Synthesis, Characterization and Performance Evaluation of Burmese Grape (Baccaurea ramiflora) Seed Biochar for Sustainable Wastewater Treatment

Highlights

What are the main findings?

• FTIR analysis showed multiple surface functional groups, e.g., R-OH, OH, -C=O, -COOH, etc., on the biochar surface.
• FESEM confirmed the randomized and porous tunnel-like structures of biochar.
• The BET specific surface area was 19.90 ± 1.20 m²/g.
• Kinetics and isotherm analysis showed alignment with pseudo-second-order kinetic and Langmuir Isotherm models.

What is the implication of the main finding?

• ~85% removal and 166.30 mg/g sorption capacity were obtained.
• The possible interaction of MB and BGS-derived biochar can be characterized by elec-trostatic attraction, H-bond and π–π conjugation.

Abstract

Biochar prepared from different bio-sources serves as a feasible solution for the decontamination of dye-contaminated wastewater. In this study, biochar was synthesized from a sustainable source, i.e., local fruit waste, Burmese grape seeds (BGSs). The seeds were collected from a local market, washed, pre-teated and finally converted into biochar by pyrolysis in a N₂ furnace. The removal efficiency of the synthesized biochar was evaluated towards a cationic industrial azo dye, methylene blue (MB). The phosphoric acid (H₃PO₄) and potassium hydroxide (KOH) pretreated BGS were pyrolized at 500 °C for 3 h in a N₂ furnace at a heating rate of 10 °C/min. The spectroscopic analysis confirmed the presence of multiple surface functional groups, e.g., R-OH, OH, -C=O, -COOH, etc. The surface of the biochar was randomized with porous tunnel-like structures. The specific surface area and pore volume obtained from BET analysis were 19.90 ± 1.20 m²/g and 5.85 cm³/g. The MB concentration (mg/L), contact duration (min) and pH were varied to assess the MB sorption phenomena. The optimum pH was found to be 8. During the first 20 min of contact time, adsorption was rapid and equilibrium was reached after 75 min. The adsorption was best described by pseudo-first-order kinetics with a good fit (R² = 0.99). The maximum removal percentage was ~85%, and per gram of BGS can adsorb 166.30 mg of MB, which supports the Langmuir adsorption isotherm model. The obtained results were compared with the reported literature, and BGS showed its excellent candidacy to be industrially utilized in the tertiary stage of wastewater treatment plants.

1. Introduction

Chemical industries now comprise a significant component of the global economy in the 21st century. Bangladesh has embraced industrialization that is focused on exports. Its main export industries include leather goods, fish, seafood, shipbuilding, jute and textiles, with the textile industry dominating in recent years [1]. However, these textile industries have caused numerous environmental hazards, with water pollution being one of the most severe problems [2]. The effluents from these industries contain numerous untreated toxic chemicals [3]. Organic dyes and highly hazardous carcinogenic contamination are among the major pollutants in the effluent of such industries. Approximately 100,000 commercially available dyes are used to manufacture approximately 7 × 10⁵ tonnes of reactive dyes per year, according to estimates [4]. The textile, leather, garments and food industries are considered to be the primary sources of dye wastewater. The most used dye for silk, cotton and wool is methylene blue (MB). Eye burns brought on by MB have the potential to permanently harm both human and animal eyes. It produces a burning sensation when consumed via the mouth and may cause methemoglobinemia, vomiting, nausea, increased sweating, gastritis and diarrhoea. It can also cause short periods of fast or difficult breathing when inhaled [5]. The treatment of wastewater carrying such colour is crucial because it has an aesthetic impact on receiving rivers.

Over the years, several methods have been developed by researchers, the most reported ones being sedimentation [6], membrane separation [7], biological treatment [8], chemical oxidation [9], coagulation–flocculation [10], photocatalysis [11,12,13,14], advanced oxidation [15,16] and adsorption [17,18,19,20,21,22,23,24]. Out of these techniques, the adsorption process is considered superior and is an extensively used technique for the advantages of high removal efficiency, low initial and maintenance cost, ease of operation, regeneration of adsorbent and simple design [25].

