Approximate Adsorption Performance Indicator in Evaluating Sustainable Bamboo-Derived Adsorbents for Biogas Upgrading

Abstract

Purifying biogas by removing contaminants and carbon dioxide (CO₂) to produce biomethane enhances its energy content, making it suitable as fuel and for injection into natural gas grids. Bamboo-derived adsorbents Bamboo-500 (pyrolyzed at 500 °C), Bamboo-700 (pyrolyzed at 700 °C), and Bamboo-A-900 (activated with CO₂ at 900 °C) were synthesized and characterized to evaluate their performance for CO₂ and CH₄ adsorption. Increasing pyrolysis temperature from 500 °C to 700 °C and further CO₂ activation at 900 °C enhanced adsorption capacities of CO₂ and CH₄ due to improved surface area and micropore structure. In this study, the novel Approximate Adsorption Performance Indicator (AAPI) approach is introduced, offering an efficient method for evaluating adsorbent performance, particularly in biogas upgrading. AAPI results suggest Bamboo-500 is suitable for biogas upgrading at very low pressures (<0.12 MPa) with low regeneration energy and acceptable CO₂ capacity (1.9 ± 0.2 mol kg⁻¹). However, Bamboo-A-900 excelled at medium and high pressures by its highest CO₂ adsorption capacity (8.0 ± 0.3 mol kg⁻¹) promoted by the high surface area (1220 m²g⁻¹) and calcium oxide presence. Finally, Bamboo-A-900 shows promise for enhancing CO₂ adsorption and biogas upgrading. Bamboo-derived adsorbents offer a sustainable solution for biogas upgrading, supporting Sustainable Development Goals by promoting clean energy transitions.

1. Introduction

Over time, carbon dioxide (CO₂) has gained recognition as a significant greenhouse gas contributing to global warming [1]. Concerns about energy security and reducing the environmental effect of fossil fuels while raising living standards have led to the development of renewable energy sources in this context [2,3]. Bioenergy, specifically biogas produced by anaerobic digestion of biomass wastes, stands out as a potential option [4,5]. Biogas, produced in an oxygen-free environment, consists primarily of methane (CH₄) (50–75%) and CO₂ (25–50%), with traces of other gases such as water vapor, ammonia (NH₃), hydrogen sulfide (H₂S), siloxanes, halogenated hydrocarbons, etc. Under standard conditions, CH₄ has a calorific value of 36 MJ m⁻³, which is higher than the 21 MJ m⁻³ of raw biogas [6]. Purifying biogas to biomethane through eliminating impurities and CO₂ enhances its energy content and purity, making it ideal for various uses such as power generation, vehicle fuel, and injection into natural gas grids.

Adsorption has become an attractive CO₂ upgrading method due to its eco-friendliness, low energy needs, safe operation, removal efficiency, and flexible design [7,8]. A variety of materials have been studied for CO₂ adsorption and biogas upgrading, such as activated carbons [9,10,11,12,13], biochars [14,15,16,17], carbon molecular sieves [18,19], metal–organic frameworks [20,21], and zeolites [22,23]. Although metal–organic frameworks and zeolites have large surface areas that are suitable for CO₂ adsorption, their use has been limited due to their high production costs and affinity for moisture [24,25]. Biochar is usually synthesized through dry pyrolysis at temperatures of 400–900 °C under inert conditions; thus, biochars’ porous structure is primarily controlled by carbonization temperature. Raw biomass has high aliphatic content and low aromaticity, reflected in its H/C and O/C ratios of 1.4–1.8 and 0.55–0.75, respectively, which decrease during carbonization, particularly at higher temperatures [26,27]. Biochar’s limited CO₂ adsorption capacity is due to its low surface area and surface functional groups [28].

Recent studies have concentrated on developing biochars and activated biochar from biomass wastes, due to their potential for cost-effectiveness and waste revalorization. Efficient use of biomass-derived adsorbents is not only promising for biogas upgrading, but it also supports a number of the Sustainable Development Goals (SDGs), especially SDG 7 and SDG 12, which promote clean energy and responsible consumption of renewable biomass resources [29]. Bamboo waste stands out as an abundant, renewable, and cost-effective agroindustrial residue. Distributed widely across tropical and subtropical regions, bamboo encompasses nearly 1500 species in 87 genera. Its high productivity (10–40 t ha⁻¹ year⁻¹) and substantial biomass density (300–900 kg m⁻³) make it a globally sustainable resource for bioenergy, with reduced logistics costs that enhance its economic feasibility [30,31]. Biochars and activated biochars produced from bamboo wastes have been used for a variety of applications, including the removal of nitrogen oxides [32], sulfur dioxide, dyes, and phenol formaldehyde [33], volatile organic compounds [34], and carbon dioxide [35,36].

Researchers have increasingly focused on enhancing CO₂ adsorption capacity and selectivity on porous carbon materials. Many research works have studied different bamboo activation methods and their effect on the CO₂ adsorption capacities. The maximum adsorption capacity of 4.1 mol kg⁻¹ was attained by chemically activating bamboo powder with K₂CO₃ [37], whereas the maximum adsorption capacity of 3.4 mol kg⁻¹ was obtained by carbonizing bamboo under N₂ and physically activating it with CO₂ [38]. Additionally, bamboo materials that were carbonized and activated with various concentrations of NaNH₂ demonstrated a good capacity of 5 mol kg⁻¹ at 0 °C and about 4 mol kg⁻¹ at 25 °C [39]. Furthermore, bamboo activated with a 50% w/v H₃PO₄ solution showed an adsorption capacity of up to 9 mol kg⁻¹ [35]. Chemical activation techniques typically result in better CO₂ adsorption efficiencies than physical activation techniques [40].

Despite the fact that bamboo-derived adsorbents have been used in numerous studies, no research has investigated their potential as raw biomass for upgrading biogas by capturing its primary constituents (CO₂ and CH₄). The present study aims to address this gap by comparing and assessing the effectiveness and adsorption performance of bamboo-derived biochars and activated biochar for CO₂ and CH₄. Raw bamboo was chosen for its abundance in Ecuador, its origin, and its widespread use in sustainable housing construction, which generates significant waste and drives the need for new valorization pathways. This study seeks to answer key questions, such as, how do surface properties and pore structure of bamboo-derived adsorbents influence their CO₂ and CH₄ adsorption performance? What is the impact of pyrolysis temperature and activation methods on their adsorption efficiency? An indicator known as the Approximate Adsorption Performance Indicator (AAPI) was developed to assess the adsorption performance of these materials based on the identification of the three main adsorption parameters: adsorption capacity, approximate selectivity, and heat of adsorption [41]. Furthermore, a range of analytical techniques were used to characterize the materials, ensuring a comprehensive evaluation of their adsorption properties. This approach aims to offer insightful information about the potential of bamboo-derived materials for enhancing biogas upgrading processes and advancing sustainability efforts.

