistry of the carbon nanofibers. For pCNFs, a characteristic absorption band was observed at approximately 1650 cm⁻¹, which can be attributed to C=N stretching vibrations associated with residual nitrogen-containing structures derived from the cyclization and carbonization of polyacrylonitrile (PAN). Another prominent peak located at approximately 1582 cm⁻¹ corresponds to aromatic C=C stretching vibrations, indicating the presence of graphitic carbon domains formed during the carbonization process [28]. The persistence of this peak suggests that the carbon framework remained stable after thermal treatment.

After activation with the H₂SO₄/HNO₃ mixture, several new absorption bands appeared in the FTIR spectrum. A distinct peak at approximately 1714 cm⁻¹ was assigned to the stretching vibration of carbonyl (C=O) groups, while the absorption band at 1377 cm⁻¹ was attributed to hydroxyl-related vibrations. In addition, the peak observed at approximately 1229 cm⁻¹ corresponds to C–O stretching vibrations of oxygen-containing surface functionalities [28]. The appearance of these peaks indicates that the acid treatment successfully introduced oxygen-containing functional groups onto the carbon surface. Compared with pCNFs, ACNFs exhibited a higher abundance of oxygenated functional groups, including carbonyl, hydroxyl, and carboxyl-related species. Similar observations have been reported for acid-treated carbon materials, where oxidation by nitric acid and sulfuric acid generated additional surface oxygen functionalities and increased the chemical activity of the adsorbent surface [29,30]. The increase in oxygen-containing groups is also consistent with the EDX results, which revealed a higher oxygen content after activation. The coexistence of graphitic carbon structures and oxygen-containing functional groups is particularly advantageous for methylene blue adsorption. The aromatic carbon domains can facilitate π–π interactions between the conjugated structure of methylene blue and the carbon surface, while oxygen-containing groups enhance adsorption through electrostatic attraction and hydrogen-bonding interactions [27,37]. Therefore, the FTIR results confirm that acid activation effectively modified the surface chemistry of carbon nanofibers and generated additional active sites that are beneficial for dye adsorption.

3.1.3. EDX Analysis

The elemental compositions of pCNFs and ACNFs obtained from EDX analysis are summarized in Table 1, while the corresponding spectra are shown in Figure 3. Carbon and oxygen were identified as the dominant elements in both samples, confirming the successful formation of carbon nanofibers.

After acid activation, the oxygen content increased from 11.62 to 12.48 at.%, whereas the carbon content decreased slightly from 88.03 to 87.52 at.%. The increase in oxygen concentration indicates the successful incorporation of oxygen-containing functional groups onto the carbon surface during H₂SO₄/HNO₃ treatment. This observation is in good agreement with the FTIR results, which revealed the appearance of C=O and C–O functional groups after activation.

In addition, trace inorganic elements detected in the pristine sample, including niobium (Nb) and zirconium (Zr), were no longer observed after activation. This result suggests that the acid treatment effectively removed residual impurities and cleaned the carbon surface. Similar increases in oxygen content and reductions in inorganic contaminants have been reported for acid-treated carbon nanomaterials and activated carbon fibers [29,30].

The combined increase in oxygen content and removal of impurities are expected to enhance the adsorption performance of ACNFs by providing additional active sites and improving interactions between the adsorbent surface and methylene blue molecules. Consequently, the EDX results further confirm the effectiveness of acid activation in modifying the surface chemistry of PAN-derived carbon nanofibers.

Table 1. Elemental composition of pristine carbon nanofibers (pCNFs) and activated carbon nanofibers (ACNFs) determined by EDX analysis.

3.2. Adsorption Performance

3.2.1. Effect of Contact Time

The adsorption behavior of methylene blue on ACNFs was investigated as a function of contact time. As shown in Figure 4, the removal efficiency increased rapidly during the initial adsorption stage and gradually approached equilibrium after approximately 120 min.

The rapid adsorption observed during the first stage can be attributed to the abundance of available adsorption sites on the adsorbent surface. As adsorption progressed, the number of vacant sites decreased and repulsive interactions between adsorbed and dissolved dye molecules became more significant, resulting in a slower adsorption rate.

The equilibrium time of approximately 120 min suggests that ACNFs possess favorable adsorption kinetics and efficient mass transfer characteristics. The enhanced adsorption performance can be attributed to the combined effects of increased surface roughness and oxygen-containing functional groups introduced during acid activation.

3.2.2. Effect of Adsorbent Dosage

The effect of adsorbent dosage on methylene blue removal is presented in Figure 5. The removal efficiency increased with increasing adsorbent dosage due to the greater number of available adsorption sites.

The highest removal efficiency of approximately 42% was achieved at an adsorbent dosage of 25 mg. Beyond this dosage, the increase in removal efficiency became less significant, suggesting that the adsorption process approached saturation under the investigated conditions.

These results indicate that adsorption performance is strongly dependent on the availability of active adsorption sites and that acid activation effectively enhanced the adsorption capacity of carbon nanofibers toward methylene blue.

