Adsorption of methylene blue onto clay/carbon composite: kinetics and isotherms study

Recently, increasing attention has been given to the technology for removing organic colorants (dyes) from wastewater (Nurani et al. 2024). The presence of organic dyes in wastewater is visually unappealing even at minimal concentrations and may exert a substantial inhibitory effect on the photosynthetic rate of aquatic flora. Methylene blue (MB), classified as a basic thiazine (cationic) dye, holds substantial significance due to its numerous applications, including printing and dyeing, rubber, coloring paper, wool, leather, and dyeing cotton (Duman et al. 2020). MB possesses a complex structure with numerous organic groups, and after entering the water, it becomes more stable and difficult to degrade (Qi et al. 2020). Acute human exposure to MB can result in increased heart rate, tissue necrosis, yellowing of the skin and eyes, quadriplegia, and vomiting (Alharby et al. 2021). Moreover, MB has been reported to be mutagenic and carcinogenic (Lagiewka et al. 2023). Hence, the removal of MB dye traces from wastewater becomes environmentally significant.

Various methods are used in wastewater treatment to remove dyes from aqueous systems, including photocatalysis, coagulation or flocculation, environmental bacteria, and physical adsorption, among others (Mohadesi et al. 2024). Adsorption is a commonly used method for removing dyes from wastewater. Compared with other methods, adsorption is a more advantageous option due to its low operating costs, minimum waste disposal, and high efficacy. In addition, adsorption techniques do not require sophisticated technologies, design characteristics, or processes to function effectively (Mohadesi et al. 2024). Recently, there has been a noticeable increase in the use of activated carbon for pollutant removal from wastewater; nevertheless, this adsorbent is still costly and difficult to regenerate. Several cost-effective adsorbents have been explored for their potential to remove MB dye from wastewater, including cellulose nanocrystals extracted from sugar cane bagasse (Raj et al. 2024) and clay (Loutfi et al. 2023).

Spent bleaching earth (SBE), a by-product of the oil refinery industry, has the potential to be a useful adsorbent for MB. SBE is mainly composed of aluminosilicate materials of Al₂O₃ and SiO₂ with a percentage of 60–80% and contains oil residue (approximately 20–40 wt.%) and other metal impurities (Abdelbasir et al. 2023). SBE is the by-product that remains after the bleaching earth has been decolored during the oil refining process. The improper disposal of SBE into the environment can result in spontaneous combustion in the air and the emission of unpleasant odors, consequently contributing to secondary pollution (Liu et al. 2021). Recent research has demonstrated that, by appropriate treatment, SBE can serve as an effective adsorbent material for heavy metals (Ke et al. 2023) or dyes (Liu et al. 2021). Treatment methods for SBE include but are not limited to aerobic calcination, acid-base and surfactant modification, and hydrothermal methods. While the aerobic calcination method can release harmful and toxic gases into the environment, the acid-base and surfactant modification causes secondary pollution and, the hydrothermal method, while capable of producing high-quality materials, often suffers from low yield and limited scalability, hindering its practical applications (Wan et al. 2021). The calcination of SBE to obtain a clay/carbon composite (SBE@C) has exhibited commendable adsorption capabilities (Wan et al. 2019, 2021; Liu et al. 2021). Reports of SBE treatment under anoxic conditions are rare, even though this approach is simple to implement and prevents secondary pollution.

In this study, we report the treatment of SBE, a solid waste from oil refineries, to produce SBE@C (500 °C) adsorbent, through pyrolysis under an anoxic environment. The effect of pyrolysis temperature (300–900 °C) on the removal capacity of MB by carbonized SBE was explored. Then, an investigation and comparison of the adsorption properties (kinetics and isotherms) of MB onto SBE or SBE@C (500 °C) were carried out. A comprehensive evaluation assessed the influences of crucial factors such as adsorbent dosage, initial pH, temperature and coexisting cations, and ionic strength on SBE and SBE@C (500 °C). Additionally, the materials were characterized to explore their potential adsorption mechanisms. This research stands out from other studies by examining the original SBE and the clay/carbon composite (SBE@C (500 °C)) in its analysis. Additionally, the study explores the levels of dissolved organic carbon (DOC) released into both pure water and the MB solution after adsorption by SBE and SBE@C (500 °C).

