Thermochemical Conversion of Biomass into Biochar: Enhancing Adsorption Kinetics and Pore Properties for Environmental Sustainability

Abstract

This study investigates the pyrolysis and adsorption properties of biochar derived from coconut shell (BC-CS), rice husk (BC-RH), and cow manure (BC-CM) under varying thermal treatment conditions. Biochar samples were produced at 800 °C with residence times ranging from 0 to 60 min. Their characteristics were analyzed using their Brunauer–Emmett–Teller (BET) surface area, total pore volume, and pore diameter measurements. BC-CM exhibited the highest BET surface area of 263.3 m²/g and a total pore volume of 0.164 cm³/g, while BC-RH and BC-CS showed maximum BET surface areas of 220.62 m²/g and 197.38 m²/g, respectively. Nitrogen adsorption–desorption isotherms revealed distinct microporous and mesoporous structures, with BC-CM demonstrating superior adsorption capacity across all relative pressures. The adsorption kinetics of methylene blue (MB) were examined at initial concentrations of 1 ppm, 5 ppm, and 10 ppm, with varying biochar doses (0.1 g, 0.3 g, and 0.5 g). The results showed that the adsorption rate constant (k) decreased with higher initial MB concentrations, while the equilibrium adsorption capacity (q_e) increased. BC-CM achieved the highest q_e of 2.18 mg/g at 10 ppm and a 0.5 g dose, followed by BC-RH-800-45 (1.145 mg/g) and BC-CS (0.340 mg/g). The adsorption process was well described by a pseudo-second-order kinetic model, indicating chemisorption as the dominant mechanism. Increasing biochar doses improved MB removal efficiency, highlighting the dose-dependent nature of adsorption. These findings underscore the importance of optimizing pyrolysis parameters to enhance biochar’s adsorption performance and identify key factors influencing its effectiveness in environmental applications.

1. Introduction

At COP28, there was a strong emphasis on the need for advanced agrifood systems and the integration of sustainable agricultural practices [1]. This includes leveraging renewable biomass resources such as plant materials and agricultural waste, which are vital for achieving the Paris Agreement’s goal of limiting global warming to 1.5 °C and reducing greenhouse gas emissions [1]. Plant biomass is one of the most abundant biomaterials on earth and is primarily made up of three components—lignin, cellulose, and hemicellulose. Lignocellulosic biomass plays an important role by converting atmospheric CO₂ into carbohydrates and maintaining the delicate balance caused by the combustion of fossil fuel, which releases excess amounts of CO₂ into the atmosphere. This transformation not only positions biomass as a renewable source of energy, but also supports its status as a carbon-neutral fuel [2,3]. Even though biomass in its raw form is useful to us, the thermochemical conversion of biomass is a technology that has been used for millennia that enhances the value of the product. Pyrolysis, in particular, is essential in transforming biomass into a variety of valuable products [4]. Pyrolysis is described as the process of decomposing organic material at elevated temperatures in the absence of oxygen, resulting in three primary components—biochar, bio-oil, and syngas. Biochar, which is the focus of this research, has been reported to have diverse applications, extending beyond energy production to include uses such as solid fuel, carbon material, soil amendment, environmental adsorbent (biosorbent), functional catalyst, or feedstock for chemicals [4,5,6,7,8]. The versatility of biochar highlights its potential in promoting sustainable agricultural methods and providing green alternatives to conventional practices [1].

Extensive research has been conducted on the use of biochar and its application in the environment, particularly the soil, air, and water. An example of this is the research conducted by Zama (2018) et al., where they explained that biochar is able to immobilize heavy metals in contaminated soils, reducing their bioavailability and toxicity [9]. In research conducted by Wang (2021) et al., biochar’s application reduced the mobility of cadmium and lead in soils, preventing their uptake by plants [10]. Biochar can also adsorb and immobilize organic contaminants, like polycyclic aromatic hydrocarbons (PAHs), thereby mitigating their environmental impact [11]. With regards to air pollution control, biochar can adsorb volatile organic compounds (VOCs) from industrial emissions, reducing air pollution and improving air quality [12]. It also effectively adsorbs sulfur dioxide (SO₂) from flue gases, demonstrating its potential in industrial air purification systems [13]. Many investigations have also focused on the use of biochar in water treatment. Biochar is reported to be highly effective in adsorbing heavy metals, pharmaceuticals, and organic pollutants from wastewater. Studies have shown that biochar can remove up to 90% of lead (Pb) and cadmium (Cd) from contaminated water [14,15]. Its high surface area and porosity make it suitable for adsorbing dyes from industrial effluents, such as methylene blue, with significant adsorption capacities [16]. Chemically modified biochar, such as phosphoric acid-treated rice husk biochar, further enhances these adsorption capabilities, expanding its applicability in various environmental settings [17].