2. Materials and Experimental Apparatuses

2.1. Raw Biomass, and the Preparation of Biochars and Activated Biochars

The initial precursor material, bamboo, was sourced from Yachay Tech University in Ecuador, where bamboo is widely used, aligning this research with efforts to revalorize waste materials. Three adsorbents were used for comparison and evaluation: one activated biochar produced at 900 °C and two pyrolyzed biochars at 500 and 700 °C (see Figure 1); these temperatures were chosen considering that they were the most effective range to obtain ideal textural properties, as they facilitate the development of a well-defined pore structure, which is important for enhancing adsorption capacity [42,43,44,45]. Physical activation with CO₂ was used because it is less expensive than chemical activation. To produce the biochar “Bamboo-500”, a quantity of raw bamboo on a dry base was subjected to a pyrolysis process up to 500 °C in an oxygen-limited environment, resulting in a mass loss of 59.1%. After that, raw bamboo was pyrolyzed at 700 °C in an argon (Ar) atmosphere at a flow rate of 10 L h⁻¹ ( identical to the conditions used for producing Bamboo-500) to produce Bamboo-700, with a mass loss of 68.7%. Pyrolysis products yield varied with increasing the temperature from 500 to 700 °C (

Table 1. Product yields of bamboo biochars obtained by pyrolysis.

Biochars	Biochar Yield (%)	Tars Yield (%)	Gas Yield (%)
Bamboo-500	29.4	32.3	38.3
Bamboo-700	27.5	59.6	12.9

). Biochar yield decreased by increasing the pyrolysis temperature; this is because at higher temperatures, there was a rapid and comprehensive decomposition of lignocellulosic components that resulted in a reduction in biochar yield. Finally, Bamboo-700 was activated with CO₂ for 120 min at 900 °C and a flow rate of 12 L h⁻¹ (heating and cooling were carried out under Ar). The activated biochar obtained from this process is named Bamboo-A-900, with a mass loss of 40.2%.

2.2. Physico-Chemical Characterizations of Bamboo Family

The bamboo family’s physico-chemical characteristics were examined using the same experimental setups under the same experimental conditions as our earlier investigation on the cocoa pod husk family [14]. Scanning Electron Microscopy (SEM) was employed to analyze the carbon morphology and pore structure of the bamboo materials, with micrographs captured using a FEI Quanta 400 microscope (Eindhoven, The Netherlands) and chemical compositions provided using an INCA x-act detector for Energy Dispersive X-ray (EDX) spectroscopy (Oxford Instruments, High Wycombe, UK). Multiple EDX acquisitions were performed over large areas, and the average values from 4 or 5 different zones were used for the final results. X-ray powder diffraction (XRD) was performed using a Bruker D8 Advance A25 diffractometer (Bruker, Karlsruhe, Germany), generating diffraction patterns by scattering X-rays through the crystal lattice, providing insights into the materials’ crystalline phases. Thermogravimetric analysis (TGA) using a METTLER TOLEDO TGA/DSC3+ (Mettler Toledo, Greifensee, Suisse) yielded information on the bamboo family’s thermal stability and breakdown temperatures through regulated heating procedures using nitrogen and air flows. For TGA runs, Bamboo-500 was repeated three times to check equipment consistency, with identical results but the other two samples were measured once. Furthermore, N₂ physisorption tests on an ASAP 2420 instrument (Micromeritics, Atlanta, GA, USA) at 77 K measured the specific surface area and pore volumes of bamboo materials. These samples were degassed to eliminate residual molecules, and the Brunauer–Emmett-Teller (BET) method used to determine the N₂ specific surface area and Dubinin–Radushkevich model applied to determine total, micro, and mesopore volumes. These analyses, which provide a comprehensive understanding of the physical properties of materials. Pore size distribution was analyzed using N₂ adsorption isotherms, employing two-dimensional nonlocal density functional theory (2D-NLDFT) with SAIEUS software (version 3.2, Micromeritics) to derive the pore size distribution and average pore width (L₀).

2.3. Adsorption Characteristics Set-Ups

2.3.1. Calorimetry Experiments

High-pressure differential scanning calorimeter (Setaram micro DSC-VII) was utilized to determine the heat of CO₂ adsorption of bamboo materials. The preparation protocol has been thoroughly described in a previous work [41]. Samples, approximately 70 mg, were weighed and introduced into the measurement cell, and the temperature program was set accordingly. Nitrogen served as a purge gas throughout the process, with a regeneration step initially removing impurities by vacuum and heating at 373 K for 24 h. Pure CO₂ was then introduced at 303 K until system pressure stabilized in the pressure range 0.1–2 MPa. By dividing the integral heat by the CO₂ adsorption capacity, the heat of adsorption (kJ mol⁻¹ of CO₂) was obtained.

2.3.2. Gas Adsorption Isotherm

Pure gas adsorption isotherms were obtained using a homemade high-pressure manometric setup [46,47,48,49] and involved an initial 24-h outgassing phase at 373 K under vacuum pressure (10⁻² Pa), after that, the adsorption isotherm settles at the study temperature (303 K). Measurements were performed using a fixed adsorbent mass of 1 g. All measurements were conducted on samples from the same preparation batch to ensure consistency and reproducibility. Following a series of helium expansions from the dose volume (${\mathrm{V}}_d$) to the adsorption volume (V_ads), the accessible volume in the adsorption cell was calculated. Helium traces were eliminated using a 4-h vacuum. Once the dead-space volume was obtained by helium calibration, a quantity of pure gases (CH₄ or CO₂) was introduced into the dosing cell, and the pressure was registered upon reaching equilibrium (constant pressure). Subsequently, the gas was expanded into the adsorption cell, and the pressure was recorded when equilibrium was achieved. The amount of gas adsorbed into the adsorbent for the first point is calculated using the following equation:

$$n_{ads}^1(T,P_1)={\displaystyle \frac{1}{m_{ads}}}((V_d{\rho }_i^1)-((V_d+V_{ads}){\rho }_f^1))$$

(1)

The adsorption isotherm was then performed using an accumulative approach, increasing the pressure by approximately 5 bar in between measurements. The following formula was used to calculate the gas’s adsorption capacity at step i:

$$n_{ads}^i(T,P_i)={\displaystyle \frac{1}{m_{ads}}}((V_{ads}{\rho }_f^{i-1}+V_d{\rho }_i^i)-((V_d+V_{ads}){\rho }_f^i))$$

(2)

where m_ads (g) is the mass of the adsorbent, ${\rho }_i$ and ${\rho }_f$ (g cm⁻³) are the molar density at initial and final pressures respectively, $n_{ads}^1$ and $n_{ads}^i$ (mol kg⁻¹) are the excess adsorbed quantities for the first point and the latter points, respectively, ${\mathrm{V}}_d$ (cm³) is the dosing cell volume. To ensure the reliability of the results, repeatability was conducted by performing each measurement two times, and any uncertainty was quantified by calculating the standard deviation from the two repetitions.

