Adsorption of Methylene Blue Using As-Developed Binderless Hot-Pressed Granular Activated Carbon Derived from Sugarcane Bagasse Residues

Abstract

This study aims to develop sugarcane bagasse-derived granular activated carbon (SCB-GAC) as a biobased adsorbent for methylene blue (MB) adsorption in an aqueous solution using a binderless hot-pressed (HP) technique. HP conditions, i.e., mechanical pressure, temperature, and holding time, were systematically fine-tuned to first produce HP-SCB pellets. HP pellets were further carbonized at 500 °C for 1 h to obtain biochar pellets, which were cut into granules prior to activation with steam under the optimum conditions of 850 °C for 30 min. The mechanical pressure showed the most significant influence on the yield and textural properties of the as-developed HP-SCB-GAC. The optimal HP condition of 20 MPa at 270 °C for 30 min resulted in a substantial yield (53.7%) of the developed HP-SCB-GAC with uniform surface texture possessing the highest strength and specific surface area (804.6 m²/g). The MB adsorption using the selected HP-SCB-GAC showed minimal sensitivity to pH adjustments. The maximum MB adsorption capacity at approximately 138 mg/g could be achieved and fitted with the pseudo-second-order kinetic model and Langmuir equilibrium isotherm behaviors. The selected HP-SCB-GAC could reach as high as 87–92% of MB adsorption efficiency for at least five consecutive cycles using ethanol as the regenerant.

1. Introduction

Activated carbon-based adsorption is an effective technique for dye removal from wastewater, offering advantages such as high specific surface area, low cost, and convenience. However, conventional activated carbon production relies on nonrenewable resources (i.e., coal) or forestry biomass (i.e., wood), making it costly and limiting its widespread application. To overcome these limitations, researchers are exploring low-cost alternatives derived from renewable agricultural biomass, particularly sugarcane bagasse (SCB). SCB is an abundant agro-industrial residue composed mainly of hemicelluloses (24.8–32.0%), cellulose (24.8–44.0%), and lignin (12.0–24.0%). It has gained attention as a renewable precursor for production of value-added activated carbon. − Granular activated carbon (GAC) is widely employed for contaminant removal from wastewater, particularly for dye adsorption, due to its high surface area and porosity. GAC, along with powdered activated carbon (PAC), represents the most utilized form of activated carbon. PAC typically exhibits a higher adsorption capacity and faster kinetics owing to its smaller particle size and larger external surface area. However, practical challenges such as difficult separation and dust pollution limit its large-scale applicability. In contrast, GAC offers advantages in terms of easier handling, lower pressure drop, and reusability but often shows lower adsorption kinetics and a reduced accessible surface area compared to PAC.

Recent studies have focused on improving the GAC performance by utilizing biomass-derived precursors. Comparative evaluations demonstrate that raw biomass (e.g., coconut shells, sawdust, or agricultural residues) has limited adsorption capability due to its low surface area. After carbonization, biochar derived from these materials shows improved performance but still falls short of commercial GAC. For instance, raw sugarcane bagasse (SCB) generally exhibits no porous structure (∼0.1–2 m²/g), while the carbonized biochar can reach ten to a few hundred m²/g, typically ∼100–300 ± 50 m²/g when pyrolyzed at 500 °C. , In contrast, commercial GACs such as Filtrasorb 400 (bituminous coal) and Norit (peat) typically exhibit a surface area exceeding 800 m²/g , and significantly enhanced adsorption capacity for dyes such as methylene blue. However, unlike high-strength biomass materials such as coconut shells, fibrous agricultural residues such as sugarcane bagasse (SCB) do not naturally form granules through simple carbonization and activation. The carbonized SCB pellets were also relatively brittle and easily broken into fibrous powder. Therefore, granulation techniques, with or without binders, are necessary to convert SCB-based PAC into GAC. To overcome the limitations of conventional methods, innovative synthesis strategies such as template-assisted and binderless pelletization under heat are being explored to enhance the porosity and functionality of GAC without compromising mechanical strength or reusability. These developments aim to bridge the gap between high surface area and operational robustness, making GAC from various types of agricultural residues a more efficient and sustainable adsorbent for dye-containing wastewater treatment.

The use of organic or inorganic binders in biomass pelletization enhances density and mechanical strength but reduces fuel properties , such as heating value by reducing carbon content. Additionally, it decreases the specific surface area and increases total manufacturing cost by approximately 12%. To avoid these issues, a binderless granulation method using the hot-pressed (HP) process has been introduced and initially developed from coal pitch-based activated carbon fiber. The HP technique was later applied to biomass-based GAC production; this process involves applying mechanical pressures of 0–20 MPa at 400 °C, followed by carbonization at 950 °C and activation at 900 °C. The results showed that increasing mechanical pressure improves bulk density from 0.30 to 1.00 g/cm³. With a fixed pressure at 10 MPa under various temperatures (25–300 °C), Miura et al. achieved a specific surface area exceeding 1000 m²/g, improving the physical properties of GACs. Further advancements in HP technology led to its solid fuel application in biomass black pellet production, with torrefaction performed at 250 °C and 70 MPa with subsequent torrefaction at 260–300 °C for 0–30 min. Higher torrefaction temperatures and extended holding times at 300 °C for 30 min resulted in increased carbon content. Additional research on mechanical pressure (10–70 MPa at 250 °C for 10 min) demonstrated enhanced carbon content and high yield (91–99 wt %) at 900 °C. Xiao et al. further explored biomass pelletization to improve fuel properties, while Wang et al. applied the HP method to rice husk plates, observing increased strength at 230 °C but brittleness beyond 245 °C due to pyrolysis. Increasing the HP pressure from 50 to 250 MPa significantly improved plate strength, confirming that both temperature and mechanical pressure clearly influenced the strength of HP biomass products.

The challenge of producing high-strength GAC from fibrous biomass residues for wastewater dye removal is the focus of this study. To overcome this issue, GAC was developed from SCB through a binderless HP approach, followed by carbonization and steam activation. Steam, an accessible byproduct from steam boilers in sugar mills, where SCB is used as fuel, was chosen as the gasifying agent. The activation conditions were optimized in a thermogravimetric analyzer (TGA) to achieve high specific surface area with a burnoff yield over 50 wt %. The study further explored how mechanical pressure, temperature, and holding time in the HP process influence GAC properties. Last, the selected HP-SCB-GAC, defined as sugarcane bagasse-derived granular activated carbon prepared by the hot-pressed and steam-activation technique, was subjected to batch adsorption studies for methylene blue (MB) removal, including evaluations of the pH effect, adsorption kinetics, equilibrium isotherms, and regeneration and recycling of the used adsorbent for potential application in dye removal in wastewater.

2. Materials and Methods

The experimental procedure for preparation of HP-SCB-GAC from raw SCB can be divided into three main steps as illustrated in Figure : (1) binderless granulation via the hot-pressed technique, (2) carbonization, and (3) steam activation in a thermogravimetric analyzer (TGA). The HP-SCB-GAC was physically analyzed for yield, pellet density, surface properties, and compressive strength. The batch adsorption test for methylene blue (MB) removal including adsorption kinetics, equilibrium isotherms, and cyclic regeneration was subsequently performed.

