How the Carbonization Time of Sugarcane Biomass Affects the Microstructure of Biochar and the Adsorption Process?

Abstract

Biochars (BCs) are very versatile adsorbents, mainly, in the effectiveness of adsorption of organic and inorganic compounds in aqueous solutions. Here, the sugarcane biomass (SCB) was used to produce biochar at different carbonization times: 1, 2, 3, 4, and 5 h, denominated as BC1, BC2, BC3, BC4, and BC5, respectively. The superficial reactivity was studied with adsorption equilibrium experiments and kinetics models; Methylene Blue (MB) was used as adsorbate at different pH values, concentrations, and temperatures. In summary, the carbonization time provides the increase of superficial area, with exception of BC4, which decreased. Equilibrium studies showed inflection points and fluctuations with different initial dye concentration and temperature; SCB showed the best adsorption capacity compared to the BCs at the three temperatures tested, varying with the increase of MB concentration, suggesting the dependence of these two main factors on the adsorption process. The proposed adsorption mechanism suggests the major influence of Coulomb interactions, H-bonding, and π-interactions on the adsorption of MB onto adsorbents, evidencing that the adsorption is led by physical adsorption. Therefore, the results led to the use of the SCB without carbonization at 200 °C, saving energy and more adsorbent mass, considering that the carbonization influences weight loss. This study has provided insights of the use of SCB in MB dye adsorption as a low-cost and eco-friendly adsorbent.

1. Introduction

The presence of dyes in water bodies poses a major threat to human health and aquatic organisms. Methylene Blue (MB) is an organic dye that has been widely used in printing industries for coloring paper, clothes, and plastics [1,2,3]; however, its improper disposal in the environment can cause a series of problems for the environment and human health due to its potential mutagenic and carcinogenic effect and great ability to color ecosystems [4,5].

Adsorption is a technique widely used in water treatment processes and has been broadly studied in the application for removing organic dyes in water [6], for being an easy and economically viable method [1,7]. Among the challenges to be faced, there is the choice of a renewable source for the production of adsorbent materials, since these can be produced from different sources.

The production of materials and composites from sustainable sources has attracted attention due to the range of application possibilities [8]. Lignocellulosic biomass is a residue of agro-industrial activities that has aroused great interest in the development of new adsorbent materials, precisely because of its large quantity and availability. Lignocellulosic waste, such as rice husk [9], palm fiber [10], and sugarcane biomass (SCB) [11], has been used for the production of adsorbent materials as well as activated carbon [12,13] and biochar (BC) [14,15,16]. BC is a product of the thermochemical transformation of lignocellulosic biomass, heated in a closed system with limited oxygen [17], and have been employed in the removal of organic compounds from wastewater, such as dye [18], medicines [19], and pesticides [9].

The combustion of biomass for BC production, a process denominated carbonization or pyrolysis, generates a material rich in carbon, considering SCB as a source of waste. SCB is constituted mostly of cellulose (1,4-β-poly-anhydroglucose), which is a polymer of D-glucose monomers; hemicellulose, composed of polymers of pentoses, such as galactans and xylans; and lignin, in which the structure varies depending on the source, generally constituted of coniferyl alcohol and others polymers [20]. The carbonization process provides the modification of the functional groups of lignocellulosic material, forming hydroxyl groups (O-H), aromatic chains (C=C), ethers (C-O), and others, on the surface (Figure S1). These functional groups have been widely studied, due to their presence on the surface being considered as an important factor in the adsorption process.

