Biochar Prepared from Steam-Exploded Bitter Melon Vine for the Adsorption of Methylene Blue from Aqueous Solution: Kinetics, Isotherm, Thermodynamics and Mechanism

Abstract

Bitter melon vine (an agricultural waste product with high fiber content) is difficult to treat and has caused problems in the environment. This research aims to produce biochar through low-temperature pyrolysis assisted by non-polluting steam explosion. The physical and chemical properties of the biochar were characterized using scanning electron microscopy (SEM) images, specific surface area measurements (BET), X-ray diffraction patters (XRD), elemental analysis (EA), and Fourier transform infrared spectroscopy (FTIR). Next, the adsorption mechanism of methylene blue (MB) on the steam-exploded bitter melon vine biochar pyrolyzed at 200 °C (qBC₂₀₀) and the effects of adsorption time, pH, initial concentration, adsorption temperature, and adsorbent dosage on the adsorption effect were investigated. Steam explosion destroyed the dense structure of the plant, increased the number of oxygen-containing surface functional groups, and improved the adsorption performance of the material. Therefore, qBC₂₀₀ more effectively adsorbed MB than untreated biochar, reaching a saturated adsorption capacity of 267.72 mg/g. The MB adsorption kinetics and isothermal adsorption process of qBC₂₀₀ align with the pseudo-second-order kinetic model and Langmuir isothermal equation (monolayer adsorption), respectively. The thermodynamic results show that MB adsorbs via a spontaneous, entropy-increasing exothermic reaction. The adsorption mechanism involves electrostatic attraction, hydrogen bonding, and π–π interactions. The prepared biomass with high fiber content is a promising new material for wastewater treatment.

1. Introduction

China is a large agricultural production country that generates a large amount of agricultural waste each year, including more than 300 million tons of vegetable waste. At present, the comprehensive utilization of vegetable waste is approximately 10%, largely owing to low human awareness of resource utilization and imperfect policy systems [1,2]. Scientific treatment and disposal of these agricultural wastes is necessary for controlling agricultural non-point source pollution, improving rural living environments, constructing beautiful countrysides, and strategically meeting the goals of carbon peak and carbon neutrality. Bitter melon (Momordica charantia), a plant of the Cucurbitaceae family, is rich in alkaloids, saponins, peptides, vitamins, minerals, and other bioactive compounds that provide basic nutrients for the human body and can prevent and treat various diseases. Bitter melon is a popular crop cultivated in China [3,4,5]. Economic and technical limitations and a lack of resourceful disposal and utilization conditions have led to a large amount of wasted bitter melon vine. Reasonable disposal and utilization of bitter melon vine would prevent wastage of resources and reduce pollution of the ecological environment. Although bitter melon is a rich source of triterpenoids, polysaccharides, flavonoids, and other nutrients [6,7], the methods for extracting these compounds are costly, complex, require operation by specialized personnel, and are not easily popularized. To deliver economic and social benefits, bitter melon must be converted to high-added-value products on a large scale at low cost.

Biomass pyrolysis is a recently developed agricultural waste-resource utilization technology that converts the unstable organic carbon in biomass to sequestered biochar. Besides providing an important route to carbon peak and carbon neutrality in agriculture and rural areas, this technology can effectively remove agricultural waste [8,9,10]. Biomass pyrolysis obtains highly aromatic biochar products with rich pore structures, large specific surface areas, and various oxygen-containing functional groups, which are now widely used in industry and agriculture [11,12,13]. Biochar can be used to remediate toxic metals from wastewater and stabilize toxic metals in contaminated soil [14,15]. As an effective agent for sequestering C, biochar can increase the efficiency of soil organic matter cycling, reduce emissions of greenhouse gas, and improve soil nutrients [16]. The quality of biochar largely depends on the hemicellulose, cellulose, and lignin contents in the biomass raw materials [16,17]. With a cellulose content above 50% (sometimes as high as 70%), bitter melon vine is an ideal raw material for biochar preparation. However, biochar prepared by direct pyrolysis of melon vine has a poor adsorption capacity, which must be enhanced through modification to improve its utilization value [18,19,20]. The current modification methods usually employ acids, alkalis, and oxidizers that contaminate the waste liquid after modification, damaging the ecological environment. These methods also involve complicated process steps. Greener and cheaper biochar preparation methods are urgently required.