1.

Schematic diagram of experimental processes for the production of HP-SCB-GAC.

2.1. Raw Materials and Chemicals

Raw SCB provided by Kaset Phol Sugar Mill, Udon Thani, Thailand, was ground and sieved into a size range of 106–250 μm and then dried in a hot-air oven overnight prior to use in all experiments. Methylene blue (MB) powder (KemAus, methylene blue 95%, color index (CI) number: 52015) was used for adsorption tests of HP-SCB-GAC. Hydrochloric acid (HCl 37%), sodium hydroxide (NaOH) pellets, and 90% ethanol solution were used for the study of the pH effect and regeneration.

2.2. Preparation of HP-SCB-GAC

2.2.1. Binderless Granulation via the Hot-Pressed Technique

One gram of SCB-powder was placed in SUS316 stainless steel molds (13 mm in diameter), and the sample was covered with force and base and then put in the SUS316 reactor tube as shown in the previous work. The sample was pressed in the reactor tube under a hydraulic compression load of 10–40 MPa continuously for 15 min at room temperature. The furnace was heated from room temperature to reach an elevated temperature with a heating rate of 10 °C/min, and then held for 0–45 min. The targeted temperatures of 230–290 °C were selected because a preliminary study using TGA revealed (see the Supporting Information) that raw SCB samples started to decompose at 230 °C with a yield of 92 wt %. The yield was still kept at above 80 wt % and then rapidly decreased after 290 °C as shown in Figure S1 (Supporting Information). The furnace was turned off to cool down rapidly under the mechanical load, which was slightly decreased to about 10 MPa lower than each set pressure. The weight and pellet density were recorded and calculated. The hot-pressed products obtained in this study were called hot-pressed (HP) SCB pellets (HP-SCB pellets). The HP samples were abbreviated as “HP mechanical pressure–temperature–holding time”. For example, HP20–270–30 represents the HP pellet prepared at 20 MPa at 270 °C for 30 min.

2.2.2. Carbonization

After the hot-pressed step, HP-SCB pellets were dried by using a hot-air oven at 110 °C overnight before carbonization. The HP-SCB pellets were placed at the center of a quartz tube reactor with an outer diameter of 27 mm under a nitrogen (N₂) atmosphere with a flow rate of 100 mL/min to keep an inert atmosphere in the system until the experiment was complete. The furnace was heated at a heating rate of 10 °C/min from room temperature to 500 °C and held for 60 min to obtain the biochar pellets called HP-SCB-biochar pellets. The furnace was then immediately turned off, and weight and pellet density were also recorded and calculated. The HP-SCB-biochar pellets obtained from this step were abbreviated as “C carbonized temperature–HP mechanical pressure–temperature–holding time”, with C500–HP20–270–30 representing the HP-SCB-biochar pellet carbonized at 500 °C using the HP-SCB pellet prepared under 20 MPa at 270 °C for 30 min.

2.2.3. Steam Activation

The HP-SCB-biochar pellets were neatly cut using a small cutting tool and sieved in a size range of 2–3 mm and called HP-SCB-biochar granules as shown in Figure prior to activation by steam in TGA. 200 mg of SCB-biochar granules was placed on a platinum basket (12 mm in diameter and 12 mm in height) inside the TGA, connecting with the steam generator through a high-performance liquid chromatography (HPLC) pump (Shimadzu, LC-10AT) to deliver steam into the TGA. This steam generator-TGA system was modified and used previously in an early study. The TGA was heated up under 100 mL/min of N₂ gas from room temperature to reach elevated temperatures (800–900 °C) with 10 °C/min to be ready for steam activation. Water vapor was injected into the TGA and further held for 10–40 min at a fixed activation temperature. During the steam activation step, N₂ was changed to a flow rate of 50 mL/min with an additional 50 mL/min of water vapor fed from the steam generator containing water boiled at 250 °C. After completing the steam activation, TGA was immediately cooled down under 100 mL/min of N₂ flow to collect the final products called SCB-granular activated carbon (HP-SCB-GAC). The HP-SCB-GAC was also abbreviated for each steam activation condition to “S steam activation temperature–activation holding time”, with S850–30 corresponding to the HP-SCB-GAC prepared under steam activation at 850 °C for 30 min.

2.3. Physical Characterization of the As-Prepared HP-SCB Pellet, HP-SCB-Biochar Pellet, and HP-SCB-GAC

2.3.1. Yield and Apparent Density of the Pellet

Yield percentage (%Yield) of each product was quantitatively calculated using eq , where Weight_final is the final weight of the HP-SCB pellet, HP-SCB-biochar pellet, or HP-SCB-GAC, measured after each step of the GAC preparation process of hot-pressing, carbonization, or steam activation, respectively. Each condition of all experiments was repeated in triplicate to confirm their reproducibility by reporting the average yield with the standard deviation (S.D.) less than 2%. The basis of represented yield can be considered as raw basis (Weight_initial = initial weight of dried raw SCB) or sample basis (Weight_initial = initial weight of the sample in each preparation step).

$$\%\mathrm{Yield}=\frac{{\mathrm{Weight}}_{\mathrm{final}}}{{\mathrm{Weight}}_{\mathrm{initial}}}\times 100$$ 1

For apparent density of the pellet, the diameter and height of the SCB-HP pellet and SCB-HP biochar pellet were measured to determine the volume of the pellet. Then, the pellet density was calculated using eq , where D (g/cm³) is the apparent density of the pellet, m (g) is the mass of the pellet, π is a constant, r (cm) is the radius of the pellet (half of the measured diameter), and h (cm) is the height of the pellet.

$$D=\frac{m}{\pi r^{2}h}$$ 2

2.3.2. Compressive Strength

A compression test was conducted on the cylindrical shaped samples using a texture analyzer (universal testing machine, TM Tech, Thailand) with a 6–8 mm diameter probe. For compressive strength, at least three samples of the HP-SCB pellet and HP-SCB-biochar pellet were tested at a constant compressive rate of 1 mm/min until the material was destroyed due to deformation, and then, compression was stopped. This test was determined by Young’s modulus using eq , where F is the compression force (N), R is initial radius of the granule (m), E is Young’s modulus (kPa), υ is the Poisson ratio (=0.5), and H is the displacement (m).

$$F=\frac{4R^{1/4}}{3}\frac{E}{1-{\upsilon }^2}{\left(\frac{H}{2}\right)}^{3/2}$$ 3

2.3.3. Surface Properties

The HP-SCB-GAC cut into a small size range of 2–3 mm in diameter as a granule form was used to analyze surface properties through the Brunauer–Emmett–Teller (BET) method with the volumetric adsorption principle under inert gas by an automated adsorption apparatus (Bell Japan Inc., BELSORP, BELSORP-mini II) to compare the specific surface areas (S _BET), pore volumes, and pore sizes of the HP-SCB-GAC products. S _BET is calculated from the linear region of the BET plot (typically in the relative pressure range P/P ₀ = 0.05–0.3) and represents the total surface area available for gas adsorption per unit mass of the material, expressed in m²/g. ,