Moreover, there are several methodologies for BC production, such as hydrothermal carbonization [21], gasification [22], and pyrolysis [10]. The pyrolysis process involves high temperatures for carbonization (>300 °C), under a flow of nitrogen gas [23]. SCB was turned in BC under a nitrogen flow with temperatures up to 300 °C [11], and the cool down to room temperature was under a nitrogen flow too; the capacity for the cooper sorption was 40.5 mg g⁻¹ for the 350 °C pyrolytic temperature of BC. A pyrolyzed rice husk was used for malachite green dye removal, the material was prepared at temperatures above 400 °C in a nitrogen atmosphere, and the authors found maximum adsorption of 37.47 mg g⁻¹ for adsorbate concentration equal to 20 mg L⁻¹ [24]. Biswas et al. 2020 performed removal tests of MB dye onto SCB; the biomass was pyrolyzed at 500 °C under a nitrogen flow. The results showed maximum adsorption of 43.97 mg g⁻¹ for adsorbate concentration equal to 20 mg L⁻¹. However, for this process high temperatures have been used under nitrogen flow (for an inert atmosphere), inducing a very expensive process, increasing the costs of the final product.

Thus, it is important to search for new alternatives for the production of low-cost BC, i.e., using low temperatures in the carbonization process without an inert atmosphere. There is a lack of information and research that concerns the influence of time in the carbonization of SCB for adsorption. Furthermore, research with biomass in natura showed very good adsorption capacities for dyes in several studies [25,26,27,28]; this brings the possibility of using the biomass itself as an adsorbent. Herein, BCs from SCB were prepared at different carbonization times (1–5 h) at 200 °C and characterized by microscopy and spectroscopy techniques, that provide insights into the structure. The novelty is related to evaluating the influence of carbonization of the SCB and identifying if it is an appropriate approach to improve the adsorption performance or the changes in the structure do not improve the adsorption. We performed batch adsorption with SCB and BCs adsorbents at various conditions, such as different initial dye concentrations, contact time, adsorbent dosage, MB solution pH, and temperature. Furthermore, we propose the mechanism of adsorption of MB onto SCB and BCs produced.

2. Materials and Methods

2.1. Synthesis and Characterization of the Adsorbents

The precursor SCB waste was collected in a sugar-alcohol industry, then dried in an oven with air circulation for 24 h at 100 °C and milled in a Willye micro mill. To analyze the thermal stability of SCB, a thermogravimetric analysis (TGA/DTG) was performed in Perkin Elmer Pyris 1 equipment. The initial mass was 3.167 mg, heated in the range from 65 to 750 °C (10 °C min⁻¹), under nitrogen flow (20 mL min⁻¹). The result is presented in the SI.

To produce the BCs, the SCB was carbonized in a muffle furnace (200 °C) at different times, as follows: 1, 2, 3, 4, and 5 h, denominated as BC1, BC2, BC3, BC4, and BC5, respectively. Subsequently, they were sieved in a granulometric mesh (0.15 mm).

The characterization setups of the SCB and BCs structures were: the functional groups of the surfaces of adsorbents were studied using the Fourier-transform infrared spectroscopy (FTIR/NIR, Perkin Elmer Frontier) with attenuated total reflectance (ATR) mode, covering the spectral range from 650 to 4000 cm⁻¹. Points of zero charge (pH_PZC) were determined as follows: 10 mg of each adsorbent was stirred for 24 h, at room temperature, with 30 mL of distilled water with initial pH (pH_i) between 2 and 12; then, the final pH (pH_f) value was measured. We plotted a graph of pH_i versus pH_f. The pH_PZC is defined as the point where the curve intersects the line pH_i = pH_f. Moreover, the morphological analysis was performed with scanning electron microscopy (SEM, JEOL IT300LV). The surface area was measured by the N₂ physisorption technique (Quantachrome Instruments, NOVA 1000e model) and Brunauer–Emmett–Teller (BET) method, with a pre-treatment under vacuum at 240 °C for 2 h.