Biochar, derived from carbon-rich agricultural products through pyrolysis, stands out as one of the most effective adsorbents for removing pollutants [17]. Recognized by the US Environmental Protection Agency (EPA) as a leading adsorption technology, biochar has been the focus of extensive research. A wide variety of biomass materials have been used in the removal of pollutants, specifically, agricultural and plant wastes such as corn cobs, citrus peels, rice husks (or hulls), rice bran, sawdust, wheat bran, wheat husks, sugarcane bagasse, coconut shells, banana stems, barley husks, hazelnut shells, walnut shells, cottonseed hulls, soybean hulls, sunflower stalks, bamboo cells, and tree bark are promising bio-adsorbents [18,19,20,21,22]. Agricultural waste is said to be one of the most feasible materials used to create biochar because of its high availability, ease of processing, and the general valorization of a waste material into a valuable product [23]. In this research, methylene blue of various concentrations was used as the pollutant source—this material was chosen because chemical dyes in waterways is one of the leading causes of environmental pollution.

To leverage the potential of locally abundant agricultural waste, this study investigated the production of biochar from three types of agricultural residues: rice husk (RH), cattle manure (CM), and coconut shell (CS). These three sources represent the cross-sectionality of biomass’s diverse sources because of their initial characteristics of being sourced from either animals (CM), commercial food production (RH), or hard woods from industry (CS). By subjecting these materials to pyrolysis at elevated temperatures, we aim to evaluate how pyrolysis conditions—specifically, temperature (800 °C) and varying residence times (0, 15, 30, 45, 60 min)—influence the yield and pore characteristics of the resulting biochar, using a standard heating rate of 10 °C/min. Our research focuses on the physical and chemical properties of the biochars and their effectiveness in adsorbing a cationic dye (methylene blue), aiming to identify the key factors that enhance adsorption performance. The guiding questions of how slight changes in residence time at elevated temperatures affect the physiochemical characteristics of biochars from different sources, and how these changes affect their adsorption performance, will be answered in the development of this research.

2. Materials and Methods

2.1. Material

Coconut shells (CS), rice husks (RH), and cattle manure (CM), employed as precursor materials for biochar synthesis, were procured from local agricultural cooperatives in Pingtung, Taiwan. The raw biomass materials were first sun-dried and subsequently oven-dried to reduce the moisture contents. The dried materials were shredded and sieved to the desired particle sizes. The methylene blue (MB) used for the adsorption experiments was procured from Koch-Light Laboratories Ltd., Colnbrook, Bucks, England. This was diluted using distilled water to create an initial stock solution, which was then diluted further to the individual concentrations used in the adsorption experiments. Initial proximate analysis revealed specific compositional characteristics for each biomass category. Coconut shells (CS) exhibited a volatile matter content of 81.58 wt% (±3.04), a low ash content of 0.45 wt% (±0.12), and a considerable amount of fixed carbon at 17.97 wt%, with an associated calorific value of 20.92 MJ/kg. Rice husks (RH) were identified as having a volatile matter content of 68.59 wt%, an ash content of 13.38 wt%, fixed carbon of 10.69 wt%, and a calorific value of 16.59 MJ/kg. Cattle manure (CM) showed the highest volatile matter content at 83.02 wt%, coupled with an ash content of 6.65 wt% and fixed carbon of 6.31 wt%, presenting a calorific value of 18.58 MJ/kg. These findings are consistent with the established literature [24]. In addition, Ultimate Elemental analysis was used to characterize the biochars produced at 800 °C with 0 to 60 min residence times using an Elemental Analyzer Unicube, Langenselbold, Germany.

2.2. Biochar Preparation

Given that temperature is a critical parameter in pyrolysis, this study synthesized biochar from coconut shells (BC-CS), rice husks (BC-RH), and cattle manure (BC-CM) at 800 °C, with residence times of 0, 15, 30, 45, and 60 min, and a heating rate of 10 °C/min under an inert nitrogen atmosphere. Approximately 5.00 g of biomass material was loaded into a stainless steel crucible and placed into a vertical tube furnace. Nitrogen gas at a flow rate of 500 cm³/min was used to purge the cylinder for 15 min prior to the initial ramping up of the desired temperature/ time, and was maintained at that flow rate for the duration of the experiment. The heating rate of 10 °C/min was chosen based on the literature and initial TGA results, as a 10 °C/min heating rate has been reported to be effective in producing quality biochar with desirable surface and pore properties. All subsequent biochars were labeled based on the conditions they were produced in, as biochar-biomass-temperature-residence time, i.e., BC-CS-800-15 [25,26].

