1 Introduction

One of the main greenhouse gases, CO2, is responsible for disturbing the cycle of normal climate conditions. It is a major contributor toward global warming. The main source for the accumulation of CO2 in the atmosphere is by combustion of fossil fuels and deforestation. The present and expected future demand for global energy suggests that it is impossible to reduce the utilization of fossil fuels. There is a need to stabilize the climate changes by controlling CO2 emission. In the twenty-first century, global climate change due to the rise of CO2 in the atmosphere has become one of the important environmental and energy concerns [1]. Therefore, to reduce the CO2 level in the atmosphere, carbon capture technologies are the major optional methods [2, 3].

Various activation methods were developed for CO2 capture such as physical [4, 5], chemical [6,7,8] and membrane technology [9,10,11]. Among these methods, adsorption by using solid adsorbents is the prominent approach due to the requirement of low energy, low cost and easy operation [9, 10, 12, 13]. Solid adsorbents such as metal–organic frameworks [14], zeolites [15], zeolitic imidazole frameworks [16], calcium oxides [17] and supported amines [18] are used for CO2 capture. The use of these solid adsorbents needs specific techniques and is associated with high cost. Under these conditions, many researchers have focused on the development of solid adsorbents from renewable sources. Waste biomass is a renewable source for synthesis of low-cost solid carbon adsorbents [19]. The solid adsorbents, activated carbons (AC) derived from plant waste biomass, could be low-cost materials and are considered as excellent materials in the adsorption process because of their high specific surface area and pore volume [20, 21]. As materials with high surface area, adequate porosity, and high mechanical strength containing active carbons required, various waste plant materials have been used to prepare these carbons [22]. Activated carbons are prepared in two ways, one by physical activation (CO2 or steam activation) [23, 24] and the second by chemical activation (KOH, ZnCl2, H3PO4, etc.) [25, 26]. The porous carbons were modified using physical and chemical activation processes to create carbon with high surface area and tunable porosity to improve CO2 adsorption capacity [27]. Carbon is exposed to a flow of steam or CO2 during physical activation procedures. More reactive carbon atoms are removed from the carbon skeleton during this process, resulting in a larger number of pores, improved surface area and porosity [28]. On the contrary, chemical activation method is frequently carried out through impregnation of the carbon with chemical activating agent, followed by carbonization at high temperature at atmospheric condition to form new pores [29]. Among these two methods, chemical activation can lead to generation of high surface area with small pore size materials than the ones prepared by the physical activation method [30].

Many researchers have recently published CO2 adsorption capability of several biomass-based porous carbon adsorbents under various conditions. For example, Khuong et al. examined CO2 capture capacity of carbon derived from bamboo and found that the solid residue had a CO2 adsorption capacity of 3.4 mmol/g at ambient conditions [31]. Yang et al. used KOH to activate coconut shell to produce narrow microporous activated carbons with CO2 adsorption capability of 4.23 mmol/g [32]. At ambient conditions, carbon produced from corn kernel biomass material had a maximum CO2 adsorption performance of 3.63 mmol/g [33]. Intan Syafiqah Ismail, et al. published that bamboo-based activated carbon produced from single-step H3PO4 activation had a maximum CO2 absorption of 1.46 mmol/g at ambient conditions [34]. Zubbri et al. found that activating rambutan peels treated with KOH resulted in a higher CO2 adsorption capacity of 2.78 mmol/g at 30 °C and atmospheric pressure [35]. However, it is required to provide a suitable activated carbon adsorbent for maximum CO2 adsorption capacity by utilizing waste biomass materials.

In continuation of our effort to prepare carbon-based adsorbents from waste biomass, subabul, a widely grown tree in the tropical region, is used as source for preparation of carbon. The scientific name of the commonly called subabul is Leucaena leucocephala. It comes under Fabaceae family and sub-family Mimosoideae [36]. It is also known as white lead tree, jumbay, river tamarind or white popinac. Subabul trees are available abundantly in the world, particularly in the southern (Mexico) and northern America, India and other tropical countries. Subabul is widely used for soil fertility, fencing, firewood, paper industry and fiber.