The choice of adsorbents mainly depends on the availability, affordability, adsorption capacity and kinetics, selectivity of particular pollutants and ease of regeneration [26]. Due to its low cost, straightforward construction, the convenience of use, insensitivity to hazardous pollutants and low concentrations of dangerous compounds, adsorption has been viewed as an appealing way to remove MB from aqueous systems [27]. Palm kernel fiber [28], fly ash [29], swelling clays [30], montmorillonite [31], pyrolyzed petrified sediment [32], mesoporous metal-oxide-doped silica nanocomposites [33,34,35], recycled nano-silica [36], silkworm exuviae [37], iron terephthalate [38], walnut shell [39], coal-based materials [40], carbon nanotubes (CNTs) [41], graphene materials [42] and metal–organic frameworks (MOFs) [43] have all been studied for the treatment of dye-contaminated wastewater. However, the reported sorbents are not always cost-friendly and reusable [44]. In recent years, biomass-based materials have been utilized in different applications [45]. Biochar, due to its remarkable effectiveness in eliminating a variety of contaminants, has drawn a lot of interest from researchers. Biochar has several applications in the field of wastewater treatment, energy production, CO₂ sequestration, soil remediation, supercapacitor generation, etc. [46,47,48,49,50,51]. Biochar has a porous and extremely amorphous structure. Its microscopic and macroscopic structure exhibits high levels of surface reactivity, mechanical stability and chemical stability [21]. Its surface activity, which depends on its pore properties and the existence of surfaces with functional groups, such as -C=O, -COOH, -OH and lactone groups, has a significant impact on its capacity to adsorb [52]. These functional groups account for the surface structure’s strong attraction to different dyes and other substances. As a result, it ranks among the best and most versatile adsorbents for water filtration [53]. Hydrogen bonding, electrostatic interaction, pi–pi interactions and hydrophilic–hydrophobic interaction are all probable mechanisms by which the reactive dye molecules engage with the biochar [54]. The primary source’s elemental composition also affects how well biochar performs as an adsorbent [52]. Utilizing readily accessible agricultural by-products as a raw material has recently been shown to be highly economically appealing. In recent years, several bio-waste materials, such as coconut shell [55], banana peel [56], pineapple stem [57], moringa oleifera seeds [22,23], rice straw [58], orange peel [59], wheat straw [60], etc., have been studied for this purpose. The source material and manufacturing technique significantly impact how the activated biochar’s pores develop and, therefore, how it ultimately behaves [61]. Hence, this study implemented the use of BGS, which is a bio-waste generated from the fruits of Burmese Grapes. It belongs to the family Phyllanthaceae, primarily grown in South Asian countries, e.g., Burma, India, Bangladesh, Vietnam and Malaysia. This abundantly available bio-waste can be highly economically beneficial in producing an efficient adsorbent for water purification. Thus, the choice of this bio-source as a raw material for biochar production can be justified by its availability in the local market, cheap or no cost and seed to fruit ratio.

The Burmese grape (Baccaurea ramiflora), a green tree with an overhead and thin skin that grows slowly and may reach heights of 25 m, is a member of the Phyllanthaceae family. This plant is widely cultivated in India and Malaysia and can be found across South Asia [62]. The fruit is gathered and used to make wine, edible fruit and medication to cure skin conditions. The wood, roots and bark are gathered because they offer therapeutic qualities. Traditional Chinese Dai medicine has traditionally used the whole plant [63]. Numerous scientific studies have demonstrated the presence of various bioactive substances in plants and their products, such as fruits, peels, leaves and seeds, which provide myriad health benefits and protection against degenerative illnesses [64]. However, until now, no study has been found to utilize its waste for sustainable wastewater treatment. The biodegradable seeds of this fruit can be utilized to be transformed into carbonaceous material, which can be used for organic contaminant removal from aqueous solution. The non-toxic material is not harmful to the aquatic environment and can be an eminent source of adsorbent for industrial application.

