Efficient Adsorption of Methylene Blue by Porous Biochar Derived from Soybean Dreg Using a One-Pot Synthesis Method

Soybean dreg is a by-product of soybean products production, with a large consumption in China. Low utilization value leads to random discarding, which is one of the important sources of urban pollution. In this work, porous biochar was synthesized using a one-pot method and potassium bicarbonate (KHCO3) with low-cost soybean dreg (SD) powder as the carbon precursor to investigating the adsorption of methylene blue (MB). The prepared samples were characterized with scanning electron microscopy (SEM), transmission electron microscopy (TEM), elemental analyzer (EA), Brunauer-Emmett-Teller (BET), X-ray diffractometer (XRD), Raman spectroscopy (Raman), Fourier transform infrared spectrometer (FTIR), and X-ray photoelectron spectroscopy (XPS). The obtained SDB-K-3 showed a high specific surface area of 1620 m2 g−1, a large pore volume of 0.7509 cm3 g−1, and an average pore diameter of 1.859 nm. The results indicated that the maximum adsorption capacity of SDB-K-3 to MB could reach 1273.51 mg g−1 at 318 K. The kinetic data were most consistent with the pseudo-second-order model and the adsorption behavior was more suitable for the Langmuir isotherm equation. This study demonstrated that the porous biochar adsorbent can be prepared from soybean dreg by high value utilization, and it could hold significant potential for dye wastewater treatment in the future.


Introduction
The increasing demand for dyes in different industries exacerbates the discharge of dye wastewater [1], as well as their toxic effect on the environment and organisms, inducing significant concern from society as a whole [2]. For example, the discharge of dye wastewater into forests or fields will directly damage the soil productivity, and the presence of dyes in water can affect light penetration and photosynthesis of aquatic plants [3,4]. Some organic dyes and their products display mutagenicity or carcinogenicity toward humans [5], which may cause different degrees of damage to some organs, including kidney, brain, reproductive system, and liver, etc. [6][7][8]. In the dye industry, the cationic dye MB is an aromatic heterocyclic compound that can burn the eyes of people and animals, and may stimulate the gastrointestinal tract [9], causing symptoms such as dyspnea, vomiting, diarrhea, insanity, and methemoglobin after ingestion [10,11]. Therefore, the effective removal of dye from wastewater and preventing it from polluting the environment is a significant challenge for industrial production that requires an urgent solution.
In the early stage of dye wastewater treatment, only simple balancing and precipitation treatment methods were adopted. Currently, conventional dye removal methods that include biological, chemical, and physical approaches are used for the treatment of dye wastewater [12], such as biodegradation [13], membrane separation [14], coagulation 2. Results and Discussion 2.1. Characterization 2.1.1. SEM and TEM SEM and TEM characterized the porous structure. According to the SEM image (Figure 1a,b), the surface of the SD appeared as an irregular fold layered structure, which was relatively smooth and dense with no apparent pores [30]. However, after hightemperature activation with KHCO 3 , an abundance of pore structures was evident on the surface of the SDB-K-X, showing a 3D framework with randomly opened pores, indicating that KHCO 3 played a positive role in the pore formation of biochar. This may be due to the K 2 CO 3 generated by pyrolysis of KHCO 3 at 130~170 • C. When the temperature reaches Molecules 2021, 26, 661 3 of 15 600 • C, K 2 CO 3 begins to decompose and expand pores, forming large and abundant pore structures. In addition, metal potassium, reduced by carbon, was converted into gaseous potassium when the activation temperature reached 800 • C, which infiltrated the inner structure of the biochar, forming a larger specific surface area and abundant hierarchically porous structures. The possible activation reactions between SD and KHCO 3 are as follows (Equations (1)-(5)). SDB-K-3 (Figure 1b) showed the most uniform pore structure, while the pores of SDB-K-2, SDB-K-4, and SDB-K-5 contained blocky impurities, which could be attributed to the different proportions of SD and activators, resulting in an incomplete reaction or skeleton collapse and pore blockage.
CO 2 + C → 2CO (5) potassium when the activation temperature reached 800 °C, which infiltrated the inner structure of the biochar, forming a larger specific surface area and abundant hierarchically porous structures. The possible activation reactions between SD and KHCO3 are as follows (Equations (1)-(5)). SDB-K-3 ( Figure 1b) showed the most uniform pore structure, while the pores of SDB-K-2, SDB-K-4, and SDB-K-5 contained blocky impurities, which could be attributed to the different proportions of SD and activators, resulting in an incomplete reaction or skeleton collapse and pore blockage.
According to the pore structure analysis with SEM, all of the adsorbents showed continuous three-dimensional pore structure, nanoscile-sized flakes, and fractional porous carbons, but only the internal graphite structure was shown in the high-resolution TEM image of the SDB-K-3 sample (Figure 1c). The pores, indicated by white arrows in Figure 1d, were distributed throughout SDB-K-3, showing distinct lattice fringes with a distance of 0.246 nm, corresponding to the graphite (101) plane. Graphene streaks in TEM images indicated the presence of graphene structures in all samples ( Figure S1); however, no clear crystal spacing was detected; this may be attributed to the by-products produced by the reaction between the activator and the carbon in different proportions, which lead to the incomplete or total collapse of the material structure and hinder the exposure of the microcrystalline particles.  According to the pore structure analysis with SEM, all of the adsorbents showed continuous three-dimensional pore structure, nanoscile-sized flakes, and fractional porous carbons, but only the internal graphite structure was shown in the high-resolution TEM image of the SDB-K-3 sample (Figure 1c). The pores, indicated by white arrows in Figure 1d, were distributed throughout SDB-K-3, showing distinct lattice fringes with a distance of 0.246 nm, corresponding to the graphite (101) plane. Graphene streaks in TEM images indicated the presence of graphene structures in all samples ( Figure S1); however, no clear crystal spacing was detected; this may be attributed to the by-products produced by the reaction between the activator and the carbon in different proportions, which lead to the incomplete or total collapse of the material structure and hinder the exposure of the microcrystalline particles.