2.3. Biochar Characterization

In this study, the pore characteristics of the biochar products, including their Brunauer–Emmett–Teller (BET) surface area, pore volume, and pore size distribution, were determined using an accelerated surface area and porosity system (ASAP 2020; Micromeritics Co., Norcross, GA, USA). The specific surface area was calculated using the BET equation across a relative pressure range of 0.05 to 0.35. Total pore volume was measured as the volume of nitrogen adsorbed at a relative pressure of approximately 0.995. Micropore area and volume were quantified using the Halsey equation through the t-plot method, following the International Union of Pure and Applied Chemistry (IUPAC) definitions, which classify micropores as having a diameter or width of less than 2 nm, and mesopores as those between 2 and 50 nm. The pore size distributions, particularly in the mesopore and small macropore range, were analyzed using the Barrett–Joyner–Halenda (BJH) method based on experimental N₂ isotherms (desorption branch) via the Kelvin model of pore filling. The surface morphology of the biochar was examined using a scanning electron microscope (SEM) system (S-3000N; Hitachi Co., Tokyo, Japan). The SEM analyses were performed by applying an accelerating potential of 15.0 kV (electron beam) in a vacuum chamber. Additionally, an EDS system (7021-H; HORIBA Co., Kyoto, Japan) was attached to the SEM to quantify the elemental compositions of the biochar surfaces. In order to observe the functional groups present on the biochar surface, an FTIR instrument (FT/IR-4600; JASCO Co., Tokyo, Japan) was employed. The biochar samples were grounded in an agate mortar and mixed with potassium bromide (KBr). About 10 mg of each biochar sample was mixed with 300 mg of the infrared grade KBr~3 wt% biochar. About 30 mg of the fine mixture of biomass/KBr was then placed in a 13 mm die kit and deposited in a hydraulic press to create the sample pellets. The transmission spectra were scanned with a resolution of 4 cm⁻¹ in the range of 4000–400 cm⁻¹ [4].

2.4. Adsorption Performance Experiments

The adsorption kinetics of the biochar derivatives BC-CS, BC-RH, and BC-CM were assessed, adhering to methodologies delineated in a precedent study [24]. These experiments were performed in batch mode using a 200 mL aqueous medium that was maintained at a constant temperature (25 °C) and stirred at 100 rpm. The experimental design varied the concentrations of methylene blue (MB) (1, 5, and 10 mg/L) and included different dosages of biochar (0.1 g, 0.3 g, 0.5 g). Throughout the kinetic trials, aliquots were extracted at designated intervals (5, 10, 20, 30, 40, 50, 60, 120 min, and 3, 4, 20, and 24 h). Each aliquot was decanted, and the MB concentration in each sample was quantified using a UH5300 spectrophotometer (Hitachi Co., Tokyo, Japan) set to a wavelength of 665 nm. The standard curve against which the concentrations were analyzed is included in the Supplementary Materials, Figure S1.

The amount of adsorbed MB was calculated as follows [4]:

$$q_e=V(C_0-C_e)/w$$

(1)

$$q_t=V(C_0-C_t)/w$$

(2)

where q_e and q_t (mg/g) are the amounts of MB adsorbed by the biochar at equilibrium and at time t, respectively; C₀, C_e, and C_t (mg/L) are the initial concentration, the equilibrium concentration, and the concentration of MB solution at time t, respectively; V (L) is the volume of the aqueous MB solution; and w (g) is the mass of biochar added.

3. Results

3.1. Characterization of the Biochars

The surface morphology of each of the three biochars is presented in the SEM images seen in Figure 1. There is a clear difference in the morphological structures presented, as at the experimental temperatures, each material was observed to react differently. The coconut shell biochar (Figure 1a) shows a dense network of pores that can fracture easily, this is primarily due to the initial composition of the biomass, which has a high percentage of lignin [27]. Figure 1b shows the rice husk biochar, where the development of layers within the husk can be observed; during the development of these layers, the evacuation of the volatiles left behind the porous biochar structure [21]. The cow manure biochar, seen in Figure 1c, shows a highly porous structure that is left behind after pyrolysis. Careful observation of the image reveals that the initial biomass plays an important role in the type of biochar obtained. For cow manure biomass, which is not homogeneous, it is necessary to homogenize the samples as best as possible when observing them on a microscopic scale [28]. The SEM images, along with the EDS spectra for the other biochar and raw biomass samples, are included in the Supplementary Materials, Figure S2.