With this goal in mind, this work focuses on using biochar made from Burmese grape seeds to remove MB from an aqueous medium. As per our knowledge, this work is the first attempt to use and characterize BGS-derived biochar and assess its applicability for wastewater treatment.In order to determine the ideal operating conditions for dye removal, process parameters, including initial dye concentration, contact duration and pH, were changed. For an in-depth analysis on the sorption system and its mechanism, kinetic and isotherm studies were executed. Comparative analysis of MB removal by different biochar and the probable mechanism of MB removal onto BGS-derived biochar were examined and discussed.

2. Materials and Methods

Burmese grape seeds were collected from the local market. The other reagents used were: (i) phosphoric acid (H₃PO₄), (ii) potassium hydroxide (KOH) and (iii) methylene blue (MB). All of these reagents were purchased from Sigma Aldrich, and DI water was used to prepare the solutions.

2.1. Preparation of BGS Biochar

The collected BGSs were chopped into small parts. Then, the sample was air-dried for 2 days to remove the surface-captured moisture. BGS was then mixed with the solution of H₃PO₄ (40%) in a weight ratio of 1:2 followed by drying in an air oven at 80 °C for 3 h. The dried samples were then mixed with 50% (w/v) KOH solution to increase the functional groups in the structure. Then, it was dried at 100 °C for 2 h. Finally, the particles were carbonized under a nitrogen gas flow at a temperature of 500 °C for 3 h to be converted into biochar. The samples were further used for characterizations and adsorption experiments.

2.2. Characterizations

To check the chemical functionality of the prepared sample, it was examined using Fourier-transform infrared spectroscopy (FTIR). To create pellets, a small quantity of the produced material was combined with KBr powder. FTIR spectra were captured in transmission mode using an FTIR-8400 analyzer in the 4000–400 cm⁻¹ region by a spectrometer from Shimadzu, Japan. SEM pictures were taken using a field-emission scanning electron microscope (FESEM). An FESEM instrument procured from JEOL Ltd. in Tokyo, Japan, was used to assess the surface morphological characteristics and elemental composition analysis. Brunauer–Emmett–Teller (BET) surface area analysis was conducted in a PulseChemiSorb 2705 instrument.

2.3. Experimental Design

The process variables, e.g., dye solution concentration (mg/L), contact duration (min) and pH, were tuned to assess the MB removal characteristics of the BGS biochar. The biochar dose for each experiment was 5 mg. The influence of pH on adsorption was investigated by altering the solution pH between 3.0 and 9.0. To alter the pH of the solution, 0.1 M NaOH and HCl solution was utilized. Kinetic tests were performed using an MB dye concentration of 60 mg/L at ambient temperature (27 ± 2 °C) and examined at a time ranging from 5 to 90 min. With initial dye concentrations of C_o = 10–80 mg/L for 90 min, concentration dependency and isotherm tests were performed. According to the Equations, the dye’s removal (%) and adsorption capacity (q, mg/g) were determined. According to Equations (1) and (2) [65]:

$$\mathrm{Removal},\%=\frac{\left({\mathrm{C}}_{\mathrm{i}}-{\mathrm{C}}_{\mathrm{e}}\right)\times 100}{{\mathrm{C}}_{\mathrm{i}}}$$

(1)

$$\mathrm{Adsorption} \mathrm{capacity}, \mathrm{q}=\frac{\left({\mathrm{C}}_{\mathrm{i}}-{\mathrm{C}}_{\mathrm{e}}\right)\times \mathrm{V}}{\mathrm{m}}$$

(2)

Specifically, C_i and C_e represent the initial and equilibrium concentrations, whereas m is the adsorbent dosage and V is the solution volume.

The most widely accepted kinetic models for studying the adsorption processes are the pseudo-first-order and pseudo-second-order models. These models were utilized to comprehend the kinetic behaviour of the adsorption process of the BGS-derived biochar particles. Adsorption is predicated on chemisorption in the pseudo-second-order model, whereas the pseudo-first-order model implies that the adsorption is regulated by the molecular diffusion on the surface and pores of the sorbents.