C₁₆H₁₈CIN₃S, HCl, NaCl, MgCl₂, AlCl₃, FeCl₃·6H₂O, KCl, and NaOH were all of analytical grade and supplied by Kermel Chemical Reagent (Tianjin, China).

SBE was obtained at no cost from United Refineries Limited (Bulawayo, Zimbabwe). The SBE was dried for 24 h, pulverized into a powder, and passed through a sieve of less than 100 mesh, allowing for a maximum particle size of 150 μm. The sifted SBE was subjected to pyrolysis at a programmed heating rate of 8 °C/min. The pyrolysis experiment was conducted at 300, 500, 700, and 900 °C for 2 h under a continuous flow of nitrogen gas at 150 mL/min (Wan et al. 2021). The final products marked as SBE@C (300 °C), SBE@C (500 °C), SBE@C (700 °C), and SBE@C (900 °C), were stored in a desiccator before and after each experiment.

Adsorption isotherms of a 100 mL MB solution (10–60 mg/L) onto 0.6 g/L for SBE or SBE@C (500 °C) were prepared in a set of 150 mL Erlenmeyer flasks. The solution was then agitated on a thermostatic oscillator (ZD-85 Shanghai Lichen, China) at 150 rpm for 300 min. The research also investigated the impact of temperatures (25, 35, and 45 °C) and conducted concurrent thermodynamic assessments. The pH of the solution was not adjusted during the experiment to ensure that the adsorption process was not influenced by the presence of ions introduced by pH adjustment.

Varying adsorbent dosages of SBE@C (500 °C) (0.3–1.5 g/L) were mixed with samples of 10 mg/L (100 mL) MB solution to assess the impact of dosage on MB adsorption. Other experimental conditions were kept constant, including a contact time of 120 min, temperature of 25 °C, pH of 6.5, and mixing speed of 150 rpm. The adsorption reaction was then carried out and samples were taken periodically to ascertain the remaining concentration of MB in solution.

An investigation was conducted to assess the impact of initial pH (4–9) on the adsorption of MB solution 10 mg/L, at a dosage of 0.6 g/L for both SBE and SBE@C (500 °C). pH measurements were conducted using a Leici pHS-3C pH meter (China). The pH levels were controlled with NaOH or HCl (0.1 mol/L). Additionally, to simulate real MB wastewater conditions, the impact of different selected cations on MB adsorption by both SBE or SBE@C (500 °C) was investigated. SBE or SBE@C (500 °C) (0.6 g/L) were mixed with a 100 mL MB solution of 10 mg/L containing cations (Na⁺, K⁺, Mg²⁺, Fe³⁺, and Al³⁺ 0.01 mol/L). Different concentrations of Na⁺ (0.01, 0.05, and 0.1 mol/L) were investigated to analyze the impact of cationic strength on MB adsorption by SBE and SBE@C (500 °C). The experimental parameters, including initial concentration (10 mg/L), adsorbent dose (0.6 g/L), contact time (120 min), temperature (25 °C), and mixing speed (150 rpm), were kept constant.

The reusability of the adsorbent SBE@C (500 °C) (referred to as SBE@C (500 °C)-de was evaluated using the desorption method. NaOH (0.1 mol/L) solution was mixed with 1 g/L of the adsorbed SBE@C (500 °C). The mixture was agitated for 24 h on a thermostatic oscillator ZD-85 Shanghai Lichen, China (150 rpm) at 25 °C. The solution was filtered and rinsed multiple times with deionized water until a neutral pH was reached. Subsequently, the sample was subjected to a 12-h drying process at 65 °C in an oven. The regenerated SBE@C (500 °C) was sequentially added to a 10 mg/L MB solution at 25 °C. This regeneration process was repeated three times. The resulting data were utilized to assess how effectively SBE@C (500 °C) could be reused.

For the detailed analysis methods used in this research, refer to Supplementary Text 1.