EA and BET
The C, O, H, N, and S content of SD and SDB-K-X were measured using an elemental analyzer. The results in Table 1 showed that, after high-temperature activation with KHCO 3 , the biochar yield of the samples exceeded 15%, among which SDB-K-3 reached the highest yield of up to 15.61%. The C content of SDB-K-X increased from 41.14% to over 66%, while the O, H, N, and S content all decreased. The O content of SDB-K-X decreased from 46.63% to about 26% and was probably due to the influence of different KHCO 3 ratios and the formation of gas during the reaction. The porous structure of SD and SDB-K-X was characterized by nitrogen sorption isothermal analysis. As illustrated in Figure 2a, SDB-K-X exhibited a typical type I adsorption-desorption isotherm, indicating the microporous structure mainly existed. Figure 2b showed that the pores of SDB-K-X were evenly distributed, and mainly concentrated at 0.75-2.0 nm. With the interaction between substances and the release of gas products in the reaction process, the framework of SDB-K-X was etched to produce a large number of micropores, thus forming a developed layered porous structure [31]. The summary in Table 2 indicates that as the ratio of SD to KHCO 3 increased, all the SDB-K-X samples displayed extremely high specific surface areas, exceeding 1500 m 2 g −1 . When the mass ratio of SD to KHCO 3 was 1:3, the subsequent SDB-K-3 exhibited the maximum specific surface area and pore volume of up to 1620 m 2 g −1 and 0.7509 cm 3 g −1 . As such, the specific surface area first increased and then decreased with the SD to KHCO 3 ration became higher, which may be ascribed to the shrinking of the carbon framework. This indicates that appropriate amount of KHCO 3 is conducive to the activation effect; with the increase of KHCO 3 dosage, the formed micropores will expand into mesopores or macropores. However, when excessive amount of KHCO 3 is used, some pores will collapse due to excessive corrosion. The analysis was consistent with the SEM and TEM results. In general, materials with a more extensive specific surface area contain more active sites, and a considerable number of micropores are used as the active adsorption sites, which is conducive to the removal of dyes [32]. Table 2. Specific surface area, pore volume, and average pore size of SDB-K-X.