Steam explosion technology, simply referred to as steam explosion, has been rapidly developed as a biomass pretreatment method in recent years. Using mainly steam, this technology heats the biomass to 160 °C–260 °C while maintaining the pressure at 0.69–4.83 MPa. After holding for a period of time, the pressure is released instantaneously to achieve the “explosion.” Thermal reactions and the physical tearing effect quickly destroy the dense structure of the material and change its chemical composition [21]. Steam explosion lowers energy consumption, cost, and pollutant generation, compared to those of other pretreatment methods, while increasing the efficiency, as desired for large-scale applications. Modifications of fruit and vegetable waste, grain processing, and the extraction of traditional Chinese medicines have greatly progressed in recent years [22,23,24,25]. Sui et al. reported that steam explosion treatment improves the nutritional value of corn straw by changing its lignocellulosic structure [24]. He et al. modified the dietary fiber of tartary buckwheat bran using steam explosion technology. They varied the steam explosion intensities and compared the resulting compositions and physicochemical properties of the dietary fiber in the bran [25]. Yu et al. found that steam-explosion-treated flaxseed provided a significantly higher oil yield than the untreated group [26]. Steam explosion treatment also increases the content of active components in flaxseed oil and improves the stability and quality of the oil. However, this technology has rarely been applied to the preparation of biochar from pretreated melon vine biomass and its impact on the physicochemical properties of biochar has not been investigated [27].

This study investigates the effect of steam-explosion-assisted low-temperature pyrolysis on the yield, surface groups and structure, chemical properties, and adsorption performance of biochar prepared from bitter melon vine as the raw material. The aim is to provide a theoretical basis and technological support for the modification and high-value utilization of melon vine biochar.

2. Materials and Methods

2.1. Preparation of Biochar from Bitter Melon Vine

Bitter melon vine was harvested from rural areas around Weifang City, Shandong Province, China. First, the collected bitter melon vines were dried outdoors, cut into small segments with scissors, pulverized with a pulverizer, and then baked in an oven at 105 °C for 24 h. The powder prepared from bitter melon vines, labeled as C₀, was packed into a crucible, covered with the crucible lid, and placed in a muffle furnace for oxygen-limited pyrolysis at 200 °C, 300 °C, 400 °C, 500 °C, or 600 °C for 3 h. Once the reaction was completed, the sample was cooled to room temperature. After calculating the yield, the sample was maintained in a sealed bag. The bitter melon vine biochars prepared at 200 °C, 300 °C, 400 °C, 500 °C, and 600 °C were labeled BC₂₀₀, BC₃₀₀, BC₄₀₀, BC₅₀₀, and BC₆₀₀, respectively.

2.2. Biochar Preparation Using Steam-Explosion-Assisted Pyrolysis

Prior to steam explosion treatment, the bitter melon vine was crushed into pieces approximately 2 cm in size and the water content was adjusted to approximately 50%. The steam explosion equipment (QBS-80; Hebi Zhengdao Qibao Industrial Co., Ltd., Hebi, China) was stabilized for 3 min and its pressure was maintained at 1.5 MPa. The steam-explosion-treated material was collected and placed in an oven at 105 °C for 24 h. The resulting sample was labeled qBC₀.

To prepare the steam-exploded bitter melon vine biochars, the crucible was filled with qBC₀, covered with a lid, placed in a muffle furnace, and pyrolyzed under oxygen-limited pyrolysis for 3 h at 200 °C, 300 °C, 400 °C, 500 °C, or 600 °C, yielding qBC₂₀₀, qBC₃₀₀, qBC₄₀₀, qBC₅₀₀, and qBC₆₀₀, respectively.