After pyrolysis at a high temperature (500 °C), SBE@C (500 °C) displays a unique sheet-like structure with significant surface pores, potentially enhancing its MB adsorption capacity compared with SBE. In contrast, SBE exhibits a rough and compact surface lacking discernible structural characteristics. The Brunauer–Emmett–Teller (BET) results and the morphological changes in SBE@C (500 °C) are in agreement. However, there was no obvious difference in the morphological structure of SBE or SBE@C (500 °C) before and after the adsorption of MB. As illustrated in Figure 1 and Supplementary Table S4, the EDS results show that C, O, Al, and Si make up the majority of the elements in SBE and SBE@C (500 °C), with very few S and Cl. In comparison to those in SBE, there were varying changes in the percentages of the six components in SBE@C (500 °C). The most notable changes were observed in C and O, which ranged from 49.56 to 58.25% and from 42.33 to 35.21%, respectively. According to the EDS results, both SBE and SBE@C (500 °C) contain C, O, Al, Si, S, and Cl both before and after MB adsorption.

The XRD patterns of the characteristic diffraction peaks show that four different substances were present in the materials where quartz predominated: mica, montmorillonite, quartz, and orthoclase. A previous study also demonstrated the same substances in SBE and SBE@C (500 °C) (Liu et al. 2020). As shown in Figure 2(a), the peaks of quartz are found at 2θ = 20.91, 26.64, 36.53, 39.66, 42.58, 45.89, 50.13, 60.0, and 68.0° (Zhou et al. 2013). Peaks at 29.1, 73.3, and 61.8° are assigned as peaks of montmorillonite; 19.9 and 35.0° indicate the peaks of mica; and the peaks at 23.5, 25.6, and 27.1° are attributed to orthoclase. The similar peaks of SBE or SBE@C (500 °C) both before and after MB adsorption indicate that the structural characteristics of the materials are unaffected by carbonization and adsorption. Compared with SBE@C (500 °C), SBE exhibits similar peaks, but the peaks at 2θ = 29.1, 61.8, and 73.3° for SBE@C (500 °C) are notably lower than the SBE peaks. This indicates that SBE@C (500 °C) differs from SBE, which is most likely due to calcination at 500 °C.

The Fourier transform infrared (FTIR) spectra of SBE and SBE@C (500 °C) before and after MB adsorption are compared in Figure 2(b). The absorption bands attributed to the organic matter in SBE are at 2,927, 2,855, and 1,466 cm⁻¹ (Tang et al. 2018). However, these characteristic absorption bands only appeared in SBE and SBE after the adsorption of MB but completely vanished in both SBE@C (500 °C) and SBE@C (500 °C) after the adsorption of MB, showing that high-temperature pyrolysis at 500 °C can fully carbonize the residual organics in SBE. The 1,633 cm⁻¹ characteristic absorption band is due to the aromatic stretching vibration of C–H (Wan et al. 2019), indicating the presence of aromatic carbon in all the materials. This suggests that the adsorption mechanism of MB onto SBE or SBE@C (500 °C) occurs via π–π interactions. The 1,743 cm⁻¹ characteristic absorption band is attributed to the C = O stretching vibration in the carboxyl group (Mana et al. 2008). Notably, the characteristic band is visible in SBE and SBE after adsorption but not in SBE@C (500 °C) or SBE@C (500 °C) after adsorption, probably because of its low residual organic matter content. Furthermore, the 3,553 cm⁻¹ band is attributed to the –OH group of the absorbed water, and the intensity of this band is significantly weakened in SBE@C (500 °C), which is indicative of dihydroxylation (Kadi et al. 2012). The FTIR results indicate that both SBE and SBE@C (500 °C) contain –COOH, –COOR, and –OH groups. The 1,039 and 797 cm⁻¹ bands are assigned to the asymmetric and symmetric stretching vibrations of Si–O–Si, respectively, and the 469 cm⁻¹ band is due to the bending vibration of Si–O (Christidis et al. 1997). Moreover, the absorption band at 521 cm⁻¹ is due to the vibration of the Si–O–Al bond (Liu et al. 2021). These characteristic absorption bands confirm the interference, which indicates that mica, quartz, and montmorillonite make up SBE and SBE@C (500 °C).

The pore structure of a material is a key indicator of its adsorption properties. Supplementary Table S5 shows the BET analysis results for SBE and SBE@C (500 °C). Pyrolysis at 500 °C significantly increased the SBE-specific surface area from 0.17 to 68.28 m²/g. Furthermore, the mesopore volume and average pore diameter of the untreated SBE increased after calcination at 500 °C from 5.82 × 10⁻⁴ to 3.4 × 10⁻¹ and 13.83 to 19.93 nm, respectively. This is primarily attributed to the release of volatile organic compounds and oils from SBE during pyrolysis at 500 °C, which significantly enhances the adsorption of MB by SBE@C (500 °C). This is mainly attributable to the release of volatile organic compounds and oils from SBE during pyrolysis at 500 °C and greatly enhances MB adsorption by SBE@C (500 °C).