2.2. Adsorption Studies of MB Removal onto SCB and BCs

The six types of adsorbents (SCB and BCs) were used in the batch adsorption experiments to investigate the removal of MB at different conditions. The equipment setup was by using a Solab Shaker for the experiments at room temperature and an incubator for the experiments at 35 and 45 °C (150 rpm for all studies). The contact time was studied ranging the agitation time from 0 to 1440 min. Adsorbent dosage was performed only for BC5, the mass ranged from 5 to 30 mg at pH 7, and temperature of 25 °C. The influence of positive and negative charges in the adsorption process was evaluated by adjusting the pH of MB solution (20 mg L⁻¹) to 2, 4, 6, 8, and 10 with NaOH (0.1 M) and HCl (0.1 M). To understand the adsorption equilibrium, the initial concentration of MB ranged from 10 to 270 mg L⁻¹ with a volume equal to 10 mL and an adsorbent mass of 5 mg, at 25, 35, and 45 °C; this experiment was conducted in duplicate to reduce experimental error.

The final concentration of MB was determined using an MB standard curve (0.1–10 mg L⁻¹) developed in a UV-Vis spectrophotometer at 664 nm (R² = 0.999). The amounts of MB adsorbed onto the adsorbents and removal efficiency were calculated according to the following equations:

$$q_e=\frac{(C_{0 }-C_e)}{m}\times V$$

(1)

$$removal efficieny=\frac{(C_{0 }-C_e)}{C_0}\times 100\%$$

(2)

where $q_e$ (mg g⁻¹) is the maximum amount of MB adsorbed by mass of adsorbent, $C_0$ and $C_e$ (mg L⁻¹) is the initial and equilibrium MB concentration, respectively; m (g) is the adsorbent mass, and V (L) is the MB volume.

3. Results and Discussion

3.1. FTIR Spectroscopy

FTIR analysis was carried out to evaluate the chemical surface of the SCB and BCs produced at different carbonization times (Figure 1). Likewise, Table S1 shows the functional groups identified with the principal bands. SCB has broadband between 3500 and 3100 cm⁻¹, in a range attributed to hydroxyl groups present of water adsorbed and linked to cellulose, hemicellulose, and lignin. To the BC1 sample, we can observe the increase of this band intensity, due to the liberation of water adsorbed, a result also observed in the DTG curve up to 150 °C; however, this band decreases when the carbonization time increases, and this is associated with dehydration. Bands related to the CO and CO₂ groups are observed at 1350 cm⁻¹, characteristic from products of slow cellulose pyrolysis, these molecules are adsorbed on the BC surface. Vibrational stretching bands between 1740 and 1026 cm⁻¹ are all associated with cellulose, hemicellulose, and lignin, the decrease in the intensity of these bands means an evident precursor degradation, generating several functional groups on the surface; however, the peak at approximately 1026 cm⁻¹ almost disappears, suggesting the almost complete decomposition of cellulose and hemicellulose [29].

The generation of functional groups on the surfaces of the adsorbents can directly affect the adsorption capacity, and the interactions that occur on the surface can be of a chemical and/or physical nature. Some of the potential adsorption mechanisms can be described by surface adsorption by covalent bonds, Coulomb interactions, hydrogen bonds, and π-interactions [30]. Carbonized adsorbents materials at 200 °C, after 1 h presents functionality characteristic of adsorbent, considering the formation of amorphous carbon, and the presence of superficial functional groups can contribute to remove organic molecules more efficiently. Moreover, the morphological study can identify the influence of residence time on pore formation.

3.2. Topography Study and Surface Area of SCB and BCs

The SEM images of the adsorbents were obtained to understand how the residence time affects the development of the pore on the surface of the adsorbents (Figure 2). To SCB, in the micrograph images, no pores are observed, just a rigid structure. The BC1 and BC2 micrographs did not show an increase in surface pores, even with the increased carbonization time.

The increase in pore quantity can be observed in the adsorbents BC3, BC4, and BC5, also showing roughness aspects, corroborating with the FTIR spectra, in which the increased carbonization time favors the degradation of the cellulose, hemicellulose, and lignin structures. Likewise, the degradation of the polymer increases the formation of internal pores, providing even more adsorption sites when added to the superficial ones, favoring the removal of the contaminants in the solution. However, the structures have the same fiber appearance, related to lignin, which is highly resistant to thermal oxidation [31]. The formation and enlargement of internal and superficial pores can be important in the adsorption process; to better understand the changes in the surfaces of the adsorbents, we studied the influence of the carbonization time in the surface area.