2.3. Characterization of Biochar from Bitter Melon Vine

The pH of the biochars was determined according to GB/T 7702.16 and the O/C, H/C, and C/N ratios were calculated from the C, H, O, S, and N contents determined by an elemental analyzer (Elementar vario EL III, Langenselbold, Germany). The morphologies and structures of the biochars were observed using a scanning electron microscope (SEM) (JSM-7800F, Tokyo, Japan) and the functional groups were characterized using a Fourier transform infrared (FTIR) spectrometer (Nicolet iS10, Waltham, MA, USA). The crystal structure of biochars were determined using an X-ray diffractometer (XRD) (Rigaku Ultima IV, Tokyo, Japan). The specific surface areas and pore structures of the biochars were determined using a fully automated specific surface area and pore size analyzer (BSD-PS1, Beijing, China).

2.4. Experiment of MB Adsorption on Biochar

Twenty milligrams of each biochar were placed in a 100 mL conical flask. After adding 50 mL (accurately measured) of methylene blue (MB) with an initial concentration of 150 mg/L and adjusting the pH to 7.0, the flask was placed in a constant-temperature shaker for the adsorption test. The speed and temperature of the shaker were set to 200 rpm and 25 °C, respectively, and the adsorption time was 12 h. The solution was filtered and the MB absorbance was determined using an ultraviolet–visible spectrophotometer. The removal rates and adsorption capacities of MB by the different biochars were, respectively, calculated as follows:

$$R={\displaystyle \frac{C_0-C_e}{C_e}}\times 100\%,$$

(1)

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

(2)

where C₀ and C_e are the initial and equilibrium concentrations of MB in the solution (mg/L), respectively, m is the mass of biochar (g), V is the volume of the solution (mL), and $q_e$ is the equilibrium adsorption amount (mg/g).

2.4.1. Effect of pH on the Adsorption Performance of Biochar

Furthermore, 20 mg of qBC₂₀₀ was added to 50 mL of aqueous solutions with different pH values (2, 4, 6, 8, or 10) and an MB concentration of 150 mg/L. The solutions were incubated at a constant temperature with shaking for 12 h and filtered for absorbance measurements. The MB removal rates and adsorption capacities of the biochars were calculated using Equations (1) and (2), respectively.

2.4.2. Effect of Initial Dosage on the Adsorption Performance of Biochar

Accurately weighed qBC₂₀₀ (5, 10, 15, 20, 25, or 30 mg) was placed in 50 mL of an aqueous solution with a pH of 7.00 and an MB concentration of 50 mg/L. After 12 h of constant-temperature shaking (25 °C, 200 rpm), the solutions were filtered for absorbance measurements. The MB adsorption amounts and removal rates of the biochars were calculated using Equations (1) and (2), respectively.

2.4.3. Adsorption Kinetics Experiment

Twenty milligrams of qBC₂₀ were weighed and added to 50 mL of a solution containing 50 mg/L MB. The solution was placed in a constant-temperature shaker at 200 rpm and 25 °C. The time was set to 1, 2, 4, 8, 12, 16, 20, or 24 h. After adsorption, the samples were filtered and the absorbances were measured. The MB removal rates and adsorption capacities of the biochars were calculated using Equations (1) and (2), respectively.

2.4.4. Adsorption Thermodynamics Experiments

Twenty milligrams of qBC200 were weighed and added to 50 mL of aqueous solutions with different concentrations of MB (30, 50, 70, 90, 110, 130, and 150 mg/L) at a pH of 7.00. The solutions were placed in a shaker with a speed of 200 rpm and different temperatures (25 °C, 35 °C, and 45 °C) for 24 h. After adsorption, the solutions were filtered for absorbance measurements. The MB removal rates and adsorption capacities of the biochar were calculated using Equations (1) and (2), respectively.

3. Results and Discussion

3.1. Characterization of Biochars

3.1.1. Elemental Compositions of the Biochars

The elemental compositions of the biochars are listed in

Table 1. Elemental compositions of the biochars.