2.4. Batch Adsorption of Methylene Blue (MB) Using the Selected HP-SCB-GAC

2.4.1. Analysis of the pH Effect, Adsorption Kinetics, and Adsorption Equilibrium Isotherms

HP-SCB-GACs were used to study the effect of pH by mixing 0.05 g of HP-SCB-GAC in 50 mL of methylene blue (MB) aqueous solution with a concentration of 50 mg/L. The MB solution was adjusted to acidic (pH 2) and basic (pH 12) conditions using hydrochloric acid (HCl 37%) and sodium hydroxide (NaOH) pellets by comparison with the controlled MB solution at pH 7. The sample was then shaken at a speed of 200 rpm at room temperature. The absorbance peak height at a wavelength of 664 nm, indicating the presence of MB in the solution, was measured and recorded after 24 h using a UV–vis spectrophotometer (SP830 Plus, Metertech, Taiwan). For the study of adsorption kinetics using the controlled MB solution at pH 7, the absorbance was measured and recorded at specific intervals during a 1–24 h period.

The % adsorption efficiency of MB is given by eq , where C ₀ (mg/L) is the initial concentration of MB before the adsorption experiment and C _t (mg/L) is the final concentration of MB after the adsorption experiment. The adsorption capacity (q _e) can be expressed by eq , where V (mL) is the volume of the MB solution and W (g) is the weight of HP-SCB-GAC used as an adsorbent.

$$\%\mathrm{adsorption}\mathrm{efficiency}=\frac{C_0-C_t}{C_0}\times 100\%$$ 4

$$q_{\mathrm{e}}=\frac{(C_0-C_t)\times V}{W}\times 100\%$$ 5

The resulting data of MB batch adsorption at each specific period follow eqs and for pseudo-first-order and -second-order kinetic models, respectively, when q _e (mg/g) and q _t (mg/g) are the MB adsorption capacity of each HP-SCB-GAC at equilibrium and at a specific time. The adsorption rate constants for the pseudo-first-order (PFO) kinetic model and pseudo-second-order (PSO) kinetic model are represented by k ₁ (h^–1) and k ₂ (g·mg^–1 h^–1), as shown in eqs and , respectively.

$$\ln (q_{\mathrm{e}}-q_{\mathrm{t}})=\ln q_{\mathrm{e}}-k_{1}t$$ 6

$$t/q_{\mathrm{t}}=1/(k_2{q_{\mathrm{e}}}^2)+t/q_{\mathrm{e}}$$ 7

The adsorption behavior between HP-SCB-GAC and MB in this study could be simply discussed by comparing equilibrium adsorption isotherm models. 0.05 g of HP-SCB-GAC sample was mixed with 50 mL of MB aqueous solution, varying the concentration from 25 to 500 mg/L. The sample flasks were shaken at 200 rpm for 24 h before analyzing the adsorption efficiency of MB using a spectrophotometer measured at a wavelength of 664 nm as explained above. In order to discuss the adsorption mechanism, the experimental adsorption data in this study was obtained by following the (i) Langmuir, (ii) Freundlich, (iii) Temkin, and (iv) Sips adsorption isotherm models, which were explained in detail in our previous study.

2.4.2. Regeneration and Recycling

For regeneration and recycling tests, ethanol was selected as a proper desorption regenerant with a consideration of low cost, low toxicity, and high desorption efficiency compared to other commonly used basic, acidic, salt-based, and polar solvents such as sodium hydroxide (NaOH), hydrochloric acid (HCl), sodium chloride (NaCl), and methanol or acetone, also discussed in previous studies. − First, 0.05 g of the selected HP-SCB-GAC was added in 50 mL of 50 mg/L MB aqueous solution at room temperature for 24 h. After the measurement of MB using the spectrophotometer as explained previously, the used HP-SCB-GAC was washed using 50 mL of 90% ethanol for 4 h in a shaker with 200 rpm for MB desorption, followed by washing with deionized (DI) water until pH reached 5–6 as an initial value of DI water. After that, the regenerated sample was dried at 105 °C overnight before being utilized in the next adsorption test. The adsorption–desorption experiments in triplicate were repeated for five consecutive cycles to ensure adsorption efficiency and capacity of the selected HP-SCB-GAC.

3. Results and Discussion

3.1. Effect of Hot-Pressed Conditions on the Physical Properties of the HP-SCB Pellet, HP-SCB-Biochar, and HP-SCB-GAC

Raw SCB used in this study was first analyzed using a thermogravimetric analyzer (TGA) (Shimadzu, TGA-50) and CHNO analyzer (J-Science, CHN JM10 micro corder, Japan) for its proximate and ultimate analyses. It showed that the contents of moisture content (MC), volatile matter (VM), fixed carbon (FC), and ash were 7.9, 77.1, 11.4, and 3.6 wt %, respectively, with the weight percentage of each element as 43.8% carbon (C), 6.8% hydrogen (H), and 47.4% oxygen (O). The hot-pressed SCB (HP-SCB) pellet was prepared from dried and sieved raw SCB under various mechanical pressures, temperatures, and holding times. The HP-SCB pellet was subsequently carbonized at 500 °C for 60 min, and the obtained HP-SCB-biochar pellet was then steam-activated, producing HP-SCB-GAC. The HP-SCB pellet, the HP-SCB-biochar pellet, and the HP-SCB-GAC produced were analyzed for their physical appearances, yields, pellet densities, and textural properties.

3.1.1. HP-SCB Pellet: Physical Properties and Yields

The mechanical compression pressure was varied at 10, 20, 30, and 40 MPa during the hot-pressed process at 270 °C for 30 min. As illustrated in Figure a, as the mechanical pressure was increased, the obtained HP-SCB pellet with darker and slightly smaller pellet morphology was apparently observed, particularly in the pellets prepared under 30 and 40 MPa. Their surfaces were rougher and partially cracked as compared to the ones prepared under lower mechanical pressures at 10–20 MPa.

2.

Physical appearances of HP-SCB pellets prepared under different HP conditions: (a) mechanical pressures (10–40 MPa) at 270 °C for 30 min, (b) temperatures (230–290 °C) at 20 MPa for 30 min, and (c) holding times (0–45 min) at 20 MPa and 270 °C.

In terms of the yield of the HP-SCB pellet as exhibited in Table , it tended to decrease as the mechanical pressure increased. The highest SCB-HP pellet yield of 89.4% was obtained at 10 and 20 MPa. The low mechanical pressure during the HP process suppressed the volatile release, resulting in high HP-SCB pellet yield. They were substantially decreased to 83.6 and 78.1% at 30 and 40 MPa, respectively. The higher mechanical pressure during the HP process accelerated the dehydration and deoxygenation reactions. It has been found that with increases in volumetric pressures (0.1–4.0 MPa) during torrefaction of Leucaena wood, volatile matters were significantly decreased, leading to a decrease in mass yield. It has also been reported that the decomposition rate of the biomass structure is highly dependent on the thermogravimetric reactor pressures (0.1–2.1 MPa) applied during the torrefaction process of wood, which resulted in decreased mass yields of the torrefied products.