Supplementary Figure S5 shows the survey scan and C1s X-ray photoelectron (XPS) spectra of SBE and SBE@C (500 °C). Supplementary Figure S5(a) indicates that four main peaks, C1s, O1s, Si2s, and Al2p, are present in both materials. To better understand the variations in the XPS spectra between SBE and SBE@C (500 °C), the C1s were identified and fitted, as illustrated in Supplementary Figure S5(b) and S5(c). The binding energies at 284.8, 286.7, 287.6, and 289.2 eV correspond to C = C (aromatic carbon group), –C–O (phenolic, alcohol, or either), C–H, and –COOR (ester), respectively (Heymann et al. 2011; Wu et al. 2014). When comparing Supplementary Figure S5(b) and S5(c), significant differences were observed at 286.7 and 289.2 eV, assigned to –C–O and –COOR, respectively, between SBE and SBE@C (500 °C). Furthermore, the characteristic diffraction peak assigned to C–H located at 287.6 eV completely vanished in SBE@C (500 °C), indicating that the volatile organic compounds on the surface of SBE were transformed into carbon species during pyrolysis at 500 °C. The results align with the outcomes of the FTIR analysis.

As depicted in Figure 3(a) and 3(c), the SBE and SBE@C (500 °C) adsorption capacities increased from 4.55 to 6.02 mg/g and 15.93 to 16.59 mg/g (approximately 1.32 and 1.04 times), respectively, when the temperature increased from 25 to 45 °C. SBE@C (500 °C) reached equilibrium more quickly than SBE. For SBE@C (500 °C) at 45 °C, as shown in Figure 3(c), a significant increase in the adsorption capacity from 7.44 to 14.94 mg/g was observed as the contact time increased from 10 to 80 min. Initially, the enhanced removal capacity of MB by SBE@C (500 °C) was due to the presence of several active adsorption sites on the material surface. The removal capacity continued to increase, slightly increasing to 16.59 mg/g after 140 min of contact. Moreover, further extension of the contact time did not lead to a notable increase in the adsorption capacity, mainly due to the limited number of active adsorption sites on the material surface, with most of these sites already occupied by MB dye molecules (Deniz 2013). Table 1 shows the results of the experimental data and correlation coefficient (R²) fitted to the three dynamic models.

In comparison to other kinetic models, the pseudo-second-order models of SBE and SBE@C (500 °C) had the highest correlation coefficients R² ≥ 0.992 and R² ≥ 0.991, respectively. These findings are in agreement with previous studies of MB adsorption onto activated carbon composited with Egyptian black sand (Elkholy et al. 2023), and MB adsorption by activated carbon from peanut shells (Sanou et al. 2024). This suggests that MB adsorption onto SBE or SBE@C (500 °C) involves chemisorption (Valentini et al. 2023). The initial sorption rate v_o was calculated using Equations (5) and (6). Based on the pseudo-second-order kinetics model, for SBE and SBE@C (500 °C) at 45 °C, there was an increase in v_o from 0.19 to 1.03 mg/(g min), respectively. Hence, the v_o for SBE@C (500 °C) increased by a factor of 5.42 after pyrolysis. The v_o for SBE and SBE@C (500 °C) increased with increasing temperature, indicating that an increase in temperature is beneficial for enhancing the v_o for MB adsorption by both materials.

As illustrated in Figure 4(a), the isotherm curves indicate a marginal increase in the MB removal capacity from 28.09 to 30.62 mg/g with increasing temperature from 25 to 45 °C. Figure 4(b) shows that the equilibrium adsorption capacity of SBE@C (500 °C) at 29.54 mg/g was approximately 2.8 times that of SBE at 10.62 mg/g. This is primarily attributable to the significantly formed pore structure and increased specific surface area following the calcination of SBE, which is reflected in the SEM and BET results. Compared with other reported adsorbents of different activated materials (Supplementary Table S2), SBE@C (500 °C) has a satisfactory MB adsorption capacity. Table 2 shows the isotherm model fitting parameters of MB adsorption by both SBE and SBE@C (500 °C).