The Fourier-transform infrared (FTIR) spectra of biochar samples from the three different feedstocks are presented in Figure 2a–c. Careful observation of the spectra shows that the surface chemistry of the same type of biochar was similar during the changes in temperature. Another observation that was made was that some common functional groups can be seen in different types of biochar; however, the intensities of these peaks were different, indicating the abundance of the identified group. The first common peak that was observed was found in the CS (Figure 2a) and RH (Figure 2b) biochars at around 3200–3600 cm⁻¹—this corresponds to O-H stretching vibrations, signifying the presence of hydroxyl groups and adsorbed water. This implies that surface hydroxyl groups are available to form hydrogen bonds with adsorbate molecules, enhancing adsorption efficiency [29]. Lower down the spectra, RH (Figure 2b) and CM (Figure 2c) displayed a common peak at around 2200–2400 cm⁻¹, and this range corresponds to the presence of the COOH functional group in the biochar. In other investigations it was suggested that the peak recorded in this range gives further evidence for the presence of the OH group on the biochar surface, normally recorded around wave number 3400± cm⁻¹ [30]_. There was a peak observed at around 1600 cm⁻¹ in the CS and CM biochars; in the literature it was explained that C=C stretching peaks normally occur around 1600–1650 cm⁻¹, emphasizing the aromatic nature of the biochar, which also engages in π-π interactions [31]. All three biomasses displayed peaks in the range of 1000–1400 cm⁻¹—CS’s occurred at around 1100 cm⁻¹, RH’s occurred at around 1400 cm⁻¹, and CM’s occurred at around 1000 cm⁻¹. The absorbance band at 1000–1400 cm⁻¹ is reported to be due to C-O stretching vibrations, indicating the presence of alcohols, phenols, ethers, and esters, which offer additional adsorption sites through various interactions [32]. These surface chemical properties greatly influence the adsorption of cationic pollutants. The carboxyl (COOH), hydroxyl (O-H), and carbonyl (C=O) groups are vital for cation adsorption, as they provide anionic sites that attract cations through electrostatic interactions, thereby enhancing adsorption capacity. The adsorption capacity of biochars is affected not only by the pore structure, but also the surface chemistry. Many of these qualities depend not only on the initial biomass, but also the treatment parameters. As treatment time increases, the content of certain functional groups may vary, affecting biochar’s ability to adsorb cations. For example, higher thermal treatment temperatures may increase carbonization, reducing surface polar functional groups and, thus, decreasing cation adsorption capacity [33]. Therefore, it is crucial to consider biochar’s surface chemical properties and preparation conditions to optimize its adsorption performance for cationic pollutants.

Elemental analysis was conducted on the three biochar materials produced under the different parameters in order to understand how the feedstock and conditions affect the biochar’s properties; these results are shown in

Table 1. Elemental analysis of biochars.

Sample	Carbon (wt%)	Hydrogen (wt%)	Nitrogen (wt%)	Sulfur (wt%)	Oxygen (wt%)	H/C Ratio	O/C Ratio
BC-CS-800-0	90.6	1.3	0.8	0.1	4.2	0.17	0.03
BC-CS-800-15	91.8	1.1	0.8	0.1	10.3	0.14	0.08
BC-CS-800-30	93.4	1.2	0.9	0.0	7.5	0.15	0.06
BC-CS-800-45	93.8	1.2	0.9	0.1	4.0	0.15	0.03
BC-CS-800-60	97.3	1.2	0.4	0.1	1.1	0.15	0.01
BC-RH-800-0	56.0	1.1	0.0	0.1	42.9	0.24	0.57
BC-RH-800-15	58.6	1.3	0.5	0.2	39.5	0.27	0.51
BC-RH-800-30	61.3	1.2	0.6	0.2	39.7	0.23	0.49
BC-RH-800-45	64.9	1.1	0.5	0.1	41.3	0.20	0.48
BC-RH-800-60	66.3	1.1	0.5	0.1	41.9	0.20	0.47
BC-CM-800-0	51.0	1.0	1.8	0.4	45.8	0.24	0.67
BC-CM-800-15	60.4	1.3	1.3	0.3	36.8	0.26	0.46
BC-CM-800-30	63.8	1.3	1.1	0.3	33.5	0.24	0.39
BC-CM-800-45	64.8	1.3	1.3	0.3	32.4	0.24	0.38
BC-CM-800-60	66.6	1.2	1.0	0.3	30.9	0.22	0.35

. Across all three biomasses, the trend of increased carbon content correlated with increases in retention time; BC-CS had the highest carbon composition, which increased from 90.6 wt% to 97.3 wt% over the change in retention time from 0 to 60 min [34]. The carbon contents of BC-RH and BC-CM reached a maximum of 66.3 wt% and 66.6 wt%, respectively, both at 800 °C at 60 min [33]. The initial biomass source plays a significant role in the final carbon content; however, the pyrolysis conditions can alter the composition [35].