1. Yields and Pellet Densities of HP-SCB and Biochar Pellets Derived from Different HP Conditions.

	HP-SCB pellet		HP-SCB biochar pellet
samples	yield (wt %)	pellet density (g/cm3)	yield (wt %)		pellet density (g/cm3)
	based on raw SCB	based on the HP-SCB pellet	based on the HP-SCB pellet	based on raw SCB	based on the HP-SCB biochar pellet
varying the HP mechanical pressure (10–40 MPa)
HP10–270–30	89.4 ± 0.1	1.1 ± 0.0	25.4 ± 0.2	22.7 ± 0.1	0.7 ± 0.0
HP20–270–30	89.4 ± 0.2	1.2 ± 0.0	26.2 ± 0.1	23.4 ± 0.1	0.8 ± 0.0
HP30–270–30	83.6 ± 2.4	1.4 ± 0.0	29.1 ± 0.2	23.9 ± 0.2	0.8 ± 0.0
HP40–270–30	78.1 ± 8.4	1.3 ± 0.1	28.1 ± 0.5	22.5 ± 2.0	0.8 ± 0.1
varying the HP temperature (230–290 °C)
HP20–230–30	90.3 ± 0.6	1.2 ± 0.0	25.8 ± 0.2	23.3 ± 0.3	0.7 ± 0.0
HP20–250–30	89.9 ± 0.2	1.2 ± 0.1	26.0 ± 0.2	23.4 ± 0.2	0.8 ± 0.1
HP20–270–30	89.4 ± 0.2	1.2 ± 0.0	26.2 ± 0.1	23.4 ± 0.1	0.8 ± 0.0
HP20–290–30	88.8 ± 0.6	1.2 ± 0.0	26.7 ± 0.6	23.7 ± 0.4	0.8 ± 0.0
varying the HP holding time (0–45 min)
HP20–270–00	96.7 ± 0.4	1.2 ± 0.0	25.7 ± 0.0	24.9 ± 0.1	0.7 ± 0.0
HP20–270–15	90.5 ± 0.0	1.2 ± 0.0	25.7 ± 0.3	23.2 ± 0.2	0.8 ± 0.0
HP20–270–30	89.4 ± 0.2	1.2 ± 0.0	26.2 ± 0.1	23.4 ± 0.1	0.8 ± 0.0
HP20–270–45	88.7 ± 0.3	1.3 ± 0.1	26.6 ± 0.1	23.6 ± 0.0	0.9 ± 0.0

The density of the HP-SCB pellet was slightly increased with the increased mechanical pressures, from 1.1 g/cm³ at 10 MPa to 1.2–1.4 g/cm³ at 20–40 MPa. The fibrous biomass particles could interlock and fold under pressing and then enhance the strength by suppressing the development of cracks. The hydroxyl-rich functional groups in SCB’s structure would act as a binder. Interestingly, the HP20–270–30 pellet prepared under 20 MPa gave the highest compressive strength among all samples, as shown in Table . The pellet using 20 MPa of mechanical pressure during the HP process could produce the nondeformed HP-SCB pellet with regular color, shape, and uniform surface texture including the relatively highest yield and compressive strength. Consequently, mechanical pressure at 20 MPa during the HP process was used as a fixed parameter for further investigation by varying the HP temperature and holding time.

2. Maximum Compressive Force and Young’s Modulus of HP-SCB Pellets and HP-SCB Biochar Pellets Prepared under Various HP Mechanical Pressures.

	HP-SCB pellet		HP-SCB biochar pellet
samples	maximum compressive force (kN)	Young’s modulus (MPa)	maximum compressive force (kN)	Young’s modulus (MPa)
HP10–270–30	2.30 ± 0.05	50.7 ± 0.1	0.31 ± 0.01	5.03 ± 0.09
HP20–270–30	3.58 ± 0.11	79.9 ± 0.3	0.58 ± 0.02	9.41 ± 0.12
HP30–270–30	2.96 ± 0.07	66.1 ± 0.2	0.32 ± 0.01	5.02 ± 0.06
HP40–270–30	2.87 ± 0.08	64.3 ± 0.1	0.23 ± 0.01	3.53 ± 0.04

The effects of HP temperatures (230–290 °C for 30 min) and holding times (0–45 min at 270 °C) on the physical properties of SCB-HP pellets were later investigated by using 20 MPa as a constant HP mechanical pressure. The change in color was clearly observed in the most severe conditions of the HP20–290–30 prepared at 290 °C and the HP20–270–45 prepared with a holding time of 45 min, as shown in Figure b,c. Nevertheless, there was no severe change in pellet morphology observed among the HP temperatures and holding times tested. When determining the yields of the HP-SCB pellets, it was found that, as the HP temperatures increased, the yields only slightly decreased from 90.3% at 230 °C to 88.8–89.9% at 250–290 °C. With increasing holding times, the yield also showed the same trend of a small decrease from 96.7% at 270 °C HP without holding time to 88.7–90.5% with a prolonged holding time of 15–45 min. It has been found that in Leucaena torrefaction, increases in torrefaction temperatures (200–275 °C) and holding times (0–15 h) reduced volatile matters, oxygen contents, and mass yields but increased the fixed carbon contents of the torrefied products. , The decrease in mass yield was suggested to be as a result of increased moisture evaporation and lignocellulose degradation caused by the increased torrefaction temperatures. Reduction in mass yield and enhancement of carbon content have been also observed when increasing torrefaction temperatures (220–300 °C) and with prolonged holding time (10 min to 2 h) during torrefaction of other biomasses. The densities of the HP-SCB pellets prepared at different HP temperatures and holding times were kept constant at 1.2 g/cm³.

Among the hot-pressed (HP) parameters investigated, the mechanical pressures had the most extensive impact on the HP-SCB pellet morphology and yield as compared with the temperature and the holding time tested in this study. This is in-part consistent with the previous findings demonstrating that pressure has a more pronounced effect on the torrefaction of biomasses than temperature. ,, The yields of HP-SCB pellets obtained in this study are competitive (even relatively higher) with torrefied mass yields derived from other different biomasses torrefied under various pressures, temperatures, and holding times.