Table 2

The isotherm model fitting parameters of MB adsorption by SBE and SBE@C (500 °C)

.	Freundlich isotherm .				Langmuir isotherm .
	T (°C) .	KF (mg/g)(L/mg)1/n .	n .	R2 .	Q0 (mg/g) .	KL (L/mg) .	R2 .
SBE	25	3.380	5.83	0.967	6.980	0.260	0.987
	35	3.310	4.38	0.994	8.980	0.190	0.999
	45	3.950	4.25	0.964	10.62	0.210	0.997
SBE@C (500 °C)	25	19.996	9.75	0.909	27.531	4.754	0.987
	35	20.310	9.35	0.905	28.312	4.493	0.984
	45	21.699	9.57	0.939	29.544	8.016	0.965

.	Freundlich isotherm .				Langmuir isotherm .
	T (°C) .	KF (mg/g)(L/mg)1/n .	n .	R2 .	Q0 (mg/g) .	KL (L/mg) .	R2 .
SBE	25	3.380	5.83	0.967	6.980	0.260	0.987
	35	3.310	4.38	0.994	8.980	0.190	0.999
	45	3.950	4.25	0.964	10.62	0.210	0.997
SBE@C (500 °C)	25	19.996	9.75	0.909	27.531	4.754	0.987
	35	20.310	9.35	0.905	28.312	4.493	0.984
	45	21.699	9.57	0.939	29.544	8.016	0.965

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Based on the isothermal adsorption process, the correlation coefficients (R²) for SBE and SBE@C (500 °C) of the Langmuir model were ≥0.987 and ≥0.965, respectively, and are better suited for describing the equilibrium MB removal than the Freundlich isotherm model, with correlation coefficients (R²) for SBE and SBE@C (500 °C) ≥0.964 and ≥0.905, respectively as shown in Table 2. Hence, the adsorption of MB by SBE or SBE@C (500 °C) is attributed to monolayer adsorption (Wang et al. 2023). These findings are in agreement with previous studies on MB adsorption by low-cost adsorbent and MB adsorption onto synthesized activated carbon where the Langmuir model was found to be more accurate when compared with the Freundlich model (Al-Asadi et al. 2023; El Jery et al. 2024). Compared with SBE, SBE@C (500 °C) exhibited an enhanced adsorption ability for MB.

Supplementary Table S3 shows the thermodynamic parameters for MB adsorption by SBE@C (500 °C). All the ΔG° values were <0, indicating a spontaneous adsorption process (Shahwan 2021). Moreover, the decreasing values of ΔG° with increasing temperature show that the MB adsorption process becomes more beneficial with increasing temperature (Golban et al. 2019). A positive ΔH° (26.39 kJ/mol) indicates an endothermic adsorption process. The (ΔS°) value is positive (121.00 J/mol), indicating an increase in disorder or randomness of the MB removal process. These findings align with previous literature (Al-Mokhalelati et al. 2021).

The impact of pyrolysis temperature (300–900 °C) on MB removal from wastewater by pyrolyzed SBE was investigated. The results, as illustrated in Supplementary Figure S1, indicate that the untreated SBE has a very low adsorption capacity at equilibrium (2.55 mg/g). Pyrolysis at 300 °C resulted in incomplete carbonization of the surface oil of the material, leading to poor adsorption performance (1.83 mg/g). When the pyrolysis temperature is 500 °C, the removal capacity significantly increases to 10 mg/g due to the full carbonization of organic material on the SBE surface. However, at 700 and 900 °C, the temperature may be too high to preserve the internal pore structure of the material and could contribute to a decrease in its adsorption capacity (9.59 and 8.43 mg/g, respectively).

The adsorbent dosage was varied (0.3–1.5 g/L) to investigate the impact of the SBE@C (500 °C) dosage on MB removal from wastewater, as shown in Supplementary Figure S3. The SBE@C (500 °C) removal efficiency (Supplementary Figure S3(a)) increased with increasing dosage (0.3 to 1.5 g/L) from 65.90 to 100%, respectively, after 120 min of contact. Compared with lower dosages of SBE@C (500 °C), higher dosages of SBE@C (500 °C) take less time to reach equilibrium due to the presence of multiple active adsorption sites (Zhang et al. 2018). As the SBE@C (500 °C) dose increased from 0.3 to 1.5 g/L, the matching adsorption capacity in Supplementary Figure S3(b) decreased from 21.97 to 6.90 mg/g, respectively. This is primarily because the adsorption sites on the adsorbent do not become saturated during the adsorption process. As a result, increasing the amount of adsorbent does not lead to a proportional increase in adsorption capacity (Chiban et al. 2016). These findings align with the results of a previous study (Ishak et al. 2021).