Biochars	Elemental Composition (%)					O/C	H/C	(N + O)/C	S/C
	N (%)	C (%)	H (%)	O (%)	S (%)
qBC	2.90	38.25	4.61	36.20	0.17	0.95	0.12	1.02	0.0044
qBC200	3.10	43.08	4.44	31.87	0.18	0.74	0.10	0.811	0.0042
qBC600	2.14	40.50	2.08	25.02	0.31	0.62	0.051	0.671	0.0076
BC0	2.61	40.97	5.21	41.65	0.07	1.02	0.13	1.080	0.0017
BC200	2.10	42.08	4.74	39.46	0.09	0.94	0.11	0.987	0.0021

. The carbon and oxygen contents of the untreated bitter melon vines were 40.97% and 41.65%, respectively, reducing to 38.25% and 36.20%, respectively, after steam explosion. Under steam explosion, the high-temperature hydrolysis of acetyl groups in hemicellulose possibly releases some acidic small molecules that destroy the glycosidic bonds in hemicellulose, partially hydrolyzing them to monosaccharides and oligosaccharides with the loss of carbon and oxygen elements [28]. The H/C ratio reflects the aromaticity magnitude of biochar, with smaller ratios indicating stronger aromaticity. Meanwhile, the O/C and (N + O)/C ratios reflect the hydrophilicity and polarity magnitudes, respectively, with larger ratios indicating stronger hydrophilicity and polarity, respectively [29,30]. The H/C ratios of qBC₂₀₀ and BC₂₀₀ pyrolyzed at the same temperature were similar, indicating similar aromaticity values of the biochars obtained from the two treatment methods, and the O/C and (N + O)/C ratios of qBC₂₀₀ were slightly smaller than those of BC₂₀₀. The H/C, O/C, and (N + O)/C ratios of the biochars prepared from steam-explosion-treated biomass gradually decreased with increasing pyrolysis temperature, indicating that increasing the pyrolysis temperature enhances the aromatic structure of the biochar surface. Meanwhile, the decreased polarity and weakened hydrophilicity indicate the partial removal of polar functional groups.

3.1.2. Infrared Spectral Analysis

The infrared spectra of the biochars treated with different methods at different pyrolysis temperatures show obvious differences (Figure 1). The absorption peaks near 3400 cm⁻¹ were caused by the –OH stretching vibrations of alcohols and phenols. The peaks at 2928 and 1640 cm⁻¹ were caused by the stretching vibrations of methylene and C=C (or C=O) groups, respectively, and the peak at 1045 cm⁻¹ is attributed to C–O and C–C stretching vibrations or C–OH bending vibrations [17,30]. These absorption peaks decreased and eventually disappeared with increasing temperature, indicating a gradual reduction in the number of oxygen-containing functional groups and increasing degree of aromatization in the biochars, consistent with the results of elemental analysis. The characteristic absorption peak of hemicellulose near 1730 cm^–1 belongs to the C=O stretching vibrations of the carboxylic acid in hemicellulose. The absorption peaks near 1250 cm^–1 were attributed to the stretching vibrations of aliphatic C–O–C in cellulose [31]. The disappearance of these peaks after the steam explosion treatment indicates a reduction in the cellulose and hemicellulose content in the steam-explosion-treated bitter melon vine. The absorption peaks of lignin near 1320, 1640, and 1330 cm⁻¹ were generally reduced after the steam explosion treatment, suggesting that steam explosion contributes to lignin decomposition [32,33]. Meanwhile, the absorption peak near 1380 cm⁻¹ was more enhanced in qBC₂₀₀ than in the other materials, suggesting that steam explosion degrades and separates the O–H plane formed by hemicellulose and lignin in the bitter melon vine [33]. Comparing the infrared spectrum of qBC₂₀₀ with that of XqBC₂₀₀ after MB adsorption, one finds that MB adsorption enhances the characteristic absorption peaks of the benzene ring near 1300–1600 cm^–1, indicating the involvement of π–π interactions in the MB adsorption process [11]. In addition, the emerging FTIR peak at 1604 and 1465 cm⁻¹ indicated the stretching vibrations of aromatic C=C/C=N and C–N, which connect with MB through π–π interactions and hydrogen bonding [34,35]. These interactions are considered as the main pathways of MB adsorption on the material.

3.1.3. SEM Analysis

During the steam explosion process, the effect of mechanical splitting disrupts the dense structure of plants [33]. Hemicellulose decomposition and lignin separation increase the porosity between the fibers, the number of surface cracks, and the specific surface area of the fibers (Figure 2B). SEM images of the biochar derived from the pristine bitter melon vine and steam-explosion-treated bitter melon vine are compared in Figure 2. Sample qBC₂₀₀ after the steam explosion treatment presented a loose structure with more pits and pores and a more complex pore structure than the untreated sample (Figure 2A), verifying that steam explosion treatment can increase the porosity and specific surface area of the biochar. The surface of the BC₂₀₀ looks very smooth, verifying that low carbonization temperatures cannot destroy its dense structure; micropores and mesopits do not form (Figure 2C).