3.1.2. HP-SCB Biochar Pellets: Physical Properties and Yields

The HP-SCB pellet was subsequently carbonized at 500 °C for 60 min, and the obtained biochar pellet was analyzed for their yields in both sample basis (% weight of HP-SCB pellet) and raw biomass basis (% weight of raw SCB), as well as for their densities. The results are shown in Table . When the HP mechanical pressure was increased from 10 MPa to a range of 20–40 MPa, biochar pellet yield analyzed on sample basis was increased from 25.4% to a range of 26.2–29.1%. It has been reported that mechanical pressures (10–70 MPa) applied during torrefaction of Leucaena wood at 250 °C could significantly increase char yield after pyrolysis at 900 °C. The higher the mechanical pressure applied, the higher the char yield obtained. The mechanical pressures were suggested to accelerate the dehydration reaction of cellulose during pyrolysis, promoting cross-linking reaction and resulting in the increases in biochar yields. Biochar yield has been predicted to be significantly affected by fixed carbon content. The increases in the SCB biochar pellet yields observed might therefore be a result of the increased fixed carbon contents in the SCB-HP pellets as a consequence of the increased HP mechanical pressures, as discussed. The HP mechanical pressure of 40 MPa tested at 270 °C for 30 min in this study might be excessive and destroy the structure of the cellulose fibers, leading to a slight decrease in the biochar yield. For analysis based on raw biomass, the overall yield of biochar pellet showed that it increased from 22.7% at 10 MPa to 23.4–23.9% at 20–30 MPa. The yield then slightly decreased to 22.5% at 40 MPa due to its significantly lowest yield of 78.1% during the HP process. The density of the prepared HP-biochar pellet was obviously decreased from the HP-SCB pellet and remained almost stable at 0.7–0.8 g/cm³ for all samples due to the volatilization of cellulose during carbonization. In addition, the HP-SCB biochar pellet prepared at 20 MPa showed the highest strength among all samples, as shown in Table .

By increasing the HP temperature from 230 to 290 °C and extending the holding times from 0 to 45 min, the yields of HP-SCB biochar pellet analyzed on sample basis were slightly increased from a range of 25.7–25.8% to a range of 26.6–26.7% as shown in Table . An increase in the SCB biochar pellet yield observed might also be due to the increased fixed carbon contents in the HP-SCB pellets as a result of the increased HP temperature and time as discussed earlier. In addition, it has been reported that cellulose volatilization was enhanced at higher torrefaction temperatures (200–300 °C), which was beneficial for char forming in subsequent pyrolysis. It has been reported that the char yield obtained during pyrolysis of torrefied Leucaena was increased by extending the torrefaction holding time (30 min to 15 h). The increased char yield was suggested to be a consequence of the more cross-linking reactions proceeded during the pyrolysis of the torrefied Leucaena as a result of the longer torrefaction holding time. , However, those yields analyzed on raw biomass basis were almost the same as 22.5–23.9% during the HP process at 10–40 MPa for 15–45 min. Only for HP20–270–00 (without holding time), the yield of the HP-SCB biochar pellet based on raw SCB could be kept as high as 24.9% due to its relatively higher yield during the HP process. The HP-SCB biochar pellet densities at different HP temperatures and holding times had no changes at 0.7–0.9 g/cm³, also similar to those obtained at different HP mechanical pressures.

The results indicated that the HP mechanical pressure had the most significant effect on the biochar pellet yield compared with the temperature and the holding time. However, those parameters had very mild effects on the biochar pellet densities. The maximum biochar pellet yield (29.1 ± 0.2%) obtained under an HP mechanical pressure of 30 MPa, temperature of 270 °C, and holding time of 30 min in this study was competitive with the previously reported biochar yields derived from other different biomasses. ,,

3.1.3. Effect of Steam-Activation Conditions on the Physical Properties of HP-SCB-GAC

To evaluate the effects of steam-activation conditions on the physical properties of HP-SCB-GAC, HP-SCB pellets were prepared by compacting SCB under selected HP conditions of 20 MPa at 270 °C for 30 min. The 20 min activation time was first selected since the preliminary TGA results revealed that the yields of HP-SCB-GAC were still higher than 50 wt % at 800–900 °C as shown in Figure S2 (Supporting Information), which was preferable for industrial applications. The effect of steam-activation time was fine-tuned by activating the same biochar granule for 10–40 min at a medium steam-activation temperature of 850 °C. The yields and specific surface areas (S _BET) of the as-produced HP-SCB-GAC on sample basis (wt % based on the initial weights of C500–HP20–270–30 for biochar yield and HP-SCB-GAC for activated carbon yield) and on raw biomass basis (wt % based on the initial weight of raw SCB) were then analyzed, and the results are shown in Figures and .

3.

Yields (wt %) and specific surface areas (S _BET) of HP-SCB-GAC produced from the C500–HP20–270–30 biochar pellet under different steam-activation conditions: (a) temperature and (b) holding time on sample basis (wt % based on the initial weights of C500–HP20–270–30 for biochar yield and HP-SCB-GAC for activated carbon yield).

4.

Yields (wt %) and specific surface areas (S _BET) of HP-SCB-GAC produced from the C500–HP20–270–30 biochar pellet under different steam-activation conditions: (a) temperature and (b) holding time on raw SCB basis (wt % based on the initial weight of raw SCB).

When analyzed on sample basis, the yields and S _BET of the HP-SCB-GAC are expressed in Figure a,b for the effect of steam-activation temperature and holding time, respectively. Among different activation temperatures tested, the highest HP-SCB-GAC yield of 72.3% was obtained at 800 °C. By raising the activation temperature to 850 and 900 °C, the HP-SCB-GAC yields were considerably decreased to 63.1 and 49.7 wt %, respectively. Conversely, the HP-SCB-GAC produced at 800 °C possessed the lowest S _BET of 520 m²/g_GAC, and with increases in the activation temperatures to 850 and 900 °C, the S _BET of the HP-SCB-GAC were substantially increased to 650.3 and 848.4 m²/g_GAC, respectively, as shown in Figure a. Steam-activation involves the reaction between char (carbon) and an activating agent (H₂O), which causes the removal of carbon to form CO and H₂, resulting in the main weight loss. This also forms a developed pore structure, causing an increase in specific surface area of the resulting activated carbon. It has been reported that in production of cassava-derived activated carbon (using gas released from carbonization as an activating agent), an increase in activation temperature (450–900 °C) led to a significant decline in activated carbon yield, which was suggested to be a result of the loss of most volatile compounds with the increased activation temperature. The yield loss was also postulated to be caused by the weakening of macromolecular structures and conversion of carbon into ash, as a consequence of partial oxidation that usually occurs at temperatures of 600–1000 °C. By increasing steam-activation time from 10 to 40 min at 850 °C, the yields of HP-SCB-GAC showed the same decreasing trend from 71.2 to 49.4 wt % with an increase in S _BET from 537.7 to 875.9 m²/g_GAC as shown in Figure b. This is because a longer activation time enables complete expulsion of volatile materials and generates more micropores (V _Micro), thus increasing the total pore volume (V _Total) and S _BET as shown in Table S1 (Supporting Information). Extension of steam activation time from 15 to 75 min has also been found to reduce the yield of activated carbon derived from coconut shell and from 2 to 4 h for those of spruce and birch. During the steam activation, the mechanism in terms of established gasification reactions refers to the carbon-steam reaction (C + H₂O = CO + H₂) including water–gas shift reaction (CO + H₂O = CO₂ + H₂) as a secondary process occurring at high temperatures (typically 700–900 °C for biomass gasification). , It has been reported that increasing the steam-activation temperature (750 °C to 800 and 850 °C) and time (30–45 min) not only increased the diameter of the existing pores but also created new pores in the olive bagasse-derived activated carbons by continual devolatilization of the chars and carbon burnoff caused by the carbon–H₂O reaction, accounting for the higher S _BET. The results indicated that the steam-activation temperatures and times were crucial factors that significantly affected the HP-SCB-GAC production yields and S _BET. Several previous studies confirmed that the higher the surface area and pore volume of the mesoporous activated carbons derived from biomass and coal, the better the adsorption capacity in dye removal. ,