In the current work, we prepared different types of carbon materials for carbon dioxide capture using subabul stems as biomass raw material. Different carbon materials are prepared by direct carbonization and chemical activated method. The carbon materials were characterized thoroughly and used as adsorbents for selective adsorption of CO2 under different conditions. The adsorption capacities of these materials were explained based on their properties.

2 Experimental

2.1 Preparation of carbon materials from subabul wood

Subabul wood was procured from local suppliers in Hyderabad, India, and H3PO4 was procured from SD Fine Chemicals, India. The procured subabul wood was milled to powder and washed with distilled water, followed by drying in an oven at 100 °C for 12 h. The derived wood powder was divided into different portions. Each portion of the powder was carbonized at different temperatures. The carbonization temperatures varied from 600 to 900 °C under nitrogen flow (40 ml/min) for 4 h. The obtained materials were denoted as SB-600, SB-700, SB-800 and SB-900, where SB indicates subabul wood and the number indicates the carbonization temperature. The second type of adsorbent was prepared by treating the wood powder with H3PO4 at room temperature. In this method, the biomass powder was treated with 88% H3PO4 solution (1:3 weight ratio) for 4 h. Then the wood mass was washed with distilled water repeatedly to get neutral pH. The solid biomass was kept in an oven for drying overnight at 100 °C and subjected to carbonization at different temperatures in the range of 600–900 °C. These samples are denoted as SBPA-600, SBPA-700, SBPA-800 and SBPA-900, where SBPA stands for subabul treated with H3PO4 and the number indicates the carbonization temperature.

2.2 Characterization

BET surface area of the adsorbents was measured from N2 adsorption–desorption data acquired on BELSORB II Instrument, Japan. Powder X-ray diffraction (XRD) patterns of the adsorbents were recorded on Rigiku Miniflex (Rigaku Corporation, Japan) X-ray diffractometer using Ni filtered CuKα radiation (λ = 1.5406 Å) in the scan range of 10–80°. FT-IR spectra of the samples were obtained on FT-IR DIGILAB Biorad spectrometer using a KBr disc method. C, H, N and S elemental analysis was carried out on a Vario Micro Cube elemental analyzer. Raman spectra of carbon adsorbents were analyzed with Horiba JobinYvon LabRAM HR spectrometer. Thermo Scientific K-ALPHA surface analysis spectrometer was used to obtain X-ray photoelectron spectra (XPS) using a monochromatic, micro-focused Al–Ka radiation (1486.6 eV). SEM images of the adsorbents were obtained on a JEOL FE-SEM-7610F microscope. The detailed procedure of all characterization techniques was presented in our previous publication [37].

2.3 Carbon dioxide adsorption measurements

CO2 adsorption studies were conducted in a dynamic adsorption flow system. The adsorption was performed in a fixed bed reactor (SS, 41 cm length: 0.9 cm id). In a typical experiment, about 1 g of carbon adsorbent was mixed with 0.5 g of quartz beads suspended in the middle of the reactor between two quartz plugs and pretreated under helium atmosphere at 200 °C for 1 h. Helium was passed through the reactor bed at the required flow rate with the help of mass flow controllers for removal of moisture content present on the adsorbent. 10% CO2 balance He gas mixture was passed over the adsorbent at the desired temperature (25–70 °C) and the gas concentration was monitored at the outlet periodically. After the sample had attained its CO2 saturation capacity, it was purged with He gas for 30 min. The outlet gas was analyzed online using a gas chromatograph (Agilent Technologies 7820A equipped) equipped with a thermal conductivity detector and a Porapak Q column. The adsorption data were collected at three different temperatures (25, 50 and 70 °C). Then the adsorption sample was flushed with pure helium gas for 30 min and desorption was carried out by increasing the temperature at a rate of 5 °C/min until it reached 160 °C.