The influence of initial pH on the equilibrium adsorption capacity of MB by SBE or SBE@C (500 °C) is illustrated in Supplementary Figure S4(a). The pH of the zero point of charge (pH_pzc) plots of SBE and SBE@C (500 °C) were also plotted, as illustrated in Supplementary Figure S4(b). The pH_pzc value is a key parameter that helps identify the type of active centers on the surface of a material. The pH at which the final pH was equal to the initial pH was considered the pH_pzc, indicating a neutral charge at this pH value. The pH_pzc values for SBE and SBE@C (500 °C) were determined to be 4.55 and 7.31, respectively. When the surface charge of SBE or SBE@C (500 °C) is negatively charged, it promotes the adsorption of cationic dyes (Bortoluz et al. 2020). The differences in pH_pzc values between these two materials could explain the observed differences in their adsorption capacities for MB.

With increasing pH (4–9), the removal capacities of both SBE and SBE@C (500 °C) increased from 3.49 to 4.71 mg/g and 15.4 to 16.50 mg/g, respectively, as shown in Supplementary Figure S4(a). The poor adsorption performance observed at pH values of 4 and 5 suggested that an abundance of H⁺ ions competed with the MB cations for available adsorption sites. At a higher pH range (6–9), the surfaces of the SBE and SBE@C (500 °C) particles are more likely to acquire a negative charge, resulting in strong electrostatic attraction with the positively charged MB cations (Bulut & Aydin 2006). The increasing trend of the MB adsorption capacity became more stable for SBE and SBE@C (500 °C) at higher pH values (6–9); therefore, the pH was optimized at a pH of 8. The pH had a minimal impact on the adsorption process, making it advantageous for effective adsorption under real wastewater conditions. A comparable trend has been noted for the adsorption of MB onto papaya bark fibers (Nipa et al. 2023).

SBE and SBE@C (500 °C) inhibited the MB removal rate in a similar order: Fe³⁺ > Al³⁺ > Mg²⁺ > K⁺ > Na⁺. Within 140 min, the ability of SBE and SBE@C (500 °C) to adsorb MB decreased from 5.70 to 0.87 mg/g and 15.21 to 9.71 mg/g, respectively, when MB coexisted with Na⁺, K⁺, Mg²⁺, Al³⁺, and Fe³⁺. Compared with the blank experiment (6.01 and 16.6 mg/g) for SBE and SBE@C (500 °C), respectively, all these values were lower. The most significant impact was observed for Fe³⁺, and the removal capacity of MB by SBE and SBE@C (500 °C) decreased from 6.01 to 0.87 mg/g and 16.60 to 9.71 mg/g, respectively, when the MB solution contained 0.01 mol/L Fe³⁺, thereby almost inhibiting the removal of MB by SBE. The affinity of these cations for SBE and SBE@C (500 °C) may be the cause of their inhibitory effects on adsorption, as they compete for available adsorption sites between the cationic MB solution and the cations present on the adsorbent surface. It has been reported that cations with very small ionic radii have a greater tendency to enter adsorbent pores and can be adsorbed more readily than those with larger ionic radii. The ionic radii of K⁺, Na⁺, Mg²⁺, Fe³⁺, and Al³⁺ are 0.133, 0.097, 0.066, 0.064, and 0.051 nm, respectively (Kilislioglu 2003). Among the five cations, Fe³⁺ and Al³⁺ have the smallest ionic radii and are more readily adsorbed onto SBE and SBE@C (500 °C), thus forming a competitive relationship with MB. Higher valence states of Fe³⁺ and Al³⁺ had greater inhibitory effects on the SBE and SBE@C (500 °C) adsorption of MB than did Na⁺, K⁺, and Mg²⁺.