To determine the optimum steam-activation temperature and holding time, the HP-SCB-GAC yields and S _BET calculated on raw biomass basis (wt % based on raw SCB) were therefore taken into account and simultaneously analyzed, with the results shown in Figure a,b. With more severe activation conditions by increasing the activation temperature and time, the yields of HP-SCB-GAC tended to gradually decrease from 16.7–16.8% to approximately 11.6% (for the samples prepared at 900 °C for 20 min and at 850 °C for 40 min). On the other hand, the S _BET of the HP-SCB-GAC were apparently increased from 87.5 m²/g_rawSCB of the sample activated at 800 °C to 95.8 and 98.0 m²/g_rawSCB for those prepared at 850 and 900 °C, respectively, as shown in Figure a. By extending the activation time from 10 to 20 and 30 min at 850 °C, the S _BET clearly increased from 90.0 to 95.8 and 101.5 m²/g_GAC, respectively, whereas it was not considerably different at 102.0 m²/g_GAC for the HP-SCB-GAC prepared at 850 °C with further increasing activation time to 40 min as shown in Figure b. The discussion on the yields and specific surface areas of the HP-SCB-GAC based on raw SCB demonstrated that steam-activation at 850 °C for 30 min could be thoroughly considered as the optimum condition producing a substantial yield of HP-SCB-GAC at 53.7 wt % possessing a sufficient S _BET of 804.6 m²/g_GAC. At this optimum condition of steam activation, the overall yield of the HP-SCB-GAC was still obtained at 12.6 wt % based on raw SCB with the % burnoff during steam activation still not too high at >50 wt %. It is also not necessary to further increase the activation temperature and time to 900 °C and 40 min with small increases in S _BET with the consideration of energy consumption. Consequently, steam-activation at 850 °C for 30 min was used in further experiments for investigation of the effect of hot-pressed conditions on the properties of HP-SCB-GAC.

3.1.4. HP-SCB-GAC: Textural Properties and Yields

The HP-SCB biochar pellet was subsequently steam-activated at a temperature of 850 °C for 30 min, which was the optimum activation condition obtained from the previous experiment as discussed earlier (Figures –). The as-produced HP-SCB-GAC was analyzed for yield and S _BET as shown in Figure . When the HP mechanical pressure was raised from 10 to 40 MPa, HP-SCB-GAC yield based on the biochar sample was slightly increased from 53.3 to 57.6% as shown in Figure a. However, the S _BET of the HP-SCB-GAC was dramatically increased from 544.6 to 804.6 m²/g_GAC as the HP mechanical pressure was increased from 10 to 20 MPa and sharply reduced to 684.9 and 524.5 m²/g_GAC with an increase in the HP mechanical pressure to 30 and 40 MPa. It has been reported that during biomass torrefaction, macromolecules, typically hemicellulose and cellulose, were partially depolymerized, and some gases and tar were released, resulting in micropore formation. With the torrefaction severity, the S _BET of the torrefied biomass was increased. , Depending on the porosity (void fraction) of the torrefied biomass precursor, further release in volatile matters during the subsequent pyrolysis could lead to the question that the formation of new pores resulted in increased S _BET, or the enlargement or collapse and consolidation of the pre-existing micropore structures resulted in decreased specific surface area of the resulting biochar. − The increase in HP mechanical pressure from 10 to 20 MPa may eventually lead to the formation of new pores during steam activation, resulting in an enhanced specific surface area of the HP-SCB-GACs. However, the HP mechanical pressures of 30 and 40 MPa may lead to the destruction of micropore structures and form large pore size during steam activation, resulting in the reduced S _BET of the HP-SCB-GACs. The detailed data of surface properties including average pore size, micropore, mesopore, and total pore volumes (V _Micro, V _Meso, V _Total) of all HP-SCB-GACs prepared under various HP conditions are reported in Table S2 (Supporting Information). For the HP-SCB-GACs derived from various HP temperatures (230–290 °C) and holding times (0–45 min), their yields were comparable, at around 53.7–54.8% and 53.7–55.4%, as shown in Figure b,c, respectively. Their S _BET were similar in a narrow range of 788.6–804.6 m²/g_GAC and 754.7–804.6 m²/g_GAC for the samples prepared at different HP temperatures and holding times as shown in Figure b,c, respectively.

5.

Yields and specific surface areas (S _BET) of HP-SCB-GACs derived from different HP conditions: (a) mechanical pressures (10–40 MPa) at 270 °C for 30 min, (b) temperatures (230–290 °C) at 20 MPa for 30 min, and (c) holding times (0–45 min) at 20 MPa and 270 °C.

The results indicate that the HP mechanical pressure had the most significant effect on the HP-SCB-GAC yields and S _BET, compared with the temperature and holding time. The results also demonstrated that hot pressing of SCB under a mechanical pressure of 20 MPa, a temperature of 270 °C, and a holding time of 30 min was the optimum HP condition producing a substantial yield (53.7%) of the HP-SCB-GAC possessing the highest specific surface area (804.6 m²/g), comparable to those of activated carbons derived from steam-activation of other biomasses. , The HP-SCB-GAC produced under this HP condition (HP20–270–30) was selected for further experiments to evaluate its MB adsorption performance.

3.2. Methylene Blue (MB) Adsorption Performance Using the Selected HP-SCB-GAC

3.2.1. Effect of pH, Adsorption Time, and Kinetics of MB Adsorption

MB adsorption at different pH values (2, 7, and 12) using 0.05 g of HP-SCB-GAC was tested to determine the optimal pH for MB adsorption performance. With almost unchanged MB removal efficiency (87.8–90.1%) and adsorption capacity (41.8–42.6 mg/g) of HP-SCB-GAC as shown in Figure a, the adsorption process showed minimal sensitivity to pH adjustments across a wide range of pH values from 2 to 12. The pH-independent adsorption of MB onto activated carbon (coconut shell AC with 1265 m²/g of surface area) over a pH range of 3–11 has also been observed in a previous study, likely because MB is always cationic across a broad pH range. At pH 3.8, it transitions from a divalent to a monovalent cation. Another supporting study reported similar trends using black olive stone activated carbon for MB removal across a pH range of 2 to 10, achieving a removal efficiency of 68–70%. The dominant adsorption mechanism was identified as hydrophobic–hydrophobic interactions, which were particularly effective at neutral pH. Therefore, conducting MB adsorption performance tests at neutral pH (pH 7) is justified, as it offers a balanced approach, is environmentally relevant, and ensures experimental consistency, making it a preferred choice in further adsorption tests in this study.

6.

MB adsorption data of the selected HP-SCB-GAC (HP20–270–30): (a) effect of pH, (b) adsorption efficiency and capacity, (c) pseudo-first-order kinetic correlation, (d) pseudo-second-order kinetic correlation, (e) effect of MB initial concentrations, and (f) adsorption capacities of the HP-SCB-GAC at various MB initial concentrations fitted with different isotherm models.