2.4 CO2 adsorption kinetics

In the present study, the kinetic analysis of CO2 adsorption is evaluated at different temperatures (i.e., 25 °C, 50 °C and 70 °C) based on pseudo-first-order, pseudo-second-order and Avrami fractional order kinetic models.

Pseudo-first-order kinetic model assumes that the rate of adsorption is proportional to the number of available active sites on the adsorbent. The integrated form of the models is expressed as:

$${Q}_{t}={Q}_{e}(1-\mathrm{exp}\left(-{k}_{f}t\right),$$

where \({Q}_{t}\) and \({Q}_{e}\) are the amount of CO2 adsorbed for unit mass of adsorbent (mmole/g) at any time, t, and at equilibrium, respectively, and \({k}_{f}\) (min−1) is the rate constant.

In pseudo-second-order kinetic model, the rate of adsorption is proportional to the square of number of free active sites on the adsorbent. The integrated form is given by:

$${Q}_{t}= \frac{{Q}_{e}^{2}{k}_{s}t}{1+ {Q}_{e}{k}_{s}t},$$

where \({k}_{s}\) (g mole−1 min−1) is the second-order rate constant.

Avrami fractional order kinetic model is expressed as follows:

$${Q}_{t}= {Q}_{e}\left[1-\mathrm{exp}{(-({k}_{A}t)}^{{n}_{A}})\right],$$

where \({k}_{A}\) (min−1) is the Avrami kinetic constant and \({n}_{A}\) is the Avrami exponent.

The accuracy of the model is evaluated based on the coefficient of determination (R2) and is expressed as follows:

$${R}^{2}=1-\frac{\sum_{i=0}^{n}{\left({Q}_{t(exp)}-{Q}_{t\left(model\right)}\right)}^{2}}{\sum_{i=0}^{n}{\left({Q}_{t(exp)}-\overline{{Q }_{t\left(exp\right)}}\right)}^{2}},$$

where \({Q}_{t(exp)}\) and \({Q}_{t\left(model\right)}\) are experimental and calculated values of CO2 absorbed. \(\overline{{Q }_{t\left(exp\right)}}\) is the average value of experimental data. n represents the total number of experimental data points.

Once the kinetic parameters are calculated for the best model, the activation energy is evaluated using rate constant (k) at different temperatures based on Arrhenius equation.

$$k= {k}_{0}{e}^{-E/RT},$$

where \({k}_{0}\) is the Arrhenius pre-exponential factor, E is the activation energy (cal mole−1), R is the universal gas constant (cal mole−1 K−1) and T is the temperature (K).

3 Results and discussion

3.1 Characterization of carbon adsorbents

The N2 adsorption–desorption data were used to determine the specific surface area and porosity of the adsorbents. The isotherms of SB and SBPA carbon adsorbent materials are presented in Fig. 1. The pore size distributions are shown in Fig. 2. According to IUPAC classification, type I isotherm was observed for SB and SBPA carbon adsorbents. Among these, SB carbon adsorbents showed hysteresis loop with type I isotherm, which indicates that the carbons have more micropores and less mesopores. The SB-800 and SB-900 samples not only show type I, but also type IV isotherm with H4-type hysteresis loop associated with monolayer—multilayer adsorption condensation in narrow slit-like pores [38]. SBPA carbons showed type I isotherm and exhibited high nitrogen uptake at low relative pressure (P/P0 < 0.1) associated with the microporous structure. SBPA-800 and SBPA-900 samples have narrow micropores with < 1 nm pore size. These SBPA adsorbents exhibited an identical sharp knee associated with similar microporosity [39]. The textural properties of the adsorbents are shown in Table. 1. The surface area and pore volume of the carbons were increased with carbonization temperature from 600 to 900 °C for both SB and SBPA samples. As the carbonization temperature was increased from 600 to 900 °C, both surface area and pore volumes of the activated carbons increased due to thermal degradation and the volatilization process of the impregnated H3PO4 present in the biomass [38]. In SBPA carbons, temperature rise also led to the generation of a large volume of narrow micropores.