2.4. Approximate Adsorption Performance Indicator

Several methods have been proposed to evaluate the performance of adsorbents using both pure component and gas mixture isotherms [50,51,52,53,54,55]. The core concept of this study is based on the use of Approximate Adsorption Performance Indicator (AAPI), which was previously applied in our recent work [41] to assess the adsorption performance of the bamboo family. This indicator is adapted from Wiersum et al.’s [54] Adsorption Performance Indicator, which relies on gas mixture isotherms. In this context, AAPI is based on pure gas adsorption isotherms and takes into account the approximate selectivity (AS), adsorption capacity of CO₂, and heat of adsorption of CO₂, can be expressed as follows:

$$AAPI={\displaystyle \frac{{(A{S}_{C{O}_2/C{H}_4}-1)}^A{n}_{ads,C{O}_2}^B}{|\mathsf{\Delta }{H}_{ads,C{O}_2}{|}^C}}$$

(3)

where AS of CO₂ over CH₄ molecules is calculated as follows:

$${AS}_{C{O}_2/C{H}_4}={\displaystyle \frac{n_{ads,C{O}_2}}{n_{ads,C{H}_4}}}$$

(4)

${\mathrm{n}}_{ads,C{O}_2}$ and ${\mathrm{n}}_{ads,C{H}_4}$ denote the adsorption capacity of the stronger adsorbate (CO₂) and CH₄, respectively, and $\mathsf{\Delta }{H}_{ads,C{O}_2}$ signifies the heat of adsorption of CO₂. The exponents corresponding to the approximate selectivity, adsorption capacity, and heat of adsorption are denoted by parameters A, B, and C, respectively. The exponent “B = 2” is used to emphasize the parameter “adsorption capacity” because the objective is to separate CO₂ from CH₄, while A and C (see Equation (3)), are set to 1. The exponent B was chosen to be 2 to achieve a balanced weighting between adsorption capacity, approximate selectivity, and heat of adsorption. A sensitivity analysis varying B from 1 to 3 showed that when B = 1, the AAPI gave more weight to selectivity and heat of adsorption, causing adsorbents with lower adsorption capacity but higher selectivity to rank higher. In contrast, when B = 3, more emphasis was placed on adsorption capacity, shifting the rankings toward adsorbents with higher CO₂ capacity, while reducing the importance of selectivity and heat of adsorption.

3. Results and Discussions

3.1. Bamboo Family Characterization

3.1.1. SEM Analysis

SEM images were obtained at various magnification scales (500 μm, 200 μm and 100 μm) to investigate the morphological changes in bamboo samples after carbonization (Bamboo-500, Bamboo-700) and subsequent activation (Bamboo-A-900) (Figure 2). The materials exhibit a fibrous structure with long, wide fibers composed of several fused thinner fibers. While SEM images of the raw bamboo are not available in this study as it focused on the characterization of processed materials, previous analysis by Sahoo et al. [56] reported an almost negligible presence of pores in raw bamboo. The materials present a fibrous structure with long fibers which are wide due to the fact that they are composed of several fused thinner fibers. Bamboo-500 exhibited a sheet-like structure with few pores due to limited thermal decomposition. Conversely, the transition from 500 °C to 700 °C yielded Bamboo-700, which, along with Bamboo-500, exhibited a honeycomb-like structure with similar macroporosity. However, Bamboo-A-900 induced significant cracking in bamboo structure, enhancing porosity and surface texture, result from thermal stress, chemical reactions, and gas release during activation. At a magnification scale of 200 μm, long fibers changed their shape resembling the form of curved fibers due to the high burn-off rates and considerable carbon removal during activation. Ji et al. [57] found that biochar derived from bamboo displays a denser porous structure and less pore distribution compared to bamboo activated with KOH. CO₂ activation, particularly, induces substantial structural alterations, making it a promising method for porous structure generation [58]. Such macropores can be beneficial for gas diffusion towards micropores, enhancing therefore the adsorption kinetics.

3.1.2. EDX Analysis

EDX analysis of the bamboo family identified various components, notably carbon (C) and oxygen (O), alongside silicon (Si), potassium (K), calcium (Ca), etc. (

Table 2. Chemical and textural properties of the bamboo family.

Elemental Analysis by EDX (wt.%)	Raw Biomass	Bamboo-500	Bamboo-700	Bamboo-A-900
Carbon	n.a	71.1	75.9	68.5
Oxygen	n.a	23.1	17.3	20.2
Silicon	n.a	3	4.1	7.5
Potassium	n.a	1.7	1.4	2.2
Calcium	n.a	0.3	0.6	0.4
Sulfur	n.a	0.2	0.3	0.5
Magnesium	n.a	0.3	0.2	0.4
Phosphorus	n.a	0.1	0.1	0.1
Aluminium	n.a	0.2	-	-
Copper	n.a	-	0.1	0.1
Manganese	n.a	-	-	0.1
Total Organic Elements	n.a	94.5	93.6	89.3
Total Inorganic Elements	n.a	5.5	6.4	10.7
Proximate analysis (wt.%)
Moisture	7.2	5.2	7.4	n.a
Volatiles	75.1	16.1	5.1	n.a
Fixed carbon	13.4	65	74.5	n.a
Ash	4.3	13.7	13	34.1
Textural properties
${\mathrm{S}}_{BET}$ (N2) (m2g−1)	n.a	n.a	365	1220
Vt (cm3 g−1)	n.a	n.a	0.16	0.6
${\mathrm{V}}_{micro}$ (cm3g−1)	n.a	n.a	0.09	0.34
${\mathrm{V}}_{meso}$ (cm3g−1)	n.a	n.a	0.07	0.26
${\mathrm{D}}_{pore}$ (nm)	n.a	n.a	0.96	0.73