For an evaluation of MB adsorption kinetic behavior using the selected HP-SCB-GAC to determine its efficiency as a function of time, as shown in Figure b, the HP-SCB-GAC revealed a rapid MB adsorption during the first 7 h of contact with MB solution due to the abundant availability of adsorption sites of the HP-SCB-GAC. The adsorption was subsequently slowed down over longer contact times because of the gradual saturation of the adsorption sites by MB with prolonged contact times. Similar kinetic behaviors of MB adsorption using various types of adsorbents have been observed in the literature. , The adsorption equilibrium was attained at 24 h with an equilibrium adsorption efficiency close to 100% and an equilibrium adsorption capacity of approximately 17 mg/g. The MB adsorption kinetics of the HP-SCB-GAC was determined by following the experimental MB adsorption data at each specific contact period for pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetic models using eqs and , as demonstrated in Figure c,d, respectively.

Table shows the theoretically calculated maximum adsorption capacities (q _e), rate constants (k ₁, k ₂), and correlation coefficients (R ²) of PFO and PSO kinetic models. The R ² value of the PSO model (0.9864) was close to 1.0000 and obviously higher than that of the PFO (0.8518). The theoretically calculated q _e of the PSO (22.22 mg/g) was also closer to the experimental q _e value (17 mg/g). The results indicate that the PSO model better fits the experimental MB adsorption data, conventionally suggesting that the overall rate of MB adsorption of the HP-SCB-GAC is predominantly controlled by chemisorption involving exchange or sharing of electrons between the positively charged MB molecules and functional groups on the surface of the HP-SCB-GAC. , Dye adsorption kinetics of previously developed adsorbents have also been found to follow the PSO kinetic model. ,,− However, recent studies revealed that the PSO model can empirically fit a large number of adsorption kinetic data from various systems due to its mathematical operation’s flexibility with linear regression and is not directly related to a specific sorption mechanism, leading to difficulty in concluding whether the surface reaction or diffusion is the rate limiting step of the adsorption process. Further studies using other kinetic models with the estimation of the thermodynamic parameters, along with the PSO model, could gain more insights into the adsorption mechanisms.

3. Pseudo-First-Order and Pseudo-Second-Order Kinetic Model Parameters for the MB Adsorption of Selected HP-SCB-GAC (HP20–270-30).

model	parameters
pseudo-first-order (PFO)	qe (mg/g)	12.31
	k1 (h–1)	0.13
	R 2	0.8518
pseudo-second-order (PSO)	qe (mg/g)	22.22
	k2 (g·mg–1·h–1)	0.01
	R 2	0.9864

3.2.2. Effect of the Initial MB Concentration and Adsorption Equilibrium Isotherms

To study the interactive behavior of MB with the HP-SCB-GAC, the adsorption isotherm of the HP-SCB-GAC was determined. Figure e illustrates a data set of various initial MB concentrations for use to analyze adsorption equilibrium isotherm behaviors. The experimental adsorption capacities of the HP-SCB-GAC were fitted to four different adsorption isotherm models, namely, Langmuir, Freundlich, Temkin, and Sips, as shown in Figure f. The results showed that the adsorption capacities of the HP-SCB-GAC increased from approximately 29 to 138 mg/g as the initial concentration of MB was increased from 25 to 500 mg/L, respectively. Increasing the initial concentration of MB promotes the concentration gradient, providing a greater driving force to overcome mass transfer resistance between MB in the solution and the adsorption sites on the HP-SCB-GAC surface, enhancing the interaction between MB and the HP-SCB-GAC, resulting in higher adsorption capacity. , As shown in Table , the experimental MB adsorption capacities of the HP-SCB-GAC were found to fit best with Langmuir isotherm models with an R ² of 0.9606. For the Langmuir isotherm, the theoretically calculated q _max of 144.9 mg/g was comparable to the experimentally determined q _max of approximately 138 mg/g. The fitness of the experimental MB adsorption capacities of the HP-SCB-GACs to the Langmuir isotherm model indicated monolayer MB adsorption on the HP-SCB-GAC surface, while the Freundlich isotherm with second-best fitted at an R ² of 0.9344 suggested the multilayered and heterogeneous adsorption. , Dye adsorption isotherms of previously developed adsorbents, particularly biomass-derived AC, have also been shown to be mostly described by the Langmuir isotherm model, ,− , while some of them were better fitted with the Freundlich − isotherm model.

4. MB Adsorption Isotherm Parameters of Selected HP-SCB-GAC (HP20–270–30).

equilibrium adsorption isotherm model	parameters
Langmuir	qmaxL (mg/g)	144.9
	KL (L/g)	0.020
	R 2	0.9606
Freundlich	1/n	0.199
	K F	35.76
	R 2	0.9344
Temkin	AT (L/g)	14.16
	BT (kJ/mol)	13.17
	R 2	0.7911
Sips	qmaxS (mg/g)	138.1
	KS (L/g)	0.080
	1/n	0.446
	R 2	0.6375

FTIR spectra (Figure S3, Supporting Information) were used to investigate changes in surface functional groups in raw sugarcane bagasse (SCB), biochars, and SCB-GACs before and after methylene blue (MB) adsorption. The raw SCB spectrum (Figure S3a, Supporting Information) displays characteristic peaks associated with lignocellulosic components: −OH stretching (3416 cm^–1), −CH stretching (2918 cm^–1), C=O stretching (1636 cm^–1), CH=CH stretching (1254 cm^–1), and C–O stretching (1049 cm^–1). After carbonization (Figure S3b, Supporting Information), the intensity of the C–H stretching band decreases, and the CH=CH band disappears, indicating degradation of lignin structures. However, −OH, C=O, and C–O bands remain evident, with peak shifts depending on mechanical pressure, suggesting retention of some functional groups essential for adsorption. After steam activation (Figure S3c, Supporting Information), the spectra still show the presence of −OH, C=O, and C–O groups with slight shifts, indicating that steam activation preserves key functional groups while enhancing textural properties.

Following MB adsorption (Figure S3d, Supporting Information), significant spectral changes are observed. The C=O band (∼1590 cm^–1) decreases in intensity, while new peaks emerge or intensify at 1625, 1629, and 1637 cm^–1 with increasing MB concentrations (25, 50, and 500 ppm). Shifts in the C–O region are also evident. These changes indicate interactions between the adsorbent surface and MB molecules, particularly through electrostatic attraction between carbonyl (C=O) groups on the SCB surface and the positively charged N⁺(CH₃)₂ moiety of MB. In our previous study, we reported that MB exhibits characteristic FTIR peaks at ∼1743 cm^–1 (N⁺(CH₃)₂), 1500–1400 cm^–1 (aromatic −C=C−), 1368 and 1360 cm^–1 (C–N), and 1212–1210 cm^–1 (C–C). Among these, the N⁺(CH₃)₂ peak appears to be the most strongly involved in interactions with the selected HP-SCB-GAC surface. These findings suggest that MB adsorption is driven by a combination of electrostatic interactions, hydrogen bonding (between −OH and MB nitrogen sites), and π–π interactions between the aromatic structures of MB and the carbonaceous adsorbent. These FTIR findings, combined with the S _BET results, demonstrate that the prepared SCB-GACs possess both favorable surface chemistry and a porous structure, supporting their effectiveness as alternative adsorbents for dye-contaminated wastewater treatment.