Sample
BET (m 2 g −1 ) Vtot (cm 3 g −1 ) Average Pore Diameter (nm) rials [34]. The ID/IG value is usually used to represent the degree of graphitization. All the SDB-K-X samples contained graphitized structures at different degrees, among which the ID/IG of SDB-K-3 was the smallest (0.96), indicating that the graphitization degree of SDB-K-3 was relatively high and the structural defects were fewer. This result can be attributed to activators at different quality levels affecting the graphite structure, leading to the formation of a defective texture.

FTIR and XPS
The FTIR spectra of SDB-K-X are illustrated in Figure 3a, indicating that functional groups exist on the surface of biochar, which is different from SD. This may be caused by the volatilization of water and organic matter, as well as the interaction between functional groups during high-temperature activation. The band at 3411 cm −1 was ascribed to -OH stretching vibration [21], while the peak at 2933 cm −1 was attributed to C-H stretching vibration. Carboxyl C=O stretching vibrations and C=C peaks were identified at 1710 and 1587 cm −1 ; the peak at 1095 cm −1 was ascribed to the C-OH stretching vibration [35]. Therefore, the samples contained abundant functional groups, which is conducive to improving the adsorption performance of organic dyes.

XRD and Raman
The SDB-K-X were subjected to XRD analysis to characterize the crystalline structures. The SDB-K-X samples all displayed typically disordered amorphous carbon, as suggested by the low-intensity and broadened peaks in the XRD patterns ( Figure 2c). There were two main diffraction peaks around 24 • and 43 • , corresponding to the (002) and (101) plane diffraction (PDF#41-1487), respectively [33].
The Raman spectra ( Figure 2d) further confirmed the formation of a graphitized SD structure. Two distinct bands were located at around 1341 cm −1 (D band) and 1585 cm −1 (G band), providing information regarding the disorder and crystallinity of sp 2 carbon materials [34]. The I D /I G value is usually used to represent the degree of graphitization. All the SDB-K-X samples contained graphitized structures at different degrees, among which the I D /I G of SDB-K-3 was the smallest (0.96), indicating that the graphitization degree of SDB-K-3 was relatively high and the structural defects were fewer. This result can be attributed to activators at different quality levels affecting the graphite structure, leading to the formation of a defective texture.

FTIR and XPS
The FTIR spectra of SDB-K-X are illustrated in Figure 3a, indicating that functional groups exist on the surface of biochar, which is different from SD. This may be caused by the volatilization of water and organic matter, as well as the interaction between functional groups during high-temperature activation. The band at 3411 cm −1 was ascribed to -OH stretching vibration [21], while the peak at 2933 cm −1 was attributed to C-H stretching vibration. Carboxyl C=O stretching vibrations and C=C peaks were identified at 1710 and 1587 cm −1 ; the peak at 1095 cm −1 was ascribed to the C-OH stretching vibration [35]. Therefore, the samples contained abundant functional groups, which is conducive to improving the adsorption performance of organic dyes. results. These results were consistent with those of the FTIR characterization, indicating that the SDB-K-X surface contained a considerable number of oxygen-containing functional groups.

The Adsorption Process
Combined with the structural characterization of biochar, MB adsorption onto all the samples was compared in Figure 4; all the samples showed high adsorption efficiency, and SDB-K-3 was selected as the best adsorbent for the removal of MB, which was attributed to its highest specific surface area, suitable pore volume, and abundant oxygencontaining functional groups. The adsorption of SDB-K-3 at different initial concentrations (1000, 1500, and 2000 mg L −1 ) was investigated, indicating that the adsorption capacity increased from 997.85 to 1171.88 mg g −1 . The adsorption kinetics, adsorption equilibrium, and adsorption thermodynamics of SDB-K-3 for MB were further illustrated in the following sections. Based on the structural characterization analysis of SDB-K-X, XPS was used to evaluate the elemental composition and surface groups of SDB-K-3, which primarily contained C and O, as well as a small amount of N ( Figure 3b). These results corresponded well with the EA findings after the peak-differentiating-imitating analysis to identify the high-resolution spectra of C1s and O1s. As shown in Figure 3c,d, the C 1s spectrum was deconvoluted into four peak components at 284.0, 284.8, 286.0, 287.8, and 289.2 eV, which were attributed to C=C (sp 2 C) (1.89%), C-C (sp 3 C) (60.65%), C-O (25.55%), C=O (4.50%), and π-π* satellite (7.40%) groups, respectively [36]. The O1s peaks confirmed the presence of C=O (27.76%), C-O (50.56%), and -OH (21.68%) at 533.7, 532.6, and 531.5 eV [37], respectively. The XPS spectra of SDB-K-2, SDB-K-4, and SDB-K-5 ( Figure S2) show similar results. These results were consistent with those of the FTIR characterization, indicating that the SDB-K-X surface contained a considerable number of oxygen-containing functional groups.