). Varying the pyrolysis temperature and activating the biochar induces changes in chemical composition, as supported by the literature [59,60]. Bamboo-500 exhibited a high carbon content (71.1%), which is attributed to bamboo’s substantial cellulose mass fraction (47.7%) [61]. Transitioning from Bamboo-500 to Bamboo-700 removes volatile compounds and enhances carbonization, elevating carbon content from 71.1% to 75.9% [62,63]. This rise in the carbon mass fraction is caused by the decrease of (-OH) surface functional groups during dehydration and the development of denser carbon structures through polymerization processes [64]. Higher pyrolysis temperatures tend to produce more stable biochars because of their condensed aromatic character [65]. Similar trends in elemental compositions of biochars at various temperatures are reported in literature [66,67]. Oxygen mass fraction reduced in Bamboo-700 from 23.1 to 17.3 % due to gas and vapor phase release, including H₂, CO, CO₂, hydrocarbons, and water [65,68], consistent with Yu et al.’s findings [56,69]. Changes in biochar chemical composition after CO₂ activation at 900 °C arise from several factors: the high activation temperature facilitates the decomposition and gasification of carbonaceous material, reducing the carbon content from 75.9% in Bamboo-700 to 68.5 wt.%, while introducing functional groups containing oxygen (O increases from 17.3% to 20.2%), which could improve adsorption properties. The rise in oxygen content can also be attributed to the increased presence of inorganic matter (Si, K, Ca), as these elements are typically bonded with oxygen in forms such as oxides and carbonates. This is further supported by the observed increase in total inorganic content, which grew from 6.4% to 10.7% (see Table 2). Carbon-based material decomposition and subsequent reaction with CO₂ during activation may be the source of the oxygen-containing groups (hydroxyl, carboxyl, or carbonyl groups) that are incorporated onto the bamboo surface [70]. Silicon (Si) dominates in Bamboo-A-900 (7.5%) which can be confirmed by the increase in the ash content in TGA analysis (Table 2), in the form of Cristobalite (SiO₂), attributed to selective carbon component removal or enhanced silicon retention in biochar matrix.

3.1.3. TGA Analysis

Initially, raw bamboo has a high volatile content (75.1%), primarily stemming from decomposition of organic compounds like cellulose (300–400 °C) and hemicellulose (220–315 °C), which vaporize at relatively low temperatures [71]. Upon pyrolysis at 500 °C, moisture decreases from 7.2% to 5.2%, and volatile content diminishes from 75.1% to 16.1% compared to untreated bamboo (Figure 3a and Table 2). This reduction reflects the evaporation of water at low temperatures (<120 °C) alongside the thermal breakdown of organic matter, leading to an increase in fixed carbon content from 13.4% to 65% and ash content from 4.3% to 13.7% (Figure 3b), yielding a carbon-rich residue. However, an increase in the moisture content from 5.2% to 7.4% was observed with an increase in pyrolysis temperature from 500 °C to 700 °C due to the improvement of porosity (S_BET) (Table 2) favoring physisorption phenomena. Additionally, further decomposition of volatile organic compounds reduces volatile content from 16.1% to 5.1% [72,73], while fixed carbon content increases from 65% to 74.5% (Table 2). Interestingly, the increase in the pyrolysis temperature from 500 °C to 700 °C does not affect ash content (13%), as observed by Palniandy et al. [73]. Notably, Bamboo-A-900 exhibits a substantial increase in ash content, i.e., 34.1% compared to 13% in Bamboo-700, which can be attributed to the concentration of inherent minerals as carbon is removed during activation. This increase is accompanied by a rise in silicon content from 4.1% in Bamboo-700 to 7.5% in Bamboo-A-900 (Table 2).

3.1.4. XRD Analysis

Changes in peak intensities, positions, and widths in the XRD patterns among samples from the bamboo family indicate variations in crystallinity and structural transformations occurring during pyrolysis and activation processes (see Figure 4). XRD patterns of bamboo biochars (Figure 4) indicate presence of inorganic minerals such as SiO₂ ( Crystallography Open Database (COD) 9008110) (Cristobalite, 2$\theta$ = 20.86°, 45°) and CaO (COD 7200686) (lime, 2$\theta$ = 38°, 55°, 65°) for Bamboo-A-900, and CaCO₃ (COD 9014524) observed for Bamboo-500 and Bamboo-700 (calcite, 2$\theta$ = 22°, 43°, 80°). For both Bamboo-500 and Bamboo-700, two broad peaks indicate the presence of disordered-like carbon at 2$\theta$ values of 20–28° and 40–45°, with more prominent peaks observed in Bamboo-500 due to lower biomass undergoing thermal degradation [74,75], similar observations were seen by Jena et al. [76]. Bamboo-500 and Bamboo-700 exhibit disordered carbon structures, indicating that the pyrolysis of biomass does not favor graphitization. This is attributed to the strong cross-linking effects, which hinder the reorganization of graphene planes into ordered structures [77]. The two broad peaks observed in Bamboo-500 and Bamboo-700 are less observed, and sharper and higher intensity peaks appear in Bamboo-A-900 at the same 2$\theta$ values due to higher ash content (see Table 2) [78]. The sharp peaks observed are predominantly due to crystalline impurities, especially SiO₂, which become more pronounced at higher ash content. This suggests that the increased visibility of impurities occurs to the detriment of carbon [79]. The presence of CaCO₃ in Bamboo-500 and Bamboo-700 indicates the presence of Ca-based species within bamboo biomass precursor. The absence of CaCO₃ in Bamboo-A-900 suggests its thermal decomposition during pyrolysis, releasing CO₂ and leaving behind CaO as a residual product. Furthermore, identification of SiO₂ in Bamboo-A-900 at 900 °C suggests presence of silicon-containing compounds in bamboo biomass, which undergo thermal decomposition and transformation to silica during activation. Such silica may originate from various sources within bamboo, including phytoliths or silicate minerals present in biomass. In conclusion, SiO₂ emerges as a dominant crystalline phase indicated by the very high silicon content (7.5%) in Bamboo-A-900 (Table 2) with a small amount of CaO.