Another observation that can be made from the results seen in Table 1 is the variation in oxygen content. The initial biomass also influences the oxygen content, as can be seen with the difference in BC-CS compared to BC-RH and BC-CM. In general, BC-CS had lower oxygen content; however, no general trend was observed with regard to oxygen content and pyrolysis conditions, and in some materials the oxygen content increased slightly and then decreased, and in others it decreased slightly and then increased. The changes observed in oxygen content at different retention times may be due to secondary reactions that were terminated prematurely at the end of the experiment, or different levels of volatile compound removal during pyrolysis [36].

The H/C and O/C ratios are other pieces of information that can be used in the characterization of the biochars. High temperatures and prolonged pyrolysis increase carbon content and reduce H/C and O/C ratios, thereby enhancing the stability of biochar [36]. The H/C ratios in BC-CS (avg. 0.152 ± 0.01) were relatively lower than the ratios recorded in BC-RH (avg. 0.228 ± 0.03) and BC-CM (avg. 0.24 ± 0.01). The higher aromaticity, indicated by lower H/C ratios, suggests a more stable and durable biochar that is less prone to degradation [37]. Higher O/C ratios were observed in BC-RH (avg. 0.504 ± 0.04) and BC-CM (avg. 0.45 ± 0.13) compared to that of BC-CS (avg. 0.042 ± 0.03). Higher O/C ratios suggest the presence of more oxygen-containing functional groups, such as the hydroxyl, carbonyl, and carboxyl groups. These functional groups play a significant role in the adsorption of pollutants, as they provide active sites for binding through hydrogen bonding, π-π interactions, and electrostatic interactions [38]. The presence of these groups enhances the biochar’s capacity to adsorb cationic pollutants, which is essential for water treatment applications.

The final observation that can be made from the elemental analysis in Table 1 is the nitrogen content. BC-CM (avg. 1.3 ± 0.308) had a much higher nitrogen content compared to those of BC-CS (avg. 0.76 ± 0.207) and BC-RH (avg. 0.42 ± 0.239). Nitrogen is a critical nutrient for plant growth, and biochar with higher nitrogen content can contribute to improved soil fertility [39,40,41,42]. The results obtained from the elemental analysis can be explained by the statement made by Ortiz (2020) et al., which states that different feedstocks and pyrolysis conditions, such as temperature and time, significantly affect the final elemental composition and functionality of biochar [40].

3.2. Pore Properties of the Resulting Biochars

The results seen in

Table 2. BET surface area and pore properties of the biochars.

Biochar Product	BET Surface Area (m2/g)			Total Pore Volume (cm3/g)		Pore Diameter (nm)
	SBET	Smicro	Sext	Vt	Vmicro	4Vt/SBET
BC-CS-800-0	81.56	69.79	11.76	0.035	0.003	2.5
BC-CS-800-15	197.38	179.54	17.84	0.099	0.09	2.01
BC-CS-800-30	90.32	78.35	11.978	0.040	0.041	2.11
BC-CS-800-45	174.51	162.27	12.23	0.087	0.081	1.99
BC-CS-800-60	149	147.23	1.77	0.076	0.076	2.04
BC-RH-800-0	64.34	52.88	11.46	0.049	0.031	2.77
BC-RH-800-15	197.26	178.66	18.6	0.119	0.095	2.08
BC-RH-800-30	187.8	176.77	11.03	0.142	0.093	2.18
BC-RH-800-45	220.62	196.62	23.99	0.127	0.014	2.18
BC-RH-800-60	194.4	171.37	23.03	0.114	0.092	2.14
BC-CM-800-0	195.64	165.4	30.24	0.113	0.086	2.31
BC-CM-800-15	251.96	200.64	51.31	0.145	0.102	2.31
BC-CM-800-30	233.52	177.64	55.88	0.138	0.092	2.36
BC-CM-800-45	118.31	91.74	26.58	0.089	0.048	2.71
BC-CM-800-60	263.3	169.55	93.75	0.164	0.086	2.49