3.2.3. Regeneration and Recycling of the Selected HP-SCB-GAC for MB Adsorption

In wastewater treatment, the regeneration and recycling of adsorbents after the adsorption process are crucial. This is not only to reduce operational costs and enhance the economic viability of the process but also to ensure its environmental sustainability. In this study, therefore, the selected HP-SCB-GAC was assessed for its stability and MB adsorption performance in several cycles of regeneration and recycling. The HP-SCB-GAC, which had been used to adsorb MB, was treated with 90% ethanol for MB desorption, and the regenerated HP-SBC-GAC was then reused for MB adsorption in 50 mg/L MB solution. The HP-SCB-GACs underwent five consecutive cycles of MB adsorption–desorption. In each cycle, the MB adsorption efficiency and capacity of the HP-SCB-GAC were evaluated. The HP-SCB-GACs exhibited consistent MB adsorption efficiency ranging from 87.8% to 91.7% with five consecutive cycles of MB adsorption–desorption, demonstrating the potential of the HP-SCB-GAC, as illustrated in Figure . The adsorption capacity of HP-SCB-GAC after the first cycle was 41.8 mg/g, significantly increasing to 46.6 mg/g after the second cycle. Subsequently, it further increased to 48.4– 48.6 mg/g after the third and fourth cycles before slightly dropping to 47.7 mg/g after the fifth cycle of regeneration and adsorption. The observed increases in MB adsorption capacities of the HP-SCB-GAC during the second, third, and fourth cycles of regeneration may be attributed to the augmentation of vacant sites on the HP-SCB-GAC surface. This is likely due to partial destruction of the HP-SCB-GAC structure caused by the regeneration process, which typically involves chemical washing and drying at elevated temperatures, along with chemical reactions with MB. These processes facilitate a greater mass transfer of MB molecules into the HP-SCB-GAC structure. However, the slight reduction in adsorption capacity during the fifth cycle of regeneration could be due to the pore obstruction resulting from the gradual coagulation of MB molecules over several regeneration cycles, as well as pore structures having collapsed caused by a constant washing and drying.

7.

MB adsorption efficiencies and capacities of selected HP-SCB-GAC (HP20–270–30) over five consecutive cycles of regeneration and recycling for adsorption.

Based on the overall findings, this study underscores the potential of developed HP-SCB-GAC as an efficient adsorbent. With a notably high specific surface area (804.6 m²/g) and a competitive MB adsorption capacity (approximately 138 mg/g), it exhibits a performance comparable to biochar (∼113 mg/g)⁵ and powdered activated carbons (PAC) derived from other biomasses (26–292 mg/g) ,− as demonstrated in Table . Furthermore, it demonstrates remarkable stability over multiple cycles of regeneration and recycling. While they may not surpass the performance of biopolymer-based adsorbents like alginate bead composites (279–769 mg/g) ,,, and benchmark commercial GACs (260–319 mg/g), the developed HP-SCB-GAC offers significant advantages. Notably, they are produced through a straightforward granulation process without the need for additional binders. With a specific surface area surpassing 800 m²/g, ongoing efforts focus on modifying the HP-SCB-GAC to achieve potential alternative adsorbents suitable for a diverse range of applications such as volatile organic compound (VOC) removal in polluted flue gas from petrochemical industries.

5. Experimentally Determined Maximum Adsorption Capacities of Methylene Blue (MB) Using Various Bio-Based Adsorbents.

raw biomass (biobased adsorbent)	type of adsorbent	SBET (m2/g)	MB initial concentration (mg/L)	mass of the adsorbent (g/L)	maximum adsorption capacity (mg/g)	references
SCB (steam-850 °C-AC)	hot-pressed (HP) GAC	804.6	500	0.05	138.1	this study
SCB (steam-900 °C-AC)	GAC with alginate	198.4	500	0.05	403.0
coffee ground (cellulose extract)	alginate bead	1078	100	0.20	400.5
C. polygonoides (acid-600 °C-AC)	AC alginate bead	NA	400	0.01	769.2
rice husk	magnetic alginate bead	NA	500	0.20	279.4
SCB (pyrolysis-500 °C)	biochar	323.0	50	0.02	113.0
	calcium-alginate bead	263.6	50	0.02	71.2
Filtrasorb 400 (bituminous coal)	commercial GAC	∼948	500	1.0	319.0
Norit (peat, steam activated)	commercial GAC	∼875	500	1.0	280.0
Picacarb (coconut shells)	commercial GAC	NA	500	1.0	260.0
pine sawdust (presteam-900 °C-AC)	PAC	962.0	75	0.01	332.0
paper mill sewage sludge (steam-850 °C-AC)	PAC	280.0	100	NA	158.7
nonpareil almond shells (CO2-500 °C-AC)	PAC	1239	500	1.0	440.0
Chandler walnut shells (CO2-500 °C-AC)	PAC	1039	500	1.0	358.0
pineapple peel (KOH-700 °C-AC)	PAC	1160.1	150	0.20	168.2
fish gill waste (KOH-800 °C-AC)	PAC	NA	50	0.20	78.5
bamboo (microwave-200W-AC)	PAC	1323	400	0.1	291.6
seaweed (NaOH-800 °C-AC)	PAC	1334	300	0.1	249.0
figure leaves (acid-350 °C-AC)	PAC	18.2	200	0.02	41.7
rice husk (acid-500 °C-AC)	PAC	244.5	200	0.6	26.3

^aGAC: granular activated carbon.

^bPAC: powder activated carbon.

^cNA = not available.

Conclusions

A binderless hot-pressed (HP) technique was used to produce sugarcane bagasse-derived granular activated carbon (HP-SCB-GAC) by varying the mechanical pressure, temperature, and holding time. The mechanical pressure showed the most significant influence on yields and textural properties of the developed HP-SCB-GAC. The optimal HP condition at 20 MPa, 270 °C for 30 min resulted in a comparable specific surface area at 804.6 m²/g of the HP-SCB-GAC with highest strength, selectively used for MB adsorption tests. The maximum MB adsorption capacity at about 138 mg/g could be attained and fitted with pseudo-second-order kinetic model and Langmuir adsorption isotherm behavior using the selected HP-SCB-GAC. Finally, MB adsorption efficiency at 87–92% could be achieved for at least five adsorption–desorption cycles. Significantly, the HP-SCB-GAC with a high surface area, produced through a simple granulation process without the use of additional binders, can be regarded as a potential alternative adsorbent suitable for a wide range of environmental applications.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c03372.

• TGA curve for pyrolysis of raw SCB powder, TGA curves for steam activation of HP-SCB biochar pellets, FTIR spectra for all samples, yields and textural properties for all HP-SCB-GAC samples (PDF)

The authors declare no competing financial interest.