3.1.5. Textural Characterization

The textural properties of the bamboo materials underwent significant alterations due to pyrolysis and CO₂ activation (Table 2). The N₂ adsorption isotherms provide insights into the pore structures of Bamboo-700 and Bamboo-A-900. However, the N₂ isotherm for Bamboo-500 is not presented due to the absence of measurable pores, falling within the detection limit and error range of the instrument. According to the classification of the International Union of Pure and Applied Chemistry (IUPAC) [80], Bamboo-700 and Bamboo-A-900 both have isotherm shapes that are very similar to type I. This indicates rapid adsorption at low relative pressures (P/P₀ < 0.05), followed by a plateau that suggests monolayer adsorption in micropores [81]. However, the appearance of a type IV isotherm for Bamboo-A-900 at P/P₀ > 0.9, in which the isotherm deviates from the parallel shape with P/P₀ (Figure 5a), suggesting the presence of small mesopores (increase of ${\mathrm{V}}_{meso}$ from 0.07 (Bamboo-700) to 0.26 cm³ g⁻¹ (Bamboo-A-900), Table 2). Compared to Bamboo-700, Bamboo-A-900 has a greater N₂ adsorption capacity because it has larger pore volumes in the micropore and mesopore regions, probably due to the opening of inaccessible pores during oxidation [82]. Activation of Bamboo-700 raises the BET surface area from 365 to 1220 m² g⁻¹, micropore volume from 0.09 to 0.34 cm³ g⁻¹, mesopore volume from 0.07 to 0.26 cm³ g⁻¹, and total pore volume from 0.16 to 0.6 cm³ g⁻¹ (see Table 2). As a result, activation causes significant textural changes, making the material more porous and significantly increasing the surface area accessible for adsorption [72]. The larger surface area offers more active sites, making the material suitable for applications such as CO₂ adsorption, thus potentially improving adsorption efficiency [83].

The pore size distributions derived from the N₂ adsorption isotherms exhibit heterogeneous structures of porous materials. Analysis reveals that both Bamboo-700 and Bamboo-A-900 exhibit multiple modal distributions in the micropore region (pores smaller than 2 nm) and the mesopore region (pores ranging from 2 to 50 nm) (Figure 5b). In the micropore region, Bamboo-A-900 curve displays a prominent narrow peak at a pore size of 0.52 nm with a maximum pore volume of 0.066 cm³ g⁻¹, the presence of this low micropore strongly influence CO₂ adsorption at low partial pressures [84]; however, Bamboo-700 exhibits a bimodal distribution consisting of two narrow peaks, with a maximum peak located at a pore size of 0.99 nm and a pore volume of 0.009 cm³ g⁻¹ (Figure 5b). In the mesopore region, both Bamboo-700 and Bamboo-A-900 present distinct minor peaks at varying pore widths. Remarkably, the pore size distribution pattern of Bamboo-A-900 is comparable to that of activated biochar derived from tabah bamboo at 820 °C [85]. The average pore widths of Bamboo-700 (0.96 nm) and Bamboo-A-900 (0.73 nm) are particularly noteworthy as they both fall within the micropore range (Table 2). This is significant because these micropores directly influence the effectiveness of CO₂ and CH₄ capture and their adsorption processes within the bamboo family due to the fact that the diameters of CO₂ (0.33 nm) and CH₄ molecules (0.38 nm) align with the micropore range [8,86].

3.2. Heat of Adsorption

The heat of CO₂ adsorption of bamboo materials at 303 K as a function of pressure (Figure 6) signifies the strength of the adsorption interaction between CO₂ molecules and the material’s surface, indicating their affinity for gas molecules. The lower heat of CO₂ adsorption for Bamboo-500 (16.7 ± 0.6 kJ mol⁻¹) compared to Bamboo-700 (19.5 ± 0.8 kJ mol⁻¹) at 0.1 MPa (Figure 6) suggests weaker interactions at the lower pyrolysis temperature (500 °C), possibly due to limited surface area and fewer active adsorption sites. The decrease in volatile matter content from 16.1% to 5.1% (Table 2) as the pyrolysis temperature increases from 500 °C to 700 °C contributes to the increase of both surface area and pore volume [87]. Additionally, the comparable heat of CO₂ adsorption for Bamboo-A-900 (19.1 ± 0.3 kJ mol⁻¹) relative to Bamboo-700 (19.5 ± 0.8 kJ mol⁻¹) suggests that CO₂ activation has minimal effect on the strength of the adsorption interaction, despite differences in surface chemistry between the materials. The heat of CO₂ adsorption into Bamboo-500, Bamboo-700, and Bamboo-A-900 decreases from 16.7 ± 0.6, 19.5 ± 0.8, and 19.0 ± 0.3 kJ mol⁻¹ to 14.7 ± 1.1, 14.2 ± 1.3, and 16 ± 1 kJ mol⁻¹ in the 0.1–1.9 MPa pressure range, respectively. For all the samples, this heat is high in the low-pressure region due to the strong interaction between CO₂ molecules and the favorable active sites of the samples surface (monolayer adsorption). As pressure increases, the heat of adsorption had a tendency to decrease and stabilize because most accessible adsorption sites are occupied, weakening the interactions between adsorbent and CO₂ molecules [88]. As the heat of CO₂ adsorption obtained for the bamboo family remains below 40 kJ mol⁻¹, the predominant adsorption interactions appear to align with physisorption rather than chemisorption [89]. The dipole–dipole interactions between the adsorbent and CO₂ molecules are mainly governed by physical mechanisms such as Van Der Waals forces [90]. Moreover, Bamboo-500 with the lowest initial adsorption heat of 16.7 ± 0.6 has less energy consumption during the desorption process, making it more kinetically feasible to regenerate.

3.3. Adsorption Isotherms

From the pure gas adsorption isotherms shown in Figure 7, the CO₂ and CH₄ adsorption capacities of the bamboo family were calculated. The preferential adsorption of CO₂ over CH₄ for all the samples (AS > 1) relies on their physical and chemical properties. CO₂, being a smaller molecule (0.33 nm) with higher polarizability (29.1 × 10⁻²⁵ cm³) and quadrupole moment (−13.71 × 10⁻⁴⁰ cm²), diffuses more easily and interacts more extensively with surface functional groups on biochars [91]. These interactions include Van Der Waals forces and hydrogen bonding, which are less favorable for the larger CH₄ molecule (0.38 nm), which has lower polarizability (25.9 × 10⁻²⁵ cm³) and zero quadrupole moment [92].