The pseudo-first-order model considers diffusion to be the dominating factor for adsorption. The following Equation (3) is an illustration of the integrated pseudo-first-order equation:

$$\ln ({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}})=\ln {\mathrm{q}}_{\mathrm{e}}-{\mathrm{k}}_1\mathrm{t} \mathrm{i}$$

(3)

In this equation, the 1st-order rate constant, k₁ (min⁻¹), can be calculated by taking the sorption data, e.g., sorption capacity at equilibrium, q_e (mg/g), and at any time, t (min), q_t (mg/g).

The pseudo-second-order model considers chemisorption as the prime factor for adsorption. It can be described by Equation (4),

$$\frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{t}}}=\frac{1}{{\mathrm{k}}_2{\mathrm{q}}_{\mathrm{e}}{}^2}+\frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{e}}}$$

(4)

The calculated parameter, k₂, is the pseudo-second-order rate constant, q_e and q_t, are the same as defined previously.

To characterize the dye-removal process by BGS-generated biochar, isotherm analysis was carried out. In order to explain the adsorption behaviour, two key isotherm models, Langmuir and Freundlich, were employed to linearize the experimental data. Monolayer adsorption is the main consideration in the Langmuir isotherm model, whereas multilayer adsorption on a heterogenous adsorption site is the prime consideration of the Freundlich model. The following are the linear expressions for the Langmuir (5) and Freundlich (6) isotherms:

$$\frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{e}}}=\frac{1}{{\mathrm{b}}_{\mathrm{L}}{\mathrm{q}}_{\mathrm{m} }}+\frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{m} }}$$

(5)

$$\ln {\mathrm{q}}_{\mathrm{e}}=\frac{1}{\mathrm{n}}\ln {\mathrm{C}}_{\mathrm{e}}+\ln {\mathrm{k}}_{\mathrm{f}}$$

(6)

In Equations (5) and (6), equilibrium dye concentration, C_e (mg/L), and equilibrium sorption capacity, q_e (mg/g), were used. The Langmuir sorption constant, b_L and maximum sorption capacity, q_m (mg/g), in Equation (5), and Freundlich adsorption constant, k_f, and heterogeneity factor, n, in Equation (6) are the calculated parameters, respectively.

3. Results and Discussions

3.1. Biochar Characterizations

Pyrolysis is a practical approach to use wastes, e.g., municipal solid wastes (MSWs), and biowastes in an environment-friendly technique to prepare biochar. Herein, dried seeds were reacted with H₃PO₄ and KOH to activate and reveal their functional groups. The impurities (metals and organics) are leached in the H₃PO_4, and KOH helps the carbonaceous structure to disintegrate. Then, the pre-treated BGSs were pyrolyzed to become biochar via the following steps shown in Figure 1.

The diffusion of stack gases (CO, H₂, N₂) perforated the biochar structure producing highly macro- and mesoporous biochar (micropores are hard to identify). The FTIR spectra of the material were captured using a spectrophotometer and are shown in Figure 2A to verify the synthesis of biochar.

The broad and strong peaks within 3100–3600 cm⁻¹ correspond to the O–H stretching mode of hydroxyl groups present in organic compounds, such as alcohols, phenols and carboxylic acids [66]. The C–H stretching (symmetric and asymmetric) vibrations for different aliphatic groups may account for the peak at roughly 2900 cm⁻¹ [67]. The aromatic C=C and C=O stretching of ketones and quinones may provide peaks in a range of 1550 cm⁻¹ to 1680 cm⁻¹ [68]. The peak demonstrates the lactone structure O=C-O vibration at 1400 cm⁻¹ [69]. In contrast to the 700 cm⁻¹ (broad) cis C–H out-of-plane bend, bands between 970 and 960 cm⁻¹ correspond to trans C–H out-of-plane bend vibrations. The peak around 1220–1180 cm⁻¹ may be related to the stretching modes of the hydrogen-bonded P=O, the O–C stretching vibrations in the P–O–C linkage and the P=OOH that arises owing to the presence of residual H₃PO₄ in the structure during pyrolysis [70]. A peak at 1300 cm⁻¹ indicates the deformation of the aliphatic CH₃. Near the apex, which is between 1100 and 1240 cm⁻¹, fragrant CO stretches. Aliphatic ether and alcohols exhibit symmetric C–O stretching, which is shown by the peak at 1021 cm⁻¹. The aromatic C–H deformation modes are shown by peaks about 670 cm⁻¹ bands [67]. The functional groups present in the BGS biochar surface highly influenced the sorption process [71].