The Adsorption Process
Combined with the structural characterization of biochar, MB adsorption onto all the samples was compared in Figure 4; all the samples showed high adsorption efficiency, and SDB-K-3 was selected as the best adsorbent for the removal of MB, which was attributed to its highest specific surface area, suitable pore volume, and abundant oxygen-containing functional groups. The adsorption of SDB-K-3 at different initial concentrations (1000, 1500, and 2000 mg L −1 ) was investigated, indicating that the adsorption capacity increased from 997.85 to 1171.88 mg g −1 . The adsorption kinetics, adsorption equilibrium, and adsorption thermodynamics of SDB-K-3 for MB were further illustrated in the following sections.

Adsorption Kinetics
To investigate the effect of the sorption behavior mechanism of SDB-K-3 on MB, the kinetic adsorption data were simulated using the pseudo-first-order and pseudo-secondorder models. The equations of the two models are expressed as follows [38,39]: where qt and qe (mg g −1 ) are the amounts of MB adsorbed at time t and equilibrium, respectively, while k1 (per min) and k2 (g/mg per min) is the pseudo-first-order and pseudosecond-order model rate constants of adsorption, respectively. Figure 5a shows that the adsorption efficiency increased with the extension of adsorption time, while the adsorption process reached a balance, and the adsorption sites were saturation, maintaining an equilibrium in the adsorption capacity [35]. As the concentration increased, the mass transfer driving force rose, the interaction between SDB-K-3 and MB improved, and the adsorption capacity increased until reaching the adsorption saturation. The maximum absorption Q (mg g −1 ) amounts of 1000, 1500, and 2000 mg L −1 MB were 996.37, 1136.83, and 1174.73 mg g −1 , and the removal efficiency R (%) were 99.64%, 75.79%, and 58.74%, respectively. The adsorption efficiency of SDB-K-3 to MB increased rapidly within 0.5 h and reached the adsorption equilibrium within 2 h. The rapid adsorption process may be ascribed to the existence of effective binding sites, as well as the formation of a hierarchical porous structure with a high SDB-K-3 surface area [40].
The model regarding the relevant kinetic parameters for MB adsorption is provided in Table 3, while Figure 5b,c illustrate the kinetic plots. Therefore, the pseudo-secondorder model displayed higher R 2 values (>0.999) in the three MB concentrations than the pseudo-first-order model. Notably, the pseudo-second-order model can describe the adsorption process better than the pseudo-first-order model, meaning that the chemical effect was also involved in the adsorption of MB on SDB-K-3 [35]. This result can be ascribed to the high surface area and surface abundance of functional groups contributing to the diffusion of organic dye molecules from the water to the surface of the adsorbent. Furthermore, the calculated values of the adsorption amounts at equilibriums of 1000, 1500, and 2000 mg L −1 MB onto SDB-6-K from the pseudo-second-order model were 1000.00, 1129.14, and 1155.79 mg g −1 , respectively, which were approximately the same as those from the experimental results (996.37, 1136.83, and 1174.73 mg g −1 ). These findings suggested that the pseudo-second-order kinetic model was more suitable for describing the adsorption behavior of MB onto SDB-K-3.