Fig. 1
figure 1

N2 adsorption–desorption isotherms of a SB samples and b SBPA samples

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Fig. 2
figure 2

Pore size distribution of a SB samples and b SBPA samples

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Table 1 Textural properties of the activated carbons
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The XRD patterns of the activated carbons are presented in Fig. 3. The results mainly showed two characteristics peaks at 2θ of 24.2° and 42.6°. These two broad peaks are recognized by the presence of partial multilayered graphitic domains corresponding to the reflection planes of (0 0 2) and (1 0 1) [40, 41]. However, when the carbonization temperature rose from 600 to 900 °C, the position of the XRD peak at 2θ = 24.2° shifted toward a higher degree. This shift suggests that, due to thermal effect at high temperature regions, a reduction in the interlayer distance of the graphite structure as well as the loss of linking functional groups [10, 42]. Nevertheless, after H3PO4 treatment, the XRD peaks at 2θ of 24.2° and 42.6° are at high intensity than those of SB carbons, indicating that presence of more graphitic structure. However, the peak intensity at 2θ = 42.6° of SBPA carbons was decreased with increase in carbonization temperature from 600 to 900 °C. It indicates that during the carbonization process, a greater degree of chemical reaction between H3PO4 and biomass components ensues, resulting in a much higher number of defects in the synthesized activated carbons [38, 42].

Fig. 3
figure 3

XRD patterns of a SB and b SBPA samples

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Laser Raman spectra of the activated carbons are presented in Fig. 4. The carbon adsorbents showed distinct peaks at 1329 and 1595 cm−1 corresponding to the D and G bands of the carbon structure [43, 44]. Naturally, the D band is attributed to amorphous or disordered carbons and the G band suggests the in-plane tangential stretching of C = C bonds in graphene sheets. The degree of graphitization of SB and SBPA carbons was calculated from the intensity ratio of the D and G band. The higher the IG/ID ratio, suggesting a higher degree of graphitization, the smaller is the IG/ID ratio, suggesting a lower degree of graphitization. The SB and SBPA carbons showed IG/ID ratio lower than 1. On increasing the carbonization temperature, the IG/ID ratio was decreased from SB-600 (0.90) to SB-900 (0.81) and SBPA-600 (0.94) to SBPA-900 (0.73), suggesting that the degree of graphitization was decreased. In SBPA carbon’s increase in carbonization temperature, the IG/ID ratio was decreased, suggesting that a greater number of structural defects increased in the samples with low graphitization. This result is in good agreement with XRD and pore size distribution analysis [45, 46].

Fig. 4
figure 4

Raman spectra of a SB samples and b SBPA samples

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FT-IR spectra of carbon samples are depicted in Fig. 5. A common band at 3325 cm−1 was noticed for SB and SBPA activated carbons corresponding to the –O–H stretching vibration of the adsorbed water. In addition, the SB carbons also exhibited different bands at 2762, 1617, 1419, and 1053 879 cm−1. The band at 2762 cm−1 corresponds to asymmetric C–H stretching vibrations. The FT-IR bands observed at 1617 and 1419 cm−1 are related to the C = N stretching vibrations of the carbon material [47]. The peaks appearing at 1053 cm−1 and 879 cm−1 could be assigned to acyclic C–O–C groups, which are conjugated to the C = C and C–S functional groups, respectively [4, 48]. From Fig. 5(a), the band appeared at 1617 cm−1 with high intensity for SB-800 carbon, indicating the presence of a greater number of C = N functional groups [25, 47]. SBPA activated carbons showed bands at 2167, 1689, 1134 and 786 cm−1. The FTIR spectra of the activated carbons showed a 2167 cm−1 absorption band associated with axial deformation present in triple bonds or accumulated double bonds. These bonds probably occurred due to the presence of carboxylic groups in the activated carbon surface [49, 50]. The strong band appeared at 1689 cm−1 ascribed to C = O stretching vibrations of carboxylic groups [4]. The band appearing at 1134 cm−1 could be assigned to the hydrogen-bonded P = O and P = OOH groups of aromatic carbon. Another band observed at 786 cm−1 related to the presence of aromatic substituted aliphatic groups of carbon [51]. The bands appearing at the 1689 and 1134 cm−1 intensity were more for the SBPA-800 sample compared to other SBPA activated carbons. These results indicate the presence of a greater number of C = O and O = P–OH functional groups on the carbon adsorbent. Among SBPA activated carbons, the common band intensity at 3325 cm−1 was decreased and the band intensity at 1689 and 1134 cm−1 was increased compared to SB carbons due to the interaction between H3PO4 and biomass during the activation process [52, 53].