The FESEM image in Figure 2B demonstrates that the prepared biochar had a hierarchical porosity and a three-dimensional cage-like structure. The biochar exhibited a complicated and changeable structure. The image shows that the biochar surface was porous and rough, with various-sized cracks, fissures and holes. The alignment of these pores was irregular, and a limited number of holes with various diameters likely appeared in the generated biochar as a result of the volatilization and dehydration of moisture and organics [72]. In the pre-treatment process, utilization of KOH may initiate the following reactions with residual KOH [73].

4 KOH + C → K₂CO₃ + K₂O + 2 H₂

(R1)

Dehydration of KOH took place at temperatures lower than 500 °C after the reaction shown in Reaction 2.

2 KOH → K₂O + 2 H₂O

(R2)

The reforming processes were maintained by the water vapor produced ((R3) and (R4)),

H₂O + C → CO + 2 H₂

(R3)

CO + H2O → CO₂ + 2 H₂

(R4)

According to reaction theory, the potassium oxide and biochar dioxide subsequently interacted to generate potassium carbonate (R5).

K₂O + CO₂ → K₂CO₃

(R5)

These SEM images show that the surface of the biochar sample was rough, porous and ruptured because of the release of large volatile molecules during the calcination process [74]. The pores on the biochar surface resemble circles and ellipses most of the time. Among the types of pore size, they are more likely macropores. In Figure 2B, complicated coral-structure macropores are observed on the surface of the BGS-derived biochar. It may be possible that these macropores and associated tunnel structures are formed due to the hydrothermal conversion stage [75]. The sorption of MB by BGS-derived biochar may be highly influenced by these macropores and rough surface of the biochar [76] in the pore diffusion stage.

Table 1. Elemental composition and BET analysis of the BGS biochar.

Elemental Composition
Sample	C (%)	N (%)	O (%)	K (%)	P (%)
BGS-biochar	70.02	17.05	12.08	0.35	0.50
BET Analysis
Sample		Surface Area(m2/g)		Pore Volume(cm3/g)
BGS-biochar		19.90 ± 1.20		5.85

presents the elemental composition and BET analysis of the BGS-derived biochar.

The C, N, O, K and P content of BGS-derived biochar obtained from EDS analysis was found to be 70.02, 17.05, 12.08, 0.35 and 0.50%, respectively. The presence of K and P shows the effect of pre-activation. The O and N content may be associated with oxygenated and nitrous functional groups in the biochar surface. The specific surface area and pore volume obtained from BET analysis were 19.90 ± 1.20 m²/g and 5.85 cm³/g, respectively. However, the literature reported a much higher surface area for some synthesized biochar than the obtained surface area in our case. Therefore, the sorption of MB by BGS-derived biochar was possibly controlled combinedly by the effect of MB interaction with surface functional groups of biochar and pore diffusion of MB through biochar mesopores.

3.2. Effect of pH, Time and Concentration

The dye solution’s pH level significantly impacts the adsorption phenomena by changing the adsorbents’ surface-active sites and the ionization of the molecules. Figure 3A displays the removal rate of MB by BGS-derived AC at various pH values, ranging from 3 to 9. Because the negatively charged OH ion predominates on the adsorbent surface, previous studies have demonstrated that positively charged (cationic) dyes may adsorb more easily at higher pH values. It encourages the cationic dye molecules to engage electrostatically with the active/reactive regions of the adsorbent [77]. In this investigation, a similar effect was shown.