3.1.4. Crystal Structure Analysis of the Biochars

Figure 3 shows the XRD spectra of the different biochar samples. The broad diffraction peaks near 14–25° in the spectra of BC₂₀₀ and qBC₂₀₀ are the characteristic diffraction peaks of disordered carbon, indicating an amorphous carbon structure in BC₂₀₀ and qBC₂₀₀ [36]. In the spectrum of qBC₃₀₀, the amorphous carbon peaks shifted from those of qBC₂₀₀ and the characteristic peaks of calcium oxalate, calcium carbonate, and organic monomer crystallization (e.g., GA cellulose) appear, indicating the thermal dissociation of long-chain polymers into organic monomers and inorganic salts that interact with the amorphous carbon. Increasing the carbonization temperature accelerated the water loss and thermal decomposition in the sample, as evidenced by the sharpening of the characteristic peaks (i.e., enhancement of crystallinity). The (101) peak around 45° in the XRD spectra of the qBC₄₀₀, qBC₅₀₀, and qBC₆₀₀ samples corresponds to graphite-ordered carbon, suggesting that the interaction between the crystals and amorphous carbon also increases with temperature, increasing the orderliness of the biochar [37].

3.1.5. Specific Surface Area and Pore Size Analysis of the Biochars

The pore structure characteristics of the biochars are listed in

Table 2. Specific surface area and pore structure parameters of biochar.

Biochar	BET/(m2·g−1)	Total Pore Volume/cm3·g−1	Average Pore Size/nm
BC200	3.0699	0.0026	3.5029
qBC200	4.2130	0.0052	3.9629

. The qBC₂₀₀ sample exhibits a higher average pore size and total pore volume and a larger Brunauer–Emmett–Teller (BET) specific surface area than BC₂₀₀, indicating that steam explosion treatment can destroy the fiber surface and produce pores. However, at such a low carbonization temperature, the volatile substances in the biomass are not precipitated, so the total pore volumes and BET specific surface areas of both biochars are relatively small and the pore filling effect is weak.

3.1.6. Effect of Preparation Conditions on the Adsorption Performance of Biochar

Table 3. Yields, pH levels, and adsorption performances of the biochars.

Biochar	qBC	qBC200	qBC300	qBC400	qBC500	qBC600	BC	BC200	BC300	BC400	BC500	BC600
yield (%)	_	76.64	46.5	39.18	33.3	29.19	_	86.22	49.6	26.42	20.54	18.67
pH	6.73	6.70	7.05	7.58	8.28	8.88	6.57	6.65	6.85	6.87	7.28	7.29
q (mg/g)	85.02	253.20	116.00	76.21	13.04	5.30	85.02	81.23	96.06	59.09	51.68	42.84

summarizes the effects of steam explosion technology and pyrolysis temperature on biochar yield, pH, and adsorption properties. At low pyrolysis temperatures, steam explosion decreases the yield, enhances the MB adsorption, and slightly increases the pH from those of the pristine bitter melon vine biochars. At higher pyrolysis temperatures, steam explosion significantly increases the yield and pH and reduces the adsorption capacity from those of the pristine bitter melon vine biochars. These findings are probably explained by the effect of lignin content in the raw material on biochar yield [38,39]. After the steam explosion treatment, most or all of the hemicellulose in the bitter melon vine was hydrolyzed into water-soluble components or volatile small molecules, but lignin (a high-molecular-weight polymer) is less easily depolymerized and its degradation rate is low. Consequently, the hemicellulose content reduces while the relative contents of lignin and cellulose increase, thereby increasing the biochar yield. The pH of the biochar also increased with increasing pyrolysis temperature, indicating a reduction and increase in the numbers of acidic and basic functional groups, respectively, on the biochar surface. In general, raising the content of acidic surface functional groups on biochar enhances the MB adsorption capacity of the biochar. At an initial MB concentration of 150 mg/L and a biochar injection concentration of 0.4 g/L, qBC₂₀₀ adsorbed 253.2 mg/g of MB (Table 3), exceeding the MB adsorptions of the other materials (including qBC, qBC₂₀₀, and qBC₃₀₀). Meanwhile, qBC₆₀₀ adsorbed the lowest amount of MB (only 5.3 mg/g). Increasing the pyrolysis temperature enhances the degree of aromatization on the biochar surface, diminishing the number of polar functional groups, surface charge, and polarity. As MB adsorption is dominated by electrostatic and polar interactions, the adsorption capacity reduces accordingly.