The results of the surface area are presented in Table S2. We observed that the surface area increases up to three hours of carbonization to 66.05 m² g⁻¹, then decreases with four hours to 37.98 m² g⁻¹, and then increases again with five hours to 90.64 m² g⁻¹. The decrease in the area of BC1 to the precursor SCB may be due to the narrowing of the pores by the formation of oxygen groups, in the pore entrances and walls, with one hour of carbonization [18]. Then, we see that the surface area increases in BC2 and BC3, which may be associated with an increase in the carbonization time, which degrades the structures of the cellulose, hemicellulose, and lignin polymers, forming internal and superficial pores.

BC4 reduces its surface area by almost half, from 66.05 to 37.98 m² g⁻¹; we can associate this feature with the conversion of micro and mesoporous to macropores due to the conditions of thermal oxidation, i.e., four hours of carbonization [18]. Though, with the increase in carbonization time to five hours, the surface area increases to 90.64 m² g⁻¹, suggesting the formation of more internal and superficial micropores in BC5 [32]. We can conclude that for the formation of a larger surface area, the ideal carbonization time is five hours; moreover, adsorption studies have been carried out to better understand the relationship between surface area and adsorption capacity, as well as the influence of functional groups on the adsorption process.

3.3. Adsorption Study

3.3.1. Effect of BC5 Dosages

The adsorbent dosage is an important parameter to study since it directly implies the cost-benefit of the material when applied in water treatment [33]. To investigate this parameter, we used BC5 as a representative sample; the mass of the adsorbent varied between 5 and 30 mg, with 25 mL of MB (25 mg L⁻¹) (Figure S2). With 5 mg of adsorbent, we noticed a removal efficiency of 83.8% with an adsorption capacity of 104.8 mg g⁻¹. However, when we increased the adsorbent mass to 10 mg, the removal efficiency and adsorption capacities were 84.2% and 52.6 mg g⁻¹, respectively. This behavior can be explained by the increase in active sites for adsorption with the increase in mass, where the adsorption capacity decreases by increasing the mass ratio of adsorbate/mass of adsorbent (mg g⁻¹) [21].

The increase in mass to 30 mg, the removal efficiency increased to 97.1% and the adsorption capacity decreased to 20.2 mg g⁻¹. This result may be due to the adsorbent–adsorbate interactions that can occur with high concentrations of adsorbent, which reduces the area available for adsorption and, consequently, the adsorption capacity [34]. With the increase in mass from 5 to 15 mg, the removal efficiency increases only 1.3%, while the adsorption capacity decreases from 104.8 to 35.5 mg g⁻¹; in this case, the ideal dosage of the mass to be used, aiming at a better cost–benefit ratio, is 5 mg (0.2 g L⁻¹).

3.3.2. pH_PZC and Effect of the Solution pH on the MB Adsorption

The pH_PZC is determinate as the pH value where the surface charge is zero; for the adsorbents in this study, the pH_PZC varied between 6.1 and 7.7 (Figure 3a). These results show that the variation in the carbonization time may be influencing the presence and/or amount of functional groups on the surface of the adsorbents, as we noticed in the FTIR spectra, because when pH_PZC < 7 represents the predominance of acid groups and above 7 the presence of basic functional groups [35]. When the pH value of the solution is below pH_PZC, the adsorbent surface is positively charged, being more efficient in attracting anions [36]; therefore, there will be repulsion between the adsorbent surface and the MB molecules, since MB is a cationic dye.