3.3. Adsorption Isotherm Analysis

3.3.1. Langmuir Isotherm Model

The Langmuir model provided an excellent fit to the experimental data, with correlation coefficients ranging from approximately 0.9566 to 0.9924. The high R² values suggest that methylene blue adsorption occurred predominantly through monolayer coverage on homogeneous adsorption sites.

3.3.2. Freundlich Isotherm Model

The Freundlich model also described the adsorption behavior; however, lower correlation coefficients were observed compared with the Langmuir model. This indicates that surface heterogeneity contributed to adsorption but was not the dominant mechanism.

3.3.3. Comparison between Langmuir and Freundlich Models

Comparison of the correlation coefficients obtained from both models revealed that the Langmuir model provided a better description of the adsorption equilibrium. The superior fitting suggests that methylene blue adsorption on ACNFs occurred primarily through monolayer adsorption on energetically similar adsorption sites. The improved surface homogeneity after activation may explain the strong agreement with the Langmuir model.

Table 2. Comparison of Langmuir and Freundlich isotherm parameters for methylene blue adsorption

3.4. Adsorption Kinetic Analysis

3.4.1. Pseudo-First-Order Model

The pseudo-first-order model showed moderate agreement with the experimental data, indicating that physical adsorption contributed to the overall adsorption process.

Table 4. Kinetic parameters obtained from pseudo-first-order adsorption models.

3.4.2. Pseudo-Second-Order Model

The pseudo-second-order model exhibited higher correlation coefficients and better agreement between calculated and experimental adsorption capacities. These results suggest that the adsorption process was primarily controlled by chemisorption involving electron sharing or exchange between methylene blue molecules and oxygen-containing functional groups on the ACNF surface.

Table 5. Kinetic parameters obtained from pseudo-second-order adsorption models.

3.4.3. Comparison of Kinetic Models

Comparison of the kinetic parameters demonstrated that the pseudo-second-order model provided a significantly better fit than the pseudo-first-order model. Therefore, chemisorption was considered the dominant mechanism governing methylene blue adsorption onto activated carbon nanofibers.

3.5. Regeneration and Reusability

The regeneration study demonstrated that ACNFs retained a substantial portion of their adsorption capacity after repeated adsorption–desorption cycles. Although a gradual decrease in removal efficiency was observed with increasing cycle number, the adsorbent maintained satisfactory performance, indicating good structural stability and reusability.

The favorable regeneration behavior highlights the potential applicability of activated carbon nanofibers as sustainable adsorbents for wastewater treatment systems.

4. Conclusion

In this study, carbon nanofibers (CNFs) were successfully fabricated from electrospun polyacrylonitrile (PAN) nanofibers through stabilization and carbonization processes, followed by chemical activation using a mixed sulfuric acid and nitric acid treatment. The effects of acid activation on the structural characteristics and adsorption performance of the resulting carbon nanofibers were systematically investigated.

SEM analysis demonstrated that both pristine and activated carbon nanofibers retained their interconnected fibrous morphology after thermal treatment and chemical activation. However, acid activation generates a rougher surface texture and additional surface defects, which are beneficial for adsorption applications. FTIR characterization confirmed the successful introduction of oxygen-containing functional groups, including hydroxyl, carbonyl, and carboxyl groups, onto the activated carbon nanofiber surface. In addition, EDX analysis revealed the removal of inorganic impurities and an increase in oxygen content after activation, indicating effective surface oxidation.

The adsorption performance toward methylene blue was significantly enhanced after activation. The activated carbon nanofibers exhibited improved dye removal efficiency due to the combined effects of increased surface roughness and the presence of oxygen-containing functional groups. Adsorption equilibrium data were better described by the Langmuir isotherm model than the Freundlich model, suggesting that methylene blue adsorption occurred predominantly through monolayer adsorption on relatively homogeneous adsorption sites. Furthermore, kinetic analysis showed that the pseudo-second-order model provided a superior fit compared with the pseudo-first-order model, indicating that chemisorption was the dominant adsorption mechanism.

Overall, the results demonstrate that acid activation is an effective strategy for improving the physicochemical properties and adsorption performance of PAN-derived carbon nanofibers. The activated carbon nanofibers developed in this study exhibit promising potential as environmentally friendly and reusable adsorbents for the treatment of dye-contaminated wastewater. Future studies may focus on optimizing activation conditions, increasing adsorption capacity, and evaluating the performance of the materials toward a broader range of organic pollutants.

Acknowledgements

The authors acknowledge the Faculty of Environment and Natural Resources, Ho Chi Minh City University of Technology (HCMUT), Vietnam National University Ho Chi Minh City (VNU-HCM), Department of Environmental Engineering & Innovation and Development Center of Sustainable Agriculture, National Chung Hsing University, Taichung, Taiwan for providing laboratory facilities and research supports.

Du Thuc Nhi^1,2, Liao Jui Pin^1,2, Kun-Yi Andrew Lin³, Le Thi Huynh Tram^1,2, Dang Vu Bich Hanh^1,2*

¹ Faculty of Environment and Natural Resource, Ho Chi Minh City University of Technology (HCMUT),

² Vietnam National University-Ho Chi Minh City (VNU-HCM)

³ Department of Environmental Engineering & Innovation and Development Center of Sustainable Agriculture, National Chung Hsing University, Taichung, Taiwan

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