3.1.7. Effect of Initial Solution pH

The pH of the solution affects the MB adsorption of biochar by influencing the surface charge characteristics of the biochar and the morphology of MB. The results of the pH analysis are shown in Figure 4. The MB adsorption amounts of the four biochars gradually increased with the increasing initial pH of the solution. The increase was vigorous at a pH below 4 and gentle at a pH above 4. This behavior might result from the large number of H⁺ ions in the solution at low pH, which compete with MB for the adsorption sites. When H⁺ attaches to the biochar surface, it introduces positive charges that repel the MB molecules, thereby inhibiting the adsorption process. In contrast, a high solution pH enhances the deprotonation process on the biochar surface, generating more negative-ion centers and enhancing the electrostatic adsorption of MB [40]. The adsorption capacity increases accordingly. Meanwhile, increasing the carbonization temperature decreases the adsorption capacity of biochar, as previously reported in the literature [41,42]. At adsorption equilibrium, the solution pH exceeds its initial value. As the initial pH of the solution increases, the pH of the equilibrated solution first increases to its maximum and thereafter follows a gentle trend with a small change range. The absorption amount exhibits a similar change trend with the increasing initial pH of the solution. It was suggested that the carboxyl and hydroxyl functional groups on the biochar surface buffer the pH of the solution. When the initial pH of the solution is low, the surface carboxyl and hydroxyl groups can bind the H⁺ ions in the solution, thus increasing the pH of the solution after equilibrium. When the initial pH of the solution is high and close to the pH of biochar, the surface carboxyl and hydroxyl groups release H⁺ ions into the solution, decreasing the pH of the equilibrated solution from that of the initial solution.

3.1.8. Effect of Initial Biochar Dosage

Figure 5 shows the effect of biochar dosage on MB adsorption to the biochar. Increasing the qBC₂₀₀ dosage gradually increases the MB removal rate by increasing the number of active sites for MB adsorption. When the qBC₂₀₀ dosage increased to 0.4 g/L, the MB removal rate approached 100%, but the adsorption capacity was reduced to 120 mg/g. Compared with the dosage of 0.3 g/L, the removal rate was increased to 4.15% and the adsorption capacity was reduced to 44.22%. Excessive biochar dosing reduces the amount of MB remaining in the solution and increases the number of vacant active sites on the biochar. As the adsorption capacity of the biochar is not fully utilized, the unit adsorption capacity reduces and a portion of the biochar is wasted. At a qBC₂₀₀ dosage of 0.3 g/L, the MB removal rate exceeded 90% and the adsorption effect and economy were improved, indicating that 0.3 mg/L is the optimal qBC₂₀₀ dosage.

3.1.9. Adsorption Kinetics

The absorption of qBC200 for MB was investigated through kinetic studies (Figure 6). As can be seen in Figure 6, qBC200 removed a significant amount of MB in the first 60 min; as the adsorption time increased, the initially fast adsorption rate of qBC₂₀₀ reduced and eventually stabilized at equilibrium. Equilibrium was reached at 4 h, when approximately 95.71% (119.64 mg/g) of the MB was removed and the adsorption amount did not obviously change with time. To evaluate the kinetic data of MB adsorption by qBC₂₀₀ under the experimental conditions examined, the adsorption amount versus time curves of qBC₂₀₀ were fitted to the pseudo-first-order and pseudo-second-order kinetic models. The obtained kinetic parameters are shown in

Table 4. Thermodynamic parameters of MB adsorption on qBC₂₀₀.