The maximum amount of dye was removed around pH 8.0, denoting that the electrostatic attraction between the cationic portion of the dye and the negatively charged sites (-OH, -COOH) on the surface of the functionalized biochar was the primary factor in the adsorption mechanism. The removal rate increased quickly in a pH range of 4–7. Positively charged dye molecules make more electrostatic interaction with negatively charged adsorbent surfaces with a rise in pH, leading to a corresponding increase in adsorption [78]. The competing adsorption between the proton and dye reduces the dye adsorption at comparatively lower pH levels [79]. The electrostatic interaction here between positively charged MB dye and the negatively charged sorbent surface rises with a pH increase, leading to a corresponding increase in adsorption [80]. The adsorption efficiency almost stayed the same when pH rose from 8.0 to 9.0.

Between 5 and 90 min, the adsorption of MB by AC generated from BGS was examined. According to Figure 3B, the adsorption capacity rose sharply in the first 30 min before gradually rising over time. After 75 min, a balance was attained. A reduction in adsorption rate at greater concentrations occurs because the adsorbates eventually occupy unoccupied active sites that were once free to occupy [81]. After 30 min, the sluggish adsorption rate might be described by a process in which the dye molecules first come into contact with the boundary layer before slowly diffusing over the top of the AC to penetrate into the pore structure [82]. With time, the molecular diffusion between the dye molecules and the AC surface slowed. The system finally reached saturation, suggesting that all of the active sites were engaged and that the system had reached an equilibrium state [83]. Once the equilibrium had been reached, the impact of contact time was not apparent.

Figure 4A displays the results of the adsorption of MB onto BGS-derived biochar at varied initial dye solution concentrations. The adsorption capacity of BGS-generated biochar rose from 42.75 mg/L to 150.75 mg/L when the starting dye concentration increased from 10 to 80 mg/L. This is a result of the driving factor between adsorbents (biochar) and dyes (MB) becoming stronger as the initial concentration rises [84]. Figure 4B represents the relationship between C_e and q_e. At a certain C_e, the adsorption capacity reaches an optimum value due to the saturation of the active sites.

3.3. Kinetics and Isotherms Analysis

Kinetics and isotherm studies are performed in order to fully comprehend the mechanism governing the adsorption of MB onto BGS-generated biochar. To predict the interaction between MB and biochar, the kinetic behaviour of the adsorption procedure was examined. Unlike the pseudo-first-order model, the pseudo-second-order model can forecast how the system will behave throughout the entire contact duration. The model’s advantage is that it can determine the preliminary adsorption rate and capacity instead of first identifying any unidentified parameters [52]. The linearized plots are shown in Figure 5. The estimated correlation coefficients (R²), R²_{Pseudo 1st} = 0.98 and R²_{Pseudo 2nd} = 0.99 indicate that the pseudo-second-order model more closely matches our experiment than the pseudo-first-order model. The agreement of our experimental results with a pseudo-second-order model showed that chemisorption, which is characterized by the exchange or sharing of electrons, dominates adsorption. The attractive force between the positively charged MB and the negatively charged surface of the BGS-derived biochar may be used to explain the chemisorption process. Probably, the functional groups, e.g., -OH and -COOH, of the BGS-derived biochar surface interacted with the dye molecules, leading to the adsorption process. Several reported bio-materials have been found to follow pseudo-second-order kinetics for the removal of MB [85]. However, the close value of R² for the two models in our case represented the dual mechanism of the adsorption process, which meant diffusion and chemical interaction both played import roles in dye capture on the adsorbent [86].