The impact of varying concentrations of Na⁺ (0.01, 0.05, and 0.1 mol/L) on the removal of MB by SBE and SBE@C (500 °C) was investigated to study the impact of cationic strength. Figure 5(b) shows that as the Na⁺ concentration increased from 0.01 to 0.1 mol/L, the removal capacity of MB by both SBE and SBE@C (500 °C) decreased from 6.01 to 4.40 mg/g and 16.60 to 11.03 mg/g, respectively. These results indicate that higher Na⁺ concentrations enhance the inhibitory effect of Na⁺ on MB adsorption by SBE or SBE@C (500 °C); this is most likely due to increased competition between Na⁺ and MB cations for available adsorption sites (Ren et al. 2020).

Based on the premise that π–π interactions can occur between aromatic rings with π bonds in both the adsorbent and adsorbate (Wan et al. 2019), this study offers evidence that supports the presence of a π bond in the MB molecule (Supplementary Table S1). The FTIR results show a characteristic absorption band of aromatic carbon at 1,633 cm⁻¹, confirming the existence of a π bond in both SBE and SBE@C (500 °C). Therefore, it is concluded that the π–π interaction serves as a key mechanism for the adsorption of MB in this study. A study on the removal of MB dye by acid-fractionated biomass materials also found comparable outcomes (Jawad et al. 2020).

The pH_pzc for SBE and SBE@C (500 °C) were found to be 4.55 and 7.77, respectively (Supplementary Figure S4(b)). Supplementary Figure S4(a) shows that the adsorption capacity for SBE toward MB increases at pH > 5, and at pH > 6 for SBE@C (500 °C). The observed behavior can be attributed to the pH_pzc values, which indicate that at pH > 4.55 for SBE and pH > 7.77 for SBE@C (500 °C), the adsorbent surface acquires a complete negative charge. This negative charge facilitates stronger electrostatic interactions with positively charged MB molecules, leading to enhanced adsorption. The results of this study suggest that electrostatic interactions play a significant role in the adsorption of MB by SBE and SBE@C (500 °C). A similar study on acid-factionalized biomass material for MB dye removal has found electrostatic interaction as a major driving force of MB adsorption (Jawad et al. 2020).

BET results revealed that SBE@C (500 °C) has a significantly larger specific surface area, and pore size compared with SBE (Supplementary Table S5). Specifically, the specific surface area significantly increased from 0.17 m²/g (SBE) to 68.28 m²/g (SBE@C (500 °C)), and the pore size increased from 13.83 for SBE to 19.93 for SBE@C (500 °C). With a minimum molecular cross-section of about 0.8 nm, the MB molecule is unable to enter pores smaller than 1.3 nm in diameter (Wang et al. 2005). Hence, the larger pore structures present in SBE@C (500 °C) allow for greater access of MB molecules, leading to a higher adsorption capacity compared with SBE.

In this study, SBE, a by-product of the oil refinery, was pyrolyzed under anoxic conditions to obtain SBE@C (500 °C). The results showed that the initial adsorption rate v_o at 45 °C increased from 0.19 mg/(g min) of SBE to 1.03 mg/(g min) of SBE@C (500 °C), while the maximum adsorption capacity of MB, Q₀ (determined by the Langmuir model), significantly increased from 10.62 mg/g of SBE to 29.54 mg/g of SBE@C (500 °C). The adsorption kinetics of MB by SBE@C (500 °C) can be effectively described with the pseudo-second-order model (R² ≥ 0.991). In addition, after three regeneration cycles, SBE@C (500 °C) retained 73.3% of its adsorption capacity, and it regenerated 11 times before its adsorption performance completely decreased. The findings from the study showed that the DOC released by SBE@C (500 °C) is minimal and meets China's drinking water quality requirements, making it a feasible adsorbent for the practical removal of MB from wastewater without causing secondary pollution to the surrounding water environment. The mechanism of MB adsorption onto SBE@C (500 °C) primarily involved π–π interactions, electrostatic interaction, and changes in physicochemical properties. In summary, this research offers a sustainable method for producing an efficient eco-friendly clay/carbon adsorbent that does not cause secondary pollution.

This study was supported by the National Natural Science Foundation of China (Nos. 52070073 and 52200013), the Science & Technology Innovation Talents in Universities of Henan Province (No. 22HASTIT009), the Key Projects of Scientific and Technological Collaborative Innovation of Zhengzhou City (No. 21ZZXTCX05), and the Innovative Funds Plan of Henan University of Technology (No. 2021ZKCJ09).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.