Adsorbents	Pseudo-First Order Model			Pseudo-Second Order Model
	qe (mg/g)	k1	R2	qe (mg/g)	k2	R2
MB	120.84	2.810	0.9990	122.14	0.112	0.9999

. Comparing the R² of pseudo-first-order and pseudo-second-order models, it was found that the pseudo-second-order model ideally demonstrates the sorption of MB to qBC₂₀₀ [35]. In addition, the pseudo-second-order kinetic model predicts an equilibrium MB adsorption amount of 122.25 mg/g, close to the actual amount adsorbed by BC₂₀₀ at 24 h, which validates the fitness of this model. Hence, the sorption of MB to the qBC₂₀₀ was based on the assumption of the pseudo-second-order kinetic model of chemisorption involving valency forces via electron sharing or transfer between MB and qBC₂₀₀ [42,43]

The pseudo-first-order and pseudo-second-order kinetic equations are, respectively, given by

$$q_{t }=q_e\left[1-\mathrm{e}\mathrm{x}\mathrm{p}(-k_{1 }t)\right],$$

(3)

$$q_t={\displaystyle \frac{k_{2 }{q}_{e }^{2 }t}{1+k_2{q}_{e}t}}$$

(4)

where q_e and q_t are the amounts of adsorbed biochar at the equilibrium time and time t, respectively, and k₁ and k₂ are the rate constants of the pseudo-first-order and pseudo-second-order kinetic models, respectively.

3.1.10. Adsorption Isotherms

To evaluate the adsorption mechanism, the adsorption isotherms were obtained at an initial MB concentration in the range of 30–150 mg/L and the adsorption temperature at 298, 308, and 318 K for the adsorption using the sorbent dosages of 0.4 g/L. The experiments were conducted with a contact time of 24 h, to be sure that equilibrium was reached in the adsorption system. The equilibrium adsorption capacity is reported as a function of the equilibrium concentration in solution after the adsorption on qBC₂₀₀, as shown in Figure 7. From Figure 7, it can be seen that the adsorption capacity increased rapidly, gradually leveled off, and then finally stabilized as the initial MB concentration increased. This trend is mainly attributed to the limited number of adsorption sites on the adsorbent, and an increasing number of MB molecules cannot occupy the saturated adsorption sites [35,43,44,45]. Figure 7 also demonstrates that as the temperature increased, there was a slight reduction in adsorption capacity. This might be the result of MB slight reduction in driving force or kinetic energy, as the adsorption temperature rose from 298 K to 318 K.

To evaluate the static adsorption date of MB on qBC₂₀₀ under the experimental examined, the Langmuir and Freundlich adsorption models were employed (Equations (4) and (5), respectively) and the fitting results are shown in

Table 5. Fitting parameters of the modeled adsorption isotherms.

T(K)	Langmuir			Freundlich
	qm (mg/g)	KL (L/mg)	R2	KF	1/n	R2
298	267.72	0.5841	0.9560	123.66	0.22	0.9526
308	265.23	0.4314	0.9821	109.79	0.24	0.9602
318	258.01	0.3901	0.9749	104.81	0.24	0.9568

. The closer the coefficient of regression (R²) to unity (1), the more the calculation model fits the data. It was noticed from R² in Table 5 that the Langmuir model (R² = 0.9560) slightly better described the adsorption process than the Freundlich model (R² = 0.9526) [41]. The theoretical maximum adsorption capacity from the Langmuir model was 267.72 mg/g, which was comparatively higher than the other biochars employed for the confiscation of MB (

Table 6. Assessment of the Qe for MB dye employing different biochars.

Biochar	Pyrolytic Temperature (°C)	Qe (mg/g)	References
Sawdust ozone biochar	500	200	[46]
Sewage sludge and grape dreg biochar	500	30.98	[47]
Rice straw biochar	400	26.87	[48]
Orange peels biochar	800	476	[40]
Banana biochar	800	390	[40]
Silica-composited biochars from rice straw	700	131.58	[46]
Silica-composited biochars from swine manure	700	143.76	[46]
Bitter melon vine biochar	200	267.72	This work

). The value of 1/n from the Freundlich model was smaller than 1, suggesting that qBC₂₀₀ was suitable for MB removal from water solution. High K_L and K_F values, which are related to adsorption capacity, also showed that qBC₂₀₀ could effortlessly adsorb MB. In addition, K_F and K_L decrease with increasing temperature, indicating that high temperature inhibits MB adsorption. From the Freundlich model fitting results, the 1/n values ranged from 0.22 to 0.24 depending on temperature, also indicating that high temperature was unfavorable for MB adsorption.