show the BET surface area, total pore volume, and pore diameter of each biochar sample derived from the three biomass materials treated under the various experimental conditions. There was a general trend of the BET surface area and total pore volume increasing with increased retention time, peaking before slightly declining. This may suggest that optimal heat treatment enhanced porosity, while excessive treatment caused structural collapse [43]. For instance, BC-CS’s BET surface area increased from 81.56 m²/g at 0 min to 197.38 m²/g at 15 min, then dropped to 90.32 m²/g at 30 min, increased further to 174.51 m²/g at 45 min, and finally reached 149 m²/g at 60 min. A similar pattern is observed for BC-RH, where the BET surface areas were 64.34, 197.26, 187.8, 220.62, and 194.4 m²/g at the respective time intervals. BC-CM did not follow similar patterns as BC-CS and BC-RH; however, it exhibited the highest BET surface area and total pore volume, peaking at 263.3 m²/g and 0.1 cm³/g at 60 min retention time [44].

The changes in micropore volume were less significant across all samples, and pore diameter variations were minimal, indicating that the treatment parameters affected the pores’ quantity and volume more than their size. An example of this can be seen in the results, where BC-RH’s micropore volume goes from 0.03 cm³/g at 0 min to 0.09 cm³/g at 15 min, then slightly decreases to 0.01 cm³/g at 45 min. The minimal variations in pore diameter and significant changes in micropore volume highlight that the conditions primarily affected pore quantity and volume rather than size, suggesting that the structural integrity of the biochar was maintained while enhancing its adsorption capabilities [43]. The varying pore structure characteristics of biochars from different raw materials can provide a basis for selecting specific types of biochar tailored to particular applications [45]. It should be noted that different raw materials result in biochars with distinct pore structure characteristics under the same conditions. Appropriate heat treatment can significantly enhance the BET surface area and total pore volume of biochar, improving its adsorption performance. However, excessive heat treatment may cause pore structure collapse [46]. This variability implies that a one-size-fits-all approach to heat treatment may not be effective, necessitating tailored treatment protocols for each type of biochar [27].

In order to improve the readability of this paper, only the three biochars with the highest BET surface area from each respective material are presented and explained in the text, and the other N₂ adsorption–desorption plots are part of the Supplementary Materials, Data S3. Figure 3 presents the nitrogen adsorption–desorption isotherms for three biochar samples (BC-CS-800-15, BC-RH-800-45, BC-CM-800-60) at different relative pressures (P/P_o). Careful observation of the plots shows that the biochars exhibit Type I and IV isotherms, indicating microporous (pore width < 2 nm) and mesoporous (pore width 2–50 nm) characteristics [47]. BC-CM-800-60 shows the highest adsorption capacity out of the three samples, reaching a maximum of about 100 cm³/g, indicating the largest specific surface area and pore volume. BC-RH-800-45 presented a moderate adsorption capacity at about 70 cm³/g, while BC-CS-800-15’s was the lowest, at around 60 cm³/g. The lower adsorption capacity of these two biochars may be a result of the pore size distribution or the pore shapes in the material [48]. At low relative pressures, all samples show a rapid increase in adsorption, indicating the presence of microporous structures. This rapid increase in adsorption at low relative pressures suggests effective microspore development, a crucial feature for high adsorption efficiency, as supported by Tan et al. (2021) [36]. In the mid to high relative pressure range (P/P_o = 0.5–0.99), BC-CM-800-60 continued to show increasing adsorption, displaying a significant upward trend at high pressures, which suggests the presence of a large mesoporous structure [49]. BC-RH-800-45 exhibited relatively smaller changes in adsorption in the mid to high pressure range, indicating that it is primarily microporous and suitable for adsorbing small molecular pollutants [47]. BC-CS-800-15 followed a similar pattern to the RH biochar across the changes in pressure, albeit at a lower adsorption capacity. Another significant observation that can be made from the isotherm plots is the open-end hysteresis loop. Out of the three samples, only BC-CM-800-60 made an attempt to close the loop around the mid-point pressure (P/P_o = 0.5). Maziarka (2021) et al. explained that open hysteresis is common for N₂ adsorption–desorption plots of biochar materials, and one of the reasons they gave is that N₂ adsorption occurs at 77 K, which causes strong stress on the pores; this stress causes deformation during measurement, which results in open hysteresis. In order to resolve this issue, CO₂ adsorption at 273 K and N₂ adsorption at 77 K can be used in conjunction with each other to eliminate the bias in measurements [50].