Adsorption Kinetics
To investigate the effect of the sorption behavior mechanism of SDB-K-3 on MB, the kinetic adsorption data were simulated using the pseudo-first-order and pseudo-secondorder models. The equations of the two models are expressed as follows [38,39]: ln q e − q t = lnq e − k 1 t (pseudo-first-order) where q t and q e (mg g −1 ) are the amounts of MB adsorbed at time t and equilibrium, respectively, while k 1 (per min) and k 2 (g/mg per min) is the pseudo-first-order and pseudo-second-order model rate constants of adsorption, respectively. Figure 5a shows that the adsorption efficiency increased with the extension of adsorption time, while the adsorption process reached a balance, and the adsorption sites were saturation, maintaining an equilibrium in the adsorption capacity [35]. As the concentration increased, the mass transfer driving force rose, the interaction between SDB-K-3 and MB improved, and the adsorption capacity increased until reaching the adsorption saturation. The maximum absorption Q (mg g −1 ) amounts of 1000, 1500, and 2000 mg L −1 MB were 996.37, 1136.83, and 1174.73 mg g −1 , and the removal efficiency R (%) were 99.64%, 75.79%, and 58.74%, respectively. The adsorption efficiency of SDB-K-3 to MB increased rapidly within 0.5 h and reached the adsorption equilibrium within 2 h. The rapid adsorption process may be ascribed to the existence of effective binding sites, as well as the formation of a hierarchical porous structure with a high SDB-K-3 surface area [40].
The model regarding the relevant kinetic parameters for MB adsorption is provided in Table 3, while Figure 5b,c illustrate the kinetic plots. Therefore, the pseudo-secondorder model displayed higher R 2 values (>0.999) in the three MB concentrations than the pseudo-first-order model. Notably, the pseudo-second-order model can describe the adsorption process better than the pseudo-first-order model, meaning that the chemical effect was also involved in the adsorption of MB on SDB-K-3 [35]. This result can be ascribed to the high surface area and surface abundance of functional groups contributing to the diffusion of organic dye molecules from the water to the surface of the adsorbent. Furthermore, the calculated values of the adsorption amounts at equilibriums of 1000, 1500, and 2000 mg L −1 MB onto SDB-6-K from the pseudo-second-order model were 1000.00, 1129.14, and 1155.79 mg g −1 , respectively, which were approximately the same as those from the experimental results (996.37, 1136.83, and 1174.73 mg g −1 ). These findings suggested that the pseudo-second-order kinetic model was more suitable for describing the adsorption behavior of MB onto SDB-K-3.

Thermodynamic Analysis
To estimate the type of adsorption for SDB-K-3 on MB, the thermodynamic adsorption parameters, such as the ΔG (kJ·mol −1 ), the ΔH (kJ·mol −1 ), and the ΔS (J·K −1 ·mol −1 ) were calculated. The thermodynamic parameters can be obtained with the following equations [43]:

Adsorption Equilibrium
The adsorption isotherm is used to describe the interaction between adsorbate and adsorbent, indicating the distribution of adsorption molecules between the liquid and solid phase when the adsorption process reaches an equilibrium state. To analyze the adsorption mechanism of the MB on the surface of SDB-K-3, the adsorption data were fitted using the Langmuir [41] and Freundlich [42] isotherm models, respectively. The Langmuir adsorption isotherm assumes that adsorption occurs in a single-layer at a specific homogeneous site inside the adsorbent. The Freundlich isotherm is an empirical equation suggested for adsorption systems on the heterogeneous surface. The linear form of the Langmuir and Freundlich isotherm equations can be written as: where C e (mg g −1 ) and q e (mg g −1 ) are the concentrations of MB in the solution and the adsorbent at the adsorption equilibrium, respectively. q m (mg g −1 ) is the MB monolayer adsorption capacity, k L is the adsorption constants, which are related to the free energy of adsorption. n F and k F are Freundlich adsorption constants related to adsorption capacity and sorption intensity, respectively. The adsorption isotherms of SDB-K-3 for MB at different initial dye concentrations and temperatures are shown in Figure 5d. By increasing the initial concentration of MB at a specific temperature, q e gradually increased, and then approached saturation adsorption. With the increase of temperature, the saturation adsorption capacity (qm) increases accordingly, confirming the active influence of temperature on adsorption capacity. Therefore, it can be inferred that the adsorption of SDB-K-3 on MB is an endothermic process.
The curves simulated in Figure 5e, indicate that the curves of the Langmuir model are more suitable for fitting the adsorption data than those of the Freundlich isotherm model (Figure 5f). Therefore, the k 1 and q e values were calculated from the plots provided in Table 4, indicating that the correlation coefficients (R 2 > 0.999) of the Langmuir model for all initial MB concentrations were more significant than those found when using the Freundlich model. The results indicated the adsorption process of SDB-K-3 on MB occurred in a single layer.