The results of the removal efficiency as a function of pH value show that at pH 2 and 4, the removal is lower than at pH 6, 8, and 10 (Figure 3b). For example, SCB had a removal efficiency of 37.1 and 89.0% at pH 2 and 10, respectively. This is due to the presence of H⁺ ions in the solution, which competes for the active sites with the MB molecules, contributing to the relatively low removal efficiency [36]. However, for BCs the removal efficiency did not change significantly with the change in pH values, demonstrating that electrostatic interactions are not the main factor in the removal of MB. It is important to consider other parameters when explaining the adsorption mechanism of MB, including the bonds that take place in the adsorption process, such as π-bonding and Coulomb interactions.

Additionally, MB solutions (20 mg L⁻¹) with pH 8 and 10 decreased after the adsorption process, keeping the pH value between 6 and 7.3 (Table S3). This decrease probably occurs due to the interactions between H⁺ ions and the hydrolyzed form of MB (MB⁺) are weak; therefore, most of the H⁺ adsorbed on the surfaces of the adsorbents are released in solution [37]. These results suggest the predominance of Coulomb interactions between MB molecules and functional groups with negative partial charges, after dissociation of H⁺ in solution.

The removal efficiency for adsorbents, on average, was higher for solutions with pH 8 (63.7%); this result can be understood by the protonation and deprotonation of the MB in solution, in the MB and MB⁺ forms [27]. El-Ahmady and Rabei reported that at pH 8, the seaweed biomass showed greater removal of MB, indicating that the surface of the adsorbent used also has protonation and deprotonation behavior, depending on the pH of the solution [27].

The removal efficiency of BCs increased by a rate of 7.8% with a change in pH from 2 to 10. This low increase can be associated with the influence of other interactions between BCs and MB molecules, such as the π-π interactions, suggesting that the dominant interaction in the adsorption mechanism [38]. As seen previously, FTIR spectra show the presence of -OH, -COOH, and C=C functional groups on the surfaces of the adsorbents. The -COOH group has pKa values between 2 and 4; therefore, its dissociation contributes to the MB removal by electrostatic interactions at acid pH; as well as the dissociation of protons from the phenolic groups -OH (pKa between 7–10) contributes to the removal at basic pH [39].

3.3.3. Effect of Contact Time and Kinetic Models

To evaluate the effect of the contact time on the adsorption capacity, we calculated the MB concentration at different contact times, varying between 0 and 1440 min. The removal efficiency is shown in Figure 4a. With only 60 min of contact, the SCB and BC4 had a removal capacity of 90%, while the BC1 with 1440 min of contact removed only 60% of the MB in solution. The decrease in the efficiency of SCB removal for BC1 may be due to the low availability of active sites for adsorption caused by the decrease in surface area from 6.54 to 2.29 m² g⁻¹ after one hour of carbonization, which implies a low availability of adsorption sites.

At pH between 8 and 10, we saw that adsorption is favored by the decrease of H⁺ on the surface of the adsorbents, suggesting the predominance of Coulomb interactions between MB molecules and functional groups with negative partial charges, after dissociation of H⁺ in solution. Hydrogen bonds occur when hydrogen is donated to a hydrogen acceptor, such as nitrogen atoms, for example. The MB molecule contains three nitrogen atoms that can donate electrons to form a bond with functional groups on the surface of the adsorbents. This suggests that these interactions occur between the groups -OH and -COOH, which are present as seen in the FTIR spectra (Figure 1).

BC2, BC3, BC4, and BC5 adsorbents had a removal rate > 96% within 360 min and gradually decreased, resulting from the diffusion of MB molecules on the surface of the particles [40], reaching equilibrium with 720 min of contact. This behavior, with high removal efficiency in a short time, maybe associated with several factors, such as (i) high availability of sites available for MB adsorption in the beginning [41], decreasing as it reaches equilibrium; and (ii) a large number of functional groups on the surface of the adsorbents and MB molecules in the initial stage, leading to greater adsorption due to the ability to conduct the mass transfer [42].