Fig.5
figure 5

FT-IR patterns of a SB samples and b SBPA samples

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The CHNS analyses of SB and SBPA activated carbons are depicted in Table 2. The SB samples contain high carbon content than the SBPA samples. Among the SB carbons, the percentage of C and N contents decreased and at the same time H and O percentages increased with increase in carbonization temperature. With increase in carbonization temperature, the polymeric structure of biomass (lignin, cellulose and hemicelluloses) decomposed and the non-carbon elements such as oxygen and hydrogen were retained. The carbon and nitrogen percentage decreased from 83.43 to 72.02% and 1.28 to 0.97%, respectively, with change in carbonization temperature from 600 to 900 ºC. At the same time, the hydrogen and oxygen percentages increased from 1.73 to 2.14 and 9.02 to 14.38%, respectively [38]. The SBPA samples also showed similar trend with change in carbonization temperature. During the acid activation process, H3PO4 might react with biomass and then the volatile matter gets diffused speedily out of the surface. This is the reason for an increase in the loss of carbon content for SBPA carbon samples. The carbon percentage of SBPA samples was decreased from 71.65 to 59.63% and the oxygen percentage increased from 15.22 to 18.56% with increase in carbonization temperature from 600 to 900 ºC. SBPA-900 carbon adsorbent showed less O, H and N content compared to SBPA-800. Further increase in the temperature from 800 to 900 °C caused an increase in the degree of aromaticity [54]. These results are in support of the observations made from Raman and FT-IR analysis.

Table 2 CHNS analysis of SB and SBPA activated carbons
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The surface chemistry of the carbon adsorbents was characterized by X-ray photoelectron spectroscopy. As shown in Fig. 6, the deconvoluted C1s XPS spectra of all four activated carbon adsorbents exhibit four different binding energy peaks. The XPS peak at 284.7 eV represents the graphitic carbon, indicating the successful generation of a graphitic structure after treatment with phosphoric acid. In addition, other binding energy peaks at 285.5, 286.6 and 288.9 eV would characterize the presence of carbon in C–OH/C–O–C/C-O-P, carbonyl (-C = O) and the carboxylic/ester/lactone functional groups, respectively [55, 56].

Fig.6
figure 6

XPS spectra of C1s for SBPA samples: a SBPA-600, b SBPA-700, c SBPA-800 and d SBPA-900

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Furthermore, the deconvoluted O1s spectra of all carbon adsorbents exhibit three different binding energy peaks, indicating different types of oxygen functional groups on the surface of the carbon. As shown in Fig. 7, the B.E at around 530.8–531.1 eV represents the carbonyl oxygen in ketone, and the B.E at around 531.9–532.2 eV characterizes the presence of ester and alcoholic oxygen atoms. The third peak at 533.3 eV could be assigned to non-carbonyl anhydride and ester oxygen atoms [4, 57]. Thus, the ketonic functional groups may act as basic sites for the adsorption of CO2. Moreover, as reported by various studies, it cannot be ruled out that π-electrons in the aromatic ring system of the activated carbon could act as Lewis basic sites for CO2 adsorption [58].