Thermodynamic Analysis
To estimate the type of adsorption for SDB-K-3 on MB, the thermodynamic adsorption parameters, such as the ∆G (kJ·mol −1 ), the ∆H (kJ·mol −1 ), and the ∆S (J·K −1 ·mol −1 ) were calculated. The thermodynamic parameters can be obtained with the following equations [43]: (12) where R is the gas constant (8.314 Jmol −1 K −1 ), Kc is the thermodynamic equilibrium constant, and T (K) is the sorption temperature, C e (mg g −1 ) and q e (mg g −1 ) are the concentrations of MB in the solution and the adsorbent at the adsorption equilibrium, respectively. The ∆H and ∆S values can be calculated from the slope and intercept of the linear plot of ln Kc and 1/T. The obtained thermodynamic parameters are shown in Figure 6 and Table 5. All acquired ∆G values were negative at all temperatures, indicating that the adsorption of MB on SDB-K-3 was spontaneous and feasible, suggesting a physisorption process [44]. The positive ∆H values implied the endothermic nature of the adsorption interactions. Furthermore, the positive ∆S value illustrated the adsorbent and the adsorption properties of some structural changes, while indicating the randomness of solid-liquid interface in the process of adsorption was increased [45].
where R is the gas constant (8.314 Jmol −1 K −1 ), Kc is the thermodynamic equilibrium constant, and T (K) is the sorption temperature,ce (mg g −1 ) and qe (mg g −1 ) are the concentrations of MB in the solution and the adsorbent at the adsorption equilibrium, respectively. The ΔH and ΔS values can be calculated from the slope and intercept of the linear plot of ln Kc and 1/T. The obtained thermodynamic parameters are shown in Figure 6 and Table 5. All acquired ΔG values were negative at all temperatures, indicating that the adsorption of MB on SDB-K-3 was spontaneous and feasible, suggesting a physisorption process [44]. The positive ΔH values implied the endothermic nature of the adsorption interactions. Furthermore, the positive ΔS value illustrated the adsorbent and the adsorption properties of some structural changes, while indicating the randomness of solid-liquid interface in the process of adsorption was increased [45].  Table 5. Thermodynamic parameters for the adsorption of MB on SDB-K-3.

Concentration
(mg L −1 )  Figure 7 shows the effects of initial pH on MB adsorption by SDB-K-3; the results show that with the increase of the initial pH of MB solution, its adsorption capacity was slightly improved, and the solution pH directly affects the surface charge and the degree  Table 5. Thermodynamic parameters for the adsorption of MB on SDB-K-3.

Concentration
(mg L −1 )  Figure 7 shows the effects of initial pH on MB adsorption by SDB-K-3; the results show that with the increase of the initial pH of MB solution, its adsorption capacity was slightly improved, and the solution pH directly affects the surface charge and the degree of ionization of adsorbents present in the solution [46]. In acidic media, the -COOH and -OH were protonated at the form of -COOH 2+ and -OH 2+ . The positive charges on the surface of carbon materials compete with MB + to occupy the adsorption sites of the channel, thus reducing the adsorption performance. As the pH of the solution increased, the positive charge on the surface of the adsorbent decreased gradually, which made the electrostatic adsorption between carbon material and MB + occur. When the pH was alkaline, the adsorption capacity of the alkaline dye MB was improved by the -COO − and -OH groups on the surface of SDB-K-3. of ionization of adsorbents present in the solution [46]. In acidic media, the -COOH and -OH were protonated at the form of -COOH 2+ and -OH 2+ . The positive charges on the surface of carbon materials compete with MB + to occupy the adsorption sites of the channel, thus reducing the adsorption performance. As the pH of the solution increased, the positive charge on the surface of the adsorbent decreased gradually, which made the electrostatic adsorption between carbon material and MB + occur. When the pH was alkaline, the adsorption capacity of the alkaline dye MB was improved by the -COO − and -OH groups on the surface of SDB-K-3.