AS was the highest in Bamboo-500 under the low pressure region due to the very low CH₄ adsorption capacity of 0.3 ± 0.1 mol kg⁻¹ at 0.1 MPa (Figure 7). This low capacity is attributed to the limited pore structure development resulting from the low pyrolysis temperature of 500 °C. However, in the high-pressure range, CH₄ is more compressed into the pores, resulting in lower AS for all the samples [93]. The increase in CO₂ and CH₄ adsorption capacities of bamboo pyrolyzed at 700 °C from 1.9 ± 0.2 mol kg⁻¹ to 3.8 ± 0.1 mol kg⁻¹ and from 0.8 ± 0.2 mol kg⁻¹ to 1.4 ± 0.2 mol kg⁻¹, respectively, compared to 500 °C at 1.9 MPa (Figure 7), can be attributed to changes in its physical and chemical characteristics. Higher CH₄ adsorption capacity on Bamboo-700 can be explained by a more porous structure with increased surface area and microporosity in this sample. The significant decrease in volatile matter from 16.1% to 5.1% (Table 2), enhancing CO₂ adsorption efficiency by reducing blockages of the pores. This suggests that the optimal pyrolysis temperature for producing bamboo-derived biochar with a higher surface area and appropriate pore structure is more than 500 °C.

On the other hand, Bamboo-A-900 exhibits improved CO₂ and CH₄ adsorption capacities (8.0 ± 0.3 and 3.4 ± 0.2 mol kg⁻¹, respectively) compared to Bamboo-700 (3.8 ± 0.1 and 1.4 ± 0.2 mol kg⁻¹, respectively) and Bamboo-500 (1.9 ± 0.3 and 0.8 ± 0.2 mol kg⁻¹, respectively) at 1.9 MPa (see Figure 7). This enhancement is linked to the increase in BET surface area from 365 m²g⁻¹ to 1220 m²g⁻¹, micropore volume from 0.09 to 0.34 cm³ g⁻¹, and mesopore volume from 0.07 to 0.26 cm³ g⁻¹ as indicated in Table 2, which offers additional adsorption sites for CO₂ molecules [38,94,95]. In addition, the presence of O-functional groups and CaO in Bamboo-A-900 enhances interactions with CO₂ molecules, making these factors particularly effective in the intermediate and high-pressure ranges. This results in increased CO₂ adsorption and contributes to higher selectivity. Based on the pore size distribution, Bamboo-A-900 shows the highest CO₂ and CH₄ adsorption capacities due to the presence of most pores in the micropore range of 0.5–0.7 nm and 1–2 nm (Figure 5b). For efficient separation of CO₂ and CH₄, the more critical pore size range is between 0.3–0.4 nm, as this range aligns with the molecular size of CO₂ and CH₄ (0.33 and 0.38 nm, respectively), which enhances selectivity for their separation [8]. A study showed that most CH₄ is adsorbed at a pore size distribution in the range of 0.5–1 nm [96], with similar findings observed in a recent study [85]. In addition, XRD analysis reveals the presence of CaO on the Bamboo-A-900 surface (Figure 4), which reacts with CO₂ under ambient conditions to form CaCO₃ [97], as shown in Figure 8, enhancing CO₂ adsorption capacity.

3.4. Approximate Adsorption Performance Indicator

AAPI was used to assess the preliminary performance of bamboo-derived adsorbents for separating CO₂ from CH₄, based on the adsorption of the pure gases CO₂ and CH₄. However, this preliminary evaluation does not fully represent the adsorption properties of the CO₂/CH₄ mixture, as gas interactions and competitive adsorption could influence the results. AAPI results indicate that the adsorption performance of the most effective adsorbent varies with pressure. At very low pressure (<0.12 MPa), Bamboo-500 is identified as the ideal adsorbent with a maximum value of 1.04 ± 0.02. This is primarily due to its highest AS (7.3 ± 0.7), lowest heat of CO₂ adsorption (16.7 ± 0.6 kJ mol⁻¹) and lowest CH₄ adsorption capacity (0.3 ± 0.1 mol kg⁻¹). These characteristics are attributed to its smaller surface area, lower porosity, and fewer favorable adsorption sites. Despite Bamboo-A-900 having the highest CO₂ adsorption capacity of 2.6 ± 0.1 mol kg⁻¹ at 0.12 MPa, its separation performance is reduced due to its higher CH₄ adsorption capacity (0.8 ± 0.1 mol kg⁻¹) and heat of CO₂ adsorption of 19.1 ± 0.3 kJ mol⁻¹. Therefore, activating the biochar appears unnecessary for CO₂ separation from CH₄ at very low pressures, reducing industrial process costs as shown in Figure 9. However, at pressures above 0.12 MPa, AAPI results indicate that Bamboo-A-900 becomes the most effective adsorbent, with an AAPI value of 5.5 ± 0.1, compared to 1.6 ± 0.2 for Bamboo-700 and 0.3 ± 0.1 for Bamboo-500 at 1.9 MPa (see

Table 3. Consolidated data from elemental analysis and adsorption parameters of bamboo-derived adsorbents at 1.9 MPa.

Parameter	Bamboo-500	Bamboo-700	Bamboo-A-900
Carbon (wt.%)	71.1	75.9	68.5
Oxygen (wt.%)	23.1	17.3	20.2
SBET (m2 g−1)	n.a	365	1220
AdC (CO2) (mol kg−1)	1.9 ± 0.3	3.8 ± 0.1	8.0 ± 0.3
AS	2.4 ± 0.3	2.6 ± 0.1	2.4 ± 0.2
$-\mathsf{\Delta }{H}_{ads,C{O}_2}$ (kJ mol−1)	14.7 ± 1.1	14.2 ± 1.3	16 ± 1
AAPI	0.4 ± 0.1	1.6 ± 0.2	5.4 ± 0.2

). This superior performance is due to Bamboo-A-900’s higher CO₂ adsorption capacity (8.0 ± 0.3 mol kg⁻¹), thanks to the highest BET surface area (1220 m²g⁻¹), increased micropores and mesopores, O-functional groups effect, and the presence of CaO, which enhances interactions with CO₂ molecules, as these factors become more effective in the intermediate and high pressure range. This approach improves the sustainability of renewable energy systems by enhancing the quality of biomethane, which can substitute fossil fuels, lowering greenhouse gas emissions and contributing to climate action and its impacts (SDG 13) in the SDGs [29].