3.3. Sorption Kinetic Performances of Resulting Biochar

The images presented in Figure 4, Figure 5 and Figure 6 show the removal efficiency of methylene blue (MB) using different doses of biochar (0.1 g, 0.3 g, and 0.5 g) over a period of one day. Close observation of the plots shows that there was a rapid decline in MB concentration for the cow manure biochar (BC-CM) within a short time (0.3 days); this decline suggests an abundance of adsorption sites. The rice husk biochar (BC-RH) had the second fastest decline, followed by the coconut shell biochar (BC-CS). These results indicate the effectiveness of the biochar’s surface characteristics in adsorption [51]. Based on the results, it can be seen that the 0.5 g dose showed the greatest removal efficiency, suggesting that higher biochar doses enhance adsorption. During the first 0.6 days, a faster decline in MB concentration is observed, indicating swift initial adsorption due to higher initial MB levels and abundant active sites on the biochar. From 0.6 days onward, the removal rate decreased slowly, likely due to the saturation of adsorption sites. The rapid decline in MB concentration for BC-CM within a short time frame can also be attributed to the biochar’s higher surface area and pore volume. BC-CM’s superior adsorption compared to BC-RH’s and BC-CS’s capacity is consistent with its higher BET surface area and total pore volume, as shown in Table 2. The adsorption process observed in this experiment generally followed three stages, as explained in the literature: (1) an initial rapid adsorption phase, (2) a slower adsorption phase, and (3) an equilibrium phase. During the equilibrium phase the adsorption capacity may have slowed down due to the gradual filling of micropores and mesopores, as well as the slow diffusion of MB molecules into the pore channels. This study applied the pseudo-second-order kinetic model to effectively describe the adsorption process using the following equation [47]:

$$\mathrm{t}/q_t=1/\left(k\times q_e^2\right)+\left({1/q}_e\right)\times t$$

(3)

where q_t is the amount of MB adsorbed at time t (mg/g), q_e is the equilibrium adsorption capacity (mg/g), k is the rate constant (mg/(g·min)), and t is the time (minutes). Additionally, the time required for the adsorbent (i.e., BC-CS-800-60) to adsorb half of the adsorbate (i.e., MB) at equilibrium (i.e., t = t_1/2 for q_t = q_e/2) can be determined using the following equation:

$$t_{1/2}=1/(k \times q_e)$$

(4)

The data indicate that increasing the biochar dose improves MB removal effectiveness, demonstrating dose dependence and time influence. The limitedness of the presented results may be because of the controlled conditions under which the experiment was conducted. The adsorption performance of biochar in real-world applications may be influenced by environmental conditions such as temperature, pH, and the presence of coexisting ions [52]. These experiments were conducted under controlled conditions, which may not fully represent the complexities of actual environmental settings [53]. Additionally, the observed phenomenon where increased adsorbent doses lead to decreased equilibrium adsorption capacity requires further investigation. This may be due to the saturation of adsorption sites, or interactions between biochar particles [54].

3.4. Adsorption Kinetics Parameters

Table 3. Kinetic parameters for methylene blue (MB) adsorption onto adsorbent at initial MB concentrations, based on pseudo-second-order model.

	Initial MB Concentration (mg/L)	k (g/mg·min)	qe (mg/g)	Correlation Coefficient	t 1/2 (min)
BC-CS-800-15	1 mg/L	0.725	0.108	0.998	12.77
	5 mg/L	0.253	0.220	0.968	17.99
	10 mg/L	0.118	0.340	0.991	24.97
BC-RH-800-45	1 mg/L	0.024	1.552	0.994	27.19
	5 mg/L	0.077	1.200	0.998	10.85
	10 mg/L	0.0601	1.145	0.956	14.54
BC-CM-800-60	1 mg/L	0.171	0.378	0.996	15.46
	5 mg/L	0.935	1.042	0.955	1.03
	10 mg/L	0.101	2.180	0.997	4.53

Adsorption conditions: adsorbent dosage = 0.5 g/0.1 L, agitation speed = 100 rpm, and temperature = 25 °C.

and

Table 4. Kinetic parameters for methylene blue (MB) adsorption onto adsorbent at various adsorbent dosages, based on pseudo-second-order model.