To characterize the MB adsorption by the produced BGS-derived biochar, adsorption isotherm studies were carried out. The experimental adsorption data were analyzed using two popular isotherm models, the Langmuir and Freundlich models, which are frequently employed to characterize adsorption behavior (Figure 6). According to Langmuir, when a dye molecule is at an active site on a surface, no more adsorption occurs there. The model has a dynamic equilibrium point when adsorption and desorption rates are identical. The physical importance of this occurrence is that it has reached saturation at all active spots on the surface [87]. The correlation coefficient value (R²) was found to be 0.99 for the Langmuir model and 0.95 for the Freundlich model. However, Using the Langmuir model, the maximum adsorption capacity (q_m) was determined, which was 166.3 mg/g. This calculated q_m is very close to the experimental saturation adsorption capacity (q_e), which was 150.75 mg/g. For the dye adsorption isotherm, it can be argued that the Langmuir model fits better, suggesting that the adsorption process is monolayer adsorption on the heterogeneous deformed graphitic structure of BGS-generated biochar. The surface charges and porous cracks may play a role in the monolayer adsorption of dye particles [88].

The MB sorption results of different biochar prepared from diverse sources are presented in

Table 2. Methylene blue (MB) sorption results of biochar derived from different sources.

Sources of Biochar	Conditions of Biochar Preparation	Experimental Conditions	Adsorption Capacity (mg/g)	References
Wheat straw	Pyrolyzed over 550 °C in N2 atmosphere for 3 h and activated with HCl	pH = 11, T = 25 °C, Dye = 80 mg/L, Contact time = 150 min.	61.6	[89]
Cattle manure biochar	Pyrolyzed over 200 °C in N2 atmosphere and activated with HCl	pH = 10, T = 25 °C, Dye = 200 mg/L, Contact time = 1.25 h	241.99	[90]
Palm oil fruit bunch	Microwaved at 800 W, reaction time of 30 min and a ratio of impregnation of 0.5 with FeCl3	pH = 10, T = 25 °C, Dye = 30 mg/L, Contact time = 120 min	31.25	[91]
Solid wastes (beer industry)	Prepared through N2 pyrolysis followed by CO2 activation at 800 °C for 60 min	pH = 6, T = 25 °C, Dye = 50 mg/L, Contact time = 2 h	161	[92]
Banana pseudo-stem	Pyrolyzed with a rate of 5 °C/min with phosphomolybdic acid	pH = 7, T = 25 °C, Dye = 50 mg/L, Contact time = 72 h	146.23	[93]
Wet-torrefied microalgae	Pyrolyzed in a N2 -purged inert condition using a modified household microwave at 800 W irradiation	pH = 8, T = 30 °C, Dye = 210 mg/L, Contact time = 7 h	113.00	[94]
Tea residue	Pyrolyzed in a muffle furnace at 700 °C with heating rate at 10 °C/min for 4 h and NaOH activation	pH = 7, T = 25 °C, Dye = 50 mg/L, Contact time = 60 min.	105.27	[95]
Moringa oleifera biochar	Pyrolyzed in a N2 furnace at 300 °C at 5 °C/min for 3 h	pH = 8, T = 25 °C, Dye = 60 mg/L, Contact time = 120 min	67	[22]
Lychee seeds	Pyrolyzed in a muffle furnace at 700 °C, KOH impregnation and microwave irradiation at 600 W of 10 min	pH = 6, T = 25 °C, Dye = 50 mg/L, Contact time = 120 min	124.5	[96]
Magnolia grandiflora leaf	Pyrolyzed in a muffle furnace at 500 °C with heating rate at 3 °C/min for 3 h	pH = 12, T = 25 °C, Dye = 50 mg/L, Contact time = 180 min.	101.27	[97]
Burmese grape seeds	Pyrolyzed at 500 °C, and preactivated with H3PO4 and KOH	pH = 8, T = 27 °C, Dye = 60 mg/L, Contact time = 70 min	166.3	This study

.