3.5. Comparative Analysis of CO₂ Adsorption Capacities of All the Studied Biochars and Activated Biochars

A comparison of CO₂ adsorption capacities among various samples at 0.1 MPa (Table 4) reveals that the Bamboo-500 demonstrated a CO₂ adsorption capacity of 1.7 mol kg⁻¹, which is slightly lower than the 2.1 mol kg⁻¹ reported by Xie et al. [98] for a similar temperature, likely due to differences in surface area and preparation methods. This value is comparable to high-performing biochars like spent coffee grounds and walnut shell at the different pyrolysis temperatures [99,100]. However, Bamboo-700 exhibits an adsorption capacity of 2.1 mol kg⁻¹, outperforming other pristine biochars in the literature at the same pyrolysis temperature (see Table 4), indicating that higher pyrolysis temperatures for the same raw material enhance biochar’s adsorption capacity. Compared to other biochars such as sawdust and walnut shell [100,101], which show lower adsorption capacities ranging from 0.4 to 1.6 mol kg⁻¹ across various temperatures, Bamboo-700 demonstrate superior performance, highlighting its potential as effective CO₂ adsorbent.

For activated biochars, Bamboo-A-900 shows a CO₂ adsorption capacity of 2.6 ± 0.1 mol kg⁻¹ at 0.1 MPa, which is higher than the 2.0 mol kg⁻¹ reported by Khuong et al. [38] for bamboo biochar activated with CO₂ at 800 °C (see Table 4). This improvement is attributed to the increased surface area and enhanced pore structure resulting from higher activation temperatures, facilitating more effective CO₂ adsorption. However, chemically activating bamboo powder with K₂CO₃ achieved an adsorption capacity of 4.1 mol kg⁻¹ [37]. In addition, it can be observed that many activated biochars with lower surface areas demonstrate higher CO₂ adsorption capacity, indicate that functional groups play a significant role, i.e., Commercial kevlar with a S_BET of 1593 m²g⁻¹ has lower CO₂ adsorption capacity (1.5 mol kg⁻¹) compared to Bamboo-A-900 (2.6 mol kg⁻¹), despite its lower S_BET of 1220 m²g⁻¹. Also, bamboo samples carbonized and activated with varying concentrations of NaNH₂ exhibited an optimal capacity of 5 mol kg⁻¹ at 273 K and around 3.5 mol kg⁻¹ at 298 K [39]. Moreover, bamboo biochars activated with KOH show significantly higher adsorption capacities, such as 4.5 mol kg⁻¹ and 4.0 mol kg⁻¹ at 873 °C and 973 °C, respectively, as reported by Wei et al. [83]. This substantial increase is likely due to the creation of additional micropores and functional groups that enhance CO₂ affinity. Bamboo-A-900 exhibits a CO₂ adsorption capacity of 2.6 ± 0.1 mol kg⁻¹, comparable to the 2.7 mol kg⁻¹ for almond shells activated with CO₂ at 750 °C by Gonzalez et al. [102] and outperforming beech wood biochar, which has an adsorption capacity of 2.1 mol kg⁻¹ at 1087 °C by Zgrzebnicki et al. [103]. Further optimization, especially through chemical activation methods (using KOH, K₂CO₃, NaNH₂, etc.) could significantly enhance the performance of these biochars, as evidenced by the higher adsorption capacities reported in other studies [57,83].

Table 4. Comparison of CO₂ adsorption into biochars derived from various feedstocks at 0.1 MPa with their properties. T_ads: Adsorption temperature; AdC: Adsorption Capacity; n.a: not available.

Samples	Tprocess (°C)	Activating Agent	C (wt.%)	O (wt.%)	${\mathrm{S}}_{\mathit{BET}}$ (m2g−1)	${\mathrm{T}}_{\mathit{ads}}$ (K)	AdC(CO2) (mol kg−1)	Ref.
Pristine biochars
Bamboo-500	500	-	71.1	23.1	n.a	303	1.7	This study
Bamboo-700	700	-	75.9	17.3	365	303	2.1	This study
Bamboo	500	-	66.2	24.8	92	298	2.1	[98]
Bamboo	600	-	66	29.2	89	298	2.2	[98]
Sawdust	450	-	82.3	14.1	8.8	303	0.4	[101]
Sawdust	750	-	97.3	1.53	11.4	303	1.0	[101]
Sawdust	850	-	93.4	5.2	182	303	1.1	[101]
Walnut shell	500	-	69.4	25.1	94.5	298	0.6	[100]
Walnut shell	700	-	80.3	17.5	265	298	1.3	[100]
Walnut shell	900	-	84.9	13.6	397	298	1.6	[100]
Spent coffee grounds	500	-	79.7	5.4	311	303	0.7	[99]
Rambutan peel	500	-	76.4	19.2	7.80	303	0.6	[104]
Rambutan peel	700	-	81.6	15.9	176	303	1.3	[104]
Rambutan peel	900	-	83.4	14.7	570	303	1.6	[104]
Perilla leaf	700	-	71.8	15.3	473.4	323	2.3	[105]
Korean oak	400	-	88.7	9.7	270.8	323	0.6	[105]
Japanese oak	500	-	89.9	7.5	475.6	323	0.4	[105]
Soybean stover	700	-	81.9	15.5	420.3	323	0.7	[105]
Activated biochars
Bamboo-A-900	900	CO2	68.5	20.2	1220	303	2.6	This study
Bamboo	800	CO2	n.a	n.a	637	298	2.0	[38]
Bamboo	873	KOH	n.a	n.a	1846	298	4.5	[83]
Bamboo	973	KOH	n.a	n.a	930	298	4.0	[83]
Bamboo	700	KOH	n.a	n.a	540	298	3.4	[57]
Bamboo	1073	K2CO3	70.4	n.a	1802	298	3.4	[37]
Bamboo	500	NaNH2	74.6	n.a	1286	298	3.5	[39]
Sargassum	800	KOH	n.a	n.a	292	298	1.1	[106]
Almond shells	750	CO2	n.a	n.a	822	298	2.7	[102]
Commercial kevlar	1000	CO2	n.a	n.a	1593	303	1.5	[9]
Commercial kevlar	1000	CO2	n.a	n.a	1586	303	1.7	[9]
Beech wood	1087	CO2	n.a	n.a	1586	303	2.1	[103]

Table 4. Comparison of CO₂ adsorption into biochars derived from various feedstocks at 0.1 MPa with their properties. T_ads: Adsorption temperature; AdC: Adsorption Capacity; n.a: not available.