Adsorption Performance Comparison with Other Adsorbents
The comparison of the maximum adsorption capacities of MB using various previously reported adsorbents is summarized in Table 6, showing that the adsorption capacity of SDB-K-3 for MB was higher than those of other adsorbents, such as banana peel [46], garlic peel [47], seaweed [48], coconut shells [49], wood [50], rattan [51], and wheat straw [52], which are considered naturally available and cost-efficient. Different adsorbents exhibit different adsorption properties for MB, which may be caused by the variances in raw material composition and preparation methods, resulting in the composition and pore structure of biochar. Consequently, considering its low cost and comparable adsorption capacity, SDB-K-3 is assumed to be a promising adsorption material to remove MB from water.

Materials and Reagents
SD was obtained from the Shandong Yuxin Biotechnology Co., Ltd. (Shandong Province, China). The SD was first dried in a vacuum oven (GZX-9030MBE, Shanghai Boxun Industrial Co., Ltd. Medical Equipment Factor, Shanghai, China) at 105 °C for 48 h to obtain the starting material. All chemicals, including KHCO3, MB, and hydrochloric acid

Adsorption Performance Comparison with Other Adsorbents
The comparison of the maximum adsorption capacities of MB using various previously reported adsorbents is summarized in Table 6, showing that the adsorption capacity of SDB-K-3 for MB was higher than those of other adsorbents, such as banana peel [46], garlic peel [47], seaweed [48], coconut shells [49], wood [50], rattan [51], and wheat straw [52], which are considered naturally available and cost-efficient. Different adsorbents exhibit different adsorption properties for MB, which may be caused by the variances in raw material composition and preparation methods, resulting in the composition and pore structure of biochar. Consequently, considering its low cost and comparable adsorption capacity, SDB-K-3 is assumed to be a promising adsorption material to remove MB from water.

Materials and Reagents
SD was obtained from the Shandong Yuxin Biotechnology Co., Ltd. (Shandong Province, China). The SD was first dried in a vacuum oven (GZX-9030MBE, Shanghai Boxun Industrial Co., Ltd. Medical Equipment Factor, Shanghai, China) at 105 • C for 48 h to obtain the starting material. All chemicals, including KHCO 3 , MB, and hydrochloric acid (HCl, 37 wt%) were of analytical grade, purchased from the Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used as received.

Preparation of SDB-K-X
The SD was mixed with KHCO 3 (SD/KHCO 3 weight ratio = 1:2, 1:3, 1:4, and 1:5) and ground evenly, after which it was activated at 800 • C at a heating rate of 10 • C min −1 in a N 2 (99.999%) atmosphere for 90 min. After the temperature had cooled to room temperature, the subsequent material was placed in a 5 wt% HCl solution and stirred continuously for 24 h, after which it was filtered and separated. The residue was washed with DI water until a neutral pH was reached, and dried at 105 • C for 24 h. The activated material that was obtained was denoted as SDB-K-X (X = 2, 3, 4, and 5 referring to the SD/KHCO 3 weight ratio = 1:2, 1:3, 1:4, and 1:5, respectively) and finally preserved in a desiccator until further use.