For different BCs, the increase in removal efficiency is seen with the increase in carbonization time. After 1440 min of contact, BC2 reached 97.6% removal and increased to 99.1 and 99.8% for BC3 and BC5, respectively. This increase may be due to the enlargement of the surface area (Table S2), a result of the increase in carbonization time. Wang et al. (2018) also observed an increase in the adsorption capacity of MB with an increase in the surface area of reed-derived biochar [43]. The authors investigated the effect of tannic acid on activated BC to remove MB; the surface area increased from 26.0 to 37.5 m² g⁻¹ after treatment, increasing the adsorption capacity from 27.2 to 37.2 mg g⁻¹. Based on the results above, the contact time of 360 min was chosen for the studies of adsorption isotherms, considering the best cost–benefit and economy of energy demand.

The kinetic models used were Pseudo-First-Order (PFO), Pseudo-Second-Order (PSO), and the Elovich model to analyze the adsorption capacity over time. The equations of the models used and their respective kinetic parameters are presented in

Table 1. Kinetics models parameters of MB onto SCB and BCs.

Model	Equation/Ref.	Parameter	Value
			SCB	BC1	BC2	BC3	BC4	BC5
PFO	$q_t=q_1 \left(1-e^{-k_{1}t}\right)$ [44]	q1 (mg g−1)	18.38	9.46	18.73	19.09	18.79	18.56
		k1 (min−1)	0.34	0.20	0.04	0.04	0.14	0.12
		R2	1.00	0.85	0.96	0.96	0.97	0.96
PSO	$q_t=\frac{{k_2{q}_2}^{2}t}{1+k_2{q}_{2}t}$ [45]	q2 (mg g−1)	18.77	10.59	19.42	19.68	19.71	19.57
		k2 (g mg−1 min−1)	0.04	0.01	0.003	0.004	0.01	0.01
		R2	1.00	0.91	0.98	0.98	0.99	0.99
Elovich	$q_t=\frac{1}{\beta }\ln \left(1+\alpha \beta t\right)$ [46]	α (mg g−1 min−1)	1012	44.1	12.3	18.1	3177	700
		β (mg g−1)	1.84	0.92	0.43	0.44	0.71	0.63
		R2	0.99	0.98	0.99	0.98	0.98	0.99

, and Figure 4a–c present the model fits. According to the results of the correlation factor (R²), experimental data were well described by all the models used, and the PSO model was better compared to the PFO model, this can be seen by the values of R² and $q_e$ q_e that were more similar to the experimental values. The Elovich and PSO model were the two models that better describe the adsorption kinetics for all adsorbents, with R² ≥ 0.98, with exception of BC1 in the PSO, which presented an R² of 0.91; this result brings the possibility of the BC1 in following a different adsorption mechanism.

The Elovich model is generally applied to chemosorption data [47] and describes heterogeneous diffusion processes that are regulated by two parameters: diffusion factor and reaction rate [48]. Thus, the good correlation of the data with the Elovich model suggests that the process of adsorption of MB in adsorbents occurs by heterogeneous diffusion. Additionally, the PSO model reveals that adsorption is a rate-limiting step; this process can be associated with chemical interactions between MB molecules and adsorbents, with changes in valence forces and sharing or exchange of electrons [49]. Therefore, MB molecules may be adsorbed on the surface through chemical bonds, which usually take a little longer to reach equilibrium [49].

3.3.4. Adsorption Mechanism

The adsorption of MB onto SBC and BCs was a dynamic process and presented different variations depending on several factors, such as pH of the solution, initial concentration of MB, and temperature, which we can consider to propose an adsorption mechanism (Figure 5). The kinetic results suggest that the adsorption process takes place by chemical bonds; however, we can deduce that covalent bonds may not be one of the main interactions that occur between MB and adsorbent molecules, due to the possibility of weak interaction forces, as discussed previously. Thus, the main interactions that occur in the studied adsorption process are Coulomb interactions, hydrogen bonds, and π-interactions, which are considered for physical adsorption processes [30].