Fig. 7
figure 7

XPS spectra of O1s for SBPA samples: a SBPA-600, b SBPA-700, c SBPA-800 and d SBPA-900

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The atomic percentages obtained from XPS analysis are shown in Table 3. The C and O atomic percentages are in the range from 77.45 to 70.62% and 20.63 to 28.15%, respectively. During the preparation, the increase in the carbonization temperature and H3PO4 ratio with biomass results in primary gasification of the surface carbon and formation of additional oxygen groups on the adsorbent surface. In the activation process with H3PO4, volatile substances diffused fast from the surface and similar findings were reported [38]. The XPS results are in good agreement with the observations made from FT-IR and CHNS analysis.

Table 3 The elemental percentage of SBPA carbon adsorbents from XPS analysis
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The morphology of SB and SBPA activated carbons was captured by FE-SEM analysis and the images are depicted in Fig. 8. Among SB carbons, SB-600 carbon showed smooth surface with irregular arrangements and SB-700 carbon showed flake-like structure. The SB-800 and SB-900 samples showed improvement in the pores with irregular arrangements [25]. SBPA carbons showed the development of microporous structure and porosity with increase in the temperature from 600 to 900 °C due to evaporation of H3PO4 during carbonization, leaving the space previously occupied by it [59]. This is the reason for the development of porosity as noticed in the SEM images. Among SBPA activated carbons, SBPA-800 showed honeycomb structure with high microporosity and these results are in good agreement with surface area and pore size distribution of the samples.

Fig. 8
figure 8

FE-SEM images of a SB-600, b SB-700, c SB-800, d SBPA-900, e SBPA-600, f SBPA-700, g SBPA-800 and h SBPA-900

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3.2 CO2 adsorption capacity over SB and SBPA activated carbons

CO2 adsorption capacity of SB and SBPA activated carbons was determined in a fixed bed down flow reactor and the adsorption profile estimated by generating breakthrough curves (BTC) for each sample at different temperatures,  are presented in Fig. 9. The CO2 capture capacity was estimated from BTC for each carbon at different temperatures ranging from 25 to 70 °C and the results are presented in Table. 4. The adsorbents were prepared after treatment with H3PO4, i.e., SBPA samples exhibited high CO2 adsorption capacity compared to their corresponding SB samples prepared without any treatment.

Fig. 9
figure 9

Breakthrough curves of SBPA-800 activated carbon at different temperatures

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Table 4 The CO2 capture capacities of SB and SBPA activated carbons at different temperatures
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The CO2 adsorption capacity was decreased with increase in adsorption temperature from 25 to 70 °C, as the CO2 capturing process is exothermic in nature. This is the reason for the decrease in the CO2 capture capacity with increase in temperature. The adsorption capacities can be explained based on their characteristics. The CO2 adsorption capacity depended on the textural properties of activated carbons, such as surface area, pore volume and pore size distribution.

The adsorbents derived from different raw materials exhibited CO2 adsorption capacities based on their pore structures. It is reported that the phenolic resin-based carbon spheres with high portion of small micropores (0.8–1.0 nm) showed high CO2 adsorption capacity under ambient conditions. In another study, carbon derived from polymer spheres treated with KOH contains fine microporosity, which resulted in superior CO2 adsorption capacity of 4.6 mmol/g at 23 °C [60, 61]. The activated carbon derived from polysaccharides and sawdust biomass treatment with KOH showed high CO2 capture of 4.8 mmol/g due to the presence of a large number of narrow micropores (< 1 nm) [62]. These studies provided experimental confirmation for the importance of small micropores in CO2 capture. Thus, the results conclude that high volume of small micropores play an important role for achieving high CO2 capture under ambient conditions [63]. The CO2 capture capacity of activated carbon prepared from H3PO4-treated cypress sawdust biomass containing micropores showed about 1.73 mmol/g CO2 adsorption capacity at 25 °C [64]. The CO2 capture capacity of granular bamboo-derived carbon with narrow micropore (diameter of < 1) showed 4.6 mmol/g [65]. A high CO2 adsorption was showed by the narrow microporous material, mainly due to the enhancement in the strength of interaction between pore walls and CO2 molecules. Therefore, to get high CO2 adsorption capacity, it is imperative to synthesize carbon adsorbents containing narrow micropores [25, 66, 67].