Characterization of SDB-K-X
The yields of biochars were calculated based on a mass balance using Equation (13) Yields (%) = m1/m2 × 100% (13) where m1 and m2 were the masses (g) of SDB-K-X and soybean dreg after drying, respectively. The morphology of SD and SDB-K-X was performed using SEM (JEOL JSM-6700F, Japan) after coating the sample with a thin layer of gold, while the hole wall microstructure was performed with TEM (JEOL JEM-2100F, Tokyo, Japan) operated at 200 kV. The EA (C, H, O, N, and S) of the samples were acquired with an elemental analyzer (Elementar Vario EL III, Hanau, Germany). The nitrogen sorption isotherms of the samples were carried out at 77 K using a nitrogen adsorption apparatus (BET, Quantachrome AUTOSORB IQ, Boynton Beach, FL, USA). The samples were degassed in vacuum at 300 • C for 12 h before testing, the specific surface area and pore size distribution of adsorbents were calculated by density functional theory (DFT) and BET equation. The crystallinity of the samples was conducted with an X-ray diffraction (Rigaku Ultima IV, Tokyo, Japan) using Cu Ka radiation at 40 kV and 40 mA; the scanning rate was 2 • /min with a scanning step of 0.2 • from 10 • to 80 • (2θ), while the Raman spectra were obtained using Raman (Bruker Optics SENTERRA, Ettlingen, Germany, excitation-beam wavelength: 532 nm). The organic structure of SDB-K-X was characterized by FTIR (SHIMADZU Type 2000, Tokyo, Japan) recorded in a wavenumber range of 4000-400 cm −1 (the sample was thoroughly mixed with KBr at a ratio of 1:100). The surface element composition and chemical state of the samples were obtained with spectra recorded using XPS (Thermo Scientific Escalab 250Xi, Waltham, MA, USA).

Adsorption Experiments
All batches were tested in a Water-bathing Constant Temperature Vibrator (WE-3, Tianjin Ounuo Instrument Co., LTD, Tianjin, China). Then, 0.025 g adsorbent was added to a 25 mL MB aqueous solution of 1000, 1500, and 2000 mg L −1 , and oscillated at 150 rpm for 2 h at 298 K. The adsorption was performed for 2 h at 150 rpm, after which the suspension was centrifuged at 4000 rpm for 10 min. Then, the supernatant was filtered through a 0.22 µm filter to measure its adsorption value (UV-vis, Agilent Cary-60, Santa Clara, CA, USA) at the maximum value of 664 nm for MB. The adsorption capacity Q (mg g −1 ) and removal efficiency R (%) of MB on the adsorbent were separately calculated with the following formulation: where Q (mg g −1 ) is the amount of MB adsorbed on the adsorbent, C 0 (mg L −1 ) and C (mg L −1 ) are the initial concentration and the equilibrium concentration of MB, respectively, V (L) is the volume of the MB solution, m (g) is the mass of the adsorbent used, and R (%) is the removal efficiency of the dye. All adsorption studies were conducted in triplicate, and the data were reported as average ± standard deviation (SD).

Adsorption Kinetics
Furthermore, to investigate the MB adsorption kinetics, 0.025 g of SDB-K-3 was added to 25 mL MB aqueous solution at concentrations of 1000, 1500, and 2000 mg L −1 , respectively, at a temperature of 298 K and a speed of 150 rpm. At a preset time point (from 0 min to 600 min), a small amount of the sample was removed with a pipette gun for determination.

Effect of Solution pH
Effect of solution pH was investigated at pH 2, 3, 4, 5, 6, 7, 8, and 9. The pH of solution was adjusted with 1N HCl and 1N NaOH solutions. Then, 0.025 g SDB-K-3 was added to each 25 mL volume of MB aqueous solution having an initial concentration of 2000 mg L −1 for a constant adsorption time of 120 min at 150 rpm at 298 K.

Conclusions
In this study, the porous biochar prepared from SD shows a highly specific BET surface area (1620 m 2 g −1 ), large pore volume (0.7509 cm 3 g −1 ), and rich oxygen-containing hydrophobic groups. As a cost-effective dye adsorbent, it has significant application potential. SDB-K-3 exhibits excellent adsorption capacity for MB, with a maximum capability of 1273.51 mg g −1 for MB at 318 K. Combined with the detailed characterization of SDB-K-3, this superior result can be ascribed to the 3D framework structure and active adsorption site, suggesting that the combination of chemical adsorption and physical adsorption, π-π interaction, hydrogen bonding, and electrostatic interaction may be possible adsorption mechanisms. These results demonstrate that the excellent adsorption performance of SDB-K-3 to MB is a possible option for practical industrial wastewater treatment. Further investigation should focus on optimizing the preparation method, reducing production costs, and realizing industrial applications.