Samples	Tprocess (°C)	Activating Agent	C (wt.%)	O (wt.%)	${\mathrm{S}}_{\mathit{BET}}$ (m2g−1)	${\mathrm{T}}_{\mathit{ads}}$ (K)	AdC(CO2) (mol kg−1)	Ref.
Pristine biochars
Bamboo-500	500	-	71.1	23.1	n.a	303	1.7	This study
Bamboo-700	700	-	75.9	17.3	365	303	2.1	This study
Bamboo	500	-	66.2	24.8	92	298	2.1	[98]
Bamboo	600	-	66	29.2	89	298	2.2	[98]
Sawdust	450	-	82.3	14.1	8.8	303	0.4	[101]
Sawdust	750	-	97.3	1.53	11.4	303	1.0	[101]
Sawdust	850	-	93.4	5.2	182	303	1.1	[101]
Walnut shell	500	-	69.4	25.1	94.5	298	0.6	[100]
Walnut shell	700	-	80.3	17.5	265	298	1.3	[100]
Walnut shell	900	-	84.9	13.6	397	298	1.6	[100]
Spent coffee grounds	500	-	79.7	5.4	311	303	0.7	[99]
Rambutan peel	500	-	76.4	19.2	7.80	303	0.6	[104]
Rambutan peel	700	-	81.6	15.9	176	303	1.3	[104]
Rambutan peel	900	-	83.4	14.7	570	303	1.6	[104]
Perilla leaf	700	-	71.8	15.3	473.4	323	2.3	[105]
Korean oak	400	-	88.7	9.7	270.8	323	0.6	[105]
Japanese oak	500	-	89.9	7.5	475.6	323	0.4	[105]
Soybean stover	700	-	81.9	15.5	420.3	323	0.7	[105]
Activated biochars
Bamboo-A-900	900	CO2	68.5	20.2	1220	303	2.6	This study
Bamboo	800	CO2	n.a	n.a	637	298	2.0	[38]
Bamboo	873	KOH	n.a	n.a	1846	298	4.5	[83]
Bamboo	973	KOH	n.a	n.a	930	298	4.0	[83]
Bamboo	700	KOH	n.a	n.a	540	298	3.4	[57]
Bamboo	1073	K2CO3	70.4	n.a	1802	298	3.4	[37]
Bamboo	500	NaNH2	74.6	n.a	1286	298	3.5	[39]
Sargassum	800	KOH	n.a	n.a	292	298	1.1	[106]
Almond shells	750	CO2	n.a	n.a	822	298	2.7	[102]
Commercial kevlar	1000	CO2	n.a	n.a	1593	303	1.5	[9]
Commercial kevlar	1000	CO2	n.a	n.a	1586	303	1.7	[9]
Beech wood	1087	CO2	n.a	n.a	1586	303	2.1	[103]

In summary, the results indicate that bamboo biochars have strong potential for CO₂ adsorption. Bamboo-700 particularly with a CO₂ capacity of 2.1 mol kg⁻¹ shows competitive adsorption capacities compared to other pristine biochars and even activated biochars (e.g., 1.5 mol kg⁻¹ for activated commercial Kevlar [9] and 1.1 mol kg⁻¹ for activated sargassum [106]). Comparison with other studies highlights that while bamboo biochars are highly effective, there is potential for further optimization, particularly through chemical activation methods to enhance their adsorption properties [40]. However, chemical activation requires more energy and incurs additional costs for washing out excess agents, and it may negatively impact the separation performance of CO₂/CH₄ mixtures [107].

4. Conclusions

Three carbon-based samples derived from bamboo were prepared, characterized through a series of physico-chemical analysis, and evaluated based of their adsorption properties in this study: Bamboo-500 (pyrolyzed at 500 °C), Bamboo-700 (pyrolyzed at 700 °C), and Bamboo-A-900 (pyrolyzed and activated with CO₂ at 900 °C). Increasing the pyrolysis temperature from 500 °C to 700 °C enhances the adsorption capacities of CO₂ and CH₄ due to the improvement of the surface area and pore structure of the bamboo-derived samples. Further activation of the pyrolyzed bamboo with CO₂ at 900 °C significantly increases the specific surface area and pore texture, as evidenced by N₂ adsorption analysis and the decrease of the carbon content (68.5%) compared to the pyrolyzed samples (71.1% for Bamboo-500 and 75.9 for Bamboo-700), thus improving the adsorption properties of the materials. SEM analysis revealed that the activated bamboo (Bamboo-A-900) displayed smaller fiber size and numerous honeycombed macropores compared to the pyrolyzed biochars (Bamboo-500 and Bamboo-700) enhancing gas diffusion kinetics. The presence of CaCO₃ in Bamboo-700 and its absence in activated biochar suggests thermal decomposition during pyrolysis, releasing CO₂ and leaving behind CaO. The heat of CO₂ adsorption indicates that the bamboo family possesses a heterogeneous surface, as evidenced by the decrease in heat with increasing pressure. At very low pressure (<0.12 MPa), Bamboo-500 exhibits superior adsorption performance due to its lower heat of adsorption, a result of its less developed structure. In contrast, Bamboo-A-900 shows the highest adsorption performance at medium and high pressures, attributed to its large surface area, and presence of CaO, which interacts with CO₂ molecules despite high CH₄ adsorption capacity and heat of adsorption. Integrating biogas upgrading and bamboo utilization represents a significant step towards sustainable energy production and environmental responsibility. The activated sample Bamboo-A-900 enhances CO₂ adsorption capacity, facilitating biogas purification. Therefore, raw bamboo is a viable biomass source, producing effective samples with favorable physicochemical characteristics that can help reduce the cost of biogas upgrading processes, making them more sustainable and economically viable for industrial applications.

5. Perspectives

Future studies should investigate chemical modifications to biochars and activated biochars to enhance adsorption performance, especially for CO₂ separation in binary and ternary mixtures. For example, Bamboo-A-900 shows the highest BET surface area (1220 m²g⁻¹) and substantial adsorption capacities for both CO₂ and CH₄ (8.0 ± 0.3 and 3.4 ± 0.2 mol kg⁻¹, respectively). Conducting chemical modification of its surface with basic chemical materials can enhance the separation of CO₂ over CH₄, and therefore the selectivity. Additionally, the AAPI approach can be further explored for its application to other types of adsorbents to assess their potential for biogas upgrading. Furthermore, future studies should assess the performance of bamboo-derived adsorbents in real biogas conditions, taking into account impurities such as H₂S and water vapor, which could influence the adsorption capacity and selectivity. For instance, moisture and H₂S may reduce the effectiveness of adsorbents by interfering with the adsorption sites and altering the material properties. Studying these effects will provide a more comprehensive understanding of the adsorbents’ suitability for biogas upgrading under practical conditions.