The adsorption process, although generally endothermic, can be exothermic; when the process is endothermic, there is an increase in adsorption raising the temperature, as in the results discussed in the previous section. Moreover, when the process is exothermic, increasing temperature will decrease the adsorption capacity [25].

Figure 6 shows the effect of temperature on MB adsorption in SCB and BCs at different concentrations. In general, the increase in temperature had a positive influence on the increase in the adsorption of the dye in the adsorbents, such as BC5, which at a concentration of 230 mg L⁻¹, at 25 °C had an adsorption capacity of only 3.6 mg g⁻¹, increasing to 69. 2 mg g⁻¹ at 45 °C. A significant increase was also observed for BC4 with MB concentration of 110 mg L⁻¹, increasing from 4.2 to 45.6 mg g⁻¹ at 25 and 45 °C, respectively. This may be because with increasing temperature there is a decrease in the viscosity of the solution, thus increasing the mobility of MB molecules [50].

We observed that in high concentrations of MB, the SCB decreases its adsorption capacity, with an increase in temperature from 25 to 45 °C (Figure 6), indicating that at high temperatures the bonding forces become weaker for this adsorbent. With an initial concentration of 230 mg L⁻¹ at 35 °C the adsorption capacity was 67.14 mg g⁻¹, and decreased to 43.1 mg g⁻¹ at 45 °C. This result may be associated with a decrease in interactions between the active sites and the MB molecules, as discussed in the study by Biswas et al. (2020) [34]. The authors investigated the adsorption of MB in composites prepared from SCB biochar and observed that with an increase in temperature from 30 to 60 °C, the removal increased from 86 to 88.93%, respectively.

Notwithstanding the decrease, SCB was still a better adsorbent than BCs under the same concentration conditions at 25 and 35 °C. Figure 6 also reveals that the increase in temperature had a greater influence on the adsorption capacity of BC5, which proved to be the best adsorbent among BCs; however, the average adsorption capacity for BC5 at 25, 35, and 45 °C were 13.9, 32.9, and 44.6 mg g⁻¹, respectively, while for SCB it was 46.2, 49.4, and 46.9 mg g⁻¹. This demonstrates that despite the carbonization time of five hours, increasing the surface area by 14× to SCB is still proved to be a better adsorbent for MB dye under the studied conditions. Besides, the similar SCB adsorption capacity between 25 and 45 °C increases its applicability and is not dependent on the temperature of the effluent contaminated with MB dye.

An interesting behavior observed in the SCB adsorption capacity in some concentrations (30, 60, 230, and 270 mg L⁻¹) is that it increases from 25 to 35 °C, and then decreases to 45 °C. This may be related to factors, such as (i) the existence of weak bonding forces between the SCB and MB molecules, which, at high temperatures (45 °C), are weakened and break, decreasing the adsorption capacity [51]; and (ii) the increase in the kinetic energy of MB molecules with increasing temperature [52]. These results suggest the dependence on factors, such as the adsorbate concentration and temperature, as being the main influencers in the adsorption process, defining the endothermic or exothermic nature of the process.

To investigate the influence of the initial MB concentration, a stock solution of 1000 mg L⁻¹ was prepared and then diluted in different concentrations (10, 30, 60, 110, 150, 190, 230, and 270 mg L⁻¹). The values calculated for $q_e$, which is the maximum degree of surface coverage, were calculated and plotted against the initial concentration in the dye; the value here is presented as the maximum adsorption capacity. There are different inflection points with increasing concentration causing fluctuations in the adsorption capacity of MB by the adsorbents; this behavior is observed for all adsorbents at different temperatures (Figure S4a). The Figure also shows that the maximum adsorption capacity was observed in different concentrations studied for the adsorbents, being 230, 230, 190, 60, 270, and 60 mg L⁻¹ for SCB, BC1, BC2, BC3, BC4, and BC5, respectively. Two tests were conducted to reject the alternative of the inflection point be due to agglomeration of the MB molecules or by interference in the filtration; they are in the SI.