The Langmuir and Freundlich equations are, respectively, given as

$$\mathrm{Langmuir} \mathrm{equation} q_{e }={\displaystyle \frac{q_{m }{K}_L{C}_e}{1+K_L{C}_e}},$$

(5)

$$\mathrm{Freundlich} \mathrm{equation} q_e=K_F{C}_e^{{\displaystyle \frac{1}{n}}}$$

(6)

where C_e is the MB concentration at adsorption equilibrium (mg/L), and q_m is the maximum amount that can be adsorbed as a complete monomolecular layer on the surface (mg/g).

Comparing the adsorption isotherms at different temperatures, we can see that MB adsorption on qBC₂₀₀ worsens with increasing temperature. To investigate the effect of temperature on the adsorption process, the thermodynamic parameters [Gibbs free energy change (ΔG⁰, KJ/mol), standard enthalpy change (ΔH⁰, kJ/mol), and standard entropy change (ΔS⁰, J/mol-K)] were calculated using the Van ‘t Hoff formulas, which reveal the change in the internal energy of the particles during the adsorption process [45]. As shown in

Table 7. Thermodynamic adsorption parameters of MB on q BC₂₀₀.

T (K)	K0	lnK0	ΔG0 (KJ/mol)	ΔH0 (KJ/mol)	ΔS0 (J/mol·K)
298	1.87 × 105	12.14	−30.07	−15.84	47.51
308	1.38 × 105	11.84	−30.30
318	1.25 × 105	11.74	−31.03

, the Gibbs free energy changed by approximately −30 kJ/mol and the ΔH⁰ was −15.84 KJ/mol after adsorption, indicating that the adsorption process is a spontaneous physical adsorption process [35,45,46,47]. The adsorption process mainly involves hydrogen bonding, the hydrophobic effect, electrostatic forces, and van der Waals forces. The negative ΔH⁰ and positive ΔS⁰ of MB adsorption on qBC₂₀₀ indicate a major role of electrostatic adsorption, although π–π interactions and hydrogen bonding also partake in the adsorption process.

The Gibbs free energy change is calculated as

$$∆G^0=-RT\mathrm{l}\mathrm{n}{K}^0,$$

(7)

$$∆G^0=∆H^0-T∆S^0$$

(8)

where R is the ideal gas constant, K⁰ is the equilibrium constant, and T denotes the absolute temperature. The biochars prepared from bitter melon vine show particular advantages for MB removal from water. First, the MB adsorption capacity of bitter melon vine biochar exceeds those of most biochars reported in the literature. Second, the raw material of the bitter melon vine biochar is a by-product of steam explosion at low carbonization temperatures, which conserves energy and produces no environmental pollution. Therefore, qBC₂₀₀ is a promisingly effective adsorbent for printing and dyeing wastewater treatments.

4. Conclusions

Steam-exploded bitter melon vine biochars were successfully synthesized in this study. The batch experimental study demonstrated that steam explosion technology can effectively improve the adsorption performance of biochar. The theoretical maximum adsorption capacities of steam-exploded biochar were two times greater than non-steam-exploded biochar, which were obtained at room temperature, pH ~7. The adsorption of MB on qBC200 was well described by the pseudo-second-order kinetic model and the Langmuir adsorption isotherm model, indicating that the adsorption process is dominated by monomolecular layer adsorption. In the adsorption thermodynamics analysis, ΔG⁰ and ΔH⁰ were both less than zero while ΔS⁰ exceeded zero, indicating that MB adsorption is a spontaneous exothermic process with electrostatic adsorption playing a major role. As a cheap, easily accessible, biodegradable, and non-hazardous modification method, steam explosion offers high potential value and large-scale application. Therefore, it is expected to solve the problems of the high cost and easy secondary pollution of modified biochar.