The present carbons prepared after H3PO4 showed the presence of reasonable amounts of micropores. As predicted, the carbon adsorbents derived from subabul wood activation using H3PO4, followed by carbonization considerably increased the specific surface area from 57 m2/g to 905 m2/g and pore volume from 0.05 to 0.27 cm3/g when compared between SB and SBPA samples. As carbonization temperature increased to 900 °C, the microporosity of the carbon materials improved. Consequently, the mean pore diameter of the pore also decreased close to very narrow micropore range (< 1 nm). Therefore, the low surface area carbon adsorbent SB-600 exhibited lower CO2 adsorption capacity. Moreover, carbon with microporosity less than 1 nm can capture more amount of CO2 due to larger adsorption potential [68]. Among these carbons, SBPA-800 carbon showed high CO2 adsorption capacity of 4.52 mmol/g, as it contains high quantity of micropores (pore size = 0.85 nm and surface area 823 m2/g). The adsorption capacities of carbon samples not only depended on porosity, but also depend on basic functional groups of nitrogen, oxygen and sulfur present in it [69]. The N-doped carbon prepared from coal-based pitchblende showed maximum CO2 adsorption capacity of 3.4 mmol/g at 25 °C [70]. The oxygen-functionalized carbon with oxygen percentage of 19.3 showed maximum CO2 adsorption capacity of 3.9 mmol/g at 25 °C, because of the specific interaction between oxygen functionalities and acidic nature of CO2 [71]. These results conclude that the basic sites of nitrogen can attract acidic CO2 more at ambient conditions. The high content of nitrogen, oxygen and sulfur present in SBPA-800 is as shown in Table 2. These might contribute to its high CO2 adsorption capacity. When the sulfur is present in the form of oxide (–SO or –SO2, etc.), the negatively charged oxygen might bind with CO2 and enhance CO2 capture [25]. These are the reasons for enhanced CO2 capture at ambient conditions for the present SBPA carbon samples.

Figure 10 indicates the relationship between CO2 adsorption capacity and micropore volume of the SBPA samples. A linear relationship between micropore volume and CO2 adsorption capacity was observed. This indicates that CO2 adsorption capacity increased with increase in the micropore volume. Micropores less than 0.8 nm pore size significantly affects the CO2 adsorption capacity at low temperature and atmospheric pressure. The reason for these narrow micropores exhibiting the strong adsorption capacity is that they can enhance the strength of interaction between pore walls with CO2 molecules. Hence, a high CO2 adsorption capacity was observed with narrow micropores [25]. One can conclude based on the result that SBPA-800 showed high adsorption capacity, mainly because of the presence of narrow micropores.

Fig. 10
figure 10

Relationship between CO2 adsorption capacity and micropore volume of SBPA samples

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3.3 Adsorption kinetics

The adsorption kinetic models are developed for SBPA-800 activated carbon of this work using the experimental data collected from CO2 adsorption studies conducted at three different temperatures (i.e., 25 °C, 50 °C and 70 °C). Figure 11 gives the complete picture of the evaluation of adsorption kinetics of all three models: pseudo-first-order, pseudo-second-order and Avrami fractional order kinetic models from experimental data evaluated at 25 °C, 50 °C and 70 °C in Fig. 11 (a–c), Fig. 11(d–f) and Fig. 11 (g–i), respectively. The models are linearized and based on the slope and intercepts of the respective graphs, the kinetic parameters of individual models are generated. The values of kinetic parameters for each model and their corresponding R2 values are listed in Table 5. It is found that pseudo-first-order kinetic model resulted in relatively high R2 values for all the temperatures compared to other kinetic models. Therefore, it is concluded that the pseudo-first-order kinetics is the best model to describe the kinetic behavior of CO2 adsorption. It describes that the adsorption process of SBPA-800 carbon adsorbent is physical adsorption [72, 73], indicating that adsorbed CO2 was easily desorbed and had stable recyclability. Figure 12 shows the comparison of kinetic models at different temperatures (i.e., 25 °C, 50 °C and 75 °C) with experimental absorption capacity. Further, the activation energy is evaluated using Arrhenius equation by considering the kinetic parameter, kf, of first-order kinetics at different temperatures and it is found to be 260.0387 cal/mole and thepre-exponential factor ko is 24.53. The activation energy is Ea > 0, meaning that when the temperature is increased, the reaction rate is increased. This positive Ea value confirms again the physical adsorption with the relatively weak attraction forces.