	Adsorbent Dosage (g/0.1 L)	k (g/mg·min)	qe (mg/g)	Correlation Coefficient	t 1/2 (min)
BC-CS-800-15	0.1 g	0.017	1.049	0.683	56.40
	0.3 g	0.176	0.330	0.979	17.24
	0.5 g	0.253	0.216	0.968	18.32
BC-RH-800-45	0.1 g	0.276	2.472	0.987	1.47
	0.3 g	0.020	2.767	0.997	17.89
	0.5 g	0.058	4.189	1.000	4.09
BC-CM-800-60	0.1 g	0.007	3.946	0.989	38.99
	0.3 g	0.011	2.107	0.979	42.00
	0.5 g	0.007	2.800	0.955	51.76

Adsorption conditions: initial MB concentration = 5 mg/L, agitation speed = 100 rpm, and temperature = 25 °C.

present the adsorption kinetics parameters for methylene blue (MB) at different initial concentrations (1 mg/L, 5 mg/L, and 10 mg/L) and a fixed biochar dose (0.5 g) (Table 3), and at a fixed initial concentration (5 mg/L) with varying adsorbent doses (0.1 g, 0.3 g, and 0.5 g) (Table 4). Observation of the data shows that as the initial concentration increased, the adsorption rate constant (k) generally decreased, particularly in BC-CS-800-15, indicating a slower adsorption rate at higher concentrations. Simultaneously, the equilibrium adsorption capacity (q_e) increased with higher concentrations, suggesting that more adsorption sites were available at higher concentrations. BC-RH-800-45 and BC-CM-800-60 exhibited significant increases in equilibrium adsorption capacity at higher initial concentrations, reaching capacities of 1.145 and 2.180, respectively, demonstrating their superior adsorption capabilities [50]. Additionally, the correlation coefficients are high across all concentrations (0.956 to 1.000), indicating a good fit of the adsorption data to the kinetic model [55].

In comparison, the effect of varying adsorbent doses at a fixed initial concentration (5 mg/L), as seen in Table 4, also shows a notable impact on adsorption performance. As the adsorbent dose increased, the adsorption rate constant (k) typically increased, while the equilibrium adsorption capacity (q_e) showed more complex variations, likely due to the saturation effect of the adsorption sites. BC-RH-800-45 achieved the highest equilibrium adsorption capacity (q_e = 4.189) at a high dose (0.5 g), indicating the strongest adsorption capacity [56]. Conversely, BC-CS-800-15 showed a decreased adsorption performance across all doses, which may be attributed to its diminished surface area, surface chemistry, and pore structure [57]. Furthermore, it was observed that increasing the adsorbent dose generally lead to a decrease in the half-life (t_1/2), indicating faster adsorption rates at higher doses [58].

These results support the potential of these biochars for environmental pollution control, especially BC-RH-800-45 and BC-CM-800-60, which exhibited decent adsorption performance, particularly at higher initial concentrations and doses, with significant increases in equilibrium adsorption capacity and faster adsorption rates [24]. The data suggest that the physical properties of biochar, such as its surface area and pore structure, along with its surface chemical properties, like the presence of functional groups, significantly influence its adsorption performance [59].

4. Conclusions

• This study demonstrates the significant impact of pyrolysis conditions on the pore structure and adsorption capabilities of biochar derived from coconut shell (BC-CS), rice husk (BC-RH), and cow manure (BC-CM).
• Optimal pyrolysis at 800 °C with varying residence times notably influenced the BET surface area, total pore volume, and pore diameter, with BC-CM exhibiting the highest BET surface area (263.3 m²/g) and pore volume (0.164 cm³/g).
• Residence time at elevated temperatures affect the various types of biochars differently. The optimum conditions of residence time are biomass-specific. Too short or too long residence times can cause the biochar’s pores to clog or collapse, leading to a reduction in surface area and pore volume.
• Nitrogen adsorption–desorption isotherms indicated that these biochars exhibit Type I and IV isotherms, characteristic of microporous and mesoporous structures, with BC-CM demonstrating the highest adsorption capacity.
• The adsorption kinetics of methylene blue (MB) followed a pseudo-second-order model, suggesting some form of chemisorption as the primary mechanism. BC-CM showed the highest adsorption efficiency, followed by BC-RH and BC-CS, and increasing biochar doses enhanced MB removal efficiency.
• The adsorption capacity of the biochar is correlated with not only the physical properties, such as surface area and pore volume, but also the chemical characteristics of the biochar’s surface.
• The presence of functional groups such as the hydroxyl, carbonyl, and carboxyl groups further improved adsorption performance.
• While controlled experimental conditions provided valuable insights, real-world applications may present challenges due to varying environmental conditions. Future research should validate these findings in practical settings and refine pyrolysis protocols to ensure consistent biochar performance.
• This study underscores the need to optimize pyrolysis parameters and feedstock selection to enhance biochar’s effectiveness in environmental remediation applications.