The SCB adsorption capacity increases between 10 and 60 mg L⁻¹, decays to 150 mg L^−1, and increases again up to 230 mg L⁻¹, followed by a decrease to 270 mg L⁻¹ at 25 °C. BC1 increases remain almost constant between 10 and 110 mg L⁻¹, decreases up to 150 mg L⁻¹, increases up to 230 mg L⁻¹, and decreases again by 270 mg L⁻¹ at 25 °C. These results are similar to those of Al-Ghouti and Al-Absi (2020) who obtained fluctuations in removal efficiencies (in percentage) with increasing concentration. The authors studied the removal of MB by green and black olive stones and attributed the observed behavior to several factors as discussed by Albroomi et al. (2017) [53]. The major factors are (i) with low initial dye concentrations, the availability of adsorption sites is great; nevertheless, the amount adsorbed becomes low, as well as the mass transfer; (ii) with the increase of the initial concentration the mass transfer tends to increase, implying a high in the adsorption on active sites; and (iii) further increasing the concentration, the proportion of MB molecules and adsorption sites reaches levels where the mass transfer is not supported.

The fluctuations and inflection points are less intense for BC1 (between 10 and 150 mg L⁻¹) and BC5 (between 110 and 230 mg L⁻¹) at 25 °C (Figure S4a). These differences may be associated with the heterogeneity of the adsorption process, with different interactions between the MB molecules and the adsorbents, due to the functional groups present on the surfaces [25]. The FTIR (Figure 1) spectra present many differences of functional groups in the adsorbent’s structures, which can be a factor that influences the adsorption at high MB concentrations, as we have discussed previously. For example, for BC1, the adsorption capacity was below 20 mg g⁻¹ to 150 mg L⁻¹ at 25 °C, which was expected since this adsorbent has a lower surface area; however, there was an increase to 42.0 and 34.7 mg g⁻¹ at 230 and 270 mg L⁻¹, respectively. In these two concentrations, BC1 outperformed the other biochar. This result may be associated with the increase in interactions between the functional groups on the surface of the adsorbent in high concentrations of dye; the reason for being greater than the other BCs may be due to the greater amount of groups -COOH and -OH, which retain the MB molecules through hydrogen bonds and electrostatic interactions [40]. BC4 showed a similar result; between 110 and 270 mg L⁻¹, the adsorption capacity increased from 9.3 to 28.8 mg g⁻¹, which can be associated with a greater driving force, by increasing the concentration, which overcomes resistance in the process of mass transfer of the dye from the solution to the adsorbent [36].

Furthermore, another interaction that may be influencing the adsorption process is the π-interactions. Figure S1 shows the presence of aromatic groups that can be formed in the carbonization process, and in the FTIR spectra, we can see the presence of C=C stretch that is associated with the presence of aromatic groups (Table S1). The C=C double bond is π-systems attractive to polar molecules and other π-systems; in addition, π-systems are electronegative nature, implying greater attraction of cations [30]. Thus, we can imply that these interactions occurred predominantly in this adsorption study.

4. Conclusions

In this study, BCs were produced at different carbonization times at 200 °C to compare their adsorption capacities with SCB of the MB dye. Although there was an increase in superficial area with an increase in carbonization time, the results show that the SCB was still a better adsorbent than any BCs produced. We show that by varying the MB initial concentration, the adsorption capacity shows inflection points and fluctuations, even at different temperatures, which predict the endothermic or exothermic nature of the adsorption process. SCB showed the better adsorbent capacity at all initial concentrations (10, 30, 60, 110, 150, 190, 230, and 270 mg L⁻¹) at 45 °C, and contact time of 360 min. With the proposed adsorption mechanism, there is a predominance of physical adsorption through Coulomb interactions, hydrogen bonds, and π-interactions. Therefore, the SCB is a low-cost alternative adsorbent for use in the treatment of wastewater contaminated with MB dye.