Fig.11
figure 11

Evaluation of adsorption kinetics from experimental data a–c at 25 °C, d–f 50 °C and g–i 70 °C

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Table 5 Kinetic parameters for models with corresponding R2 values for SBPA-800
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Fig.12
figure 12

Experimental value of absorption capacity (Qt) and corresponding fit to the kinetic models with time at temperatures: a 25 °C, b 50 °C, c 70 °C and d Arrhenius plot

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3.4 Comparison with other carbon adsorbents

The present SBPA-800 activated carbon adsorbent for CO2 is compared with other adsorbents derived from different biomass sources. The CO2 capture capacity determined at 25 °C of the reported adsorbents is presented along with that of the present adsorbent in Table 6. Carbon derived from coconut shell and olive stones after carbonization at 800 °C showed less CO2 adsorption capacity of about 3–3.9 mmol/g than the presented one. The carbon derived from bamboo and other few biomasses showed relatively less or equivalent carbon to SBPA-800. The results show that the carbons derived from different biomasses exhibited low CO2 adsorption capacity compared to the present adsorbent derived from subabul wood.

Table 6 Comparison of activated carbons prepared from different sources for CO2 capture
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3.5 Recyclability of SBPA-800 activated carbon adsorbent

The SBPA-800 activated carbon adsorbent was tested for its recyclability by conducting adsorption and desorption of CO2 at 25 °C and 160 °C, respectively and the results are presented in Fig. 11. The CO2 capture capacity followed by desorption at 160 °C has sustained consequently for ten times. The results show that there is no change in the CO2 capture capacity during the recycles. A marginal decrease in CO2 capture capacity for the first six cycles was noticed. Thereafter, the same CO2 capture capacity was noticed up to ten consecutive Fig. 13 cycles. The results support that the SBPA-800 activated carbon derived after treatment with H3PO4 showed not only high capture of CO2, but also consistent activity on reuse.

FIG. 13
figure 13

Recyclability of SBPA-800 activated carbon for CO2 capture at 25 °C and atmospheric pressure

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4 Conclusions

Summing up, the present research describes the synthesis of low-cost carbon adsorbents from subabul wood with and without treatment at different carbonization temperatures. The carbon adsorbents synthesized with treatment of H3PO4 showed high CO2 capture capacity than the carbons derived without any treatment. The nature of pretreatment of the raw biomass and carbonization temperature directs the surface—structural features of the adsorbents. The carbon derived after acid treatment, followed by carbonization at 800 °C exhibited high CO2 adsorption of 4.51 mmol/g at ambient temperature and atmospheric pressure. The excellent CO2 capture ability of carbon derived after acid treatment and carbonization at 800 °C was mainly related to its high surface area (823 m2/g), micropore volume (0.17 cm3/g), high content of nitrogen and presence of more basic sites of oxygen functional groups. The present carbon adsorbents derived from subabul exhibited consistent adsorption–desorption of CO2 up to ten cycles. The CO2 adsorption process on the SBPA carbon adsorbents follows pseudo-first-order kinetics and the activation energy (Ea) has positive value. These confirm that the adsorption process on SBPA carbons is physical adsorption with the relatively weak attraction forces. Finally, our data imply that SBPA-800 carbon adsorbent could be a feasible CO2 capture material.