In Table 2, several sources of biochar synthesis, different preparation conditions, experimental conditions for MB removal and maximum sorption capacity are listed. Wide-ranging studies are reported in the literature; however, research on biochar potentiality for organic contaminant sequestration is still ongoing. The synthesized BGS-derived biochar showed excellent potentiality compared to different reported biochar for MB removal. Different straws, fruit shells, seeds, leaves, solid wastes, etc., have been applied to prepare biochar, as the choice of the parent source for biochar preparation depends on the availability, function ability, cost feasibilities, etc. Therefore, low-cost and highly abundant BGS is an excellent source for biochar synthesis and, in the future, its different applicability, e.g., efficiency in real waste water, photocatalytic activity, nitrogen/phosphorus co-doping, capacitive deionization potential, extraction of radioactive materials, etc., must be assessed [98,99].

4. Adsorption Mechanism

The prediction of the mechanism or interaction routes for the adsorbent–contaminant system is critical to develop an improved version of the catalyst (adsorbent) [100,101]. Figure 7 illustrates various adsorption routes, which include pore saturation via the mesopores of the biochar, H bonds, e-static interactions and π–π conjugation of the surface functional groups and aromatic moieties of the biochar with the MB molecules [24]. Interfacial layers, which may be split into the surface of the biochar and MB, from where the adsorptive motive generates, are the frequent points for adsorption. Relying on the type of forces involved, they may be categorized as either physical or chemical [102].

The micropore and mesopore filling process provides diffusion pathways for dye molecules to move into surface pores. The SEM image detects that the surface of BGS-derived biochar is rough and disordered with coral-shaped mesopores. The higher adsorption capability in BGS-derived biochar may be caused by the MB molecules diffusing through these pores [103]. The hydroxyl groups in the attachment of -COOH groups may produce the C=O groups of the biochar surface. As an alternative, the oligosaccharides or monosaccharides would undergo intramolecular dehydration, which would result in C=C bonds and aromatic character. These species are associated with the functional groups of surface-oxygenated functional groups [104]. The surface of the biochar exhibited several functional groups, including R-OH, -COOH and -OH, and structures, such as phenol, quinone and esters, according to the FTIR spectra. These functional groups and aromatic compounds directly influence the adsorption process, which also serve as the building blocks for different interactions [105]. In this work, a basic solution with a pH of 8 was shown to promote the greatest dye adsorption onto biochar made from BGS. The oxygenated biochar surface promotes the deprotonation of carboxylic, hydroxyl and ester groups at basic pH because -OH groups are present in the solution [106], which enhanced the electrostatic force of +ve MB molecules and −ve biochar surface [107]. The electronegative N atom (MB molecules) and polar H atoms in the carboxylic (-COOH), hydroxyl (-OH), epoxy (-O-), carbonyl C=O), etc., groups of the biochar surface may also contribute to the sorption process [108]. The reported literature on carbonaceous materials with post-activation of KOH also supports similar observations as ours for the sorption of organic components [109]. Similar interactions are commonly seen when MB dye is adsorbed by different carbon materials, such as low-rank coal [110] and bamboo chips [111], etc.

5. Conclusions

In this work, biochar was synthesized from a bio-source, Burmese grape seeds (BGSs), and characterized with FT-IR and FESEM. To determine the produced biochar’s’ adsorptive characteristics, adsorption tests were conducted. It was discovered that changes in process variables, such as contact duration, dye concentration and pH, significantly influenced adsorption. The sorption rate was very high for the first 20 min, and equilibrium was established with a clearance percentage of around 85%. The adsorption process is boosted at a pH value of 8 by improved interactions (+ve/−ve interactions, electronegative bonding and aromatic conjugations) between the MB molecules and biochar surface. However, after the blocking and saturation of the sorption sites of the biochar surface, sorption achieves a maximum value (q_m). The sorption kinetics followed the pseudo-second-order kinetic nature, and the Langmuir isotherm model described the sorption phenomenon well. The highest sorption of 166.30 mg/g was obtained from the Langmuir model, and from experimental data, it was 150.75 mg/g. Therefore, the synthesis of biochar from BGS has advantages of simultaneous utilization of biowaste and wastewater treatment, which can be realized on an industrial scale in the future.